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Micro-computed tomography of the holotype of the early tetrapod Ichthyerpeton bradleyae (Huxley in Wright and Huxley, 1866) from the Pennsylvanian of Ireland

Citation

Ó Gogáin, Aodhán; Wyse Jackson, Patrick N. (2021), Micro-computed tomography of the holotype of the early tetrapod Ichthyerpeton bradleyae (Huxley in Wright and Huxley, 1866) from the Pennsylvanian of Ireland, Dryad, Dataset, https://doi.org/10.5061/dryad.5qfttdz4k

Abstract

Ichthyerpeton bradleyae is one of the seven tetrapods originally described by Huxley from the Jarrow Coal (Pennsylvanian; Langsettian Regional Substage equated with the Bashkirian International Stage) in south-eastern Ireland. The holotype, one of only two specimens considered to represent the taxon, consists of the post-cranial skeleton, which has been highly compressed and has undergone extensive replacement of bone by carbonaceous material. The holotype is studied using microcomputed tomography, which reveals that the vertebral column has at least 25 diplospondylus vertebrae with cylindrical centra. Neural arches and a haemal arch are described for the first time. Neural arches in the caudal region are paired and neural spines only contact one another dorsally. The haemal arch is fused and wraps around the ventral margin of the centrum. A stout femur and tibia are described. The morphology of the femur is unique for early tetrapods, with fibular and tibial condyles of similar length and lacking an adductor crest. The morphology of the femur, and its length relative to the tibia suggests that the holotype of I. bradleyae preserves an immature individual. The tibia is a flat bone characteristic of stem-tetrapods. Phalanges from the right and left pes are present. Because the phalanges are disarticulated, a phalangeal count cannot be determined. Despite the new anatomical information, the systematic position of I. bradleyae is still difficult to establish; however, it does not belong within either the colosteids, temnospondyls or embolomeres to which it has previously been assigned.

Methods

The holotype of Ichthyerpeton bradleyae (TCD.T87) was scanned using the Nikon XTH 225 ST computed tomography scanner at the University of Bristol, UK. The scan was run at 220kV and 375uA with a 1mm plate of Cu in front of the source.

3D models of TCD.T87 were rendered using the freeware SPIERS (Sutton et al., 2012). Individual elements were isolated using the masking function.

 

Funding

Geological Society of London, Award: William George Fearnsides Fund

SFI Research Centre for Applied Geosciences (ICRAG)

SFI Research Centre for Applied Geosciences (ICRAG)