Friday, 18 October 2013

Theropoda Sympatrico Pt.3



So we now know that palaeoenvironments with sympatric large theropods are not that unusual and that there are many examples of such ecosystems in the fossil record. The most spectacular examples of these are the previously mentioned  Late Cretaceous fauna of the Kem Kem Formation in Morocco and, perhaps even more remarkable, the Late Jurassic fauna of the Morrison Formation in North America. 
So how was it possible for so many large predators to co-exist in the same ecosystems? Well the most obvious solution to reduce competition amongst contemporary large theropods would be a form of niche partitioning. This may seem potentially easy to spot when comparing spinosaurids with generic build theropods but it is not so easy to discriminate when comparing, for example, tyrannosaurids of similar size and morphologies. This is a common problem when trying to identifying niches for sympatric theropods since the majority of them are actually quite similar to each other.

If you have been following this series of posts then you will know that Andrea Cau has pointed out that niche partitioning is somewhat complicated by the fact that many theropod genera, but particularly the largest theropods, were morphologically variable  throughout their ontogeny and were probably changing their feeding habits as the grew (eg Tsuihiji et al 2011). This is particularly noticeable in tyrannosaurids as we have seen in the past and is one of the principle arguments against the validity of Nanotyrannus.
Simple enough to postulate perhaps but does this not complicate things even further throughout the trophic ladder? Specifically this means that there were several carnivores of similar stature and morphology, whether at a specific ontogenetic stage or adult morphs, that were surely competing for the same resources? As usual these forms of questions are incredibly difficult to answer.
We still have to assume that there may be an element of bias in the taphonomic indicators and stratigraphy of these formations and this should be stressed time and time again. Perhaps more intriguing is the possible taxanomic distortion that undoubtedly exists and that we should consider the possibility that some named theropod taxa may actually be different ontogenetic stages of the same species - and how many times have we been down this road over the last few years?
The simplest solution may be that there was simply enough meat to go around and that this abundance of prey species was such that it was driving theropod diversity and numbers. This is perhaps indicated by the various ways in which it appears theropods may have despatched their prey. Tyrannosaurs most likely killed with their crushing bite, allosauroids with multiple slashing bites and claw raking and the smaller theropods probably used a similar methodology although there is a much greater variety of form in these taxa. And I am not even considering the thought here of the possibility of group hunting theropods.
Size is generally thought of as a governing factor when considering how sympatric theropods may have coexisted. It seems reasonable to assume that big carnivores would require large prey and the theropod body plan, in general, is actually quite conservative and does not enable us to identify differences easily. So perhaps the more likely answer to this issue may lie in behavioural and intraspecific processes whereby the trophic ladder is reinforced and a simple pecking order was enough to maintain the status quo.
As we pointed out in the previous post we noted that large theropods would very likely have needed extended home territories to be able to maintain their nutritional requirements. Now regardless of individual, or whether there were group territories involved, it is almost certain that large theropod territorial overlap would have taken place so this would have limited the amount of large theropods that a territory could support otherwise there would simply not be enough prey animals to go around.
Things now start to get a little complicated. It would seem that there must have been a precarious natural balance to maintain viable breeding populations of large theropods since the larger the animal gets, then the fewer animals would have been supported by the ecosystem which then reduces the chances of perhaps finding a mate. This gets really tricky if large theropods were endothermic since their nourishment requirements would be even greater.
What is seldom realised when discussing this issue is that the same trophic relationship between large theropods and their prey is repeated between their prey and the flora they feed upon. That is that there was a greater mass of available vegetation when compared to the biomass of herbivorous animals that fed on it which, in turn, would have affected the necessary increases in the range of large theropod territories.
So what we have is something of a paradox. That is what would appear to be required to enable a simple niche partitioning hypothesis to hold true actually does not hold water very well. Not that niche partitioning was unlikely, far from it, but there are many other factors which appear to contradict it and make the whole issue of sympatric large dinosaurs, in general, problematic let alone considering sympatry in large theropods.
Niche partitioning is not only restricted to large theropods and the same processes in large herbivorous dinosaurs has been looked at over the last few years and it appears that they did indeed evolve a number of adaptations in both their feeding and masticating abilities as well as cropping plants at different heights (eg Mallon et al 2012, Mallon & Anderson 2013).   However it seems that, in real terms, these adaptions are actually relatively minor and that this may be a primary reason why so many dinosaurian species had a relatively brief temporal distribution and longevity – usually in the region of one million years – sometimes considerably less, sometimes a little more (sampling bias allowing). It is certain that competition between these herbivores accelerated faunal turnover and drove evolution forward.
So now we can get an idea, perhaps, of how rapidly dinosaurs and other animals evolved due to the pressures of intense competition and natural selection. It is again important to highlight how all of these dinosaurian clades interacted with each other and unquestionably affected each other throughout the entire ecosystem.  And sometimes, when the environment they inhabited became isolated due to earth movements, climate change or river formation then this faunal provincialism becomes more amazing.
Nowhere else is this scenario dramatically highlighted than in the provinces of Laramidia where it appears that some of these provinces were incredibly small and yet were able to support a great biomass of large dinosaurs. We have looked at Laramidia before on this blog so we will not go into great detail here but it is obvious that dinosaur provincialism was extremely likely and that a great many pocket ecosystems flourished successfully for millions of years.
In the end it is not simply a matter of hypothesising about sympatric large theropods because that focusses on too narrow an area. Instead we need to look at the entire trophic package and come to realise just how unique dinosaurs were. We have to strive to understand dinosaurs more in every respect and at all levels which leads us onto their physiology.
It appears that although we understand a great deal about dinosaurian metabolic rates we still cannot say with any degree of certainty exactly what they were. I actually wonder now that perhaps the dinosaurs displayed a combination of metabolic rates – that is a kind of “niche partitioning” in dinosaurian physiology. In other words, some dinosaurs were endothermic, some were ectothermic and, as is the general belief these days, that they were something “in between”.  Whether any dinosaur could be described as truly ectothermic is unlikely since all dinosaurs appear to be fast growing and very active animals (as bone histology quite clearly shows) and yet, even if there were only the two variable metabolic rates utilised by dinosaurs, this could still help sustain unusually high populations of large animals.
For example, different dinosaurs could eat at different rates and at different times and probably reproduce in different ways during different seasons. Plants, by their very nature, would also be represented by an enormous variety and there is no doubt that some flora was highly nutritious while other forage was not so good. It seems that the herbivorous dinosaurs were able to utilise all kinds of vegetation and variable metabolism would have certainly helped their ability to do this and may have been another primary driver of the various feeding, jaw and cropping mechanics developed by these dinosaurs.
In conclusion, we have to accept that there are still so many questions that remain unanswered. We can be certain, however, that an incredible combination of factors which include geographical constraints, climate and micro-climates, plant evolution and growth, and ultimately the extraordinary physiology and evolution of the dinosaurs allowed for these remarkable ecosystems to flourish in must what have been a constant state of flux.
It is no wonder that dinosaurs evolved into so many varied forms and that species turnover was so rapid.

