Water Reptiles of the Past and Present

CHAPTER II

CLASSIFICATION OF REPTILES

There is very much doubt, very much uncertainty, among paleontologists about the classification of reptiles. No two writers agree on the number of orders, or the rank of many forms. Some recognize twenty or more orders, others but eight or nine. And this doubt and uncertainty are due chiefly to the many discoveries of early forms that have been made during the past twenty years. The many strange and unclassifiable types which have come to light in North America, South Africa, and Europe have thrown doubt on all previous classificatory schemes, have weakened our faith in all attempts to trace out the genealogies of the reptilian orders; and classification is merely genealogy. It is only the paleontologist who is competent to express opinions concerning the larger principles of classification of organisms, and especially of the classification of reptiles. The neozoölogist, ignorant of extinct forms, can only hazard guesses and conjectures as to the relationships of the larger groups, for he has only the specialized or decadent remnants of past faunas upon which to base his opinions. About some things we can be quite confident; about some groups opinions have crystallized, and we all agree, except perhaps on trifles. The dinosaurs, the pterodactyls, the crocodiles, for instance, offer only minor problems to perplex the systematist, but the origin and the relations, not only of these, but also of nearly all the others, are still involved in obscurity. The question, whence came the ichthyosaurs, the plesiosaurs, the turtles, etc., seems almost as far from solution as it did fifty years ago. With every problem solved a dozen more intrude themselves upon us. Hence, classification simply represents the present condition of our knowledge, our present opinions as to genealogies. It was the fashion a dozen years ago to draw all sorts of genealogical trees on the slightest pretext, to trace in beautifully clear lines the precise descent of all kinds of animals; and very few have been worth the paper on which they were printed. When facts are numerous enough, conclusions are patent even to the novice; when facts are few and obscure, one can guess about as well as another. In general, it may be said that the older a group of animals is the more abstruse are the problems presented; first, because of the lack of abundant material; second, because the forms speak to us in an unfamiliar language that we cannot easily interpret. The classification of the mammals approaches more nearly the ultimate truth than does that of any other group of organisms, because we know more about the extinct forms than we do of any other class, and also because we know more about the living forms than we do about any other living animals.

Species of reptiles are, for the most part, vague quantities in paleontology; they can be determined with assurance only by the comparison of abundant material. Adult characters in mammals are apparent in the ossification of the skeleton, and size can be used within moderate limits in the determination of species; but size in reptiles means but little; no one could possibly say that the skeleton of an alligator six feet in length is not that of an adult animal if he knew nothing else about the Crocodilia. So also the compression and malformations of bones from the processes of fossilization obliterate specific characters in great part. Nor are specific characters easily distinguishable in the skeletons of living reptiles. The genus, therefore, among fossil reptiles is practically the unit, and we may be sure that for every well-defined genus we discover there existed numerous minor variations, which, had we the living animals to study, we should call species. We classify the living Crocodilia into two families, about four well-defined genera-perhaps even five or six-and about twenty-five species. Of the living lizards there are about eighteen hundred species, twenty families, and four larger groups or suborders. In all probability the lizards have never been more abundant and more varied than they are at the present time. Possibly these proportions of species, genera, families, and suborders may represent approximately the proportions that have existed at some time or other in most of the other groups which we call orders-approximately only, for we can never be quite sure that we evaluate the structural characters of different groups of organisms quite equally. The absence of a molar tooth in a mammal would ordinarily indicate a genus, the absence of a tooth in a reptile might not indicate even a variety or a race. Whence it follows that classification of organisms is not and never will be an exact science. The value of characters used in classification is very unequal, as we have seen. No two persons see these characters from the same viewpoints, and in consequence no two persons whose opinions are worth while ever wholly agree as to classification.

The following scheme differs only in minor details from the more conservative of the generally accepted views, and those differences are, for the most part, the writer's own opinions, to be taken for what they are worth. It may be said decisively that no classification of the reptiles into major groups, into super-families or subclasses that has so far been proposed is worthy of acceptance; there is no such subclass as the Diapsida or Synapsida, for instance. And we have very much more to learn about the early reptiles before any general classification of the reptiles can be securely founded. It is very probable that the primary radiation of the reptiles into the various lines of descent, into its main branches, occurred much earlier than we have been disposed to believe; that before the close of Paleozoic time, perhaps before the close of the Carboniferous, all the great groups of reptiles had gone off from the main stem, and that since then only smaller and smaller branches have appeared. There have been no new orders of reptiles in all probability since Triassic times, and perhaps none since Permian.

Taxonomists are often disposed to cut the Gordian knots of relationships by raising the ranks of the animals they study to independent positions. More than thirty independent orders of reptiles have been proposed by different students, and quite as many of mammals and of birds; possibly after more forms have been discovered there will be as many proposed for the amphibians. Sometimes, indeed, it is better to make such independent groups than to unite lesser ones on doubtful evidence. But the writer, for one, believes that it is more worthy of the thoughtful scientific student to seek for relationships than for differences. It is far easier to destroy than to construct, to make new genera, families, and orders than to unite those already proposed. To raise every proposed suborder of reptiles to an order, as has been proposed by various writers, and the orders to subclasses, only leaves classification where it was; nothing has been added to taxonomy save a lot of new names to perplex and annoy the student.

In the following scheme of classification three groups provisionally called orders are prefixed by an asterisk.

Primitive reptiles with notochordal vertebrae, imperforate temporal region, persistent intercentra; two coracoids; plate-like pelvis, with all or most of the amphibian skull elements; short legs and short neck; phalangeal formula primarily 2, 3, 4, 5, 3(4).

Temporal region imperforate. Head and limbs more or less retractile within a box formed chiefly by the exoskeleton.

Primitive reptiles with notochordal vertebrae, perforate temporal region, persistent intercentra; two coracoids; plate-like pelvis with median vacuity; no free dermosupraoccipitals in skull; longer legs and neck; phalangeal formula 2, 3, 4, 5, 3(4).

Reptiles with a single temporal perforation on each side; vertebrae not notochordal; intercentra not persistent; pelvis with vacuity; skull bones reduced; teeth heterodont; phalangeal formula, 2, 3, 3, 3, 3.