Shringasaurus

The skull of Shringasaurus is not completely known, but what's preserved indicates that the skull was small and boxy, with a short, deep snout with rounded jaw tips and bony nostrils fused into a single, confluent opening at the front of the snout. This is broadly similar to the completely known skull of Azendohsaurus, however the lower jaw of Shringasaurus has a more conspicuous taper towards the tip compared to the deep, down-turned dentary of Azendohsaurus.[1][3]

The horns of Shringasaurus closely resemble those seen in ceratopsid dinosaurs, despite being totally unrelated to each other. The horns are attached to the frontal bones on the roof of the skull over the eyes, and sit across almost the entire breadth of the skull. These horns are pointed up and curve forwards from the skull, with slight variation in size and orientation between large individuals. Smaller and younger individuals had smaller, more gracile horns, indicating that the horns did not fully develop until the animals were mature. Intriguingly, at least one small specimen lacks horns entirely, whereas another similarly small specimen has small but well developed horns. It is suggested then that Shringasaurus was sexually dimorphic, and that possibly the females lacked horns.[1]

The horns themselves have a rough, grooved texture that implies they were covered with a keratinous sheath of horn in life, also like ceratopsid horns, and so would have likely been longer than the bony cores indicate.[4][5] The bones of the skull beneath the horns are unusually thick, and in the larger individuals the bones of roof of the skull (the nasal, prefrontal, frontal and postfrontal) are fused together on each side.[1]

Life restoration

The teeth of Shringasaurus are low and leaf-shaped (lanceolate) with large denticles on either side, similar in shape to those of Azendohsaurus but lacking the prominent expansion above the root, like the teeth of Pamelaria. Because the skull and jaws are incompletely known, the total tooth count of Shringasaurus is unknown, but like Azendohsaurus it had four teeth in each premaxilla. Shringasaurus also had numerous palatal teeth (though known only from the vomer thus far), and like Azendohsaurus they are uniquely as well developed as the marginal teeth along the edge of the jaw. Like them, they were leaf-shaped and serrated, but in Shringasaurus the palatal teeth are even more lanceolate than the marginal rows.[1] Such palatal teeth are unusual, as most other herbivorous reptiles with them have much simpler, domed palatal teeth, and palatal teeth identical to those of the jaw margins are otherwise only found in the related allokotosaurs Azendohsaurus and Teraterpeton.[3][6]

Femur, tibia and fibula of Shringasaurus.

The vertebral column is well known in Shringasaurus, including the whole cervical series, various dorsal vertebrae, both sacral vertebrae and some caudal vertebrae. Like other azendohsaurids, the first-through-middle cervical vertebrae are characteristically elongated, giving Shringasaurus a long, raised neck, although it is proportionately shorter than in Azendohsaurus and Pamelaria. The neck is also much taller than in other azendohsaurids, with tall, prominent neural spines. This trend continues into the dorsals of the back, which although are not as long as the cervicals have neural spines twice the height of their centra. The 2nd–5th cervicals of Shringasaurus sport prominent epipophyses, structures for supporting neck musculature, suggesting Shringasaurus had strong neck muscles.[7] The first twelve dorsals are also marked by various well-defined laminae that bound deep fossae (depressions in the bone), similar to those found on the vertebrae of sauropods. Like Azendohsaurus, Shringasaurus has two sacral vertebrae with well-developed ribs that articulate with the ilia.[1]

The shoulder and forelimb are broadly similar to those of Azendohsaurus, with a tall scapula that is concave along the front with an expanded tip, and an interclavicle with a long paddle-like process on the back and a short forward-pointing process (an unusual feature for archosauromorphs but also found in Azendohsaurus). The coracoid articulates with the scapula to form a glenoid (shoulder socket) that faces out to the sides and back, a feature in Azendohsaurus suggested to indicate the forelimb was held more upright than a full sprawl.[2] The humerus is likewise similar, with broad ends and a narrow midshaft, and a very well-developed deltopectoral crest half as long as the whole bone, indicating powerful forelimbs. The ulna, however, can be distinguished by a lower olecranon process below the elbow than in Azendohsaurus.[1]

The hips and hind limbs are very similar to those of Azendohsaurus. The ilium has a prominent, semi-circular process at the front while the rear process is longer and thinner, and the acetabulum (hip socket) is also solid, unlike the perforated hip socket of dinosaurs. The femur is robust and slightly s-shaped, held out to the sides in a sprawl, with a robust tibia and a fibula only half as wide in the lower leg. The foot is typical for early archosauromorphs, including Azendohsaurus.[1][2]

The horns of Shringasaurus are its most prominent feature, and so some focus was placed on their role and function in its initial description. Its describers considered its horns to be likely products of sexual selection, not primarily for defence or species recognition (as has been proposed for the head ornaments of dinosaurs).[13] The horns grow notably larger and more robust in large adults, while smaller individuals have shorter and more graceful horns. The possibility that Shringasaurus was sexually dimorphic, with probable females lacking horns, further supports this interpretation. This would be similar to modern horned bovids, but unlike ceratopsid dinosaurs, and indeed other archosauromoromorphs, which do not appear to have been dimorphic.[1]

Furthermore, the size and shape of the horns has been compared to those of other horned animals, and the relatively short, curved horns were most similar to those of animals that wrestle with their horns, as opposed to ramming, stabbing and fencing. Male Shringasaurus then may have competed for mates by locking their horns together, remarkably similar to what is inferred for the horned ceratopsid dinosaurs (particularly Triceratops)[14] and the modern reedbuck antelope.[1][15]

One specimen of Shringasaurus is known to have had a pair of malformed vertebrae in its neck. The two cervicals are partially fused together, interpreted as either the result of a birth defect, spondyloarthropathy (a type of arthritis), or possibly a bacterial or fungal disc infection. The vertebrae belonged to a large adult animal, so it is unlikely that the quality of life for the individual was severely affected by the disorder, and it was probably not fatal to the animal. One of the vertebrae also preserves a healed fracture, although the cause for this injury is unknown.[10]