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

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.