A previously unreported form of dorsal rib pneumaticity in Apatosaurus (Dinosauria: Sauropoda) and implications for pneumatic variation among diplodocid dorsal ribs

ABSTRACT

Postcranial pneumaticity is interpreted as a weight saving adaptation in sauropod dinosaurs, especially in the vertebral column. In some derived sauropods pneumatic features also occur on vertebral ribs. While pneumatic ribs are considered diagnostic of the clade Titanosauriformes, they are also infrequently found in diplodocid sauropods. Here, we describe a partial dorsal rib IV or V referable to Apatosaurus sp. that exhibits a series of superficial pneumatic fossae along its posterior surface. These fossae differ from the morphology found in other pneumatic dorsal ribs of diplodocids, including other apatosaurines and Supersaurus. Moreover, the pneumatic features of this rib are more distally located from the capitulum and tuberculum than in other diplodocids and titanosaurs. Based on our findings, we propose that rib pneumaticity among apatosaurine sauropods (and potentially all diplodocids) is individually variable, in addition to being a function of ontogeny. More broadly, we conclude that rib pneumaticity among diplodocids is morphologically variable when present and individually expressed rather than being ubiquitous throughout the clade. Our findings are consistent with the hypothesis that pneumatic ribs evolved independently between Diplodocidae and Titanosauriformes and make for poor clade-level characters among diplodocids.

ACKNOWLEDGMENTS

The authors are deeply appreciative to the volunteers and summer field crews of the Museums of Western Colorado. We also extend our thanks to the Dinamation International Society for their efforts as well as for their work in excavating and recovering specimens from Mygatt-Moore Quarry. The work at Mygatt-Moore Quarry would not be possible without the cooperation and access granted by the Bureau of Land Management, for which the authors are also thankful. We also thank Daniela Schwarz, Michael D’Emic, Yan Yin, and one anonymous reviewer for their time and helpful comments on this manuscript.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

AUTHOR CONTRIBUTIONS

L. K., J. M., and M. W. devised the project. L. K. described the material and wrote the first draft of the manuscript. J. M. provided access to the specimen. M. W. provided background information and specialist information about pneumaticity. All authors provided input and writing to the final draft.