A new marrellomorph arthropod from southern Ontario: A rare case of soft tissue preservation on a late Ordovician open marine shelf

Ordovician open marine lagerstätten are relatively rare and widely
dispersed, producing a patchy picture of the diversity and biogeography of
non-mineralized marine organisms, and challenging our understanding of the
fate of Cambrian groups. Here, for the first time, we report soft-bodied
fossils, including a well-preserved marrellomorph arthropod, fragmentary
carapaces, and macroalgae, from the late Ordovician (Katian) Upper Member
of the Kirkfield Formation near Brechin, Ontario. The unmineralized
elements and associated exceptionally preserved shelly biota were entombed
rapidly in storm deposits that smothered the shallow, carbonate-dominated
shelf. The marrellomorph, Tomlinsonus dimitrii gen. et sp. nov., is
remarkable for its ornate, curving cephalic spines and pair of
hypertrophied appendages, suggesting a slow-moving, benthic lifestyle.
Re-evaluation of marrellomorph phylogeny using new data favours an
arachnomorph affinity, though internal relationships are robust to
differing outgroup selection. Clades Marrellida and Acercostraca are
recovered, but the monophyly of Marrellomorpha is uncertain. The new taxon
is recovered as sister to the Devonian Mimetaster, and, as the second
youngest known marrellid, bridges an important gap in the evolution of
this clade. More generally, the Brechin biota represents a rare window
into Ordovician open marine shelf environments in Laurentia, representing
an important point of comparison with contemporaneous lagerstätten from
other paleocontinents, with great potential for further discoveries.