Fossil Friday: Hemichordate Body Plan and Lifecycle Goes Back to the Cambrian Explosion
This Fossil Friday we will discuss the abrupt origin of yet another animal phylum during the famous Cambrian Explosion. It is the marine invertebrate phylum Hemichordata, which is represented by the pterobranchs (including the extinct graptolites) and the acorn worms (enteropneusts) as well as the enigmatic Planctosphaeroidea, which might just be planktic larva of some unknown deep sea acorn worms. Like chordates, hemichordates are deuterostome animals and considered to be the closest relatives (sister group) of echinoderms such as sea urchins and starfish. They have a tripartite body with three body cavities. While pterobranchs are sessile filter feeders, acorn worms are detritivores living in U-shaped burrows in the sea floor. The fossil record of Hemichordata goes back to the Early/Middle Cambrian (Maletz 2014, Nanglu et al. 2020).
The oldest known hemichordate and oldest pterobranch is the zooid fossil Galeaplumosus abilus from the 525-518 million year old Lower Cambrian Chengjiang Konservat-Lagerstätte of southern China (Hou et al. 2011, also see Hou et al. 2017).
Only very few fossil enteropneusts have been described yet in just eight fossil species (Cameron 2018, Yang et al. 2024) from the Cambrian (Walcott 1911, Caron et al. 2013, Nanglu et al. 2016, Yang et al. 2024), the Carboniferous (Bardack 1997, Maletz 2014, Cameron 2016), and the Jurassic periods (Arduini et al. 1984, Alessandrello et al. 2004, Bechly & Frickhinger 1999). Possible trace fossils of acorn worms have been reported from the Lower Triassic of Italy by Twitchett (1996). This rarity is quite remarkable because some other soft-bodied worm-like organisms that burrow in the sea floor are much better represented in the fossil record. Actually, the only enteropneust specimen from the Upper Jurassic Solnhofen limestone of Bavaria in Germany was described by myself as Mesobalanoglossus buergeri (also see Bechly 2015). The featured image shows the holotype specimen (no. SNSB-BSPG 1998-I-15), which is 68.8 cm long and 2.6 cm wide, and deposited at the Natural History Museum in Munich.
Abrupt Appearance, Yet Again
Recently, 39 specimens of the previously unknown acorn worm Cambrobranchus pelagobenthos were described from the Hayiyan Lagerstätte in China (Yang et al. 2024), which belongs to the famous Lower Cambrian Chengjiang biota. The scientists could also describe larvae and juveniles and thereby document the characteristic indirect development with a pelago-benthic lifestyle already for these earliest known representatives of acorn worms.
Thus, both major subgroups of the phylum Hemichordata are known from Lower Cambrian fossils with completely modern morphology and life cycle, which confirms the overall pattern of the abrupt appearance of animal phyla in the Cambrian Explosion. Furthermore, the putative stem-hemichordate Gyaltsenglossus senis was described by Nanglu et al. (2020) from the Cambrian Burgess Shale of Canada. However, with an estimated age of 506 million years, it is 10-20 million years younger than the oldest crown group representatives discussed above and thus requires an ad hoc explanation in terms of ghost lineages to be accommodated within a Darwinian paradigm
References
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Arduini P, Pinna G & Terruzzi G 1981. Megaderaion sinemuriense n.g. n.sp., a new fossil enteropneust of the Sinemurian of Osteno in Lombardy. Atti della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano 122(1-2), 104–108. https://www.biodiversitylibrary.org/part/325194
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Nanglu K, Caron J-B & Cameron CB 2020. Cambrian Tentaculate Worms and the Origin of the Hemichordate Body Plan. Current Biology 30(21), 4238–4244.e1. DOI: https://doi.org/10.1016/j.cub.2020.07.078
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Yang X, Kimmig J, Cameron CB, Nanglu K, Kimmig SR, de Carle D, Zhang C, Yu M & Peng S 2024. An early Cambrian pelago-benthic acorn worm and the origin of the hemichordate larva. Palaeontologia Electronica 27(1): a17, 1–19. DOI: https://doi.org/10.26879/1356
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