Cambroernid


The Cambroernida are a clade of Paleozoic animals with coiled bodies and filamentous tentacles. They include a number of early to middle Paleozoic genera noted as "bizarre" or "orphan" taxa, meaning that their affinities with other animals, living or extinct, have long been uncertain. While initially defined as an "informal stem group," later work with better-preserved fossils has strengthened the argument for Cambroernida as a monophyletic clade.

Description

Cambroernids encompass three particular types of enigmatic animals first appearing in the Cambrian: Herpetogaster, Phlogites, and the Eldonioidea. They are united by a set of common features including at least one pair of bifurcated or divided oral tentacles, and a large stomach and narrower intestine enclosed together in a clockwise-coiled sac.

Taxonomy and evolution

Body coiling increased throughout this group's evolution. Herpetogaster has a segmented and clockwise-curved body attached to the substrate via a narrow and partially mobile stolon. Phlogites was even more simple, with a thick immobile stolon leading up to a tentacle-bearing calyx, with internal gut coiling. The eldoniids were diverse and disc-shaped, commonly described as "medusiform", i.e. jellyfish-shaped. Though the lifestyle of eldoniids is still debated, it can be agreed that they had a large curved stomach and no stolon.
The lack of a post-anal tail in cambroernids suggests that, contrary to long-held assumptions, this feature was not present in the common ancestor of deuterostomes. This is congruent with the significant differences between the post-anal tails of chordates and hemichordates. This and other features of cambroernids suggest that post-anal tails, gill bars, and a U-shaped gut evolved multiple times in the deuterostomes through convergence.
Segmentation, as seen in Herpetogaster, is a notable characteristic of chordates not seen in other ambulacrarians, indicating that it might be a trait of ancestral deuterostomes.

Phylogeny

Phylogenetic analysis offers strong support for Cambroernida as a clade of stem-group ambulacrarians. The following cladogram is simplified from Li et al. 2023; only a sampling of eldonioids were included in the analysis:

Internal classification

Genera whose family placement is tentative are preceded with.
  • Herpetogaster
  • Phlogites
  • Class Eldonioidea
  • * Family Eldoniidae
  • ** Eldonia
  • * Family Rotadiscidae
  • ** Rotadiscus
  • ** Pararotadiscus
  • ** Vellumbrella
  • ** Seputus
  • * "Paropsonemids"
  • ** Paropsonema
  • ** Discophyllum
Note that some authors continue to treat Stellostomites as a separate taxon.

History of identification

Previously, some cambroernids were compared to members of the broad invertebrate clade Lophotrochozoa. In particular, they were allied with the lophophorates, a subset of lophotrochozoans bearing a crown of ciliated tentacles known as the lophophore. However, this interpretation has more recently been considered unlikely, insofar as cambroernids are interpreted as deuterostomes, whereas lophophorates are protostomes.

Works cited

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Category:Burgess Shale fossils