Veracity of Ikejiri's (2005) biochronology for Camarasaurus

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 However, unpublished cladistic results in Tschopp et al.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. 

About the Transylvanian titanosaur paper by Diez Diaz et al. (2025)

The lithostrotian titanosaur Magyarosaurus has long been significant for demonstrating the presence of dwarf titanosaurs in the latest Cretaceous of Europe, but until recently an up-to-date re-appraisal of the titanosaur material referred to Magyarosaurus in the past was lacking, although the description of the similarly dwarf-sized Paludititan by Csiki et al. (2010) signaled that more than one taxon of titanosaur existed in the Late Cretaceous of Romania. Now, Diez Diaz et al. (2025) have come out with the long-overdue revision of nominal titanosaur taxa assigned to Magyarosaurus from the latest Cretaceous of Transylvania, confirming suspicions about more than one genus being represented in the material referred to Magyarosaurus by von Huene (1932).

Lectotype and paralectotype of Petrustitan hungaricus (NHMUK R.3853) (from Diez Diaz et al. 2025)

As a prelude to the "Systematic palaeontology" section of their paper, Diez Diaz et al. provide a relatively short overview of a number of publications making the case for the existence of more than one titanosaur species in Transylvania, including Stein et al. (2010) and the paper by Csiki et al. (2010) describing the new titanosaur taxon Paludititan nalatzensis. They note that McIntosh (1990) found it conceivable that more than one taxon is represented in the titanosaur material assigned by von Huene (1932) to Magyarosaurus, and the paper by Mocho et al. (2023) identifying four distinct titanosaur caudal morphotypes from the Hațeg Island region provides the authors with an impetus to clarify the taxonomy of Transylvanian titanosaurs based on overlap and shared characters between type specimens and referred material for Magyarosaurus dacus, M. hungaricus, and M. transsylvanicus.

The assortment by Diez Diaz et al. of name-bearing and "Rosetta Stone" Transylvanian titanosaur specimens into "individuals" and "assemblages" in quite creative. The authors rely on locality data for these specimens and specimen preservation and registry notes when labeling the type material of Magyarosaurus dacus as an assemblage and the type remains of Petrustitan hungaricus as an individual while referring to a number of non-type specimens from select localities with duplicate elements as assemblages. This is important because the type material of P. hungaricus comprises only a left fibula and tibia, the former which nevertheless allowed for Diez Diaz et al. (2025) to compare NHMUK R.3853 to Magyarosaurus dacus by overlapping with the fibulae included in type material for M. dacus. Paradoxically, Diez Diaz et al. neglect to designate one of the fibulae included in the hypodigm for Magyarosaurus transsylvanicus by von Huene (1932) as the lectotype of transsylvanicus even though they demonstrate that the hypodigm for this species is a composite of Magyarosaurus and indeterminate titanosaur remains, with the fibulae belonging to M. dacus and the vertebrae and forelimb remains being indeterminate beyond Titanosauria.

The phylogenetic analysis of Transylvanian titanosaurs by Diez Diaz et al. (2025) is quite breathtaking. Although Cury Rogers (2005) recovered Magyarosaurus as a lithostrotian titanosaur, her cladistic analysis found that taxon to be of uncertain placement within Lithostrotia. Given that Magyarosaurus is a dwarf taxon, its placement within Saltasauridae by Diez Diaz et al. indicates that Magyarosaurus itself arose from African saltasaurids that evolved insular dwarfism after colonizing island chains in southern Europe. The authors' recovery of Paludititan as closely related to or within Lognkosauria differs from the placement of Paludititan as a member of Lirainosaurinae in the phylogeny by Mocho et al. (2024), On the other hand, Nemegtosaurus falls outside Saltasauridae in the cladistic analysis by Diez Diaz et al. (2025), while the equal weights analysis finds Malawisaurus to be sister to a clade composed of Andesaurus, Baotianmansaurus, Daxiatitan, Dongyangosaurus, Huabeisaurus, and Xianshanosaurus. The placement of Nemegtosaurus outside Saltasauridae is most likely a fluke because it is coeval with Opisthocoelicauda, but the Diez Diaz et al. cladistic analysis is in lockstep with the phylogeny in Han et al. in placing Huabeisaurus as a late-surviving basal titanosaur. The recovery of Petrustitan as sister to Antarctosaurus, Vahiny, and Jainosaurus could attest to Antarctosaurus-like titanosaurs evolving insular dwarfism after entering Europe from North Africa because Diez Diaz et al. (2025) assess Petrustitan as a dwarf titanosaur. 

