Tooth replacement patterns in the Early Triassic epicynodont Galesaurus planiceps (Therapsida, Cynodontia)

Abstract

Sixteen specimens of the Early Triassic cynodont Galesaurus planiceps (including eight that were scanned using micro-computed tomography) representing different ontogenetic stages were assembled to study the dental replacement in the species. The growth series shows that the incisors and postcanines continue to develop and replace, even in the largest (presumably oldest) specimen. In contrast, replacement of the canines ceased with the attainment of skeletal maturity, at a basal skull length of ~90 mm, suggesting that Galesaurus had a finite number of canine replacement cycles. Additionally, the functional canine root morphology of these larger specimens showed a tendency to be open-rooted, a condition not previously reported in Mesozoic theriodonts. An alternating pattern of tooth replacement was documented in the maxillary and mandibular postcanine series. Both postcanine series increased in tooth number as the skull lengthened, with the mandibular postcanine series containing more teeth than the maxillary series. In the maxilla, the first postcanine is consistently the smallest tooth, showing a proportional reduction in size as skull length increased. The longer retention of a tooth in this first locus is a key difference between Galesaurus and Thrinaxodon, in which the mesial-most postcanines are lost after replacement. This difference has contributed to the lengthening of the postcanine series in Galesaurus, as teeth continued to be added to the distal end of the tooth row through ontogeny. Overall, there are considerable differences between Galesaurus and Thrinaxodon relating to the replacement and development of their teeth.

Histology and μCT reveal the unique evolution and development of multiple tooth rows in the synapsid Endothiodon

“…In the anomodonts examined here, the CT scans show spaces around the 'postcanine' teeth that were originally occupied by the periodontal ligament and were infilled by matrix after death (Figs. 4, 5, 6, 7, 8, 9, 10) 69,89 . This is also the case in the histological sections of Endothiodon sp.…”

Computed Tomography elucidates ontogeny within the basal therapsid clade Biarmosuchia

“…In dicynodonts, the presence of a supernumerary tusk is considered pathological ( Fröbisch, 2005 ; Jinnah & Rubidge, 2007 ; Fröbisch & Reisz, 2008 ). In theriodonts (Gorgonopsia, Therocephalia and Cynodontia), replacement canines are found late in ontogeny but are not found in the largest specimens ( Kermack, 1956 ; Hopson, 1964 ; Van den Heever, 1980 ; Norton, 2020 ; Norton et al, 2020 ) except for Thrinaxodon ( Abdala, Jasinoski & Fernandez, 2013 ). As such, double canines are expected to be found in, but are not exclusive to, juveniles.…”

“…As such, double canines are expected to be found in, but are not exclusive to, juveniles. Specimen CGS MJF 22, and the holotypes of Lende chiweta and Lemurosaurus pricei do not show a replacement caniniform and as such, are better interpreted as mature specimens compared to SAM-PK-K11126 and RC 55 (see Norton, 2020 ; Norton et al, 2020 ).…”

“…14 ). The presence of two erupted maxillary caniniforms is considered a plesiomorphic condition in synapsids as sphenacodontians present continuous replacement with two functional maxillary caniniforms ( Romer & Price, 1940 ; Van den Heever, 1980 ; Sigogneau-Russell, 1989 ; Norton, 2020 ; Norton et al, 2020 ).…”

Diphyodont tooth replacement of Brasilodon—A Late Triassic eucynodont that challenges the time of origin of mammals

“…Hence, in this context, seeking dental information amongst closely related non‐mammaliaform cynodonts will be highly informative. Recently, micro‐computed tomography (μCT) scanning techniques were employed to image and study the intra‐mandibular teeth of Thrinaxodon and the nature of the dentition of extinct cynodonts (Abdala et al, 2013; Norton et al, 2020). However, the rendered virtual sections provided do not permit the visualisation of the dentine‐enamel junction and other critical histological enamel characters, also none of the details of bone resorption and rhizolysis.…”