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  Section: Anatomy of Vertebrate Animals » The Classification and the Osteology of the Reptilia
 
 
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The Plesiosauria

 
     
 

In some of the Plesiosauria, the head, not more than one-twelfth or one-thirteenth of the length of the body, is mounted upon a neck as long, in proportion, as that of a Swan; but in others, the head is large and massive, and the neck much shorter. The hind-limbs are longer than the fore-limbs, and there is a comparatively short tail. The integument was certainly devoid of any scutes; and was, probably, smooth and possessed no scales.

The cervical vertebrae may exceed forty in number, though they are generally fewer; and as none of the ribs appear to have been connected with the sternum, or if such connection existed it cannot now be traced, it becomes difficult to distinguish between cervical and dorsal vertebrae, and one is obliged to have recourse to some method of separating the two, differing from that already adopted. Now, in these animals, the neurocentral suture persists for a considerable period, if not throughout life; and the surfaces for the articulation of the cervical ribs, which are at first altogether below the neurocentral sutures, gradually rise, in the posterior parts of the neck, until they first are cut by, and then rise above, the suture. It is very convenient, and harmonizes very well with some facts to be mentioned by-and-by in the structure of the Crocodilia, to take the last of the vertebrae in which the costal articular surface is cut by the neurocentral suture, as the last of the cervical series.

The two anterior cervical vertebrae, as thus defined, constitute the atlas and axis, and are frequently anchylosed together. The centre of the other cervical vertebrae have slightly concave anterior and posterior surfaces; well-developed neural arches; anterior and posterior oblique processes, or zygapophyses, of the ordinary character; and stout, but somewhat short, spinous processes.
The centrum presents, upon each side, an oval rugose pit, sometimes more or less divided into two facets. This is the costal articular surface, which has been already adverted to. Into it fits the thickened head of a costal rib, which may have corresponding facets, but is otherwise undivided. The rib is continued backward into a short and straight body, and the angle, or the part at which the neck and the body of the rib join, is produced forward, so that the cervical ribs of the Plesiosauria have a strong general resemblance to those of the Crocodilia. In the posterior part of the neck and the anterior part of the dorsal region, the ribs become somewhat longer, and lose their anterior processes, gradually acquiring the rounded and curved form of ordinary ribs. Their proximal ends remain simple, and the facets, with which they articulate, become raised, and thrown outward, as transverse processes, developed from the arches of the vertebrae. (Fig. 68, C.)

In the anterior dorsal vertebrae, these transverse processes rapidly acquire their full length; and they are continued under this form, descending somewhat lower upon the arches of the vertebrae toward the sacrum, to the end of the dorsal region. The neural spines acquire greater length, the zygapophyses are well developed, and the articular surfaces of the centra retain the form which they possessed in the cervical region. There are usually between twenty and twenty-five dorsal vertebrae. The sacral vertebrae are two, and resemble the others, except that the sacral ribs are large and broad for the attachment of the ilium. The caudal vertebrae, usually between thirty and forty in number, become, as usual, reduced to little more than centra at the end of the tail; but, in the fore part of the tail, they have well-developed spines and articular processes, with ribs which become anchylosed to the bodies of the vertebrae, only late in life. Well-developed chevron-bones are attached between the ventral margins of successive centra of the caudal vertebrae.

As has been mentioned, there appear to be no sternal ribs, but there is a well-developed system of ossifications of the wall of the abdomen, arranged in transverse rows from before backward; each row consists of a median bone, slightly bent upon itself, thick in the middle, and thin at each end-and of six other bones, three on each side, which are elongated and pointed at each end, and so disposed that their pointed ends overlap one another. (Fig. 68, C.)

In some Plesiosauria, as already stated, the skull (Fig. 68, A) is very small in proportion to the body, not having more than a twelfth, or a thirteenth, of the length of the latter; but, in other species, the skull is much larger. The snout is tapering and depressed, and the nasal apertures are situated, not at its extremities, but just in front of the orbits-the latter, like the supra-temporal fossse, being wide. The occipital condyle is almost wholly developed from the stout basi-occipital. The ex-occipitals give off elongated parotic processes, and the basisphenoid is a thick bone, which ends in front in a long rostrum.

