Water Reptiles of the Past and Present

PROTOROSAURUS

The genus Protorosaurus is of peculiar interest, as one of the first, if not the first, known fossil reptiles, described by Spener as long ago as 1710 as a crocodile, from fragmentary remains found in 1706 in the Permian deposits of Thuringia. Numerous other skeletons or parts of skeletons attracted the attention of naturalists of the eighteenth century, but were very imperfectly described. No name was given to the animal represented by the various specimens until 1840, when Herman von Meyer restudied all the known material and described it under the name Protorosaurus speneri. The position of the genus among reptiles always has been and yet is uncertain, for the reason that the structure of the skull, and especially the structure of the temporal region, has never been satisfactorily determined. Seeley, in 1887, described more fully the original specimen of Spener, now preserved in the museum of the College of Surgeons of London, and because of certain peculiarities which it showed proposed for its reception the order Protorosauria. He thought that he detected an upper temporal vacuity, like that of lizards, but was very uncertain about the structure of the lower part of the temporal region. The writer, who has examined this type specimen, must admit that the structure of the region here is very doubtful. Under the general assumption, however, that all old reptiles must be related to Sphenodon, the Protorosauria have generally been classified as a suborder of the Rhynchocephalia. It is merely another instance of the proclivity we all have to propose hypotheses, and then, speedily forgetting that they are hypotheses, to accept them as facts.

Protorosaurus was long supposed to be an aquatic reptile, but we now know that it was a strictly terrestrial one, probably with climbing habits; and the genus concerns us only by reason of its possible relationships to distinctly aquatic reptiles of a later age.

A few years ago the writer described a very slender, lizard-like reptile about two feet in length from the Permian of Texas under the name Araeoscelis, so named because of its slender legs. The structure of both the skull and the skeleton of this reptile is now quite satisfactorily known, so well known indeed that the accompanying restoration (Fig. 62) has little that is conjectural about it, at least so far as the form is concerned. The skull has a single, upper temporal opening, quite like that of lizards, but the quadrate is not loose below. And this is really what we should expect in the ancestral lizards; and everything else of the skeleton, except perhaps one character, is what would be expected. That one character is the elongation of the cervical vertebrae, which are about twice the length of the dorsal vertebrae following them. The cervical ribs are very slender bones, articulating by a single head with the centrum only. In these and other characters, so far as they are known, Araeoscelis seems to agree with Protorosaurus, and both have very hollow bones.

PLEUROSAURUS

We may for the present be justified in maintaining the order Protorosauria for those reptiles having a single, typically upper temporal opening on each side, with a fixed quadrate, not including the ichthyosaurs. It is not improbable, however, that when more is known of the ancestors of the lizards, the whole group will find its most natural place among the Squamata. This definition will include a peculiar aquatic reptile that has been known for many years, but which has been wrongly classed in the same family as Sphenodon, on the purely gratuitous assumption that it has two temporal openings on each side; we now know that it has but one. This reptile, known scientifically as Pleurosaurus, was described originally by H. von Meyer in 1843, but we are indebted to M. Lortet for a more precise knowledge of the animal, and for the figure (Fig. 63) which is here given of the skeleton. Not a few excellent skeletons are preserved in the museums at Lyons and Munich. The specimen here figured, as actually preserved, measures about three feet in length; a part of the tail is missing, which is known from other specimens to have been remarkably long.

The figures show clearly some of the remarkable aquatic adaptations of the animal, especially the short neck, the very long and narrow body, and the extraordinarily long and flattened tail. The head is elongate triangular in shape, resembling very much that of the mosasaurs; and the external nostrils are likewise situated remotely from the end of the snout, as in the mosasaurs. The extremity of the snout has a beak-like projection. The teeth are much longer, more pointed, and more recurved than is the case with most land reptiles, indicating their use for the capture and retention of slippery, quick-moving prey.

The single-headed ribs are short, proving that the body was slender and doubtless cylindrical, more like that of a snake. The tail was not only enormously elongated, but it was also compressed into a flat and effective propelling organ in the water. This flattening of the tail is apparent from the skeleton, with its elongated chevrons below and spines above, and it is also proved by the fortunate preservation of the extremity of the tail of one specimen, showing not only the impressions of the scales in the matrix, but also the outlines that the soft parts had in life. To quote from Lortet, in translation: “The tail was covered wholly with small scales, regularly hexagonal in shape, shining and nacreous, larger on the under side than above. The upper border of the tail was surmounted by a broad crest, extending to its extremity, and composed of large, oval scales.” The body doubtless was wholly covered with scales, though it is not probable that the caudal crest continued along the back.

The limbs begin to show an aquatic adaptation, though not very pronounced. They are much shorter and smaller than are those of land-crawling reptiles; and the bones of the second series, that is, the radius and ulna, tibia and fibula, are relatively short, the beginning of adaptation to water habits. It is very probable that the feet were webbed, though the fifth digit, as usual, is shorter than the fourth. Doubtless on land the creature moved about in a serpentine way, for it could not have progressed very rapidly by the aid of its legs alone. The hind legs are longer than the front legs, and they were connected firmly with the body by means of a sacrum. The number of vertebrae in the neck is only five. The number of dorsal vertebrae is forty-three, a larger number than is known in any other air-breathing vertebrate with legs.

Upon the whole, these lizard-like, almost snake-like pleurosaurians present some very curious adaptations to water life. In water they were doubtless speedy, swimming in serpentine undulations, with the small legs for the most part folded against the body and only of occasional use. Doubtless, too, had the pleurosaurs lived longer in geological history, they would have become quite snake-or eel-like, just as have some modern salamanders.

In all probability the pleurosaurs lived habitually in fresh-water, perhaps visiting the shores for refuge, or for the hatching of their young. That they were not on the way toward a terrestrial snake-like body is evident from the flattened tail, and especially the crest of scales above; the tail was like that of the sea-snakes of the present time. Pleurosaurus, then, affords the solitary instance among reptiles of aquatic adaptation by the diminution of both front and hind extremities and the acquisition of a snake-like body and snake-like habits.