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Re: (fwd) Mammals as a cause of Dinosaur extinction



In a message dated 97-12-18 03:06:48 EST, jwoolf@erinet.com writes:

<<  Mammals were an ever-present factor in the world of the dinosaurs. >>

Perhaps something I wrote in the early editions of Mesozoic Meanderings #2
might shed some light on the mammal-dinosaur interaction. Here the term "core
group" refers to an assemblage of small, diverse and rapidly evolving, loosely
related tetrapods from which lineages of larger, more slowly evolving
tetrapods occasionally evolve. When an externally generated extinction occurs,
it suddenly and preferentially eradicates the more slowly evolving lineages of
large animals (for a number of reasons). In the aftermath, the supply of large
forms is replenished from the rapidly evolving core groups, which survive the
extinction decimated but basically intact. Core groups undergo extinction,
too, but seldom if ever in externally generated extinctions. Rather, they
slowly compete with one another over millions of years, sometimes changing
into core groups of more derived animals (as happened with small mammals,
which changed from being primarily multituberculates to being primarily
placentals during the Cretaceous, and with small theropods, which changed from
being dino-birds to being birds of a much more modern appearance) and
sometimes simply petering out (as happened with small--say, Echinodon-
size--ornithischians and small pterosaurs during the Cretaceous). Anyway--

The Cretaceous-Tertiary (K-T) Extinction

This extinction event was by far the most significant for the archosaurs.
Quite simply, all the extant pterosaur and dinosaur groups were eradicated, to
the last species, never to return. Of the archosaurian lineages only
Crocodylia and modern Aves survived. Since the archosaurs encompassed
practically all the terrestrial megafaunal niches, the K-T event left the
earth essentially devoid of medium-size to large predators and medium-size to
large herbivores for the first time since the P-T extinction at the beginning
of the Mesozoic Era. The abruptness and sweeping universality of the
extinction have generated intense popular interest and scientific debate
concerning possible causes. The proximate cause seems to have been the impact
of a large (6?10 km) asteroid at the edge of the Yucatan peninsula in the
southern Gulf of Mexico, but I do not believe this tells the whole story.

The most interesting question about this extinction event is not, "What caused
it?" but, "Why didn't the archosaurs return, suitably altered, as they had
done six times before?" In terms of the model outlined here, the answer is
that the Late Cretaceous vertebrate core groups no longer included a
dinosaurian Bauplan. The small arboreal and cursorial dinosaurs of the
Triassic and Jurassic Periods were decimated during the Early Cretaceous
through adverse competition from metatherian and placental mammals?the
surviving theropsid core groups?leaving only a single, increasingly well-
adapted, aerial core group, Aves, as survivors. Large, flightless avian
predators and herbivores (Diatryma, Phorusrhacos, Onactornis, and Dinornis)
did evolve at various times from several avian groups during the Tertiary, and
they bore exactly the same relationship to their volant core-group ancestors
that the large theropods bore to volant core-group theropodomorphs. But
without the functional, grasping forelimbs, such flightless avians were no
longer theropod dinosaurs. So, after serving as dinosaur food for two-thirds
of the Mesozoic Era, mammals managed to gain some measure of revenge.