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

(simplified from;^[10] only those that are most relevant to the evolution of mammals are described below)

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]

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".^[28]

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."^[29] 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,^[30] 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

(based on Cynodontia:Dendrogram - Palaeos)

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.^[31] 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:^[32]

• 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: ^[33]

• 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

Cladogram after.^[28] († marks extinct groups)

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

Color vision

Early amniotes had four opsins in the cones of their retinas to use for distinguishing colors: one sensitive to red, one to green, and two corresponding to different shades of blue.^[34][35] 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.^[34] 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.^[36] Some placentals and marsupials, including humans, subsequently evolved green-sensitive opsins; like early crown mammals, therefore, their vision is trichromatic.^[37][38]

Australosphenida and Ausktribosphenidae

Ausktribosphenidae is a group name that has been given to some rather puzzling finds that:^[39]

• appear to have tribosphenic molars, a type of tooth that is otherwise known only in placentals and marsupials.^[40]
• 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).^[41]

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).^[42]

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.^[42] 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.^[43]

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:^[39]

• 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.^[44]
• 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.^[45][46] 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.^[47] 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:^[48]

• no interclavicle.^[44]
• 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.^[40]

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).^[49][50]

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.^[51]

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.^[52]

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.^[53]
• 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 has prevented the forelimbs from evolving into paddles or wings and has therefore prevented the appearance of aquatic or truly flying marsupials (although there are several marsupial gliders).

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:^[54]

• 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.^[55][56] Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.^[57]

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.^[58] Fossils of Eomaia from 125 million years ago in the Early Cretaceous have also been classified as eutherian.^[59] 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.^[60] 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.^[61] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.^[62]
• 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:^[63]

• 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,^[64] ankles and feet.^[65]

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.^[32]
• 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).^[66]
• 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.^[67]
• 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).^[68][69]

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:^[70]

• 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.^[71]
• 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 vegetarians.
• 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:^[72][73] 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 ^[74][75] and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.^[76]
• 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.^[77] 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.^[78][79] 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:^[80]

• 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.^[81][82]
• 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.^[83]

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.^[84] Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic.^[85] 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.^[86] On the other hand, there is fossil evidence from 85 million years ago of hoofed mammals that may be ancestors of modern ungulates.^[87]

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.^[88] 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.^[89][90]

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.^[91] 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.^[42]

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):^[92][93][94]

• 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.^[95] 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.^[96] 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.^[32] 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.^[28]
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.^[97][98] 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][99] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.^[100] A trace fossil from the Lower Triassic had been erroneously regarded as a cynodont footprint showing hair,^[101] but this interpretation has been refuted.^[102]

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.^[103]

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.^{[104] [105]}

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.^[106]

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).^[107] 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.^[108]

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. ^{[97] [109][110]}

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.^{[97][98][111]} 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.^[97][98] 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.^[112] 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.^[98][113] 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.^[114] 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.^[115] 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.^[115] This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.^[115]

19-year study of trillions of meals shows GE crops do not harm food-producing animals, humans

Jon Entine | September 10, 2014 | Genetic Literacy Project

 

Original link:  http://www.geneticliteracyproject.org/2014/09/10/19-year-study-of-trillions-of-meals-shows-ge-crops-do-not-harm-food-producing-animals-humans/#.VBBiY1s_akc.google_plusone_share

 

Although there have been more than two thousand studies documenting that GMOs do not pose an unusual threat to human health, questions about the safety of genetically modified foods remain in the minds of many consumers.

Gilles-Eric Séralini, in his retracted GMO corn study (later republished in a pay for play journal without peer review), claimed rats fed genetically engineered corn developed grotesque cancerous tumors—the kind no farmer would miss among his animals if the cause-effect was genuinely in place.

Anti-GMO crusader Jeffrey Smith, on his personal website, the Institute for Responsible Technology, lists more than a dozen cases in which he claims test animals fed GMOs exhibited abnormal conditions, including cancer and early death. He also references his own self-published book and anecdotal activist web site posts in claiming that pigs fed GM feed turned sterile or had false pregnancies and sheep that grazed on BT cotton plants often died.

“Nearly every independent animal feeding safety study on GM foods shows adverse or unexplained effects,” he writes. “But we were not supposed to know about these problems either—the biotech industry works overtime to try to hide them. Industry studies described above, for example, are neither peer-reviewed nor published.”

The American Academy of Environmental Medicine—an alternative medicine group that rejects GMOs and believes that vaccines are dangerous, and characterized as a “questionable organization” by Quack Watch and Science Based Medicine—claims, “Several animal studies indicate serious health risks associated with GM food,” including infertility, immune problems, accelerated aging, faulty insulin regulation, and changes in major organs and the gastrointestinal system.”

