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

• Alessandrello A, Bracchi G & Riou B 2004. Polychaete, sipunculan and enteropneust worms from the Lower Callovian (Middle Jurassic) of La Voulte-sur-Rhône (Ardèche, France). Memoire della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano (Fascicolo I) 32, 1–16.
• 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
• Bardack D 1997. Wormlike animals: Enteropeusta. pp. 89–92 in: Shabica CW & Hay AA (eds). Richardson’s Guide to the fossil fauna of Mazon Creek. Northeastern Illinois University, Chicago (IL), 308 pp.
• Bechly G 2015. [Chapter] Eichelwürmer (Hemichordata: Enteropneusta). p. 324 in: Arratia G, Schultze HP, Tischlinger H & Viohl G (eds). Solnhofen – Ein Fenster in die Jurazeit. 2 vols. Pfeil Verlag, Munich (Germany), 620 pp. [In German] https://pfeil-verlag.de/publikationen/solnhofen-ein-fenster-in-die-jurazeit/
• Bechly G & Frickhinger KA 1999. Acorn worms. pp. 76–79 in: Frickhinger KA (ed.). The Fossils of Solnhofen 2: New specimens, new details, new results. Goldschneck-Verlag, Korb (Germany), 190 pp. [German PDF]
• Cameron CB 2016. Saccoglossus testa from the Mazon Creek fauna (Pennsylvanian of Illinois) and the evolution of acorn worms (Enteropneusta: Hemichordata). Palaeontology 59(3), 329–336. DOI: https://doi.org/10.1111/pala.12235
• Cameron CB 2018. Class Enteropneusta: Introduction, Morphology, Life Habits, Systematic Descriptions, and Future Research. Treatise Online 109, 1–22. DOI: https://doi.org/10.17161/to.v0i0.7889 (dead link)
• Caron J-B, Conway Morris S & Cameron CB 2013. Tubicolous enteropneusts from the Cambrian period. Nature 495, 503–506. DOI: https://doi.org/10.1038/nature12017
• Hou X-g, Aldridge RJ, Siveter DJ, Siveter DJ, Williams M, Zalasiewicz J & Ma X-y 2011. An Early Cambrian Hemichordate Zooid. Current Biology 21(7), 612–616. DOI: https://doi.org/10.1016/j.cub.2011.03.005
• Hou X-g, Siveter DJ, Siveter DJ, Aldridge RJ, Cong P-y, Gabbott SE, Ma X-y, Purnell MA & Williams M 2017. Hemichordata. Chapter 22, pp. 250–251 in: The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life. 2nd Edition. John Wiley & Sons, Chichester (UK) / Hoboken (NJ), xii+316 pp. DOI: https://doi.org/10.1002/9781118896372.ch22
• Maletz J 2014. Hemichordata (Pterobranchia, Enteropneusta) and the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 398, 16–27. DOI: https://doi.org/10.1016/j.palaeo.2013.06.010
• Nanglu K, Caron J-B, Conway Morris S & Cameron CB 2016. Cambrian suspension-feeding tubicolous hemichordates. BMC Biology 14: 56, 1–9. DOI: https://doi.org/10.1186/s12915-016-0271-4
• 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
• Twitchett RJ 1996. The Resting Trace of an Acorn-Worm (Class: Enteropneusta) from the Lower Triassic. Journal of Paleontology 70(1), 128–131. https://www.jstor.org/stable/1306375
• Walcott CD 1911. Cambrian Geology and Paleontology II: No. 5 – Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57, 109–145. https://repository.si.edu/handle/10088/34820
• 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