The Mammals Emerge

THE MAMMALS

ABOUT THE MORNING OF 26 DECEMBER; ABOUT 985,000 METERS UP THE LINE

We are now about five days from the end of the year. In the aftermath of the terrible events of the Great Dying, a new crop of biological winners has assumed its place in the world. Despite the appalling loss of species in the extinction, no phylum was completely wiped out, which was significant, and the unconscious contest for dominance of the biosphere has taken unexpected turns. The synapsids have radiated across much of Pangea, competing for habitats with the reptiles. The long line of non-mammalian synapsids has advanced through many evolutionary stages and is now on the threshold of producing the first true mammals. Those first mammals will be unimpressive animals, thoroughly inferior in strength and size to the massive archosaurians that are evolving. But they will show a tenacity and resilience that belies their seeming fragility. In the huge transformations to come, it is they who will emerge as the victors.

Credible Definitions of the Term Mammal

T. S. Kemp has pointed out that the definition of the term mammal is a somewhat arbitrary one, for at what point in its evolutionary history did the lineage of non-mammalian synapsids acquire a suite of traits sufficient for it to be labeled mammalian?  Further, Kenneth Rose has observed that much of what we consider mammalian in modern animals of the class cannot be inferred from the fossil record, except in the rarest cases. These traits would include external hair (in all but a few types), milk production by means of mammary glands in those females which have given birth, a four-chambered heart that facilitates the reoxygenation of the blood by channeling part of the blood flow to the lungs, the presence of a diaphragm to assist in breathing, endothermy (the ability to maintain a constant internal body temperature) and along with endothermy a relatively high metabolic rate, well-developed sensory apparatuses,  and brains generally larger and more complex than those of other vertebrates.¹ There are, however, numerous features of the bony anatomy of fossilized animals that can identify them as mammals. A distinct kind of lower jaw (all one bone), heterodont dentition (meaning simply that there are different types of teeth present), the ossicles (small bones) of the inner ear, five distinct sections of the vertebral column, a distinct kind of pelvic girdle, a distinct kind of nasal structure, and many other features.² Complications arise, however, due to the fact that certain features of these kinds apparently evolved independently several times, making the job of tracing lines of descent more difficult.

In tracing the sequence of non-mammalian lines of animals that led to mammals, we would say, in general, that the order of their emergence was the basal synapsids, which are generally but not universally known as the pelycosaurs, from them the therapsids, and from them the cynodonts.³ It is the general opinion of paleontologists that from the cynodonts the first true mammals evolved.

The Therapsids

The therapsids were at one time one of the most numerous of all land animals. Many different genera and species of them existed, and some of them grew to considerable size, particularly a kind known as dinocephalians. The therapsids appear to have been very widespread by the Late Permian period (about 260-250 million ybp). In general the larger versions of them possessed powerful jaws, prominent canine teeth, greater mobility and thicker skulls than pelycosaurs, and lived mainly in middle or higher latitudes rather than the tropics. They may have possessed some means of regulating their internal temperature, but in all probability it was not as advanced as those of the mammals that evolved from them.⁴ A great many varieties of therapsid evolved in the Late Permian. The dominant type in that period was a kind of therapsid known as a dicynodont. Some 90% of the therapsid specimens we have collected from the Late Permian are of this type. Dicynodonts were noteworthy because they were the first truly widespread herbivores in the Earth’s history. They had short snouts, somewhat box-like skulls, and probably possessed beak-like structures with which to shear vegetation.⁵ The fossil evidence also indicates the dicynodonts appear to have possessed a secondary palate, that part of the upper mouth’s structure that connects the mouth and the nasal passages. This allows an animal to chew food and breathe simultaneously.⁶

Near the end of the Permian, about 250 million ybp, almost around the time of the mass extinction, a new variation evolved: the cynodonts—the line from which the true mammals emerged. Although the therapsids were ravaged by the great dying, enough of them survived to keep the therapsid lineage going.

The Cynodonts

The earliest examples of cynodonts have been found in South Africa, although another early variety has been found as far away as Russia, showing the tremendous range of these animals. Varieties of cynodont radiated extensively in the Middle Triassic Period, and the evidence shows that some cynodonts were chiefly carnivorous, some primarily herbivorous, and some omnivorous.⁷ The cynodonts, over the course of their evolutionary history, increasingly accumulated traits that we consider mammalian. Among these traits were an enlargement of the teeth, the completion of a bony secondary palate, the evolution of complex, occluding back teeth, changes in the connection of the jaw to the bony structures of the side of the head, changes in the structure of the shoulder blade, and the development of more mammal-like toes. Studies of cynodont skulls indicate that these animals appear to have had well-developed cerebellums, a possible indicator of advanced neuromuscular control mechanisms, and well-developed senses of smell and hearing as well.⁸ Several genera of a variety of cynodont known as a tritylodont have been unearthed in China. [By now it should be apparent from the persistent use of the stem -dont that many of these animals are classified in large part according to their teeth.] The Chinese samples appear to be from the Early Jurassic Period (about 200-175 mya) and reveal animals that were small (and in some cases very small) with distinct and diverse kinds of dentition. They have also been found in proximity to the remains of animals believed to be among the earliest true mammals.⁹ (See below). Tritylodonts did not necessarily give rise to mammals, although it is possible a population branched off from the main tritylodont lineage and did so. There were many genera of cynodonts that existed in the Late Triassic Period, and it is not yet known which of them gave rise to Class Mammalia. But the similarity between the size and features of tritylodonts and those of the earliest mammals is suggestive.

