Evolution of mammals

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Restoration of Procynosuchus, a member of the cynodont group, which includes the ancestors of mammals

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes.^[1] Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.

Mammals are the only living synapsids.^[2] The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago.^[3] The sauropsids are today's reptiles and birds along with all the extinct animals more closely related to them than to mammals.^[3] This does not include the mammal-like reptiles, a group more closely related to the mammals.

Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores.
In the subsequent Triassic period, however, a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.

Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; but, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.

Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and warm-bloodedness.

Definition of "mammal"

Figure 1: Mammalian and non-mammalian jaws. In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists only of the dentary bone.

While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.

One such feature available for paleontology, shared by all living mammals (including monotremes), but not present in any of the early Triassic therapsids, is shown in figure 1: mammals use two bones for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian tetrapods use this system including amphibians, turtles, lizards, snakes, crocodilians, dinosaurs (and their descendants the birds), and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal (another small skull bone). In the Jurassic, their quadrate and articular bones evolved into the incus and malleus bones in the middle ear.^[4][5] Mammals also have a double occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.^[4]

In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molar occlusion was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved."^[6] Today, most paleontologists consider the animals satisfying this criterion to be mammals.^[7]

The ancestry of mammals

Tetrapods  Amphibians  Amniotes  Sauropsids (including dinosaurs)  Synapsids  Caseids  Cotylorhynchus  Eupelycosaurs Edaphosaurids  Edaphosaurus   Sphenacodontians  Sphenacodontids    Dimetrodon  Therapsids      Mammals

Pelycosaurs

Amniotes

The first fully terrestrial vertebrates were amniotes — their eggs had internal membranes that allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the Surinam toad, have evolved other ways of getting around this limitation). The first amniotes apparently arose in the middle Carboniferous from the ancestral reptiliomorphs.^[8]

Within a few million years, two important amniote lineages became distinct: mammals' synapsid ancestors and the sauropsids, from which lizards, snakes, crocodilians, dinosaurs, and birds are descended.^[3] The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus, respectively) date from about 320 to 315 million years ago. It is difficult to be sure about when each of them evolved, since vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier.^[9]

Synapsids

The original synapsid skull structure has one hole behind each eye, in a fairly low position on the skull (lower right in this image).

Synapsid skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:

• made the skull lighter without sacrificing strength.
• saved energy by using less bone.
• probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.

The synapsid pelycosaurs included the largest land vertebrates of the Early Permian, such as the 6 m (20 ft) long Cotylorhynchus hancocki. Among the other large pelycosaurs were Dimetrodon grandis and Edaphosaurus cruciger.

Therapsids

Therapsids descended from pelycosaurs in the middle Permian and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.^[10]

The therapsid lineage that led to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with some that could easily be mistaken for mammals:^[11]

• gradual development of a bony secondary palate. Most books and articles interpret this as a prerequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectotherms use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing.^[12] The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, since chewing makes it possible to digest food more quickly. In mammals, the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
• the dentary gradually becomes the main bone of the lower jaw.
• progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was erratic and very slow — for example: all herbivorous therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact, modern monotremes still have semi-sprawling limbs.

Therapsid family tree

Therapsids

Biarmosuchia Eutherapsida Dinocephalia Neotherapsida Anomodonts Dicynodonts Theriodontia Gorgonopsia Eutheriodontia Therocephalia Cynodontia (Mammals, eventually)

Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.

Biarmosuchia

The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.^[13]

Dinocephalians

Dinocephalians ("terrible heads") included both carnivores and herbivores. They were large; Anteosaurus was up to 6 m (20ft) long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs.
In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather "reptilian".^[14]

Anomodonts

Lystrosaurus, one of the few genera of dicynodonts that survived the Permian-Triassic extinction event

The anomodonts ("anomalous teeth") were among the most successful of the herbivorous therapsids — one sub-group, the dicynodonts, survived almost to the end of the Triassic. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians. ^[15]

Theriodonts

The theriodonts ("beast teeth") and their descendants had jaw joints in which the lower jaw's articular bone tightly gripped the skull's very small quadrate bone. This allowed a much wider gape, and one group, the carnivorous gorgonopsians ("gorgon faces"), took advantage of this to develop "sabre teeth". But the theriodont's jaw hinge had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.

The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalians ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).^[16]

Cynodonts

Artist's conception of the cynodont Trirachodon within a burrow

The cynodonts, a theriodont group that also arose in the late Permian, include the ancestors of all mammals. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.^[17]

Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon; the animals are thought to have been drowned by a flash flood. The extensive shared burrows indicate that these animals were capable of complex social behaviors.^[18]

Triassic takeover

The catastrophic Permian-Triassic mass extinction slightly more than 250 million years ago killed off about 70 percent of terrestrial vertebrate species and the majority of land plants.

As a result,^[19] ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, who were dominant predators in the late Permian,^[20] the cynodonts' principal competitors for dominance of the carnivorous niches were a previously obscure sauropsid group, the archosaurs, which includes the ancestors of crocodilians and dinosaurs.

The archosaurs quickly became the dominant carnivores,^[20] a development often called the "Triassic takeover."

Their success may have been due to the fact that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea, as mammals do today; considerable water is required to keep urea dissolved.^[21]

The Triassic takeover may have been a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches that had minimal competition from the archosaurs: the tritylodonts, who were herbivores, and the mammals, most of whom were small nocturnal insectivores (although some, like Sinoconodon, were carnivores that fed on vertebrate prey, while still others were herbivores or omnivores).^[22] As a result:

• The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
• As the body length of the mammals' ancestors fell below 50 mm (2 inches), advances in thermal insulation and temperature regulation may have become necessary for nocturnal life.^[23]
• Acute senses of hearing and smell became vital.
  • This accelerated the development of the mammalian middle ear.
  • The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight.^[24] Brain tissue requires a disproportionate amount of energy.^[25][26] The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
• Probably as a side-effect of the nocturnal life, mammals lost two of the four cone opsins, photoreceptors in the retina, present in the eyes of the earliest amniotes. Paradoxically, this may have improved their ability to discriminate colors in dim light.^[27]

This retreat to a nocturnal role is called to nocturnal bottleneck, and is thought to explain many of the features of mammals.^[28]

From cynodonts to crown mammals

Fossil record

Mesozoic synapsids that had evolved to the point of having a jaw joint composed of the dentary and squamosal bones are preserved in few good fossils, mainly because they were mostly smaller than rats:

• They were largely restricted to environments that are less likely to provide good fossils. Floodplains as the best terrestrial environments for fossilization provide few mammal fossils, because they are dominated by medium to large animals, and the mammals could not compete with archosaurs in the medium to large size range.
• Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including fungi and bacteria) and by being trodden on.
• Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.

