It can only take the most passing familiarity with dinosaurs to notice that some of them were really, really big. Huge numbers of dinosaurs have been described and an awful lot of them were large, and especially when put in the context of modern terrestrial animals. African elephants don't even get to five tons in mass that often, yet there were dozens of dinosaur species for whom that would represent a runt, and a number of species that were ten times that value in size.
For all their vaunted huge size though, the fossil record also reveals a good number of small, even tiny, dinosaurs. We have plenty of species that were the size of a chicken or magpie say, so it's not like they were all big, or that there isn't a very broad range of sizes. The question though, is how does this marry up to what we might expect for body sizes in other groups of organisms? Do dinosaurs simply have a good number of giants in their midst but are otherwise rather normal? Or is the whole make-up of dinosaur body sizes unusual?
To try and investigate this question, we collected data on dinosaur body sizes and also data on a number of living and extinct animal groups. Note that here by the word 'dinosaur' we are only talking about the extinct group of terrestrial forms. Modern birds are literally living dinosaurs, but we did not include any birds (living or extinct) in the dinosaur data set since, well, we'd expect them to be evolving rather differently and being under different pressures what with the whole flying thing.
Looking at the data it was clear that the living groups we examined (fish, mammals, reptiles and birds) all had the same basic pattern. There were lots and lots of small species, but very few large ones -- lots of different mice species, few elephant species. The dinosaurs however showed something really rather different -- they were replete with large species, but the smaller ones were few and far between. It's basically bordering on the polar opposite of what we see in other vertebrate groups.
Now there are problems with the fossil record and one that is well known is that, on average, smaller specimens tend to be rare. So might these results just be linked to this phenomenon? If the smaller dinosaurs are rarely preserved and harder to find, then surely that will give a bias towards the big ones? In this case, we don't think this bias has a major effect for several reasons.
First of all different groups within the dinosaurs have somewhat different patterns -- if this were a universal effect it should hit them all equally and it doesn't. The herbivores displayed many more larger species than the carnivores, relatively speaking. Secondly, we looked at fossil mammals and while their distribution patterns do show more large species, they were clearly nothing like dinosaurs and very much like living mammals -- the bias doesn't seem to be affecting them that much. Finally, we also looked at the pterosaurs, the flying reptiles that lived along side the dinosaurs and were close relatives of them. Their distribution was also pretty normal, again suggesting that it's the dinosaurs that are odd and it is not just an issue of them being fossils.
So dinosaurs are genuinely weird. They have an awful lot of big species and relatively few small ones. What might cause this? Here, our work builds on existing research by other dinosaur experts that dovetails rather nicely with our conclusions. Most significantly is the fact that dinosaurs seem to have had a different approach to growth and nurturing their young than most other groups.
There is good evidence for parental care in many dinosaurs, one or both parents would have looked after the eggs and babies for at least a while after hatching. However, even the very largest dinosaurs could only lay eggs of a limited size, meaning hatchlings were small and despite growing fast, had a very, very long way to get to the size of their parents. A 5 kg hatchling could grow into an adult that was between 1 and 50 tons. That means that there would be years where the young animals were independent of their parents yet critically, only a fraction of their size. They would have been filling different ecological niches too: a one ton Tyrannosaurus is not going to be tackling the same prey as a 10 ton adult; and while a 10 meter long Diplodocus might sound like a big animal with a big reach, it would be less than half the size of an adult and clearly not capable of reaching so high into the trees. Baby dinosaurs would be doing different things to adults and, during their growth and development, might potentially take on several different ecological roles.
So the answer to the question "Where are all the small dinosaurs?" could well be -- they are there, just as juveniles. The juveniles of large species of dinosaur are occupying the niches that might normally be there for adults of smaller species. However, the unique dinosaur pattern of growth means they also had a unique ecological make up. Odd though this might sound, it's probably linked to the size of the eggs dinosaurs laid. Eggs do have a major limitation as, if they are too large, the shell has to be so thick to avoid breaking under the weight of the contents that gas exchange can't occur effectively and the developing embryo will asphyxiate. So even the largest dinosaurs had to lay (relatively small eggs). That answers that question, but why or how did dinosaurs get so much bigger than other living groups?
Here enters the vexed question of dinosaur physiology. To put it in not-very-technical terms -- were they hot or cold blooded? This has been the subject of huge amounts of research and scientific debate, but it is probably fair to say that our current best data suggests that a number of smaller ones at least (including the ancestors of birds and animals like the famous Velociraptor) were warm blooded, but that the huge beasts like Diplodocus were not. However, this does not make them slow, sluggish, inactive animals, but merely means that they were not generating their own heat internally. However, what heat they did have (absorbed from the environment, or produced by muscle activity and the like) would bleed off very slowly -- they had a large volume and small surface area so would keep warm anyway. This is actually something we see in big sharks and tuna, you can sort of cheat yourself to warm bloodedness without the penalties of burning up tons of calories to stay hot.
On top of that, once you start to get really big (like say over five tons or so) then you also get more efficient. A lot of plants are tough and hard to digest which is why many large herbivores either eat huge amounts of greenstuffs, or have to really work to break it down and digest it (like cows chewing the cud). But if you're big enough and have a long enough digestive track, the plants will eventually get broken down and you get more energy back again without having to effectively put extra effort in. This greater digestive efficiency at large size might also account for the size differences we see between the herbivores and carnivores. The latter would not benefit from this strategy as and had enough food among the smaller prey (mammals, lizards and the like) to live successfully at somewhat smaller sizes though big predators would be out there tackling larger prey.
In short, dinosaurs exhibit a number of unique features which collectively gives them this odd profile of many large species and few small ones. Large dinosaurs proliferated as very large sizes made them super efficient as herbivores (and naturally large carnivores would evolve to exploit them). With eggs limited in size, this made the gap between hatchling size and adult size ever larger meaning babies had more growing to do and would fill more and more ecological niches, thus changing the overall pattern of body sizes as there were limited spaces for small adults. Dinosaurs weren't just big, their whole evolution and ecology was centered around just how large they got. For dinosaurs, size mattered.
Dave Hone is an associate researcher at the University of Bristol and Eoin O'Gorman is a postdoctoral research fellow at Queen Mary University of London.
For more information about Hone's current research project on cannibalism in tyrannosaurs go here.