In fact, a striking attribute of the therapsids in general was their reduction in size over time. The last surviving cynodonts were no bigger than squirrels, and, as we will see below, the first genuine mammals to branch off from the therapsid line were no bigger than mice. Some paleontologists have hypothesized that the selection pressure for small size may have been the presence of aggressive, predatory dinosaurs. The ability to hide from such threats would have been of paramount importance.¹⁰ Small size was probably coupled with another cynodont innovation: the ability to burrow into the ground. Evidence from South Africa indicates that a cynodont known as Thrinaxodon liorhinus had this capacity at 251 million ybp. Thrinaxodon’s ability to burrow may have been of crucial importance in the era when the mass Permian-Triassic extinction was sweeping the globe. It is of great interest, by the way, to note that at least 50% of all existing mammal species are burrowing animals. It was the ability of certain cynodonts to effectively hide themselves that may have allowed the mammalian lineage to evolve in the first place.¹¹ It would appear therefore, that mammals and their immediate forebears learned a very important strategy very early on: when in danger, dig in.

The First True Mammals

The definition of a mammal is, as we noted above, a somewhat arbitrary one.  The matter hinges on the issue of which animals deserve to be designated as the first “true” mammals. There are animals known as mammaliaforms, those animals thought to have first displayed crucial mammalian characteristics (such as the later cynodonts), and then there are those animals considered to be the earliest genuine mammals. There are also people who consider only the eutherians, those animals that are generally (but somewhat misleadingly) labeled “placental” mammals, to be the original members of the class. I will adopt a more inclusive definition of the term mammal, with the understanding that the border between mammaliaforms and mammals is somewhat unclear. 

As we have just seen, Class Mammalia can be traced at least as far back as the early Jurassic Period, almost 200 million ybp, and there is some evidence that primitive mammals may have been present in the late Triassic, from around 225 million to 200 million ybp. Despite misconceptions on the part of some people, who think mammals appeared only after the dinosaurs had died out, the two groups shared a long history. If we assume mammals of some type existed at 200 million ybp, just to be careful in our estimates, this means that mammals and dinosaurs shared the Earth for about 135 million years. This is a period more than 50 times longer than the length of time the genus Homo is thought to have existed. In broader geological terms, we would say that the mammals evolved in the Mesozoic Era and were in the shadow of the great reptiles until its end. The period from 65 mya to the present is known as the Cenozoic Era—the Age of Mammals (about which I will have more to say in the next chapter).

The oldest animal that might have been a mammal is, according to Rose, Adelobasileus, of the Late Triassic Period and dated to 225 million ybp. Its mammalian characteristics include the arrangement of some of the cranial foramina [the openings in the skull that serve as pathways for the cranial nerves], the shape of the occipital condyles [structures at the back and lower sections of the skull that interact with the uppermost vertebra to allow the turning of the head], and certain structures in the inner ear. Unfortunately, no teeth or lower jaw of this animal have been found, so its status in relationship to other animals is still uncertain.¹²

Another of the first identifiably mammalian groups was the genus Morganucodon. The remains of one of its species, M. watsoni, have been found largely in Wales. Fragments of the skull, teeth (either individual teeth or partial sets) and parts of the postcranial [other than the skull] anatomy have been uncovered in great numbers.¹³ Two other species of the Morganucodon genus have been found in China, both of which confirm the anatomical information gleaned from the Welsh finds, and the Chinese specimens are relatively complete and well-preserved. The members of this genus were about the size of mice.¹⁴ Morganucodon samples have been uncovered in a variety of widely separated regions, which tells us that this genus must have radiated throughout Pangea.¹⁵ A more primitive, and yet more recent mammalian genus known as Sinoconodon has also been unearthed in China. The specimens of Sinoconodon have dentition which is less mammal-like in character than other finds, although the features of the skull and lower jaw are definitely mammalian in character.¹⁶

