Friday, 21 August 2015

Bringing the Chengjiang Fossils Into Focus

Burgess Shale fossils are not perfectly flat, but instead the highly
compressed three dimensional remains of the original creature
The Burgess Shale fossils occupy a hallowed place in the history of life, for their sheer morphological diversity and the insight they provide into the nature of evolution itself.

For decades it was thought that the fossils were preserved as thin films of alumina and carbon on the surface of the shale. It was only in the 1970s that palaeontologists realised that while these films were thin, they were not perfectly flat. The creatures of the Burgess shale were still preserved in three dimensions, albeit highly compressed.

This opened up the exciting possibility of being able to 'dissect' the specimens and reveal hitherto unknown anatomical elements. Harry Whittington, Derek Briggs and Simon Conway Morris revolutionised our understanding of the Burgess Shale organisms. Yet the technique is destructive and undesirable if a specimen is rare or one of a kind.

A recent study, however, conducted on fossils from the Burgess Shale's Asian counterpart the Maotianshan Shale, focused on the reconstruction of previously obscured anatomical elements without resorting to destructive methods. Specimens from Maotianshan can extend several millimetres below the surface of the rock, so a micro-CT was used to bring these details into focus for the first time.

A light micrograph (left) and a micro-CT image (right) of a 520 million year old specimen of Xandarella spectaculum from the Chengjiang lagerstatte.
Micro-CT, or computed microtomography, involves taking radiographs of the specimen from multiple angles and mathematically compiling the data into a single three dimensional image. The analysis was conducted on Xandarella spectaculum, an arthropod with certain similarities to trilobites. The species is rare and known exclusively from Chengjiang, China, making it a perfect test case of an irreplaceable specimen with potentially unresolved anatomy. 'Microtomography is a powerful technique for the analysis of the three-dimensionally preserved specimens recovered at Chengjiang,' said Yu Lui from the Department of Biology at Ludwig-Maximilians-Universitaet in Munich.

Micro-CT is now a widely used imaging technique in palaeontology owing to its ability to reconstruct features which are small, delicate, obscured or otherwise impossible to excavate. As such its applications are wide ranging. Yet they have yet to be extensively applied to Chengjiang specimens. With the proof of concept in place, we may well expect a new revolution in thinking similar to that which sparked Harry Whittington and his team to dissect the Burgess Shale fossils for the first time. The Chengjiang fossils have already yielded many surprises. Combined with this powerful form of analysis, new discoveries will surely be made.