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Ichthyosaur fossil record (free pdf)

Ben Creisler

A new paper in open access:

Terri J. Cleary, Benjamin C. Moon, Alexander M. Dunhill and Michael J.
Benton (2015)
The fossil record of ichthyosaurs, completeness metrics and sampling biases.
Palaeontology (advance online publication)
DOI: 10.1111/pala.12158

Ichthyosaurs were highly successful marine reptiles with an abundant
and well-studied fossil record. However, their occurrences through
geological time and space are sporadic, and it is important to
understand whether times of apparent species richness and rarity are
real or the result of sampling bias. Here, we explore the skeletal
completeness of 351 dated and identified ichthyosaur specimens,
belonging to all 102 species, the first time that such a study has
been carried out on vertebrates from the marine realm. No correlations
were found between time series of different skeletal metrics and
ichthyosaur diversity. There is a significant geographical variation
in completeness, with the well-studied northern hemisphere producing
fossils of much higher quality than the southern hemisphere.
Medium-sized ichthyosaurs are significantly more complete than small
or large taxa: the incompleteness of small specimens was expected, but
it was a surprise that larger specimens were also relatively
incomplete. Completeness varies greatly between facies, with
fine-grained, siliciclastic sediments preserving the most complete
specimens. These findings may explain why the ichthyosaur diversity
record is low at times, corresponding to facies of poor preservation
potential, such as in the Early Cretaceous. Unexpectedly, we find a
strong negative correlation between skeletal completeness and sea
level, meaning the most complete specimens occurred at times of global
low sea level, and vice versa. Completeness metrics, however, do not
replicate the sampling signal and have limited use as a global-scale
sampling proxy.