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Nothosaurus and Pistosaurus (Triassic sauroptergyians) long bone histologies (free pdf)



From: Ben Creisler
bcreisler@gmail.com


A recent paper not yet mentioned on the DML. The paper is open access.

Anna Krahl, Nicole Klein & P Martin Sander (2013)
Evolutionary implications of the divergent long bone histologies of
Nothosaurus and Pistosaurus (Sauropterygia, Triassic).
BMC Evolutionary Biology13:123
DOI: 10.1186/1471-2148-13-123
http://link.springer.com/article/10.1186/1471-2148-13-123



Background
Eosauropterygians consist of two major clades, the Nothosauroidea of
the Tethysian Middle Triassic (e.g., Nothosaurus) and the
Pistosauroidea. The Pistosauroidea include rare Triassic forms
(Pistosauridae) and the Plesiosauria of the Jurassic and Cretaceous.
Long bones of Nothosaurus and Pistosaurus from the Muschelkalk (Middle
Triassic) of Germany and France and a femur of the Lower Jurassic
Plesiosaurus dolichodeirus were studied histologically and
microanatomically to understand the evolution of locomotory
adaptations, patterns of growth and life history in these two
lineages.

Results
We found that the cortex of adult Nothosaurus long bones consists of
lamellar zonal bone. Large Upper Muschelkalk humeri of large-bodied
Nothosaurus mirabilis and N. giganteus differ from the small Lower
Muschelkalk (Nothosaurus marchicus/N. winterswijkensis) humeri by a
striking microanatomical specialization for aquatic tetrapods: the
medullary cavity is much enlarged and the cortex is reduced to a few
millimeters in thickness. Unexpectedly, the humeri of Pistosaurus
consist of continuously deposited, radially vascularized fibrolamellar
bone tissue like in the Plesiosaurus sample. Plesiosaurus shows
intense Haversian remodeling, which has never been described in
Triassic sauropterygians.

Conclusions
The generally lamellar zonal bone tissue of nothosaur long bones
indicates a low growth rate and suggests a low basal metabolic rate.
The large triangular cross section of large-bodied Nothosaurus from
the Upper Muschelkalk with their large medullary region evolved to
withstand high bending loads. Nothosaurus humerus morphology and
microanatomy indicates the evolution of paraxial front limb propulsion
in the Middle Triassic, well before its convergent evolution in the
Plesiosauria in the latest Triassic. Fibrolamellar bone tissue, as
found in Pistosaurus and Plesiosaurus, suggests a high growth rate and
basal metabolic rate. The presence of fibrolamellar bone tissue in
Pistosaurus suggests that these features had already evolved in the
Pistosauroidea by the Middle Triassic, well before the plesiosaurs
radiated. Together with a relatively large body size, a high basal
metabolic rate probably was the key to the invasion of the
Pistosauroidea of the pelagic habitat in the Middle Triassic and the
success of the Plesiosauria in the Jurassic and Cretaceous.