Thoughts on Danison et al. (2024) paper on Saurophaganax

Diplodocids have been previously documented from the Morrison Formation of Cimarron County, western Oklahoma, thanks to postcranial remains described by Stovall (1938) and Carpenter & McIntosh (1994), and while most of these specimens are juvenile, dorsal vertebra OMNH 1670 reported by Stovall (1938) represents a giant individual compared to other apatosaurine specimens found in western Oklahoma. Now, Danison et al. (2024) have published a paper demonstrating that the material assigned to Saurophaganax maximus Chure, 1995 includes remains from the caudal region belonging to a diplodocid and that the holotype dorsal neural arch OMNH 1123 is more likely to be a sauropod rather than a theropod. 

Holotype dorsal neural arch of Saurophaganax maximus (from Danison et al. 2024)

The morphological criteria employed by Danison et al. (2024) evaluating the systematic placement of the holotype dorsal neural arch of Saurophaganax maximus is quite breathtaking. As noted by these authors, the accessory lamination present in this specimen is typical of sauropods and while upturned diapophyses are seen in theropods, this feature in OMNH 1123 is also present in juvenile diplodocid vertebrae. In his paper describing Saurophaganax maximus, Chure (1995) considered the accessory laminae autapomorphic for Saurophaganax but did not compare OMNH 1123 to the dorsal vertebrae of any sauropods from the Morrison Formation. By comparing OMNH 1123 with Apatosaurus dorsal vertebra OMNH 1366 (also found in western Oklahoma), Danison et al. effectively leave open the possibility that OMNH 1123 could be a diplodocid sauropod when taking serial position and ontogeny into account, bearing in mind the fact that no valid theropod taxon from the Morrison Formation has accessory laminae on the vertebrae. 

The re-identification by Danison et al. of six mid-caudal chevrons (OMNH 1102, 1104, 1438, 1439, 1684, and 1685) assigned to S. maximus as belonging to Diplodocidae is worthy of attention. As pointed out by Dansion et al., no quarry map for Kenton Quarry 1 (type locality for Saurophaganax maximus) is known to exist, calling into question Chure's (1995) referral of the chevrons to the same taxon as OMNH 1123 and the allosaurid remains for which the authors erect the name Allosaurus anax. Since there are duplicate hindlimb elements assigned to the S. maximus hypodigm, it made perfect sense for Danison et al. to evaluate the question of whether the mid-caudal chevrons represent an individual different from OMNH 1123. The fact that Danison et al. assign the atlas OMNH 1135 to Neosauropoda indeterminate is quite noteworthy given the presence of Camarasaurus in Kenton Quarry 1 (Carpenter and McIntosh 1994), which shows that two different neosauropod clades co-exist in the Morrison Formation of western Oklahoma. In other words, the atlas and mid-caudal chevrons constitute the new addition to the fossil record of Morrison sauropods from western Oklahoma.

References:

Carpenter, K., and McIntosh, J., 1994. Upper Jurassic sauropod babies from the Morrison Formation. pp. 265-278. In Carpenter, K., Hirsch, K., and Horner, J., (eds.) Dinosaur Eggs and Babies. New York, NY: Cambridge University Press.

Chure, D.J., 1995. A reassessment of the gigantic theropod Saurophagus maximus from the Morrison Formation (Upper Jurassic) of Oklahoma, USA; pp. 103–106 in A. Sun and Y. Wang (eds.). Sixth Symposium on Mesozoic Terrestrial Ecosystems and Biota, Short Papers. China Ocean Press: Beijing, China.

Danison, A., Wedel, M., Barta, D., Woodward, H., Flora, H., Lee, A., and Snively, E., 2024. Chimerism in specimens referred to Saurophaganax maximus reveals a new species of Allosaurus (Dinosauria, Theropoda). Vertebrate Anatomy Morphology Palaeontology 12 (1). https://doi.org/10.18435/vamp29404

Stovall, J.W., 1938. The Morrison of Oklahoma and its dinosaurs. Journal of Geology 46:583-600.