Pleurocoelus: the birth of excellence
April 24, 2008
Othniel Charles Marsh, who was always careful to base all of his hundreds of new taxa on the best, most diagnostic material available (Alert: Sarcasm detected!), named Pleurocoelus nanus based on a handful of junenile sauropod vertebrae centra from the Arundel clays of Maryland (Marsh 1888). Here’s the dorsal. As you can see, it is loaded with unique features like big pneumatic fossae, which at the time were only known in all other sauropods (we have since found some with less pneumaticity in the dorsals, or none at all), and the absence of a neural arch, which is shared with any sufficiently immature vertebrate.
Here’s a cervical, which was not figured by Marsh (1888). These views are after Lull (1911:pl. 15), as modified by Wedel (2003:fig. 10); pfs stands for pneumatic fossa.
And a sacral, again from Marsh (1888).
To be fair, the criteria for “diagnosably distinct” in the 1880s were different than they are now. Wilson and Upchurch (2003) addressed this in their revision of Titanosaurus: as we find and describe more fossil taxa, characters that originally diagnosed small taxonomic groups (like species and genera) are often found to be more broadly distributed. For example, the original diagnosis of Titanosaurus ended up applying to almost everybody in the clade Titanosauria. It is conceivable that in the future we will discover an entire clade of xenoposeidonids with identical weird dorsals and all of their diagnostic characters elsewhere in the skeleton, and the longish list of weird characters that diagnose Xenoposeidon will turn out to be present in all xenoposeidonids. There’s not much we can do about this, other than to keep working, revisit old diagnoses from time to time and see if they need updating, and generally be nice about it.
I am cool with not being nice about Pleurocoelus, though, because of what happened later. But that’s a story for another post.
Note: In 2005 Carpenter and Tidwell sunk Pleurocoelus into Astrodon, which is totally cool by me, and which makes Astrodon the correct name for the poorly-known Arundel titanosauriform, just like Apatosaurus is the correct name for the Morrison diplodocine that is built like a brick outhouse. But in this series I am Telling a Tale about the Days of Yore, past tense, pre-2005, so I’m using Pleurocoelus.
- Carpenter, K., and Tidwell, V. 2005. Reassessment of the Early Cretaceous sauropod Astrodon johnsoni Leidy 1865 (Titanosauriformes). Pp. 78-114 in. Tidwell, V., and Carpenter, K. (ed.) Thunder-lizards: The Sauropodomorph Dinosaurs. Indiana University Press, Bloomington.
- Lull, R.S. 1911. Systematic paleontology of the Lower Cretaceous deposits of Maryland: Vertebrata. Lower Cretaceous Volume, Maryland Geological Survey, 183–211.
- Marsh, O.C. 1888. Notice of a new genus of Sauropoda and other new dinosaurs from the Potomac Formation. American Journal of Science, 3rd Series 35:89-94.
- Wedel, M.J. 2003. The evolution of vertebral pneumaticity in sauropod dinosaurs. Journal of Vertebrate Paleontology 23(2):344-357.
- Wilson, J.A., and Upchurch, P. 2003. A revision of Titanosaurus (Dinosauria – Sauropoda), the first ‘Gondwanan’ dinosaur genus. Journal of Systematic Palaeontology 1:125–160.