The morphology and ontogeny of Typothorax coccinarum (Archosauria, Stagonolepididae) from the upper triassic of the American Southwest

The aetosaur Typothorax coccinarum in known from abundant material from the Late Triassic Chinle Formation and Dockum Group of the American southwest, particularly in Arizona, New Mexico, and west Texas. Redondasuchus reseri is a smaller aetosaur taxon with several similarities to Typothorax that may indicate it is a juvenile. The purposes of this thesis are to provide a thorough, well-illustrated description of Typothorax coccinarum, describe the ontogeny of the taxon, and to determine if Redondasuchus is a valid taxon distinct from Typothorax.

The Canjilon Quarry (Upper Petrified Forest Formation, Chinle Formation, north-central New Mexico) Typothorax material includes three large concentrations of scutes and skeletal material (probably representing individuals) and other isolated elements, collected from two distinct stratagraphic layers. The original field maps and field numbers recorded on the bones allow the association of much of the material to be established. The Post Quarry (Cooper Canyon Formation, Dockum Group, west Texas) yielded a subadult specimen of Typothorax (TTUP 9214). Although detailed quarry maps and field notes for the Post Quarry are currently unavailable, the general consistency in the material supports its assignment to a single, unusually small individual of Typothorax. The Apache Canyon Quarry (Redonda Formation, Dockum Group, eastern New Mexico) material of Redondasuchus consists entirely of isolated scutes probably found in at least two different stratigraphic levels within the quarry.

The scutes of TTUP 9214 possess the diagnostic features of Typothorax coccinarum, while its small size and the incomplete ossification of some elements suggest it is a subadult. There are no major differences between the scutes of Typothorax and Redondasuchus apart from size; arching is seen in the presacral as well as caudal dorsal paramedian scutes of Typothorax, and the alleged down-turned lateral edge of the holotype scute of Redondasuchus is in fact is the medial edge, placing arching at the center of ossification as in Typothorax. Redondasuchus reseri is assigned to Typothorax, but kept as a separate, smaller taxon, Typothorax reseri, due to the absence of T coccinarum material from the Redonda Formation.

The only axial skeletal material described here is from TTUP 9214, which includes a squamosal, quadrate, braincase, dentary, and vertebrae from all regions of the column except for posterior caudals. The braincase has an incompletely enclosed trigeminal foramen and basal tubera that are deeply split by the basioccipitalbasisphenoid suture. The dentary is extremely shallow, with ten aveoli. The transverse processes of the vertebrae grow extremely wide in the dorsal region. Appendicular skeletal material is well represented, and many areas of muscle attachment present in crocodilians can be plausibly inferred. The scapulocoracoid, clavicle, humerus, illium pubis, ischium, femur, tibia, fibula, astragalus, calcaneum, metapodials, and phalanges are all known from the Canjilon Quarry material and/or TTUP 9214. The dorsal paramedian and lateral scutes of Typothorax are the best-known elements, and show variation relating to their anteroposterior placement on the body. The ordering of these scutes is inferred based on aetosaur specimens of other taxa with articulated scutes, and two blocks of articulated Typothorax scutes from the pelvic and caudal region of one of the Canjilon Quarry specimens. All dorsal paramedians have circular or oval ornamentation in a random pattern, a raised anterior bar, and most have a thick ventral keel and are extremely wide. The lateral scutes in the precaudal region are angulated, have a triangular dorsal flange, and lack a horn. In the caudal region the lateral scutes are flat plates with the flanges lying in roughly the same plane. Appendicular and ventral scutes of Typothorax are known, but their arrangement on the body is not understood due to the lack of articulated material.

Bivariate ontogenetic allometry was explored for the femur and tibia of Typothorax coccinarum. The femur of Typothorax may have lengthened more rapidly then the tibia. Log squared measurements of femur and tibia length were plotted against each other, yielding a good correlation, but failing to pass the p-test (probably due to the small sample size of individuals having both elements). Log-squared measures for the femur and tibia were then plotted against size scores on the first principal component (PCI) to describe the relative growth of various dimensions of these bones. Several of the femur measures for Typothorax coccinarum have good correlation coefficients that pass the p-test for, generally indicating that the bone grew robust more rapidly distally and at the midshaft then it lengthened. All tibia measures showed good correlation coefficients that passed the p-test, generally indicating that the robustness of the tibia increased faster then its length in all areas of the bone, especially proximally and at the midshaft.

Many ontogenetic changes in Typothorax coccinarum seem to represent recapitulations, with likely plesiomorphic traits such as a less compact neck, slightly narrower transverse processes on the vertebrae, femora that are not so long relative to the crus, a narrower carapace, and a raised ridge on the dorsal paramedian scutes being lost with age and increase in size. The young of Typothorax may have been more active then the older individuals. There is insufficient evidence that posture became more erect in Typothorax ontogenetically, or between aetosaur taxa phylogenetically, as a graviportal adaptation. However, this possibility cannot be entirely ruled out for aetosaurs as a group, and it is possible that erect posture evolved in archosaurs in response to more then one selective pressure.