Phytosaur

Abstract: The sequence of neurocentral suture closure is one criterion for the determination of ontogenetic stage in extant crocodylians. This pattern is frequently used to assess ontogeny for a variety of fossil archosaurs that may or may not follow the same sequence and timing of suture closure. Phytosaurs are one of the few basal archosaur groups with a sample size large enough to help test whether the crocodylian pattern of suture closure is plesiomorphic for Pseudosuchia and Archosauria. Analysis of a large sample of North American phytosaur specimens confirms that phytosaurs share the crocodylian state of closure, and so this pattern is probably plesiomorphic for the Pseudosuchia. An additional independent ontogenetic trend observed in phytosaurs is that the lateral fossae on cervical vertebrae in phytosaurs deepen with ontogeny. A preliminary survey indicates that there is considerable variation of both the sequence and timing of neurocentral suture closure in other archosaur clades. Therefore, it i...

Abstract: Study of casts of Stegomosuchus, a small reptile from the Upper Trias of the Connecticut Valley, hitherto regarded as a pseudosuchian thecodont, suggests that the skull of Protosuchus includes two pairs of supraorbital bones, and as a consequence the narrow interorbital region indicates a closer relationship to the South African genus Notochampsa than has previously been thought. These three genera of late Triassic crocodilians are placed in the family Stegomosuchidae von Huene, 1922. Proterochampsa, from the Ischigualasto Formation of Argentina, described by Reig (1959) and Sill (1967) as an ancestral crocodile, is considered to be an extremely primitive phytosaur.

Abstract: The long-snouted, crocodile-like, amphibious reptiles of the Late Triassic known as phytosaurs are assigned to 5 genera on the basis of cranial characters. The most primitive genus, Paleorhinus, has external nares well forward of the antorbital fenestrae, the posttemporal arch at level of the skull roof, a slender rostrum without crests, homodont dentition, a low quadrate, and no trace of a posterior squamosal process. Mystriosuchus, which has highly specialized posterior nares, tall quadrate, and depressed posttemporal bar, retains the primitive form of the squamosal, homodont dentition, and slender (much longer) rostrum of Paleorhinus from which it may be derived through Francosuchus (the European subgenus of Paleorhinus). The remaining phytosaur genera are characterized by rounded processes of the squamosal bone projecting well behind the occipital surface of the skull, and by the development of heterodont dentition and generally more robust skulls. Angistorhinus, the most primitive member of this lineage, retains a primitive posttemporal arch at the level of the skull deck, but is advanced over Paleorhinus in its posteriorly placed external nares. Its rostrum is slender and lacks crests; the dentition is only slightly differentiated. Phytosaurus and Rutiodon, the more specialized members of this group, have depressed posttemporal arches and tall quadrates. Rostra of Rutiodon vary from slender and uncrested in small skulls to moderately robust with high dorsal crests in the posterior half in the largest individuals; teeth vary from moderately to strongly heterodont. The squamosal processes are slender and show progressive phyletic elongation. The ratio of rostral to postrostral length varies inversely with size. Phytosaurus skulls are characterized by massive rostra with continuous dorsal crests and have a relatively constant rostral ratio independent of size; the squamosal processes are short and deep; dentition is strongly heterodont; considerable evidence suggests that the dorsal armor plates differed from other phytosaurs.

Abstract: Recently developed rigorous and testable stratigraphic models, improved vertebrate systematics, and radioisotopic calibration allow the Late Triassic land vertebrate “faunachrons” of western North America to be extensively revised and codified Recognizing that reliable biochronology depends on well-documented and testable biostratigraphic models, we propose a simple set of three units to serve as a biochronology “starter kit”: one local biostratigraphic unit (a multiple taxon interval teilzone or biozone), one regional or global chronostratigraphic unit (an estimated holochronozone), and one biochronologic unit (an estimated holochron) These units are applied to the Otischalkian, Adamanian, Revueltian, and Apachean “faunachrons,” with all units being defined by multiple phytosaur taxa representing the appearance of monophyla Best estimates for the holochrons are 227–224 Ma (earliest Norian) for the Otischalkian; 224–215 Ma (early to earliest middle Norian) for the Adamanian; 215–207 Ma (earliest middle Norian to Rhaetian) for the Revueltian; and 207–202 Ma (Rhaetian) for the Apachean