Water Reptiles of the Past and Present

by Samuel Wendell Williston

VERTEBRAE AND RIBS

The spinal column or backbone of reptiles, as in all air-breathing vertebrates, is made up of a variable number of separate segments called vertebrae, permitting flexibility. Each vertebra is composed of a body, or centrum, and an arch on the dorsal side for the protection of the spinal cord. Various projections from the vertebra, called processes, serve for the attachment of ligaments or muscles, for articular union with adjacent vertebrae, or for the support of ribs, and these processes have characteristic differences in different reptiles. The pair in front and behind, for articulation with the adjoining vertebrae, may become obsolete or even lost in swimming reptiles, as we shall see; they are called zygapophyses. In not a few reptiles there is an additional pair for zygapophysial articulation in front and behind, called zygosphene and zygantrum, for the greater strengthening of the column; they are especially characteristic of snakes and certain lizards. In certain other reptiles, especially the long-necked dinosaurs, there is an additional pair arranged differently from the zygophene, that have received the names hyposphene and hypantrum.

Fig. 12.—Procoelous vertebra of snake: za, zygantrum; zs, zygosphene; ps, posterior zygapophysis.

On the top of the arch is the spine or spinous process, which may vary enormously in size and length; sometimes it is flattened or dilated above for the support of an exoskeleton, or it may be heavy and massive for the attachment of strong muscles and ligaments. In the modern basilisk lizards and in the ancient Dimetrodon and Edaphosaurus from the Permian rocks of Texas these spines are of enormous length, some of them nearly four feet long in reptiles not twice that length. Slender crawling reptiles usually have no spines, or only vestigial ones. On the sides of the arch there may be a distinct transverse process for the articulation of the rib.

In all early reptiles the ends of the body or centrum are concave, as they are in nearly all fishes. Such a conformation, called amphicoelous, gives great flexibility to the spinal column, but only moderate strength, since the intervening spaces are filled with cartilage in life. In all living reptiles, with few exceptions, the body is concave, like a saucer, in front and correspondingly convex behind, and the intervening cartilage has largely disappeared. Such a mode of union, called procoelous, adds greatly to the strength of the backbone, enabling it to receive greater shocks or greater pressure without dislocation; or to sustain the greater strain of muscles used in running swiftly or in climbing. Among living reptiles, only the gecko lizards and the tuatera have biconcave vertebrae. Some extinct reptiles, such as some of the dinosaurs, animals that walked erect upon their legs, had their vertebrae convex in front and concave behind (opisthocoelous). Birds, though walking erect, have a very different and more complicated articulation of the cervical vertebrae, and certain reptiles, like the turtles, have very complicated cervical vertebrae.

In the embryos of all vertebrate animals there appears first an elongated fibrous rod, called the notochord, in the place of the future spinal column. This rod may persist through life, never ossifying, as was the case with all the earliest fishes, and is the condition in some living ones. As the embryo grows, however, the separate segments, or vertebrae, ossify about this rod in all reptiles, forming bony rings, perforate at first in the middle for the more or less constricted notochord. This stage was the permanent condition in all the earliest reptiles and in some later ones. Such animals are said to have notochordal vertebrae, the notochord more or less continuous, like a string of beads, the beads representing the enlargements between the contiguous vertebrae.

Fig. 13.—Notochordal cervical vertebrae, with intercentra, of Ophiacodon, a primitive theromorph reptile from the Permocarboniferous of New Mexico: pa, proatlas; an, arch of atlas; o, odontoid; ax, axis.

In many early amphibians, and probably in all the earliest ones, as well as in the fishes from which they were derived, the vertebra is more complicated in that it is composed of at least three pairs of separate bones, two of which united with each other, the third finally disappearing in modern animals, or at the most represented by a mere vestige called the intercentrum. The dorsal pair of these bones, called the neurocentra, forms the arch of the vertebra. The ventral posterior pair, called the pleurocentra, increases in size and unites to form the centrum or body of the vertebra; while the ventral anterior pair, early united with each other, is called the hypocentrum or intercentrum, persistent in all early reptiles as a vestige between the centra on the ventral side. This divided condition of the vertebra is persistent in the first vertebra, the atlas of all higher animals, in which the so-called body is the hypocentrum or intercentrum, the arch is the neurocentrum, while the pleurocentra have fused more or less with the anterior part of the next vertebra, the axis, to form the so-called odontoid. That this is the real explanation of the structure of the atlas is proved by the various stages of its evolution in the reptiles, from the earliest ([Fig. 15]) in which it scarcely differs from rhachitomous—as this structure is called—vertebrae of an early amphibian, to the modern in which the structure is nearly like that of mammals.

