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Fossil Friday: Fossil Tunicate Confirms Cambrian Explosion

Photo: Megasiphon thylakos holotype, after Fig. 2 in Nanglu et al. 2023, fair use.

The so-called Cambrian Explosion is an iconic event in the history of life about 520 million years ago, marked the sudden appearance of more than 20 bilaterian animal phyla with all their different body plans. However, body plans are not restricted to the phylum level. For example within the phylum Chordata we find three different subphyla (i.e., lancelets, tunicates, and vertebrates) with very different body plans and different life cycles. All three subphyla appear abruptly and together in the Lower Cambrian Stage 3, about 518 milion years ago, including the oldest putative tunicates such as Shankouclava anningense from the famous Maotianshan shales in South China (Shu et al. 2001Chen et al. 2003).

Even within tunicates we find quite different body plans such as free living salps and larvaceans as well as sessile sea grapes and sea squirts. Of course, this raises the interesting question of the origin and antiquity of these tunicate body plans, especially because tunicates are nowadays considered to be the closest relatives of vertebrates. Unfortunately, tunicates are generally very rare in the fossil record and previously only one early tunicate showed anatomical details with soft-tissue preservation.

Confirming Life’s Big Bang

Recently, a new study was published by Nanglu et al. (2023) about a fossil tunicate from the mid-Cambrian Marjum Formation of Utah, which is dated to an age of about 500 million years. The remarkably well-preserved animal was named Megasiphon thylakos and documents an early biphasic life cycle, with a planktonic larva and a sessile epibenthic adult. The authors consider that Megasiphon could even be a modern crown group tunicate, which would place the basal divergence into free living and sessile tunicates “50 million years earlier than currently estimated based on molecular clocks.” Of course, the phylogenetic analysis was, as all too often is the case, not conclusive and could not decide between two alternative scenarios. But even if Megasiphon were to fall only within the stem group of all tunicates, the authors emphasize that it clearly “demonstrates that fundamental components of the modern tunicate body plan were already established shortly after the Cambrian Explosion.” In commentary for the journal Science, Heidt (2023) even claims that the discovery could “push back origins of vertebrates, including humans,” which would agree with equally ancient fishlike animals such as Metaspriggina.

Almost on a monthly basis new fossil evidence corroborates the abruptness of the Cambrian Explosion as a genuine “Big Bang” of life and a fatal problem for Darwinian evolution. Far from solving the enigma of the Cambrian Explosion and other such abrupt events in the history of life, our exponentially growing knowledge of the fossil record instead makes the problems bigger and bigger, which clearly shows that they are not based on mere artifacts of preservation or sampling bias.