Snake-like limb loss in a Carboniferous amniote

A new taxon of molgophid recumbirostran from the Carboniferous of Illinois, Nagini mazonense, suggests that the forelimb-first pattern of limb reduction that characterizes modern snakes also occurs early on in amniote evolution. Among living tetrapods, many lineages have converged on a snake-like body plan, where extreme axial elongation is accompanied by reduction or loss of paired limbs. However, when and how this adaptive body plan first evolved in amniotes remains poorly understood. Here, we provide insights into this question by reporting on a new taxon of molgophid recumbirostran, Nagini mazonense gen. et sp. nov., from the Francis Creek Shale (309-307 million years ago) of Illinois, United States, that exhibits extreme axial elongation and corresponding limb reduction. The molgophid lacks entirely the forelimb and pectoral girdle, thus representing the earliest occurrence of complete loss of a limb in a taxon recovered phylogenetically within amniotes. This forelimb-first limb reduction is consistent with the pattern of limb reduction that is seen in modern snakes and contrasts with the hindlimb-first reduction process found in many other tetrapod groups. Our findings suggest that a snake-like limb-reduction mechanism may be operating more broadly across the amniote tree.

Automated summary

The discovery of a new species called Nagini mazonense provides insights into the early evolution of limb reduction in amniotes. This species, which lacks forelimbs entirely, represents the earliest occurrence of complete limb loss in amniotes. The findings suggest that a snake-like limb-reduction mechanism may be more widespread across the amniote tree.