Amphisbaenian

An amphisbaenian is a member of Amphisbaenia, a lineage of predominantly burrowing Squamata. The elongated body often has rings of scales, small eyes and a skull specialized for moving through soil. Most species lack external limbs, but Bipes retains forelimbs. Their wormlike appearance does not make them Annelida, and a limbless body does not make them snakes.

Their evolutionary relationships illustrate the problem of convergent evolution. Burrowing can favor similar reductions and skull modifications in separate reptile lineages, making an anatomy-only grouping misleading. An integrated 2015 study compared morphological characters, molecular sequences and fossil taxa, and recovered amphisbaenians with lacertids in Lacertoidea. Including DNA allowed the investigators to test similarities that might instead reflect similar modes of life.^[ref]

The example shows why a shared loss of limbs must be interpreted on a phylogenetic tree, alongside other evidence, before it is treated as evidence of close ancestry.

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- Bipes Amphisbaenian

Bipes

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Bipes is a taxonomic genus of burrowing amphisbaenians distinguished by retaining external forelimbs despite an elongated body and absence of external hindlimbs. It is an informative comparison with other amphisbaenians, which lack external limbs: external appearance alone does not reveal how much internal anatomy remains.^[ref]

In a 2019 anatomical study, investigators used iodine-enhanced computed tomography to distinguish soft tissues and reconstruct shoulder muscles as well as bones. Bipes retained an almost complete shoulder girdle, while other sampled amphisbaenians had much more reduced bony structures. Nevertheless, all sampled amphisbaenian forms retained the full set of 17 shoulder muscles, sometimes fused or rearranged. Muscle attachment could shift from bone to surrounding soft tissue.^[ref]

The result challenges a tempting shortcut: losing a visible limb does not necessarily delete all its associated muscles at once. Bipes supplies the limbed comparison that helps reveal which internal components have persisted or changed during the evolution of the burrowing body form.