Unranked clade Cambroernida (stem group ambulacrarians)
Eldonia, together with other discoidal or pedunculate fossils such as Herpetogaster, probably belongs in the stem group to a clade known as the Ambulacraria, represented by both echinoderms and hemichordates (Caron et al., 2010).
Eldonia – from Eldon, a train stop on the Canadian Pacific Railway 30 km east of Field. Eldon is named after a town in County Durham, England, and means “Aelle’s hill.”
ludwigi – after Hubert Ludwig, a German echinoderm expert who described many fossil holothurians.
Lectotype – USNM 57540 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: Stellostomites eumorphus (Sun and Hou, 1987), from the Lower Cambrian Chengjiang fauna, was redescribed as Eldonia eumorpha (Chen et al., 1995). However, S. eumorphus is retained in the literature as the only valid species (Zhu et al. 2002); E. berbera was described from the Upper Ordovician of Morocco (Alessandrello and Bracchi, 2003). If confirmed it would be the youngest stratigraphic occurrence for the genus.
Middle Cambrian, Bathyuriscus-Elrathina Zone to Bolaspidella Zone (approximately 505 million years ago).
Burgess Shale and vicinity: Walcott and Raymond Quarries on Fossil Ridge.
Other deposits: Middle Cambrian Spence Shale and Marjum Formation in Utah (Conway Morris and Robison, 1988).
Brief history of research:
Described by Walcott in 1911, Eldonia was originally interpreted as a holothurian (sea cucumber within the echinoderms), a view that was accepted by some eminent experts at the time (Clark A.H., 1913) and upheld by later re-examination of the material (Durham, 1974). However, this interpretation has always had detractors (Clark H.L., 1912; Dzik, 1991, 1997; Madsen, 1956, 1957, 1962; Paul and Smith, 1984), and the lack of key echinoderm features prohibits a close relationship with that group (Conway Morris, 1993; see also Zhu et al., 2002). Despite their resemblance to jellyfish (scyphozoans) there is a wide consensus that eldoniids do not share any affinities with cnidarians. A connection to “lophophorates” (e.g., brachiopods, phoronids) has been argued in more detail (Chen et al., 1995, Dzik, 1997), but this status remains rather problematic. The description of Eldonia’s close relative Herpetogaster provides a possible link to the Ambulacraria, a group that contains the echinoderms and hemichordates (Caron and Conway Morris, 2010).
Fragments of the reflective gut have been extracted by acid maceration and analyzed for taphonomic studies (Butterfield, 1990).
Eldonia has a discoidal body with both anus and mouth opening ventrally. Fine rays radiate from a central point within the disc. The gut coils clockwise (viewed from the dorsal surface) around the centre of the organism and is clearly separated into a pharynx, stomach (the darker area), and narrow intestine. There is a pair of relatively stout tentacles around the mouth which probably were used for feeding.
Walcott collected hundreds of specimens of Eldonia in a single fossil layer within the Phyllopod Bed that he called the Great Eldonia Layer. Additional specimens have since been collected from the Walcott Quarry, where they comprise 0.4% of the community (Caron and Jackson, 2008).
Eldonia has conventionally been interpreted as a free-floating filter-feeder. However, based on its morphology, preservational patterns, and its similarity with Herpetogaster, a benthic lifestyle has also been proposed, with its tentacles either collecting food from the water, or sweeping the sea floor for particles of detritus (Caron and Conway Morris, 2010). It is unclear whether the animal could move at least occasionally or was permanently stationary (sessile).