Triassic Life on Land by Hans-Dieter Sues

Author:Hans-Dieter Sues
Language: eng
Format: epub
Tags: SCI054000, Science/Paleontology
Publisher: Columbia University Press
Published: 2010-04-28T00:00:00+00:00

CHINLE STRATIGRAPHIC INTERVAL A

Lucas and Huber (2003) regarded Chinle stratigraphic interval A as corresponding to the Otischalkian LVF, which Lucas (1998) characterized by the presence of Metoposaurus, the phytosaurs Parasuchus (formerly known under its subjective junior synonym Paleorhinus) and Angistorhinus (= ? Rutiodon), and the aetosaur Longosuchus. As an additional index fossil, Lucas proposed the archosauriform Doswellia. This unusual taxon was first described from the Richmond basin of the Newark Supergroup in Virginia (Weems 1980; Dilkes and Sues 2009) and was later reported from Texas (Long and Murry 1995). The Otischalkian LVF is based on a tetrapod assemblage from the Dockum Group at sites just north of the abandoned community of Otis Chalk, near Big Springs in Howard County, Texas. Lucas (1998) considered the Otischalkian late Carnian in age, which is consistent with previously reported palynological information (Litwin, Ash, and Traverse 1991).

Hunt (1993) considered Metoposaurus bakeri distinct from Koskinonodon (under the preoccupied name Buettneria), but Sulej (2002) referred it to the latter genus.

Phytosaurs referred to Parasuchus have a much greater stratigraphic range than previously assumed (Parker 2005). It is important to emphasize that Parasuchus represents a grade of basal phytosaurs that share the plesiomorphic position of the external nares in front of the antorbital fenestrae, and the phylogenetic relationships of the various taxa assigned to “Parasuchus” remain yet to be determined. Basal phytosaurs referred to “Parasuchus” also share dorsoventrally flattened skulls with largely dorsally facing orbits and slender snouts (chapter 5). By contrast, Angistorhinus has a deeper skull with the external nares situated level with the antorbital fenestrae, as is the case in all other phytosaurs more derived than those assigned to “Parasuchus.”

The aetosaur Longosuchus reached a length of about 3 meters. Its dorsal dermal armor is distinguished by lateral osteoderms that form distinct, spikelike structures throughout the neck, trunk, and anterior tail region (Sawin 1947; Hunt and Lucas 1990).

According to Lucas (1998), the Otischalkian LVF comprises the oldest Late Triassic tetrapod assemblages in the American West. The Popo Agie Formation of Wyoming has yielded a very similar assemblage, with the metoposaurid Koskinonodon, the phytosaurs “Parasuchus” and Angistorhinus, the aetosaur Desmatosuchus, the “rauisuchian” Poposaurus, a hyperodapedontine rhynchosaur, and the dicynodont Placerias (Lucas 1994).

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