Abnormal development of paired enigmatic structures in the derived dicynodont Lystrosaurus murrayi (Therapsida, Anomodontia).

Derived dicynodonts, including Lystrosaurus murrayi, have edentulous lower jaws. As an interesting exception to this conventional wisdom, a well-preserved specimen of L. murrayi (MGGC- 8850/1RE13 F) has abnormal, paired mandibular structures similar in overall morphology to dental teeth. The specimen pertains to the historical collections of the Museo G. Capellini (Bologna, Italy) and was collected in 1929 by M. Gortani during prospecting activities near Harrismith, South Africa. The specimen is articulated and includes a nearly complete skull, the first eight vertebrae and the proximal end of the right scapula. Reconstruction based on a Dual-Beam CT revealed unusual, paired structures similar to mandibular teeth, encased within the lower jaw, lingual to maxillary tusks. Different interpretations are possible for the abnormal mandibular structures in MGGC-8850/1RE13 F: 1) the dental lamina in the lower jaw, which would normally degenerate in derived dicynodonts, remained active and potentiated the odontogenesis; 2) the dental lamina was potent in the lower jaw throughout the ontogeny across dicynodonts but activated differentially; or 3) the mandibular patterning in our specimen was dorsalized, which resulted in an ectopic formation of upper jaw dentition in the lower jaws. Supranumerary or ectopic teeth are a commonly screened phenotype in modern mammalian models. Such variation distributes in a continuum – rarely does an edentulous jaw develop fully functional teeth that are present in outgroups. The absence of mandibular dentition in other specimens of L. murrayi and in most other bidentalian dicynodonts suggests that MGGC- 8850/1RE13 F represents a rare developmental abnormality or a case of atavism.