INVERTEBRATE TRACE FOSSILS INSIDE VERTEBRATE SKULLS: THE WORMS CRAWL IN, AND THE WORMS CRAWL OUT

As with biotaxa (body fossils), ichnotaxa (trace fossils) may exhibit an appreciable range of intrageneric and intraspecific variations in their characteristic shapes. Stenomorphic trace fossils, whose morphologies deviate irregularly from the expected norm, can result from environmental stress (e.g., elevated or lowered salinity), heterogeneous substrate (e.g., heterolithic or incompletely cemented rock), or constricted habitat space (e.g., in small cavities in rockgrounds or vertebrate skulls).

After a vertebrate animal dies and its soft tissues begin to decay, the cavities inside the skull often are filled with sediment. The endocranial microenvironment of a dead animal offers a very special habitat for burrowing organisms, such as insects and oligochaetes (on land) and crustaceans and polychaetes (in marine settings), whose burrows and ichnofabrics represent the post-mortem infaunal community within the bony confines of the vertebrate's remains. This unusual habitat is an extremely confined space for burrowers, so their burrows typically are quite stenomorphic. The post-depositional history of a vertebrate carcass, such as its possible transport from the living environment into a very different depositional environment, can be determined by examining the burrows inside the sediment-filled skull. Also, the pre-cementation residence time of a dead body on the sea floor or river bottom can be interpreted on the basis of the texture and structure of the sediment fill inside the skull, including the complexity of the endocranial ichnofabric.

Of course, endocranial trace fossils are completely hidden from normal view, which presents a difficult problem for ichnologists. Trace fossils are three-dimensional entities, yet most trace fossil studies are accomplished by means of two-dimensional views, and reconstruction of their three-dimensional morphology generally is done by piecing together various views of multiple specimens or by the time-consuming and specimen-destroying process of serial sectioning. The non-destructive technique of high-resolution computed tomography (CT-scanning) of fossil specimens allows us to view endocranial trace fossils in full relief in exquisite detail and to reconstruct the complex ichnofabrics in three dimensions.