THE ANCESTORS OF MAMMALS

In the Permian and Triassic Periods lived the

therapsids and the ictidosaurs, a curious group

of reptiles with many manlmalian characteristics

IT IS difficult to see much in common

between a modern reptile, such as a crocodile, and a modern mammal,

such as a dog. Anatomically and physio­logically they seem to be about as far apart as possible for four-footed animals. But if we go back in geologic time we find a close connection between some of the early mammals and certain reptiles. Improbable as it may seem, the fossil record shows that the earliest mammals were descended from reptilian ancestors.

It was somewhat more than a century ago that the bones of mammal-like rep­tiles were first discovered in South Africa by Andrew Geddes Bain, a well­known fossil collector of the period. Bain's specimens were noted and de­scribed by the great English anatomist and paleontologist, Sir Richard Owen. The significance of Bain's findings was overlooked for decades.

Charles Darwin's disciple Thomas Huxley suggested that the mammals had arisen from amphibians, a conclu­sion to which he was led by his studies in comparative anatomy. But in the years between about 1870 and 1884 Owen and the brilliant U. S. paleontologist Edward Drinker Cope, after studying certain fossil reptiles from South Africa, independently reached another conclu­sion which has been strengthened with every passing year-namely that the an­cestry of the mammals is to be found in these fossil reptiles of the Permian and Triassic Periods, some 150 to 230 mil­lion years ago.

THE physiological, reproductive and anatomical differences between liv­

ing reptiles and mammals are readily apparent. First, the reptiles are "cold­blooded" animals in which the internal body temperature varies more or less directly with the temperature of their environment; mammals are "warm­blooded," with a fairly constant body heat and an outer covering of hair to insulate them. The reptiles, as a result of their lack of temperature control, are for the most part sluggish, by compari­son with the active mammals. Most of the reptiles lay eggs from which the young are hatched, though some retain the eggs within the body of the female

40

by Edwin H. Colbert

so that the young are born alive. In most mammals, the embryo develops and is nourished within the uterus of the moth­er. And the mammals of course are dis­tinguished by the property from which they derive their name: they suckle their young.

Many of the anatomical differences between modern reptiles and mammals are reflections of physiological or repro­ductive differences. The reptiles are typified by a relatively small and simple brain, whereas mammals have a large brain. Modern reptiles have a single bony joint at the base of the skull, the occipital condyle, to articulate the head with the backbone; mammals have two condyles. The lower jaw in reptiles con­sists of several elements, one of which, the articular bone, works against the quadrate bone of the skull to form' the articulation between skull and jaw. In mammals there is a single jawbone, the dentary, which articulates directly with the squamosal bone of the skull. Reptiles have a single bone in the middle ear; mammals have a chain of three. The teeth of a reptile are generally pretty much alike, and they are renewed by many "generations," so that a replace­ment is on hand for any tooth that may drop out. The teeth of mammals are dif­ferentiated into incisors, canines, pre­molars and molars, and a mammal is limited to two sets-the "milk" teeth and the later permanent teeth.

There are also important differences in the rest of the skeleton. The verte­brae of a reptile are fairly uniform, whereas in a mammal they are strongly differentiated in the neck, thorax and lumbar regions. A reptile's long bones usually can continue to grow through­out the life of the animal; a mammal, on the other hand, has separate "epiphyses" at the ends of the bones which become fused with the bones as the animal reaches maturity and prevent further growth. In reptiles the number of bones in the fingers and toes varies, while in mammals they are limited to two bones in the thumb and big toe and three bones in each of the other fingers and toes.

Because of these many differences, the early anatomists failed to see the rela­tionship between mammals and reptiles.

But with the discovery of many bones of mammal-like reptiles in South Africa, notably by the physician-paleontologist Robert Broom, a multitude of likenesses appeared. Although by far the greatest number and variety of these reptiles has been excavated from South Africa, by now they have also been found in many parts of the world, including North and South America, Russia, England and western China. It is therefore apparent that during the last stages of the Paleo­zoic Era and the first stages of the Meso­zoic they were spread over almost all the earth.

The reptiles from which the mammals are believed to have come belong to two orders, knovvn as the therapsids and the ictidosaurs. Of particular interest among the therapsids is the suborder called the theriodonts, so named from their "beast"­like or mammal-like teeth. An especially important genus of theriodont is Cynog­nathus (meaning dog-jaw).

