A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs

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Abstract
The vertebrae of sauropods are characterized by numerous bony struts that connect the costovertebral and intervertebral articulations, centrum, and neural spine of the presacral, sacral, and anterior caudal vertebrae. A nomenclature for sauropod vertebral laminae is proposed that: 1) utilizes the morphological landmarks connected by the laminae (rather than their spatial orientation); and 2) provides the same name for serial homologues. This landmark-based nomenclature for vertebral laminae, which establishes the first criterion of homology (similarity), is the first step towards interpreting their phylogenetic significance. Nineteen different neural arch laminae are identified in sauropods, although all are never present in a single vertebra. Vertebral laminae can be divided into four regional categories, with each distinct lamina abbreviated with a simple four-letter acronym: diapophyseal laminae; parapophyseal laminae; zygapophyseal laminae; and spinal laminae. The distribution of neural arch laminae in presacral, sacral, and caudal vertebrae is evaluated to assess homology in sauropods and other saurischians. Five diapophyseal laminae and six zygapophyseal laminae characterize saurischian dinosaurs. Parapophyseal laminae and spinodiapophyseal laminae are unique to a subgroup of sauropods that includes Barapasaurus, Omeisaurus, and Neosauropoda. The presence of diapophyseal laminae in caudal vertebrae characterizes diplodocids. Vertebral laminae probably partitioned pneumatic diverticuli on the neural arch and provided structural support for the axial column. Their basic architecture evolved in saurischians prior to the Late Triassic (Carnian), 25 million years before the first known sauropod.