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)[1] 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,"[2] later work with better-preserved fossils has strengthened the argument for Cambroernida as a monophyletic clade.[3]
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.[2]
Taxonomy and evolution
From left: Herpetogaster collinsi, Phlogites longus, Eldonia ludwigi
Body coiling increased throughout this group's evolution.[4]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[2][5] (also known as eldonioids[6][7][8] or eldonids[1][7]) 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.[6][9][10][7]
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.[11]
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.[11]
Phylogeny
Phylogenetic analysis offers strong support for Cambroernida as a clade of stem-group ambulacrarians.[4] The following cladogram is simpllified from Li et al. 2023; only a sampling of eldonioids were included in the analysis:[12]
Note that some authors continue to treat Stellostomites as a separate taxon.[14]
History of identification
Previously, some cambroernids were compared to members of the broad invertebrate clade Lophotrochozoa. In particularl, they were allied with the lophophorates, a subset of lophotrochozoans bearing a crown of ciliated tentacles known as the lophophore.[6][15] However, this interpretation has more recently been considered unlikely, insofar as cambroernids are interpreted as deuterostomes, whereas lophophorates are protostomes.[2]
↑ 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)
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