Mesozoic aquatic reptiles, with two pairs of pentadactyle limbs, firmly fixed quadrate bones, single temporal arches, numerous alveolar teeth, and ribs which articulate only with the centra of the biconcave vertebrae.
The Plesiosauria comprise the Mesosauri, Nothosauri, and Plesiosauri in an ascending order of development, which concerns especially the changes from a semi-terrestrial to an absolutely aquatic life;–elongation of the neck with corresponding shortening of the tail, and the gradual transformation of the limbs into hyperphalangeal paddles.
The skull varies considerably in length. Seen from above it shows the nostrils, orbits, very large supratemporal foramina, and the interparietal hole. The nostrils lie rather far back, in front of the orbits, between the elongated premaxillaries, short nasals, and the usually large maxillaries. The orbits are rather small, bordered behind by the postfrontals and postorbitals, which two bones fuse together in the Plesiosauri. The temporal bridge is long, and is formed by the junction of the two bones just mentioned with the squamosal mass, which overlaps the greater portion of the quadrate, and perhaps contains the quadrato-jugal. The dorsal branch of the squamosal joins a corresponding diverging branch of the parietal, and completely shuts off the posterior region of the supratemporal foramen. The interparietal hole is small and placed far back. The palate possesses a row of teeth on the pterygoids in Lariosaurus. The choanae open separately between the vomers and maxillaries. The pterygoids are very long; posteriorly they join the quadrates, anteriorly they extend right up to the vomers, separating the palatines from each other thereby. Palatal vacuities are absent in Nothosaurus; small and oval, between the palatines, pterygoids, ectopterygoids and maxillaries in Lariosaurus; still smaller in the Plesiosauri.
The vertebrae are mostly biconcave, in the Triassic genera still perforated by the chorda, while in many Plesiosauri the centra are solid, with almost plane articulating surfaces. The neural arches are usually firmly sutured, or quite fused with the centra. Intercentra are absent, except as chevrons in the tail. Although the cervical and some of the thoracic ribs of the Triassic genera have typical capitula and tubercula, they articulate exclusively upon the centra, and not upon the neural arches also. The number of cervical vertebrae amounts to nine in Mesosaurus; in Lariosaurus it is increased to about twenty; and in some Plesiosauri to between thirty and forty. The cervical ribs are very short, but they increase gradually towards the thorax, which is well protected by long and strong ribs, which decrease again very gradually, being still long in the lumbar region. There is, properly speaking, no sacrum, because the one to four sacral ribs remain quite separate. The tail is still long in Lariosaurus, consisting of about forty much shortened vertebrae; considerably shorter than the neck in most of the Plesiosauri. A sternum is absent, but the belly is protected by many strong abdominal ribs, crowded together, and consisting each of a median and two pairs of lateral pieces.
The shoulder-girdle is very strong, composed of scapulae, very strong coracoids, clavicles, and an interclavicle. The precoracoids are indicated by a process and a notch in the Triassic genera; in the later forms they are abolished. The coracoids always meet in the median line, and often produce a strong symphysis. The scapulae possess a very prominent and large acromial process, upon which rest the dorsal or lateral ends of the clavicles. In some Plesiosauri the shoulder-girdle has undergone an absolutely unique modification. The correct interpretation has been given by C. W. Andrews after the examination of exquisitely preserved specimens of Cryptoclidus from the Oxford clay of the Middle Oolite, near Peterborough. The dorsal portion or main shaft of the scapula is reduced to what now looks like a dorso-lateral process, while the broad acromial process is much elongated, and lies in Plesiosaurus upon the ventral surface of the clavicle; the latter and the irregularly T-shaped interclavicle being, however, still visible from below. In Cryptoclidus the two acromial processes meet each other and form a long ventral symphysis, which meets that of the much-enlarged coracoids, the latter enclosing with the scapulae a pair of roundish foramina. The clavicles are not visible from below; they rest upon the dorsal surface of the scapular symphysis, and the interclavicle seems to be suppressed. Young Cryptoclidus (Fig. 113, B) and various species of Plesiosaurus show intermediate conditions.
Fig. 113.–A, Restored outlines of a Plesiosaurus, × 1⁄50; B, dorsal view of the pectoral arch of an immature Cryptoclidus, from the middle Oolite; C, fore-limb of a Plesiosaurus, from the Lias. A, Acromial process of scapula; Cl, clavicle; Co, coracoid; H, humerus; i, carpale intermedium; M1 to m5 first to fifth metacarpals; p, pisiform bone; R, radius; r, radial carpal; S, scapula; U, ulna; u, ulnar carpal.
This unique arrangement is correlated with the enormous development of the fore-limbs, although nothing of the kind has taken place in the Ichthyosauri, which have similar large paddles. The limbs exhibit considerable differences in the various groups of Plesiosauria, but they are all pentadactyle. In the oldest, the Mesosauri and Nothosauri, the limbs are still of the terrestrial type, although fitted for swimming; the chief bones are still slender and elongated, and none of the five fingers and toes have more than five phalanges, the usual number of which seems to be 2, 3, 4, 5, 3 for the first to fifth digits respectively. In the Plesiosauri the limbs are transformed into long hyperphalangeal paddles, unfit for progression on land, rather like those of the Ichthyosauria, with much shortened radius and ulna, tibia and fibula; but the phalanges, which increase to about ten, are always longer than broad, and there is no indication of an increase of the number of fingers, or of additional, lateral, phalanx-like nodules. The pelvis is very strong; the broad pubes and ischia meet in the middle line, and they either enclose one wide undivided foramen, or the two symphysial portions meet, and there are then two obturator-foramina. The pubes are generally much larger, especially broader, than the ischia; and although partaking in the formation of the acetabulum, they do not articulate with the ilia, at least not in Plesiosauri. The ilia are always small; in Plesiosauri attached to only one or two sacral ribs; to three or four in the Triassic genera.
Ichthyosauri and Plesiosauri were combined as "Enaliosauria" by Conybeare. Owen recognised their fundamental differences, and separated them as "Ichthyopterygia" and "Sauropterygia," according to the structure of the limbs. We now know that the paddles of the Ichthyosauri bear but a superficial resemblance to the fins of fishes, and are fundamentally referable to the pentadactyle type, as are the paddles of the Plesiosauri, although the latter retain more of the typical features of reptilian limbs. It was soon recognised that the Nothosauri are allied to the Plesiosauri, but the Mesosauri (until then vaguely grouped with the Rhynchocephalia, or linked with Protorosauri as Proganosauria) have only recently[[145]] received their proper place in the system as members of the Plesiosauria, which we divide into two main groups.
Order I. NOTHOSAURI.[[146]]