Leveraging these allegations, anti-GMO groups regularly post blogs alleging that animals fed GMOs have developed health problems that could show up in humans. “Monsanto’s GMO Feed Creates Horrific Physical Ailments in Animals,” screamed a typical headline, in AlterNet, a popular fringe alternative site. It touted “new research” but as is typical of many such articles, in neither cited a study or linked to any independent research.

Is there any basis to these allegations? After all, globally, food-producing animals consume 70 percent to 90 percent of genetically engineered (GE) crop biomass, mostly corn and soybean. In the United States alone, animal agriculture produces over 9 billion food-producing animals annually, and more than 95 percent of these animals consume feed containing GE ingredients. The numbers are similar in large GMO producing countries with a large agricultural sector, such as Brazil and Argentina.

Estimates of the numbers of meals consumed by feed animals since the introduction of GM crops 18 years ago would be well into the trillions. By common sense alone, if GE feed were causing unusual problems among livestock, farmers would have noticed. Dead and sick animals would literally litter farms around the world. Yet there are no anecdotal reports of such mass health problems.

But we don’t need to depend on anecdotes to address these concerns. Writing in the Journal of Animal Science [NOTE: article behind paywall], in the largest study ever conducted, Alison Van Eenennaam and Amy E. Young, geneticists with the Department of Animal Science at the University of California-Davis, reviewed 29 years of livestock productivity and health data from both before and after the introduction of GE animal feed. The field data represented more than 100 billion animals.
What did they find?

There were no indications of any unusual trends in the health of animals since 1996 when GMO crops were first harvested. Considering the size of the dataset, it can reasonably be said that the debate over the impact of GE feed on animal health is closed: there is zero extraordinary impact.
The authors also address the implications of their study on human health.

No study has revealed any differences in the nutritional profile of animal products derived from GE-fed animals. Because DNA and protein are normal components of the diet that are digested, there are no detectable or reliably quantifiable traces of GE components in milk, meat, and eggs following consumption of GE feed.

The authors go on to warn about the fractured regulatory process in which countries eager to export “second generation” GE crops are likely to get approvals before import countries give their okay to receive the new varieties.

“GE crops with altered output traits for improved livestock feed [are] in the development and regulatory pipeline,” they write. “Additionally, advanced techniques to affect targeted genome modifications are emerging, and it is not clear whether these will be encompassed by the current GE process-based trigger for regulatory oversight.”

They argue for the harmonization of both regulatory frameworks for GE crops and governance of advanced breeding techniques to prevent widespread disruptions in international trade of livestock feedstuffs in the future.

Jon Entine, executive director of the Genetic Literacy Project, is a senior fellow at the World Food Center, Institute for Food and Agricultural Literacy, University of California-Davis. Follow @JonEntine on Twitter

Smoking Gun Evidence of an Ancient Earth: GPS Data Confirms Radiometric Dating

September 10, 2014 by Natural Historian

Original link: http://thenaturalhistorian.com/2014/09/10/smoking-gun-evidence-of-an-ancient-earth-gps-data-confirms-radiometric-dating/

My last post, on the tectonic origins of the Dead Sea and Jordan Valley, brought me face-to-face once again with one of the most striking pieces of evidence for an ancient earth that I am aware of. Take a look at this graph.

This graph shows the smoking gun.

Smoking gun evidence of what? That the earth’s plates have been moving slowly for millions of years.

Therefore the Earth is very, very old.

Radiometric/Geological estimates vs Geodetic/GPS estimates of continental plate motions. Figure from AlReheji et al. 2010.

This graph should cause creationists to lose sleep at night, wondering why God provided such contrary data to a young earth.  Some will grasp for an escape chute and claim that these data are the result of God’s creating with the appearance of age while others will cast doubts on the data themselves but at the end of the day, both of these claims are without theological or scientific merit.

This graph shows that the so-called historical sciences and observational sciences that Ken Ham touts can both reach the same conclusions.

The graph includes data that are far from novel. The data don’t reveal anything that hasn’t been common knowledge among geologists for a long time.  But this graph won’t make an appearance in your recent issue of Creation Magazine or be highlighted on the Answers in Genesis website.

Basic diagram of the crustal plates of the Earth and their general direction of motion. Plates are moving at different speeds with some moving away from each other and other running into each other. How the plates interact explains much about the origins of earthquakes and volcanoes.

So what is this a graph of?  

