Wednesday, 5 August 2015

Viviparity in extinct reptiles

Mesosaur - an aquatic reptile from the Palaeozoic
Nobu Tamura (CC BY 2.5 via Wikimedia Commons)

Dan Blackburn and Christian Sidor have written an interesting paper (here)summarizing the fossil evidence for viviparity in the Late Palaeozoic and Mesozoic Eras. They document a minimum of six separate evolutionary origins of viviparity. Most are aquatic reptiles including mesosaurs (picture), sauropterygians, ichthyopterygians, choristoderans and mosasauroids. The sixth example is a Cretaceous lizard Yabeinosaurus.

Mosasaurus hoffmannii - an aquatic reptile from the Late Cretaceous
 Nobu Tamura (CC BY 3.0 via Wikimedia Commons)

It cannot be established with certainty that the fetuses were supported by a placenta, since soft tissues usually do not fossilize. The probability is great, however, given the advanced stage of development at birth and by analogy with present day snakes and lizards. Some have extremely complex placentas an example being the New World lizard Mabuya (here).

Materpiscis attenboroughi - a live-bearing fish from the Late Devonian 
Sularko - Museum Victoria (Licensed under CC BY-SA 3.0 via Wikimedia Commons)

My own article on fossil evidence for viviparity (here) was prompted by the discovery of an extremely well preserved fish with a putative umbilical cord (here).

The chicken or the egg

 As an afterthought it is of interest that the original amniote egg had a leathery shell like many reptiles today. A calcified shell was evolved at least four times (here) including in the dinosaur lineage that led to the birds. The calcified egg came before the chicken!    

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