Eldonia

Eldonia is an extinct soft-bodied cambroernid best known from the Fossil Ridge outcrops of the Burgess Shale, particularly in the 'Great Eldonia layer' in the Walcott Quarry. In addition to over 550 specimens collected by Walcott, 224 specimens of Eldonia are known from the Greater Phyllopod bed, where they comprise 0.43% of the community. Species also occur in the Chengjiang biota, Siberia, and in Upper Ordovician strata of Morocco.

Description

It takes the form of a round, medusoid disk (which originally led to suggestions of a jellyfish affinity) with a C-shaped gut trace. The gut is recalcitrant and can be extracted using Hydrofluoric acid.

A specimen from the Lower Ordovician Madaoyu Formation in Hunan, South China, can be interpreted as the incomplete body of Eldonia or the similar animal. However, its annulation, the structure of the intestine and the shape of the body are more similar to those of Ottoia.

Classification

Walcott's original interpretation as a holothurian was rapidly disputed. Alternative affinities to be suggested, which did not stand the test of time, included the siphonophores and a coelenterate medusa.

Eldonia and the other Eldoniidae form a clade, Eldonioidea, with the Rotadiscidae and the informal group known as the paropsonemids. The Eldonioidea are, in turn, part of the stem-ambulacrarian clade Cambroernida.

There are three species within Eldonia, although some authors consider E. eumorpha to be a distinct genus, Stellostomites, as S. eumorphus.

Paleoecology

The organism is frequently found in association with the lobopod Microdictyon, which is presumed to have fed on Eldonia.

• Eldonia berbera
  Eldonia berbera
• Eldonia ludwigii, Walcott Quarry. About 4 cm long
  Eldonia ludwigii, Walcott Quarry. About 4 cm long

See also

• Paleobiota of the Burgess Shale