The potential of x-ray and synchrotron CT scanning in determining soft tissue anatomy in early vertebrates

Abstract

Vertebrate hard parts are potentially a rich source of informatio about the biology of the extinct animals. However, established techniques for studying vertebrate fossils recover only a small part of this information and the techniques are also destructive, which severely limits their utility particularly with rare material. Recently, the use of conventional CT scanning has been used because non-destructive "serial sections" can now be produced rapidly and three-dimensional models reconstructed electronically. However, the resolution provided by most conventional CT scanners is insufficient to study histological features. Very recently, the advent of synchroton CT scanning has been used to visualize sublte texture differences with negligible absorption contrast.Here we present its use to systematically survey, document and reconstruct soft-tissue contacts such as arrested growth surfaces and Sharpey's fibres. Integral to this project is the fossils from the Gogo Formation in Western Australia as they preserve actual soft tissue structures. Recently small areas of muscle tissue were identified in the placoderm Eastmanosteus, which included blood vessels and nerve fibres (Trinajstic et al. 2007) and the preserved umbilical cord connecting the embryo of the ptyctodont Materpiscis to a yold sac (Long et al. 2008). New discoveries have revealed large areas of phosphatised muscle preserved beneath dermal plates in placoderm fishes. For the first time we can map the postcranial musculature in an extinct placoderm fish. In addition structures interpreted as the heart, liver and abdominal tract have been identified in palaeoniscoid fishes. The excellent 3D preservation in these fossils has provided proof of the synchrotron technique.