When the Dinosaur Renaissance gathered momentum during the Seventies, its biggest proponent and champion was one Robert T. Bakker – a palaeontologist who needs no introduction. Bakker was instrumental in displaying dinosaurs in a new light. Gone were the old depictions of cold blooded, slow moving, dim witted reptiles of the primeval swamps and in their place came this new dynamic clade of animals – animals that were fast, agile, rapidly evolving and, above all, warm blooded. Indeed, things would never be the same again and we all got swept away in this new glorification of the dinosaurian race.
Chief amongst Bakker’s evidence for proclaiming the existence of endothermian dinosaurs were the use of predator/prey ratios (PPR’s). Simply put, in any specific ecosystem, the amount of carnivores that can be supported by any given population of herbivores can be measured as a ratio. Because cold blooded carnivores generate their body heat from the sun they can eat much less frequently and, as a result, larger numbers of carnivores can exist contemporarily. But because warm blooded carnivores need to generate their heat internally they must eat much more frequently so the amount of carnivores that can be supported in the ecosystem is much much lower.
Bakker argued at the time that by comparing, not only extant mammalian communities, but also Permian reptilian faunas with dinosaurian faunas, that because PPR’s in dinosaur communities were so low, and those in the Permian were so high, that dinosaurs must have been warm blooded. At the time this was seemingly ground breaking research and there were many believers but this was clearly wrong and we now know that there are many variables that rendered the hypothesis redundant. Not only is it flawed to compare extant mammalian communities with extinct ones but sampling in the different formations is highly variable and problematic and thus it is difficult to formulate any ratio with any confidence.
Dinosaurs may very well have been warm blooded and there is a weight of evidence now that supports that they were, at the very least, more than ectothermic if not fully endothermic. However, there was a curious by product of the predator/prey hypothesis and that was that it was commonly accepted that you would generally expect to find, in any sampled dinosaurian fauna, only the one large predator in residence. This persisted for some years probably perpetuated by the fact that Tyrannosaurus rex, as one of the best known and well researched theropods, is, indeed, the only large carnivore in the Late Maastrichtian of North America.
It was strange too that the revelation that Daspletosaurus and Albertosaurus were contemporary large tyrannosaurs in both the Dinosaur Park and Two Medicine Formations was simply accepted as unusual. The most popular explanation was that this was evidence of niche partitioning with Albertosaurus primarily feeding on hadrosaurs while the more heavily built Daspletosaurus preyed upon ceratopsids (Russell 1970).
But eventually, as more and more formations became better sampled, it became apparent that, actually, sympatric large theropods are the norm with, perhaps, the most extreme example being the population of the Late Cretaceous Kem Kem Formation in Morocco which has produced Carcharodontosaurus, Deltadromeus, and Spinosaurus amongst others.
These co-habiting theropod populations bring their own unique problems to the table and continue to fascinate palaeontologist’s in trying to work out how these large carnivores managed to share out the various resources available. These issues are amplified when we look at the amazing populations of Laramidia which displays several much smaller pocket environments where it appears virtually impossible for there to be enough resources to go around for all dinosaurs – let alone the carnivores.
Dinosaurs are freaks then and this is precisely why you cannot simply compare extant faunal communities with dinosaurian faunas. It appears almost impossible in some cases for dinosaurs to be able to coexist at all – but they obviously did and what a magnificent race of creatures they are. So how can we explain how large theropods co-existed successfully in what would appear to be extraordinary circumstances? There are many things to consider but perhaps we should look at the nuts and bolts of the carnivorous dinosaur – what is a carnivorous dinosaur, what were their functions and, most importantly of all, how important were they to the ecosystem?
Theropod dinosaurs are spectacular – from the smallest raptorial forms right through to the largest tyrannosaurs. And it is safe to assume that, when compared to their herbivorous contemporaries, that they were relatively more “intelligent”. I have put the word intelligent in quotation marks because it is important to make the distinction between intelligence, as displayed by extant mammals, and that hypothesised for dinosaurs for it is unlikely that any dinosaur was capable of intelligent reasoning in comparison, for example, to a dolphin.
But it is obvious that they would have been smarter than their prey because their predatory nature demanded it and, I believe, it is safe to assume that this would also entail a degree of complexity. How complex, again, remains a matter of conjecture but it is a possibility (some of us believe likely) that there was a degree of social complexity within Theropoda whether they were solitary hunters or hunted in groups. This is highly suggestive that there would have been a significant variety in their behavioural patterns which would have included their hunting techniques, their courtship and mating rituals and even how they may have raised their young. We only have to look at birds to get, perhaps, a hint of what may have been a variety of different behaviours displayed by their dinosaurian ancestors.
Large theropods must truly have been a sight to behold. The largest of them would certainly have nothing to fear in their ecosystem aside, perhaps, from another of their species. Other dinosaurs may have been as equally impressive and many were indeed far larger than even the biggest theropod but for me they have no equal. And despite the fact that we know so much about them it is still the vast amount of information that we don’t know about them which is so fascinating. But the one thing we can be sure of, as with any ecosystem, theropods, and large theropods in particular were at the top of the food chain - the very peak of the trophic pyramid.
Carnivorous dinosaurs were essential components of their ecosystems and it is easy to think that all they did was eat prey animals – but this is far from the truth. Without the carnivores then chaos would soon ensue. Theropods evolved to eat their contemporaries -not withstanding therizinosaurs which we will come to a little later – and they were obviously very successful at it since they were dominant throughout the dinosaurs reign but we often forget, that throughout the Mesozoic, there were a multitude of different environments. Theropods evolved in tandem with deserts, forests, coastal plains, uplands, lowlands and everything else in between and every single ecosystem depended on them to function correctly.
Theropods were the major controllers of herbivorous dinosaur populations and it is safe to assume that the rules that govern the trophic pyramids of today would have applied to those of the past. I find it particularly interesting that we consider many aspects of dinosaur extinction, not only at the K-T boundary, but also the various phases of dinosaur extinction throughout the Mesozoic without often considering environmental disruption because a species, particularly a carnivore, became extinct. We all consider environmental change due to volcanoes, earthquakes, mountain building and even the occasional meteor strike, and no doubt these were considerable influences on the environment, but extinction due to the disappearance of species is well documented – even today.
In the next part of this mini-series we will look at the cause and effects of theropod extinction on an ecosystem and consider whether this too was a major driver in dinosaurian evolution and species turnover.
Russell, Dale A. (1970). "Tyrannosaurs from the Late Cretaceous of western Canada". National Museum of Natural Sciences Publications in Paleontology 1: 1–34.