Fossils: A Story of the Rocks and Their Record of Prehistoric Life

by Harvey C. Markman

Marine Reptiles: Plesiosaur (Lower Skeleton) and Mosasaur

The Mesozoic Era produced many types of reptiles besides the dinosaurs. Two of the marine forms are shown in this illustration, both from Cretaceous beds of the western United States. Plesiosaurs were the giants of the seas in their time, lengths of forty to fifty feet being not uncommon. A long flat tail provided locomotion for the mosasaurs whereas the plesiosaurs resorted to the peculiar limb structures known as flippers or paddles.

THE AGE OF REPTILES

The Mesozoic, or era of middle life, was a long stretch of time during which there was marvelous development among the reptiles. Many strange types were produced and most of them became extinct before the end of the era. The reptilian stock branched out in many directions. Types emerged which differed from one another so widely that their mutual relationships have become obscure. Hideous and fantastic creatures suggesting sea serpents and dragons were worldwide in distribution. Reptiles of the air and seas acquired large size and weird forms, but greater advances were made upon land.

The flying reptiles or pterosaurs flourished in Jurassic times with some of the larger varieties surviving until near the close of the Cretaceous. Although these winged lizards were the first of the vertebrates to fly they are not to be confused with birds. They were without feathers, and the earlier forms were provided with long tails bearing a flattened rudder-like tip. One of the best known of this type had a length of about eighteen inches. Its jaws were long and provided with sharp teeth. The wings were membranes attached to body and legs, stretched and manipulated by means of greatly elongated fingers. In later types there was a reduction in tooth equipment and length of tail. Pteranodon, found in Kansas, had a wing spread of twenty-five feet, a large toothless beak, a short body, and a mere stub of a tail. It was one of the last of these winged monsters.

Several types of marine reptiles appeared during this era, among them the plesiosaurs which first appeared in Triassic seas. These peculiar animals were serpent-like with regard to the character of head, neck, and tail, but in other respects were quite different, the short barrel-shaped body being provided with four large paddles corresponding to the usual limbs of quadrupeds. Fossil remains of these animals are common in many Jurassic and Cretaceous deposits, some of the largest exceeding forty feet in length. Mosasaurs, also marine carnivores, inhabited shallow Cretaceous seas throughout the world and are especially abundant as fossils in the Kansas chalk beds. These were elongated forms with a resemblance to salamanders in some respects but provided with long pointed jaws and sharp teeth. Swimming was accomplished largely by the tail though probably aided to some extent by four webbed paddles or flippers. The ichthyosaurs were more fish-like in construction, as the name implies. The limbs were short and broad, and there was usually present a well-developed tail-fin as well as a large fin on the back. They were especially abundant in Jurassic time. Fossils are fairly common in marine deposits of western North America. Mosasaurs and ichthyosaurs were about half as long as the plesiosaurs.

DINOSAURS

Most spectacular of the prehistoric reptiles were the dinosaurs, a large group of animals varying greatly as to size, form, and habits. They were adapted for a life on land though many of them probably spent much of their time partly submerged in the waters of lakes and streams. There is little that can be said of the group as a whole other than that all of them were reptiles. Further than that it is necessary to regard them as belonging to several different subdivisions of the Reptilia. Classification has been difficult and the names used for the various subdivisions are often misleading to the layman who tries to understand the terminology.

Ancestral reptiles were five-toed and five-fingered but among the dinosaurs there were many departures from the standard formula. Three or four of the digits were commonly well developed, the others when present being shortened or reduced to mere rudiments. Early in the history of dinosaurs there was a division of the stock into two main branches, each of which includes a variety of types and sizes, and is again subdivided. The two main groups are best recognized by the construction of the bony framework which comprises the pelvic girdle or hip region of the skeleton. In order to avoid technical difficulties, however, the remaining discussion of these interesting reptiles will be confined to a few names and descriptions which serve to illustrate roughly the great amount of variation that developed from the comparatively simple ancestral pattern. The plan according to which the dinosaurs are usually classified is barely suggested by the types described.