A SILESAURID (ARCHOSAURIA: DINOSAURIFORMES) FROM THE TRIASSIC OF THE ATACAMA DESERT, CHILE

RUBILAR-ROGERS, D.; EZCURRA, M.; IRMIS, R.; DESOJO, J.B.; SOTO-ACUÑA, S

Congreso; Society of vertebrate paleontology; 2013

Silesaurids are dinosauriform archosaurs hypothesized to be the sister-taxon of Dinosauria. The clade includes species from the Middle and Late Triassic of North and South America, Europe, and Africa, and consists of herbivorous/omnivorous quadrupedal forms with beaked lower jaws. In South America, silesaurids are currently restricted to one taxon from the early Norian of Brazil and preliminary reports from the late Carnian of Argentina. Here, we substantially expand the South American silesaurid record with the description of a partial postcranial skeleton from the Triassic strata of the Atacama Desert in northern Chile. The specimen (SGO.PV.22250) comes from the ?Estratos El Bordo?, a sedimentary unit that can currently only be constrained to the Triassic; it has also yielded the pseudosuchian Chilenosuchus fortae. The specimen is mainly preserved as natural moulds in a silicified limestone block. The slab includes a sequence of ten articulated presacral vertebrae, partial ilia and hindlimbs, and several ribs. The ilium has a preacetabular process that does not extend beyond the anterior margin of the pubic peduncle and the acetabulum is partially closed, with a gently concave ventral margin. The femur is sigmoidal in posterior view and possesses a prominent fourth trochanter. The fibular shaft is only slightly transversely thinner than that of the tibia. The femoral head is trapezoidal in posterior view and separated from the shaft by a distinct notch. A deep popliteal fossa extends along at least one-quarter of the length of the bone. The latter three femoral characters are apomorphies of Silesauridae, but the fragmentary nature of the specimen prohibit us from determining its phylogenetic relationships within Silesauridae. However, the presence of a partially closed acetabulum distinguishes the Chilean form from other silesaurids with preserved ilium (e.g. Silesaurus, Sacisaurus, Asilisaurus). This specimen increases the diversity and biogeographic range of Silesauridae, and represents only the second tetrapod lineage known from the Triassic of Chile.