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Re: plesiosaurs et al



>Last night's PaleoWorld discussed plesiosaurs, icthyosaurs and (was it)
>mososaurs, and mentioned that they had all back-evolved into sea
>creatures.

Since all these forms are reptiles (or sauropsids, if you prefer that
term), they evolved from terrestrial animals.

>Have any of their terrestrial forerunners been unearthed?

The plesiosaurs' closest relatives, forms called nothosaurs, placodonts,
and pachypleurosaurs, all had crocodile-like legs (with fingers, probably
webbed).  Like crocodiles, these relatives were almost certainly modile on
land and in the water.  Most evidence suggests that all these forms
(collectively called the Sauropterygia) are more closely related to
lizards, snakes, and tuataras than they are to the archosaurs, but some
have suggested that they branched off before the lepidosaur-archosaur
split.

Ichthyosaurs also might be lepidosauromorphs, but their ancestry is the
hardest to trace of the marine reptiles.

Mosasaurs are true lizards, more closely related to modern monitor lizards
and Gila monsters than they are to most other groups (skinks, iguanas,
etc.).  As such, they descended from a more typical lizard form.

>Did
>they branch off from archosaur stock or before? Also, it seemed to imply
>that these creatures died off due to gradual changes in the food chain
>because of tectonic drift; any validity to this?

Could be.  Another factor is the draining of the inland seaways, which
would have (and almost certainly did) have drastic effects on oceanic
conditions.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.