In the past 40 years, however, the number of Mesozoic fossil mammals has increased decisively; only 116 genera were known in 1979, for example, but about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".^[29]

Mammals or mammaliaforms?

Some writers restrict the term "mammal" to the crown group mammals, the group consisting of the most recent common ancestor of the monotremes, marsupials, and placentals, together with all the descendants of that ancestor.

In an influential 1988 paper, Timothy Rowe advocated this restriction, arguing that “ancestry... provides the only means of properly defining taxa” and, in particular, that the divergence of the monotremes from the animals more closely related to marsupials and placentals ”is of central interest to any study of Mammalia as a whole.”^[30] To accommodate some related taxa falling outside the crown group, he defined the Mammaliaformes as comprising "the last common ancestor of Morganucodontidae and Mammalia [as he had defined the latter term] and all its descendants." Besides Morganucodontidae, the newly defined taxon includes Docodonta and Kuehneotheriidae. Though haramiyids have been referred to the mammals since the 1860s,^[31] Rowe excluded them from the Mammaliaformes as falling outside his definition, putting them in a larger clade, the Mammaliamorpha.

Some writers have adopted this terminology noting, to avoid misunderstanding, that they have done so. Most paleontologists, however, still think that animals with the dentary-squamosal jaw joint and the sort of molars characteristic of modern mammals should formally be members of Mammalia.^[7]

Where the ambiguity in the term "mammal" may be confusing, this article uses "mammaliaform" and "crown mammal".

Family tree — cynodonts to crown group mammals

Cynodontia	Dvinia Procynosuchidae Epicynodontia Thrinaxodon Eucynodontia Cynognathus Tritylodontidae Traversodontidae Probainognathia Tritheledontidae Chiniquodontidae Prozostrodon Mammaliaformes Morganucodontidae Docodonta Hadrocodium Kuehneotheriidae crown group Mammals

Morganucodontidae and other transitional forms had both types of jaw joint: dentary-squamosal (front) and articular-quadrate (rear).

Morganucodontidae

The Morganucodontidae first appeared in the late Triassic, about 205M years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints.^[32] They were also one of the first discovered and most thoroughly studied of the mammaliaforms outside of the crown-group mammals, since an unusually large number of morganucodont fossils have been found.

Docodonts

Reconstruction of Castorocauda. Note the fur and the adaptations for swimming (broad, flat tail; webbed feet) and for digging (robust limbs and claws).

Docodonts, among the most common Jurassic mammaliaforms, are noted for the sophistication of their molars. Though they were generally herbivorous and insectivorous, an exception is the fish-eating Castorocauda ("beaver tail"), which lived in the mid Jurassic about 164M years ago and was first discovered in 2004 and described in 2006. Castorocauda was not a crown group mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the "small nocturnal insectivore" stereotype:^[33]

• It was noticeably larger than most Mesozoic mammaliaform fossils — about 17 in (43 cm) from its nose to the tip of its 5-inch (130 mm) tail, and may have weighed 500–800 g (18–28 oz).
• It provides the earliest absolutely certain evidence of hair and fur. Previously the earliest was Eomaia, a crown group mammal from about 125M years ago.
• It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammaliaforms were from the Eocene, about 110M years later.
• Castorocauda's powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the platypus, which also swims and digs.
• Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey.

Hadrocodium

The family tree above shows Hadrocodium as an "aunt" of crown mammals. This mammaliaform, dated about 195M years ago in the very early Jurassic, exhibits some important features: ^[34]

• The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
• In therapsids and early mammaliaforms the eardrum may have stretched over a trough at the rear of the lower jaw. But Hadrocodium had no such trough, which suggests its ear was part of the cranium, as it is in crown-group mammals — and hence that the former articular and quadrate had migrated to the middle ear and become the malleus and incus. On the other hand, the dentary has a "bay" at the rear that mammals lack. This suggests that Hadrocodium's dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that Hadrocodium or a very close ancestor may have been the first to have a fully mammalian middle ear.
• Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. Hadrocodium's braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain — in other words it now became possible for mammaliaforms both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.

Earliest crown mammals

The crown group mammals, sometimes  called 'true mammals', are the extant mammals and their relatives back to their last common ancestor. Since this group has living members, DNA analysis can be applied in an attempt to explain the evolution of features that do not appear in fossils. This endeavor often involves molecular phylogenetics, a technique that has become popular since the mid-1980s.

Family tree of early crown mammals

Crown group mammals	Australosphenida Ausktribosphenidae † Monotremes Eutriconodonta † Allotheria    Multituberculates † Spalacotheroidea † Cladotheria Dryolestoidea † Theria Metatheria    Marsupials Eutheria    Placentals

Colour vision

Early amniotes had four opsins in the cones of their retinas to use for distinguishing colours: one sensitive to red, one to green, and two corresponding to different shades of blue.^[35][36] The green opsin was not inherited by any crown mammals, but all normal individuals did inherit the red one. Early crown mammals thus had three cone opsins, the red one and both of the blues.^[35] All their extant descendants have lost one of the blue-sensitive opsins but not always the same one: marsupials and placentals (except for cetaceans) retain one blue-sensitive opsin while monotremes retain the other.^[37] Some placentals and marsupials, including humans, subsequently evolved green-sensitive opsins; like early crown mammals, therefore, their vision is trichromatic.^[38][39]

Australosphenida and Ausktribosphenidae

Ausktribosphenidae is a group name that has been given to some rather puzzling finds that:^[40]

• appear to have tribosphenic molars, a type of tooth that is otherwise known only in placentals and marsupials.^[41]
• come from mid Cretaceous deposits in Australia — but Australia was connected only to Antarctica, and placentals originated in the northern hemisphere and were confined to it until continental drift formed land connections from North America to South America, from Asia to Africa and from Asia to India (the late Cretaceous map here shows how the southern continents are separated).
• are represented only by teeth and jaw fragments, which is not very helpful.

