Cambroernid
Extinct clade of animals
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The Cambroernida are a clade of Paleozoic animals with coiled bodies and filamentous tentacles. They include a number of early to middle Paleozoic (Cambrian to Devonian)[2] 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,"[3] later work with better-preserved fossils has strengthened the argument for Cambroernida as a monophyletic clade.[4]
Caron, Conway Morris, & Shu, 2010
| Cambroernids Temporal range: | |
|---|---|
| |
| Herpetogaster | |
 | |
| Eldonia | |
Scientific classification  | |
| Kingdom: | Animalia |
| Stem group: | Ambulacraria |
| Clade: | †Cambroernida Caron, Conway Morris, & Shu, 2010 |
| Subdivisions | |
| Synonyms[1] | |
| |
Description
Cambroernids encompass three particular types of enigmatic animals first appearing in the Cambrian: Herpetogaster (the type genus), 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.[3]
Taxonomy and evolution
Body coiling increased throughout this group's evolution.[5] Herpetogaster has a segmented and clockwise-curved body attached to the substrate via a narrow and partially mobile stolon (stalk). Phlogites was even more simple, with a thick immobile stolon leading up to a tentacle-bearing calyx (cup-shaped main body), with internal gut coiling. The eldoniids[3][6] (also known as eldonioids[7][8][9] or eldonids[2][8]) 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.[7][10][11][8]
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.[12]
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.[12]
While Phlogites was at one point assigned to a newly proposed phylum, Dendrobrachia, this was discarded for the larger group in favor of cladistic approach to stem groups as ICZN precedence rules do not apply to phyla. A further explanation given was that defining a new phylum was seen as "essentialist" and serving to "place problematic taxa in phylogenetic limbo."[1]
Phylogeny
Phylogenetic analysis offers strong support for Cambroernida as a clade of stem-group ambulacrarians.[5] The following cladogram is simplified from Li et al. 2023; only a sampling of eldonioids were included in the analysis:[13]
| Ambulacraria |
| ||||||||||||||||||||||||||||||||||
Internal classification
Genera whose family placement is tentative are preceded with (?).
- Herpetogaster
- Phlogites
- Class Eldonioidea[4][14]
- Family Eldoniidae
- Eldonia (=Stellostomites; =Yunnanomedusa)
- Family Rotadiscidae
- Rotadiscus (=Brzechowia)
- Pararotadiscus
- (?) Vellumbrella
- (?) Seputus
- "Paropsonemids" (informal group)
- Paropsonema
- Discophyllum
- Family Eldoniidae
Note that some authors continue to treat Stellostomites as a separate taxon.[15]
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.[7][16] However, this interpretation has more recently been considered unlikely, insofar as cambroernids are interpreted as deuterostomes, whereas lophophorates are protostomes.[3]