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

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

Los Angeles

Congreso; 73th Annual Meeting of the Society of Vertebrate Paleontology; 2013

Silesaurids are dinosauriform archosaurs hypothesized to be the sister-taxon ofDinosauria. The clade includes species from the Middle and Late Triassic of North andSouth America, Europe, and Africa, and consists of herbivorous/omnivorous quadrupedalforms with beaked lower jaws. In South America, silesaurids are currently restricted toone taxon from the early Norian of Brazil and preliminary reports from the late Carnianof Argentina. Here, we substantially expand the South American silesaurid record withthe description of a partial postcranial skeleton from the Triassic strata of the AtacamaDesert in northern Chile. The Museo Nacional de Historia Natural specimenSGO.PV.22250 comes from the ?Estratos El Bordo?, a sedimentary unit that cancurrently only be constrained to the Triassic; it has also yielded the pseudosuchianChilenosuchus forttae. The specimen is mainly preserved as natural moulds in a silicifiedlimestone block. The slab includes a sequence of ten articulated presacral vertebrae,partial ilia and hind limbs, and several ribs. The ilium has a preacetabular process thatdoes not extend beyond the anterior margin of the pubic peduncle and the acetabulum ispartially closed, with a gently concave ventral margin. The femur is sigmoidal inposterior view and possesses a prominent fourth trochanter. The fibular shaft is onlyslightly transversely thinner than that of the tibia. The femoral head is trapezoidal inposterior view and separated from the shaft by a distinct notch. A deep popliteal fossaextends along at least one-quarter of the length of the bone. The latter three femoralcharacters are apomorphies of Silesauridae, but the fragmentary nature of the specimenprevents us from determining its phylogenetic relationships within Silesauridae.However, the presence of a partially closed acetabulum distinguishes the Chilean formfrom other silesaurids with preserved ilia (e.g. Silesaurus, Sacisaurus, Asilisaurus). Thisspecimen increases the diversity and biogeographic range of Silesauridae, and representsonly the second tetrapod lineage known from the Triassic of Chile.