Australosphenida is a group that has been defined in order to include the Ausktribosphenidae and monotremes. Asfaltomylos (mid- to late Jurassic, from Patagonia) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout Gondwanaland (the old Southern hemisphere super-continent).^[42]

Recent analysis of Teinolophos, which lived somewhere between 121 and 112.5 million years ago, suggests that it was a "crown group" (advanced and relatively specialised) monotreme. This was taken as evidence that the basal (most primitive) monotremes must have appeared considerably earlier, but this has been disputed (see the following section). The study also indicated that some alleged Australosphenids were also "crown group" monotremes (e.g. Steropodon) and that other alleged Australosphenids (e.g. Ausktribosphenos, Bishops, Ambondro, Asfaltomylos) are more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below).^[43]

Monotremes

Teinolophos, from Australia, is the earliest known monotreme. A 2007 study (published 2008) suggests that it was not a basal (primitive, ancestral) monotreme but a full-fledged platypus, and therefore that the platypus and echidna lineages diverged considerably earlier.^[43] A more recent study (2009), however, has suggested that, while Teinolophos was a type of platypus, it was also a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to remain attached to their mothers and would drown if their mothers ventured into water (though there are exceptions like the water opossum and the lutrine opossum; however, they both live in South America and thus don't come into contact with monotremes). Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M, when they made their transition from semi-aquatic to terrestrial lifestyle.^[44]

Monotremes have some features that may be inherited from the cynodont ancestors:

• like lizards and birds, they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole").
• they lay eggs that are leathery and uncalcified, like those of lizards, turtles and crocodilians.

Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.

Of course these features are not visible in fossils, and the main characteristics from paleontologists' point of view are:^[40]

• a slender dentary bone in which the coronoid process is small or non-existent.
• the external opening of the ear lies at the posterior base of the jaw.
• the jugal bone is small or non-existent.
• a primitive pectoral girdle with strong ventral elements: coracoids, clavicles and interclavicle. Note: therian mammals have no interclavicle.^[45]
• sprawling or semi-sprawling forelimbs.

Multituberculates

Skull of the multituberculate Ptilodus

Multituberculates (named for the multiple tubercles on their "molars") are often called the "rodents of the Mesozoic", but this is an example of convergent evolution rather than meaning that they are closely related to the Rodentia. They existed for approximately 120 million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene.

Some authors have challenged the phylogeny represented by the cladogram above. They exclude the multituberculates from the mammalian crown group, holding that multituberculates are more distantly related to extant mammals than even the Morganucodontidae.^[46][47] Multituberculates are like undisputed crown mammals in that their jaw joints consist of only the dentary and squamosal bones-whereas the quadrate and articular bones are part of the middle ear; their teeth are differentiated, occlude, and have mammal-like cusps; they have a zygomatic arch; and the structure of the pelvis suggests that they gave birth to tiny helpless young, like modern marsupials.^[48] On the other hand, they differ from modern mammals:

• Their "molars" have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molars of uncontested early crown mammals.
• The chewing action differs in that undisputed crown mammals chew with a side-to-side grinding action, which means that the molars usually occlude on only one side at a time, while multituberculates' jaws were incapable of side-to-side movement—they chewed, rather, by dragging the lower teeth backwards against the upper ones as the jaw closed.
• The anterior (forward) part of the zygomatic arch mostly consists of the maxilla (upper jawbone) rather than the jugal, a small bone in a little slot in the maxillary process (extension).
• The squamosal does not form part of the braincase.
• The rostrum (snout) is unlike that of undisputed crown mammals; in fact it looks more like that of a pelycosaur, such as Dimetrodon. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the nasal the top, and the tall premaxilla at the front.

Theria

Therian form of crurotarsal ankle. Adapted with permission from Palaeos

Theria ("beasts"), is the clade originating with the last common ancestor of the Eutheria (including placentals) and Metatheria (including marsupials). Common features include:^[49]

• no interclavicle.^[45]
• coracoid bones non-existent or fused with the shoulder blades to form coracoid processes.
• a type of crurotarsal ankle joint in which: the main joint is between the tibia and astragalus; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach. (The other well-known type of crurotarsal ankle is seen in crocodilians and works differently — most of the bending at the ankle is between the calcaneum and astragalus).
• tribosphenic molars.^[41]

Metatheria

The living Metatheria are all marsupials (animals with pouches). A few fossil genera, such as the Mongolian late Cretaceous Asiatherium, may be marsupials or members of some other metatherian group(s).^[50][51]

The oldest known metatherian is Sinodelphys, found in 125M-year-old early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.^[52]

Didelphimorphia (common opossums of the Western Hemisphere) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-arboreal unspecialized omnivores.^[53]

The best-known feature of marsupials is their method of reproduction:

• The mother develops a kind of yolk sack in her womb that delivers nutrients to the embryo. Embryos of bandicoots, koalas and wombats additionally form placenta-like organs that connect them to the uterine wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.^[54]
• Pregnancy is very short, typically four to five weeks. The embryo is born at a very early stage of development, and is usually less than 2 in (5.1 cm) long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune system will attack the embryo.
• The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a nipple, which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her mammary glands, as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple was historically thought to have restricted metatherian evolution, as it was assumed that the forelimb couldn't become specialised into structures like wings, hooves or flippers. However, several bandicoots, most notably the Pig-footed bandicoot, have true hooves similar to those of placental ungulates, and several marsupial gliders have evolved.

Skull of thylacine, showing marsupial pattern of molars

Although some marsupials look very like some placentals (the thylacine or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals:^[55]

• Some, including the thylacine, have four molars; whereas no known placental has more than three.
• All have a pair of palatal fenestrae, window-like openings on the bottom of the skull (in addition to the smaller nostril openings).