Note 
Just a reminder that these posts are just an example of what goes through your head sometimes when you confront the success of these animals. Sure there is a lot of science in here but there is still a great amount of conjecture as well. In other words, it may not necessarily be so – but there just may be a possibility that it is and so on…

References

Jordan C. Mallon, David C. Evans, Michael J. Ryan, Jason S. Anderson, Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada, Palaeogeography, Palaeoclimatology, Palaeoecology, Volumes 350–352, 15 September 2012, Pages 124-138, ISSN 0031-0182, http://dx.doi.org/10.1016/j.palaeo.2012.06.024.
Mallon JC, Anderson JS (2013) Skull Ecomorphology of Megaherbivorous Dinosaurs from the Dinosaur Park Formation (Upper Campanian) of Alberta, Canada. PLoS ONE 8(7): e67182. doi:10.1371/journal.pone.0067182
Tsuihiji, Takanobu , Watabe, Mahito , Tsogtbaatar, Khishigjav , Tsubamoto, Takehisa , Barsbold, Rinchen , Suzuki, Shigeru , Lee, Andrew H. , Ridgely, Ryan C. , Kawahara, Yasuhiro and Witmer, Lawrence M.(2011) 'Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia', Journal of Vertebrate Paleontology, 31: 3, 497 — 517 DOI: 10.1080/02724634.2011.557116

11 comments:

Duane Nash said...

I think we need to get away from the whole paradigm of competition/exclusion/partitioning and start looking at how commensal/keystone species concept can be applied to dinosaurs. For instance with the reintroduction of wolves in Yellowstone we saw a net gain for grizzly bears. Although they both explicitly compete for prey/carcasses the overall higher input of carcasses from wolf predation has benefitted the grizzly. The beaver is another great example of how a keystone species can benefit myriad animals for example moose. Moose and beaver both explicitly "compete" for willow but overall the presence of beaver is a net positive for the moose due to its habitat modification/creation of wetland. I suspect similar interactions dominated in the Mesozoic, even more so than the typical "niche partitioning" put forth. Of course teasing apart these potential interactions from the vagaries of the fossil record is the challenge. Not trying to plug my blog but I go into more detail at antediluviansalad.blogspot

Mark Wildman said...

