Sunday, December 23, 2012

Dinosaurs: Stranger Than You Think


(Picture from here.)

I was going to put up a post on the biological necessity of government and how it was a compromise between the twin human drives of competition and cooperation. How boring is that? Instead, something better came across my desk. What could be more exciting than dinosaurs?

A few necessary ideas that lay the groundwork.

First: all existing vertebrate classes are supposed to follow a size rule: a lot of little guys, less middle guys and not very many big ones. This is true for birds, amphibians, reptiles, etc.

Second: In terms of biomass, mammals rule the roost regarding large animals. "Large" being defined as greater than a few hundred grams. There are a few big bugs but starting about the size of a shrew and going on to whales mammals are the dominant terrestrial animals. Not to say that birds, amphibians and reptiles aren't important. But we live in something called the Age of Mammals for a reason. In the water it's a different story.

Third: There's a tendency for animals to reside in a given trophic class and ecological niche. Like the pirate code, it's more of a suggestion, really, and it's far more true for terrestrial life than aquatic life. But rats tend to compete against things that are rat like. Cod compete with things that are cod like. It's not often that rats compete with cod.

Fourth: For mammals (remember we're in the Age of Mammals) and birds competition is usually between adults. Both rear their young and toss them out of the nest when they are approaching adulthood. Competition is between adults or adult equivalent, not between young. This isn't as hard and fast outside of mammals and birds. The young of cod and frogs occupy a different ecological niche than adult cod and frogs. Competition is between different species.

As you might have guessed, new data suggests that for dinosaurs It Ain't Necessarily So.

David Hone and associates at Queen Mary University have just published a paper that details aggregate sizes of adult vertebrates. (Here is the paper. Here and here are good discussions of it.) It turns out that the size distribution I described above does not hold with dinosaurs. All other classes of vertebrates skew towards few large species and many small species. Dinosaurs skew in the opposite direction: many large species, few small ones.

The article is very good and readable and I suggest you go read it in its entirety. It's not a simple statistical correlation. Theropods have a somewhat different skew than the other two groups of dinosaurs. Reptiles have a bump in the middle occupied by the crocodilians. The figure at the left is from the article. It's pretty unmistakable. Upper left is the Dinosauria. Look at all of the others. All skewed to the left except dinosaurs which are skewed to the right.

There are a lot of implications here and possible explanations of odd bits of dinosaur data. Let's explore some of these and then see how this new data fits.

One issue is how many dinosaurs there actually were. Peter Dodson and Steve Wang have a nice paper about this problem. There are, of course, all sorts of problems with any sort of diversity estimation such as bias in the fossil record, estimating the diversity at any one time versus the whole 150 million years, etc. The maximum number of genera looks to range from less than fifty to close to 250 over the duration of the dinosaurs. (From figure 2 in the article.)  Compare that to the 1,229 genera of modern mammals. In addition, it's estimated that the number of species/genus in dinosaurs is about 1.2 with the vast majority of genera represented by a single species. Modern mammals have about 4 species/genus. It's clear that dinosaurs were less diverse than mammals.

Another problem is predator/prey ratios. This has been discussed in the context of whether or not dinosaurs are warm blooded or not for a long time. (See here.) Cold blooded predators have a predator/prey ratio of about 1:4. Because they're not keeping a warm body temperature they don't need to eat so much and so there can be more of them. Think crocodiles. Mammals historically have a p/p ratio of about 1:30 and modern mammals a ratio of 1/100 or even less. Dinosaurs have a ratio of 1/14. Better than crocodiles but not as good as mammals.

And, finally, there's the problem of why dinosaurs died out at the end of the Cretaceous and mammals lived.

Let's describe the scene Hone's paper suggests.

We know that a lot of large dinosaurs started out small and got bigger. The Titanosaurs might have been the largest animal to ever walk the earth but they started out about the size of a basketball or smaller. They grew fast but this was years fast, not weeks fast. So for a long time they were bumbling around in the brush just growing.

This means that they were in a different trophic level and niche than the adults. Mammals have a lot of rat equivalents eaten by cat equivalents, deer equivalents eaten by leopard equivalents. Rats are only competing with other rat equivalents. They never compete with deer. Our mammalian strategy precludes this. We're defined by the fact we nurture our young until they're ready to go out on their own. At which point the compete effectively as adults. Deer against antelope against zebra. Not zebra competing against muskrats.

This means in the case of dinosaurs that the niches we ordinarily think of as being occupied by other species can be, in fact, occupied by the juvenile of species.

We see this in any species where the young and adults are extremely different in size. The aforementioned cod, for instance. It's not surprising that larval forms can compete against other larval forms before they compete with different adult species. But on land the situation is quite different-- mostly. We do see it in crocodiles where the adult form is many many times larger than the young. Young crocodiles compete with each other and not with adults. There is some rearing in crocodiles. They protect an egg nest. A female will come in blazing if it hears a baby crocodile of distress. But it's marginal and only protects the very small. It doesn't take long for baby crocs to get big enough to compete with the adults and at that point we have a different picture. And this is not the Age of Crocodiles.

This might be a reason that the species diversity of the dinosaurs were so low compared to mammals. The ecological niches we now see occupied by many different species were occupied by the young of just a few species.

More evidence for this came from another paper that came out a year ago by Werner and Griebeler discusses how the reproductive biology of dinosaurs might drive them towards larger size. Dinosaurs had much larger litter sizes than comparable sized mammals. They show a mathematical model how this might drive the species to a larger size.

Predator/prey ratios might also come into account since they are typically calculated in terms of species-- a bias we have from our mammalian studies. It looks like dinosaurs followed a different tack. Dinosaur predators might have had a much larger prey diet of immature dinosaurs-- both their own and other species.

Finally, though their large size made them able to muddle through several previous extinctions (referring to the Werner paper mentioned above), their limited diversity made them vulnerable as a class to a very large extinction event like a meteor strike. Mammals had a lot of different species to choose from so species that were more suited to the new post-meteor world had a better shot. In addition, rearing to an adult organism meant that the young were protected by their parents beyond the point where dinosaur parents would have left them to forage. It meant not only the dinosaurs as organisms were vulnerable, the whole dinosaur ecology was vulnerable.

I love papers like this for a few reasons. For one, the way it turns our thinking on its head. We are mammals. We tend to think that the world before us was mammalian. It was not. Dinosaurs were not mammals. They didn't think like mammals. They didn't have an ecology like mammals. They didn't rear like mammals. For another it shows how science self corrects. We had an idea about dinosaurs. This paper confronts that idea. Now scientists will discuss it, poke holes in it and eventually it will be fit into the crazy quilt that is our understanding of the world.

And, finally, it shows how the world is so much more interesting and incredible than anything we could make up.

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