Cambroernid

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 Temporal range: Cambrian–Devonian PreꞒ Ꞓ O S D C P T J K Pg N
Herpetogaster
Eldonia
Scientific classification
Kingdom:	Animalia
Stem group:	Ambulacraria
Clade:	†Cambroernida Caron, Conway Morris, & Shu, 2010
Subdivisions
Herpetogaster Phlogites Eldonioidea

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.^[2]

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.^[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]

Ambulacraria	Cambroernida Herpetogaster Phlogites Rotadiscus Eldonia eumorpha Eldonia ludwigi Pararotadiscus Eldonioids Ambulacraria Hemichordata Echinodermata (crown group)
(total group)

Internal classification

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

• Herpetogaster
• Phlogites
• Class Eldonioidea^[3][13]
  • Family Eldoniidae
    • Eldonia (=Stellostomites; =Yunnanomedusa)
  • Family Rotadiscidae
    • Rotadiscus (=Brzechowia)
    • Pararotadiscus
    • (?) Vellumbrella
    • (?) Seputus
  • "Paropsonemids" (informal group)
    • Paropsonema
    • Discophyllum

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 particular, 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]