To ANYONE familiar with modern reptiles, Cynognathlls seems most un­

reptilian. In life this animal must have been very different in appearance and in actions from the reptiles of today. Even the fossilized bones show this, for in Cynognathus many anatomical charac­ters bridge the structural gap between reptile and mammal.

Cynognathus was a rather large ani­mal with a long, doglike skull, as big as the skull of a wolf. Evidently it was carnivorous, for its skull is armed with sharp, strong teeth well adapted to seiz­ing and tearing its prey. These teeth, quite unlike those of the carnivorous crocodile, are not evenly spaced and uni­form but rather are separated into dif­ferentiated groups like the teeth of many mammals. They must surely have func­tioned like mammals' teeth. In the front of the jaws are small, conical incisor teeth for nipping or biting. Behind the incisors there is a gap, followed by a

Single, large, daggerlike canine in each of the upper and lower jaws. Like the canine in present-day wolves or foxes, it must have been the great, slashing knife that formed the principal weapon of the animal. Behind it are the cheek teeth, which in mammals are known as the pre-

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Crocodilians

Pterosaurs

Saurischian dinosaurs

Ancestral birds

Thecodonts

Cotylosaurs,

the stem reptiles

MAMMAL-LIKE REPTILES occupy a relatively minor position in the entire reptilian line of descent. At lower right are the therapsids, with the ictidosaurs one of the

Turtles

Rhynchacephalians

Mammals

Pelycosaurs

reptilian orders with mammalian characteristics. The early mammal which is shown in thc Cretaceous Period (near top) is rather similar to the present-day opossum.

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LYCAENOPS is a typical spccies among the mammal­like reptiles . .It is one of the theriodonts, meaning; that it had beastlike teeth. The theriodonts are a suborder of the therapsids. One of the distinguishing; features of

Lycaenops, and other mammal-like reptiles, is a pair of long teeth used as slashing instruments. This restoration was made hy the author and artist John C. Germann at New York's American Museum of Natural History.

molars and molars. In C!fllognatillls, as in thc mammals, these tceth are complex, with several cusps forming the crown of each tooth. Evidently they werc useful for cutting food into small pieccs, so that it could be quickly assimilated by the digcstivc system. This is indeed a con­trast to thc living reptiles, which bolt thcir food and then digest it slowly.

There are various other mammal-like characters in the skull of C!fllognathus. For instance, as a corollary to its per­fected dentition, this animal, like the mammals, had a secondary bony palate separating the respiratory tract from the alimentary passage. This obviously added to the efficiency and speed of eat­ing' which were important to a relatively active animal. Again, in Cljnognathlls as in the mammals, there are two condyles

. joining the skull to the Rrst vertebra of the hackbone.

Various mammal-like characters ap­pear in the skeleton of Cljnognathlls behind the skull. Thus therc is a con­sidcrable degree of specialization of thc vertebrae: those in the neck are diffcrent from the rib-bearing trunk ver­tebrae, and one can see the beginnings of a l:ibless lumbar region, as in the mammals. The shoulder blade has a strong spine along its front edge-some­thing quite new for the reptiles, and a forerunner of the spine that is so distinc­tive of the mammalian shoulder blade. In the pelviC girdle the ilium is elon­gatcd. and much of this clongation is fnrward, so that the bone hegins to de­velop a shape prophetic of the mam­malian ilium. The limbs and the feet are in ccrtain respects rather mammal-likc. Evidently Cynog'l1atlws had a somewhat mammal-like posture, with the body car­ried well above the ground and the feet pullcd in toward the midline to give

42

strong support 'and increase the effi­ciency of walking.

Yet in spite of all these advances, Cynognathlls is distinctly a reptile, and it retains many of its ancient reptilian features. Thus it has a full complement of reptile bones in the skull, and to a large degree in the skeleton as well. There is little of the loss and coalescence of bones so typical of mammalian struc­ture. The lower jaw is formed of several bones instead of a single one, and the skull is hinged to the lower jaw in the typical reptilian arrangement. The Rnger and toe bones are entirely reptilian.

For all that, C!fnognatlllIs represents a great forward step in evolutionary de­velopment. Moreover, certain other the­riodont reptiles related to Cljnognatlws show advances in characters where CIj­nognatlltls was conservative. For in­stance, a theriodont known as J3allTia, which in many respects is less mammal­like than CynognatllUs, has the same number of bones in its toes as a mam­mal has. Thus while no one theriodont reptile approaches completely the mam­malian type of structure, the theriodonts as a group clearly exhibit a trend in that direction.