Marsupials also have a pair of marsupial bones (sometimes called "epipubic bones"), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.^[56][57] Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.^[58]

Eutheria

The time of appearance of the earliest eutherians has been a matter of controversy. On one hand, recently discovered fossils of Juramaia have been dated to 160 million years ago and classified as eutherian.^[59] Fossils of Eomaia from 125 million years ago in the Early Cretaceous have also been classified as eutherian.^[60] A recent analysis of phenomic characters, however, classified Eomaia as pre-eutherian and reported that the earliest clearly eutherian specimens came from Maelestes, dated to 91 million years ago.^[61] That study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the Cretaceous–Paleogene boundary, about 66 million years ago.
Eomaia was found to have some features that are more like those of marsupials and earlier metatherians:

Fossil of Eomaia in the Hong Kong Science Museum.

• Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals — early mammaliaforms, non-placental eutherians, marsupials, and monotremes — as well as in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion.^[62] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.^[63]
• A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
• Five incisors in each side of the upper jaw. This number is typical of metatherians, and the maximum number in modern placentals is three, except for homodonts, such as the armadillo. But Eomaia's molar to premolar ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials.

Eomaia also has a Meckelian groove, a primitive feature of the lower jaw that is not found in modern placental mammals.

These intermediate features are consistent with molecular phylogenetics estimates that the placentals diversified about 110M years ago, 15M years after the date of the Eomaia fossil.

Eomaia also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.

Placentals' best-known feature is their method of reproduction:

• The embryo attaches itself to the uterus via a large placenta via which the mother supplies food and oxygen and removes waste products.
• Pregnancy is relatively long and the young are fairly well-developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.

It has been suggested that the evolution of placental reproduction was made possible by retroviruses that:^[64]

• make the interface between the placenta and uterus into a syncytium, i.e. a thin layer of cells with a shared external membrane. This allows the passage of oxygen, nutrients and waste products, but prevents the passage of blood and other cells that would cause the mother's immune system to attack the fetus.
• reduce the aggressiveness of the mother's immune system, which is good for the foetus but makes the mother more vulnerable to infections.

From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth,^[65] ankles and feet.^[66]

Expansion of ecological niches in the Mesozoic

There is still some truth in the "small, nocturnal insectivores" stereotype, but recent finds, mainly in China, show that some mammaliaforms and crown group mammals were larger and had a variety of lifestyles. For example:

• Castorocauda, a member of Docodonta which lived in the middle Jurassic about 164 million years, was about 42.5 cm (16.7 in) long, weighed 500–800 g (18–28 oz), had limbs that were adapted for swimming and digging and teeth adapted for eating fish.^[33]
• Multituberculates are allotherians that survived for over 125 million years (from mid Jurassic, about 160M years ago, to late Eocene, about 35M years ago) are often called the "rodents of the Mesozoic". As noted above, they may have given birth to tiny live neonates rather than laying eggs.

Repenomamus sometimes preyed on young dinosaurs

• Fruitafossor, from the late Jurassic period about 150 million years ago, was about the size of a chipmunk and its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably termites, as ants had not yet appeared).^[67]
• Volaticotherium, from the boundary the early Cretaceous about 125M years ago, is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like that of a modern flying squirrel. This also suggests it was active mainly during the day.^[68]
• Repenomamus, a eutriconodont from the early Cretaceous 130 million years ago, was a stocky, badger-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, one more than 1 m (39 in) long and weighing about 12–14 kg (26–31 lb), the other less than 0.5 m (20 in) long and weighing 4–6 kg (8.8–13.2 lb).^[69][70]

Evolution of major groups of living mammals

There are currently vigorous debates between traditional paleontologists and molecular phylogeneticists about how and when the modern groups of mammals diversified, especially the placentals. Generally, the traditional paelontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the Cretaceous) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genetics. These debates extend to the definition of and relationships between the major groups of placentals — the controversy about Afrotheria is a good example.

Fossil-based family tree of placental mammals

Here is a very simplified version of a typical family tree based on fossils, based on Cladogram of Mammalia - Palaeos. It tries to show the nearest thing there is at present to a consensus view, but some paleontologists have very different views, for example:^[71]

• The most common view is that placentals originated in the southern hemisphere, but some paleontologists argue that they first appeared in Laurasia (old supercontinent containing modern Asia, N. America and Europe).
• Paleontologists differ as to when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous. Molecular biologists argue for a much earlier origin, even suggesting appearance in the Middle Jurassic.^[72]
• Most paleontologists suggest that placentals should be divided into Xenarthra and the rest, but a few think these animals diverged later.

For the sake of brevity and simplicity, the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals — X marks extinct groups. The diagram also shows the following:

• the age of the oldest known fossils in many groups, since one of the major debates between traditional paleontologists and molecular phylogeneticists is about when various groups first became distinct.
• well-known modern members of most groups.

Eutheria

Xenarthra (late cretaceous) (armadillos, anteaters, sloths) Pholidota (late cretaceous) (pangolins) Epitheria (latest Cretaceous) (some extinct groups) X Insectivora (latest Cretaceous) (hedgehogs, shrews, moles, tenrecs) Anagalida Zalambdalestidae X (late Cretaceous) Macroscelidea (late Eocene) (elephant shrews) Anagaloidea X Glires (early Paleocene) Lagomorpha (Eocene) (rabbits, hares, pikas) Rodentia (late Paleocene) (mice & rats, squirrels, porcupines) Archonta Scandentia (mid Eocene) (tree shrews) Primatomorpha Plesiadapiformes X Primates (early Paleocene) (tarsiers, lemurs, monkeys, apes including humans) Dermoptera (late Eocene) (colugos) Chiroptera (late Paleocene) (bats) Carnivora (early Paleocene) (cats, dogs, bears, seals) Ungulatomorpha (late Cretaceous) Eparctocyona (late Cretaceous) (some extinct groups) X Arctostylopida X (late Paleocene) Mesonychia X (mid Paleocene) (predators / scavengers, but not closely related to modern carnivores) Cetartiodactyla Cetacea (early Eocene) (whales, dolphins, porpoises) Artiodactyla (early Eocene) (even-toed ungulates: pigs, hippos, camels, giraffes, cattle, deer) Altungulata Hilalia X Perissodactyla (late Paleocene) (odd-toed ungulates: horses, rhinos, tapirs) Tubulidentata (early Miocene) (aardvarks) Paenungulata ("not quite ungulates") Hyracoidea (early Eocene) (hyraxes) Sirenia (early Eocene) (manatees, dugongs) Proboscidea (early Eocene) (elephants)

This family tree contains some surprises and puzzles. For example:

• The closest living relatives of cetaceans (whales, dolphins, porpoises) are artiodactyls, hoofed animals, which are almost all pure herbivores.
• Bats are fairly close relatives of primates.
• The closest living relatives of elephants are the aquatic sirenians, while their next relatives are hyraxes, which look more like well-fed guinea pigs.
• There is little correspondence between the structure of the family (what was descended from what) and the dates of the earliest fossils of each group. For example the earliest fossils of perissodactyls (the living members of which are horses, rhinos and tapirs) date from the late Paleocene, but the earliest fossils of their "sister group", the Tubulidentata, date from the early Miocene, nearly 50M years later. Paleontologists are fairly confident about the family relationships, which are based on cladistic analyses, and believe that fossils of the ancestors of modern aardvarks have simply not been found yet.

