Journal of Paleontology, 91(5), 2017, p. 1047–1059 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.18
A juvenile Koskinonodon perfectus (Temnospondyli, Metoposauridae) from the Upper Triassic of Arizona and its implications for the taxonomy of North American metoposaurids
Bryan M. Gee1 and William G. Parker2
1Department of Biology, University of Toronto at Mississauga, Ontario L5L 5L1, Canada ⟨
bryan.gee@
mail.utoronto.ca⟩ 2Division of Science and Resource Management, Petrified Forest National Park, Arizona 86028, USA ⟨
william_parker@nps.gov⟩
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 meto- posaurids 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 combi- nation 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.
Introduction
Metoposaurids are Late Triassic temnospondyl amphibians with a global distribution and are some of the most commonly recovered fossils in freshwater depositional settings (Colbert and Imbrie, 1956). Three species of metoposaurids are currently recognized from North America: (1) Koskinonodon perfectus Case, 1922; (2) “Metoposaurus” bakeri Case, 1931; and (3) Apachesaurus gregorii Hunt, 1993. Both K. perfectus and A. gregorii are known from the Chinle Formation exposures of the Petrified Forest National Park (PEFO) (Long and Murry, 1995). Koskinonodon was previously distinguished from Metoposaurus Lydekker, 1890 solely by the presence of a lacrimal that contacts the orbit (Hunt, 1993); however, the utility of this character is uncertain because this feature was identified in the holotype of M. diagnosticus Meyer, 1842 from Germany (Sulej, 2002; but see Lucas et al., 2007) and subsequently in M. krasiejowensis (Sulej, 2007) and M. algarvensis (Brusatte et al., 2015). The diagnostic features of Apachesaurus that are most frequently preserved include the presence of shallow otic notches, an absence of tabular horns, an occiput that does not project posteriorly, and elongate intercentra (Hunt, 1993; Spielmann and Lucas, 2012). However, the ontogeny of North American metoposaurids is poorly understood because of a paucity of juvenile specimens, rendering it difficult to verify that
these diagnostic features of Apachesaurus are not found in immature forms of already known metoposaurids, such as Koskinonodon. Additionally, complete skull material of A. gregorii is very rare, with the full set of diagnostic cranial characters known only from the holotype, and additional specimens referred to the taxon are too damaged to permit additional evaluation of the validity of these characters (Hunt, 1993; Spielmann and Lucas, 2012). The vast majority of post- cranial material consists of the small, elongate intercentra, which are rarely associated or articulated with any diagnostic cranial material (Spielmann and Lucas, 2012). Finally, both size and stratigraphic position are used as informal characters to identify metoposaurid material, even though neither should be considered to be reliable for species discrimination (e.g., Irmis, 2007; Bell et al., 2010). Given these considerations, additional study of cranial material of small metoposaurids is especially important for helping to identify taxonomically informative features that are not confounded by ontogeny and that can distinguish juveniles of a large species from adults of a small species.
metoposaurid (PEFO 35392) from the Petrified Forest Member (Upper Triassic, Norian) of the PEFO. Material from Norian- age sediments that can be definitively assigned to K. perfectus is rare. Additionally, the presence of intercentra that are more
Here we describe cranial and postcranial material of a small 1047
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