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Archaic rhomaleosaurid plesiosaurs from Jurassic high palaeolatitudes



Ben Creisler
bcreisler@gmail.com


A new online paper:

Roger B.J. Benson, Nikolay G. Zverkov, and Maxim S. Arkhangelsky (2015)
Youngest occurrences of rhomaleosaurid plesiosaurs indicate survival
of an archaic marine reptile clade at high palaeolatitudes.
Acta Palaeontologica Polonica (in press)
doi:http://dx.doi.org/10.4202/app.00167.2015
http://app.pan.pl/article/item/app001672015.html



Rhomaleosaurid plesiosaurians were a common and ecologically
significant component of Early Jurassic marine faunas, primarily as
large-bodied predators. They declined in abundance and made their last
fossil appearance in the Middle Jurassic. However, the geographic
pattern of rhomaleosaurid extinction has thus far been obscured by
spatial bias in the Middle Jurassic marine reptile fossil record,
which is strongly focussed on low-latitude European assemblages. We
report two rhomaleosaurid specimens from the Callovian (late Middle
Jurassic) of the UK and Russia. Along with Borealonectes from Arctic
Canada, these are the youngest-known occurrences of rhomaleosaurids.
The UK specimen is the first identified from the Callovian of Europe,
despite intensive fossil sampling over almost 200 years and the
recovery of hundreds of other plesiosaurian specimens. Its discovery
indicates that rhomaleosaurids were present, but extremely rare, at
low palaeolatitudes of the Callovian. The Russian specimen is one of
relatively few marine reptile specimens from its mid-palaeolatitude
assemblage, as with the high latitude taxon Borealonectes.
Furthermore, we suggest that a mid latitude southern hemisphere
occurrence from the Callovian of Argentina, previously referred to
Pliosauridae, in fact represents a rhomaleosaurid. These findings
suggest that rhomaleosaurids were actually common elements of mid-high
palaeolatitude marine faunas, indicating a geographically staggered
pattern of declining rhomaleosaurid abundance, and demonstrating the
apparent persistence of an archaic marine reptile group in cool,
mid–high latitude environments of the Middle Jurassic. It is therefore
possible that sustained Middle–Late Jurassic global warming
accelerated the ultimate extinction of rhomaleosaurids. Our findings
suggest that widening the geographical breadth of fossil exploration
could considerably enhance current knowledge of Jurassic marine
reptile evolution.



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