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Re: Radiation of species at late mid-Triassic
My reflexive response to this question - i.e, what was so
unusual about the Triassic other than that it came after the P-T mass
extinction - is to say that nothing really was. However, you have to
keep in mind that the P-T event was absolutely the worst ever. Numero
uno, the Big One, the Mother of All Mass Extinctions, etc. A recent
estimate (Jablonski 1994, Phil Trans R Soc Lond 344,11) puts the
species-level extinction percentage at 95 +/- 2. This applies to
marine macroinvertebrates like brachiopods, crinoids, snails,
ammonites, etc., but you can easily imagine that whatever did those
guys in was just as bad for terrestrial organisms. The runner-up, by
the way, is the end Ordovician event (84 or 85%, depending on how you
calculate it), with the K-T event coming in a distant fifth (70 or
76%)!! Of course, the fifth worse extinction event in 535-odd million
years is nothing to sneeze at...
My point here is that after a really bad extinction, you
always get an impressive and rapid radiation event. This is a simple
consequence of how logistic growth curves work (the logistic growth
model is routinely applied to paleontological diversity data). The
extraordinary radiation of mammals in the early Paleocene is a good
example: all of the major living orders (lagomorphs, rodents,
carnivores, artiodactyls, perissodactyls, cetaceans, insectivores,
presumably most of the marsupial orders) appear to have branched off
from each other within a few million years of the K-T. This was
simply a side-effect of the rapid speciation/adaptive radiation that
was going on at the time, and I'm sure the apparently coincident
origin of several major tetrapod groups (dinosaurs, mammals, turtles,
frogs, salamanders, possibly the squamates, etc.) is a by-product of
the recovery from the P-T event.
Incidentally, the "thecodonts" are just a stem-group for the
archosaurs (e.g., pterosaurs, dinosaurs, crocodylians), and likewise
the "therapsids" are a stem group (one of several) for the mammals.
So it's basically redundant to talk about the origin of "thecodonts"
AND dinosaurs or of "therapsids" AND mammals happening at the same
time - in fact, the "therapsids" go back well into the Permian. Also,
the birds weren't around until much later (late Jurassic:
Archaeopteryx, of course).