Cambroernid

Cambroernids
Temporal range:
Herpetogaster
Eldonia
Scientific classification
Kingdom: Animalia
Stem group: Ambulacraria
Clade: †Cambroernida
Caron, Conway Morris, & Shu, 2010
Subdivisions
Synonyms[1]
  • Dendrobrachia Hou et al. 2006

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]

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

From left: Herpetogaster collinsi, Phlogites longus, Eldonia ludwigi
te: tentacles, ph: pharynx, dt: digestive tract, ics: inner coiled sac, ocs: outer coiled sac, an: anus, st: stolon

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
(total group)

Internal classification

Genera whose family placement is tentative are preceded with (?).

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]

References

  1. ^ a b Caron, Conway Morris & Shu 2010, pp. 1–2
  2. ^ a b Hagadorn & Allmon 2019
  3. ^ a b c d Caron, Conway Morris & Shu 2010, p. 2
  4. ^ a b Li et al. 2023
  5. ^ a b Li et al. 2023, p. 2362
  6. ^ Zhu, Zhao & Chen 2002
  7. ^ a b c Dzik, Yuanlong & Maoyan 1997
  8. ^ a b c Lefebvre et al. 2022
  9. ^ Chen, Zhu & Zhou 1995
  10. ^ Caron, Conway Morris & Shu 2010, p. 7
  11. ^ MacGabhann & Murray 2010
  12. ^ a b Li et al. 2023, pp. 2362–2364
  13. ^ Li et al. 2023, p. 2363
  14. ^ Schroeder, Paterson & Brock 2018
  15. ^ Lieberman et al. 2017, p. 12
  16. ^ Zhang et al. 2006 (Note: Cites the 1999 discovery publication Early Cambrian Chengjiang Fauna from Kunming Region China (Luo Hui-Lin, Hu Shi-Xie, Chen Liang-Zhong, Zhang Shi-Shan, Tau Yong-Shan), which is in Chinese and not available online)

Works cited

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