TAPHONOMY OF THE EARLY CAMBRIAN EMU BAY LAGERSTÄTTE, KANGAROO ISLAND, SOUTH AUSTRALIA
Department of Geology and Geophysics, University of Adelaide, South Australia 5005
|Nedin, C. 1997. Taphonomy of the Early Cambrian Emu Bay Shale Lagerstatte, Kangaroo Island, South Australia. International Cambrian Explosion Symposium, Nanjing, China, April 1995, Programme and Abstracts, p. 20-21.|
The Early Cambrian Emu Bay Shale Lagerstätte at Big Gully is the only site in Australia so far to yield Burgess Shale-type fossils and offers an important basis for comparison with other Burgess Shale-type assemblages, particularly the Chengjiang fauna in China, which is the closest geographically.
The Emu Bay Shale outcrops at two localities on the north coast of Kangaroo Island, at Big Gully and at Emu Bay some 7 km to the west. At Emu Bay the assemblage consists of isolated trilobite exuvia, dominantly cranidia, with hyolithids and phosphatic brachiopods in what appears to be current-swept deposits. This is in marked contrast to the assemblage at Big Gully, which is dominated by the essentially complete specimens of the trilobites Redlichia takooensis and Hsuaspis bilobata and the enigmatic form Myoscolex ateles, a probable soft-bodied arthropod, superficially similar to the Burgess Shale animal Opabinia, rather than an annelid worm as previously described. Other common elements are the phyllocarids Isoxys communis and Tuzoia australis, with the remainder of the assemblage comprising the rarely to very rarely occurring forms Anomalocaris, Palaeoscolex, Naraoia and Xandarella, forms which provide a link with the Chengjiang fauna. The Lagerstatte differs from other Cambrian Konservat-Lagerstätten in that it appears to be a shallow water deposit containing a very diversity-poor assemblage of some 9 genera, in which sessile forms appear to be absent, preserved via mineralization by calcium carbonate, either alone or surrounding muscle tissue preserved by apatite.
Both preservational styes occur in Myoscolex where discrete muscles are evident, especially those running dorso-ventrally, surrounded by a loose meshwork of pink to pinkish-white fibrous calcite. Such preservation can be compared with recent laboratory studies on the fossilization of shrimp muscle (Briggs & Kear 1994), showing the main controlling factor in calcium phosphate precipitation is consistently low pH levels during early decay under anoxic conditions, resulting in the mineralization of labile tissues. Once this phase is complete, a resultant rise in pH levels initiates calcium carbonate precipitation around the margins of the organism. Where labile tissues were absent, as in moults, or environmental conditions unsuitable for phosphate mineralization, only calcium carbonate appears to have been the precipitated.
The taphonomy of the Emu Bay Shale Lagerstätte appears similar to that found in laboratory studies, suggesting a depositional environment with anoxic conditions at or immediately below the sediment-water interface, which allowed decay induced phosphatization, followed by the formation of calcium carbonate around the outside of the specimen. In moults, such as Anomalocaris appendages, only carbonate mineralization is found.
This environment provides an explanation for the lack of sessile organisms in the assemblage, despite their occurrence in the coeval deposit in Emu Bay, since an anoxic sediment column would affect the immediately overlying benthic boundary layer, effectively halting larval settlement and recruitment.
Recent laboratory experiments combined with the first record of the phosphate mineralization of muscle tissues from a Cambrian LagerstStte, enable the chemical environment during the deposition of the Emu Bay Shale Lagerstatte to be constrained, providing a basis for the reconstruction of the palaeoenvironment.
Briggs, D.E.G. & Kear, A. (1994) Decay and mineralization of shrimps. Palaios, 9: 431-456.