1902 Encyclopedia > Pleiosaurians

Pleiosaurians




PLESIOSAURIANS. The remarkable extinct marine reptiles included in the group of the Plesiosauria (or Sauropterygia, as they are sometimes called) existed during the whole of the Mesozoic period, that is, from Triassic into Cretaceous times, when they appear to have died out. The best known of these reptiles, and that which gives its name to the group is the Plesiosaurus, a genus established by Conybeare in 1821, and including numerous species, some of which may have attained a length of as much as 20 feet. The nearly allied Elasmosaurus of North America, however, reached a much greater size, its remains indicating an animal about 45 feet in length. Several almost perfect skeletons of Plesiosaurus having at different times been found, the general proportions of the body are well known. Although the different species vary in regard to proportions, the small size of the head and extreme length of the neck are always striking points in the skeleton of a Plesiosaurus, while the tail is propor-tionately short. The limbs, both fore and hind, are well developed and modified for swimming, the forms of the various bones making it clear that the digits of each limb were not separate, but enclosed in one covering of integu-ment, as in the flippers of a whale or a turtle. The exterior of the body, there is every reason to believe, was smooth as it is in Cetacea, and not provided with either bony or horny scutes or scales as in the living crocodiles and turtles. The internal skeleton therefore is the only part available for study.

The skull of Plesiosaurus has a tapering and depressed snout, and in consequence of the large size of the pre-maxillary bones the nasal apertures are placed far back, just in front of the orbit, as in birds. There is a distinct parietal foramen, as in lizards. The orbit is completely surrounded by bone, and there are supra- and infra-temporal fossae. The single occipital condyle is formed almost entirely by the basi-occipital bone. The basi-sphenoid is well developed, and is produced into a long rostrum. On the base of the skull four fossa? are to be seen; the front pair of these are bounded behind by the palatine bones, and are regarded as the true posterior nares. The teeth are slender, sharp, curved, and striated; they have single fangs, and are placed loosely in separate alveolar sockets.





The spinal column is composed of a large number of vertebrae, some species having ninety or more in the entire series. The centrum of each vertebra has the fore and hind surfaces slightly concave; the neural arch is con-nected with the centrum by a suture, which seems never to have been entirely obliterated. The cervical vertebras vary in number from twenty-four to upwards of forty in different species. Each is provided with a pair of ribs, closely resembling those found in the cervical region in the crocodile, but with a single articular head only. Towards the hinder part of the neck the ribs become more elongated, and take on the form of dorsal ribs; but, as none of the ribs join the sternum, the usual means of dis-tinguishing the dorsal and cervical regions is wanting. There may be from twenty to thirty dorsal vertebrae. True sternal ribs have never been detected ; but abdominal ost front and outside of which is a peculiarly shaped scapula with a plate extending dor-sally from the glenoid cavity, and a second process directed inwards


FIG. 2.—Pectoral arch of Plesiosaurus, seen from below (after Hulke). co, coracoid; sc} scapula; pc, precoracoid; ost, omosternum.

FIG. 3.—Pelvic arch of Plesiosaurus, from above (after Huxley), is, ischium; pb, pubis ; il, ilium.

bones, or ribs, are well developed. The sacrum consists of two vertebrae, with stout broad ribs for attachment to the iliac bones. The caudal vertebrae, between thirty and forty in number, have distinct chevron bones, which are attached between the successive vertebrae. The pectoral arch (fig. 2) consists of a large coracoid on each side, in

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The orbit is completely surrounded by bone, and there are supra- and infra-temporal fossae. The single occipital condyle is formed almost entirely by the basi-occipital bone. The basi-sphenoid is well developed, and is produced into a long rostrum. On the base of the skull four fossa? are to be seen; the front pair of these are bounded behind by the palatine bones, and are regarded as the true posterior nares. The teeth are slender, sharp, curved, and striated; they have single fangs, and are placed loosely in separate alveolar sockets.

The spinal column is composed of a large number of vertebrae, some species having ninety or more in the entire series. The centrum of each vertebra has the fore and hind surfaces slightly concave; the neural arch is con-nected with the centrum by a suture, which seems never to have been entirely obliterated. The cervical vertebras vary in number from twenty-four to upwards of forty in different species. Each is provided with a pair of ribs, closely resembling those found in the cervical region in the crocodile, but with a single articular head only. Towards the hinder part of the neck the ribs become more elongated, and take on the form of dorsal ribs; but, as none of the ribs join the sternum, the usual means of dis-tinguishing the dorsal and cervical regions is wanting. There may be from twenty to thirty dorsal vertebrae. True sternal ribs have never been detected ; but abdominal ost front and outside of which is a peculiarly shaped scapula with a plate extending dor-sally from the glenoid cavity, and a second process directed inwards and downwards. The latter process is now regarded as the precoracoid by Mr J. W. Hulke, who also considers the plate of bone—originally of two pieces—found in the middle and in front of the coracoids to be the homologue of the omosternum of Batrachia. If this interpretation be correct, Plesiosaurus has neither clavicles nor interclavicles. In the fore limb all the characteristic bones are present. The humerus is an elongated bone with the anterior border nearly straight and the hinder border concave; it is rounded at the upper end, and flat-tened below, where it is articulated to two much shorter bones, the radius and ulna. Next to these is a row of three carpal bones—the radiale (scajshoid), the ulnare (cuneiform), and the intermedium (lunar); a second row of four bones succeeds these, three of which are carpals, but the outer one may be a metacarpal; next comes a row of five metacarpals. The digits are five in number, and with the exception of the first are made up of numerous separate ossicles, or phalanges.

