Eldonia ludwigi Walcott, 1911 (diameter ~3 to ~3.5 cm across), preserved as a carbonized film in slightly metamorphosed shale from the Burgess Shale (Middle Cambrian) of southwestern Canada (YPM 5824, Yale University’s Peabody Museum, New Haven, Connecticut, USA).

The Middle Cambrian-aged Burgess Shale is the most famous fossil deposit on Earth. It is located near the town of Field in Yoho National Park, southeastern British Columbia, western Canada. The deposit is famous for its spectacular soft-bodied preservation - the organisms have had their appendages & internal organs preserved. Many tens of thousands of fossils have been collected from the Burgess Shale Formation over the last century. Including known, but unnamed species, and excluding known or demonstrable junior synonyms, the Burgess Shale biota totals at least ~280 species.

Many claim that Charles Walcott discovered the Burgess Shale Lagerstätte (as soft-bodied fossil deposits are called by paleontologists) in 1909. However, it was actually discovered in 1886 or 1888 by Richard McConnell, based on anomalocarid appendage material from Mt. Stephen, in the Campsite Cliff Member of the Burgess Shale Formation. The main collecting localities have been two quarries (Walcott Quarry & Raymond Quarry) on the western side of the ridge connecting Mt. Field and Wapta Mountain a little north-northeast of Field. Numerous other smaller localities have been identified in the same area & for many, many kilometers to the south. Collecting at the Burgess Shale was most intense in 1910-1917 (Charles Walcott), 1925-1930 (Harvard’s Museum of Comparative Zoology), 1966-1967 (Geological Survey of Canada), and 1975-2000s (Royal Ontario Museum).

Eldonia ludwigi is a good example of a problematic fossil organism (= not easily taxonomically classified). Eldonia and Eldonia-like fossils are known from scattered localities around the world, but they are concentrated in Cambrian soft-bodied fossil deposits, such as British Columbia’s Burgess Shale and China’s Chengjiang deposit.

This slab from the Burgess Shale shows a specimen of Eldonia ludwigi Walcott, 1911. Eldonia had a nonmineralized, medusiform, disc-shaped body with radiating structures, apparent tentacular organs, and a coiled gut (usually very dark-colored). Early interpretations held that Eldonia was a holothurian (a sea cucumber). This view may be correct, but Eldonia and Eldonia-like organisms are often now considered to be lophophorates (Superphylum Lophophorata, which includes several familiar groups: brachiopods, bryozoans, entoprocts, and phoronids). Who knows?

Stratigraphy: Walcott Quarry Member, Burgess Shale Formation, Ptychagnostus praecurrens Interval-zone, lower Marjuman Stage, middle Middle Cambrian.