New Vertebral Laminae and Patterns of Serial Variation in Vertebral Laminae of Sauropod Dinosaurs

Vertebral laminae connect various projections on the neural arch (costovertebral and intervertebral articulations, neural spine) and centrum of the presacral, sacral, and anterior caudal vertebrae of sauropods and other saurischian dinosaurs. The nomenclature applied to vertebral laminae is based on the landmarks they connect. Along the vertebral series and especially through regional transitions (e.g., cervical-dorsal), the landmarks anchoring laminae vary in their relative positions. As a result, vertebral laminae change serially in shape and orientation, and some may be restricted to certain vertebral regions. Without complete vertebral series, however, understanding this variation can prove difficult and can lead to misidentification of serial variation as an interspecific or higher-level difference. Here I review recently recommended changes in laminar nomenclature and describe two patterns of serial variation bracketing laminae that have a restricted distribution in the vertebral column. I term these patterns ‘lamina capture’ and ‘lamina cutoff. ’ Patterns of serial variation of vertebral laminae can be coded as cladistic characters in phylogenetic analysis. Contributions from the Museum of Paleontology, University of Michigan 1Museum of Paleontology and Department of Earth & Environmental Sciences, University of Michigan, 1109 Geddes Avenue, Ann Arbor, Michigan 48109-1079, U.S.A. (wilsonja@umich.edu) iNtroduCtioN Vertebral laminae are bony struts that connect major projections on the neural arch and centrum of saurischians and various other tetrapods (Wilson, 1999; Gower, 2001). They provide structural support for the projections of the neural arch, and sets of laminae enclose spaces that housed air sacs in life (Britt, 1993; Wedel, 2003). Vertebral regions are defined by the nature of their ribs (e.g., thorax = ribs contact sternum; sacrum = ribs contact ilium), which means that transitions between vertebral regions are characterized by substantial changes in the position of the rib articulations and their attendant vertebral laminae. The laminae of sauropod vertebrae have long been recognized to be complex (e.g., Seeley, 1870; Phillips, 1871; Hulke, 1880). Variation in the arrangement of vertebral laminae is apparent at very fine levels, such as on right and left sides of the same vertebra (Fig. 1) or along the vertebral series of a single individual (Fig. 2), and variation is also evident at individual, specific, generic, and suprageneric levels. The earliest nomenclature for vertebral laminae appeared at the close of the nineteenth century (Osborn, 1899). Despite the ambiguity of the terms (e.g., ‘oblique lamina,’ ‘horizontal lamina’), they were commonly employed in English-language descriptions of sauropods until late in the twentieth century, particularly those describing sauropods from the Morrison Formation of the western United States (e.g., Hatcher, 1901). Janensch (1929) proposed a far more practical, landmark-based nomenclature for vertebral laminae that never gained currency in the literature, despite partial adoption by McIntosh (1990). A century after Osborn (1899), Wilson (1999) proposed a nomenclature for laminae that, like Janensch’s, is landmarkbased. Nineteen laminae, organized into four major groups (i.e., diapophyseal, parapophyseal, zygapophyseal, spinal), are specified by seven landmarks (i.e., diapophysis, parapophysis, prezygapophysis, postzygapophysis, neural spine, anterior and posterior centrum). Each lamina can be described with a fourletter acronym based on the landmarks and major groupings. For example, the lamina connecting the neural spine and