Vertebrate Anatomy, Morphology, Palaeontology (Dec 2024)
What exactly is a nuchal ligament and who exactly has one?
Abstract
Nuchal ligaments are relatively well understood and have venerable histories of recognition in extant euungulates, canids, elephants, and humans, but whether any anatomical structures in other taxa, both extant and extinct, qualify as nuchal ligaments is unclear because the term ‘nuchal ligament’ lacks a clear, narrow, consistently applied definition. Possible definitions of the term could be etymological, taxonomic, compositional, or morphological/topological, or a combination thereof. Currently, a de facto morphological/topological definition of ‘nuchal ligament’ sensu stricto seems most common: a nuchal ligament is an epaxial, cervical ligament with a funiculus that is elevated above the cervical spinous processes and connected to them only via laminae. However, many references to ‘nuchal ligaments’ in both extant and extinct taxa instead seem to employ a broader, etymological definition that encompasses numerous different compositions, morphologies and topologies. Several, largely untested assumptions have been made about functional and osteological correlates of a nuchal ligament, such as possessing a ‘large’ or ‘heavy’ head and/or a ‘long’ neck, possessing specific features on the occipital region of the skull, and possessing specific morphologies or dimensions of the cervical and cranial thoracic spinous processes. These assumptions have led to corollary assumptions that many extinct tetrapods—particularly those phylogenetically far removed from taxa known to possess them—had nuchal ligaments, but until these presumed correlates are tested and demonstrated in extant taxa, such assumptions remain purely speculative, and alternative cranio-cervical support mechanisms also must be considered. Depending on the definition applied, attributions of nuchal ligaments to extinct taxa, and even to some extant taxa (including humans), may be references to other sorts of morphologically and topologically distinct epaxial structures such as supraspinous ligaments and fibrous septa/raphes that occupy similar anatomical positions as nuchal ligaments sensu stricto. ‘Nuchal ligament’ requires a narrow definition to understand what, if any, features correlate with the presence of the ligament, as well as what taxa have convergently evolved the structure.
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