Book - Vertebrate Zoology (1928) 41
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XLI The Evolution Of The Mammalia
In considering the evolution of the mammals it is necessary to revert to the Theromorph reptiles, in the more highly developed members of which, such as Cynognathus, it was found that the following characters were present. The skull had two occipital condyles, a false palate, heterodont teeth in sockets with the mammalian method of replacement. The dentary was large, the remaining membrane-bones of the lower jaw were small, and the jaw articulation was beginning to be taken on by means of the squamosal ; the quadrate was loose and small. The limb girdles were of the mammalian type, and the limbs were long and supported the body clean off the ground. These characters point unmistakably to the fact that the mammals were derived from ancestors which were Theromorph reptiles.
The dominant factor in mammalian evolution appears to have been the development of the brain along the lines of increase in size of the roof of the cerebral hemispheres, and the formation of a special area of cerebral cortex called neopallium, which was no longer under the dominance of the fibres coming from the olfactory centres. The neopallium became an organ for the retention of past sensations and for the delicate co- ordination of the activities of the body of the animal, which thus became capable of more efficient response to external sets of circumstances, and capable of profiting by experience. It enabled the animal to improve the speed and precision of its method of locomotion with the help of the long and delicately formed limbs ; and the fact that the skin lost its hard horny scales and became supple and covered with hairs, enabled it to increase its sensitiveness. The hair covering, furthermore, was a non-conductor of heat, and this fact together with the greater activity of the animal and more intense metabolism enabled the mammals to become warm-blooded. Later on, with the development of the sweat-glands in the skin, the mammals were able to regulate their loss of heat, and so become constant- temperatured or homothermous. The modification of some of the skin-glands into mammary glands made it possible for the young mammals to pass through a protected period of infancy during which the finishing touches to their development were put on, and they became apprenticed under the care of the family to the conditions of their adult life.
The transition from Theromorph reptiles to mammals probably took place in the Permian period, for in the Triassic, fossils are found which show an advance in grade of structure. Of these, the Multituberculata are a group which persisted until the Eocene. They advanced in general evolution and grade of structure as far as the Marsupials. The pelvis was narrow as in the reptiles, and the lower jaw which contained a single bone, had inflected angles. The single bone (dentary) in the lower jaw is a characteristic mammalian feature. The Multituberculata were, however, specialised, and possessed molar teeth with a large number of cusps. They are probably a divergent line which evolved parallel with but independently from the remaining mammals.
At this stage it must be imagined that the primitive mammals had seven cervical vertebrae as a constant number, and that they had evolved the characters enumerated above, together with the diaphragm and the non-nucleated red blood- corpuscles. The epipterygoid had been converted into the alisphenoid, and the quadrate (incus) and articular (malleus) into auditory ossicles. They retained, however, the reptilian characters of the presence of the coracoid, precoracoid, and interclavicle, the cloaca and the habit of laying eggs. They had not yet evolved the viviparous habit or the formation of a placenta, epiphyses were not yet well developed in the bones, the mammary glands were unprovided with teats, and the two halves of the neopallium were not connected by that special transverse commissure : the corpus callosum.
The Monotremes must have diverged from the main stem at this point, and they are represented to-day by Ornithorhynchus and Echidna, to which the description just given fits well. They are inhabitants of the Australasian region.
The remaining mammals were the ancestors of the Marsupials and of the Placentals. These two groups are fairly closely allied, and have the following characters in common : the mammary glands have teats, they are viviparous, a placenta of some kind is present, the ear has an external pinna, the bones mostly have epiphyses ; the coracoids,interclavicle, and cloaca have been lost.
It is clear from the reduction of the milk- dentition and of the allantoic placenta (which is only preserved in Perameles) in Marsupials, that they are derived from a stock with two sets of teeth and with a well-formed allantoic placenta. On the other hand, some primitive Placentals show evidence of descent from forms with marsupioid characters, such as alleged traces of a marsupial pouch, of coracoids, and other features. The conclusion to be drawn is that Marsupials and Placentals had a common ancestor perhaps in the Jurassic. Now, in the Jurassic the fossil Trituberculata are found, and they are regarded as related to the Marsupials by some, and to the Insectivores (Placentals) by others. The number of teeth was large. The Trituberculata which derive their name from the pattern of the cusps on the molars were probably related to the common ancestors of Marsupials and Placentals. It is a significant fact that the arrangement of the cusps on the molar teeth in several primitive groups of mammals is of this type, regardless of the diet for which the teeth of the higher members of these groups are modified. Molars with separate cusps like this are called bunodont.
In the Eocene period, the Marsupials had a wide distribution over the earth, but at the present day they are restricted to the Australian and southern and central American regions. These regions are characterised by their isolation and the comparative absence of mammals of the Placental type. If it had not been for the latter fact, there is little doubt that the Marsupials would have become extinct, for they cannot compete with the Placentals. Instead, in the security of their isolation, they radiated out into a number of types which are especially interesting in that they have evolved parallel with several groups of Placentals, and by becoming adapted to equivalent biological environments have developed a convergent resemblance to these Placentals. Nearly all Marsupials have a marsupial pouch in the female and epipubic bones supporting it.
The opossum (Didelphys) and Cagnolestes are American ; all the remainder are restricted to Australasia, though fossils related to these are also found in South America. Dasyurus is the Marsupial equivalent of the cats, while the dogs are represented by Thylacinus, Perameles (the bandicoot) is an attempt at a rabbit, Petaurus (the phalanger) resembles the flying squirrels, while Notoryctes is a remarkable imitation of the mole. Phascolarctos (the koala) is the " marsupial bear," Phascolomys (the wombat) is the " marsupial rodent," the extinct Thylacoleo was the " marsupial lion," while Macropus (the kangaroo) represents the swift-moving Ungulates.
