A juvenile Koskinonodon perfectus (Temnospondyli, Metoposauridae) from the Upper Triassic of Arizona and its implications for the taxonomy of North American metoposaurids

Abstract

Abstract.-Metoposaurids are temnospondyl amphibians that are well known from Upper Triassic deposits in North America, Europe, India, and Africa. Two species of metoposaurids, Koskinonodon perfectus and Apachesaurus gregorii, are among the most common fossils found in the Petrified Forest National Park (PEFO) of the southwestern United States. The two are distinguished on the basis of several cranial traits and the morphology of their intercentra, and are more informally distinguished by their overall size and biostratigraphic range. While material of adult metoposaurids is extremely common in Upper Triassic deposits, described material of juveniles is very rare, which limits the study of ontogenetic trajectories that would assist in assessing whether diagnostic traits of the smaller A. gregorii are also ontogenetically influenced traits in juveniles of K. perfectus. Here we report on material from a juvenile specimen of K. perfectus from the Petrified Forest Member (Norian) of the Chinle Formation in the PEFO. This specimen is significant because it represents the highest known occurrence of the species within the PEFO, documents a rare occurrence of a juvenile specimen of a North American metoposaurid, and records a rare North American metoposaurid specimen that includes associated cranial and postcranial material. Additionally, the combination of cranial features diagnostic of K. perfectus and intercentra of proportions intermediate between K. perfectus and A. gregorii in this specimen raises questions about the utility of several characters for species discrimination in North American metoposaurids.

Redescription of Arganasaurus (Metoposaurus) azerouali (Dutuit) comb. nov. from the Upper Triassic of the Argana Basin (Morocco), and the first phylogenetic analysis of the Metoposauridae (Amphibia, Temnospondyli)

“…and Buffa et al . , appendix 3): this supports the hypothesis of Gee & Parker (, , b) and Gee et al . () for whom ‘ Apachesaurus ’ gregorii may be a juvenile form of a large metoposaurid, potentially K. perfectus .…”

“…These authors argued that the group underwent a trend in size diminution during the Norian, as diminutive taxa such as A. gregorii would have been better suited to arid climates, which is why they are more represented in these horizons. This trend in size diminution has been refuted by Gee & Parker (, ) on the basis of A. gregorii being a juvenile metoposaurid, and evidence shows that the metoposaurids could withstand some variety of climates (Konietzko‐Meier & Klein ). However, metoposaurids are indeed rarer in the higher Norian horizons.…”

“…Apachesaurus gregorii Hunt, from the Norian of North America has recently been considered as a juvenile form of a large metoposaurid, potentially Koskinonodon perfectus (Gee & Parker , , b; Gee et al . ).…”

“…nov. as a diagnostic character. However, as size is subjected to many biases, we follow Hunt () and Gee & Parker () for whom it cannot be considered a diagnostic character for metoposaurids. Schoch & Milner () also proposed the small teeth in the palatal series of A. azerouali comb.…”

“…These otic notches are similar in shape to that of A. lyazidi . A. gregorii has extremely shallow otic notches and no tabular horn – an argument to state that A. gregorii is a juvenile metoposaurid (Gee & Parker , , b; Gee et al . ).…”