Sunday, 30 August 2015

Defining the genus Homo

Reconstruction of Homo erectus georgicus by √Člisabeth Daynes
Licensed under CC BY-SA 3.0 via Wikipedia Commons
A thought provoking essay in the current issue of Science (here) asserts definitions of the genus Homo and the species assigned to it remain "as fuzzy as ever." They drive home their point by showing, on the one hand, specimens attributed to Homo with australopithecine features and, on the other, australopithecine fossils with features hitherto claimed to be confined to Homo.

This is a brief but instructive read. The take home message is that hominid systematics needs to be rethought. In the process we may need to scrap "the iconic list of names in which fossil specimens have historically been trapped."

Cover of Third Edition 1956
One name that may have to go is Homo habilis (previous post), the handy man. Louis Leakey, it is suggested, was keen (perhaps too keen) to identify this fossil as the maker of simple tools following the dictum of Man the Toolmaker. It was a pleasant surprise to find Kenneth P. Oakley's booklet cited as an influence on previous and current thinking. My well thumbed copy dates from my schooldays.

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!