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



At 07:00 PM 12/17/97 GMT, you wrote:
>On 17 Dec 1997 20:34:38 -0500,  "Stan Engel"
><thorisporn@worldnet.att.net> wrote:
>
>
>As a follow-up to my first post, the general response is that my
>supposition
>is untestable and not supported by data. So here is a reiteration of
>"known"
>facts.
>
>1. The first placental (I repeat placental, placental, placental)
>mammals
>are from 85-90 mya. Is it coincidence that the decline in the number
>of
>dinosaur species began about that time? It sounds suspicious to me.

Ummmm, no it didn't!  The current known peak of dinosaur diversity worldwide
is the Campanian (83-71 Ma), and the Maastrichtian, although less known
diversity than the Campanian, is still greater than any other stage.  Only
the Kimmeridgian comes close.

Of course, these diversity counts are highly regionally based (a good
percentage of the Campanian diversity comes from the Judith River Group of
North America and the Djadokhta and equivs from Mongolia).  Nevertheless,
they are based on observed data.  What is the basis for your claim that
decline in dino species began in the early Late Cretaceous?

(Also, why whould plancentals be any more severe dino ovivores than other
groups?).

>2. In all probability some meteor struck the Earth 65 mya and had
>climatic
>effects. From what I had read, the dinosaur record ceases some 100,000
>years
>before the KT boundary.

This is VERY suspect.  Dinos are rare fossils, so the lack of their fossils
within 100,000 years of the boundary might just reflect poor sampling.
(Futhermore, some dino fossils are known within a meter or so of the boundary).

>3. Long ago, giants walked the Earth oblivious to the small beasties
>that
>would be their successors. The big guys were the Therapsids and their
>competition were Thecodonts. No one has a problem accepting this. Why
>is
>there a problem with mammals eating Dinosaur young?

Read almost anything by Benton on the Late Triassic extinction for a more
detailed look at the therapsid-archosaur "competition" model.  It is a lot
weaker than certain beareded photogenic paleontologists give it credit for
in certain 1986 books.

>4. As far as the fossil record is concerned, Therapsid fossils cease
>and
>Dinosaur fossils begin about the same time in the Triassic.

See above.

>5. It is accepted that Molluscs caused the decline in Brachiopods.

No, it isn't (or not as such).  Many standard models for the decline in
post-Paleozoic brachiopod and pelmatozoan diversity is that the arrival of
MANY groups of advanced, mobile predators (neogastropods, advanced
echinoids, decapod crustaceans, teleost fish, etc.) put a massive selective
pressure on all forms of epifaunal invertebrates.  Some, like bivalves, had
the potential to hide. Other, attached, non-motile forms like brachiopods
and crinoids became marginalized.  They are still around, just in generally
cooler and/or deeper waters.

Consult recent historical geology or invert paleo books, under the heading
"Mid-Mesozoic marine revolution".

>6. Lots of species and genera have been forced into extinction from
>competition with other lifeforms.This is the case for
>Multituberculates,
>Trilobites, Creodonts to name a few. The Dinosaurs, however, would
>have
>"reigned" until now had it not been for a fortuitous meteor. They were
>oddly
>immune from competition.

Competition is very much a debated topic.  Also, exactly what was competing
with trilobites?

>7. Small animals _can_ kill large ones. Watch enough nature flicks and
>you'll come across the episodes where a pack of Hyenas is in the
>process of
>killing an infant Rhinoceros.

No disagreement with you there.  However, do we have any direct evidence for
hyaenid species driving rhinoceratid species to extinction?

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661