Vertebrate Succession in the Ischigualasto Formation

MARTÍNEZ, RICARDO NESTOR; APALDETTI, CECILIA; ALCOBER, OSCAR ALFREDO; COLOMBI, CARINA E.; SERENO, PAUL; FERNÁNDEZ, ELIANA; SANTI MALNIS, PAULA; CORREA, GUSTAVO; ABELIN, DIEGO

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Lugar: Lawrence; Año: 2013

0272-4634

The Ischigualasto Formation has yielded the richest record of fossil vertebrates during the initial phase of the evolution of dinosaurs in the Late Triassic (Carnian-Norian). Ash layers within the formation have generated a chronostratigraphic framework to calibrate the distribution of abundant and diverse vertebrate remains. Intensive geologic work has subdivided the formation into four members and three abundance biozones. We describe two new basal dinosauromorphs, a lagerpetonid Trilicosaurus rapax and a silesaurid Ignotosaurus fragilis, adding important small-bodied taxa to the fauna in association with the earliest dinosaurs. To date published reports cite only 19 of the 29 vertebrate taxa now known in the formation and record their temporal presence in the formation only very generally by large-scale biozones. We provide a census of 848 fossil specimens representing 26 vertebrate taxa sampled across 50 m intervals of the formation. This taxonomically enlarged, temporally registered census shows that faunal changes within the Ischigualasto Formation are not sudden events but rather record a gradual decrease in abundance and diversity related to climatic change or deterioration. The singular, apparently sudden shift in faunal composition occurs at the end of the Exaeretodon biozone, when this abundant cynodont disappears and is replaced by the dicynodont Jachaleria, which is the most common vertebrate in the taxonomically depauperate, less fossiliferous lowermost portion of the Los Colorados Formation.