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| Ursactis comosa Osawa, Caron & Gaines, 2023 DOI: 10.1098/rsos.221400 drawing by Danielle Dufault / Royal Ontario Museum. |
Abstract
Early annelid evolution is mostly known from 13 described species from Cambrian Burgess Shale-type Lagerstätten. We introduce a new exceptionally well-preserved polychaete, Ursactis comosa gen. et sp. nov., from the Burgess Shale (Wuliuan Stage). This small species (3–15 mm) is the most abundant Cambrian polychaete known to date. Most specimens come from Tokumm Creek, a new Burgess Shale locality in northern Kootenay National Park, British Columbia, Canada. Ursactis has a pair of large palps, thin peristomial neurochaetae and biramous parapodia bearing similarly sized capillary neurochaetae and notochaetae, except for segments six to nine, which also have longer notochaetae. The number of segments in this polychaete range between 8 and 10 with larger individuals having 10 segments. This number of segments in Ursactis is remarkably small compared with other polychaetes, including modern forms. Specimens with 10 segments show significant size variations, and the length of each segment increases with the body length, indicating that body growth was primarily achieved by increasing the size of existing segments rather than adding new ones. This contrasts with most modern polychaetes, which typically have a larger number of segments through additions of segments throughout life. The inferred growth pattern in Ursactis suggests that annelids had evolved control over segment addition by the mid-Cambrian.
Keywords: Burgess Shale, polychaete, development, stem-group annelid, gregarious behaviour, body tagmatization
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| Photos of the holotype, ROMIP 66770.1, of Ursactis comosa gen. et sp. nov. from the lower quarried interval of the Tokumm Creek locality. The specimen has 10 chaetigerous segments and stout palps, peristomial neurochaetae, and longer notochaetae on the sixth to ninth segments. It is presumably preserved dorsally. (a) Composite image of part and counterpart. Squares with dashed lines indicate the area shown in (b–d). (b) Close-up of the first four segments in the part. (c) Close-up of segments 8–10 and the pygidium in the counterpart. (d) Close-up of the head in the counterpart, showing dark patches within the head. Acronyms: ch, chaetae; chP, peristomial chaetae; mo, mouth; pal, palp; pr, parapodium; py, pygidium; numbers following the acronyms denote segments, counting from anterior, excluding peristomium. Scale bars: (a): 5 mm; (b–d): 1 mm. |
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| Distribution of Ursactis comosa gen. et sp. nov. (a) Maps with localities yielding Ursactis indicated by stars. The detailed map comes from the grey area in the general map. (b) Generalized stratigraphic column for the Burgess Shale Formation at the Tokumm Creek locality. The stars and the adjacent numbers represent the locations of two quarried intervals that yielded specimens of Ursactis and the number of specimens collected at each interval, respectively. (c) Outcrop photo of the lower quarried interval; the quarried interval is marked with dashed lines. (d–f) Specimens collected from the Collins Quarry on Mt. Stephen (d), the upper interval of the Tokumm Creek locality (e) and the lower interval of the Tokumm Creek locality (f). (d) Ursactis sp., ROMIP 66882. (e) Ursactis comosa, ROMIP 66873. (f) U. comosa, ROMIP 66874.1. Scale bars: (d): 1 mm; (e,f): 5 mm. |
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| Ursactis comosa gen. et sp. nov. from the lower quarried interval of the Tokumm Creek locality. (a) Overview of one of the multiple slabs of ROMIP 66771B, preserving specimens ROMIP 66771.28–48,74,119–121. Squares with dashed lines indicate areas shown in close-up in (b–d). (b) ROMIP 66771.33–42. A cluster of at least 10 specimens. Counterpart to figure 3d. (c) ROMIP 66771.120. A ventrally preserved specimen showing the biramous state of parapodia, posterior longer notochaetae and the mouth. Close-up image of the head and parapodia is in figure 5k. (d) ROMIP 66771.48. An enrolled specimen. Acronym: ch, chaetae, with numbers denoting the segments, counting from the anterior, except for the peristomium. Scale bars: (a): 5 cm; (b–d): 5 mm. |
Systematic palaeontology
Phylum: Annelida Lamarck, 1809.
Ursactis comosa gen. et sp. nov.
Etymology: Ursa for the constellation Ursa Major, and actis, a Greek word for a ray, referring to the species' starry appearance and the clustered occurrence of fossils; comos, a Latin word meaning ‘having long hair,’ for its long capillary chaetae, in particular some notochaetae on segments six to nine.
Locality and stratigraphy: One specimen of Ursactis sp. (figure 1d) comes from the Collins Quarry on Mt. Stephen, Yoho National Park, Kicking Horse Shale Member. All the specimens of U. comosa come from two stratigraphic intervals within the upper part of the Burgess Shale Formation, Cambrian (Miaolingian Series, Wuliuan Stage), Ehmaniella biozone, at the Tokumm Creek locality of the Marble Canyon area, northern Kootenay National Park, British Columbia, Canada.
Diagnosis for genus and species: Polychaete worm possessing maximum 10 chaetigers, excluding the peristomium, with biramous parapodia yielding simple capillary chaetae. Approximately 8–12 chaetae on each neuropodium and notopodium, and among chaetigers one to eight. The last two chaetigers have approximately five and two chaetae, respectively. Up to five notochaetae on chaetigers six to nine, double the length of other chaetae on the same chaetigers.
Conclusion:
The known disparity and ecological diversity of Cambrian annelids have expanded in recent years thanks to the discovery of new species, including tube-dwelling forms [Chen, et al., 2020; Nanglu & Caron, 2021] and forms living in symbiosis with other taxa [Nanglu & Caron, 2021]. The clustering behaviour of Ursactis also implies that some polychaetes probably occupied substantial space and resources in some areas, suggesting a greater ecological role in local ecosystems.
The morphology of Ursactis may provide important clues to understanding the developmental patterns of the ancestral annelid. First, the occurrence of longer chaetae on specific segments suggests an early example of simple body tagmatization in annelids. Second, the surprisingly small number of segments and minor variation in segment number in this new polychaete implies that the addition of segments was terminated earlier in ontogeny than in modern polychaetes. Continued study of the modes of growth in fossilized annelids and of post-larval development of extant polychaetes would provide important clues to understanding the evolution of developmental patterns in the Annelida.
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| life reconstruction of Ursactis comosa gen. et sp. nov. drawing by Danielle Dufault / Royal Ontario Museum. |
Hatena Osawa, Jean-Bernard Caron and Robert R. Gaines. 2023. First Record of Growth Patterns in A Cambrian Annelid. Royal Society Open Science. DOI: 10.1098/rsos.221400