Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 52.—Ichthyosaurus quadricissus.
Photograph of specimen in Senckenberg museum,
from Dr. Dreverman.

Nevertheless we have yet very much more to learn about the order Ichthyosauria as a whole—whence they came and how they originated; what their nearest kin were among other reptiles; and especially, more about the connecting links between them and terrestrial reptiles. They have, as an order, so isolated a position, are so widely separated from all other reptiles in structure, that they have long been a puzzle to paleontologists. Like the whales and other cetaceans among mammals, we know the ichthyosaurs well in the plenitude of their power and the fulness of their development, but have yet only an imperfect knowledge of their earlier history, and none whatever of their earliest. However, as will be seen farther on, the recent discoveries by Merriam have shed much light on some of the stages of their evolution. So nearly perfectly were all the later ichthyosaurs adapted to their life in the water that it was believed by nearly all paleontologists until about a score of years ago that they had descended directly from fishes. But this belief has been quite abandoned by all, not only because the recent discoveries of the earlier ichthyosaurs have demonstrated a positive increase in the aquatic adaptations of the later forms, but also because a double origin of any type of animal life is quite out of accord with all known facts and principles of paleontology. It is quite possible for animals, in becoming adapted to peculiar environmental and food conditions, to acquire certain resemblances to other animals, but quite impossible for them to acquire their actual structure. The ichthyosaurs are true reptiles, and all reptiles must have had a common origin.

Fig. 53.—Baptanodon (Ophthalmosaurus). Skull from the side, from above, and from below (after Gilmore): ang, angular; bs, basisphenoid; d, dentary; fr, frontal; j, jugal; la, lacrimal; mx, maxilla; na, nasal; oc, occipital condyle; p, palatine; pa, parietal; pm, premaxilla; po, postorbital; ps, parasphenoid; pt, pterygoid; pf, prefrontal; sa, surangular; sp, splenial; sq, squamosal; st, supratemporal; q, quadrate; qj, quadratojugal.

Fig. 54.—Occiput of Baptanodon (Ophthalmosaurus): pa, parietal; soc, supraoccipital; sq, squamosal; exoc, exoccipital; op.o, paroccipital; sta, stapes; st, supratemporal; qu, quadrate; qj, quadratojugal; pt, pterygoid; bs, basisphenoid; sag, surangular; ag, angular; art, articular; pra, prearticular. (After Gilmore.)

We are sometimes in doubt, however, as to whether characters resembling those of other animals are really acquired as adaptations to peculiar environments, that is, parallel, convergent, or homo-plastic characters, or whether they are due to heredity from remote ancestors. The reptilian characters of the ichthyosaurs, however, are so emphatic that they can only be ascribed to heredity. Ichthyosaurs are as truly reptilian as crocodiles or snakes, notwithstanding their fish-like form and habits. The ichthyosaur ancestors were once truly land reptiles—of that we are as sure as we well can be. Some have thought that those ancestors were the primitive Rhynchocephalia, but most are now convinced that they were among the most primitive of reptiles, a branch probably from the cotylosaurs or cotylosaurian ancestors. Probably of all the extinct forms that we know the Proganosauria come the nearest; indeed it is not impossible that they may have been the actual forbears of the ichthyosaurs.

The ichthyosaurs varied in length from two to thirty feet, but the different species, especially all the later ones, resembled each other pretty closely in shape; the beak was more slender in some than in others, and the shapes of the fins and paddles varied not a little, as we shall see. The jaws were long and slender, provided with numerous rather small but sharp and recurved teeth, especially well fitted for the seizure and retention of slippery prey. The teeth were inserted, not in separate sockets, as are those of the crocodiles and many other reptiles, but in long, deep grooves, and were easily lost, indeed so easily lost that one late American form was originally described as edentulous, and it was not till a number of years had elapsed that the teeth were found. The nostrils were small, and situated far back on the sides of the face, near the eyes. The eyes were very large, not only in proportion to the size of the skull, but, in the largest species, actually attaining in some, perhaps, the size of a human head. The eyeball was surrounded in front by an extraordinarily large and strong ring of ossifications in the sclerotic membrane, giving not only protection to the eye under the varying pressure of the water, but also greater control over vision. The neck was very short, so short, in fact, that no construction was visible in the living animal between the head and body; it was capable of only slight movement. The trunk was elongated and relatively slender, sometimes with more than fifty vertebrae in it. The tail also was long and flattened, ending in all the later species in a large fleshy fin, resembling the caudal fin of many fishes in shape and doubtless also in function. There was also a large dorsal fin, supported by hardened or calcified sinews, in shape like the dorsal fin of most fishes and many cetaceans. This character is absolutely unique among reptiles, so far as is known, and was one of the extreme specializations of water life. The hind limbs were smaller, often much smaller than the fore ones, and both were quite fin-like in life, or rather flipper-like, though not at all fin-like in structure. The skin was smooth and bare. In brief, to quote Fraas’s words:.