Fig. 14.—Rhachitomous dorsal vertebra of Eryops: n, neurocentrum or arch; pl, pleurocentrum; i, intercentrum or hypocentrum; az, anterior zygapophysis; pz, posterior zygapophysis; d, diapophysis, for tubercle of rib; p, parapophysis, for head of rib.

In front of the atlas, that is, between it and the skull, there was, in all early reptiles, as well as in some later ones, like the crocodiles and tuatera, the remnant of what is believed to have been another vertebra, of which only the arch remains, and which is called the proatlas. In its earliest condition it articulated with the skull in front and the arch of the atlas behind.

As in mammals, the vertebrae of the different regions have received distinctive names, cervical, dorsal, lumbar, sacral, and caudal. The numbers of each region are far more variable than they are among mammals, the total number of vertebrae in the column varying from about thirty to more than five hundred, in certain snakes. Nor are the different regions always easily distinguishable, especially those in front of the sacrum. In the earliest reptiles there was practically no neck, and only two vertebrae, the atlas and axis, that properly can be called cervical. Very soon, however, the reptiles developed a longer neck with seven vertebrae, a number that has remained singularly constant in higher animals, especially in the mammals. In most modern reptiles there are from seven to nine; in a few lizards, five. But the number was much more inconstant among the older reptiles; some of the plesiosaurs had as many as seventy-six cervical vertebrae; some of the older lizards even had as many as eighteen.

Ordinarily the cervical vertebrae differ from those behind them only in the small size or fusion of their ribs; sometimes, however, as in the Protorosauria and Pterosauria, the vertebrae may be much elongated. The dorsal vertebrae of reptiles vary in number from ten in turtles and some dinosaurs to forty-three in Pleurosaurus; and under the name dorsal we include the so-called lumbar, as there is seldom any real distinction between the two series, save the smaller size or the co-ossification of the ribs of the latter.

Fig. 15.—Ophiacodon,
a primitive theromorph
reptile: proatlas, atlas,
and axis, with ribs.

Fig. 16.—Sacrum of Chelone.

The sacrum in reptiles primitively consisted of a single vertebra, which bore a large rib on each side for the support of the pelvis. Very early, however, a second or even a third vertebra was added to it from behind. The number two is the rule among reptiles, both ancient and modern; among crawling reptiles the number never exceeds three, but among ambulatory and flying reptiles the number may be as great as in any mammal.

The number of caudal vertebrae in reptiles is exceedingly variable, from a dozen or fifteen up to a hundred and fifty or more. In snakes but two regions are distinguishable, the caudal and precaudal, and the number altogether may reach nearly five hundred. With the exception of the first few basal caudal vertebrae (pygals) and the minute ones at the extreme tip, all caudal vertebrae of reptiles bear a slender, usually Y-shaped bone below in the interval between the centra, for the protection of the vessels and nerves. Because of their shape they have been called chevrons, and are really outgrowths from the intercentra.

The ribs of reptiles are of more importance in classification than one would suppose. The primitive rib was a slender, curved bone, with the vertebral end dilated to articulate continuously with the intercentral space—that between the centra and the anterior part of the arch. And this is the condition still remaining in the tuatera. Very soon, however, the lower end of the articular surface (capitulum) became separated from the upper (tubercle) by a notch, and the ribs became distinctly double-headed. And this mode of articulation is the rule among mammals. Among later reptiles, however, there were many modifications. In nearly all the head migrated a little backward on the centrum. By the loss of the tubercle in lizards, the head became truly single-headed, and attached solely to the body; and this condition is characteristic of the order Squamata. In another large group the head of the rib gradually migrated up on the arch and on the transverse process (diapophysis), so that both head and tubercle are attached to the diapophysis; and this condition is equally characteristic of the orders of reptiles known collectively as the Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, the ribs are single-headed and attached to the end of the diapophysis. Finally in most ichthyosaurs the capitulum and tubercle both articulate with the body of the vertebra.

Fig. 17.—Ostodolepis, a primitive theromorph reptile. Vertebrae from in front and side, with primitive double-headed rib and intercentrum.

Ribs primitively were probably attached to all the vertebrae to the end of the tail. In the earliest reptiles that we know they are present on all vertebrae as far back as the tenth or twelfth caudal only, those of the caudal for the most part co-ossified with the centra. The ribs of the neck vertebrae more quickly disappeared, or became fused with the vertebrae, and only in the crocodiles among living reptiles are there ribs on the atlas. The sacral ribs, on the other hand, became much larger and stouter and developed an articulation at their outer ends for the support of the ilium ([Fig. 16]).

The so-called ventral ribs are slender ossifications in the connective tissue under the skin, on the under side of the body, and are characteristic of most reptiles. The anterior ones doubtless fused together more or less to form the sternum or breast bone, which was otherwise absent in the early reptiles.