Anatomical Terminology for the Sacrum of Sauropod Dinosaurs

The sacrum represents a key anatomical element for understanding Dinosauria, beginning with the first use of the taxon name in 1842. A five-vertebra sacrum was originally considered diagnostic of dinosaurs, and variation in the number of sacral vertebrae has been used to diagnose smaller groups of dinosaurs. Despite its centrality to early definitions of Dinosauria, utility for diagnosing smaller groups, and manifest complexity in certain groups (e.g., sauropods and theropods), the sacrum has played a relatively minor role in determining phylogenetic relationships among dinosaurs. Part of this oversight may be due to the absence of nomenclature to describe the parts of the sacrum. Here I present a nomenclature for the sauropod sacrum that is designed to aid description of the complex structures involved and to facilitate development of novel character data. Although based on sauropods, it is applicable to closely related taxa, such as other sauropodomorphs and theropods. 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 481091079, U.S.A. ([email protected]) INTRODUCTION Owen (1842: 102-103) diagnosed the original triumvirate of Dinosauria, which he considered “gigantic Crocodile-lizards of the dry land,” by “a large sacrum composed of five anchylosed vertebrae of unusual construction.” Since that time, the sacrum has furnished mostly meristic characters, which have been applied at higher and lower taxonomic levels, but little of its “unusual construction” has contributed to traditional taxonomies or numerical cladistic analyses of dinosaurs. Here I provide a nomenclature for the sacrum of sauropod dinosaurs that is intended to facilitate description of the complex elements involved and foster development of new phylogenetic characters. I begin by discussing the definition of the sacrum and changing opinions on the bony elements that it comprises. I then briefly examine the contribution of sacral characters to both traditional taxonomy and cladistic analysis before presenting a nomenclature for the units, junctions, apertures, and vertebral laminae of the sacrum in sauropod dinosaurs. DEFINITION AND COMPONENTS OF THE SACRUM The word ‘sacrum’ is derived from the Latin term sacer, which means holy thing or temple (Brown, 1954). ‘Sacrum’ is itself a translation of the Greek word hieron, meaning sacred or large (Oscar, 1987). Most explanations of its adoption into human anatomy have focused on these two meanings, either relating to: (1) its use in sacrificial rituals or its proximity to organs of procreation; or (2) its relatively large size (Singer, 1954). More recently, Oscar (1987) has suggested that ‘sacrum’ and ‘hieron’ draw their sacred meanings from Egyptian mythology, specifically the story of Osiris. After being killed by his brother Set, Osiris was resurrected by his sister and wife Isis. Set killed Osiris a second time and then chopped his body into pieces that were scattered throughout the Levant. Osiris was then resurrected a second time by Isis from his sacrum. In Egyptian symbology, Osiris’ sacrum is represented by the djed column, which is an elongate, columnar structure resembling the sacrum and lumbar vertebrae of an ox (Budge, 1918; Gordon and Schwabe, 2004: fig. 6.8). The role of the djed in burial rituals preparing the dead for the afterlife may have contributed to the sacred origins of the term ‘sacrum.’