New - Eggs and viviparity of the choristoderan Hyphalosaurus

[Jocelyn Falconnet <j.falconnet@gmail.com>]

Hou L.-H., Li P.-P., Ksepka D.T., Gao K.-Q. & Norell M.A. In press. 
Implications of flexible-shelled eggs in a Cretaceous choristoderan 
reptile. Proceedings of the Royal Society B 277(1685):1235-1239.

Flexible, or soft-shelled, eggs are almost unknown in the fossil record, 
leaving large gaps in our knowledge of the reproductive biology of many 
tetrapod clades. Here, we report two flexible-shelled eggs of the 
hyphalosaurid choristodere Hyphalosaurus baitaigouensis from the Early 
Cretaceous of China, one containing an embryo and the second associated 
with a neonate. Choristoderes are an enigmatic group of aquatic reptiles 
that survived the K–T extinction but died out in the Miocene. 
Hyphalosaurids, a specialized clade of Choristodera, resemble miniature 
plesiosaurs and are considered to be primarily aquatic in habit. 
Scanning electron microscopy of samples from the eggs reveals a thin, 
non-columnar external mineralized layer characterized by rounded nodes 
and tentatively identified poorly structured irregular pores, with an 
underlying amorphous layer presumably representing decomposed protein 
fibrils. While the relationships of Choristodera remain controversial, 
eggshell microstructure more closely resembles that of Lepidosauromorpha 
(the lineage including lizards) as opposed to that of Archosauromorpha 
(the lineage including birds and crocodiles).

http://rspb.royalsocietypublishing.org/content/277/1685/1235.full
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Ji Q., Wu X.-C. & Cheng Y.-N. 2010. Cretaceous choristoderan reptiles 
gave birth to live young. Naturwissenschaften 97(4): 423-428.

Viviparity (giving birth to live young) in fossil reptiles has been 
known only in a few marine groups: ichthyosaurs, pachypleurosaurs, and 
mosasaurs. Here, we report a pregnant specimen of the Early Cretaceous 
Hyphalosaurus baitaigouensis, a species of Choristodera, a diapsid group 
known from unequivocal fossil remains from the Middle Jurassic to the 
early Miocene (about 165 to 20 million years ago). This specimen 
provides the first evidence of viviparity in choristoderan reptiles and 
is also the sole record of viviparity in fossil reptiles which lived in 
freshwater ecosystems. This exquisitely preserved specimen contains up 
to 18 embryos arranged in pairs. Size comparison with small free-living 
individuals and the straight posture of the posterior-most pair suggest 
that those embryos were at term and had probably reached parturition. 
The posterior-most embryo on the left side has the head positioned 
toward the rear, contrary to normal position, suggesting a complication 
that may have contributed to the mother’s death. Viviparity would 
certainly have freed species of Hyphalosaurus from the need to return to 
land to deposit eggs; taking this advantage, they would have avoided 
intense competition with contemporaneous terrestrial carnivores such as 
dinosaurs.

http://www.springerlink.com/content/47727132vv60374q/?p=8c7516042406403283e502874ff8764e&pi=8