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Here's what y'all were staying tuned for.
We were? [I can see your brow furrowing. Stop that, your face is gonna
freeze up that way.]
Why did mammals increase in body size after dinosaurs went extinct? First,
keep in mind that dinosaurs did not exactly go extinct. The Cenozoic has
been called the Age of Mammals but a better appellation would probably be
the Age of Flying Dinosaurs. Within a surprisingly short time after the
extinction of non-avian dinosaurs, every group of amniotes increased in
diversity. Birds have done impressively and today outnumber by species
every other group of tetrapods.
What went extinct at the K-T boundary was almost everything large. Large
animals, with their low fecundity and difficulty secluding themselves, are
vulnerable to extinction, but that's another story. Nevertheless, in many
taxa there is the trend of increasing body size. For herbivores, there are
two good reasons to get large. One is to dissuade your predators from
inviting you to lunch. The other has to do with energy. Plant material is
less energy-dense than animal, so herbivores have to eat a lot to sustain
themselves, particularly if they are endotherms. One way to reduce the
need is to get large, because as size goes up, metabolic rate goes down.
So we often see increasing size trends among endothermic herbivores.
The problem for mammalian herbivores in the Mesozoic is those damn
dinosaurs. Large dinosaurian predators made life very difficult for
mammals. Increasing size doesn't help you if you're a little rodent-sized
herbivore and there are big speedily running razor blades all over the
landscape. It's best to stay small so you can bolt for a hole when one
heads your way.
Then, quite suddenly, all those running razor blades disappear. (I can
almost hear the collective sigh of furball relief.) There's lots of plant
material free for the chomping (getting dangerously close to that
competition thing here, Dave) and now your predators are mostly about your
size or smaller. Time for the evolution of herbivorous specializations and
increasing size! Another "arms race" is on, with mammalian predators
trying the match the rapidly increasing size of mammalian herbivores. I
believe this race is still with us. However, we should not necesarily
expect to see mammals reach the enormous sizes of herbivorous dinosaurs.
If I am correct in assuming that there has been an increase in low-growing
vegetation since the end of the Jurassic (apparently a questionable
assumption), and if I am correct in my belief that a mammal shaped like a
rhinoceros but the size of a large sauropod is an impossibility, then we
may never see mammals reach 30+ ton size. In this regard, I must confess I
have never seen original material of Indricotherium, but the restorations I
have seen make it considerably more gracile than any living rhinoceros,
with a rather long neck and slender legs. This would give it a
considerably higher surface area/volume ratio than smaller rhinos. Just a
brief digression here to note that a large endotherm's overheating problem
comes from within, not from without. It requires a very cold climate
indeed to draw heat quickly enough out of a large endotherm to make
insulation worthwhile. In hot climates, insulation and thermal inertia
accomplish nothing, because the overheating problem does not originate
outside. Notice that Indricotherium, the largest known mammal, is also one
of the closest in morphology to sauropods. Notice how basically similar
all large sauropods are in morphology, in contrast to smaller herbivorous
dinosaurs. The constraints of such hugeness may be such that any huge
endothermic herbivore must be a high browser.
There is, I believe, a broad sort of competitive release at the K-T
boundary. But it does not specifically involve dinosaurs and mammals. ALL
amniotes show a dramatic increase in diversity during the Cenozoic. With
the loss of non-avian dinosaurs, a lot of terrestrial habitat was indeed
opened up. But mammals were not the only beneficiaries. We tend to look
at mammals because some of them are large. But most mammals have remained
small, and small animals dominate all terrestrial environments today.
"Dinosaurs" are in most cases the most diverse of all, and are conspicuous
in virtually every terrestrial system, from Arctic to Antarctic.
One rather striking thing is that during the hundreds of millions of years
of the Mesozoic, no dinosaur that we know of ever became small. Dinosaurs
were consistently big, remarkably so in a way. Perhaps this is our bias
(or the fossil record's) against avian dinosaurs at work. But the fact is,
large animals evolve from small animals, large animals evolve from large
animals, and small animals evolve from small animals. Small animals do not
generally evolve from large ones. This is what we would expect when we
consider that small animals, with their high rates of fecundity and short
generation times, probably evolve much more rapidly than large ones.
Speciation rates probably increase exponentially with decreasing body size.
Wasn't that worth staying tuned for? Thank goodness it's all REALLY
settled now. If y'all would just realize that I'm always right, everything
would be fine.