1056


Journal of Paleontology


in North American metoposaurids, especially A. gregorii (Spielmann and Lucas, 2012). The identification of isolated intercentra should be discouraged given the paucity of articulated or associated skeletons of A. gregorii that possess both elongate intercentra and diagnostic cranial material and of juvenile speci- mens of largemetoposaurids (Long and Murry, 1995; Spielmann and Lucas, 2012). Additionally, past workers have emphasized cranial features over post-cranial features in species determina- tion, which we continue to follow here given the greater phylo- genetic utility of cranial characters (Hunt, 1993; Long andMurry, 1995). Presently, the only species of the genus is K. perfectus; although “Metoposaurus” bakeri has sometimes been placed in the genus, it lacks a lacrimal that enters the orbit, which is the diagnostic feature of Koskinonodon (Hunt, 1993). Given that recent authors no longer consider this character valid for distin- guishing between the two genera, the taxonomic placement of the species remains unresolved (Sulej, 2002;Brusatte et al., 2015; but see Lucas et al., 2007). Additional discussion of the character is beyond the scope of this paper, but regardless of its taxonomic placement, material of “M.” bakeri in the southwest of North America ismostly restricted to the DockumGroup of Texas,with a single documented occurrence in Nova Scotia (Case, 1932; Hunt, 1993; Long and Murry, 1995). Koskinonodon perfectus is the most widespread and most commonly recovered metopo- saurid, and skulls of adult specimens are known to exceed 60cm (Colbert and Imbrie, 1956). Accordingly, PEFO 35392, with a length of 15.4cm from the posteromedial margin of the skull to the posterior margin of the orbit, can be designated as a juvenile of the species (Long andMurry, 1995; Zanno et al., 2002). This is further supported by the histological analysis, which shows that the associated intercentra are relatively immature in spite of the larger size of the skull in comparison to the allegedly mature holotype specimen of A. gregorii. Koskinonodon perfectus is a commonly recovered fossil from the Adamanian land vertebrate faunachron (LVF) of the PEFO, but documentation of diagnostic material is rare in the Revueltian LVF (Hunt, 1993; Long and Murry, 1995;Heckert and Lucas, 2002; Parker andMartz, 2011). At present, PEFO 35392 represents the youngest confirmed occurrence of cranial material of K. perfectus within the park. Although material belonging to adult specimens of the


large North American metoposaurid taxa (K. perfectus and “M.” bakeri) is extremely common in both the Chinle Formation and the Dockum Group, reports of material that can be conclusively identified as belonging to juvenile individuals of the two species are rare (Sulej, 2007). Material ascribed to juveniles has occa- sionally been documented (e.g., Hunt, 1993), but Zanno et al. (2002), who described partial cranial material ascribed to K. perfectus, featured the only previous description of such material. Additionally, that specimen (MNAV8415) is from the Blue Mesa Member, which is well below the horizon containing PEFO 35392, and it was assigned solely on the now invalid character of a lacrimal contacting the orbit (Zanno et al., 2002). Cranial material of small metoposaurids is not uncommon in Norian-age sediments, but it is often highly fragmentary and lacking in any diagnostic characters that allow for a designation at the genus level. The occurrence of a juvenile K. perfectus in younger sedi-


ments than previously documented for the taxon is important for improving our understanding of the Adamanian-Revueltian


faunal turnover, thought to correlate with a climatic shift to an increasingly arid regime with more pronounced seasonality (Dubiel et al., 1991; Parker and Martz, 2011; Nordt et al., 2015). Material of K. perfectus is common in Adamanian strata (Blue Mesa Member, lower Sonsela Member) of the park and rare in Revueltian strata (upper Sonsela Member, Petrified Forest Member), while material of A. gregorii demonstrates the inverse distribution pattern (Hunt and Lucas, 1993), which is thought to reflect a climate-associated transition from large metoposaurids to small ones (Parker and Martz, 2011; Atchley et al., 2013). Past workers have cited several features of A. gregorii, inclu- ding its co-occurrence with a predominantly terrestrial assem- blage, poorly developed lateral line system, elongate intercentra, and pelvic structure, as evidence of adaptations for a more terrestrial lifestyle that would have allowed for this transition (Hunt, 1993). However, it should be noted that elongate inter- centra, the most commonly recovered material of the taxon, are also found in the much larger Dutuitosaurus ouazzoui (Dutuit, 1976), which is considered fully aquatic (Steyer et al., 2004). Additionally, the presence of a poorly developed lateral line should not be considered equivalent to the absence of a lateral line, as would be expected of truly terrestrial forms. Finally, A. gregorii shares far more features with other metoposaurids in contrast to more definitively terrestrial stereospondyls from the Upper Triassic, such as Rileymillerus Bolt and Chatterjee, 2000, which, in addition to being found with predominantly terrestrial fauna, as with A. gregorii, entirely lacks a distinct lateral line system. We consider it plausible that both the morphological differences and the hypothesized ecological transition could also be explained by life stage niche partitioning. This possibi- lity was previously suggested based on strong negative allometry of Koskinonodon limb bones (Rinehart et al., 2008) and is attractive for several reasons. Firstly, changes in habitat occupation and skeletal mor-


phology that are associated with the life cycle are common among both extant and extinct amphibians (Wilbur, 1980; Schoch, 2014). Given the pronounced seasonality that char- acterized the equatorial regions of the Upper Triassic, it would not be unexpected for aquatic taxa, particularly desiccation- prone amphibians, to acquire adaptations that would allow them to tolerate more arid conditions during the dry season (Dubiel et al., 1991; Martz and Parker, 2010; Atchley et al., 2013; Nordt et al., 2015). For example, workers have postulated that M. diagnosticus, which features more robust forelimbs and musculature relative to Dutuitosaurus ouazzoui (two taxa with well-described articulated skeletons), may have been adapted for burrowing into lake sediments during the dry season (Konietzko-Meier and Sander, 2013). Although any transition from a semi-aquatic to an aquatic lifestyle was not likely as dramatic as the full metamorphosis of extant amphibians, changes to the vertebral column and girdles to accommodate different modes of locomotion and to the otic region to account for different auditory reception are not unreasonable to infer. Secondly, occupation of different environments would


increase the potential for taphonomic bias in addition to that associated with hydrodynamic sorting, which would explain the stark difference in relative abundances between the two taxa (e.g., Loughney et al., 2011). Such a bias is clear in the Blue Mesa Member (middle Norian) at the PEFO based on the


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