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.