Preliminary analysis of macroevolutionary patterns in the variation of cranial morphotypes in the proterochampsid archosauriforms from the Triassic of South America

TROTTEYN, MARÍA JIMENA; EZCURRA, MARTÍN D.

Buenos Aires

Jornada; XXVI Jornadas Argentina de Paleontología; 2012

Proterochampsids are Triassic archosauriforms documented from the well-sampled Los Chañares (Ladinian) and Ischigualasto (Carnian–Norian) formations of Argentina and coeval levels of the Santa María Formation of Brazil. The anatomy and phylogenetic relationships of proterochampsids have been studied in detail, but the macroevolutionary patterns within the group have been mostly ignored. Thus, our aim was to analyse variations observed in proterochampsid cranial morphological diversity through time in order to characterize some aspects of their macroevolutionary history. Three cranial morphotypes were recognized qualitatively, which may represent different ecomorphotypes: 1) strongly dorsoventrally compressed skulls (Chanaresuchus Romer, Proterochampsa Reig); 2) dorsoventrally tall skulls (Gualosuchus Romer); and 3) moderately compressed skulls (Cerritosaurus Barberena, Rhadinosuchus Huene, Tropidosuchus Arcucci). The apex of morphological diversity is observed in the Los Chañares Formation with the presence of all three morphotypes, whereas in the Ischigualasto Formation the diversity is restricted to morphotype 1 only. By contrast, morphotypes 1 and 3 are both recorded in the Brazilian beds coeval to the Los Chañares (Dinodontosaurus AZ) and Ischigualasto (Hyperodapedon AZ) formations. Accordingly, a decrease in cranial morphological diversity is observed in the Argentinean basin during the Carnian–Norian relative to the Ladinian, but morphological diversity is constant through time in the Brazilian basin. These patterns may reflect different macroevolutionary histories occurring in the two basins, in particular the probable extinction of morphotypes 2 and 3 in the Late Triassic of Argentina. The hypotheses proposed here will be tested in the future with the continuously improved sampling of the South American Triassic beds.