Jonkeria

Jonkeria is an extinct genus of dinocephalians. Jonkeria was a large and omnivorous animal, from the Tapinocephalus Assemblage Zone, Lower Beaufort Group, of the South African Karoo.

Description
The overall length was 3.5 m or more (up to 4 or), the skull about 55 cm long. The skull is nearly twice as long as wide, and the snout is elongated and provided with sharp incisors and large canines. The cheek teeth were small. The body is robustly built, and the limbs stout. Jonkeria cannot be distinguished from its relative Titanosuchus on cranial grounds, but only in limb length; Jonkeria having short and squat limbs, and Titanosuchus long ones.

The limb and rib bones of Jonkeria display thickened bone walls and infilling of the medullary cavity with bone tissue. This is similar to the bone structure of the modern hippopotamus and the extinct aquatic reptile Claudiosaurus, and implies that, like them, Jonkeria was semiaquatic. The long rostrum of Jonkeria was well-innervated and sensitive, possibly to detect changes in pressure underwater. Young Jonkeria individuals grew rapidly, similarly to endothermic animals, whereas adults experienced cyclical growth rates.

Evidence of femoral osteomyelitis has been described in a fossilised specimen of Jonkeria. The authors attributed the cause of the pathology, characterised by bony spicules growing perpendicular to nonpathological fibrolamellar bone tissue, to a bacterial infection resulting from an attack by a predator, as evidenced by puncture marks on the femur.

Discovery
The holotype specimen, TM 212, was collected in 1916 from the Abrahamskraal farm in the Prince Albert Local Municipality in South Africa. It is currently located in the Ditsong National Museum of Natural History.

Classification
About a dozen species have been named, including the type species, J. truculenta. At least some of the other species were synonymised by Boonstra 1969, and the remaining species were synonymized into J. truculenta in 2024.

The cladogram below depicts the results of a phylogenetic analysis of a selection of dinocephalians representing the various recognised subgroups, including Jonkeria, performed by Fraser-King et al. (2019). Under their results and systematic terminology, Jonkeria was found to be a tapinocephalian closer to tapinocephalids than are Styracocephalus and Estemmenosuchus. The cladogram below is simplified from their full analysis, focused only on the relationships of dinocephalians.