Diagram showing: the structure of the most important parts of the skeleton of Plesiosaurus. - A, the skull: Na, nasal aperture. - B, the left fore-limb: II, humerus; U, ulna: R. radius; r. i. u., radiale, intermedium, and ulnare, in the proximal row of carpal bones; 1, 2, 3, distal carpal bones; Mc, metacarpus; Ph, phalanges. - C, a dorsal vertebra with ribs (R.) and ventral ossifications (V.o)-D, the left hind-limb: F, femur: T, tibia; F. fibula; t. i. f., tibiale, intermedium, and fibulare, in tlie proximal row of tarsal bones; 1, 2, 3, distal tarsal bones; Mt, metatarsus; Ph, phal. nges.-E, the pectoral arch: Sc, scapula; Co, coracoid; a, clavicles and interclavicle (?). - F, the pelvic arch: Pb, pubis; II, ilium; Is, ischium
Fig. 68. - Diagram showing: the structure of the most important parts of the skeleton of Plesiosaurus. - A, the skull: Na, nasal aperture. - B, the left fore-limb: II, humerus; U, ulna: R. radius; r. i. u., radiale, intermedium, and ulnare, in the proximal row of carpal bones; 1, 2, 3, distal carpal bones; Mc, metacarpus; Ph, phalanges. - C, a dorsal vertebra with ribs (R.) and ventral ossifications (V.o)-D, the left hind-limb: F, femur: T, tibia; F. fibula; t. i. f., tibiale, intermedium, and fibulare, in tlie proximal row of tarsal bones; 1, 2, 3, distal tarsal bones; Mt, metatarsus; Ph, phal. nges.-E, the pectoral arch: Sc, scapula; Co, coracoid; a, clavicles and interclavicle (?). - F, the pelvic arch: Pb, pubis; II, ilium; Is, ischium.
There is a well-marked parietal foramen, and the parietals send off comparatively short processes backward, which become connected with the large squamosals. The latter unite with the postfrontals, which separate the orbits from the temporal fossa, and the orbit is completed behind by the junction of the postfrontal with the jugal. The jugal bone is continued backward into a slender bar, which extends as far back as the lower end of the quadrate, and probably contains a quadratojugal, so that there is a distinct infra-temporal fossa. The most obvious circumstance in which the skull of Plesiosaurus differs from that of most Reptilia is in the great size of the premaxillaries, which constitute a large proportion of the snout.

The under-surface of the skull is rarely well exposed in its anterior part; posteriorly, it exhibits a broad and long expansion, formed by the pterygoid bones, which unite in the middle line, and send processes outward and backward to the quadrate bone. On each side of the middle line of this region of the skull, is seen an ovoidal fossa or depression. The pterygoids are continued forward, and are united externally with transverse bones, and more anteriorly with flattened palatine bones. When the fore-part of the under-surface of the skull is exposed, two other fossae are visible, one on each side of the middle line, bounded behind by the palatine bones, and separated by what appear to be the vomers. I conceive that these are the true posterior nares, and that the posterior apertures are simply spaces left between the pterygoid bones and the basis cranii.

At the sides of the base of the skull, specimens of Plesiosaurus occasionally exhibit two styliform bones, which lie parallel with the axis of the skull; these may be parts of the hyoidean apparatus. No trace of any sclerotic ring has been found.

The teeth of the Plesiosauria are sharp-pointed, curved, and the outer surfaces of their crowns striated. Each tooth is lodged in a distinct alveolus, with which, as in the Crocodilia, it does not become anchylosed.

The pectoral arch (Fig. 68, E) is one of the most remarkable parts of the organization of the Plesiosauria. It consists, in the first place, of two very large coracoids, the long axes of which are parallel with one another, while their inner edges meet, without overlapping, throughout the greater part of their extent. In this respect they differ from any of the Lacertilia, which are provided with well-developed limbs. In these the long axes of the coracoids always cut one another at a large angle, open posteriorly-a circumstance which results from the manner in which the coracoids are received into grooves in the anterolateral edges of the rhomboidal part of the sternum. Hence it would appear that the Plesiosauria, like the Chelonia, did not possess any thing corresponding to this rhomboidal part of the sternum, but that the intercoracoid part of the sternum was either absent, or reduced to a mere band, as in some Batrachia.

The scapulae are unlike the corresponding organs in any other reptile. The glenoidal end, stout and strong, is continued horizontally forward and inward, as a bony prism, with a somevvhat concave inner edge, and flat inferior surface. The outer surface, rising up at right angles to the ventral surface, gives rise to a well-defined edge; at a short distance from the glenoidal end, the part of the bone which bears this surface is produced upward and backward, into a low recurved plate. This part appears to represent the proper body of the scapula in other Reptiles, while the horizontal prolongation answers to that pregleuoidal process of the scapula, which extends forward and inward as a free bony bar in many Lacertilia - for example. Iguana.

