Tail regeneration in the Paleozoic tetrapod Microbrachis pelikani and comparison with extant salamanders and squamates

Abstract

AbstractAmong extant tetrapods, salamanders are the only group capable of full limb and tail regeneration, the latter including the re‐establishment of a fully functional tail with the axial skeleton and associated musculature. Tail regeneration is associated with autotomy (self‐amputation) in some salamander taxa but is also possible in salamander taxa that lack tail autotomy. Modern squamates also frequently show tail autotomy as a defense or decoy mechanisms against predation, but contrary with salamanders, the tail skeleton is replaced by a cartilaginous rod in the regenerate and the associated musculature is rather unusual. Here, we describe details of the tail regeneration in a fossil early tetrapod, the microsaurian lepospondyl Microbrachis pelikani in comparison to two salamander species, Desmognathus monticola and D. ocoee. Two specimens of Microbrachis represent an early and a later stage of tail regeneration and clearly show that this microsaur was capable of salamander‐like tail regeneration. The differentiation of the axial skeleton proceeds in a similar way to modern salamanders from proximal to distal and in a reversed order relative to development, i.e. with an early establishment of the vertebral centra, followed by the neural arches. Moreover, the morphological analysis suggests that Microbrachis autotomized its tails along an intravertebral autotomization plane, which is similar to modern squamates but differs from intervertebral tail autotomy seen in salamanders. The findings highlight that patterns of tail autotomy evolved independently from the overall capacity to regenerate the tail and the capacity to re‐establish the axial skeleton. The phylogenetic distribution of salamander‐like tail regeneration suggests that this may represent an ancient feature of sarcopterygians, which is in line with recent data on the evolution of full limb regeneration in tetrapods.