The Cretaceous strata of Mongolia have revealed fossils of apparently Placental mammals, of which Deltatheridium is an example, and which can be regarded as intermediate between the Jurassic Trituberculata and the true Placentals of the Eocene.
The Placentals are characterised by the possession of an allantoic placenta, a corpus callosum joining the two halves of the neopallium, and a typical dental formula of if, c], p|, m|. This number of teeth is, however, often modified and reduced.
At the beginning of the Eocene period there appeared a group of true Placentals which were primitive in that they were of small size, with tritubercular short-crowned molars, five fingers and toes, and walking on the flat of the hand and foot. Among them can be recognised some with a tendency to modification of the teeth for a carnivorous diet — the Creodonta, — others for a herbivorous diet — the Condylarthra. Others again were generalised Insectivora. Very early, a branch diverged from the Condylarthran stock and gave rise to the Amblypoda, large, clumsy, premature rhinoceros-like forms such as Uintatherium, and which soon went extinct.
In the later Eocene divergent evolution has progressed, and it is very interesting to notice that a number of the Orders of Mammals have become differentiated, and that these are not yet split up into the various families. The Creodonta had given rise to the Carnivora, which branched out into the Pinnipedia or seals, and Fissipedia or dogs, cats, bears, civets, and badgers. The Rodentia came off from near the primitive Insectivora, as did also the Primates (Lemuroids and Tarsioids) and the Edentates. The Perissodactyla or odd-toed Ungulates emerged from a stock intermediate between Condylarthra and Insectivora, and blossomed out into the huge Titanotheres which soon went extinct, the horses, tapirs, and rhinoceroses. The even- toed Ungulates or Artiodactyla emerged from some form between the Creodonta and the Insectivora, and, apart from a number of short-lived groups, radiated out into the pigs and hippopotamuses on the one hand, and the camels, antelopes, deer, cattle, and giraffes on the other. Related to the Ungulates are the conies (Hyracoidea) and the elephants (Proboscidea). The whales (Cetacea) are regarded as having arisen from a stock related to the Creodonta, and the Sirenia may have a common descent with the Proboscidea. South America became inhabited by a peculiar collection of archaic forms which were all doomed to extinction, but of which some such as the Thoatheria had evolved into a very remarkable imitation of the horses. The Edentata include armadillos, sloths, and ant-eaters.
The Cheiroptera or bats are related to the Insectivora, while the Dermoptera have affinities with the Insectivora and Primates. In some cases sufficient fossil forms are known from successive strata to enable lines of descent to be traced with considerable precision. This applies especially to the horses, the camels, and the elephants. The evolution of the horses from Eohippus (Eocene) through Mesohippus (Oligocene), Miohippus (Miocene), Pliohippus (Pliocene) to Equus, was accompanied by a progressive increase in size, lengthening of the teeth which become " rootless, development of ridges on the molars, fusion of ulna with radius and tibia with fibula, specialisation of the wrist and ankle joints into articulations allowing movement in only one plane, enlargement of the 3rd digit in hands and feet, and reduction of all the other digits until their disappearance.
The evolution of the camels from the Eocene Protylopus through Poebrotherium (Oligocene), Procamelus (Miocene) to the present day is likewise a history of gradual increase in size, increase in length of the teeth and development of selenodont ridges on the molars, reduction of the upper incisors, enlargement of the 3rd and 4th digits in hands and feet with suppression of the remainder, and fusion of the 3rd and 4th metacarpals and metatarsals.
So far as is known the history of the elephants starts with the Eocene Moeritherium, of about the size of a pig, and with the primitive dental formula of i|, c^, pf, nig. Its ridged (lophodont) molars had only two ridges. In the upper Eocene, Palseomastodon was larger, and had a not inconsiderable trunk. The canines and all the incisors except one pair in each jaw had disappeared, and the molars had three ridges. Tetrabelodon from the Pliocene was still larger and its incisors were elongated into tusks, with persistent pulps. The molars had as many as six ridges and were so large that there was not room in the jaws for more than two teeth in each jaw on each side. Furthermore, instead of being replaced from beneath as in ordinary mammals, they were replaced from behind, the new tooth pushing the old one out forwards in front of it. It is worth noticing that although these animals grew large and tall, their necks were very short, and it is only by means of the long trunk that they were able to reach down to the ground for eating and drinking. The next step, shown by the Pliocene Mastodon, was accomplished by a shortening of the lower jaw and the loss of the lower incisor- tusks. Lastly in Elephas the grooves between the ridges on the molars become filled with cement. The ridges may be a dozen in number, and the maximum number of molars on each side in each jaw in use at one time is one and a half.
The history of the Primates is reserved for the next chapter.
Literature
Broom, R. On the Origin of Mammals. Philosophical Transactions of Royal Society, Ser. B, vol. 206, 1914.
Gregory, W. K., and Simpson, G. C. Cretaceous Mammal Skulls from Mongolia. Nature, vol. 118, 1926.
Matthew, W. D. The Evolution of the Mammals in the Eocene. Proceedings of the Zoological Society of London, 1927.
Osborn, H. F. The Age of Mammals. Macmillan, New York, 19 10.
Weber, M. Die Saiigetiere. Fischer, Jena, 1927.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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