In 2001 yet another very small mammal-like animal found in China was announced. Hadrocodium wui appears to have lived at the boundary between mammaliaform and mammal. Dated from about 195 million ybp, it displays a number of derived characteristics. (Derived in this context means a relatively novel trait that is passed on to the animal’s descendants.) Features that scientists at one time believed to have evolved much later in mammalian history are present in H. wui.  In particular the features of the mandible and its related structures (especially the temporomandibular joint), the size of the cranial vault, and the structure of the middle ear are highly derived. How diminutive was H. wui? The length of its skull was 12 millimeters and its estimated body mass was a mere 2 grams.¹⁷

Geographical Distribution of Mesozoic Mammals

Although the Mesozoic Era lasted far longer than the Cenozoic has thus far, the fossil record of Mesozoic mammals is, in the words of one paleontologist, “frustratingly sparse”.  There are significant gaps, both chronological and geographical, in that record, and such gaps can have an influence on how the general evolution of mammals is evaluated.¹⁸ Given this understanding, in what locales have we uncovered Mesozoic mammals?

In regard to the Late Triassic and Early Jurassic Periods, numerous finds have been made. In Europe, specimens (largely teeth or parts of them) have been unearthed in Germany, Switzerland, France, Luxembourg, Belgium, and the United Kingdom. There are some 20 locales in the UK that have yielded specimens, and in Wales, jaws and other parts of the anatomy have been discovered as well as teeth. In Asia, Yunnan Province’s Lufeng Basin, in southern China, has been a treasure trove of vertebrate fossils, including several mammals or mammaliaforms we have already described. At least 10 areas in China, two in Japan, and several locations in India have yielded fossils. African specimens have come from regions as diverse as Morocco, Lesotho, South Africa, Libya, Madagascar, and Cameroon. In South America, disappointingly, only trackways have been discovered. Greenland has yielded some excellent, and particularly old sets of mammalian remains. In North America, the United States (Texas, Arizona, Alaska), and Mexico have yielded discoveries. Unfortunately, far fewer remains have been discovered from the Middle Jurassic Period, the most poorly documented in mammalian history (when Pangea was still relatively intact). Important finds have been made in western Europe, China, Madagascar, and South America, however. ¹⁹

In the Late Jurassic, Pangea began to split up (which we will discuss below). Some lines of animals began diverging noticeably because of this. Late Jurassic mammals have been found in North America, western Europe, Mongolia, China, and Africa. Some of the most significant Late Jurassic finds have come from Portugal, where a variety of (now extinct) mammal known as multituberculates (rodent-like animals) has been uncovered in large numbers. Finally, specimens from the Cretaceous Period have been unearthed in Iberia, Uzbekistan, Russia, and Mongolia (in addition to locales in China, the UK, and other regions where older mammal fossils have been uncovered.) And for the first time we see Australia represented in the finds.²⁰ It is therefore safe to say that any class of animal that left tracks in South America, dug itself into southern Africa, made a life in Mongolia, and wandered through both prehistoric Wales and Arizona was remarkably successful at the game of adaptive radiation, even given the fact that most of the Earth’s land was in one large supercontinent for most of the time of that radiation.

The Evolution of the Eutherians

From a reproductive standpoint, mammals fall into three different categories, or subclasses. The smallest subclass, and probably the most ancient, is known as Prototheria, the egg-laying mammals, which are now comparatively rare, represented by just a few species. Some 99.9% of all mammals fall into the two largest categories. One of these subclasses is known as Metatheria, the marsupials. The other subclass, Eutheria—the somewhat misleadingly labeled “placental” mammals—contains the vast majority of all living mammals. (I say somewhat misleading because the females of some marsupial species do develop a placenta during gestation; it just doesn’t last very long.) Since humans are part of Eutheria, we are naturally interested in its origins. Those origins are the product of some rather revolutionary developments.

The great advantage eutherian mammals have over other animals is the relatively long period in which embryos are sheltered within the mother’s body. The dangers of leaving one’s offspring in nests of eggs are avoided, naturally, as are the disadvantages inherent in giving birth to offspring which are in an extremely immature stage of development, as is the case with the marsupials. It was the evolution of genuine pregnancy that made the rise of the eutherians—and by extension, us—possible.

Among the evolutionary changes that differentiate mammals from reptiles is the presence, in metatherian and eutherian  females, of an endometrium that lines the uterine wall in preparation for the implantation and nurturing of a fertilized egg. (In prototherians, the endometrium serves a different function.) In the uteruses of metatherian and eutherian females there are endometrial stromal cells (ESCs) that respond to signals sent by progesterone and other chemicals. The endometrial cells, upon being chemically signaled to prepare to receive a fertilized egg for implantation, undergo decidualization, which means they begin to form a membrane in preparation for implantation. (The layers of this membrane are called decidua.)  Placental mammals have a different kind of ESC. Researchers have found that the origin of this difference may have been brought about by a change in the network of genes that controls the expression of the endometrium. About 1,500 genes that are expressed only in placental mammals (even though they also exist in marsupials), are implicated in this change. These genes apparently were originally recruited by transposons—pieces of DNA that can plant themselves at different points in a cell’s genome. Transposons are often called “junk DNA” and often function in a parasitic way.