Hi Duane and thanks for your comments. I agree with you and this is the point of this series of posts - the overall interaction of a whole ecosystem is incredibly hard to appreciate even for an extant system of today - let alone for those of the past.
We all too often concentrate on a specific "reason" to explain the success or failure of a particular organism without really considering the whole package. The two examples you point out of the wolf and beaver again highlight just how intricately organisms are entwined with each other. We tend to focus on dinosaurs because they are our passion but, as you suggest, the amount of total interaction in any one ecosystem must have been completely mind blowing - at every level.

Alessio said...

Great article, and i agree with you and Duane, there were probably an helluva lot of interactions between species more than "simple" competition we'll sadly never know...

Andrea Cau said...

"the overall interaction of a whole ecosystem is incredibly hard to appreciate even for an extant system of today - let alone for those of the past."

Agreed. In fact, it's even difficult to demostrate that a particular fossil association was an "ancient ecosystems". This is why I usually avoid to call the fossil set from a geological formation as "an ecosystems", since it isn't. We have just the fossilized minority (with a component of autoctonous elements due to depositional events) from a now-lost biological association, that accumulated during a (often unknown) time span.

Duane Nash said...

There will always be various degrees of uncertainty for any fossilized fauna. But if you have repeated and consistent patterns of species fossilized over time and time again then is it not outside the bounds of rationality to suggest that they were part of the same ecosystem? Is that really too much of a stretch? I get what several of you are saying about the vagaries cast upon the fossil record- taphonomic bias/time sampling. But if we have robust evidence for coeval associations repeated again and again, then do we not have a more robust platform to suggest ecological associations from? Or is that too much of a straw man argument? Is "degree of certainty" the really issue?

raptor_044 said...

@Mark Wildman

"So how was it possible for so many large predators to co-exist in the same ecosystems? Well the most obvious solution to reduce competition amongst contemporary large theropods would be a form of niche partitioning. This may seem potentially easy to spot when comparing spinosaurids with generic build theropods but it is not so easy to discriminate when comparing, for example, tyrannosaurids of similar size and morphologies."

For what it's worth, the Morrison ecosystem seems to be a good example of niche partitioning among sympatric large theropods (See the Bakker quote, although I recommend reading the whole essay).

"The simplest solution may be that there was simply enough meat to go around and that this abundance of prey species was such that it was driving theropod diversity and numbers. This is perhaps indicated by the various ways in which it appears theropods may have despatched their prey. Tyrannosaurs most likely killed with their crushing bite, allosauroids with multiple slashing bites and claw raking and the smaller theropods probably used a similar methodology although there is a much greater variety of form in these taxa."

Don't forget about the "grapple-and-slash" repetoire of dromaeosaurids. You already covered the other 2 "major predatory repetoires" ( http://dml.cmnh.org/1994Nov/msg00148.html ).

-Hadiaz

Quoting Bakker ( http://books.google.com/books?id=YuC0fRUwnUgC&pg=PA343&dq=%22Bakker+Wyoming%22&hl=en&sa=X&ei=MqxkUqbwHfbJ4AOxtIHgCQ&ved=0CC0Q6AEwAA#v=onepage&q=%22Bakker%20Wyoming%22&f=false ): "So where did the Jurassic meat-eaters hunt? Nearly all of the teeth found where the soil was once dry came from allosaurs. They were feeding on gigantic, long-necked herbivorous dinosaurs: Apatosaurus, Diplodocus, Brachiosaurus.
Since megalosaurs were heavier and had stronger teeth, we thought these carnivores would have fed on giant prey, too. But we were wrong. Megalosaurs left their teeth at sites that were once swampy. Their teeth are found with the fossils of chewed-up turtles and crocodiles.
Ceratosaurs shed their teeth in spots jammed with the remains of giant fish and crocodiles. This not only tells us what they might have eaten but also suggests something about the hunting habits of these camivores. Ceratosaurs have a tail that is almost unbelievably strong and flexible. Why? "Bullets" solved the mystery. Ceratosaurs must have used their tails to swim after prey in the Jurassic waters."

raptor_044 said...

