Palacrodon sp.

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 : 121-122

publication ID	https://doi.org/ 10.4202/app.00426.2017
DOI	https://doi.org/10.5281/zenodo.10986605
persistent identifier	https://treatment.plazi.org/id/03FF265F-4F4E-766E-B0B1-FAB9FE41FE92
treatment provided by	Felipe
scientific name	Palacrodon sp.
status

Treatment

Palacrodon sp.

Fig. 3 View Fig .

1980 Procolophonidae ; Jacobs and Murry 1980: 63.

1983 Reptilia; Tannenbaum 1983: 63, fig. 109.

1993 Clevosaurus latidens ; Fraser 1993: 138.

1994 Reptilia; Kaye and Padian 1994: 189, fig. 9.109.

2000 Procolophonidae ; Edwards 2000: 320–321, fig. 4.6.3A.

Material. — MNA V3684, fragment of marginal dentition with two complete teeth; MNA V11247, isolated tooth. Obtained from MNA 207-2, the Placerias Quarry, St. John’s, USA. A fossiliferous horizon with abundant macrovertebrate and microvertebrate bones and carbonaceous nodules in a clay rich mudstone matrix ( Fiorillo et al. 2000) from the Chinle Formation, dated to 219.39 ± 0.16 Ma based on U-Pb detrital zircon dates ( Ramezani et al. 2014). These dates place the occurrence solidly in the Adamanian Land Vertebrate Estimated Holochron ( Martz and Parker 2017). Detailed locality information is on file at Museum of Northern Arizona and is available to qualified researchers.

Diagnosis. —Differs from Palacrodon browni in having teeth that have a bicuspid apex and a mesially recurved labial profile. The base of the distal shelf is concave in labial view, halfway towards the apex becoming convex to the apex of the tooth. The mesial tooth shelf is concave in labial view, and the apical edge of the tooth reaches an angle just greater than 90°. The labiolingually-broadened apex of the tooth possesses two cusps, one on the labial side, and one on the lingual side. These cusps are joined by a narrow ridge that forms a saddle between the cusps in mesiodistal view.

Description. — MNA V3684 is a tooth-bearing fragment of a maxilla or dentary ( Fig. 3A View Fig ). The assignment to either element is difficult because the bone is broken along all margins except for the tooth-bearing surface, and will be referred to as a dentary in this description. Two complete teeth are preserved. The jaw fragment has a mesiodistal length of 7.5 mm, and a labiolingual width of 3.6 mm, and is incomplete on along its labial, lingual, ventral, posterior, and anterior surfaces. Measurements of the teeth are found in Table 1 View Table 1 .

Two teeth are present. One of the two complete teeth is larger than the other ( Table 1 View Table 1 ). Palacrodon teeth increase in size distally, indicating that the larger tooth was further distal 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, and are bulbous, bulging out slightly from the margins of the jaw. No tooth root is apparent in the jaw where it is fractured less than 1 mm below the tooth bases and it can be assumed that if rooting is present, it is very shallow. There is a row of small pits on the lingual side of the distal tooth, each with a diameter of 0.01 mm where the lingual margin of the tooth base meets bone, similar to the pitting seen in PEFO 37247 ( Fig. 2C View Fig ).

The teeth are rectangular in occlusal view with most of their surface taken up by mesial and distal shelves that meet at the apex of the tooth. The base of the mesial shelf starts as a flange that meets and fuses to the tooth distal to it, and is concave in labial view. Halfway towards the apex the shelf becomes convex. The distal tooth shelf is concave in labial view and reaches an angle just greater than 90° with respect to the jaw, creating a recurved profile in labial view. The labiolingually-broadened apex of the teeth possesses two cusps, one on the labial side, and one on the lingual side Fig. 3A View Fig 2 View Fig ). These cusps are joined by a narrow ridge that forms a concave saddle between the cusps in mesial view Fig. 3B View Fig 1 View Fig ). The mesial and distal shelves are concave basally, with maximum concavity at the labiolingual midline of the teeth forming a slight basin. The presence of these basins on the mesial and distal sides of the apex creates a constriction of the apical ridge at the labiolingual midline of the teeth, and the ridge expands to its widest at the cusps on either side of the apex. The apex of the posterior tooth exhibits a lemniscate wear facet spreading across both cusps and the saddle between them. The apex of the anterior tooth has slight wear facets on the tips of both cusps, but no wear on the saddle connecting them. These wear facets cut through a thin layer of enamel exposing dentine below.

The tooth-bearing fragment described herein is likely from the posterior or middle of the maxilla or dentary, because as seen in BP /1/5296 and SAM-PK- 6215, the teeth in Palacrodon maxillae and dentaries become increasingly cylindrical and peg-like towards the anterior end of the jaw and lose their mesiodistal contact.

MNA V11247 ( Fig. 3B View Fig ) is an isolated tooth attached to a fragment of jaw. The tooth is 2.7 mm in mesiodistal length and 3.2 mm in labiolingual width, and 2.1 mm at its greatest apicobasal height. Remnant fragments of adjacent teeth can be seen fused to both the mesial and distal margins of this tooth and a small fragment of bone is attached to the base of the tooth on its labial or lingual side. Like the anteriormost tooth in MNA V3684, this tooth is worn largely on the apex of both cusps exposing dentine, and the wear facets extend onto the saddle connecting the cusps creating a lemniscate shape.