Friday, 16 March 2012

The Amazing Complexity of Laramidia


The March issue of Scientific American contains an article by Scott Sampson in which he describes the latest theories about the dinosaurs of Laramidia. In it, he describes his continuing research in the Grand Staircase-Escalante National Monument in Utah – primarily in the Kaiparowits Formation. Of course, to many of us in the palaeocommunity, and readers of blogs like this and many others, the mysteries of Laramidia are well documented.




To begin with, Laramidia itself is remarkably small compared to the total land mass of the United States today and the most interesting debate centres on how so many different dinosaur taxa managed to survive and proliferate with what would appear to be limited resources.

In addition to that, as more fossils are recovered from the south to compliment the already substantial record from the north, the intrigue increases since the faunas are quite distinct from each other which is highly indicative of faunal endemism. These differences in species range across the board, from the smallest troodontids to the largest hadrosaurs, ceratopsians and tyrannosaurs.  

The notion of a barrier(s) separating the different dinosaur provinces have been hard to prove. For instance, it has been suggested that climatic conditions alone may have been enough to segregate the communities or, as seems more likely, a geological barrier may have perhaps prevented faunal exchange. Initially thought to be a mountain range, Sampson points out that geologists are now suggesting a number of large rivers, flowing in series, may have been enough to have kept the communities apart.

From Sampson et al 2010

Of course, this is assuming that there were only two communities from the north and south but it is likely there may have been more. Add to this that other most mysterious of problems to solve – how on earth was there enough fodder available to provide food for so many mega herbivores on such a relatively small piece of land?

Firstly, there is, without doubt, sufficient evidence to support the hypothesis of faunal provincialism within dinosaurs. At first glance the faunas from both the northern and southern provinces are remarkably similar and yet sufficient morphological differences exist that confirm that the taxa are indeed different although Sampson points out that a specimen of Gryposaurus appears very similar to G. notabilis from Alberta and is still under scrutiny.

To back this up, radiometric dating of various Larimidian formations, suggests that the Kaiparowits Formation was temporally coeval with the Dinosaur Park Formation of Alberta suggesting an age between 76.5 and 75.5 million years old and this adds further evidence for different species evolving in different provinces but at the same time.

So we have at  least two different dinosaur faunas, both very similar, both containing many multi tonned animals, being able to survive on what was already a small  land mass which was further reduced in size because of various barriers which may have included, mountains, rivers and lakes. These dinosaurs survived in extraordinarily large numbers in unique conditions which, when compared with extant terrestrial communities of today, does not make sense. It appears impossible and yet it happened. What IS clear is that these dinosaurs flourished and proliferated in a way that exceeds any mammalian equivalent of today.

So this brings us nicely to that old chestnut – dinosaur physiology and metabolic rates. Sampson is very much a champion of the "Goldilocks" theory – that is that dinosaurs possessed a metabolic rate somewhere in between cold blooded ectotherms and warm blooded endotherms.   A fully endothermic dinosaur would need a greatly increased food intake whilst a fully ectothermic dinosaur could get by on considerably less. As we have noted only recently, dinosaur bone histology clearly indicates they were very active, fast growing animals and the indications are highly suggestive of endothermian dinosaurs.

But the "Goldilocks" theory hypothesises that dinosaurs with a middle-of-the-road metabolism could have got by on considerably less food than fully endothermic animals. This theory enables the Laramidian environment to sustain a much greater biomass of large dinosaurs which has been hypothesised to be up to five times greater than present day levels in Africa.

The problem, for me, is that this is all very convenient and does, on the face of it, present a rational theory to describe how so many animals flourished in so small a land area – and it is quite possible that this proposal is correct. But this simply dismisses all the evidence to the contrary, that dinosaurs were likely fully endothermic animals living at fully endothermic rates. For me, there is something else going on here and it appears to be a combination of things.

The Cretaceous is known to have been a time of extremes. The temperature was hot, there was high humidity and significant rainfall. CO2 levels were at a high – 1000 parts per million as opposed to today’s levels of 393 parts per million. It was truly like living in a greenhouse.  And, as in a greenhouse, plant growth was extreme, lush and grew at an astonishing rate with some estimates suggesting the conditions enabled a doubling of global forest productivity throughout the Cretaceous (Peralta-Medina & Falcon-Lang 2012). The proliferation of angiosperms and the increase in rapid pollination and fertilization also contributed to this explosion in the biomass.


