A specimen-level cladistic analysis of Camarasaurus (Dinosauria, Sauropoda) and a revision of camarasaurid taxonomy

Tschopp, E., Mateus O., Kosma R., Sander M., Joger U., & Wings O. (2014).  A specimen-level cladistic analysis of Camarasaurus (Dinosauria, Sauropoda) and a revision of camarasaurid taxonomy. Journal of Vertebrate Paleontology. Program and Abstracts, 2014, 241-242.

Camarasaurus is considered one of the best known sauropod dinosaurs from the Upper Jurassic Morrison Formation of the USA. Numerous finds are referred to four widely accepted species: C. supremus (type species), C. grandis, C. lentus, and C. lewisi. The osteology of the genus is considered completely known, but this knowledge is mostly based on specimens referred to Camarasaurus without using phylogenetic methods. The state of the holotypic material is often deemed unfavorable for phylogenetic methods. Type specimens were found mingled with other specimens (AMNH 5760 with 5761; YPM 1901 with 1902 and 1905), and two species are based on juvenile material (C. grandis, C. lentus). Phylogenetic studies thus generally include Camarasaurus as genus. As such, intrageneric variation is excluded a priori, and the possibility that some specimens used for scoring might be erroneously referred to the genus is ignored. In order to assess the species taxonomy of Camarasaurus, a specimen-level cladistic analysis was performed with all holotype specimens formerly proposed to belong to Camarasaurus, and the most complete referred skeletons. The ingroup counts more than 20 specimens, all but one (GMNH-PV 101) scored based on personal observations. Outgroup taxa cover early eusauropods, Diplodocoidea and titanosauriforms. The final cladogram shows the classical type specimens of Camarasaurus as sisterclade to a group with the Cathetosaurus type specimen, and the specimens GMNH-PV 101 and SMA 0002. In order to assess taxonomic issues, two earlier proposed numerical approaches were tested: apomorphy counts between sister-clades (proposed in diplodocids), and character dissimilarity (proposed in plesiosaurs). Additionally, homoplastic rates for the diverging characters were incorporated in the apomorphy counts, in order to include a value for taxonomic significance of the trait in question. These methods support the validity of the three historic Camarasaurus species (C. supremus, C. grandis, C. lentus), and corroborate proposals for the re-establishment of Cathetosaurus as distinct genus. The revised character scoring for genera and species refines their diagnoses, and will help to resolve the phylogenetic position of unstable taxa like Europasaurus, Jobaria, or Haplocanthosaurus.