Postcranial anatomy and histology ofSeymouria, and the terrestriality of seymouriamorphs

Abstract

Seymouria is the best known of the seymouriamorphs, a group of Permo-Carboniferous reptiliomorphs with both terrestrial and aquatic taxa. The majority of research on Seymouria has focused on cranial anatomy, with few detailed descriptions or illustrations of the postcrania. We utilized neutron computed tomography (nCT) and histological sampling to provide updated, detailed figures that clarify details of the postcranial anatomy and to assess the development and histology of Seymouria through specimens from the early Permian Richards Spur locality. The correlation of morphological and histological data indicate rapid metamorphosis in this terrestrially capable stem amniote, with the youngest specimen being postmetamorphic despite being distinctly younger than premetamorphic individuals of Discosauriscus, the only other seymouriamorph to have been histologically sampled. The microanatomical data (e.g., semi-open medullary cavity) also substantiate the hypothesis that Seymouria was terrestrial based on interpretation of external features, although the persistence of a modestly developed medullary spongiosa in comparison to either Discosauriscus or to other co-occurring terrestrial tetrapods suggests additional nuances that require further exploration. In the absence of clearly recognizable postmetamorphic stages in several seymouriamorph taxa, it is difficult to determine the evolutionary trajectory of terrestriality within the clade. Our analysis provides the first histological characterization of the life history of Seymouria and highlights the need for further study of seymouriamorph ontogeny.

Evidence of large terrestrial seymouriamorphs in the lowermost Permian of the Czech Republic

“…Bazzana et al . 2020 also confirmed the rapid metamorphosis of Seymouria , in contrast to Discosauriscus , by using histological sampling. Additionally, a study of 3D bone histology showed that S. sanjuanensis and D. austriacus had relatively rapid bone growth and dynamics in contrast to Devonian stem‐tetrapods and temnospondyls of equal size.…”

“…The absence of skeletal remains pertaining to adult individuals of discosauriscids in the lacustrine deposits of the Boskovice Basin also appears to be best explained by a biphasic life cycle: a shift in their habitat occupancy from an aquatic to a terrestrial setting during ontogenetic development, which is also assumed for Seymouria (Berman 2000; Eberth et al . 2000; Bazzana 2020) and Karpinskiosaurus (Klembara 2011). Furthermore, the presence of a single discosauriscid individual that could represent an early adult stage (SNM Z 25814) indicates that the habitat shift occurred around the age of 10 years or possibly slightly earlier in most individuals, given that SNM Z 25814 already lacks the sensory grooves (Klembara 2009 b ).…”

Osteohistology of Greererpeton provides insight into the life history of an early Carboniferous tetrapod

“…While early tetrapods are often compared to modern amphibians (e.g., Clack, 2012; Molnar et al, 2020; Sanchez, de Ricqlès, et al, 2010; Sanchez & Klembara, 2008), there are discrepancies in body size and the extremely derived life histories of modern amphibians likely confounds many morphological and/or physiological comparisons of growth through ontogeny. Thus, the applicability of this comparison remains questionable (Bazzana et al, 2020), and we broaden our modern vertebrate group comparison in deciphering likely explanations for the avascular zone, all while acknowledging the limitation of drawing comparisons between Greererpeton and modern taxa. Given these constraints, in our consideration of possible life history events, we establish the following criteria determined by the bone histology observed in our sample: (1) the event must incur reduced bone deposition and/or metabolic stress, (2) it must happen after limb ossification is initiated but well before skeletal maturity, and (3) it must occur as a singular event that is not repeated during the lifetime of the individual.…”

“…With some exception (Laurin et al, 2007; Meunier & Laurin, 2012), these analyses have been conducted on the humerus, an element that is taxonomically informative for these groups yet is highly disparate from the morphology of modern tetrapod limb bones that have been used to validate paleohistological interpretations (Castanet et al, 2000; De Margerie et al, 2002; Margerie et al, 2004; Montes et al, 2007). Permian crown tetrapods, on the other hand, have been extensively sampled and have demonstrated a wide range of established ecologies and life history strategies (e.g., Bazzana et al, 2020; Boitsova et al, 2019; Estefa et al, 2020; Gee, Haridy & Reisz, 2020; Huttenlocker & Shelton, 2020; Konietzko‐Meier et al, 2016; Sanchez & Klembara, 2008).…”

Evolutionary Change in Locomotion Close to the Origin of Amniotes Inferred From Trackway Data in an Ancestral State Reconstruction Approach

“…whether they are linked to their early relatives through a line of terrestrial ancestors (see e.g., discussion in Bazzana et al, 2020).…”