Molecular phylogenetics based family tree of placental mammals

Molecular phylogenetics uses features of organisms' genes to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.

Molecular phylogeneticists have proposed a family tree that is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas about various details, but here is a typical family tree according to molecular phylogenetics:^[73][74] Note that the diagram shown here omits extinct groups, as one cannot extract DNA from fossils.

Eutheria

Atlantogenata ("born round the Atlantic ocean") Xenarthra (armadillos, anteaters, sloths) Afrotheria Afroinsectiphilia (golden moles, tenrecs, otter shrews) unnamed Macroscelidea (elephant shrews) Tubulidentata (aardvarks) Paenungulata ("not quite ungulates") Hyracoidea (hyraxes) Proboscidea (elephants) Sirenia (manatees, dugongs) Boreoeutheria ("northern true / placental mammals") Laurasiatheria Erinaceomorpha (hedgehogs, gymnures) Soricomorpha (moles, shrews, solenodons) Cetartiodactyla (camels and llamas, pigs and peccaries, ruminants, whales and hippos) Pegasoferae Pholidota (pangolins) Chiroptera (bats) Carnivora (cats, dogs, bears, seals) Perissodactyla (horses, rhinos, tapirs). Euarchontoglires Glires Lagomorpha (rabbits, hares, pikas) Rodentia (late Paleocene)(mice & rats, squirrels, porcupines) Euarchonta Scandentia (tree shrews) Dermoptera (colugos) Primates (tarsiers, lemurs, monkeys, apes including humans)

Here are the most significant of the many differences between this family tree and the one familiar to paleontologists:

• The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. However, analysis of transposable element insertions supports a three-way top-level split between Xenarthra, Afrotheria and Boreoeutheria ^[75][76] and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.^[77]
• Afrotheria contains several groups that are only distantly related according to the paleontologists' version: Afroinsectiphilia ("African insectivores"), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria that paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
• Insectivores are split into three groups: one is part of Afrotheria and the other two are distinct sub-groups within Boreoeutheria.
• Bats are closer to Carnivora and odd-toed ungulates than to Primates and Dermoptera (colugos).
• Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).

The grouping together of the Afrotheria has some geological justification. All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the Indian elephant, which diverged from an African lineage about 7.6 million years ago.^[78] As Pangaea broke up, Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago.^[79][80] So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated into all the available ecological niches.

Nevertheless these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:^[81]

• Mitochondrial DNA's mutation rate in mammals varies from region to region — some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species.^[82][83]
• Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.^[84]

Timing of placental evolution

Recent molecular phylogenetic studies suggest that most placental orders diverged late in the Cretaceous period, about 100 to 85 million years ago, but that modern families first appeared later, in the late Eocene and early Miocene epochs of the Cenozoic period.^[85] Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic.^[86] Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock runs fast during major evolutionary radiations.^[87] On the other hand, there is fossil evidence from 85 million years ago of hoofed mammals that may be ancestors of modern ungulates.^[88]

Fossils of the earliest members of most modern groups date from the Paleocene, a few date from later and very few from the Cretaceous, before the extinction of the dinosaurs. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolate backwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous.^[89] However, statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene.^[90][91]

Evolution of mammalian features

Jaws and middle ears

Hadrocodium, whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the dentary and squamosal bones while the articular and quadrate move to the middle ear, where they are known as the incus and malleus.

One analysis of the monotreme Teinolophos suggested that this animal had a pre-mammalian jaw joint formed by the angular and quadrate bones and that the definitive mammalian middle ear evolved twice independently, in monotremes and in therian mammals, but this idea has been disputed.^[92] In fact, two of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that Teinolophos was a full-fledged platypus, which means it would have had a mammalian jaw joint and middle ear.^[43]

Lactation

It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):^[93][94][95]

• While the amniote egg is usually described as able to evolve away from water, most reptile eggs actually need moisture if they are not to dry out.
• Monotremes do not have nipples, but secrete milk from a hairy patch on their bellies.
• During incubation, monotreme eggs are covered in a sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards, a fourth layer appears with a composition different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
• If so, that may explain why the patches from which monotremes secrete milk are hairy. It is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.

Later research demonstrated that caseins already appeared in the common mammalian ancestor approximately 200–310 million years ago.^[96] The question of whether secretion of a substance to keep egg moist translated into actual lactation in therapsids is open. A small mammaliomorph called Sinocodon, generally assumed to be the sister group of all later mammals, had front teeth in even the smallest individuals. Combined with a poorly ossified jaw, they very probably did not suckle.^[97] Thus suckling may have evolved right at the pre-mammal/mammal transition.

Hair and fur

The first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic.^[33] As both extant mammals and Castorocauda have a double coat of hair, with both guard hairs and an undercoat, it may be assumed that their last common ancestor did as well. This animal must have been Triassic as it was an ancestor of the Triassic Tikitherium.^[29]

In the mid-1950s, some scientists interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels that supplied blood vessels and nerves to vibrissae (whiskers) and suggested that this was evidence of hair or fur.^[98][99] It was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae; the modern lizard Tupinambis has foramina that are almost identical to those found in the non-mammalian cynodont Thrinaxodon.^[12][100] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.^[101] A trace fossil from the Lower Triassic had been erroneously regarded as a cynodont footprint showing hair,^[102] but this interpretation has been refuted.^[103] Fur may have evolved from whiskers.^[104] Whiskers themselves may have evolved as a response to nocturnal and/or burrowing lifestyle.

