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Fossil Friday: Rhenocystis and the Controversial Calcichordate Hypothesis

Photo: Rhenocystis latipendunculata, (c) Christel Schuhmacher, Hunsrück Museum Simmern, CC-BY-NC-SA 4.0.

This Fossil Friday features the fossil echinoderm Rhenocystis latipendunculata from the 403 million-years-old Devonian Hunsrück shale in Germany. It looks a bit like a tadpole with body and tail, and this indeed points towards one of the great scientific controversies of the 20th century. The maverick paleontologist Richard Jefferies, who worked as a curator at the prestigious British Museum of Natural History, introduced and strongly promoted for 35 years a controversial theory for the origin of chordates, — the so-called ‘Calcichordate Hypothesis’ (Jefferies 1967, 1968, 1971, 1975, 1981, 1982, 1986, 1997, Jefferies et al. 1996, and Dominguez et al. 2002). He suggested that chordates are most closely related to echinoderms, and that lancelets, tunicates, and vertebrates independently evolved from a Paleozoic group of asymmetrical calcified organisms called carpoids (today named HomalozoaStylophora, including the subgroups Soluta, Cornuta, and Mitrata), which were usually considered as oldest and most basal echinoderms. Jefferies provided very elaborate reinterpretations of the anatomy of these fossils, in which he even claimed to recognize homologues of gill slits and the vertebrate brain with its main nerves. What he considered as a chordate tail, was considered by other paleontologists as an echinoderm feeding appendage. In other words: the expert disagreement was also about what is up and down and front and back in these fossil organisms, as well as the complete internal anatomy.

Hotly Debated

Jefferies’ calcichordate hypothesis was hotly debated in the late 1980s and 90s, but rejected by most other scientists as unfounded, debunked, and based on false methodology and phantastic speculation (Philip 1979, Fisher 1982, 1993, Jollie 1982, Sprinkle 1987, Peterson 1995, Nielsen 1995, Bergström et al. 1998, Ruta 1999, Simonetta et al. 1999, Lefebvre 2000, and Lefebvre et al. 2019). I still remember vividly that during my study at the University of Tübingen in the early 1990s, Jefferies’s hypothesis was just considered as an object of ridicule, together with other fringe ideas like Gardiner’s and Lovtrup’s haemothermia hypothesis of a close relationship of birds and mammals. The monumental textbook on the phylogenetic systematics of vertebrate animals authored by my greatly admired teacher Gerhard Mickoleit (2004), did not even consider Jefferies’ hypothesis worth mentioning. Critically referring to Jefferies’ kind of reasoning, Peterson (1995) boldly concluded that “fossils are unable to dictate relationships”. Few other paleontologists supported Jefferies’ interpretations. A notable exception were Eaton (1970a, 1970b), Shu et al. (2001), and especially Nature editor in chief Henry Gee, who strongly defended the calcichordate hypothesis in a long chapter of his book (Gee 1996). He later suggested a kind of compromise hypothesis (Gee 2001). In a study co-authored with Jefferies and published in Nature, Dominguez et al. (2002) described a stylophoran fossil as clearly possessing the predicted paired gill slits and attributed it to the stem group of tunicates. This latest and potentially final defense of the calcichordate theory was criticized as highly controversial by Bertrand Lefebvre (quoted by MacAndrew 2003), who rather considered the alleged gill slits as part of the genital organs (gonoducts).

An Interesting Twist

An interesting twist to this whole story was based on the circumstance that the opponents of the calcichordate hypothesis considered as their most substantive critique that this hypothesis implies a closest relationship of tunicates with vertebrates, which was generally considered as refuted by numerous independent phylogenetic studies (Peterson 1995, Donoghue et al. 2003). However, the comprehensive phylogenomic analysis by Delsuc et al. (2006) and several other modern studies (e.g., Bourlat et al. 2006, Putnam et al. 2008, Sawalla & Smith 2008, Giribet 2018, and Kocot et al. 2018; but see Stach 2008, Zupanc 2008, and Braun et al. 2020 for an opposing view) surprisingly suggested that tunicates are indeed closer related to vertebrates. Delsuc et al. (2006) explicitly mention that this would be compatible with Jefferies’ theory. However, they also emphasized that this does not mean that Jefferies’ interpretation is necessarily true. Most paleontologists and evolutionary biologists still considered stylophorans as echinoderms and the calcichordate hypothesis as a failed and obsolete idea. This was mainly for the following two reasons (Lefebvre et al. 2019): Even though the calcichordate hypothesis correctly predicted a sister group relationship of tunicates and vertebrates, it also relies on a sister group relationship of echinodermates and chordates that was refuted by modern phylogenomics, which rather suggests that echinodermates are the sister group to hemichordates. Furthermore, the unique stereom skeleton of echinodernates would have to be interpreted as secondarily reduced in chordates, which is an unparsimonious ad hoc assumption, for which there is no independent evidence. Finally, Lefebvre et al. (2019) could document the typical water-canals in the appendage, which strongly suggests a homology with an echinoderm arm and not with a chordate tail. It looks like we can definitely put the calcichordate hypothesis to rest. Even though the early critics seemingly were proven right, this does of course not excuse their intolerant opposition to a unconventional new view in a time when the jury was still out.

What is our take home message from this controversy? Well, at least these points: Fossils are often open to very different alternative reconstructions and interpretations (Donoghue & Purnell 2009), and phylogenetic trees often change with every new data instead of converging to the one true tree of life. This suggests that we should not place too much confidence in the currently favored scientific consensus in these fields. Also, unconventional ideas are often met with fierce opposition and ridicule in the scientific community, which is something we ID proponents know all too well. Maybe both points are related: if the evidence is weaker than pretended, the subconscious insecurity may lead to a more angry defense of the preferred but relatively weakly founded position. Keep this in mind when discussing with an angry defender of Darwinism next time.


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