A Juvenile Specimen of the Trematopid Acheloma From Richards Spur, Oklahoma and Challenges of Trematopid Ontogeny

Abstract

Trematopids are a clade of terrestrial dissorophoid temnospondyls documented primarily from terrestrial Permo-Carboniferous environments in North America and Europe. Here we describe the complete skull and articulated mandibles of a diminutive trematopid specimen (OMNH 79318) from the Early Permian karst deposits near Richards Spur, Oklahoma. Based on aspects of the neurocranium (e.g., unossified sphenethmoid, prootics, epipterygoids), the specimen represents one of the best examples of a markedly immature trematopid, an important data point for understanding the early ontogeny of trematopids. Specifically, it provides evidence that variation in otic notch structure can be ontogenetically influenced, not only among eucacopine dissorophids but also among trematopids. We provisionally refer the specimen to cf. Acheloma based on the presence of a denticulate vomerine ridge and other qualitative features. However, we emphasize that the taxonomic referral is complicated by several factors that more broadly confound trematopid taxonomy. This includes a low sample size (n = 1) of many taxa and marked size, and presumed ontogenetic, disparity between the known size range of different taxa. Complementary reexamination of both Acheloma cumminsi and Acheloma dunni as part of this study also reveals that the former possesses lateral exposures of palatal bones, the presence/absence of which was the only formal character that previously differentiated the two species, although other qualitative features (e.g., size of the internarial fontanelle) may differentiate these two species. With respect to OMNH 79318, the taxonomic referral is tentative because the specimen also shares many qualitative attributes with Phonerpeton pricei, a trematopid represented only by small-bodied, probably immature individuals. However, many of these shared features are likely to be influenced by ontogeny or size. The subsequent challenges that we encountered in our taxonomic referral suggest that ontogeny may be confounding taxonomy in both diagnoses and phylogenetic analyses of trematopids and emphasize the need for careful study of how this affects our understanding of trematopid intrarelationships.

The amphibamiformNanobamus macrorhinusfrom the early Permian of Texas

“…Despite substantial improvements in our knowledge of dissorophoids in recent decades, unequivocal larval forms of olsoniforms remain virtually unknown (see Milner, 2018 for further discussion and an example of a larval specimen), and juvenile forms are rare in comparison to more mature (‘adult’) stages. Although various specimens of small-bodied (presumably juvenile) trematopids have been described (see, e.g., Olson, 1941; Dilkes, 1993; Gee et al, 2019), they are much larger than UCMP 203686 and are clearly postmetamorphic.…”

The disadvantage of derivation: conserved systematic flaws in primary data have repeatedly biased the phylogenetic inference of Temnospondyli (Tetrapoda, Amphibia)

“…The neurocranium remains a poorly sampled region, in large part because this is a historically difficult area to access and thus to code. It is hoped that additional tomographic work in temnospondyls (e.g., Robinson et al, 2005; Čerňanský et al, 2016; Arbez et al, 2017, 2022; Gee et al, 2019a; Gee, 2020a; Marsicano et al, 2021) will produce further clarity in the way that it has for ‘microsaurian’ lepospondyls (Maddin et al, 2011; Huttenlocker et al, 2013; Pardo et al, 2015; Szostakiwskyj et al, 2015; Pardo and Anderson, 2016; Gee et al, 2019b, 2020; MacDougall et al, 2021). There are also avenues for expanding certain characters that only capture generalized trends.…”

A redescription of the late Carboniferous trematopid Actiobates peabodyi from Garnett, Kansas

“…peabodyi (and thus no information regarding ontogeny), these lines of evidence presume that at least some general attributes of other trematopids' ontogeny are applicable here. For example, the otic capsule of small individuals of Acheloma consists of relatively small and distinct elements that become far more developed and that co‐ossify in large individuals (e.g., Gee et al, ; Maddin et al, ). All large trematopids ( Acheloma , Ecolsonia cutlerensis , Mattauschia laticeps ) have a posteriorly closed otic notch by means of a ventrally extensive tabular horn, and small individuals of these taxa (when known) have an open notch, like in Act.…”

“…All large trematopids ( Acheloma , Ecolsonia cutlerensis , Mattauschia laticeps ) have a posteriorly closed otic notch by means of a ventrally extensive tabular horn, and small individuals of these taxa (when known) have an open notch, like in Act. peabodyi (Berman et al, ; Gee et al, ; Milner, ). The otic notch also becomes more dorsoventrally constricted in large trematopids, with a nearly horizontal ventral margin, but smaller individuals and Act.…”

“…cumminsi exceed 15 cm (Berman et al, 2011;Dilkes & Reisz, 1987;Milner, 2019;Polley & Reisz, 2011). As elaborated upon by Milner (2018) and Gee, Bevitt, and Reisz (2019), this disparity poses major challenges for properly inferring evolutionary relationships among trematopids because of the confounding influence of ontogeny on phylogeny. Previous analyses (Berman et al, 2011;Polley & Reisz, 2011;Ruta et al, 2003) produce conflicting results that are likely due to these factors, as well as other variables such as different taxon sampling and character matrix construction.…”