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Yabeinosaurus, earliest viviparous squamate

From: Ben Creisler

New online article:

Yuan Wang and Susan E. Evans (2011)
A gravid lizard from the Cretaceous of China and the early history of
squamate viviparity. 
Naturwissenschaften (advance online publication)
DOI: 10.1007/s00114-011-0820-1

Although viviparity is most often associated with mammals, roughly one
fifth of extant squamate reptiles give birth to live young. Phylogenetic
analyses indicate that the trait evolved more than 100 times within
Squamata, a frequency greater than that of all other vertebrate clades
combined. However, there is debate as to the antiquity of the trait and,
until now, the only direct fossil evidence of squamate viviparity was in
Late Cretaceous mosasauroids, specialised marine lizards without modern
equivalents. Here, we document viviparity in a specimen of a more
generalised lizard, Yabeinosaurus, from the Early Cretaceous of China. The
gravid female contains more than 15 young at a level of skeletal
development corresponding to that of late embryos of living viviparous
lizards. This specimen documents the first occurrence of viviparity in a
fossil reptile that was largely terrestrial in life, and extends the
temporal distribution of the trait in squamates by at least 30 Ma. As
Yabeinosaurus occupies a relatively basal position within crown-group
squamates, it suggests that the anatomical and physiological preconditions
for viviparity arose early within Squamata. 

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