New Mexico Geological Society Annual Spring Meeting — Abstracts

The Los Esteros Member of the Santa Rosa Formation contains a diverse vertebrate fauna in Santa Fe County, New Mexico. The dominant elements in this fauna, as in all Chinle Group faunas, are phytosaurs. Other members of the fauna include the aetosaurs Desmatosuchus and Stagonolepis, the metoposaur Buettneria, rauisuchians, a dicynodont cf. Ischigualastia sp., a pterosaur and several undescribed taxa. Two genera of phytosaurs are present in the Los Esteros fauna. Rutiodon is most common and is represented by skulls and much postcrania. The skulls have depressed supratemporal fenestrae and posteriorly rounded squamosal processes diagnostic of Rutiodon. The other taxon is represented by the posterior portion of one skull. This specimen has a combination of supratemporal fenestrae at the level of the skull roof and posteriorly projecting, and rounded, squamosal processes. These features are diagnostic of Angistorhinus. This report represents the first confirmed co-occurrence of the phytosaurs Rutiodon and Angistorhinus and the first occurrence of Angistorhinus in New Mexico.

Case reported Brachysuchus from the Tecovas Member of the Dockum Formation in Texas, and this taxon has been considered a junior synonym of Angistorhinus by most recent workers. The Tecovas also contains Rutiodon. However, the "Brachysuchus" from the Tecovas actually represents the taxon "Rutiodon" gregorii and therefore does not demonstrate a co-occurrence of Rutiodon and Angistorhinus.

Two phytosaur biochrons can be discriminated in the late Carnian of Laurasia. These are an early Tuvalian Paleorhinus/Angistorhinus biochron and a late Tuvalian Rutiodon biochron. There are only two instances of Paleorhinus or Angistorhinus co-occurring with Rutiodon: (1) there is one partial skull of Paleorhinus, together with many specimens of Rutiodon, in the Placerias quarry (Blue Mesa Member of the Petrified Forest Formation, northeastern Arizona); (2) the Los Esteros skull of Angistorhinus found with Rutiodon specimens. Both of these co-occurrences are low in the sequence of strata that are of Rutiodon-biochron-age. These co-occurrences indicate that there is a transition zone between the two late Carnian biochrons in which relatively rare individuals of Paleorhinus and Angistorhinus co-existed with Rutiodon. Thus, these co-occurrences do not refute the phytosaur-based biochronology of the late Carnian, they actually refine it.