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Oldest Pathology in Tetrapod Bone and Origin of Terrestrial Vertebrates + Ophiacodon growth

Ben Creisler

New in PLoS ONE:

Peter J. Bishop , Christopher W. Walmsley, Matthew J. Phillips ,
Michelle R. Quayle, Catherine A. Boisvert , Colin R. McHenry  (2015)
Oldest Pathology in a Tetrapod Bone Illuminates the Origin of
Terrestrial Vertebrates.
PLoS ONE 10(5): e0125723.

The origin of terrestrial tetrapods was a key event in vertebrate
evolution, yet how and when it occurred remains obscure, due to scarce
fossil evidence. Here, we show that the study of palaeopathologies,
such as broken and healed bones, can help elucidate poorly understood
behavioural transitions such as this. Using high-resolution finite
element analysis, we demonstrate that the oldest known broken tetrapod
bone, a radius of the primitive stem tetrapod Ossinodus pueri from the
mid-Viséan (333 million years ago) of Australia, fractured under a
high-force, impact-type loading scenario. The nature of the fracture
suggests that it most plausibly occurred during a fall on land.
Augmenting this are new osteological observations, including a
preferred directionality to the trabecular architecture of cancellous
bone. Together, these results suggest that Ossinodus, one of the first
large (>2m length) tetrapods, spent a significant proportion of its
life on land. Our findings have important implications for
understanding the temporal, biogeographical and physiological contexts
under which terrestriality in vertebrates evolved. They push the date
for the origin of terrestrial tetrapods further back into the
Carboniferous by at least two million years. Moreover, they raise the
possibility that terrestriality in vertebrates first evolved in large
tetrapods in Gondwana rather than in small European forms, warranting
a re-evaluation of this important evolutionary event.


Preprint paper in PeerJ:

Christen Shelton & P. Martin Sander (2015)
Ophiacodon long bone histology: the earliest occurrence of FLB in the
mammalian stem lineage.
PeerJ PrePrints 3:e1262
doi: https://dx.doi.org/10.7287/peerj.preprints.1027v1

The origin of mammalian endothermy has long been held to reside within
the early therapsid groups. However, shared histological
characteristics have been observed in the bone matrix and vascularity
between Ophiacodontidae and the later therapsids (Synapsida).
Historically, this coincidence has been explained as simply a
reflection of the presumed aquatic lifestyle of Ophiacodon or even a
sign of immaturity. Here we show, by histologically sampling an
ontogenetic series of Ophiacodon humeri, as well as additional
material, the existence of true fibrolamellar bone in the postcranial
bones of a member of ‘Pelycosauria’. Our findings have reaffirmed what
previous studies first described as fast growing tissue, and by proxy,
have disproven that the highly vascularized cortex is simply a
reflection of young age. This tissue demonstrates the classic
histological characteristics of true fibrolamellar bone (FLB). The
cortex consists of primary osteons in a woven bone matrix and remains
highly vascularized throughout ontogeny providing evidence to fast
skeletal growth. Overall, the FLB tissue we have described in
Ophiacodon is more derived or “mammal-like” in terms of the osteonal
development, bone matrix, and skeletal growth then what has been
described thus far for any other pelycosaur taxa. Ophioacodon bone
histology does not show well-developed Haversian tissue. With regards
to the histological record, our results remain inconclusive as to the
preferred ecology of Ophiacodon, but support the growing evidence for
an aquatic lifestyle. Our findings have set the evolutionary origins
of modern mammalian endothermy and high skeletal growth rates back
approximately 20 M.Y. to the Early Permian, and by phylogenetic
extension perhaps the Late Carboniferous.