Palacrodon browni Broom, 1906

Kligman, Ben T., Marsh, Adam D. & Parker, William G., 2018, First records of diapsid Palacrodon from the Norian, Late Triassic Chinle Formation of Arizona, and their biogeographic implications, Acta Palaeontologica Polonica 63 (1), pp. 117-127 : 120-121

Palacrodon browni Broom, 1906

Fig. 2 .

Material.— PEFO 37247, fragment of a right maxilla with two complete teeth and two partial teeth. Obtained from PFV 396, the “coprolite layer” near Billing’s Gap in Petrified Forest National Park, Arizona, USA. A fossiliferous light green matrix-supported siltstone in the upper Blue Mesa Member of the Chinle Formation, between 223.036 ± 0.059 and 220.123 ± 0.068 Ma ( Ramezani et al. 2011). These dates place the occurrence within the Adamanian Land Vertebrate Estimated Holochron ( Martz and Parker 2017; previously the Adamanian Land Vertebrate Faunachron of Lucas and Hunt 1993). Detailed locality information is on file at Petrified Forest National Park and available to qualified researchers.

Diagnosis. —Diapsid reptile with bulbous, labio-lingually broadened teeth that are ankylosed directly onto the apex of the jaw with acrodont implantation. Teeth are fused to each other mesiodistally along their entire labiolingual width. The margin of each tooth is flanged on its mesial and distal edges. The teeth have a pyramidal lateral profile with concave mesial and distal crown surfaces. Wear facets are present on the apex of each tooth. Tooth margins form a rectangular profile in occlusal view, and have a trapezoidal profile in anteroposterior view. This combination of characters is unique to Palacrodon browni .

Description. — PEFO 37247 is a partial right maxilla with four preserved teeth, two of which are complete, and two are largely fragmented. PEFO 37247 has an anteroposterior length of 7.2 mm, a greatest dorsoventral depth of 3.8 mm, and a mediolateral width of 1.1 mm. The maxilla is incomplete along its dorsal margin and both anteroposterior ends because of damage during processing. Two nutrient foramina with a width of 0.5 mm are located on the labial side of the maxilla and open into a vacuity in the interior of the element ( Fig. 2B 1 ); these foramina are similar to those found on BP /1/5296. A slot opens posteromedially at the posterodorsal end of the bone, likely where the jugal articulated onto the maxilla.

Four teeth are present, with the two middle ones being complete. One of the two complete teeth is larger than the other ( Table 1 ). This size difference indicates that the larger tooth was further back in the jaw than the smaller one ( Gow 1992, 1999). The teeth are ankylosed directly onto the apical surface of the dentary in an acrodont fashion. There is a row of small pits each with a diameter of 0.01 mm where the labial and lingual margins of the base of each tooth meets the maxillary bone ( Fig. 2C ). The tooth bases bulge out 0.3 mm from the labial and lingual margins of the maxilla, making the teeth wider labiolingually than the maxilla itself. PEFO 37247 is identical to SAM-PK- 6215 in that the teeth are pyramidal in lateral view and each complete tooth has a flat lemniscate wear facet on its apical surface ( Fig. 2A 1 ), cutting through the thin layer of enamel that covers the tooth surface and exposing the dentine center of the tooth. The wear facets on PEFO 37247 are nearly identical in shape to those found in the partial dentary of BP /1/5672. Concave shelves are present on the mesial and distal sides of the wear facet on each tooth, extending two-thirds of the way down the tooth ( Fig. 2B 2 ). A flange extends below the mesial and distal shelves on each tooth to meet the flange of the adjacent tooth. The teeth contact each other mesiodistally and the respective flanges are firmly fused together along this line of contact ( Fig. 2A 2 ); three such tooth-to-tooth contacts are present in this specimen.

Stratigraphic and geographic range. —Currently known from Cynognathus Assemblage Zone rocks ( South Africa), the lower Fremouw Formation ( Antarctica), and the Chinle Formation (Arizona, USA).

Figures

Fig. 2. Diapsid Palacrodon browni Broom, 1906 (PEFO 37247) from Norian, Petrified Forest National Park, Arizona, USA; in occlusal (A), labial (B), lingual (C), and oblique (D) views. Arrows point in anterior direction. Photographs (A1, B1, C, D), explanatory drawings (A2, B2).

Fig. 1. Geographic and stratigraphic context of Palacrodon occurrences in Arizona. A. Map of Arizona showing the locations of PFV 396 and MNA 207-2. Map of North America modified from www.lakodosajta.info. B. Stratigraphic chart of the Chinle Formation at PEFO and the Placerias Quarry, with detrital zircon dates from Ramezani et al. (2011).