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Re: Dinos on microcontinents ( was Re: Theropod "migrations")



At 10:55 AM 4/27/99 -0400, Larry Febo wrote:
>>To my knowledge, no one has reported terrestrial sediments or fossils from
>>the microcontinents which accreted onto North America (or elsewhere) in the
>>Mesozoic.  They might be extraordinarily interesting!!  (And it would be
>>more important to simply document what you find there, than look
>>speciifically for critters of a single particular type that conform to a
>>single particular hypothesis).
>
>It seems that India would be an ideal candidate for this type of study also,
>but from what I`ve looked into already (briefly), it has hardly been studied
>in depth. From the "Encyclopedia of Dinosaurs" I get  a rather short list of
>theropods: Compsosuchus, Jubbulporia, Laevisuchus, Dryptosauroides,
>Coeluroides and Ornithomimoides, all of which are considered as Nomen Dubium
>( at  T. Michael Keesey`s site), and based on a few fragments of
>vertebrae....(not much to go on).

Well, there are fragmentary limb bones, too, but not much to go on.

>Of course there is Indosaurus
>matleyi,...considered an Abelisaurid from the Maastrichtian,....now where
>did that come from?  There are known Abelisaurids from S America, but how
>did they get into India which was well isolated at that time?

First off, Mesozoic India was not your typical isolated microcontinent:
until the mid-K, it was part of Gondwana, one of the two supercontinents.
On "traditional" paleogeographies (e.g., Smith et al.'s 1994 Atlas of
Mesozoic & Cenozoic Coastlines) there was still direct land-to-land
connections between all Gondwanan continents up to the Barremian.  The
SUNY-Stony Brook Madagascan expeditions have used illustrations from a
forthcoming paleogeography, however, that show a possible land-to-land
connection from South America-to-Antarctica-to-Indomadagascar well into the
Late Cretaceous.

Secondly, shared presence of a group on two isolated landmasses *could*
indicate migration.  However, there is an even easier explanation: the group
was already present prior to the break up of the landmass!  If abelisaurids
are the sister taxon to _Elaphrosaurus_ (as suggested by various studies),
then proto-abelisaurs were already present back in the Late Jurassic, when
Gondwana was fully assembled.  Therefore, early abelisaurs could represent
part of the "original" Cretaceous fauna of the southern continents, and
their subsequent distribution on the various landmasses after the break-up
of Gondwana reflects their shared survival on these newly isolated continents.

However, things are more complicated: not for India but for neighboring
Madagascar.  Local boy _Majungatholus_ is a kissing cousin..., er, sister
taxon to Argentine _Carnotaurus_.  In fact, _Majungatholus_ seems to be much
more closely related to _Carnotaurus_ than either is to _Abelisaurus_.  So,
it could be that the ancestral carnotaurine population was found throughout
Gondwana, and we have simply sampled only a mid- (Late?) K Argentine and
Late K Madagascan representative.  However, Sampson et al. offer the
alternative explanation of a later landbridge between South America and
Madagascar (via Antarctica), leaving Africa out of the range of the
carnotaurines.

Interesting stuff.

The microcontinents I was referring to in an earlier post were landmasses
such as Sonomia (now in parts of California, Oregon, and Nevada), which may
not have been part of the Pangaean landmass (and in any case, whose
pre-accretion tectonic and paleogeographic affinities are uncertain).

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