Spinal Cord Supports in Caecilian Vertebrae and Their Significance for Amphibian Systematics.

Spinal cord supports (medially projecting bony protuberances within the neural canal) are found in several lineages of fossil and extant salamanders and have been interpreted as a synapomorphy of Caudata. Previous studies that have reported the absence of spinal cord supports in caecilian amphibians were based on an incomplete survey. Here, we re-examine the condition in caecilians using micro-CT scans and histological sections of vertebrae. We find that spinal cord supports are widespread among caecilians, occurring at least in the atlas of most examined species. In caecilians, the supports are most prominent in the atlas and appear to form through the expansion of the perichondral ossification of the septal bundles, consistent with the condition observed in salamanders. The presence of spinal cord supports in caecilians requires a reassessment of current hypotheses regarding the evolution of this character in tetrapods and may help clarify the phylogeny of Lissamphibia. Variation within Gymnophiona may provide novel characters of use in caecilian systematics.