Extinction Event Caused Sauropod Hiatus?

There has always been something about the Cloverly Formation that I have found fascinating. I have to confess that I am not quite sure when this fascination began or why but whenever there are new discoveries or literature to be assessed regarding these units I am always eager to find out as much as possible.

The Cloverly Formation is an Early Cretaceous formation comprised of non-marine strata that stretch from Wyoming to Montana and has revealed a diverse community of vertebrate fossils that are, of course, dominated by dinosaurs. The most famous residents of these beds are unquestionably Deinonychus and Tenontosaurus but there are many more specimens that have come to light over the years and some of these are giants.

Michael D’Emic, of Georgia Southern University, has featured here on this blog a few times now and with Brady Foreman, of the University of Wyoming, has just published a paper in the latest edition of the Journal of Vertebrate Paleontology focussing on new insights into the origins of the so-called sauropod hiatus that occurred during the Early Cretaceous.

The absence of sauropod remains from the mid-Cenomanian until the Maastrichtian has proven to be problematic with no satisfactory explanation.  Suggestions as to why this hiatus took place include an extinction event brought about by the expansion of the Western Interior Seaway, competition from more advanced herbivores such as hadrosauroids who, in turn, were evolving at the same time as another suggested factor in the disappearance of sauropods  – the angiosperms.

Not everyone has been convinced by this selective extinction process and suggest that it may simply be a sampling bias (Mannion & Upchurch 2010, 2011) in as much that there is no evidence for sauropods during this period simply because the environments that they preferred were not conducive for fossilisation. To help try and solve the mystery, the authors decided to re-examine material from North American sauropods from before the hiatus and have come up with some pretty interesting results.

Material referred to Paluxysaurus jonesi, Sauroposeidon proteles and other undiagnosed elements were reappraised. It appears that differences between Paluxysaurus and Sauroposeidon are minor and that some characters support the synonymising of the two taxa, with Sauroposeidon taking nomenclatural priority. For example, autapomorphies are displayed in the centrum of the mid-cervical vertebrae, the spinoprezygapophyseal laminae of the anterior caudal vertebrae and the morphology of the scapula in Paluxysaurus – all referable to Sauroposeidon.

Other material is still difficult to assess. Teeth, by way of example, display characters found in brachiosaurids and titanosaurs, as well as Sauroposeidon. Other elements including vertebrae, limb bones and some juvenile material also remain undiagnosed. Indeed, the authors’ report one caudal vertebra has been mis-identified as belonging to a sauropodomorph but is actually more likely to belong to Tenontosaurus.

Since this research has highlighted the likelihood that Paluxysaurus and Sauroposeidon are one and the same taxon, cladistic analysis (D’Emic, in press) has recovered Sauroposeidon, perhaps a little surprisingly, as a basal somphospondylan. Sauroposeidon lacks certain synapomorphies of both brachiosaurids and titanosaurs yet displays other characters attributable to the Somphospondyli. This supports a possible Laurasia-Gondwana faunal interchange which, the authors point out, compliments current thinking on the origins of Acrocanthosaurus (Brusatte & Sereno 2008).

This research also leaves a dearth of evidence in support of Early Cretaceous titanosaurs in North America. The authors suggest the first proper evidence for titanosaurs on the continent resides with the fossils of Alamosaurus from the Maastrichtian and, the authors point out that the lack of titanosaurs pre-hiatus also renders any sampling bias, as a method of explaining the hiatus, unlikely since it appears that titanosaurs were never there in the first place.

This line of research, therefore, favours an extinction event during the mid-Cretaceous to explain the sauropod hiatus. It is worth pointing out, as the authors do, that two other groups also disappear at the same time – allosauroids and basal iguanodontians and this is despite the fact that it appears that their habitat remained stable long after they had disappeared.

The authors favour, with appropriate caveats, that a combination of the transgressions of the Western Interior Seaway and competition from hadrosauroids were likely factors contributing to the sauropods extinction. There is evidence for a fall in sea temperature and a not insignificant extinction of marine invertebrates during the transgression. However, what this actually means and how it relates to the sauropod hiatus is unclear at this time.

Hadrosauroids were beginn ing to flourish and diversify at this point and despite what would appear to be two groups of animals which display widely differing feeding techniques, the authors submit that competition may have arisen between the two groups at different stages throughout ontogeny. I thought that this was a really interesting point and was something that is easily overlooked when considering competition between groups since we tend to consider only adult animals when the subject is discussed.

Different ontogenetic growth stages in large dinosaurs would inevitably demand different food resources and the fast evolving hadrosauroids, with their relatively advanced feeding and “chewing” techniques would make for stern competition for the sauropods. And the fact is that the Early Cretaceous sauropods did indeed disappear at the same time as these hadrosauroids were proliferating.

This combination of transgression events, hadrosauroid diversity and sauropod disappearance may indeed be linked but there is much more research and sampling to be done before a conclusive answer can be decided upon. And I expect some counter arguments to be published in the not too distant future too.  Mannion and Upchurch (2010, 2011) came in for particular scrutiny in this paper and it will be interesting to hear and/or read their latest thoughts on the subject.

References

Brusatte, S. L., and P. C. Sereno. 2008. Phylogeny of Allosauroidea (Dinosauria:Theropoda): comparative analysis and resolution. Journal of Systematic Palaeontology 6:155–182. 

Mannion, P. D., and P. Upchurch. 2010. A quantitative analysis of environmental associations in sauropod dinosaurs. Paleobiology 36:253–282. 

Mannion, P. D., and P. Upchurch. 2011. A re-evaluation of the ‘mid-Cretaceous sauropod hiatus’ and the impact of uneven sampling of the fossil record on patterns of regional dinosaur extinction. Palaeogeography, Palaeoclimatology, Palaeoecology 299:529–540. 

Michael D. D'Emic & Brady Z. Foreman (2012): The beginning of the sauropod dinosaur hiatus in North America: insights from the Lower Cretaceous Cloverly Formation of Wyoming, Journal of Vertebrate Paleontology, 32:4,883-902.