The trilobite Redlichia from the lower Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia: systematics, ontogeny and soft-part anatomy

Abstract

The trilobite Redlichia Cossmann, 1902 is an abundant element of the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) Konservat-Lagerstätte on Kangaroo Island, South Australia. Well-preserved, fully articulated specimens from this deposit are known to reach lengths of up to 25 cm, representing one of the largest known Cambrian trilobites. Until now, all Redlichia specimens from the EBS have been referred to Redlichia takooensis Lu, 1950, a species originally described from South China. Previous work recognized considerable differences in exoskeletal morphology among specimens of varying sizes, which was attributed to ontogeny. However, close examination of a large collection of recently acquired specimens shows that this variation actually represents two distinct morphs, interpreted here as separate species: R. takooensis, and a large, new species, Redlichia rex sp. nov. An analysis of morphological variation in holaspides (‘adults’) of the more common R. takooensis reveals considerable ontogenetic change occurred even during this later phase of growth. Some specimens of both Redlichia species from the EBS also exhibit exceptionally preserved soft-part anatomy, particularly the antennae and biramous appendages. Here, appendages (antenniform and biramous) and digestive structures are described, and biramous appendage reconstructions of R. rex sp. nov. are presented, which show a striking resemblance to some early Cambrian trilobites from South China. In particular, R. rex has a tripartite exopodite, as well as a dorsoventrally deep protopodite with gnathobasic spines used to shred or crush food items. Based on recent phylogenetic analyses, it is possible that an exopodite with tripartite subdivisions represents the plesiomorphic condition for Artiopoda (trilobites and kin). The digestive system of R. takooensis exhibits a series of paired digestive glands in the cephalon and anterior thorax, similar to those described for a number of other Cambrian and Ordovician trilobites.

http://zoobank.org/urn:lsid:zoobank.org:pub:507BEAFC-4AFA-43F4-A5C4-49E4B58C658E

Keywords:

• Arthropoda
• Artiopoda
• Redlichiida
• geometric morphometrics
• biramous appendage
• antennae

Acknowledgements

This research has been supported by an RTS PhD scholarship and a Constance Fraser Scholarship from the University of Adelaide to J.D.H., as well as grants from the Australian Research Council (LP0774959 and FT120100770 to J.R.P.; FT130101329 to D.C.G.B.). Additional financial assistance was provided by Beach Energy Ltd and the South Australian Museum. Thanks to the other regular members of the Emu Bay Shale research team: Greg Edgecombe, Jim Gehling, Jim Jago and Mike Lee. Thanks also to palaeoartist Katrina Kenny for producing the 3D reconstructions of appendages, Darcy Holmes for assistance in rendering these, and Russell Bicknell, Harriet Drage, Rudy Lerosey-Aubril, Javier Ortega-Hernández and Emma Sherratt for useful discussions. We appreciate the help of Ronda Atkinson, Marissa Betts, Mary-Anne Binnie, Glenn Brock, Aaron Camens, Allison Daley, Robert Gaines, Mike Gemmell, Trevor and Carol Ireland, Pierre Kruse, Ben McHenry, Lily Reid and Natalie Schroeder, who have assisted with collection efforts at Big Gully. SeaLink have provided logistical support. Thanks to Tae-Yoon Park and Mark Webster for their constructive reviews that significantly improved the manuscript. Finally, special thanks to landowners Paul and Carmen Buck – without their continuing support, the ongoing Emu Bay Shale research project would not be possible.

Supplemental material

Supplemental material for this article can be accessed here: http://dx.doi.org/10.1080/14772019.2019.1605411.

Associate Editor: Greg Edgecombe