Ruben & Jones (2000) note that the Harderian glands, which secrete lipids for coating the fur, were present in the earliest mammals like Morganucodon, but were absent in near-mammalian therapsids like Thrinaxodon.^[105]

Insulation is the "cheapest" way to maintain a fairly constant body temperature, without consuming energy to produce more body heat. Therefore, the possession of hair or fur would be good evidence of homeothermy, but would not be such strong evidence of a high metabolic rate.^{[106] [107]}

Erect limbs

Understanding of the evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. Some scientists think that the parasagittal (non-sprawling) limb posture is limited to the Boreosphenida, a group that contains the therians but not, for example, the multituberculates. In particular, they attribute a parasagittal stance to the therians Sinodelphys and Eomaia, which means that the stance had arisen by 125 million years ago, in the Early Cretaceous.^[108]

Warm-bloodedness

"Warm-bloodedness" is a complex and rather ambiguous term, because it includes some or all of the following:

• Endothermy, the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
• Homeothermy, maintaining a fairly constant body temperature. Most enzymes have an optimum operating temperature; efficiency drops rapidly outside the preferred range. A homeothermic organism needs only to possess enzymes that function well in a small range of temperatures.
• Tachymetabolism, maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature because of the Q₁₀ effect: biochemical processes run about half as fast if an animal's temperature drops by 10 °C.

Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism.

Modern monotremes have a low body temperature compared to marsupials and placental mammals, around 32 °C (90 °F).^[109] Phylogenetic bracketing suggests that the body temperatures of early crown-group mammals were not less than that of extant monotremes. There is cytological evidence that the low metabolism of monotremes is a secondarily evolved trait.^[110]

Respiratory turbinates

Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with mucous membranes that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in advanced Triassic cynodonts, such as Thrinaxodon and Diademodon, which suggests that they may have had fairly high metabolic rates. ^{[98] [111][112]}

Bony secondary palate

Mammals have a secondary bony palate, which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates.^{[98][99][113]} But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not.^[12]

Diaphragm

A muscular diaphragm helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates.^[98][99] On the other hand, these mammal-like rib cages may have evolved to increase agility.^[12] However, the movement of even advanced therapsids was "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaurs.^[114] So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.

Limb posture

The therapsids had sprawling forelimbs and semi-erect hindlimbs.^[99][115] This suggests that Carrier's constraint would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs.^[116] Advanced therapsids may therefore have been significantly less active than modern mammals of similar size and so may have had slower metabolisms overall or else been bradymetabolic (lower metabolism when at rest).

Brain

Mammals are noted for their large brain size relative to body size, compared to other animal groups. Recent findings suggest that the first brain area to expand was that involved in smell.^[117] Scientists scanned the skulls of early mammal species dating back to 190-200 million years ago and compared the brain case shapes to earlier pre-mammal species; they found that the brain area involved in the sense of smell was the first to enlarge.^[117] This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.^[117]

Evolution of birds

From Wikipedia, the free encyclopedia

Archaeopteryx at Paläontologisches Museum München

The evolution of birds is thought to have begun in the Jurassic Period, with the earliest birds derived from a clade of theropoda dinosaurs named Paraves. Birds are categorized as a biological class, Aves. The earliest known is Archaeopteryx lithographica, from the Late Jurassic period, though Archaeopteryx is not commonly considered to have been a true bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria.

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx,^[1] or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods).^[2] If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still under debate.

Origins

The mounted skeleton of a Velociraptor, showing the very bird-like quality of the smaller theropod dinosaurs

There is significant evidence that birds emerged within theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.^[3] As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. This was noted already in the 19th century, with Thomas Huxley writing:

We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.^[4]

Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the microraptorian dromaeosaurid Sinornithosaurus. This has contributed to this ambiguity of where to draw the line between birds and reptiles.^[5]  
Cryptovolans, a dromaeosaurid found in 2002 (which may be a junior synonym of Microraptor) was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes. Cryptovolans seems to make a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of feathered dromaeosaurs. At any rate, it is fairly certain that flight utilizing feathered wings existed in the mid-Jurassic theropods. The Cretaceous unenlagiine Rahonavis also possesses features suggesting it was at least partially capable of powered flight.

Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs if the dinosaurian origin theory is correct. They thus arrived at their hip structure condition independently. In fact, a bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.

An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama.^[6] This theory is contested by most other paleontologists and experts in feather development and evolution.^[7]

Mesozoic birds

Reconstruction of Iberomesornis romerali, a toothed enantiornithe

The basal bird Archaeopteryx, from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century. Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the very first bird might have looked. It may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings.^[4] While Archaeopteryx and its relatives may not have been very good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing.

The evolutionary trend among birds has been the reduction of anatomical elements to save weight. The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers. Confuciusornis is an example of their trend. While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds. A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms. The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches.^[8]

The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight. Another improvement was the appearance of an alula, used to achieve better control of landing or flight at low speeds. They also had a more derived pygostyle, with a ploughshare-shaped end. An early example is Yanornis. Many were coastal birds, strikingly resembling modern shorebirds, like Ichthyornis, or ducks, like Gansus. Some evolved as swimming hunters, like the Hesperornithiformes – a group of flightless divers resembling grebes and loons. While modern in most respects, most of these birds retained typical reptilian-like teeth and sharp claws on the manus.

The modern toothless birds evolved from the toothed forefathers in the Cretaceous.^[9] While the earlier primitive birds, particularly the Enantiornithes, continued to thrive and diversify alongside the pterosaurs, all but a few groups of the toothless Neornithes were cut short at the Chicxulub impact. The surviving lineages of birds were the comparatively primitive Paleognathae (ostrich and its allies), the aquatic duck lineage, the terrestrial fowl, and the highly volant Neoaves.

Adaptive radiation of modern birds

Haast's eagle and New Zealand moa, the eagle is a Neognath, the moas are Paleognaths.

Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the paleognaths and neognaths.

