Cambridge Natural History Mammalia/Chapter IV
CHAPTER IV
THE DAWN OF MAMMALIAN LIFE
The animals that we considered in the last chapter, though showing certain unmistakable likenesses to the mammals, are nevertheless unquestionably not mammals but reptiles. In the Triassic strata, however, we first meet with the remains of undoubted mammals. The Mammalia first appeared upon the earth in a tentative and hesitating way: they had not cast off many of the characters of their supposed reptilian forefathers; they shrank from observation and destruction by their small size, and apparently, so far at any rate as their teeth afford a clue, by an omnivorous diet. The world abounded at that period in large and carnivorous reptiles, which may indeed have been the principal enemies with which the first mammals had to cope. These early mammals lingered on to so late a period as the Eocene; but the majority of the genera were Triassic, Jurassic, and Cretaceous. Certain of the primitive mammalian forms have been referred to the Marsupials, and their resemblances to the Monotremata have also been pointed out. The current view of the present time, however, is that they form a special order, which may possibly have embraced the ancestors of both Marsupials and Monotremes; for it is reasonable to explain in this way the combination of characters of these two orders which they present. For this group the name Allotheria has been proposed by Marsh, and Multituberculata by Cope; the latter term is the less suitable, in that the Monotremata (Ornithorhynchus) are also "multituberculate." The group is known in a very imperfect fashion. The remains are but few and fragmentary; and for the most part we have only a few teeth to speculate upon. This is natural enough, for the harder teeth might easily be supposed to have resisted the decay which would more readily affect the softer bones. Where there are bones it is frequently the lower jaw alone which has been preserved for us—a bone which has also been preserved in the case of some of the contemporary Marsupials.
It has been pointed out (from the observation of dead dogs floating in canals) that the lower jaw is occasionally detached from the carcase. It is the most readily separable part which contains a skeleton. It may be, therefore, that the remains of these early mammals, floating down some river to the sea, may have lost their jaws while in the river, or at furthest in the shallow waters of the sea, the rest of the carcase floating out to a greater distance, and being finally entombed in the stomach of some carnivorous fish, or in the mud at the bottom of a deep ocean, which has never since seen the light.
The characters of this group are really more those of the Monotremata than of the Marsupials. The undoubted likeness which their molar teeth show to the temporary teeth of the Platypus have already been commented upon. Like the Monotremes the Allotheria appear to have possessed a large and independent coracoid; the evidence for this rests upon the discovery of the lower end of a scapula of Camptomus, a Cretaceous genus from North America upon which there is a distinct facet for the articulation of what can have been nothing else than a coracoid. On the other hand they differ from the Monotremata by the presence of incisor teeth which were Rodent-like in form, and not very different from those of certain Marsupials. This point of difference cannot be regarded as of very first-rate importance; no one would relegate the Sloth and the Armadillo to different orders on account of their tooth differences, which are about on a par with those to which we have just referred. It seems indeed likely that it will be ultimately necessary to rub out the boundary line which now divides the Allotheria and the Monotremata.
The Plagiaulacidae are unquestionably mammals, and they are placed by most naturalists in this at present uncertain group of Multituberculata, which will be retained here in deference to the distinguished authorities who have instituted the group, though there are but few characters by which it can be defined. This family though appearing in the Trias, extends down in time to the Eocene. The type-genus, that which has given its name to the family, is Plagiaulax. As it is not Triassic, the consideration of its characters will be deferred until later. Microlestes is a Rhaetic genus, known from rocks in Germany and England; but it is entirely based upon molar teeth. M. antiquus has a two-rooted molar of an elongated form with a row of tubercles on either side of a median groove, which traverses the long axis of the tooth. To some extent the teeth of the ancient form resemble those of Ornithorhynchus. Microlestes has been sometimes spoken of as a Marsupial, but Mr. Tomes[1] has found that it does not show one very universal character of the Marsupial teeth: it has not those continuations of the dentinal tubes which traverse the enamel in all Marsupials that have been examined with the sole exception of the Wombat.
The rarity of the remains of mammals in these earliest rocks of the Secondary epoch has been accounted for in another way from that which has been suggested above. It may be that the group Mammalia was not evolved in Europe at all, and that the stray remains which have been found in that continent represent the fragmentary remnants of a few scattered immigrants which heralded the later invasion of more numerous genera during the Jurassic period.
The Mammals of the Jurassic Period.—Some of the Allotheria or Multituberculata described in the last section occur in the rocks of this early part of the Secondary epoch. They are doubtful in position, as already stated; some of them indeed, as for instance Tritylodon and Dromatherium, are possibly not mammals at all, while the remainder probably belong to a non-existent order of mammals. Along with these dubious creatures are the fragmentary remains of small animals which are not merely mammals, but in all probability definitely Marsupials. It is true that here again we have little beyond lower jaws and teeth to deal with; so that there may be less certainty in referring them to the Marsupials than appears to be the opinion of the majority of Palaeontologists.
Professor Osborn in fact considers that the Mesozoic mammals consist of three groups: (1) The Multituberculata, including the Bolodontidae, Stereognathidae, Plagiaulacidae, Polymastodontidae, and possibly the Tritylodontidae (which, however, are regarded by him and by others as more probably reptiles of the Theromorphous group). (2) The Triconodonta, which were Marsupials, though in all probability with a complete succession of teeth and with an allantoic placentation. This group will include the genera Phascolotherium and Amphilestes, as well as Triconodon and Spalacotherium. Finally we have (3) the Trituberculata (or Insectivora Primitiva) with the genera Amphitherium, Peramus, Amblotherium, Stylacodon, and Dryolestes.
We shall take these three groups in order. The Multituberculata have already been to some extent defined, if such a word can be used to express the summation of the very scanty information at our disposal. Of this group, Plagiaulax is a genus which occurs in the Purbeck beds; it is only known by lower jaws implying an animal of the size of a Rat or rather smaller. The jaws have in front a large incisor which looks Rodent-like, and also like those of the Diprotodont Marsupials; but it is held that these teeth did not grow from persistent pulps, and there is in any case no anterior thickened coating of enamel. Canines are absent; the diastema is followed by four premolars increasing progressively in size and possessing somewhat complicated grinding surfaces. These surfaces are formed by several obliquely-set ridges. The succeeding teeth are termed molars on account of their difference in structure, and there are but two of them on each side. The molars are of a pattern common in the Multituberculata; the centre is hollowed, and the raised rim is beset by tubercles. Other Jurassic genera of Multituberculates are Bolodon, Allodon, and Stereognathus. All of these possess the same multituberculate molars.
Of the Triconodonta the type-genus is Triconodon. This genus is better known than most Jurassic mammals, since both the upper and the lower dentition have been described. It appears to have possessed the typical Eutherian dentition of forty-four teeth, to which a fourth molar is added in some species. The great difference between the molars and premolars argues a complete tooth-change. The genus is American as well as European.
Spalacotherium has more molars, five or six.
Phascolotherium bucklandi, on the other hand, is a much older type in the form of its teeth. There are, however, not so many of them as in Amphitherium; Phascolotherium has but two premolars and five molars, making a total of forty-eight teeth. The teeth are of the triconodont form, the three cusps being in line, and the middle one the largest.
Amphilestes has teeth of the same pattern but has more of them, the premolars and molars being respectively four and five. All these animals had the lower jaw inflected. Whether they are all Marsupials or not, it is clear that Phascolotherium and Amphilestes should be united and placed away from Amphitherium on account of the more primitive form of their teeth.
We next come to the Trituberculata.
Among the most celebrated of these remains are a few jaws discovered in the Stonesfield slates near Oxford, and examined by Buckland, Cuvier, and some of the most eminent naturalists of the beginning of the last century. These jaws have been lately submitted to a careful re-examination by Mr. Goodrich,[2] who has increased our knowledge of the subject by exposing from the rocky matrix in which the jaws lie fresh details of their structure; it is probable therefore that now all that there is to be learnt from these specimens has been recorded.
