Pathological Features in Upper Permian and Middle Triassic Dicynodonts (Synapsida, Therapsida)

Abstract

Morphological peculiarities that are attributable to pathologies are described in an Upper Permian dicynodont from Tanzania (Geikia locusticeps) and in a Middle Triassic species from Brazil (Stahleckeria potens). In ventral view, the Geikia skull (GPIT/RE/7187) presents an unusual feature between the right maxilla and jugal, a perfectly concave circular pit with a very prominent rim. This lesion could correspond to a slowly growing benign process that produced a pressure erosion of the bone, such as an epidermal inclusion cyst, probably post-traumatic, or a parasite infection caused by Echinococcus, also known as hydatid disease. In Stahleckeria (GPIT/RE/8001, mounted skeleton), various pathologies appear in different bones of distinct individuals, particularly on articular surfaces of the limb bones. The anterodorsal portion of the scapula bears a lesion that is interpreted as caused by an epidermal inclusion cyst or a muscular avulsion. The deltopectoral crest of the right humerus bears lesions that are attributed to fungal disease or to muscular avulsion. The entepicondyle of the same humerus bears lesions that are attributed to a fungal disease or a parasite infection caused by Echinococcus. The great trochanter of the left femur also shows signs of fungal disease. The distal extremity of the same femur, particularly the popliteal fossa and lateral condyle, show features that could correspond to healed osteomyelitis, fungal disease or a parasitic infection caused by Echinococcus. The proximal end of the left tibia and its cnemial crest show evidence of an infection caused by osteomyelitis or a fungal disease. The proximal end of the left fibula also may have been affected by fungal disease. The identification of these features as pathologies prevents their misinterpretation as original morphologies, reducing the risk of incorrect taxonomic, phylogenetic, and functional interpretations.