@Everyone

"There will always be various degrees of uncertainty for any fossilized fauna. But if you have repeated and consistent patterns of species fossilized over time and time again then is it not outside the bounds of rationality to suggest that they were part of the same ecosystem? Is that really too much of a stretch? I get what several of you are saying about the vagaries cast upon the fossil record- taphonomic bias/time sampling. But if we have robust evidence for coeval associations repeated again and again, then do we not have a more robust platform to suggest ecological associations from? Or is that too much of a straw man argument? Is "degree of certainty" the really issue?"

This, so much this.

-Hadiaz

Mark Wildman said...

Hi All,

I just want to say how grateful I am for all your comments on what is obviously an extremely interesting subject. I understand everyone's point of view but we must be extremely careful when interpreting what information is available to us.

I agree that that there are obviously good examples of coeval communities in the fossil record but we must accept that there is still the taphonomic aspects and sampling issues to contend with. Sampling is without doubt the biggest single factor that affects all hypotheses. Indeed, I know that Roger Benson has the bit between his teeth concerning sampling right now and a cursory glance at this years' SVP abstracts raises more, some might say alarming, factors that sampling has on palaeontology - particularly phylogenetics.

As noted, theropods did indeed despatch their prey in a variety of ways but I do not believe that we can reasonably infer from teeth found in swampy terrain, in association with the fossils of turtles and crocodiles, that ceratosaurs swam after their prey - even if the tail may have been an excellent swimming aid.

I would rather assume that some theropods visited a swamp as a matter of course and, since both turtles and crocs were fairly slow moving when compared to an agile bipedal theropod, then it may have been a matter of convenience to take some easy prey - perhaps.

We have to be careful with "smoking guns" - they are extremely important but only when they are found in the hand that fired them.

Andrea Cau said...

Before quoting Bakker, it would be nice to test whether Bakker's arguments are statistically supported beyond his own idiosyncrasy.

"But if you have repeated and consistent patterns of species fossilized over time and time again then is it not outside the bounds of rationality to suggest that they were part of the same ecosystem?"

What does "repeated and consistent pattern of species fossilized over time and time again" mean? I would like some quantification, since it is too vague to be used. For example, Bakker found consistent associations in the Morrison Formation that others have not found. Who is right?
Also, "ecosystems" are so vague even among living beings that I find very hard to use such concept in fossils...
For example, is the invertebrate fauna in human beds an ecosystem? Since human skin is part of the trophic input, is Homo sapiens part of that ecosystem?

Note that the mere association of animals is necessary but not sufficient to define an ecosystem, in particular when that association is known entirely from death bodies, which may be together due to physical events not linked to their biotic interactions. This is why I've never found problematic the presence of several large theropods in the Cenomanian of Morocco: we have first to show that they lived together and interacted biologically, and this is not evident from the available geological evidence. In fact, the only thing we know is that these fossils are from the Cenomanian, which represents at least 6 million years. That is the temporal dimension of paleontology, but ecosystems belong to a much narrower time dimension. "Six million years" of time span means that we cannot exclude that the different species (and their ecosystems) were separated each other by millions of years. Thus, although it is a very interesting and intriguing topic, arguing "sympatry" among the Kem Kem theropods is a hypothesis to test, something that may eventually result wrong.

Duane Nash said...

Sorry for painting with too broad a stroke. What I implied with "repeated and consistent patterns of species..." were some of the more well sampled sites such as Dinosaur Provincial Park from which several recent studies have addressed herbivore niche partitioning (Mallon, Anderson 2013), Better yet might be the various Mongolian Nemegt nesting grounds fossilized by collapsing sand dunes since these document a single catastrophe.

I forgot about Bakker and his smoking gun shed tooth study. "Bakker found consistent associations in the Morrison Formation that others have not found. Who is right?" Please point me in the direction of these other studies I was not aware others have followed up on his premise!!!

Agree totally with the Kem Kem dilemma that Andrea summarizes. But still fun to speculate, even if outside the bounds of rigorous science- as long as that disclaimer is put forth why not go there?

For me we need both types of input in paleontological (and maybe all scientific discussions) questions. Boundary pushing, informed speculative ventures as well as the empirically driven, testable, and rigorous approach. Where the two approaches meet is the cool part. Of course, paleontology has been marred too often in the past by the overly speculative "because I said so" approach in the past. But we need both.

Andrea Cau said...

"Please point me in the direction of these other studies I was not aware others have followed up on his premise!!!"
Gates T.A., 2005. The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a Drought-Induced Assemblage Palaios 20(4):363-375.

"For me we need both types of input in paleontological (and maybe all scientific discussions) questions."
This is why I write both a blog (where a bit more speculation is allowed) and technical papers (where I try to be less speculative).

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