Fossil Friday: An Extinct Animal Body Plan from the Cambrian Explosion
This Fossil Friday features Herpetogaster collinsi from the Middle Cambrian Burgess Shale in Canada. It is as an example of an extinct group of animals called Cambroernida, that originated in the Early Cambrian and disappeared in the Late Devonian.
Eldonia — The Medusa That Wasn’t
The Cambrian fauna of the famous Burgess Shale includes numerous enigmatic fossils that for a long time eluded any attempts by evolutionary scientists to place them in the tree of life. One of these fossils is Eldonia ludwigi, which was a soft-bodied animal with disc-shaped body, a coiled gut, and a ring of feeding tentacles around the oral opening. More than hundred years ago, Charles Walcott (1911) collected 550 specimens from the Burgess Shale and many more have meanwhile been found, including specimens from the Early Cambrian of Chengjiang in southwest China (Sun & Hou 1987, Chen et al. 1995, Zhu et al. 2002), the Middle Cambrian of Utah (Conway Morris & Robinson 1988) and Siberia (Friend 1995, Ivantsov 1998, Friend et al. 2002), and the Late Ordovician Erfoud sandstones of Morocco (Alessandrello & Bracchi 2003). Furthermore, seven other genera of eldoniids were described from different localities and strata. The latest fossil record is the putative eldoniid Paropsonema from the Upper Devonian of New York (Hagadorn & Allmon 2019), which was first described by Clarke (1900) as an echinoderm, which is an attribution that was still supported by Conway Morris (1993). However, the systematic affinities and lifestyle of eldoniids remained enigmatic. Because of the medusoid appearance some experts suggested an identification as cnidarian siphonophore or jellyfish (Madsen 1956, 1957, 1962, Lemche 1960, Seilacher 1961, Sun & Hou 1987), but the most favored alternative interpretations included an attribution to lophophorates (Dzik 1991, Chen et al. 1995, Dzik et al. 1997, Zhu et al. 2002) or to holothurian echinoderms (sea cucumbers) (Walcott 1911, Clark 1913, Durham 1974). Zhu et al. (2002) said that “the new anatomical information emphasize close phylogenetic relations with lophophorates (U-shaped intestine, circumoral tentacles and ectodermal, marginal accreted disc), though some features (e.g. dendritic tentacles, ventral pustules that may relate to reduced podia) do not exclude affinities with echinoderms.” Because of this strange and ambiguous combination of characters, Conway Morris & Robinson (1988) had concluded that “the higher taxonomic affinities of this genus are best regarded as uncertain”, but only a few years later Conway Morris (1993) considered eldoniids as pre-echinoderm deuterostomes (also see Friend 1995, Caron et al. 2010, and MacGabhann 2012), which happens to represent the currently preferred view (see below). Even the lifestyle of eldoniids was reconstructed very differently (see discussion in Caron et al. 2010): some scientists considered them as pelagic filter feeders (e.g., Clark 1913, Durham 1974, Chen et al. 1995, Zhu et al. 2002), while others re-interpreted them as sedentary benthic deposit feeders (Luo et al. 1999). Looks like there is not much we really know for sure about ancient life on Earth.
Herpetogaster — An “Alien” Life Form from Deep Time
Herpetogaster is another very strange fossil organism known from more than hundred specimens from the Middle Cambrian of British Columbia and Nevada as well as the Early Cambrian Chengjiang biota of China (Caron et al. 2010, Kimmig et al. 2019, Yang et al. 2020, 2023). It had a long and narrow stalk, a curved and segmented sac-like body, and pair of branched feeding tentacles around the apical mouth. Herpetogaster was about 3-4 cm long and was often found attached to fossil sponges. The quite similar genus Phlogites (= Cheungkongella) was originally described as urochordate (tunicate) by Shu et al. (2001), but re-interpreted as a tentaculate similar to entoprocts (Kamptozoa) by Chen et al. (2003) and Hou et al. (2006), and finally identified as a stem ambulacrarian by Caron et al. (2010) and Li et al. (2023). Shu et al. (2010) disagreed and still considered a tunicate affinity. These determinations seemingly jump between major groups of animals like in a wild guessing game.
