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Re: Dinosaur extinction



> If the diversity of size range is as claimed, a very tidy hypothesis can
> be made.  And here it is: mammals and/or birds outcompeted or preyed on
> juvenile forms of small to mid-size dinos.  Previous to this, the "lawn
> mower" ecology had prevented (particularly) small mammals from becoming
> bigger.  Once the small to mid size dinos were gone, size constraints were
> relaxed and mammals became bigger (there does seem to be such a relaxation
> going on in NA toward the K/T since some Didelphodon and Cimolestes
> species are the _largest_ known for their respective lineages).

I wouldn't rely so much on the fossil record. -- The Early Cretaceous
monotreme *Steropodon* is said to have been 1 m long, though it is hard to
imagine how it could have competed with dinosaurs...

Small size, however, is not a foolproof way to escape mass extinction, as
marsupials, lizards and birds, which were severly hit, show.

> In extant
> mammals there seems to be a threshold size above which they can cause
> misery to the eggs and hatchlings of species whose adults can kill
> them. this threshold appears to have been reached in late K/T taxa.
> Birds are another story: two hypotheses vie for attention.  Birds were the
> lucky survivors of an environmentqal ctastrophe; or, they outcompeted
> their putative competitors, the enantiornithines.

I don't know if enantiornitheans are known from right under the K-T boundary
(but they are known from the Maastrichtian), but the catastrophe was
undoubtedly there...
        Competition seems very unlikely given the distribution of ecological
niches between Enantiornithes and Euornithes -- as far as we know today.
        How old is *Bambiraptor*??? It was quite small, wasn't it?

> If the latter scenario
> were true, it is quite possible that improvements which led to their
> dominance also impacted dino hatchlings.  Indeed, the turnover of
> terrestrial taxa may have been a result the burgeoning of a predatory
> guild, namely, taxa which can expoit nonconcealed nests.

Most known enantiornitheans were arboreal and therefore very likely nested
in trees, whereas shorebirds frequently don't. BTW, how hidden are loon
nests? Loons survived the K-T boundary, as shown by Chatterjee's LK
Antarctic loon *Polarornis*. (Whatever *Neogaeornis* is, *Polarornis* _is_ a
loon, there is a beautifully preserved toothless skull and a cnemial crest
composed like in loons, unlike in grebes or hesperornithiforms...)

> In any case, the
> descendants of these birds and mammals certainly pack a wallop such that
> that particular strategy is spectacularly unsuccesful today.

AFAIK, lots of birds today nest on open ground.

> Whether or
> not this provides an inference for the K/T, I leave to the biases of each
> person.

Wise B-) ;-)

> however, it would be lovely to have evidence of such behavior at
> the K/T.

to have _any_ evidence...

> The gradual disappearance of small to mid-size dinos would be
> circumstantial evidence worth having, for sure.

Definitely, but given the leakiness of the fossil record...

> Bromham, L., M. J. Phillips, and D. Penny.  1999.  Growing up with
> dinosaurs: molecular dates and the mammalian radiation.  Theoretical
Review
> of Evolutionary Ecology 14:113-118.

Anyway, I must find time and try to find this paper...

BTW: I don't trust molecular clocks when they predict that a group sailed
through a mass extinction untouched, because in the recovery phase after a
mass extinction, when ecological niches are empty, many more mutations than
usual are likely to survive...