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Re: Shastasaurus as suction-feeding ichthyosaur



bh480@scn.org <bh480@scn.org> wrote:


> New in PLoS ONE:
>
> Sander, P.M., Chen X., Cheng L., Wang X. (2011)
> Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic
> Diversification of Suction Feeding Ichthyosaurs.
> PLoS ONE 6(5): e19480. doi:10.1371/journal.pone.0019480
> http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0019480


At the end of the paper, the authors speculate on why these
suction-feeding shastasaurs went extinct at the end of the Triassic:


"Suction-feeding ichthyosaurs of the genus _Shastasaurus_ did not
survive into the Jurassic. This may have been because they lost
their competitive advantage as atmospheric oxygen rose again,
although there are no other large suction feeders known among
Early Jurassic marine vertebrate [sic]."


There may in fact have been quite a number of large (or large-ish)
suction-feeding vertebrates around in the Jurassic, and perhaps the
Triassic.  It's been suggested (Tomita, 2010) that the jaw protrusion
of extant sharks, including predatory species, evolved from by way
suction-feeding.  In other words, the impressive jaw mobility seen in
modern sharks (Neoselachii) was inherited from suction-feeding
ancestors.  It may not be a coincidence that the Neoselachii underwent
a major adaptive radiation in the Jurassic.


Nevertheless, the authors are strictly correct in that there are no
*known* large suction-feeding vertebrates around in the Early
Jurassic.  Suction-feeding has been inferred for the fossil sharks
_Hybodus_ and _Protospinax_, with the former outside the crown clade,
and the latter possibly belonging to the Squalea branch of
Neoselachii.  These mid-late Jurassic sharks were quite small (under
2m), and were found in shallow marine sediments (_Protospinax_ is from
Solnhofen).  Tomita proposed that suction-feeding evolved
independently in three shark lineages (Hybodontiformes, Galeomorphii,
and Squalea), so there was likely to be suction-feeding sharks around
at the same time as _Shastasaurus_ - although perhaps not in the open
ocean.




Cheers

Tim