One of the few sauropods I am in anyway familiar with is Giraffatitan (formerly Brachiosaurus) and as a result I have developed more of an interest in the brachiosaurs than other sauropods. I was excited to learn that a new taxon had recently been unearthed and described and, against the odds, there’s good skull material. Lead author on the study, Dan Chure, was gracious enough to put together a guest blog post on the subject and here is it:
An Ancient Treasure Chest of Skulls
When the Carnegie Museum was excavating at the site that later became Dinosaur National Monument, they found a nicely preserved skeleton of the sauropod Barosaurus. Starting at the base of the tail they excavated along the pelvis, dorsals, scapulocoracoids, and the cervical series all the way up to cervical 2, which was exposed at the top of the sandstone cliff. Had the skull been there and destroyed by erosion or was it lost before the skeleton was buried in the river sands? We’ll never know. It was another headless wonder — and to this day no Barosaurus skull has been found. This is a fine example of a major problem with sauropod research, a dearth of skulls.
Skulls are complex structures yet built of surprisingly thin bones. They can tell us a great deal about the evolution of a dinosaur, its relationships, and its paleobiology. For many groups of dinosaurs skull characters play a very important role in phylogenetic analysis. Yet in sauropods well preserved skulls are almost unknown and out of necessity, their phylogeny is based heavily on postcranial features. There are approximately 120 valid species of sauropods, ranging from the Late Triassic to the end of the Cretaceous, yet only between 8 and 12 have complete skulls (depending on where you draw the line on “complete”). Of those known, the vast majority is from the Jurassic. One could easily go through an entire paleontological career without finding one. It is just one of the rules of the game for sauropods.
However, at Dinosaur we’ve been able to break the curse, so to speak, for at least one sauropod, and one that’s new to science to boot. The Early Cretaceous Cedar Mountain Formation is rich in fossils in Utah and the focus of considerable research effort by several institutions. However, in Dinosaur it is singularly fossil poor, with one exception. Locality literally across the road from the famous Carnegie Quarry and some 40 million years younger in age, DNM 16 has proven to be a site of exceptional significance.
Other than a few small shed theropod teeth, DNM 16 has yielded only sauropod remains. Although we don’t find intact skeletons, we do find major subunits of the skeleton — articulated humerus, radius, ulna, and manus; femur, tibia, fibula, astragalus, and metatarsals; sacrum, pelvis, and caudal vertebrae. All told, parts of most of the skeleton are now known. However, the gems of the quarry, and certainly the greatest surprise at DNM 16, are the skulls.
To date, two incomplete and two incomplete skulls have been discovered. Of the complete ones, one is three dimensional and only slightly compressed. The other is complete but disarticulated so that each bone can be studied from any angle and any view. Of the incomplete ones the first is the snout in front of the orbits slightly crushed, and the fourth, more in keeping with typical sauropod skull discoveries, is a braincase and skull roof.
All the sauropod material from DNM 16 belongs to a single taxon belonging to a new genus and species, Abydosaurus mcintoshi. Phylogenetic analysis indicates that its closest known relative is the late Jurassic Brachiosaurus. This is not too surprising given that a marked change in sauropod faunas occurs at the Jurassic Cretaceous boundary. In North America the Late Jurassic sauropod fauna is very well known from the Morrison Formation. That fauna is dominated by diplodocid sauropods (Apatosaurus, Barosaurus, and Diplodocus) and the maconarian Camarsaurus. The “cetiosaurid” Haplocanthosaurus is rare, but known from several specimens. Brachiosaurus, first described from material in the Morrison, is among the rarest of sauropods in the formation.
Jump ahead 40 million years to the Cedar Mountain formation and the picture is quite different. Diplodocids are extinct, not just in North America but worldwide. Titanosauriform sauropods are the ascendant and will be the only group of sauropods throughout the Cretaceous, diversifying to fill the many niches vacated by the dipldocoids. In the Cedar Mountain Formation brachiosaurid sauropods have replaced diplodocids and several genera have been described. However, the reasons for dramatic change are poorly understood. This is in part due to the fact that an unconformity of 20-30 million years marks the boundary between the Morrison and the overlying Cedar Mountain. During that missing period of time the faunal change occurred.
Chure, Daniel, Brooks B. Britt, John A. Whitlock and Jeffrey A. Wilson. 2010. First complete sauropod dinosaur skull from the Cretaceous of the Americas and the evolution of sauropod dentition. Naturwissenschaften (online, unpaginated). doi:10.1007/s00114-010-0650-6
More coverage of this new taxon over at SV-POW!, and more sauropods to come soon courtesy of Phil Mannion, stay tuned!