A particular transposable element, MER20, seems to control about 13% of the recruited genes. Scientists examining this phenomenon now believe that MER20 brought about a new regulatory network, one that allowed for the rise of extended pregnancy. (See below.) MER20, in other words, was an evolutionary blockbuster. A group of transposons in effect took over a collection of genes and allowed them to function, in this case possibly by shielding them from proteins which normally act to repress gene expression.²¹ Evolution often acts through the gradual accumulation of small changes, and sometimes it acts through rapid changes. This an example of the latter. In a particular kind of mammal, no more would the metatherian pattern of giving birth to extremely immature offspring be followed. An implanted egg would now have an environment in which it could develop at length within the body of a female. A layer of endometrial cells known as the decidua basalis is the point at which a fertilized egg attaches to the uterus. It is the decidua basalis that comprises the base layer of the only human organ that has a fixed time limit for existence: the placenta. It was the evolution of the placenta that helped revolutionize the process of gestation.

The placenta serves as the connection between the mother and the embryo, a lifeline through which nutrients, gases, and waste products pass. Research on the placenta’s evolution indicates that when a placenta is in its early stages of development it is chiefly using ancient genes, ones mammals have in common with other classes, that have been repurposed. As the placenta grows, more recently evolved genes are called into service to construct it, and it appears that through the duplication and divergence of these genes different kinds of placentas can develop, ones in line with the reproductive strategies of the animals in which they are found.²² Research from other scientists has addressed the question of why the placenta is not attacked by the body’s immune system as if it were an invader. These researchers found that a change in a single protein involved in transcription, HoxA-11 [coded by the HoxA-11 gene], keeps the immune system in placental mammals from destroying the placenta. Pregnancy in placental mammals depends on the expression of the hormone prolactin and the prolactin receptor (PRLR). Prolactin has not been detected in the oviducts of fish, amphibians, lizards, or snakes, even though PRLR has been. Further, prolactin has not been detected in the marsupials tested, nor is it in the oviduct of chickens. It is present, however, in the placental tissue of African elephants, and researchers conclude therefore that prolactin expression in the placental uterus is a derived feature. Prolactin represses genes that can negatively affect a pregnancy. It would appear, therefore that prolactin allows for a long gestational period without triggering the body’s immune response. MER20 [which we noted above] is the transposable element that drives prolactin expression in eutherian endometrial stromal cells. (On the basis of this research, done some years ago, these scientists estimated that MER20 must have come into action between 166 and 155 mya, which, as we will see, was a pretty good estimate.) MER20’s influence coincides with the evolution of a change in the eutherian HoxA-11 regulatory gene that took place some time after the placental-marsupial divergence. All evidence points to the fact that such an evolutionary change did not take place in marsupials.²³

So from the repurposing of ancient genes, ones mammals have in common with birds and reptiles, to the harnessing of genes for new purposes by pieces of “junk” DNA, and a change in a single regulatory protein, it became possible to build a structure within the mammalian uterus to nurture a fertilized egg for an extended period of time, one that allowed a far greater degree of embryonic maturation. From these changes, which are not yet fully understood and need much further study, came the placental mammals. From the placental mammals came the primates. And from the primates came the only being on this planet capable of tracing this story’s course.

The earliest eutherian that has been discovered comes from the Jurassic Period of China. The find has been named Juramaia sinensis—“Jurassic mother from China”—and has been dated at approximately 160 million ybp.  The find itself is unusually complete, and includes the full set of teeth (which is crucial), an incomplete but still significant skull, virtually the entire anterior [front] section of the postcranial skeleton, and some soft tissues, including hairs. The teeth and the jaw are definitely eutherian, very much distinct from those of metatherians. Its weight is estimated to have been 15-17 grams. Based on its teeth, it is thought to have been an insectivore. Juramaia is considered to be a basal eutherian, one of the very earliest. As the authors of the article in which Juramaia was announced put it, “Juramaia is more closely related to extant placentals than all metatherians of the Cretaceous…” [The Cretaceous Period began at about 145 million ybp.] Juramaia has provided solid evidence to back up molecular dating estimates for the divergence between placentals and marsupials. Up until its discovery, there was a gap between the oldest fossil of a placental mammal that had been discovered (from about 125 million ybp) and the estimate of when the basal eutherian must have existed based on a calculation of the rate of molecular change in mammalian DNA. Quoting the authors once again, “Because Juramaia is unambiguously placed on the placental side of the marsupial-placental divergence, the marsupial-placental divergence must have occurred before Juramaia.” Juramaia also gives us evidence that other mammalian clades of the Jurassic may have been derived as well.²⁴

