Re-description of the Spence Shale palaeoscolecids in light of new morphological features with comments on palaeoscolecid taxonomy and taphonomy

Abstract

The middle Cambrian (Miaolingian Series; Wuliuan Stage) Spence Shale of Utah and Idaho preserves a diverse assemblage of biomineralized and soft-bodied taxa. Among the rarest specimens of this fauna are palaeoscolecid worms. Until recently, only one specimen was known from the Spence Shale, the holotype specimen of Palaeoscolex ratcliffei Robison, 1969, later included in the genus Wronascolex. This specimen is preserved as part and counterpart but missing both the posterior and anterior terminations. A new specimen, discovered by Riley Smith, preserves an everted proboscis with spines. Based on new data collected using scanning electron microscopy and energy dispersive X-ray spectrometry (SEM–EDS), and optical microscopy, the species is thought to represent a new genus, Utahscolex gen. nov., rather than a species of Palaeoscolex or Wronascolex as previously suggested. The new genus differs from the other two genera in the lack of node ornamentation of the plates, as well as the absence of microplates and platelets. Based on this case study, it is recommended that future revisions of palaeoscolecid taxonomy require knowledge of plate, platelet, and microplate ornamentation, as well as the arrangement of the plates, platelets, and microplates on the cuticle. In addition to the improved morphological information provided by the new specimen, it also advances our knowledge of the taphonomic pathways in the Spence Shale and in palaeoscolecid worms in general. The preservation of the plates of the two specimens of this species differ in elemental composition and somewhat in quality. While both the holotype and new specimen show localized magnesium and phosphorus within the plates, the holotype has a substantial iron component, whereas the new specimen instead shows elevated calcium. In addition, kerogenization, pyritization, aluminosilicification, and phosphatization can be observed throughout the specimen. The preservation varies not only between the specimens, but also within, demonstrating the high variability of preservational pathways within a Burgess Shale-type deposit, and providing insights into the circumstances that lead to soft-bodied preservation in the Spence Shale.

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Acknowledgements

We would like to thank C. Levitt-Bussian and R. Irmis (UMNH) for access to their specimen, and P. Thapa for his assistance operating the KU SEM. We thank B. Lieberman for comments on a previous version of the manuscript. We thank Jean Vannier, Timothy Topper, and editor Mike Reich for helpful reviews. We thank the USDA Forest Service for permits. This work was supported by an Association of Earth Science Clubs of Greater Kansas City Research Grant, a University of Kansas Biodiversity Institute Panorama Grant, a Geological Society of America Graduate Student Research Grant, and a Paleontological Society Kenneth E. and Annie Caster Student Research Award to AW and a Paleontological Society Arthur James Boucot Grant to JK. JDS is supported by NSF CAREER 1652351, and the University of Missouri X-ray Microanalysis Core by NSF IF 1636643.

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Whitaker, A.F., Jamison, P.G., Schiffbauer, J.D. et al. Re-description of the Spence Shale palaeoscolecids in light of new morphological features with comments on palaeoscolecid taxonomy and taphonomy. PalZ 94, 661–674 (2020). https://doi.org/10.1007/s12542-020-00516-9

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Keywords

  • Utah
  • Great basin
  • Priapulida
  • Burgess shale-type preservation
  • SEM–EDS