In well-preserved specimens, a broad hoop of substance (Fig. 68, E, a), which seems to have been but imperfectly ossified, extends across the middle line of the body, from the preglenoidal process of one scapula to that of the other, and is continued backward in the middle line, to the junction of the two coracoids. This corresponds very nearly in form and position to the epicoracoidal ossifications of the Lacertilia, combined with the clavicles and interclavicles; but I have never been able to detect any distinct clavicular, or interclavicular, elements in any Plesiosaurus, though they appear to have been well developed in Nothosaurus.

The humerus is a stout bone-prismatic, and with a rounded head at its proximal end, flattened and broad distally. (Fig, 68, B.) Its anterior margin is nearly straight, or even slightly convex, while the posterior is concave. Distally, it presents two facets, meeting at an angle, with which the broad and short radius and ulna articulate. The ulna differs in shape from the radius, being convex posteriorly, and concave in front. The two bones are of equal length, and much shorter than the humerus. There are six rounded carpal bones,(It may be a question whether the fourth distal bone in the carpus and tarsus (Fig. 68, B and D) belongs to carpus and tarsus, or to metacarpus and metatarsus; or whether it is formed by the confluence of elements belonging to both regions.) arranged in two rows; and to these succeed five digits, composed of metacarpals and phalanges, which are elongated and constricted in the middle. The middle digits have numerous phalanges.

The pelvic arch has very large dimensions, in correspondence with the size of the hind-limb, which is usually longer than the fore-limb. (Fig. 68, F and D.) The ilium is a vertically elongated bone, narrower below than above, where it becomes connected with the sacral ribs. Inferiorly, it unites with the pubis and with the ischium, to form the acetabulum. The pubes are very broad quadrate bones, much larger than the ischia, and they meet in a median symphysis. The ischia, triangular and expanded, also unite in a ventral symphysis. The femur resembles the humerus in its general form, although both its sides are straighter, and the other bones of the hindlimb are so like those of the fore-limb, as to need no special description.

There can be little doubt that all the bones of the limbs were, like those of the Cetacea, enclosed within a common sheath of integument, so as to form a paddle.

Such is the general organization of the skeleton of the Plesiosauria, which are long extinct animals, entirely confined to the Mesozoic Rocks, from the Trias to the Chalk, inclusive. They may be divided into two groups, according as they are Triassic, or Post-Triassio, in age.

The Post-Triassic group contains the genera Plesiosaurus and Pliosaurus, the different species of which appear to differ in little more than the proportions of the head to the trunk, and the relative length and degree of excavation of the centra of the vertebrae. In the species which have been named Pliosaurus, the vertebrae are wide in proportion to their length, and deeply excavated in front and behind. Pliosaurus attained gigantic dimensions, paddles of some individuals reaching a length of not less than six feet.

The Triassic genera, Nothosaurus, Simosaurus, Pisto saurus (for a knowledge of the organization of which we are chiefly indebted to the labors of Hermann von Meyer), appear to have differed from Plesiosaurus principally in the following respects:

The connection of the neural arches with the centra of the vertebrae seems to have been looser. The supra-temporal fossse in the skull appear to have been larger in proportion. In these animals, the under-surface of the skull has the same structure as in Plesiosaurus, but apparently lacks the posterior fossae; while there is no doubt whatsoever that the true posterior nares are situated far forward, in the position assigned to them in Plesiosaurus.

The pectoral arch of Nothosaurus, again, presents a very interesting deviation from the Plesiosaurian type. The coracoids, indeed, are greatly expanded, and meet by their inner edges, so that the rhomboidal part of the sternum seems to have been wholly absent, and the scapulae have a horizontal prolongation, not quite so long as in Plesiosaurus, with an upstanding proper scapular part of corresponding shape. But then the ends of these preglenoidal processes are connected together by, and indeed suturally united with, a stout, curved, transverse bar of bone, consisting of three pieces, one small and median, and two very large and lateral, all united firmly together by sutures. There can be little doubt that the constituents of this bony bar correspond with the interclavicles and clavicles of Lacertilia and Ichthyosauria.

 
     
 
 
     



     
 
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