The paleognaths include the tinamous (found only in Central and South America) and the ratites, which nowadays are found almost exclusively on the Southern Hemisphere. The ratites are large flightless birds, and include ostriches, rheas, cassowaries, kiwis and emus. A few scientists propose that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages.^[10] In any case, the available data regarding their evolution is still very confusing, partly because there are no uncontroversial fossils from the Mesozoic.

The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (chickens, turkeys, pheasants, and their allies). The presence of basal anseriform fossils in the Mesozoic and likely some galliform fossils implies the presence of paleognaths at the same time, in spite of the absence of fossil evidence.

The dates for the splits are a matter of considerable debate amongst scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanserae and the other neognaths - the Neoaves - occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining neognaths occurred before or after the extinction of the other dinosaurs.^[11] This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation, a small and equivocal neoavian fossil record from Cretaceous, and most living families turning up during the Paleogene. Attempts made to reconcile the molecular and fossil evidence have proved controversial.^[11][12]

On the other hand, two factors must be considered: First, molecular clocks cannot be considered reliable in the absence of robust fossil calibration, whereas the fossil record is naturally incomplete. Second, in reconstructed phylogenetic trees, the time and pattern of lineage separation corresponds to the evolution of the characters (such as DNA sequences, morphological traits etc.) studied, not to the actual evolutionary pattern of the lineages; these ideally should not differ by much, but may well do so in practice.

Considering this, it is easy to see that fossil data, compared to molecular data, tends to be more accurate in general, but also to underestimate divergence times: morphological traits, being the product of entire developmental genetics networks, usually only start to diverge some time after a lineage split would become apparent in DNA sequence comparison - especially if the sequences used contain many silent mutations.

Classification of modern species

The diversity of modern birds

The phylogenetic classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised).
A preponderance of evidence suggests that most modern bird orders constitute good clades. However, scientists are not in agreement as to the precise relationships between the main clades. Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. As of the mid-2000s, new fossil and molecular data provide an increasingly clear picture of the evolution of modern bird orders, and their relationships. For example, the Charadriiformes seem to constitute an ancient and distinct lineage, while the Mirandornithes and Cypselomorphae are supported by a wealth of anatomical and molecular evidence.
The understanding of the interrelationships of lower level taxa also continues to increase, particularly in the massively diverse perching bird group Passeriformes.

Bird classification and phylogenetic analysis is still under debate and requires more research. A 2008 study published in Science examined DNA sequences from 169 species of birds that represented all of the major extant groups. The findings may necessitate a wholesale restructuring of the avian phylogenetic tree. The findings also supported unestablished relationships between orders and confirmed disputes over particular groupings.^[13]

Current evolutionary trends in birds

Evolution generally occurs at a scale far too slow to be witnessed by humans. However, bird species are currently going extinct at a far greater rate than any possible speciation or other generation of new species. The disappearance of a population, subspecies, or species represents the permanent loss of a range of genes.
Another concern with evolutionary implications is a suspected increase in hybridization. This may arise from human alteration of habitats enabling related allopatric species to overlap. Forest fragmentation can create extensive open areas, connecting previously isolated patches of open habitat. Populations that were isolated for sufficient time to diverge significantly, but not sufficient to be incapable of producing fertile offspring may now be interbreeding so broadly that the integrity of the original species may be compromised. For example, the many hybrid hummingbirds found in northwest South America may represent a threat to the conservation of the distinct species involved.^[14]

Several species of birds have been bred in captivity to create variations on wild species. In some birds this is limited to color variations, while others are bred for larger egg or meat production, for flightlessness or other characteristics.

Message from the Big Bang --"Confirms Quantum Origin of the Universe"

Data collected by Planck telescope have confirmed beyond any logical uncertainty a theory of the quantum origin of structure in the Cosmos. What precisely occurred after the Universe was born? Why did stars, planets and gigantic galaxies appear? These are the problems that cover Viatcheslav Mukhanov, a cosmologist at Ludwig-Maximilians-Universitaet (LMU) in Munich is a professional in the Theoretical Cosmology. He has used the idea of so-called quantum variations to make a theory that offers a exact picture of the vital early stage of the evolution of our Universe: Without the slight variations in energy density that result from the minute but inevitable quantum fluctuations, one cannot account for the creation of stars, planets and galaxies that illustrate the Universe we witness today. The Planck Consortium has now issued new survey of data resumed by the Planck Space Telescope that has measured the dispersal of the cosmic microwave background radiation (CMB), which, in principle, defines what the Universe looked like about 400,000 years after the Big Bang. These up-to-date results are in complete agreement with the forecasts of Mukhanov's theory - for instance, his calculation of the rate of the so-called spectral catalog of the initial inhomogeneities.

 

Image credit: Wikigag.com

The notion that quantum fluctuations must have played a part in the very initial stage of the history of the Cosmos is implied in Heisenberg's Uncertainty Principle, according to Mukhanov. 

Heisenberg presented that there is an exact boundary to the accuracy with which the position and the momentum of a particle can be resolute at any given moment. This in turn suggests that the early matter scattering will unavoidably show tiny inhomogeneities in density. Mukhanov's calculations first validated that such quantum fluctuations could give upsurge to density alterations in the early Universe, which in turn could assist as seeds for the galaxies and their clusters. Certainly, without quantum fluctuations, whose nature and magnitude Mukhanov quantitatively described, the detected dispersal of matter in the Universe would be bizarre.

In March 2014, a group of researchers informed the detection of the long-sought pattern. Though, doubts soon arose concerning this interpretation. Now a combined study by the Planck and BICEP2 teams has determined that the data do not truly provide observational proof for gravitational waves. In the spring of 2014 Mukhanov had already stated that, if the theory is right, then the BICEP2 and Planck teams could not both be correct.

Anti-intellectualism Is Killing America

Post published by David Niose on Jun 23, 2015 in Our Humanity, Naturally

 

Original link:  https://www.psychologytoday.com/https://www.psychologytoday.com/blog/our-humanity-naturally/201506/anti-intellectualism-is-killing-americablog/our-humanity-naturally/201506/anti-intellectualism-is-killing-america

Source: dylann roof facebook

The tragedy in Charleston last week will no doubt lead to more discussion of several important and recurring issues in American culture—particularly racism and gun violence—but these dialogues are unlikely to bear much fruit until the nation undertakes a serious self-examination. Decrying racism and gun violence is fine, but for too long America’s social dysfunction has continued to intensify as the nation has ignored a key underlying pathology: anti-intellectualism.

