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Fossil Friday: Cambrian Fossils Turned Upside Down Yet Again

July 12, 2024, 6:46 AM
Photo credit: Bruce Martin, CC0, via Wikimedia Commons.

This Fossil Friday features the iconic fossil Pikaia, a supposed vertebrate ancestor from the Middle Cambrian of the famous Burgess Shale of the Canadian Rocky Mountains, which offers a marvelous glimpse into the output of the Cambrian Explosion. Recently, this famous fossil organism has been totally reinterpreted and its reconstruction literally turned upside down. Such major reinterpretations of fossil animals from the Burgess Shale have happened before more than once.

Fossil fragments of isolated body parts of giant anomalocaridid arthropods (Radiodonta) were originally considered as very different animals: the circular mouth was described as assumed jellyfish Peytoia, the body was described as assumed sea cucumber or sponge Laggania, and the cephalic appendages were identified as abdomen of a fossil phyllocarid shrimp Anomalocaris. Only after discovery of more completely preserved specimens were the previous erroneous interpretations recognized and the body reconstruction revised into what we know today as the large Cambrian predator Anomalocaris and its relatives. Stephen Jay Gould (1989) commented in his bestselling book Wonderful Life that the story of Anomalocaris is “a tale of humor, error, struggle, frustration, and more error, culminating in an extraordinary resolution that brought together bits and pieces of three “phyla” in a single reconstructed creature, the largest and fiercest of Cambrian organisms.”

A Reconstruction Revised

Another iconic fossil from the Burgess Shale is the lobopod Hallucigenia, that was reconstructed as an alien worm-like creature walking on spine-like stilts, with a unpaired series of tentacles on its back, and a bulbous head (Conway Morris 1977). Later this reconstruction was revised turned upside down and head to tail: now the tentacles were identified as paired walking leglets (in a paper funnily published on April 1st by Ramsköld 1992), the spines as dorsal armature, and the bulbous structure as decay fluids from the rear end (Ramsköld & Hou 1991, Ramsköld 1992). Finally, this new reconstruction was again revised by Smith & Caron (2015), who also recognized the presence of simple eyes on the newly identified head that was previously interpreted as tail. 

Based on the then only known holotype specimen, Nectocaris was first interpreted as either a problematic fossil of unknown affinity, or as a crustacean, or as a lancelet-like chordate. Later new material was discovered that lead to a total reinterpretation of its body as a primitive squid-like cephalopod with two arms and a funnel (Smith & Caron 2010). However, based on the very same evidence an alternative view re-interpreted the animal very differently as a radiodont arthropod similar to Anomalocaris (Mazurek & Zatoń 2011). Thus, even with the new evidence of almost hundred well-preserved specimens the scientists still wondered “what the heck is this thing” (Black 2011). I discussed this stunning problem in a previous article in great detail (Bechly 2022).

A New Study

Recently, Pikaia “suffered” a similar fate as Hallucigenia and was turned upside down (Yirka 2024, Zimmer 2024). Pikaia is a fossil animal from the Burgess Shale that resembles living lancelets and was cherished by Conway Morris (1979) as not “far removed from the ancestral fish,” and by Gould (1989) as “the first recorded member of our immediate ancestry.” Based on a thorough analysis of fossil specimens in the collection of the Royal Ontario Museum, a new study by Mussini et al. (2024) suggested a dorsoventrally inverted interpretation of Pikaia, which allowed for the identification of a dorsal nerve cord (previously regarded as a ventral blood vessel) and a gut canal. Therefore, the authors attributed Pikaia (together with Yunnanozoon and the enigmatic Vetulicolia) to the stem group of chordates, supporting a revised version of Romer’s “somatico-visceral” hypothesis of chordate origins from bipartite ancestors. Sounds sophisticated and very scientific, but the new interpretation has some very strange and unique features that are found nowhere else among early chordates, such as a small bilobed head with ventral ‘antennae’ (implying that lancelets secondarily lost their head) and dorsal external gills. This makes this new interpretation at least somewhat questionable and I would not be surprised if we will see another revised interpretation in the future. After all, even staunch Darwinists like Carl Zimmer (2024) called the new interpretation just a “provocative theory … [which] may have resolved some mysteries, however, it also created new ones” and called their phylogenetic conclusion “a new — and controversial — family tree”. Harvard paleontologist Karma Nanglu (quoted in Zimmer 2024), a distinguished expert on Cambrian fossils, commented that he has “a harder time imagining that swimming along the sea floor”. This should at least raise some eyebrows if not ring all alarm bells, especially considering the weird fact that it basically seems to make no big difference for the alleged transitional position of a fossil organism if its interpretation is dorsoventrally reversed or not.

“Wonderful” Reinterpretations

What is the take home message from all these “wonderful” reinterpretations, apart from the self-evident recognition of scientific progress based on new fossil material and new methods of studying them? The above revisions were only possible because luckily we did discover more and better preserved fossil material. What about the much more numerous cases, where fossil species and their reconstructions are exclusively based on isolated, often incomplete and poorly preserved fossils? These cases rather represent the rule than an exception. We should always maintain some healthy skepticism in all those bold reconstructions of fossil organisms and the far-reaching evolutionary hypotheses built upon them. Most of these reconstructions are based on very weak evidence and are highly speculative, which includes many alleged missing links and transitional forms and the associated evolutionary scenarios. It is not for nothing that we see on a regular basis sensational media headlines like “New Fossil Rewrites the Evolution of …”

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