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Anarosaurus (Sauropterygia) from Triassic of the Netherlands
From: Ben Creisler
A new online paper:
Nicole Klein (2012)
Postcranial morphology and growth of the pachypleurosaur Anarosaurus
heterodontus (Sauropterygia) from the Lower Muschelkalk of
Winterswijk, The Netherlands.
Paläontologische Zeitschrift (advance online publication)
Sauropterygia from the Muschelkalk are only found in lag deposits
known as bone beds, and most of the material consists of isolated
bones. Alpha taxonomy of Sauropterygia from the Germanic Basin which
include Pachypleurosauria is thus based mainly on skull morphology of
a few specimens. Articulated or associated postcranial material of
pachypleurosaurs, associated with diagnostic skull material, is very
rare in the Germanic Basin and currently occurs in larger numbers only
in the Lower Muschelkalk of Winterswijk (Gelderland Province, The
Netherlands), which continuously produces new material. For the first
time, the morphology of several partially articulated skeletons of the
pachypleurosaur Anarosaurus heterodontus is described and compared.
Some of those specimens have skull material attached; others were
identified as pachypleurosaurs on the basis of their long bone
histology. The current study revealed that postcranial bones of A.
heterodontus feature a diverse morphology reflecting differences
during ontogeny. Thus, A. heterodontus specimens could be assigned to
size classes (I–III). However, on the basis of morphology, histology,
and maximal known size of isolated skulls and humeri, none of these
specimens represent fully grown individuals. Growth mark counts of
midshaft-femur samples, morphologically assigned to size class III,
document that this size class was reached within the first year of
life. Size class III continued into the second year of life, and then
afterwards skeletal maturity was reached. Thus, a juvenile A.
heterodontus grew very fast, which is also indicated by its bone
tissue type, composed of a high number of radial vascular canals and a
fast-deposited bone matrix. The assignment of isolated bones from
Lower to Middle Muschelkalk localities to A. heterodontus is now
possible with an extensive amended diagnosis of this taxon. This
largely contributes to the understanding of taxonomical diversity and
distribution. Morphological comparison of the postcranial skeleton of
A. heterodontus with that of the two other valid pachypleurosaurs from
the Germanic Basin, Anarosaurus pumilio and Dactylosaurus, supports
their close phylogenetic relationship. Furthermore, the skeleton of A.
heterodontus has no morphological or histological aquatic adaptation
such as pachyostosis or pachyosteosclerosis and thus represents the
least degree of aquatic adaptation within Pachypleurosauria.
Some historical info about Anarosaurus at: