Dinocephalia

Description

Size

Dinocephalians were generally large in size, and some the largest animals of the Permian period, and the first synapsids to reach a body size around one tonne. The herbivorous or omnivorous Jonkeria has been suggested to have weighed 989 kilograms (2,180 lb), while the large carnivore Anteosaurus has been estimated to weigh 400 kilograms (880 lb) based on scaled 3D models.

Skull and jaws

The skulls and lower jaws of dinocephalians are unified by a number of characters, including the loss of the vomerine process of the premaxilla (the tooth-bearing bone at the front of the upper jaw) such that the vomer bone borders the main part of the premaxilla, the transverse flange of the pterygoid bone of the palate is forward of the eye socket (orbit), there is lack of fossae (depressions) or ridges on the outward-facing (lateral) surface of the reflected lamina (a plate of bone found on the lower jaw of synapsids) of the angular bone, and the presence between prearticular and angular on the lower jaw of a foramen (an opening for blood vessels, nerves, etc) visible on the inner surface.

The temporal fenestra at the back of the skull is relatively enlarged in dinocephalians,¹¹ with the quadrate and quadratojugal bones at the back of the skull also being enlarged.³⁸ The premaxillary bones at the front of the snout unusually among therapsids have a long posteriorly directed spur-like process between the paired nasal bones, which in some dinocephalians causes the nasal bones to not contact each other, due the premaxillae reaching all the way to the frontal bones. The septomaxilla (a skull bone associated with the nostril opening) generally has a small exposure on the skull surface. There is no preparietal bone unlike some other therapsids. The prefrontal and lacrimal bones are generally large and form part of the border of the orbit (eye socket). The tabular bone at the back of the skull is generally large. The zygomatic arch is made up of the squamosal bone. The size and shape of the single temporal fenestra is variable, from small and slit-like to large. The quadratojugal is elongate which supports the quadrate to its sides, which is large elongated forwards (anteriorly) and downwards (ventrally). The stapes are short and thick^38-41

Most dinocephalians also developed a thickened (pachyostotic) and ornamented skull roof and braincase of varying and diverse form, sometimes with horn-like projections, making their skulls highly strong and resistant to fracture.

All dinocephalians are characterised by the interlocking incisor (front) teeth, though this was formerly thought to be unique to the group, it is also found in biarmosuchians. In tapinocephalids all teeth interlock rather than just the front incisors, though the back teeth interlock less closely. Dinocephalians ancestrally have a pair of enlarged "canine" teeth, towards the front of the mouth in both the upper and lower jaws, though in some dinocephalians such as many tapinocephalids no such teeth are present. The teeth generally have a morphology including a pointed "talon" and a "heel", with the heel serving to cut or crush food,^{41-45, 57} with this "talon and heel" morphology best developed in tapinocephalines, especially tapinocephalids, which show precise occlusion (interlocking) to effectively shear food. Some tapinocephalines like titanosuchids and estemmenosuchids have leaf-shaped post-canine teeth. In anteosaurs and titanosuchids, which have differentiated incisors and canines, there are generally 4-5 pairs of incisors and pair of canines in the upper and lower jaws, the number of postcanine teeth varies from 8-11 in both the upper and lower jaws in anteosaurs to 19 in the upper jaw and 18-21 in the lower jaw of titanosuchids. In tapinocephalids, the teeth generally show little differentiation from each other with often no distinct canines present, with 3-5 teeth in the premaxilla, 8-17 in the maxilla and 14-19 in the lower jaw.^{41-45, 57} Dinocephalians ancestrally possess palatal teeth (teeth on the roof of the mouth) on the pterygoid and palatine bones (with Estemmenosuchus also having teeth on the vomer uniquely among therapsids). Palatal teeth were completely lost in advanced tapinocephalids. The coronoid bones and the corresponding coronoid process on the lower jaw were lost in dinocephalians.^11,38

Postcranial skeleton

A proatlas bone between the atlas vertebra and the skull was probably present in dinocephalians. The vertebrae of dinocephalians are concave on both faces (amphicoelous). The neck typically has seven or eight cervical vertebrae, with around 33 vertebrae in total forward of the sacrum.⁴⁶ The length of the tail and the corresponding number of caudal vertebrae varies considerably in dinocephalians, from seven caudals in the tapinocephalid Tapinocaninus, to over 60 in some anteosaurs. The caudal vertebrae get progressively shorter towards the tip of the tail.⁴⁶ The caudal vertebrae and the atlas and axis vertebrae had associated intercentra, though these do not appear to have been present between the remaining cervical or dorsal (trunk) vertebrae.⁴⁷ The cleithrum is reduced in size, The dorsal process of the coracoid has been lost.⁴⁶ Dinocephalian limb bones are generally optimised for muscle strength. Dinocephalians are likely to have had a sprawling posture, or a posture intermediate between sprawling and upright.

Soft tissue

The dinocephalian Estemmenosuchus is well known for being one of the few non-mammalian therapsids (alongside the dicynodont Lystrosaurus) to have preserved skin impressions. The skin appears to be scaleless, and covered in numerous small tubercles around 0.3–0.45 millimetres (3⁄256–9⁄512 in) across, which may represent glands.