A new basal archosauriform diapsid from the Early Triassic of Argentina

MARTIN EZCURRA; AGUSTINA LECUONA; AGUSTIN MARTINELLI

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Lugar: Lawrence; Año: 2010 vol. 30 p. 1433 - 1450

0272-4634

The best known South American Early Triassic archosauriform belongs to a putative proterosuchid briefly reported by José Bonaparte in 1981, coming from the Quebrada de los Fósiles Formation (Puesto Viejo Group, Argentina). This specimen consists of well-preserved natural external molds of a partial postcranium that preserves dorsal vertebrae, osteoderms, a dorsal rib, a possible gastralium, a chevron, a humerus, an ilium, and pedal ungual. We re-describe this specimen and identify autapomorphies that allow us to recognize Koilamasuchus gonzalezdiazi gen. et sp. nov. A cladistic analysis positions Koilamasuchus within Archosauriformes and more crownwards than Proterosuchus and Euparkeria, as the sister taxon of the clade that includes Erythrosuchidae and Archosauria. The presence of an iliac blade with a slightly convex dorsal margin and with a maximum length more than 3 times its maximum height places Koilamasuchus within Archosauriformes. Koilamasuchus is found as more derived than Proterosuchus because of the presence of an iliac preacetabular process, a pubic peduncle that forms an angle lower than 45° to the longitudinal axis of the ilium, and dorsal body osteoderms, and even more derived than Euparkeria and Osmolskina due to the presence of a very well-developed preacetabular process. Koilamasuchus is more basal than erythrosuchids and more derived archosauriforms because of the absence of mostly dichocephalous dorsal ribs. The new taxon does not fit with the currently known lineages of non-archosaur archosauriforms, probably representing a still cover group which increases the diversity of the clade during the biotic recovery after the Permo-Triassic mass extinction.