A great amount of the sauropod record is based on caudal vertebrae. Morphological analyses of tail bones until recently were essentially focused on centrum shape, with other anatomical features being poorly studied. A detailed description of anterior caudal transverse processes (ACTP) is presented here in order to improve the scarce knowledge of this tail part. Within Sauropoda there are four different ACTP morphological types; one simple (single element projected on the vertebral side) and three complex (lateral bony laminar plates supported by bony bars). The identity of the ACTP elements includes a rib, a synapophysis and laminar components, which are renamed here. The ventral laminae include the anterior centroparapophyseal lamina (acpl) and the posterior centrodiapophyseal lamina (pcdl), whereas the dorsal ones are the prezygodiapophyseal lamina (prdl) and the postzygodiapophyseal lamina (podl) when present. ACTP morphology is not informative for higher-level phylogeny, but at lower levels the in-group relationships of Diplodocoidea can be improved through analyzing these structures. ACTP morphological variation between the first and consecutive caudal vertebrae should presumably be interpreted as a consequence of the relative development of the M. caudofemoralis brevis. The "sacralization" of the ACTP is related to the attachment of soft tissues associated with the tail, among which the Mm. caudofemorales should be the most important muscle group.
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