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Ichthyosaur Long Bone Microanatomy and Histology

From: Ben Creisler

New in PLoS ONE:

Alexandra Houssaye, Torsten M. Scheyer, Christian Kolb, Valentin
Fischer, P. Martin Sander (2014)
A New Look at Ichthyosaur Long Bone Microanatomy and Histology:
Implications for Their Adaptation to an Aquatic Life.
PLoS ONE 9(4): e95637.


Ichthyosaurs are Mesozoic reptiles considered as active swimmers
highly adapted to a fully open-marine life. They display a wide range
of morphologies illustrating diverse ecological grades. Data
concerning their bone microanatomical and histological features are
rather limited and suggest that ichthyosaurs display a spongious,
“osteoporotic-like” bone inner structure, like extant cetaceans.
However, some taxa exhibit peculiar features, suggesting that the
analysis of the microanatomical and histological characteristics of
various ichthyosaur long bones should match the anatomical diversity
and provide information about their diverse locomotor abilities and

Methodology/Principal Findings

The material analyzed for this study essentially consists of
mid-diaphyseal transverse sections from stylopod bones of various
ichthyosaurs and of a few microtomographic (both conventional and
synchrotron) data. The present contribution discusses the histological
and microanatomical variation observed within ichthyosaurs and the
peculiarities of some taxa (Mixosaurus, Pessopteryx). Four
microanatomical types are described. If Mixosaurus sections differ
from those of the other taxa analyzed, the other microanatomical
types, characterized by the relative proportion of compact and loose
spongiosa of periosteal and endochondral origin respectively, seem to
rather especially illustrate variation along the diaphysis in taxa
with similar microanatomical features. Our analysis also reveals that
primary bone in all the ichthyosaur taxa sampled (to the possible
exception of Mixosaurus) is spongy in origin, that cyclical growth is
a common pattern among ichthyosaurs, and confirms the previous
assumptions of high growth rates in ichthyosaurs.


The occurrence of two types of remodelling patterns along the
diaphysis, characterized by bone mass decrease and increase
respectively is described for the first time. It raises questions
about the definition of the osseous microanatomical specializations
bone mass increase and osteoporosis, notably based on the processes
involved, and reveals the difficulty in determining the true
occurrence of these osseous specializations in ichthyosaurs.