Cambridge Natural History Mammalia/Chapter III
CHAPTER III
THE POSSIBLE FORERUNNERS OF THE MAMMALIA
The relationship of Mammals to Vertebrates lying below them in the scale, their origin in fact, is a much-debated question, with many attempted solutions. To enter into this large question in detail would involve a great deal of useless statement of arguments founded upon misleading or upon quite inaccurate "facts." It will perhaps be sufficient if we reflect here the current view most in vogue at the present, i.e. that which would refer the Mammalia to reptiles belonging to the extinct Permian and Triassic group of the Theromorpha (also called Anomodontia). These have been explored lately to a very large extent, and chiefly by Professor Seeley.[1] The very fact that a genus Tritylodon, only known by the forepart of the skull, has been called Mammalian and Anomodont by various authors, shows at least the difficulty of differentiating the two groups when the material for study is imperfect. As a matter of fact these Theromorpha are without doubt reptiles; they show, for example, a lower jaw formed out of several distinct pieces, of which the articular articulates with a fixed quadrate on the skull. They possess the characteristic reptilian bones, the "transverse," the pre- and post-frontals, and there are various other points of structure which leave no room for doubt as to their truly reptilian nature. There are, however, numerous indications of an evolution in the mammalian direction in all parts of the skeleton, to the more important of which some reference will be made here. It may be as well to clear the ground by mentioning the fact that among the Theromorpha four distinct types of reptiles are included, which are considered to form four orders, i.e. the Pareiasauri, the Theriodontia, the Anomodontia (Dicynodontia), and the Placodontia.
The first of these divisions includes what seem to be basal forms. These reptiles show numerous points of likeness to the Amphibian Labyrinthodonts.[2] On the other hand the third division, that of the Dicynodontia, are highly-specialised Theromorpha, from which no further evolution would appear to have been possible. Thus the dentition was either completely lost, or reduced to tusks as in Dicynodon. We need not therefore concern ourselves in the present volume with these Anomodonts. It is with the Theriodonts that our business lies. The very name, be it observed, is aptly chosen on the hypothesis to be explained here; but it is not only in the teeth that these reptiles show likenesses to the Theria or Mammals, but in almost every feature of their organisation. Unlike other reptiles, the Theromorpha in general were lifted comparatively high above the ground on legs of fair length and of mammalian relationship in the position of the segments of the limbs. The typical reptile grovels upon the earth with legs sprawling out, as indeed the very name suggests. One bar to the Theriodonts being on the direct line of mammalian ancestry has been urged as a preliminary difficulty, and that is their large size. The earliest undoubted mammals were small creatures, comparable to a Rat or a Mouse in size; whereas a good-sized Bear or a Wolf is a better standard of size for some of the best-known genera of Theriodonts. It has, however, been quite permissibly suggested that living in company with these large Theriodonts were less obtrusive genera, from which the mammals might have sprung. It is so familiar a fact that a given group of animals generally contains giants, dwarfs, and members of intermediate size, that this suggestion may almost be accepted as a fact. It need at least present no difficulties to us in our comparisons.
The most salient "mammalian" feature of the Theriodonts is the heterodonty of the teeth, the pattern of the "molars," and the limited number which constitute the series. The fact, too, that they are limited to the dentary bones below and to the maxillae and the premaxillae above, is a sine qua non for mammalian comparison. In the more basal Theromorpha the teeth are not so limited in position. Finally, to complete the remarkable mammalian resemblance of the teeth of these reptiles, it must be mentioned that in Tritylodon and Diademodon the roots of the molars, as we may fairly term them, though not actually divided after the mammalian fashion, were deeply marked by a groove, which suggests an incipient division or a fusion of two distinct roots. Some of these facts of structure may now be considered in further detail. As to the incisors and canines, it is sufficient to say that the numbers of the former, and the shape of the latter, are in perfect consonance with a derivation of the Mammalia from this group. The molar series can be divided into premolars and molars, at least in so far as regards their shape; for the anterior teeth are often smaller and less complicated than those which follow, as is often the case with the two series in mammals. The molar series also consist of teeth in close apposition to each other and separated from the canines by a diastema, which is a character of mammalian teeth. The fact that in the reptile Cynognathus and the mammal Myrmecobius there are nine of these molar teeth in each half of each jaw is perhaps not a point upon which it is desirable to dwell with too much weight; but the general fact that the molars are further reduced in some genera of Theriodontia than in that which has been mentioned, is clearly a matter of significance when the ancestry of the mammals is under consideration.