To me, what is most interesting about this find comes from an analysis of its shoulder girdle and its forelimb features. The structure of the scapula includes a hypertrophied acromion, the part of the shoulder that connects to the collarbone. The forelimbs are also distinct in structure. Not only are these features definitely eutherian, lacking traits present in metatherians, they suggest something else. In conjunction with the structure of the manual phalanges—the fingers—they suggest that Juramaia had a capacity most mammals didn’t—the ability to climb trees.  Such a capacity is known to paleontologists and zoologists as a scansorial adaptation. Juramaia may have preferred arboreal environments. As the study’s authors say,

The earliest known eutherians, Juramaia and Eomaia [found at 125 million ybp]  and the earliest metatherians Sinodelphys are scansorial mammals, and differ from contemporary Mesozoic mammals, most of which are terrestrial. This suggests that the phylogenetic split of eutherians and metatherians and their earliest evolution are accompanied by major ecomorphological diversification, notably scansorial adaptation, which made it possible for therians to exploit arboreal niches.”²⁵

To say the least, the evolution of an ability to climb trees was of the greatest significance in the telling of our story. It is the preference for such environments by a certain kind of mammal that we will examine in some detail in the next chapter.

The Breakup of Pangea and Its Impact

Pangea, the largest supercontinent in the Earth’s geological history, reached its greatest extent somewhere around 250 mya. It was surrounded by the largest ocean that has ever existed, Panthalassa. On the eastern shore of Pangea was a smaller section of Panthalassa, a body of water known to geologists as Tethys. But as is the case with all things in this world, Pangea was subject to a constant process of change brought on by the actions of blind physical forces. The landmasses of the Earth, so imposing in size to the small creatures that inhabit them, are, as we have seen, simply pieces of the Earth’s relatively thin crust. They “float”, so to speak, on the hot, flexible mantle that exists beneath them. The plates on which the continents and oceans rest are apparently deeply affected by heat convection from the mantle, as well as other factors not completely understood. The full mechanism that explains how and why they move has not yet been uncovered. But where, over the last few hundred million years, they have moved, is now fairly well known. In the case of Pangea, regions of Gondwanaland continued to break off and drift northward toward Laurasia during the Mesozoic Era. Tethys began to change shape significantly around 180 mya, as bodies of water that had begun to appear in the middle of Pangea started pushing sections of the great supercontinent farther apart. These bodies of water were to become the Atlantic and Indian Oceans, the full formation of which would take more than 120 million years in each case. Huge rifts in the floors of these oceans today give testimony to the stages by which the oceans were constructed. The huge mass of territory that was to become Eurasia began its long turn from a north-south to an east-west  orientation, and what was to be Europe moved away from the Equator. Panthalassa was reduced in size, but remains a considerable body of water today. It is better known as the Pacific Ocean.²⁶

Pangea’s breakup resulted in the separation of Australia and Antarctica, which occurred over the course of 50 million years from 150 to 100 mya. And there were collisions of landmasses other than those associated with Pangea, or collisions between land that had been part of the supercontinent and land that had not been. China and Mongolia began plowing into each other about 150 mya. India and Asia collided about 55 mya.²⁷ The split between South America and Africa was complete by about 105 mya and also by that time Antarctica was firmly astride the South Pole and what would later be Madagascar had split from the African mainland. By 75 mya a map of the world that seems vaguely familiar to us had formed, although major changes were still taking place.²⁸ Only in geological terms would we ever see these (from the human standpoint)  incredibly slow-motion events as “rapid”. Only in the perspective of our condensed calendar could we see such events as only having taken “days” in which to occur. Interestingly, the breakup of Pangea can be said to be continuing right to this very day.

Pangea’s fragmentation had huge consequences for the Earth. Research has shown that the breakup had a profound effect on the Earth’s climate. Climate is affected by many factors, of course. Among them are the levels of silicate weathering on the planet’s land surfaces. Over 90% of the rocks on the Earth’s surface are silicates. When these rocks (of various types) weather, they absorb atmospheric carbon dioxide. Much of this CO² is then carried to the oceans via water runoff, where it is stored as carbonates. Volcanic and highly tectonically active islands contribute a disproportionate share of this runoff material.²⁹ This process acts as a gigantic CO² regulator. Studies now indicate that the breakup of Pangea triggered a major increase in the amount of runoff, which increased the amount of silicate weathering (since this process operates in the manner of a feedback loop), which increased the absorption of CO². This decrease in atmospheric carbon dioxide caused a reduction of the Earth’s mean temperature from about 20 degrees Celsius (about 68 degrees F.) in the Middle-Late Triassic Period to about 10 degrees C. (about 50 degrees F.)  in the late Cretaceous. This was a transformation from a world that was hot and relatively arid in the Triassic to one that was much cooler and wetter by the late Cretaceous.³⁰ Further, the fragmentation of Pangea’s landmass and the subsequent collisions of pieces that had broken off resulted in major episodes of mountain building, such as the formation of the Himalayas and the Andes that helped usher in the climatic patterns we see in the world today. And many landmasses that had been in subtropical regions during the Permian Period moved northward. By the Cretaceous, these land masses were the scene of much cooler summers and colder winters.³¹

