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.
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…
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