Abstract
The Carboniferous Pennsylvanian-aged (309–307 Ma) Mazon Creek Lagerstätte produces some of the earliest fossils of major Palaeozoic tetrapod lineages. Recently, several new tetrapod specimens collected from Mazon Creek have come to light, including the earliest fossorially adapted recumbirostrans. Here, we describe a new long-bodied recumbirostran, Joermungandr bolti gen. et sp. nov., known from a single part and counterpart concretion bearing a virtually complete skeleton. Uniquely, Joermungandr preserves a full suite of dorsal, flank and ventral dermal scales, together with a series of thinned and reduced gastralia. Investigation of these scales using scanning electron microscopy reveals ultrastructural ridge and pit morphologies, revealing complexities comparable to the scale ultrastructure of extant snakes and fossorial reptiles, which have scales modified for body-based propulsion and shedding substrate. Our new taxon also represents an important early record of an elongate recumbirostran bauplan, wherein several features linked to fossoriality, including a characteristic recumbent snout, are present. We used parsimony phylogenetic methods to conduct phylogenetic analysis using the most recent recumbirostran-focused matrix. The analysis recovers Joermungandr within Recumbirostra with likely affinities to the sister clades Molgophidae and Brachystelechidae. Finally, we review integumentary patterns in Recumbirostra, noting reductions and losses of gastralia and osteoderms associated with body elongation and, thus, probably also associated with increased fossoriality.
Highlights
Recent resurgence in the study of certain ‘microsaurs’ known as recumbirostrans has resulted in their renewed relevance in the origin of amniotes [1,2]
These authors settled on a preferred terminology based on structural differences that is followed here: ‘osteoderms’, plate-like dermal ossifications that can bear pits on the outer surface; ‘dermal scales’, thinner than osteoderms but still composed of bone, ovular or round in profile, and can be overlapping; and ‘gastralia’, ventrally located elongate ossifications often collectively arranged into a chevron pattern
The thin, rod-like gastralia of early amniotes are probably derived from the dermal gastral scales of basal tetrapods [35,39,40,41]
Summary
Recent resurgence in the study of certain ‘microsaurs’ known as recumbirostrans has resulted in their renewed relevance in the origin of amniotes [1,2]. 318 Ma) strata of Joggins, Nova Scotia [5,6] Already at this stage, and onward through the Permo-Carboniferous, recumbirostrans are morphologically diverse, showing a range in degree of development of a variety of stereotypical cranial and postcranial adaptations for a fossorial lifestyle [7,8,9,10,11,12,13]. Recent studies [13,14,15,16] on tetrapod fossils from the Moscovian-aged (309–307 Ma) Mazon Creek Lagerstätte have revealed a diverse ‘recumbirostran-amniote’ assemblage. Joermungandr is exceptionally preserved, at the higher end of the quality scale for Mazon Creek fossils. It includes alongside the full skeleton visible in dorsal and ventral aspects, a detailed soft tissue impression of the body complete with scales. Detailed examination of the scales of Joermungandr reveals a unique ultrastructural pattern, and provides the first comprehensive description of scale morphology of this kind in a Palaeozoic tetrapod (figure 4)
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