DECAY AND COMPOSITION OF THE HEMICHORDATE RHABDOPLEURA - IMPLICATIONSFOR THE TAPHONOMY OF GRAPTOLITES

Although the graptolites lacked biomineralised tissue, their skeletons are abundantly preserved in deeper-water mudstones. Decay experiments and observations on the closely related living hemichordate Rhabdople ura demonstrate that the periderm and stolen are highly resistant to d ecay, remaining intact for months, whereas the zooids are unrecognizab le within days. The extreme rarity of the preservation of traces of th e zooids in graptoloids reflects their planktic lifestyle; the zooids had normally decayed before burial. Curie-point-gas-chromatography (Py -GC) and Curie-point-gas-chromatography-mass spectrometry (Py-GC-MS) o f the periderm of Rhabdopleura confirms that proteinaceous organic mat ter is a major constituent. Untrastructurally preserved graptolite per iderm (Ordovician, Oklahoma; Silurian, Arctic Canada), on the other ha nd, is a highly altered kerogen-like substance rich in aliphatic bioma cromolecules. The composition of the preserved graptolite periderm ref lects diagenetic replacement by components probably mainly derived fro m algal cell walls.