Cambroernida — A Novel Animal Body Plan from the Paleozoic
Caron et al. (2010), who first described Herpetogaster collinsi from the Burgess Shale biota, already recognized that this organism likely represents a deuterostome animal, which they tentatively attributed to the stem group of hemichordates and echinoderms that are united in a clade called Ambulacraria. They also recognized that the enigmatic eldoniids, in spite of their discoidal shape, seem to be closely related to Herpetogaster and share a similar body plan. These animals are so distinct and so different from all other known animal phyla, that Caron et al. erected a new clade named Cambroernida to accommodate them. They did not formerly rank it as a new phylum, but it is very clear that this clade would deserve such a high rank in the taxonomic hierarchy as one of the numerous distinct body plans of bilaterian animals that originated in the Cambrian Explosion (see Bechly 2024).
To understand how weakly supported the phylogenetic and evolutionary hypotheses really are, it is worthwhile to quote from the discussion in Caron et al. (2010):
Arriving at a precise phylogenetic position for the cambroernids, therefore, has proved difficult. On balance a place amongst the tentaculate lophotrochozoans seems to be less persuasive. Given a place within the ecdysozoans is even less plausible, then the final possibility must be to look to the deuterostomes. Here, as noted the options revolve around a series of possibilities, including a stem- group echinoderm, a hemichordate or an ambulacrarian. Whilst this list of possibilities might seem to leave the matter largely unconstrained, it is important to stress that from a Cambrian perspective the morphological differences between these various alternatives were probably insignificant. If, for the sake of the argument, the position of the cambroernids does indeed lie near the branching point of the two main ambulacrarian clades that led ultimately to the echinoderms and hemichordates, then we should not be surprised that it seems reminiscent of both pterobranchs and pre-radial echinoderms. … Finally, if accepted as some sort of deuterostome then these fossils have some further interesting implications. … Whilst many of the evolutionary steps involved in this process are still hypothetical, we suggest that animals similar to Herpetogaster may, in terms of the fossil record, be our best current glimpse of a very primitive ambulacrarian.
This is a lot of uncertainty as shaky foundation for a house of cards of evolutionary speculations. It is likely the reason why, despite Caron et al.’s work, some other experts still think that “the taxonomic affinities of these groups remain uncertain” (Hagadorn & Allmon 2019), even though some others had readily excepted the cambroernid-hypothesis (MacGabhann 2012). The most recent work by Li et al. (2023) is a perfect example for the dubious evolutionary hypotheses built on such shaky assumptions. The latter author concluded that:
As Herpetogaster has been recovered at the base of the Ambulacrarian tree in recent phylogenies, a planktonic larval stage is suggested, which implies, that the last common ancestor of the Ambulacraria might have already had planktonic larvae or that such larvae developed multiple times within the Ambulacraria
Note the typical Darwinian gobbledygook with key words like “suggested” and “might,” combined with the anything goes approach of either the character is homologous or it developed multiple times. The real information content of such statements is basically zero.
Anyway, the cambroernid-hypothesis by Caron et al. has more recently been corroborated by Li et al. (2023), who analyzed the discoidal metazoan Rotadiscus grandis from the Early Cambrian Chengjiang biota, a putative eldoniid. According to these scientists, the results of their research implied that “key traits of extant forms, such as a post-anal region, gill bars, and a U-shaped gut, evolved through convergence.” Wait a moment and read this carefully again. It means nothing less than the following: characteristic similarities of one of the two major branches of metazoan animals (i.e., Protostomia and Deuterostomia) are not based on common descent but independently acquired. Furthermore, the scientists found “Rotadiscus exhibits a chimeric combination of ambulacrarian and chordate characters.” However, these chordate-like features (e.g., a serialized body) are not known from living hemichordates and echinoderms, implying secondary loss or another convergence. The more we know about the distribution of certain anatomical features in fossil organisms, the more the initially apparent congruent distribution of similarities among recent organisms evaporates as a mirage, an artifact of incomplete knowledge. In reality, many traits exhibit a highly incongruent pattern of similarities that do not easily align with a nested hierarchy that could be translated into a tree of life. One of the strongest arguments in favor of Darwinian evolution gets more and more dismantled, which totally vindicates the critique by Michael Denton that evolution is a theory in crisis, in spite of the desperate attempts of denialism by mainstream academia.
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