Amphitherium prevostii was a creature about the size of a Rat. Its jaw was first brought to Dean Buckland about the year 1814, and described six years later. Buckland thought the jaw to be that of an Opossum, an opinion in which Cuvier concurred. The jaw, however, is marked by a groove running along its length, and this groove was regarded by de Blainville as evidence of the composition of the jaw out of more than one element, which would naturally lead to its being regarded as the jaw of a reptile.[3] This species and another named after Sir Richard Owen have a dental formula which, like that of the Marsupials, is large as compared with that of the Placental mammals; it runs: I 4, C 1, Pm 5, M 6—i.e. 64 teeth altogether. This is a larger number than we find in any existing Marsupial. But as in Marsupials, and in certain Insectivora also, the angle of the jaw is inflected. These teeth are of the tritubercular pattern with a "heel." They are in fact closely like those of the living Myrmecobius; but not, it should be remarked, unlike those of certain Insectivora.
The Mammals of the Cretaceous Period.—At one time there was a totally inexplicable gap between the Jurassic and the basal Eocene, a series of strata which occupy an enormous expanse of time in the history of the earth having appeared to be devoid of mammalian remains. This gap, however, has been filled up by the discovery of mammalian remains in the North American Laramic formation, which seems to be clearly of Cretaceous age. Furthermore, it is held by some that the Purbeck beds are more properly to be placed with the Cretaceous, which would then necessitate the consideration under the present heading of some of the types already dealt with; and if, as is suggested in the following section, the lowest so-called Eocene beds are really referable to the Cretaceous, there is no lack of mammalian remains in that period. And, moreover, it was in that case the Cretaceous period which witnessed the evolution of the existing orders of Placental mammals. Otherwise the mammalian remains of the Cretaceous agree with those of the Jurassic. We find remains of the Multituberculata in fragments of Plagiaulacidae and Polymastodontidae. Ptilodus is a genus which has two premolars; and Meniscoessus is another multituberculate from the same Laramic formation. The other detached fragments of mammals are thought by Osborn to represent both Placentals and Marsupials.
The Mammals of the Tertiary Period.—Unless the lowest beds of the earliest Tertiary period, the Eocene, such as the Torrejon of North America, should be in reality referred to the Cretaceous, there is no evidence that the modern groups of Mammalia existed before the present epoch of the earth's history. It is probable, however, that the Eutheria as a group were Mesozoic. The fossil jaws that have been considered in the last chapter may quite probably be primitive Eutherians, or even divisible, as believed by Professor Osborn, into Marsupials and Insectivores. In the Tertiary, however, apart from the question as to the nature of the Puerco and Torrejon formations, and as to certain South American strata whose fossil contents have been investigated by Professor Ameghino, we find the first traces of mammals definitely referable to existing orders, or to be distinctly compared with existing orders. Since, however, representatives of types which have obvious relationships to modern types appear in considerable profusion in the very earliest strata of the Eocene, it seems clear that much remains to be discovered in beds earlier than these. Confining ourselves, however, to facts and to comparisons which can be made on more than a few lower jaws and scattered teeth, which is practically all that we possess of earlier mammals, we must arrive at the general conclusion that two of the existing larger groups of the Eutherian, non-Marsupial, mammals were differentiated at quite the beginning of the Eocene, and were represented by forms from which it is possible to derive at least the existing Carnivora, Insectivora, Artiodactyla, and Perissodactyla. These were the Creodonta and the Ungulate Condylarthra. In addition to these we may enumerate as very early types the Lemuroidea, represented by such forms as Indrodon in the New World, and (though later) by Necrolemur, etc., in the Old World, and the Edentata, if we are to allow as their ancestors the Ganodonta.