The most interesting fact about the molar series in the Theriodontia is that we meet with the two types of molars that occur in the mammals. Cynognathus and other genera have molars which consist of a main cusp, and of one cusp before and one after the main cusp; in fact these teeth are triconodont as in certain early mammals, a state of affairs which is believed by the "trituberculists" (see p. 56) to have preceded the tritubercular tooth. There are also "multitubercular" teeth, especially well developed in Tritylodon, where they exactly resemble those of certain Multituberculata, and whose structure originally led to the placing of Tritylodon among the mammals of that group. If there is any question about the mammalian nature of this fossil, there remain several other Theriodontia in which the multituberculism is well marked. It is so in Trirhackodon and in Diademodon for instance. This incidentally lends some support to the idea that the Mammalia have been evolved from two sources, a way of looking at the origin of the group that will coincide with the views of some authors like the late Dr. Mivart, and will at the same time reconcile the trituberculists and the multituberculists. For we should then assume that the Eutheria and Triconodontia had originated from some such form as Cynognathus; and the Multituberculata and the existing Monotremes from some form like Diademodon. It is not of great use to point out that Diademodon is really of the trituberculate pattern, because in its molars, though multituberculate, the trituberculate main cones can be recognised; for that state of affairs could just as well have been brought about by a reduction from the multituberculate type. The skull of these Theriodonts shows some well-marked approximations to the mammalian type. There is in the first place a commencing consolidation and reduction of the individual bones, which is so distinguishing a feature of the mammalian skull as opposed to the skull of lower vertebrates. In Cynognathus the postorbital is fused with the jugal, and the supratemporal with the squamosal, forming apparently one bone. In the lower jaw the splenial is often reduced to the thinness of paper, thus indicating a commencing disappearance. In many Theromorpha the squamosal shares largely in the formation of the articular facet for the lower jaw, obviously an important mammalian characteristic; this is brought about by the reduction of the quadrate, which latter bone, moreover, acquires in certain particulars the appearance of the mammalian malleus, with which it is, according to many, homologous. But this subject has been already dealt with on page 26. A very pronounced likeness to the mammalian skull is that there are two occipital condyles. That this has been brought about by the further development of a tripartite condyle such as occurs in tortoises, and that by the suppression of the basi-occipital part, does not affect the resemblance to the mammalian skull; in fact it explains the origin of two condyles from the typical reptilian single condyle, and disposes of the necessity for believing, with Huxley and others, the Amphibia to be on the main line of mammalian evolution on account of their two condyles. The general aspect of the skull in Cynognathus has been compared to that "of Thylacinusor Dissacus." No one can examine the actual sketches of the skull of that Theriodont without endorsing that opinion. As a curious detailed point of likeness to certain Mammalia may be mentioned "a small descending process of the malar bone, which may be a diminutive representative of the descending element of the malar seen in Elotherium, Nototherium, Diprotodon, Macropus, certain Edentata, such as Glyptodon, Megatherium, Mylodon, Bradypus, but unparalleled so far as I am aware in fossil reptiles." (Osborn.) The zoologist cannot help being impressed with the significance of small details of similarity, which do not seem to be due in any way to surrounding conditions of life, and thus referable to mere convergence, like the fish-like form of Whales and Seals.
The rest of the skeleton of the Theriodontia is by no means so well known as the skull and teeth. But from what is known, other mammalian characters can be pointed out. Perhaps the most striking mammalian feature is to be found in the scapula of Cynognathus. It is in this creature somewhat narrow and elongated; but it has a well-marked spine, ending in a hooked acromion. Now it is to be noted in support, so far, of the diphyletic origin of mammals, that in the Monotreme, as in Whales indeed, the spine forms the anterior border of the scapula, and is coincident with it, there being thus no prescapula at all in the Monotreme, and only a trace of it in certain Whales.[3] Whether the multituberculate Tritylodon or Diademodon had a scapula after the Monotreme pattern is not known; but it is clear that the scapula of the triconodont Cynognathus is quite after the pattern of the Eutherian scapula. Furthermore, Professor Seeley is of opinion that the coracoid was relatively small, and indeed smaller than the same bone in Edentates, and a fortiori than in Monotremes. Another fact of structure which points also, possibly, in the direction of a diphyletic origin for the Mammalia, is the double-headed ribs of Cynognathus. As is well known, the ribs of the Monotremata have only the central head, the capitulum.
As a general mark of affinity with mammals the reduction of the intercentra in Cynognathus may be noted, and also the existence of a small though perfectly obvious obturator-foramen, separating the pubis from the ischium. There are further details which tend in the same direction. And we shall probably not go far wrong in the present state of our knowledge if we assign the origin of the mammals to some type which would be included in the order Theriodontia or at least in the sub-class Theromorpha.