The basal macronarian Camarasaurus is the most common sauropod genus from the Morrison Formation of western North America, with numerous specimens spanning the long vertical range in the Morrison. Twenty years ago, Ikejiri (2005) devised a biostratigraphic framework for Camarasaurus using Turner and Peterson's (1999) idea of stratigraphically correlating Morrison dinosaur quarries, creating five biozones for the genus: (1) No Camarasaurus zone (early-middle Kimmeridgian); (2) Camarasaurus grandis zone (late Kimmeridgian); (3) Camarasaurus lentus zone (late Kimmeridgian-early Tithonian); (4) Transition Zone (Kimmeridgian-Tithonian boundary); and (5) Camarasaurus supremus zone (Tithonian). However, a number of recent papers afford me the opportunity to test the validity of the biochronological framework for Camarasaurus created by Ikejiri (2005).
In his discussion of Camarasaurus biochronology, Ikejiri notes that the holotype of Camarasaurus lentus (YPM 1910) falls within the C. grandis Zone, as does the holotype of Camarasaurus lewisi (BYU 9047), and that the type locality for C. lentus is stratigraphically outside the C. lentus zone. He therefore raises the possibility that C. lentus and C. lewisi could be synonyms of C. grandis, stressing that some characters cited as diagnostic for C. lewisi by a number of authors could result from intraspecific variation. However, unpublished cladistic results in Tschopp et al. (2014) find Camarasaurus lewisi to be a distinct species, and Woodruff and Fowler (2017) provide no info on the stratigraphic position within the Morrison Formation of a juvenile Camarasaurus specimen described by the authors from Montana. Thus, more than one Camarasaurus species is present in the C. grandis zone, and overlap between the C. grandis and C. lentus calls into question the distinctness of these two biochronological zones.
When establishing the No Camarasaurus Zone, Ikejiri (2005) notes that Haplocanthosaurus is the only sauropod genus reported from Dinosaur Zone 1 of Turner and Peterson (1999). Notwithstanding the fact that the presence of Haplocanthosaurus in this zone reported by Turner and Peterson is undescribed, the absence of Camarasaurus from Dinosaur Zone 1 may hold water because of the paucity of dinosaur localities from the lower part of the Salt Wash Member, but also the fact that knowledge of sauropod evolution in North America during the Bathonian-Oxfordian interval is limited. Future studies could demonstrate that any potential camarasaurid discoveries in the lower part of the Salt Wash Member could constitute a taxon generically distinct from Camarasaurus. The Transition Zone is problematic because Ikejiri (2005) refers Camarasaurus specimens from Oklahoma to C. supremus in the text of his paper but lists them as Camarasaurus sp. and possibly referrable to C. supremus in the "Systematic Paleontology" section of his paper. Thus, a re-appraisal of the Oklahoma Camarasaurus material is needed to determine if it referable to C. supremus or C. lentus. The Camarasaurus supremus Zone, however, can be judged to be valid because occurrences of Camarasaurus supremus are restricted to Dinosaur Zone 4 of Turner and Peterson (1999). In their description of the skull of Camarasaurus specimen BHI 6200, Woodruff et al. (2021) mention Camarasaurus supremus as a possible candidate for the identity of BHI 6200, but the authors provided no info on where in the Morrison Formation BHI 6200 comes from, so it is unclear if BHI 6200 is from the C. supremus zone.
In summary, Ikejiri's (2005) biochronology for Camarasaurus is somewhat tenuous, with only the No Camarasaurus Zone and Camarasaurus supremus Zone standing up to scrutiny, and the validity of the C. grandis and C. lentus Zones being undercut by the distinctness of C. lewisi. The Transition Zone does not hold up to scrutiny because the Camarasaurus material found in Oklahoma and used by Ikejiri to infer a biochronological overlap between C. lentus and C. supremus is yet to be re-evaluated, and the No Camarasaurus Zone and Camarasaurus supremus Zone are apparently valid based on current fossil evidence.
References:
Ikejiri, T., 2005, Distribution and biochronology of Camarasaurus (Dinosauria, sauropoda) from the Jurassic Morrison Formation of the Rocky Mountain Region. New Mexico Geological Society, 56th Field Conference Guidebook, Geology of the Chama Basin 2005: 367-379.
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: 241-242.
Turner, C.E. and Peterson, F., 1999. Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western Interior, U.S.A. pp. 77–114. In: Gillette, D.D. (ed.), Vertebrate Paleontology in Utah. Utah Geological Survey Miscellaneous Publication 99-1.
Woodruff, D.C., and Foster, J.R., 2017. The first specimen of Camarasaurus (Dinosauria: Sauropoda) from Montana: The northernmost occurrence of the genus. PLoS One 12:e0177423.
Woodruff, D.C., Wilhite, D.R., Larson, P.L., and Eads, M., 2021. A new specimen of the basal macronarian Camarasaurus (Dinosauria: Sauropoda) highlights variability and cranial allometry within the genus. Volumina Jurassica 19: 109–30. http://dx.doi.org/10.7306/vj.19.5.
