Sunday, 1 July 2012

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.

3 comments:

Hadiaz said...

"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."

Maybe b/c (like me) the Clovery Formation reminds you of the African Savannah w/brachiosaurs & ankylosaurs in place of elephants & rhinos as the dangerous herbivores, Tenontosaurus in place of zebra & wildebeest as the mid-sized herbivore, Deinonychus in place of hyenas & jackals as the mid-sized predator, Acrocanthosaurus in place of lions as the apex predator, etc.

BTW, who's the artist responsible for this post's pic? The style looks familiar.

-Herman Diaz

Mark Wildman said...

Hi Herman. The picture is part of a magnificent mural in a building display demonstrating the plants (well at least their extant equivalents) that shared the dinosaurs environment.

The display is part of the Botanical Garden in Lisbon which is next door to the Natural History Museum. Generally speaking the garden is in a poor condition although there are still some mightily impressive displays to be seen.

I did look for the artists name but it was either hidden from view or I missed it. Either way, it is a super mural and fairly recent as well since it features Concavenator - published in 2010.

Charles Weber said...

The Cretaceous ocean predators were very large. I suspect that the productivity implied by this was caused by a flow of phosphorus toward the ocean from the savannas (seasonal rainfall areas) permitted by erosion of phosphorus rich runways of plant smothering termites in the Amitermitinae starting in late Jurassic in Australia where the first ocean phosphorite deposits occurred. Anoxic conditions in the oceans were also probably caused by this. This anoxic bottom condition probably helped reduce the ammonites also, in addition to competition from phosphorus enhanced vertebrates. The savanna herbivore dinosaurs declined in armor, teeth, and quite a bit in bony structure across the Cretaceous outside of South America, especially in southeast Asia. Many even lost teeth. I suggest it was due to this same phosphorus famine created by erosion of the soil of the runways of plant smothering termites. Pterosaurs and birds probably lost teeth primarily because of the young eating iron oxide and bauxite in the flying reproductive soil borne termites’ guts, which bound the phosphates. You may see this discussed in more detail starting in http://www.angelfire.com/nc/isoptera/termites.html and its links, which links explore the possible affect that ant evolution had upon them. You may see a journal article in http://gsjournal.net/Science-Journals/Essays-Paleontology/Download/4717 . By the time the Cretaceous ended the world ended up with tiny savanna vertebrates, most of them mammals, which were able to give their young phosphorus in milk at that critical stage. They were a far cry from the massive, well boned Stegosaurs, etc., which roamed around the Jurassic, and had diminished tooth structure at first. They were a long time starting to increase in size (several million years).
You may see the affects on soil discussed in more detail in http://www.angelfire.com/nc/isoptera/soil.html .
Sincerely, Charles Weber

PS It is conceivable that you would also find interesting a hypothesis of my son explaining the Decca (or Deccan) lava flows as disruption of the crust by the disruption of the crust at the antipode (opposite side of a sphere) by a huge meteorite impact. You may see my version in http://charles_w.tripod.com/antipode.html .
Sincerely, Charles Weber

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