Death near the shoreline, not life on land: Ordovician arthropod trackways in the Borrowdale Volcanic Group, UK

Abstract

The Borrowdale Volcanic Group of northern England hosts Late Ordovician arthropod trackways that are frequently cited as the earliest unequivocal evidence for animal life on land, and provides a key geological locality for our understanding of myriapod evolution and terrestrialization. Original fieldwork at the site has identified four additional bedding surfaces that yield 121 new trackways (of a total 158 known individuals), permitting better sedimentological and paleoecological contextualization of the trackways. Contrary to published claims, none of the trackways occur in sandstones but are invariably preserved on dacitic tuff bedding planes. Trackway morphology comprises either short straight segments with discernable footprint impressions (Diplichnites), long looping traces suggesting dragging and footprint amalgamation (Diplopodichnus), or transitional forms. Morphometric comparison of the Diplopodichnus with fossil and recent mortichnia (death traces) suggests they record immediate pre-mortem locomotion of dying myriapods, and modern analogue reveals mechanisms by which abundant volcanic ash would have elevated arthropod mortality rates. Sedimentary structures including wave ripples, bedding sags and detached large- scale ball-and-pillow structures, show that the tuffs were deposited subaqueously. One instance where trackways intermittently traverse 3-5 cm tracts of subaerially-exposed wave-ripple crestlines attests that the tracemakers were capable of surviving out of water over limited distances. The low diversity ichnofauna has greater similarity with other Cambrian and Ordovician sites, recording semi-subaqueous pioneer tracemakers, than it does with the earliest fully-terrestrial ichnofaunas from the Silurian and Devonian.