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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.

Natascha Westphal coauthored a 2019 study of shoulder muscles in amphisbaenians, asking whether reductions of limbs and supporting bones are accompanied by equivalent losses of muscles. The researchers used iodine-enhanced computed tomography to visualize soft tissues in three dimensions and compare them with the skeleton.^[ref]

All investigated amphisbaenian taxa retained the full set of seventeen shoulder muscles, although some muscles were fused or reorganized. Bipes retained forelimbs and a nearly complete shoulder girdle, while other sampled lineages showed greater skeletal reduction. In severely reduced forms, muscle attachments could shift to skin, connective tissue or other muscles. The result supports a decoupling of muscle and bone evolution: losing a skeletal element does not by itself establish that the associated musculature has vanished.^[ref]