The impact on the world’s land animals was also significant. The fragmentation of such a huge mass of land, the shifting of so much territory from one range of latitudes to another, the changes in the absorption of sunlight brought about by these shifts in position, and the associated changes in climate, all led to a new diversification of terrestrial life forms. As patterns of migration were slowly but inexorably changed, and as certain populations became increasingly genetically isolated from one another, new opportunities for speciation arose (although not just from isolation—the actions of speciating genes may also have been at work.) This took place with a slowness that to our eyes seems inconceivable. But the breakup of the supercontinent led to the emergence of countless new habitats, and struggles for life between life forms whose evolutionary adaptations were literally a life-or-death matter. No category of animals was more deeply affected by these changes than the great archosaurians that ruled so much of the Triassic, Jurassic, and Cretaceous worlds—the dinosaurs.

The Reign of the Archosaurians

Dinosaurs lasted an enormously long time, from about 250 million ybp to about 65 million ybp, and produced a tremendous array of different animals over that period. Dinosaurs went through major evolutionary changes and became adapted to a surprising array of different environments. The most famous varieties of dinosaur, the ones that usually end up in popular fiction, did not all live at the same time, and were in fact often separated from each other by tens of millions of years. Dinosaurs are now thought to have formed two large groups, or clades. One, the Saurischia, included the fearsome Tyrannosaur. Saurischia was in turn divided into two groups, one of which, Theropoda, ultimately produced the birds.³² (See below) The other, the Ornithischia,  is perhaps best known for including within its ranks the Stegosaurus and Triceratops. Ornithischia is itself divided into numerous subclades.³³ It has been ascertained that the dinosaurs were monophyletic (again, evolved from a single source), and the largest number of basal dinosaur remains have been found in South America [which leads many paleontologists to surmise that this is where dinosaurs first evolved]. The evidence points to the fact that the saurischians evolved fairly rapidly and radiated quickly as well. One particular Triassic saurischian has been found not only in South America but in North America, southern and northern Africa, and India. It would seem that saurischians were distributed all over the western regions of Pangea.³⁴ The Triassic dinosaurs were largely carnivores, such as Eoraptor and Herrerasaurus, upright, bipedal, and perhaps relatively intelligent animals.³⁵

The splitting up of Pangea helped stimulate the diversification of the various lines of dinosaurs. Herbivorous dinosaurs in this changing world took advantage of the spread of plant life. Gymnosperms vastly increased their range in the Triassic Period, and continued to be enormously widespread in the following periods. Evidence shows they were a major food source for the herbivores. Many paleontologists hypothesize that dinosaurs and plants had a co-evolutionary effect on each other, and perhaps the evolution of the angiosperms [the flowering plants; see p. 264], which was approximately coincident with the radiation of several major groups of dinosaurs, was an example of this. Some argue that the enormous quantities of low-lying vegetation dinosaurs ate would have been a massive selection pressure in favor of plants that could disseminate seeds and grow offspring rapidly, unlike the comparatively slow-growing conifers. Evidence for this conjecture has been hard to come by. But it is suggestive that in the Cretaceous Period the herbivorous dinosaurs and the angiosperms were both proliferating rapidly.³⁶

As we saw above, the theropods are now thought to have given rise to the birds. In fact, many scientists now refer to the extinct varieties of dinosaurs as the non-avian dinosaurs, the implication being that the modern Class Aves consists of highly evolved, greatly derived dinosaurs. The evidence is so overwhelming that dinosaurs gave rise to birds that no counter-argument to this can hope to stand. The widely-known and equally-widely misunderstood Archaeopteryx was a dinosaur—a feathered dinosaur—and not a bird. Anatomically, Archaeopteryx shared a host of features with the theropods. It had a bony tail, teeth, dinosaurian vertebrae, dinosaurian pelvis, dinosaurian claws, and many others. And the contention that birds evolved from dinosaurs does not rest on Archaeopteryx alone. The Liaoning fossil beds in China have yielded astounding specimens—Sinosauropteryx, Caudipterix, and many others, which were all feathered dinosaurs, demonstrating that feathers were a common feature for many theropods. Additionally, paleontologists have uncovered dozens of fossil specimens that illustrate the transition from dinosaur to bird, such as Ichthyornis, which displayed both dinosaurian and avian traits. There can be doubt: birds have their origin in the archosaurians of the Mesozoic Era.³⁷

