Postcanine replacement in Brasilodon and Brasilitherium (Cynodontia, Probainognathia) and its bearing in cynodont evolution

AGUSTIN MARTINELLI; JOSE BONAPARTE

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

The discovery of Brasilodon quadrangularis and Brasilitherium riograndensis in the Late Triassic of Rio Grande do Sul State, Brazil has provided fresh? data for a better? understanding of cynodont evolution, as they represent the sister group of Mammaliaformes in recently proposed hypotheses (Bonaparte et al., 2003, 2005). Several well preserved specimens of these species are available including juvenile to adult individuals. In this preliminary contribution, we focus on the postcanine replacement of Brasilodon and Brasilitherium. The analysis of several individuals has revealed that upper and lower postcanines of Brasilitherium and Brasilodon have an overall alternate tooth replacement as in other cynodonts (e.g., Thrinaxodon; Crompton, 1963); however, a precise alternate pattern (i.e., odd- and even-numbered alternate replacement) is break down in different positions among the studied specimens (e.g., UFRGS-PV786T). Furthermore, one specimen of Brasilodon (UFRGS-PV716T) has strong and uniform dorsolingual wear along the entire upper postcanine row, indicating that the postcanines replacement stopped long before the death of the individual, similar to the condition in Probainognathus (Creo que en el trabajo de Romer del 70 o en algunos de los de Crompton esta dicho esto sobre probaino). In upper and lower tooth rows, the anterior postcanines are shed and their alveoli are closed, whereas new postcanines are added at the back of the tooth row. This patter was observed in Thrinaxodon, gomphodonts, and Sinoconodon (e.g., Hopson, 1971; Luo et al., 2004); however, in the two latter taxa, the replacement is not alternate. The evidence of the Brazilian species suggests that the lost of the anterior postcanines is faster than in Thrinaxodon, but similar to that of Sinoconodon, producing an enlargement of the diastema between the canine and the first postcanine during growth. Assuming the alternated position of eruption, the teeth of the same wave erupted at different times, starting the eruption from back to front. According to the tooth morphology, younger postcanines have a complex pattern, which is more evident in the lower dentition. In addition, crown complexity decreases from back to front. The knowledge of tooth replacement in Brasilodon and Brasilitherium will be useful to test the taxonomic validity of these genera. Furthermore, their sister group relationships to mammals suggests that mammalian teeth replacement was achieved later to the establishment of several other derived craniodental features in cynodont evolution.