Sunday, February 24, 2008

Thingodonta: a salient lesson in analysis

So much to do, so little time. I would do more on non-paleontological subjects, but they keep getting crowded out...

New Zealand's SB mammal fossil has been interpreted as more archaic than therians (marsupials and 'placentals', referred here as metatherians and eutherians), although on a relatively similar par with monotremes, Australia's earliest known mammals and the world's most ancient extant lineage.

Mike Archer (et al)'s book Australia's Lost World includes a relevant anecdote about mammal classification.

The mammal is now called Yalkaparadonta, but started out as Thingodonta, because the teeth, uncovered first, were unlike any known clades of mammals.

A few months later, a jawbone turned up. It demonstrated three molars, where all plesiomorphic (both primitive and derived) metatherians had four [apart from some specialised groups] but eutherians properly had only three. Further, dental action was much more akin to that of eutherians, ie this would be the first early 'placental' in Australia.

In the third step, a skull turned up. Evidence now suggested it was a metatherian albeit quite primitive, and with a front section unlike any other.

Classification had switched back and forward; fortunately publication hadn't happened until the fuller story had emerged.

This creature isn't even listed on Paleos, although there's an entry at the Australian Museum site Australia's Lost Kingdoms and Wikipedia has some good (sourced but broken-linked) discussion on the classification difficulties. That discussion ultimately placed it in its own order, with some dissent from one Frederick Szalay who placed it as a (primitive) diprotodontan metatherian.

Some distinctiveness, some alignment with metatherians and with eutherian insectivores, with a good dose of convergent evolution thrown in.

Ah, convergence. Must be the bane of a taxonomist's life. But the lesson is that classification can be disrupted by hitherto unseen characteristics. For example, in the SB mammal, the femur suggests an abducted, or sprawling gait, although not as much so as monotremes. Yet that abduction must help place it, as it is placed, close to monotremes.

One must easily accept that the paper on the mammal brings together all current knowledge - at the time of publication. Yet it's understandable if there's much keenness to uncover more evidence, as a femur and two jaw fragments tell a tantalisingly curtailed story.


Archer M, Hand S J, Godthelp H (1991): Australia's Lost World: Riversleigh, World Heritage Site. Reed, Sydney.

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