The early Eocene strata also contain representatives of at least one order, the Amblypoda, which increased subsequently, but has died out without descendants, unless we are to believe with some that the Elephants are to be derived from these Eocene "pachyderms." In later Eocene times the great majority of the existing orders, and even subdivisions of orders, are to be met with; and there are in addition such totally extinct orders as the Typotheria, Ancylopoda, and Tillodontia. Coupled with this gradual specialisation in the orders of Eutherian mammals, there is naturally a vast increase in the number of generic and family types. This culminates perhaps in the Miocene, from which time there has been a gradual decline in mammalian variety, so that it is justly said that we live now in an epoch which is impoverished of mammals. This gradual decay has persisted until to-day, as is witnessed by the extinction of the Rhytina and the Quagga, and the growing rarity of the White Rhinoceros and the American Bison.
The early Eutherian stock consisted of small mammals with small heads and slender, long tails. The limbs were pentadactyle, ensheathed in claws or broader hoofs. The fore-limbs may have been partly prehensile. The teeth were forty-four, completely differentiated into incisors, canines, molars, and premolars; and there appears to have been a complete diphyodontism. The canines were not greatly enlarged, and no diastema separated any of the teeth. The molars were bunodont or of a more cutting pattern, with some five or six tubercles. These animals were, moreover, very small-brained. This early stock is represented by Creodont and Condylarthrous animals, the exact boundaries between which are hardly marked in the very early types. Professor Osborn has argued that from this early Eutherian stock there were two waves of progress, or, as he expresses it, "two great centres of functional radiation."[4]
The first was largely ineffective, the second has produced all the Eutherian orders of to-day. These two divisions are termed by him "Mesoplacentalia" and "Cenoplacentalia." The first division embraces the Amblypoda and their descendants the Coryphodonts and Dinocerata, many of the Condylarthra, the bulk of the Creodonts and the Tillodonts. These creatures persisted for a time, but died out in the Miocene. They were mainly distinguished by the smallness of their brain; the great specialisation of structure which they exhibit having left that organ unaffected, and therefore tending in the long run to render them unable to cope with changes in the inorganic and organic world. The successful division of the primitive Eutheria comprises the groups which exist at the present day, and is not connected directly with those small-brained Mesoplacentals; it has apparently originated, however, from the least specialised of their ancestors. Professor Osborn thinks, moreover, that the Lemurs and the Insectivores are persistent descendants of the earlier wave of Eutherian life. It appears in fact as if Nature had created the existing Ungulate, Unguiculate, and other types on a defective plan, and, instead of mending them to suit more modern requirements, had evolved an entirely new set of similarly-organised types from some of the more ancient and plastic forms remaining over. The Marsupials may be the only group of the early wave remaining, and they have been able to hold their own for the geological reason that Australia was early cut off from communication with the rest of the world. That they are disappearing seems to be shown by their gradual diminution as we pass from Australia towards the continent of Asia, through the islands of the Malay Archipelago. Competition has here decimated them, as it may do in the remote future in Australia.
It is often said, but with some looseness of statement, that ancient quadrupeds are huger than their modern representatives. This statement is partly true in fact, but largely wrong in implication. For it suggests that—and the suggestion is often expressed in books that are not authoritative—huge animals have left a dwarfish offspring; that there were giants of old, and that there is a puny race to-day. As a matter of fact, the study of the gradual evolution of the early Tertiary Mammalia into their descendants of later times shows very plainly the truth of this interesting generalisation: That the primitive types were all small creatures, and that in those instances where we can trace a pedigree, there was a gradual increase in size up to a point where greater increase led to extinction. We point out later on a number of facts illustrating this matter in detail. It has been ascertained, for instance, that the pedigree of the Horses, the Camels, the Rhinoceroses, and many other groups, commences with small forms and culminates in large ones. It may be urged that such animals as the Tapir are to-day smallish forms, and that related to them in the past were the gigantic Titanotheres; but in this and similar cases it will be found that the extinct giants were not in the direct line of pedigree, but represented side-branches which waxed huge on their own account and then disappeared.