It compares two different estimates of how quickly the earth’s plates are moving in the Middle East. The first method of measurement (X axis) employs radiometric dating of rocks (that are millions of years old) to calculate the average rate of plate motion over that time.  The second method measures the rate of plate motion based on GPS readings of the precise locations of the plates (Y axis).  These GPS measurements were recorded over a 10 year period.  This allowed the rates of plate motions to be directly measured as well as an average estimated over that time period.

Plates that move a few millimeters in a few years?  Yes, initially this may seem a rather dull finding.  But these data are remarkable for one reason:  In almost all cases the estimates derived from these independent methods are in full agreement!

Why is this agreement significant and how does it prove that the world is ancient?

It is significant because it shows how predictions in science can provide powerful confirmation of a theory, when those predictions are confirmed by future observations.   The geological slip rates were determined by several methods, involving multiple research teams over the past 50 years. I could spend 5000 words describing exactly how these rates were calculated, but it makes no difference how this was done or that your understand the methods.   All that matters is that estimates of rates have been produced by multiple labs over decades.  These rates were calculated using conventional – old earth –  assumptions about radiometric dating. This includes producing dates for for 1 to 20 million years were obtained and used to back-calculate rates in millimeters per year movement.  These studies produced rates of plate motion that represent the speed of plates over millions of years in the past but were predicted to represent the present day motion of the plates.

When these millions of years rates were calculated, there were no instruments precise enough to measure the present-day motion of the plates.  But now we have lasers and global positioning satellites that can track precise movements of the earth’s crust down to hundredths of millimeters per day.  This allowed geologists to directly test the rates predicted by radiometric dating of millions of year old rocks.

The key results:  dates based on old rocks predict the same rates as GPS measurements of real-time plate motions.  What does this mean?
 
It means that current plate motions are the same, or nearly the same, as plate motions from 100, 10,000 or 1 million years ago.  The probability that these rate numbers, based on two independent methods of estimation, happen to be the same as a result of chance alone is so small that it makes winning the PowerBall look like a sure bet.  This is as confident as one can be that tectonic plates have been slowly moving for vast periods of time.  This is smoking gun evidence that these processes have been occurring continually for millions of years.

How does this fit a young earth model of the earth?

It doesn’t. The evidence against the young-earth model, based on this data alone, is very compelling.  Please think about this for a minute and let the significance set in.   Young earth creationists (YECs) have long railed against the validity of radiometric dating.  They have argued that the method is unreliable and can’t give accurate dates and always overestimate because of erroneous assumptions.

But in this case, YECs can make up any excuse they want for why they don’t believe those dating methods and why the estimates of rates based on them are flawed.  It doesn’t matter.   In fact, the more problems they say they find with radiometric dating methods, the worse their problem becomes.  This is because if the dating methods and rate estimates are bogus, then there should be no correlation with rate estimates based on real-time GPS measurements.

What young earth model would predict that rates based on “bogus” million-year old dates should yield the same rates measured by technology that we have in our cell phones?  None.  In fact most young earth models include some sort of accelerated plate motions in the past, with plate motions only slowing to their current rates in the past several thousand years.

So, according to current YEC hypotheses, estimates of past plate-movement rates should not equal rates measured in the present.  These data falsify the accelerated plate tectonics model of flood geology.

Radiometric/Geological estimates vs Geodetic/GPS estimates of continental plate motions. Labels indicate names of places where rates have been estimated using both methods.  Figure from AlRaheji et al. 2010.

Back to the graph, I want to highlight just one of the data points on this graph as an example.  I have been writing about the Dead Sea and the origin of the Jordan Valley (The Origins of the Dead Sea Part III: The Levant – a Land Literally Ripped Apart).  I mentioned there that the Dead Sea is formed by a rift fault between two plates and that the plates are sliding past each other at a bit over 4 mm/year.

On the graph, this is the SDSF (South Dead Sea Fault) point.  The vertical bar represents the variance possible (sort of a confidence interval) of the estimates based on geological estimates (radiometric mostly).  These standard deviation bars are wider than the vertical bars, which represent the possible range of error for the GPS estimates, because the latter is so much more precise.  Nonetheless, what you can see is that the GPS and geological estimates overlap for all the sites included in this study, including the Dead Sea fault.  If the Dead Sea fault had moved hundreds or thousands of feet in a year even just a few thousand years ago, as YECs might expect,  then the estimate based on GPS should not be close to the estimate based on movements averaged over long periods of time.  But they do match and so we know beyond reasonable doubt that the Arabian plate on the east side of the Dead Sea valley has been slowly moving north for millions of years.

Lets widen our view a bit.  Below (fig 2) is another example of the same type of data but this is for plate motions of plates from all over the earth.