In summary, the paper by Diez Diaz et al. (2025) is kind of a magnum opus with respect to the alpha-taxonomy of Transylvania's titanosaur fauna because it re-evaluates all nominal referred species of Magyarosaurus in light of the description of Paludititan over a decade ago. Moreover, the authors of this paper demonstrate that the titanosaur fauna from Hațeg Island comprises both insular dwarfs and large-bodied forms judging from the large size of the newly described taxon Uriash. By refining the cladistic position of the Transylvanian titanosaurs, this paper will be useful for future discussions of the biogeography of derived titanosaurs as they relative to the inter-relationships of lithostrotians. 

References:

Curry Rogers, K. A., 2005. Titanosauria: A Phylogenetic Overview. pp. 50-103. In: Curry Rogers and Wilson (eds), The Sauropods: Evolution and Paleobiology. University of California Press: Berkeley.

Csiki, Z., Codrea, V., Jipa-Murzea, C., and Godefroit, P., 2010. A partial titanosaur (Sauropoda, Dinosauria) skeleton from the Maastrichtian of Nǎlaţ-Vad, Haţeg Basin, Romania. Neues Jahrbuch fur Geologie und Palaontologie – Abhandlungen 258 (3), 297–324.

Díez Díaz, V., Mannion, P.D., Csiki-Sava, Z., and Upchurch, P., 2025. Revision of Romanian sauropod dinosaurs reveals high titanosaur diversity and body-size disparity on the latest Cretaceous Hațeg Island Island, with implications for titanosaurian biogeography. Journal of Systematic Palaeontology 23 (1): 2441516. doi:10.1080/14772019.2024.2441516

Han, F., Yang, L., Lou, F., Sullivan, C., Xu, X., Qiu, W., Liu, H., Yu, J., Wu, R., Ke, Y., Xu, M., Hu, J., and Lu, P., 2024. A new titanosaurian sauropod, Gandititan cavocaudatus gen. et sp. nov., from the Late Cretaceous of southern China. Journal of Systematic Palaeontology 22 (1): 2293038. doi: https://doi.org/10.1080/14772019.2023.2293038.

McIntosh, J.S., 1990. Sauropoda. pp. 345-401. In: Weishampel, D.B., Dodson, P., and Osmólska, H., (eds.) The Dinosauria, 1st edition. Berkeley: University of California Press. 

Mocho, P., Pérez-García, A., and Codrea, V. A., 2023. New titanosaurian caudal remains provide insights on the sauropod diversity of the Hațeg Island (Romania) during the Late Cretaceous. Historical Biology 35 (10): 1881–1916.

Mocho, P., Escaso, F., Marcos-Fernández, F., Páramo, A., Sanz, J. L., Vidal, D., and Ortega, F., 2024. A Spanish saltasauroid titanosaur reveals Europe as a melting pot of endemic and immigrant sauropods in the Late Cretaceous. Communications Biology 7: 1016. doi:10.1038/s42003-024-06653-0

Stein, K., Csiki, Z., Curry, K., Weishampel, D. B., Redelstorff, R., and Carballido, J. L., 2010. Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria). Proceedings of the National Academy of Sciences of the United States of America 107 (20): 9258–9263.

von Huene, F., 1932. Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte. Monographien zur Geologie und Palaontologie, Series 1, 4: 1–361.