The pelvic arch (fig. 3) is large, and ventrally consists of a pair of flattened more or less quadrate pubes, and a pair of somewhat triangular ischia. The iliac bones are elongated, narrower where they form part of the acetabular articula-tion and becoming broader above where' they join the sacral ribs. The hind limb very closely resembles the fore limb. The front and back margins of the femur are straighter than they are in the humerus; but the other parts almost exactly repeat the corresponding bones of the fore limb.





With regard to the probable habits of the Plesiosaurus we are not without some indications. The paddle-like form of the limbs leaves no doubt as to its aquatic mode of life, and judging from the fossils with which it is usually associated it must have been an inhabitant of the sea; it is highly probable, however, that some species at least ascended rivers, for remains of Plesiosauria are found in the Wealden freshwater deposits. The comparatively small tail and large paddles render it probable that the limbs were the chief means of propulsion. The long neck would tend to impede its progress through the water, and it would be better adapted, therefore, for swimming on or near the surface. It is unlikely that the Plesiosaurus could move as rapidly through the water as the Ichthyosaurus; but this slower movement would be compensated for by the rapidity with which its long and flexible neck could be darted at its prey. Seeing that the marine turtles and seals of the present day make their way on shore, it is quite possible that the Plesiosaurus may also have occasionally visited the land. The sharp and slender teeth would be admirably adapted for catching and holding a slippery prey, and there is no doubt that fishes formed in part, if not altogether, its natural food. Indeed the scales and teeth of fishes have been found, in one case at least, just below the vertebrae, in the region which must have been occupied by the creature's stomach.

It is of interest to note the differences which exist between the Plesiosaurus and the ICHTHYOSAURUS (q.v.), the latter being the type of another group, the Ichthyosauria, which is by some paleontolo-gists included with the Plesiosauria in a larger group called Enalio-sauria. In outward form the Ichthyosaurus must have resembled some of the recent Cetacea, inasmuch as the head is proportionately large, and without any appearance of a neck joins directly on to the trunk. The hind limbs are smaller than the front ones, and the bones of both limbs are much more shortened and flattened than in Plesiosaurus ; in addition to this there are supernumerary rows of bones, besides the five typical digital series. The pectoral arch differs in having distinct clavicles and interclavicles. The vertebra? are short from back to front, deeply biconcave, and their neural arches never have a bony connexion with the centra. There is no sacrum. The teeth are placed in a groove, and not in separate sockets. The eye-ball was protected by a series of bony sclerotic plates, which are not found in Plesiosaurus.

The group Plesiosauria includes several other genera besides the Plesiosaurus; but most of these are only represented by such imperfect specimens that the distinctions between them, as at present known, are far from satisfactory. The characters which have been relied upon for their separation are to be found chiefly in the structure of the pectoral arch, limbs, and vertebra?. Plesiosaurus is only certainly known to have existed from the time of the Lower Lias to the Chalk ; and it is especially characteristic of the Lias. More than fifty species, sometimes placed in several subgenera, have been described from different localities in Britain, some of which are represented by remarkably perfect specimens, and others by fragments only. This genus has a wide geographical distribution, species having been named from Secondary strata, on the con-tinent of Europe, in India, Australia, South America, and North America. The closely allied and gigantic form Pliosaurus is chiefly characteristic of the English Oolites.

In European Triassic beds, Plesiosauria are represented by such genera as Notliosaurus, Simosaurus, and Pistosatirus, in all of which the neural arches seem to have been less closely united to the vertebral centra than in Plesiosaurus. Neusticosaurus is another Triassic form, remarkable, not only on account of its small size, being less than 12 inches in length, but also because its limbs seem to show a transitional condition ; for, while the structure of the hind limb resembles that of a land reptile, the fore limb seems to have had more the structure of a paddle.

A number of forms closely related to the Plesiosaurus have been described from rocks of Cretaceous age in North America under the following generic names — Gimoliasaurus, Elasmosaurus, Oligo-simus, Piratosaurus, and Polycotylus. Of these the Elasmosatirus is better known than any of the others. It was an extremely elon-gated form, as may be gathered from the fact that the snake-like neck alone consisted of more than sixty vertebra?,—the entire body, as we have noticed above, being more than 45 feet in length.

See Conybeare, Trans. Geol. Soc, ser. 1, vol. v. p. 559, 1821, and ser. 2, vol. i. p. 103, 1824; Owen, Brit. Assoc. Rep., 1S39, p. 43; Hawkins, Great Sea Dragons, 1840; Phillips, Valley of the Thames, 1871; Huxley, Anat. of Vert. Anim., 1871, p. 208; Nicholson, Paleontology, vol. ii. p. 218, 1879; Sollas, _Quart. Journ. Geol. Soc., vol. xxxviii. p. 440, 1881; Hulke, Presidential Address, Geol. Soc., 1883 ; Leidy, "Fossil Vertebrates," in Report U. S. Geol. Surv. Territories, vol. i., 1873 ; and Cope, ibid., vol. ii., 1875. (E. T. N.)



The above article was written by: E. T. Newton.



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