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Alligatorellus (Late Jurassic crocodyliform) revised (free pdf)

Ben Creisler

A new online paper in open access:

Jonathan P. Tennant & Philip D. Mannion (2014)
Revision of the Late Jurassic crocodyliform Alligatorellus, and
evidence for allopatric speciation driving high diversity in western
European atoposaurids.
PeerJ 2:e599
doi:  http://dx.doi.org/10.7717/peerj.599

Atoposaurid crocodyliforms represent an important faunal component of
Late Jurassic to Early Cretaceous Laurasian semi-aquatic to
terrestrial ecosystems, with numerous spatiotemporally contemporaneous
atoposaurids known from western Europe. In particular, the Late
Jurassic of France and Germany records evidence for high diversity and
possible sympatric atoposaurid species belonging to Alligatorellus,
Alligatorium and Atoposaurus. However, atoposaurid taxonomy has
received little attention, and many species are in need of revision.
As such, this potentially high European diversity within a narrow
spatiotemporal range might be a taxonomic artefact. Here we provide a
taxonomic and anatomical revision of the Late Jurassic atoposaurid
Alligatorellus. Initially described as A. beaumonti from the
Kimmeridgian of Cerin, eastern France, additional material from the
Tithonian of Solnhofen, south-eastern Germany, was subsequently
referred to this species, with the two occurrences differentiated as
A. beaumonti beaumonti and A. beaumonti bavaricus, respectively. We
provide a revised diagnosis for the genus Alligatorellus, and note a
number of anatomical differences between the French and German
specimens, including osteoderm morphology and the configuration and
pattern of sculpting of cranial elements. Consequently, we restrict
the name Alligatorellus beaumonti to include only the French remains,
and raise the rank of the German material to a distinct species:
Alligatorellus bavaricus. A new diagnosis is provided for both
species, and we suggest that a recently referred specimen from a
coeval German locality cannot be conclusively referred to
Alligatorellus. Although it has previously been suggested that
Alligatorellus, Alligatorium and Atoposaurus might represent a single
growth series of one species, we find no conclusive evidence to
support this proposal, and provide a number of morphological
differences to distinguish these three taxa that appear to be
independent of ontogeny. Consequently, we interpret high atoposaurid
diversity in the Late Jurassic island archipelago of western Europe as
a genuine biological signal, with closely related species of
Alligatorellus, Alligatorium and Atoposaurus in both French and German
basins providing evidence for allopatric speciation, potentially
driven by fluctuating highstand sea levels.