THE approach toward the mammals is c.:arried even further in the ictido­

saurs, of which we unfortunately know all too little. The ictidosaurs possess to an even more advanced degree the various mammal-like characters that are distinc­tive of the theriodonts; in addition, they show trends toward the mammals in cer­tain characters that are still thoroughly rcptilian in the theriodonts. For instance, thc ictidosaurs have lost some of the skull bones characteristic of the theriodonts and have a verv much more mammalian skull pattern . .'rhey show a further ad-

vance in thc bony secondary palate. The ictidosaurs also have a greatly enlarged dentary bone in the lower jaw-a step toward the single jawbone of the mam­mals. But they still have other bones in the lower jaw and a typically reptilian hinge between skull and jaw.

Discoveries in two localities on oppo­site sides of the carth in recent years have extended greatly our knowledge of the ictidosaurs. In China, Dr. Chung Chien Young has described ictidosaur skulls and bones that he unearthed in Yiinnan; in England Walter Kuhne, by the most painstaking methods, has also obtained skulls and parts of skeletons of these very importan t predecessors of the mammals. These ictidosaurs bear many structural resemblanc.:es to the platypus of Australia, the most primitive of living mammals .

At what stagc in their evolution did these mammal-like reptiles cross the threshold into full mammalian status? We can assume that the reptile ceased to be a reptile and became a mammal when it had established a constant body temperaturc and an insulating coat of hair, and when reproduction had reached an advanced stage of develop­ment, especially when the female had begun to lactate and suckle its young. Unfortunately we can obtain no direct cvidencc on these changes, for tempera­ture controls, hair, mammary glands and other soft parts of the anatomy are not preserved as fossils.

ConSidering only the bones, we can say that the evolving animal had reached a mammalian stage when it achieved the combination of structural features al­ready indicated as characteristic of mam­mals, that is, a full differentiation of the vertebrae, a fused pelvis, perfected feet, epiphyses on thc long bones, a double

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occipital condyle at the base of the skull, a perfected secondary palate, a chain of three small bones in the middle ear, and a single lower jawbone articulating with the skull. Of these features, the last is perhaps the most significant.

In the mammal-like reptiles, there was a progressive reduction of the quadrate and articular bones which formed the reptilian articulation between skull and jaw; in the ictidosaurs these elements were very small indeed. Now from a synthesis of embryological and pale­ontological evidence we know that in the final transition from reptile to mam­mal, there was a remarkable trans­formation in these bones. As the reptile became more nearly a mammal, the two bones finally abandoned their functions as articulating elements between the skull and the jaw, and entered the mid­dle ear. The quadrate became the mam­malian incus bone, the articular became the mammalian malleus, and together with the stapes the three bones formed the chain of ear ossicles that is charac­teristic of the mammals. At the same time a new joint was formed between the squamosal of the skull and the dentary, the only bone left in the lower jaw.

None of the ictidosaurs had quite at­tained this stage of evolution, and so by definition they can still be regarded as reptiles. Yet at that point the distinction between reptile and mammal had be­come so narrow that the line of demarca­tion between them must be based, perhaps arbitrarily, on this difference be­tween a single ear ossicle and three ear ossicles. Though seemingly inSignificant, the difference is so constant that it serves admirably as a reference point at which to draw the very fine line between rep­tiles and mammals.

THE first mammals probably looked very much like the mammal-like rep­

tiles that were their immediate ancestors. It was in the Triassic Period of earth his­tory that the change from reptile to mammal was made, and from the suc­ceeding JuraSSic Period on, we can see mammals sharing the earth with reptiles. The Triassic Period was still early in the Mesozoic Era, the great age of dinosaurs. For a long time, geologically speaking, the early mammals were destined to live in a world full of reptiles. It was a lush, tropical world, in which the giant rep­tiles were supreme and the mammals relatively inSignificant. But as the con­ditions on the land changed, the earth became more suitable for mammals than for reptiles. By the time the mammals became dominant on the earth, their reptilian ancestors had long been extinct.

•

Edwin H. Colbert is a paleontologist at the American Museum of Natural His­tory. He is also professor of vertebrate paleontology at Columbia University.

DEVELOPMENT of secondary palatc \ black) illustrates evolutionary position of the mammal-like reptiles. Early mammal-likc reptile Dimetrodon (A) had tiny palate, with nostrils opening into month. Later mammal-like

reptile Cynognathus (B) had closed nasal passage. Dog has full palate.

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