The Cretaceous-Paleogene Extinction

The Cretaceous–Paleogene (K/Pg) Extinction, or as it is often still referred to, the Cretaceous-Tertiary (K/T) Extinction, was the most recent of the large extinction events that have periodically swept away large numbers of the Earth’s life forms. There is a division of opinion among paleontologists as to whether the non-avian dinosaurs were in decline prior to their extinction. There is less controversy over the fate of the non-avian dinosaurs after the events of approximately 65.5 million ybp. Although it has been an issue of intense debate, it now appears reasonably certain that an extraterrestrial object, an asteroid about 10 kilometers (a little over 6 miles) in diameter, struck Mexico’s Yucatan Peninsula at that time. Known as the Chicxulub asteroid impact, it left a huge crater,  between 180 and 200 kilometers in diameter. The massive release of gases from this event very probably had a catastrophic impact on the world’s climate. Significantly, a major and prolonged episode of volcanism in the landmass that ultimately became India was occurring in the same period (the Deccan flood basalt volcanism), an episode that lasted, off an on, for a million years. There are scientists in fact who argue that it was these eruptions that triggered the climate change. But many paleontologists, based on a thorough examination of the evidence, now believe that the Chicxulub event triggered the extinction. They have examined the geological deposits from that era, focusing on what is known as the K/Pg boundary. More than 350 sites where this boundary can be seen have been found. At these sites, there is an unusually high amount of iridium and related elements in the deposits found at the boundary itself. Further, the thickness of the deposits in the regions surrounding Chicxulub is very great, and deposits at sites away from the blast area show a decrease in thickness in relation to their distance from Mexico. Moreover, the evidence indicates these deposits were laid down quickly. All of these data are consistent with an asteroid strike of gigantic proportions.³⁸

The scientists studying the impact event believe that it generated earthquakes in excess of magnitude 11, huge tsunamis, and caused massive ejections of material, some of it traveling upward so fast that it was actually blasted into space. The impact is estimated to have immediately released 100 to 500 billion tons of sulfur as well. The sulfur in the atmosphere probably turned into an aerosol form that absorbed sunlight and caused an atmospheric temperature plunge of as much as 10 degrees C., perhaps for decades. Acid rain from the huge sulfur cloud also very likely inflicted severe damage on marine life. The amount of sulfur ejected may have exceeded what the Indian eruptions put out in 1 million years—and it did so within minutes of the strike. The non-avian dinosaurs, both marine and flying reptiles, and a large number of marine animals in general, were ultimately wiped out.  Marine losses were driven not just by acidification, but by the destruction of phytoplankton in the ocean, which had catastrophic effects on the food chain. Among the plants, many forest communities suffered major losses as well.³⁹

There is by no means universal agreement among scientists that the Chicxulub event was the decisive element in the loss of the dinosauria. If the dinosaurs were already in decline prior to the strike, it may have been the factors causing this decline along with the effects of the impact that caused them to disappear. The dinosaurs may have been weakened by disease as well. It is also quite possible that relict populations—survivors living in isolated areas—continued for many tens of thousands of years after the main dying had occurred. But one thing is clear—large dinosaur fossils have been recovered right up to the end of the Cretaceous—and then, suddenly (in geological terms) there are no more.

The disaster that struck the huge reptiles 65 million years ago was a golden opportunity for the mammals, who had been confined to relatively limited ecological niches by the dominant dinosauria. But these mammals were still small in size, even after 140 million or so years of evolutionary development. As Kemp has put it:

For two-thirds of the whole of their history, mammals remained small animals with the largest being barely larger than a cat, and the vast majority of the size of living shrews, mice, and rats. With hindsight, the most important evolutionary event was the origin of the modern mammalian kind of molar tooth, known as the tribosphenic tooth, and with it the roots of the two major modern taxa, marsupials and placentals.⁴⁰

To state the obvious, a great number of very considerably sized animals eventually evolved out of the various mammalian lineages. One particular kind of mammal began, in large numbers, to exploit a new territory. This mammal was adapted for climbing, and its earliest members were hardly bigger than average-sized rodents. But in making the unconscious decision to make a life in the forests of the warmer regions of the world, they were setting the stage for the ultimate emergence of the animal that would come to dominate all others. It is now time to turn our attention to that tree-dwelling order of mammals.

It is time for us to examine the emergence of the primates.

1.   Rose, The Beginning of the Age of Mammals p. 41

2.   Rose, pp. 41-42

3.   Rose, p. 44

4.   Cowen, Richard, History of Life, pp. 133-134

5.   Cowen, pp. 134-136

6.   Cowen, p. 214

7.   Chinsamy-Turan, Anusuya, Forerunners of Mammals: Radiation - Histology – Biology, pp. 223-225

8.   Chinsamy-Turan, pp. 228-229

9.   Lucas, Spencer G., Chinese Fossil Vertebrates, pp. 133-135

10. Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski, Mammalogy, Fifth Edition, p. 53

11. Ross Damiani, Sean Modesto, Adam Yates, and Johann Neveling,  “Earliest evidence of cynodont burrowing” in Proceedings of the Royal Society, 19 June 2003

