Monday, 29 October 2012

A Cretaceous Coelacanth

Coelacanths are anachronistic. Still around today, they inhabit deep underwater caves by day and hunt only at night. While they are comparatively rare nowadays, their fossil record stretches back 360 million years. Apart from the fact that they have barely changed since this time, giving us great insight into their origins and evolutionary history, they also shed light on how the first ever land-dwelling vertebrates evolved.

A modern day coelacanth with its distinctive blue colour
The coelacanths are part of a group of fish which have bones in their pectoral fins, similar to those found in the legs of land-dwelling vertebrates, properly called tetrapods. This confirms a connection between fish and amphibians. Since those early days in coelacanth history, different species have come and gone. Latimeria chalumnae is still around and comes in at over a metre in length. Rebellatrix was as large, and the only member of its group to adapt to a fast predatory lifestyle similar to that of a shark.

The 100 million year old skull of Reidus hilli,
a newly discovered species of coelacanth
Now, a new species has been uncovered from 100 million year old rocks in Texas. Fragments of a skull were found near Fort Worth by Robert. R. Reid, an amateur palaeontologist, who donated it to the Southern Methodist University. It was studied by John. F. Graf, a graduate student, who quickly identified it as a completely new species. 'When I found it, I could tell it was a bone but I didn't think it was anything special,' said Reid. 'I certainly didn't think it was a coelacanth.'

The skull from another angle
The specimen itself came from the Duck Creek Formation, a 15 metre thick cliff composed of alternating bands of limestone and shale deposited n the Cretaceous sea some 100 million years ago. The skull was identified as belonging to a coelacanth on account of the lozenge-shaped plates of bone on the underside of the jaw which are unique to the group. Graf named it Reidus hilli, in honour of its finder and of Robert. T. Hill, the so called 'Father of Texas' geology and the first to study the Duck Creek Formation.

What is interesting is that this creature was small for a coelacanth, especially considering that some of its close relatives were over 3 metres in length. It was no more than 40 centimetres long. The skull was just 4.5 in length and half as wide. It belonged to a now extinct group of coelacanths called the diplurids, which were also named by Graf.