The plants themselves were also likely to be highly nutritious and the oft mentioned work of Carole Gee (2011) looking at the food values of the Morrison Formation flora indicates that the plants were very capable of promoting rapid growth in sauropods and by that inference alone, since many of the plants and trees from the Jurassic were also prominent in the Cretaceous, suggests that rapid growth and sustainability in dinosaurs would have been maintained or even surpassed.

It may be that Laramidia was not simply a landmass containing dinosaurs in the north and south and I suggest that it is possible that there were multiple provinces each containing communities of dinosaurs that may be either  identical, have subtle or maybe even more marked morphological differences. Even the plants and trees were likely to have displayed different trends in different provinces.

How is this possible? The combination of different barriers such as mountains, rivers, forests and even the Western Interior Seaway itself could have created multiple enclaves each with its own distinct microclimate, generated by the hothouse Cretaceous climate, and, depending on the type of barriers, would have almost certainly have seen the evolution of flora and fauna with subtle but distinct differences. It’s just possible that such conditions would have sustained large populations of fully endothermic dinosaurs. But this is just my personal opinion so certainly don’t take it as fact.

The answer will almost certainly lie in yet more detailed sampling throughout the Laramidian formations and, as Sampson postulates, the unique provincialism of Laramidia will provide many more new species of dinosaur to help our understanding of this lost land.

References

Gee, C. 2011. Sauropod Herbivory During Late Jurassic Times: New Evidence for Conifer-Dominated Vegetation in the Morrison Formation in the Western Interior of North America. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp115. 

Peralta-Medina, E and Howard J. Falcon-Lang (2012). Cretaceous forest composition and productivity inferred from a global fossil wood database. Geology 40(3) doi: 10.1130/G32733.1

Sampson SD, Loewen MA, Farke AA, Roberts EM, Forster CA, et al. (2010) New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLoS ONE 5(9): e12292. doi:10.1371/journal.pone.0012292 

Sampson, S.C. (2012). Dinosaurs of the lost continent. Scientific American Vol.306(3) pp34-41



  Note


All of these landscape images are from the south island of New Zealand. I was lucky enough to stumble into this wonderful Gondwanan-like valley and the surrounding mountains and escarpments are how I imagine Laramidia may have been partitioned up back in the Late Cretaceous. 



5 comments:

Andrea Cau said...

I prefer not to mention words like "ectothermic", "endotermic" and so on, because we know nothing about cellular metabolism in extinct dinosaurs. At the same time, I find no strong evidence that dinosaurs were fully endotermic as modern placental mammals and neornithine birds are.
Inferred growth rates comparable to modern precocial birds and the probable presence of an avian-like ventilatory system suggest that dinosaurs were more bird-like than non-avian reptiles in growth and respiration, but that does not mean they had developed the same metabolic level of derived extant birds. The Sampson model is coherent with the notion, based on several lines of evidence (reproductive strategy, osteology, locomotory module and so on), that dinosaurs were biologically intermediate between crocodiles and modern birds: that "mixed" condition was, perhaps, the key of their success, because it allowed dinosaurs to use the positive aspects of both "models" without the negative aspects.

Mark Wildman said...

Hi Andrea and thanks for the comments.I don't doubt that the intermediate hypothesis could be correct for one minute. Indeed I have been swayed as to its possible validity on more than one occasion.

What I think is easily missed, however, is that there is plenty of (if you like)circumstantial evidence that suggests that dinosaurs did enjoy elevated metabolic rates at least equivalent to the avian model and maybe surpassing it.I just don't think this line of evidence should be dismissed lightly.

One line of study that I would be fascinated to learn about is how the atmospheric and climatic conditions of the Cretaceous (or the Jurassic for that matter)affected and promoted metabolic and physiological development of the dinosaurs.

For instance, what did the high CO2 levels have on the advanced dinosaurian respiratory system? What about isometric pressure or gravity? It's quite likely that this form of research would have huge implications in our determining the dinsaurian physiological profile.

Anonymous said...

"What I think is easily missed, however, is that there is plenty of (if you like)circumstantial evidence that suggests that dinosaurs did enjoy elevated metabolic rates at least equivalent to the avian model and maybe surpassing it.I just don't think this line of evidence should be dismissed lightly."

Wouldn't that count as an example of non-maniraptoran dinos using "the positive aspects of both "models" without the negative aspects"?

-Herman Diaz

Mark Wildman said...

Quite possibly. It could definitely explain how dinosaurs dominated the planet for so long. All I maintain is that, despite all the research to the contrary, we must not fall into accepting the middle-of-the-road physiological dinosaurs as the default option because its neat and tidy. But I reiterate that I accept that it is an entirely possible hypothesis.

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