Dinomammals

I’ve been giving a lot of thought lately to bears, and how their large size and thick frames are reminiscent of the megafauna that occupied North America several thousand years ago. What evolutionary forces shaped the megafauna, and what forces caused them to become extinct? It seems like the bears might be the last remainder of that class of animals.

I’m wondering if the evolutionary forces that shaped the megafauna were the same as those that shaped the dinosaurs, and that, if left unchecked, would have resulted in larger and larger mammals until they approached the size of dinosaurs. I’ve coined the term “dinomammals” to refer to these theoretical creatures.

It might be fun to conjecture what these animals would have looked like, but I’m not going to do that here. Instead, I’m going to conjecture on what it was that caused these animals’ extinction.

Any particular ecological niche is only capable of supporting so much biomass. That biomass can be concentrated in a few large individuals, or divided between a larger number of smaller animals. In most environments, it makes more sense to divide your biomass for your species into multiple packages. This will give you the greatest chance of having some members of your species survive to pass their genetic material on to the next generation. This is particularly true in environments in which your predators are overwhelmingly more powerful than the prey. In this case, the qualities that enable a member of the prey to survive are those that avoid encounters with the predator, such as small size or camouflage. By the same token, there isn’t much pressure on the predator to be more powerful, but by dividing the biomass into smaller more numerous packages increases the potential of encountering a member of the prey species.

In environments where predator and prey are more evenly matched, the attributes that will be selected for are those that help you survive an encounter, in the case of the prey, or for the predator to subdue it’s prey. In this case, size might well be an advantage.

Among males, competition for mates will also favor attributes that help you to subdue a rival. In this case, the individuals who are competing are almost by definition equally matched since they are both of the same species.

However, once you introduce the notion of animals working together in groups, you blow this out of the water. Suddenly you can have the best of both worlds. You can divide your genetic material into several individuals, and you can group these animals together to attack a much more powerful single individual of another species. Forming together in herds might give prey the same advantage.

This might in fact be common knowledge shared by all paleontologists everywhere, but I’m not a paleontologist, so it’s new to me. My theory is that it’s the type of group hunting practiced by wolves and people today that caused the extinction of the megafauna and prevented them from continuing the evolutionary path towards becoming dinomammals.

Of course, there are also theories that some dinosaurs may have acted in groups. I’ve heard it theorized that Ceratopsians moved in herds and, when attacked, formed circles with their pointy ends to the outside to defend against predators. I’ve also heard it theorized that Velociraptors attacked in groups. However, I don’t believe these dinosaurs evolved until after the really big dinosaurs like the Brontosaurus (or whatever its new name is) evolved, and that towards the later part of the dinosaur period the dinosaurs started shrinking in size. I also think that mammals, with their greater intelligence, are able to coordinate better than dinosaurs, and so are much more effective at working together in groups. It may be that dinosaurs could work together in groups, but just not effectively enough to completely counter the benefit of larger size.