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Mosasaur ribs with unusual histology and morphology

Ben Creisler

A new online paper:

Michael D. D'Emic, Kathlyn M. Smith and Zachary T. Ansley (2015)
Unusual histology and morphology of the ribs of mosasaurs (Squamata).
Palaeontology (advance online publication)
DOI: 10.1111/pala.12157

We report the presence of two previously unrecognized features in the
dorsal ribs of mosasaurs: first, the presence of extremely dense,
pervasive extrinsic fibres (anchoring soft tissue to bone, sometimes
called Sharpey's fibres); and second, high intraspecific variation in
costal bone compactness. Extensive extrinsic fibres are developed in
the dorsal ribs of the mosasaurs Tylosaurus proriger and Eonatator
sternbergi. The dorsal ribs of these mosasaurs are also characterized
by a longitudinally ridged texture that almost completely covers the
bone. Pervasive extrinsic fibres and ridged textures are absent in the
mosasaur Selmasaurus russelli as well as the dorsal ribs of extant
semi-aquatic reptiles (e.g. crocodyliforms) and mosasaurs' close
extant relatives and analogues (e.g. snakes and varanids). Similar
ridged textures characterize the dorsal ribs of several other mosasaur
taxa but are developed to a lesser extent (e.g. Mosasaurus, Clidastes,
Platecarpus and Ectenosaurus), but in no other taxa have pervasive
extrinsic fibres been reported. We interpret these osteohistological
features in T. proriger and E. sternbergi as evidence of tendinous
attachment of extensive and highly differentiated axial musculature
capable of producing great stresses, most likely related to
stabilization of the trunk relative to contralateral movements of the
tail during carangiform locomotion. We also report the compactness
indices (percentage of space occupied by bone rather than cavities)
for these large mosasaur ribs, which are much higher than previously
reported. This suggests high intraspecific variation in bone
compactness that complicates its use in reconstructing mosasaur