Abstract
Recumbirostran ‘microsaurs,’ a group of early tetrapods from the Late Carboniferous and Early Permian, are the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. However, understanding of the diversity of fossorial adaptation within the Recumbirostra has been hindered by poor anatomical knowledge of the more divergent forms within the group. Here we report the results of μCT study of Quasicaecilia texana, a poorly-known recumbirostran with a unique, broad, shovel-like snout. The organization of the skull roof and braincase of Quasicaecilia is found to be more in line with that of other recumbirostrans than previously described, despite differences in overall shape. The braincase is found to be broadly comparable to Carrolla craddocki, with a large presphenoid that encompasses much of the interorbital septum and the columella ethmoidalis, and a single compound ossification encompassing the sphenoid, otic, and occipital regions. The recumbirostran braincase conserves general structure and topology of braincase regions and cranial nerve foramina, but it is highly variable in the number of ossifications and their extent, likely associated with the reliance on braincase ossifications to resist compression during sediment compaction and mechanical manipulation by epaxial and hypaxial musculature. Expansion of the deep ventral neck musculature in Quasicaecilia, autapomorphic among recumbirostrans, may reflect unique biomechanical function, and underscores the importance of future attention to the role of the cervical musculature in contextualizing the origin and evolution of fossoriality in recumbirostrans.
Highlights
The earliest tetrapods to show morphological adaptations to headfirst burrowing are the recumbirostrans, a group of tuditanomorph ‘microsaurs’ for which monophyly has been reasonably established [1]
This novel use of the skull imposes a number of important functional constraints: the skull must resist forces imposed by the soil compression phase of the excavation cycle, the cross-sectional area of the skull must be minimized in order to reduce the amount of soil that must be displaced, the cervical musculature must be expanded in order to power the digging stroke, and the jaw must be modified to permit feeding in confined spaces
Gross morphology consistent with that seen in modern headfirst burrowers has been identified in ostodolepid recumbirostrans for some time [21], but it is only recently that adaptations consistent with headfirst burrowing have been more broadly recognized within recumbirostrans in general [9, 10, 22]
Summary
The earliest tetrapods to show morphological adaptations to headfirst burrowing are the recumbirostrans, a group of tuditanomorph ‘microsaurs’ for which monophyly has been reasonably established [1]. Head-first burrowing, where the skull itself is employed as a locomotory organ to displace or compact soil, is generally achieved via conspicuous morphological adaptations to the musculoskeletal system of the head and neck [13, 14] This novel use of the skull imposes a number of important functional constraints: the skull must resist forces imposed by the soil compression phase of the excavation cycle, the cross-sectional area of the skull must be minimized in order to reduce the amount of soil that must be displaced, the cervical musculature must be expanded in order to power the digging stroke, and the jaw must be modified to permit feeding in confined spaces. This effort has largely been limited to identification of morphology consistent with headfirst burrowing rather than variation in skull shape that may indicate variation in burrowing mode or substrate preference among recumbirostrans
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