Figure 2.  Rates of plate motions based on GPS and laser methods vs radiometric dating methods for all of the plates of the world.  Figure modified from Robbins et al. 1993.

Let’s take point A as an example of how to read this graph. Here a particular plate on Earth was estimated to be moving at about 90 mm/year in a particular direction, an estimate based on radiometric methods.  Many years later, laser methods (i.e., GPS technologies) were used to estimate the current rate of motion.  The fact that point A falls nearly on the line means that modern-day plate movement is nearly identical to the motions estimated to have occurred for millions of years.
Plates predicted to have fast motions away from each other in the past have the same rate of motion today. Plates running into each other at a particular speed, measured in negative values, are moving toward each other at the same rates today.

I risk being redundant, but this point is important to emphasize.  These values should NOT be the same if the worlds tectonic plates are only 4500 years old as predicted by flood geology.

Why would radiometric dating, which supposedly is a useless tool for estimating the true age of the earth by YEC reckoning, provide near-precise estimates of current plate motions, which are confirmed by a completely unrelated form of measurement?

For the YEC hypothesis, they shouldn’t.  And yet they do.

The only reasonable explanation for what we see here is that the radiometric dating methods provide faithful estimates of the real ages of the rocks. The GPS data are yet another independent confirmation of the validity of radiometric dating. The fact that radiometric based dates predicted rates that were confirmed later by another method serve to confirm the former method.

This is no conspiracy.  Forty years ago, scientists could not have faked the radiometric dating to derive estimates of plate motions that they knew we would find in the future.  How could anyone have known that one plate should move 120 mm/year and another one only 2 mm/year?  They simply calculated ages, did simple division, and derived a rate.  Because no one could have known the modern rate of plate motion, there was no way for any scientists to fudge numbers and bend dates to particular assumptions about rates, as YECs have long claimed.  The plate-movement rates are unbiased, and I think we can be reasonably sure that satellite measurements are unbiased recordings of the rates as well.  The fact that any of the dates, much less the majority of them, match one another is very strong confirmation of the constant motion of the plates over long periods of time.

Technology found in our cell phones has pounded a rather strong nail—among the thousands already there—in the young earth creationist’s coffin.

An Addendum:

Let me provide another example to further show this is not just some cherry-picked data.  Below (Fig. 3) is one additional example that I have used in one of my lectures. It shows the relationship between the distance of an Hawaiian Island or seamount from the active volcano.  By dividing the age of the island as determined by radiometric dating by the distance you can estimate the speed at which the pacific plate is moving toward the northwest.   This was done more than 40 years ago. In the past decade we have been able to measure the current rate of motion via GPS technology and it is very close to 8.6 cm/year.  This is an incredible coincidence if radiometric dating methods are unreliable or if there has been accelerated decay of nucleotides as some YECs have claimed.   Rather than a coincidence these similar estimates are a powerful testimony to the accuracy plate tectonic models and of the accuracy of radiometric dating techniques.


References:

Figure 1:  ArRajehi, A., McClusky, S., Reilinger, R., Daoud, M., Alchalbi, A., Ergintav, S., … & Kogan, L. (2010). Geodetic constraints on present‐day motion of the Arabian Plate: Implications for Red Sea and Gulf of Aden rifting. Tectonics,29(3).

Figure 2:  Two sets of plate motion rates compared.  The vertical (left) axis shows rates averaged over a few to a dozen years of increasing (positive) or decreasing (negative) distance (some continents are being pushed toward each other and thus the negative rate) on 149 lines between 20 space geodetic sites (very long baseline inferometry or satellite laser ranging).  The bottom axis shows rates predicted from a global plate motion model called NUVEL-1 which averages motion over about 3 million year time periods.  These modeled time estimates were determined using the ages of geomagnetic reversals from the timescale of Harland et al. 1982  A geological time scale. Cambridge University Press.  Estimates of plate motion in the past are also determined by radiometric dating of island chains such as Hawaii seen in Figure 9.  The line represents where data points should be if estimates of past and present motions were identical.   Large deviations from that line would signify very different rates in the past and the present.  Figure modified from: Robbins, J. W., D. E. Smith, and C. Ma, Horizontal crustal deformation and large scale plate motions inferred from space geodetic techniques, in Contributions of Space Geodesy to Geodynamics: Crustal Dynamics, edited by D. Smith and D. Turcotte, pp. 21-36, AGU, Washington D.C., 1993.

Figure 3: Clague, D. A. and G. B. Dalrymple. 1987.  The Hawaiian-Emperor volcanic chain Part I:  Geologic evolution.  U.S. Geological Survey Professional Paper 1350:5-54.