The Emu Bay Shale is a geological formation located in Emu Bay, South Australia. It contains a significant fossil bed known as a Konservat-Lagerstätte, which preserves soft tissues of ancient organisms. It is one of only two places in the world that have Redlichiidan trilobites. The Emu Bay Shale is dated to the Cambrian Series 2, Stage 4, which corresponds to the upper Botomian Stage of the Lower Cambrian.
The way fossils are preserved in the Emu Bay Shale is similar to that of the Burgess Shale. However, the larger grain size of the rocks in the Emu Bay Shale results in less detailed preservation compared to the Burgess Shale. Over 50 species of trilobites, non-biomineralized arthropods (including megacheirans, nektaspids, and hymenocarines), radiodonts, palaeoscolecids, a lobopodian, a polychaete, vetulicolians, nectocaridids, hyoliths, brachiopods, sponges, chancelloriids, several problematica, and a chelicerate have been identified in the Emu Bay Shale.
Description
The Emu Bay Shale on Kangaroo Island, South Australia, is Australia's only known Burgess-Shale-type fossil site. It contains fossils of creatures like Anomalocaris, Tuzoia, Isoxys, and Wronascolex, which are also found in other Burgess-Shale-type sites, such as the Chengjiang Biota in China. Some types of arthropods, including Squamacula, Kangacaris, and Tanglangia, are only found in the Emu Bay Shale and Chengjiang. The site also has high-quality trilobite fossils, such as Redlichia takooensis, Emuella polymera, Balcoracania dailyi, Megapharanaspis nedini, Holyoakia simpsoni, and Estaingia (=Hsuaspis) bilobata. Balcoracania and Emuella are the only known members of the Redlichiina family Emuellidae, which has trilobites with the most thoracic segments ever found (up to 103 in one specimen). These trilobites are only found in Australia and Antarctica.
Most Burgess-Shale-type fossil sites were formed in deep water on the outer shelf of the ocean. In contrast, the Emu Bay Shale appears to have formed in shallow, restricted basins on the inner shelf. This suggests that soft tissue preservation occurred in different environments during the Cambrian period. For example, Estaingia and "petalloids" lived in deeper areas, while Balcoracania lived in shallow, intertidal areas like tide pools. Some fossils from Emu Bay show soft tissues preserved as blocky apatite or fibrous calcium carbonate, including the oldest known phosphatized muscle tissue, similar to finds in Greenland. Mid-gut glands in arthropods like Isoxys and Oestokerkus are preserved in calcium phosphate, just as in related species from the Burgess Shale.
The Emu Bay Shale is exposed on the east side of Emu Bay, where it lies above the White Point Conglomerate. This area has many fossils, including Estaingia, Redlichia, hyolithids, brachiopods, and Chancelloria. At Big Gully, 8 km east of White Point, the Emu Bay Shale is found above the White Point Conglomerate and contains soft-bodied fossils, such as Anomalocaris, Echidnacaris, Isoxys, Tuzoia, Emucaris fava, Kangacaris zhangi, Wronascolex, Myoscolex, "petalloids," Vetustovermis, and other rare species. Trilobites at Big Gully rarely show non-biomineralized tissue, though some Redlichia fossils have visible antennae. Other fossils from inland areas include Oestokerkus, a relative of Leanchoilia; Wisangocaris, an early chelicerate; and Australimicola, a type of artiopodan arthropod. A single specimen of a Luolishaniidae lobopodian closely resembles Collinsovermis monstruosus from the Burgess Shale.
In 2011, scientists discovered seven large, isolated compound eyes from the inland quarry at Emu Bay, as well as the first well-preserved visual surfaces of Anomalocaris eyes. These findings support the idea that anomalocaridids are closely related to arthropods. The eyes had 24,000 lenses and were 30 times more powerful than those of trilobites, which were previously thought to have the most advanced eyes of their time. The resolution of these eyes would have been nearly as sharp as that of a modern dragonfly, which has 28,000 lenses in each eye.