Ordovician

Published on May 16th, 2022 | by David Marshall

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Episode 139: Marrellomorphs

Marrellomorphs are the group of early Paleozoic arthropods that get their name from the well-known Burgess Shale fossil Marrella splendens. They have for a long time been considered to be closely related to the trilobites, based on similarities in their gills, whilst other studies have suggested they could be closer related to mandibulate arthropods (crustaceans, insects & myriapods), although their strange appearance means other relationships might still be plausible.

Since they have a soft exoskeleton, marellomorphs have a very poor fossil record and so the discovery of any new specimens outside of the Burgess Shale can be incredibly significant. In this interview, we speak to Joe Moysiuk of the University of Toronto and Royal Ontario Museum about his newly described species Tomlinsonus dimitrii.

Brechin is a lagerstätte in Southern Ontario. These rocks were formed 450 millions ago in the Ordovician period, close to the equator, in a shallow tropical sea. Brechin is interesting, not just for its fossils, but for how they are preserved in situ in what being dubbed a ‘paleo Pompeii’.
Image: (1) Stratigraphic column in the study area. (2) Photographs of the site showing the beds in which soft-tissue preservation has been observed (3) Topographic map of the main study plot. High relief mounds (prefixM) are numbered and shaded dark
while surrounding basins are shaded lighter. (4) Generic richness subdivided by major fossil group in Bed 3. From Moysiuk et al. 2022. © ROM
Some of the fossils from Brechin Lagerstätte. (1) Large branching alga. (2) Partial arthropod carapace. (3) Large slab covered in various hard-bodied animals. (1, 2) Scale bars = 10 mm; (3) scale bar = 50 mm. From Moysiuk et al. 2022. © ROM
Overview of Tomlinsonus dimitrii (ROMIP 66233), composite image (part and counterpart superimposed) and interpretive line drawing of specimen. © ROM
Anatomical reconstruction of T. dimitrii.
a.s = anterolateral spine; c.aX = cephalic appendage X; en = endopod of trunk appendage; hs = head shield; hy = hypostome; m.s = mediolateral spine; pX = podomere number X; s.s = secondary spine; sp = spine on p8 of hypertrophied limb; a.m = possible arthrodial membrane; et = endite; po = pores; ex = trunk exopod. From Moysiuk et al. 2022. © ROM
Life reconstruction of T. dimitrii.
Art by Christian McCall. © ROM
T. dimitrii is a very significant fossil as it helps paint a picture of marrellomorph diversity in the Ordovician period. It’s the second youngest marrellid after Mimetaster.
Joe Moysiuk (right) in the field with co-author Alejandro Izquierdo-López. Courtesy of ROM.
Willner Madge Gallery, Dawn of Life.
Ontario section of the new Dawn of Life gallery, which includes Tomlinsonus alongside other fossils from Brechin. © ROM

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