Gee and Parker—Juvenile Metoposaurid from Arizona


paucity of juvenile individuals of K. perfectus in contrast to the very common adults (Parker and Martz, 2011). Material of A. gregorii is almost exclusively found in blue paleosol horizons at the PEFO, which also preserve the majority of rare taxa in the park, often in a greater degree of articulation than in non-blue paleosol horizons (Loughney et al., 2011). These horizons have been interpreted as abandoned ponds and channels that were fundamentally different in both habitat and depositional settings than the dominant floodplain environments of the Chinle Formation; accordingly, ecological association with different depositional settings between life stages could have produced a taphonomic bias in metoposaurid taxa within the PEFO (Loughney et al., 2011). Such a bias would explain why the assemblages containing A. gregorii are predominantly terrestrial in nature, and could explain both why large metoposaurid spe- cimens are rare in the latest Norian and why this specimen preserves a rare occurrence of associated cranial and postcranial material in contrast to the vast majority of other North American metoposaurid specimens (Hunt, 1993; Heckert and Lucas, 2002; Parker and Martz, 2011). Finally, a semi-aquatic life stage offers one possible explanation for the wide dispersal of large, fully aquatic metoposaurids, which, in addition to the southwest region of North America, are also known from Pennsylvania, Nova Scotia, Europe, Morocco, and India (Hunt, 1993; Long and Murry, 1995). The documented co-occurrence with terres- trial fauna and modestly elongate intercentra in a juvenile K. perfectus lend support to the hypothesis of niche partitioning and a semi-aquatic-to-aquatic transition from juvenile to adult life stages.


Conclusions


The ontogeny of North American metoposaurids is poorly known, largely because of the disarticulated nature of specimens and a lack of identifying cranial features that could differentiate immature specimens of K. perfectus from mature specimens of A. gregorii. PEFO 35392, which represents a rare occurrence of associated cranial and postcranial material, contributes addi- tional insight into potential ontogenetically variable features and challenges some of the existing characters used for species discrimination in Metoposauridae. Based on both the external morphology of the skull and the histological analysis of the associated intercentra, otic notches of a more intermediate depth and intercentra of an intermediate degree of elongation may be juvenile features of K. perfectus. Accordingly, this specimen highlights the need for more conservative classification of metoposaurid material from western North America, rather than identification based solely on overall size of elements.Given the scarcity of juvenile specimens of K. perfectus and the poor understanding of ontogeny of the North American taxa, making a distinction between juvenile individuals of K. perfectus and adult specimens of A. gregorii is impossible without clear identifying features, which are predominantly cranial features. Additionally, although small size, as well as the lack of otic notches and tabular horns, is the most common feature used to identify A. gregorii, caution should be exercised in assigning specimens with clearly weathered posterior margins because this may remove the tabular horns and make the otic notches appear shallower. Given the current scarcity of well-preserved


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for species discrimination, existing classifications of small metoposaurids should be rigorously reassessed (Sulej, 2007; Brusatte et al., 2015). It is important that specimens not be assigned to A. gregorii simply because they lack this single, currently invalid, diagnostic character of K. perfectus, the only other metoposaurid known from middle to late Norian sedi- ments of western North America. For example, the specimen (UCMP 175145) ascribed to A. gregorii by Zanno et al. (2002) was classified based on a lacrimal excluded from the orbit and its overall diminutive size; the former is no longer considered valid and the latter has never been a formal character (Sulej, 2002; Brusatte et al., 2015). Specimens must possess diagnostic features of A. gregorii in order to be assigned to the taxon because of the potential morphological overlap with juveniles of another taxon; otherwise, they should be referred only to the most inclusive taxon the characters allow, in this case simply Metoposauridae (Nesbitt and Stocker, 2008). At the same time, caution should also be exercised in utilizing the existing cranial characters, considering that nearly all of the diagnostic cranial features of A. gregorii are based on the holotype skull, with additional specimens needed to corroborate their utility (Hunt, 1993; Spielmann and Lucas, 2012). Additionally, until a better understanding of ontogenetic variation can be achieved in North American metoposaurids, the use of size to identify specimens should be discouraged. Finally, the documentation of a juvenile K. perfectus in an assemblage of predominantly terrestrial ver- tebrates, and that includes intercentra that are of intermediate proportions to those of A. gregorii and K. perfectus, raises questions regarding the validity of the A. gregorii as a species. Because the ontogeny of North American metoposaurids is not well known, features identified by Hunt (1993) in support of Apachesaurus as an adult, rather than a juvenile of a known taxon, require additional study for better evaluation, especially since some assertions were made on the basis of non- metoposaurid temnospondyls or on a low sample size. Given that ontogenetic changes in limb bone morphology associated with niche partitioning are already documented in K. perfectus (e.g., Olsen, 1951), variation in other features may also be associated with occupation of different environments, such as the intercentra or the depth of the otic notches. The hypothesis of life-stage niche partitioning requires further exploration, but offers solutions to several unresolved questions, such as the markedly different relative abundances of the two taxa and the paucity of juvenile specimens of K. perfectus. Therefore, these variable features should not be considered for species discrimination unless such variation can be clearly identified as interspecific rather than ontogenetic.


skulls of A. gregorii, many of the diagnostic characters require further specimens to verify their utility in species discrimination. Given the invalidity of the lacrimal position as a character


Acknowledgments


Thanks to M. Smith (PEFO) for granting access to collections for the duration of a National Park Service (NPS) summer internship awarded to BMG, to C. Lash (PEFO) for assistance with molding and casting of the intercentra, and to B. Traver (PEFO) for granting permission for the destructive histological


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