Is Caulodon a synonym of Camarasaurus?

The non-titanosauriform macronarian Camarasaurus is one of the American public's favorite macronarian sauropods from the Jurassic, being thought of as the most common macronarian from the Morrison Formation. Very few sauropod workers, however, have tackled the alpha-taxonomy of the genus and its various constituent taxonomy since the papers by Ikejiri (2005) and Osborn and Mook (1921). The result has been the tendency to treat Camarasaurus as a genus in numerous phylogenies of Sauropoda (Upchurch et al. 2004; Wilson 2002) without a comprehensive analysis of intrageneric variation of the genus.

In recent abstracts published at the SVP 2013 and 2014 meetings, Mateus and Tschopp (2013) and Tschopp et al. (2014) work to address the question of intraspecific variation in Camarasaurus. The abstract by Mateus and Tschopp (2013) deals with a newly discovered camarasaurid (SMA 0002) that is referred to Cathetosaurus lewisi based on shared characters with BYU 9047 (holotype of C. lewisi). On the other hand, Tschopp et al. (2013) conduct a specimen-level analysis of all Camarasaurus specimens including the type specimens of currently recognized species of the genus (C. grandis, C. lentus, and C. supremus). The phylogenetic results reported by Tschopp et al. (2014) seem to not only reaffirm the distinctness of Cathetosaurus from Camarasaurus, but also support the validity of the three historical species of the latter genus.

These preliminary results have prompted me to revisit the synonymy of Caulodon with Camarasaurus performed by Osborn and Mook (1921) and followed by subsequent authors. When placing the two nominal species of Caulodon (C. diversidens and C. leptoganus) in synonymy with Camarasaurus supremus, Osborn and Mook noted that AMNH 5768 and AMNH 5769 were similar to the maxillary teeth of a referred specimen of Camarasaurus supremus (AMNH 5761) in being robust and spatulate-shaped, but were hesitant to rule out the possibility of two different sauropods with spatulate teeth from the Morrison Formation. However, they did not compare the holotypes of Caulodon diversidens and C. leptoganus with the teeth of Giraffatitan or USNM 5730 (referred to Brachiosaurus sp. by Carpenter and Tidwell 1998).

Holotype teeth of Caulodon diversidens (AMNH 5768) (after Osborn and Mook 1921)

Since the 1921 monograph by Osborn and Mook, robust spatulate teeth have been described for several non-neosauropod and macronarian sauropods, including Europasaurus, Mamenchisaurus, Turiasaurus, Jobaria, and the brachiosaurid skull USNM 5730 (Carpenter and Tidwell, 1998; Marpmann et al. 2014; Ouyang and He 2002; Royo-Torres and Upchurch 2012; Russell and Zheng 1994; Sereno et al. 1999). Given that robust spatulate teeth are no longer considered diagnostic for Camarasaurus supremus or other camarasaurid species and are widely distributed among non-neosauropod and non-titanosauriform sauropods, the synonymy of Caulodon and its nominal species with Camarasaurus supremus should be regarded as untenable and the two Caulodon species instead must be considered nomina dubia at Eusauropoda indeterminate.

     
References:

Carpenter, K., and Tidwell, V., 1998, Preliminary description of a Brachiosaurus skull from Felch Quarry 1, Garden Park, Colorado. In: Carpenter, K., Chure, D. J., and Kirkland, J. I. (eds.), The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Modern Geology 23 (2): 69-84.

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.

Marpmann, J., Carballido, J., Sander, P., Knötschke, N. 2014. Cranial anatomy of the Late Jurassic dward sauropod Europasaurus holgeri (Dinosauria, Camarasauromorpha): ontogenetic changes and size dimorphism. Journal of Systematic Paleontology. DOI: 10.1080/14772019.2013.875074

Mateus, O., & Tschopp E. (2013). Cathetosaurus as a valid sauropod genus and comparisons with Camarasaurus. Journal of Vertebrate Paleontology, Program and Abstracts: 173.

Osborn, H. F., and Mook, C. C., 1921, Camarasaurus, Amphicoelias, and other Sauropods of Cope: Memoris of the American Museum of Natural History, new series, v. 3, part 3, p. 249-387.

Ouyang, H., and Ye, Y., 2002, The first mamenchisaurian skeleton with complete skull: Mamenchisaurus youngi: Sichuan Science and Technology Press: Chengdu. 111pp.

Rafael Royo-Torres & Paul Upchurch, 2012, The cranial anatomy of the sauropod Turiasaurus riodevensis and implications for its phylogenetic relationships. Journal of Systematic Palaeontology, 10(3): 553-583.

Russell, D. A., and Zheng, Z., 1994, A large mamenchisaurid from the Junggar Basin, Xinjiang, People’s Republic of China: In: Results from the Sino-Canadian Dinosaur Project. Canadian Journal of Earth Sciences 30: 2082-2095.

Sereno, P, C., Beck, A. L., Dutheil, D. B., Larson, H. C. E., Lyon, G. H., Moussa, B., Sadleir, R. W., Sidor, C. A., Varricchio, D. J., Wilson, G. P., and Wilson, J. A., 1999, Cretaceous Sauropods from the Sahara and the Uneven Rate of Skeletal Evolution Among Dinosaurs. Science 286: 1342-1347.

Steel, R., 1970, Saurischia: Handbuch der Palaoherpetology. Volume 14. Gustav Fischer Verlag.

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.

Upchurch, P.M., Barrett, P.M., and Dodson, P. (2004). Sauropoda. pp. 259-322. In: Weishampel, D.B., Dodson, P., and Osmólska, H. (eds.). The Dinosauria (2nd edition). University of California Press: Berkeley. 

Wilson, J. A., 2002, Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136: 217-276.