America is killing itself through its embrace and exaltation of ignorance, and the evidence is all around us. Dylann Roof, the Charleston shooter who used race as a basis for hate and mass murder, is just the latest horrific example. Many will correctly blame Roof's actions on America's culture of racism and gun violence, but it's time to realize that such phenomena are directly tied to the nation's culture of ignorance.

In a country where a sitting congressman told a crowd that evolution and the Big Bang are “lies straight from the pit of hell,” (link is external) where the chairman of a Senate environmental panel brought a snowball (link is external) into the chamber as evidence that climate change is a hoax, where almost one in three citizens can’t name the vice president (link is external), it is beyond dispute that critical thinking has been abandoned as a cultural value. Our failure as a society to connect the dots, to see that such anti-intellectualism comes with a huge price, could eventually be our downfall.

In considering the senseless loss of nine lives in Charleston, of course racism jumps out as the main issue. But isn’t ignorance at the root of racism? And it’s true that the bloodshed is a reflection of America's violent, gun-crazed culture, but it is only our aversion to reason as a society that has allowed violence to define the culture. Rational public policy, including policies that allow reasonable restraints on gun access, simply isn't possible without an informed, engaged, and rationally thinking public.

Some will point out, correctly, that even educated people can still be racists, but this shouldn’t remove the spotlight from anti-intellectualism. Yes, even intelligent and educated individuals, often due to cultural and institutional influences, can sometimes carry racist biases. But critically thinking individuals recognize racism as wrong and undesirable, even if they aren’t yet able to eliminate every morsel of bias from their own psyches or from social institutions. An anti-intellectual society, however, will have large swaths of people who are motivated by fear, susceptible to tribalism and simplistic explanations, incapable of emotional maturity, and prone to violent solutions. Sound familiar?

And even though it may seem counter-intuitive, anti-intellectualism has little to do with intelligence. We know little about the raw intellectual abilities of Dylann Roof, but we do know that he is an ignorant racist who willfully allowed irrational hatred of an entire demographic to dictate his actions. Whatever his IQ, to some extent he is a product of a culture driven by fear and emotion, not rational thinking, and his actions reflect the paranoid mentality of one who fails to grasp basic notions of what it means to be human.

What Americans rarely acknowledge is that many of their social problems are rooted in the rejection of critical thinking or, conversely, the glorification of the emotional and irrational. What else could explain the hyper-patriotism (link is external) that has many accepting an outlandish notion that America is far superior to the rest of the world? Love of one’s country is fine, but many Americans seem to honestly believe that their country both invented and perfected the idea of freedom, that the quality of life here far surpasses everywhere else in the world.

But it doesn’t. International quality of life rankings (link is external) place America far from the top, at sixteenth. America’s rates of murder (link is external) and other violent crime dwarf most of the rest of the developed world, as does its incarceration rate (link is external), while its rates of education and scientific literacy are embarrassingly low (link is external). American schools, claiming to uphold “traditional values,” avoid fact-based sex education, and thus we have the highest rates of teen pregnancy (link is external) in the industrialized world. And those rates are notably highest where so-called “biblical values” are prominent. Go outside the Bible belt, and the rates generally trend downward (link is external).

As this suggests, the impact of fundamentalist religion in driving American anti-intellectualism has been, and continues to be, immense. Old-fashioned notions of sex education may seem like a relatively minor issue to many, but taking old-time religion too seriously can be extremely dangerous in the modern era. High-ranking individuals, even in the military (link is external), see a confrontation between good and evil as biblically predicted and therefore inevitable. They relish the thought of being a righteous part of the final days.

Fundamentalist religion is also a major force in denying human-caused climate change (link is external), a phenomenon that the scientific community has accepted for years. Interestingly, anti-intellectual fundamentalists are joined in their climate change denial with unusual bedfellows: corporate interests (link is external) that stand to gain from the rejection of sound science on climate.

Corporate influence on climate and environmental policy, meanwhile, is simply more evidence of anti-intellectualism in action, for corporate domination of American society is another result of a public that is not thinking critically. Americans have allowed their democracy to slip away, their culture overtaken by enormous corporations that effectively control both the governmental apparatus and the media, thus shaping life around materialism and consumption.

Indeed, these corporate interests encourage anti-intellectualism, conditioning Americans into conformity and passive acceptance of institutional dominance. They are the ones who stand to gain from the excessive fear and nationalism that result in militaristic foreign policy and absurdly high levels of military spending (link is external). They are the ones who stand to gain from consumers who spend money they don’t have on goods and services they don’t need. They are the ones who want a public that is largely uninformed and distracted, thus allowing government policy to be crafted by corporate lawyers and lobbyists. They are the ones who stand to gain from unregulated securities markets. And they are the ones who stand to gain from a prison-industrial complex that generates the highest rates of incarceration in the developed world.

Americans can and should denounce the racist and gun-crazed culture that shamefully resulted in nine corpses in Charleston this week, but they also need to dig deeper. At the core of all of this dysfunction is an abandonment of reason.

Published measurements of climate sensitivity declining

 

Original link: https://landshape.wordpress.com/2015/06/20/6921/

The climate sensitivity due to CO₂ is expressed as the temperature change in °C associated with a doubling of the concentration of carbon dioxide in Earth’s atmosphere. The equilibrium climate sensitivity (ECS) refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric carbon dioxide concentration.  The transient climate response (TCR) is defined as the average temperature response over a twenty-year period centered at CO₂ doubling in a transient simulation with CO₂ increasing at 1% per year. The transient response is lower than the equilibrium sensitivity, due to the “inertia” of ocean heat uptake.

Scientists made numerous estimates of climate sensitivity over the last few decades and have yet to determine the correct value.  The figure shows the change in published climate sensitivity measurements over the past 15 years (from here).  The ECS and TCR estimates have both declined in the last 15 years, with the ECS declining from 6C to less than 2C.  While one cannot extrapolate from past results, it is likely that the true figure is below 2C, and may continue to decline.  Based on this historic pattern we should reject the studies that falsely exaggerated the climate sensitivity in the past and remember that global warming is not the most serious issue facing the world today.