12. Rose, pp. 50-51

13. Kielan-Jaworowska, Zofia, Richard Cifelli, and Zhe-Xi Luo, Mammals from the age of dinosaurs: origins, evolution, and structure, pp. 174-176

14. Lucas, Spencer G, Chinese Fossil Vertebrates, p. 152

15. Kielan-Jaworowska, et al, p. 175

16. Lucas, p. 152

17. Zhe-Xi Luo, Alfred W. Crompton and Ai-Lin Sun, “A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics”, in Science, 25 May 2001

18. Kielan-Jaworowska, et al, p. 19

19. Kielan-Jaworowska, et al, pp. 19-64

20. Kielan-Jaworowska, et al, pp. 19-64

21. Vincent J. Lynch, Robert D Leclerc, Gemma May and Günter P Wagner, “Transposon-mediated rewiring of gene regulatory networks contributed to the evolution of pregnancy in mammals” in Nature Genetics, 25 September 2011; “Invasion of Genomic Parasites Triggered Modern Mammalian Pregnancy, Study Finds” in Science Daily, 25 September 2011; Schubert, Charlotte, “Transposable Element May Have Jump-Started Pregnancy” in Biology of Reproduction, October 5, 2011

22. Kirstin Knox and Julie C. Baker, “Genomic evolution of the placenta using co-option and duplication

and divergence” in Genome Research, 13 March 2008

23. Vincent J. Lynch, Andrea Tanzer, Yajun Wang, Frederick C. Leung, Birgit Gellersen , Deena Emera, and Gunter P. Wagner, “Adaptive changes in the transcription factor HoxA-11 are essential for the evolution of pregnancy in mammals” in PNAS, September 30, 2008

24.  Luo, Zhe-Xi, Chong-Xi Yuan, Qing-Jin Meng, and Qiang Ji, “A Jurassic eutherian mammal and divergence of marsupials and placentals” in Nature, Volume 476, August 24, 2011

25.  Luo, et al.

26.  Rogers, John J. W., and M. Santosh, Continents and Supercontinents, pp. 131-135

27.  Condie, Kent C. The Earth as an Evolving Planetary System, p. 319

28.  Christopher R. Fielding, Tracy D. Frank, and John L. Isbell, Resolving the Late Paleozoic Ice Age in Time and Space, pp. 20-26

29.  Sigurdur R. Gislason, and Eric H. Oelkers, “The geochemistry of silicate rock weathering” from The International Geological Congress, 2008

30.  Y. Donnadieu, Y. Goddéris, R. Pierrehumbert, G. Dromart, F. Fluteau, R. Jacob, “A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea breakup” in Geochemistry Geophysics Geosystems, Vol. 7, 2006

31.  Peter J. Fawcett, and Eric J. Barron, “The Role of Geography and Atmospheric CO² in Long Term Climate Change: Results from Model Simulations for the Late Permian to the Present” in Tectonic Boundary Conditions for Climate Reconstructions, edited by Thomas J. Crowley, Kevin Burke, pp. 21-27

32.  Weishampel, David B., and Osmólska, Halszka, The Dinosauria, pp. 21-22

33.  Weishampel and Osmólska, pp. 323-324

34.  Weishampel and Osmólska, pp, 40-46

35.  Parsons, Keith M., The Great Dinosaur Controversy: A Guide to the Debates, p. 148

36.  Fastovsky, David E., and Weishampel, David B., Dinosaurs: A Concise Natural History, pp. 283-284; pp. 286-288

37.  Prothero, pp. 257-268

38.  Peter Schulte, Laia Alegret, Ignacio Arenillas, José A. Arz, Penny J. Barton, Paul R. Bown, Timothy J. Bralower, Gail L. Christeson, Philippe Claeys, Charles S. Cockell, Gareth S. Collins, Alexander Deutsch, Tamara J. Goldin, Kazuhisa Goto, José M. Grajales-Nishimura, Richard A. F. Grieve, Sean P. S. Gulick, Kirk R. Johnson, Wolfgang Kiessling, Christian Koeberl, David A. Kring, Kenneth G. MacLeod, Takafumi Matsui, Jay Melosh, Alessandro Montanari, Joanna V. Morgan, Clive R. Neal, Douglas J. Nichols, Richard D. Norris, Elisabetta Pierazzo, Greg Ravizza, Mario Rebolledo-Vieyra, Wolf Uwe Reimold, Eric Robin, Tobias Salge, Robert P. Speijer, Arthur R. Sweet, Jaime Urrutia-Fucugauchi, Vivi Vajda, Michael T. Whalen and Pi S. Willumsen, “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary” in Science, 5 March 2010, Vol. 327

39.  Schulte, et al

40.  Kemp. p. 3

Lillegraven, Jason A., Zofia Kielan-Jaworowska, and William A. Clemens, editors, Mesozoic Mammals: The First Two-Thirds of Mammalian History was used as a general reference.