Sauropod bone growth has been misunderstood

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com

A new online paper:


Koen Stein & Edina Prondvai (2013)
Rethinking the nature of fibrolamellar bone: an integrative biological
revision of sauropod plexiform bone formation.
Biological Reviews (advance online publication)
DOI: 10.1111/brv.12041
http://onlinelibrary.wiley.com/doi/10.1111/brv.12041/abstract



We present novel findings on sauropod bone histology that cast doubt
on general palaeohistological concepts concerning the true nature of
woven bone in primary cortical bone and its role in the rapid growth
and giant body sizes of sauropod dinosaurs. By preparing and
investigating longitudinal thin sections of sauropod long bones, of
which transverse thin sections were published previously, we found
that the amount of woven bone in the primary complex has been largely
overestimated. Using comparative cellular and light-extinction
characteristics in the two section planes, we revealed that the
majority of the bony lamina consists of longitudinally organized
primary bone, whereas woven bone is usually represented only by a
layer a few cells thin in the laminae. Previous arguments on sauropod
biology, which have been based on the overestimated amount,
misinterpreted formation process and misjudged role of woven bone in
the plexiform bone formation of sauropod dinosaurs, are thereby
rejected.

To explain the observed pattern in fossil bones, we review the most
recent advances in bone biology concerning bone formation processes at
the cellular and tissue levels. Differentiation between static and
dynamic osteogenesis (SO and DO) and the revealed characteristics of
SO- versus DO-derived bone tissues shed light on several questions
raised by our palaeohistological results and permit identification of
these bone tissues in fossils with high confidence. By presenting the
methods generally used for investigating fossil bones, we show that
the major cause of overestimation of the amount of woven bone in
previous palaeohistological studies is the almost exclusive usage of
transverse sections. In these sections, cells and crystallites of the
longitudinally organized primary bone are cut transversely, thus cells
appear rounded and crystallites remain dark under crossed plane
polarizers, thereby giving the false impression of woven bone. In
order to avoid further confusion in palaeohistological studies, we
introduce new osteohistological terms as well as revise widely used
but incorrect terminology.

To infer the role of woven bone in the bone formation of fast-growing
tetrapods, we review some aspects of the interrelationships between
the vascularity of bone tissues, basal metabolic rate, body size and
growth rate. By putting our findings into the context of osteogenesis,
we provide a new model for the diametrical limb bone growth of
sauropods and present new implications for the evolution of fast
growth in vertebrates. Since biomechanical studies of bone tissues
suggest that predominant collagen fibre orientation (CFO) is
controlled by endogenous, functional and perhaps phylogenetic factors,
the relationship between CFO and bone growth rate as defined by
Amprino's rule, which has been the basis for the biological
interpretation of several osteohistological features, must be revised.

Our findings draw attention to the urgent need for revising widely
accepted basic concepts of palaeohistological studies, and for a more
integrative approach to bone formation, biomechanics and bone
microstructural features of extant and extinct vertebrates to infer
life history traits of long extinct, iconic animals like dinosaurs.