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 global myriapod evolution and terrestrialization. Original fieldwork at the trace fossil site has identified four additional bedding surfaces that yield 121 new trackways at the site (of a total 158 known individuals), and permit better sedimentological and paleoecological contextualization of the trackways. Contrary to published claims, none of the trackways occur in sandstones but are invariably preserved on bedding planes of dacitic tuffs. Trackway morphology comprises either short straight track 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 within the environment 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 newly-discovered 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 very short distances. However, the low diversity, semi-subaqueous ichnofauna has greater similarity with other Cambrian and Ordovician sites, recording pioneer tracemakers with limited tolerance of subaerial environments, than it does with the oldest preserved fully-terrestrial trace fossil communities from the Silurian and Devonian. TRACKWAYS OF THE BORROWDALE VOLCANIC GROUP The Borrowdale Volcanic Group (BVG) is a 6 km-thick Upper Ordovician (Sandbian-Katian) succession of volcanic and related rocks that crops out in Cumbria, NW England, and which accumulated in an arc setting associated with subduction of the Iapetus Ocean (Barnes et al., 2006). Trace fossils were first discovered within the group by Mitchell (1956, p. 434), who reported "animal-tracks" on bedding planes at Lum Pot; a 150 m-long outcrop along the River Lickle, which exposes a near-continuous 25 m-thick stratigraphic section from the upper BVG (likely within its constituent Wallowbarrow Tuff Formation (Fig. 1; see Appendix)). The trace fossils were later identified as Diplichnites and Diplopodichnus by Johnson et al. (1994), who described them on the basis of 34 trackways on a 1.9 m2 bedding plane at Lum Pot and 3 trackways on the 0.1 m2 surface of a loose clast from Sour Milk Gill, 19 km north of the original locality. Johnson et al. (1994) documented that the trackways occurred in sandstone, which they interpreted as having been deposited in a freshwater lacustrine environment. S...

Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record

“…These arthropod trackways were formerly regarded as subaerial in origin (38), and the gait of the tracemaker is compatible with locomotion by penicillate millipedes (39). However, sedimentary structures in the trace-bearing strata indicate locomotion on ash deposited subaqueously (40), thereby weakening the case for a crown group millipede as the tracemaker. The inferred pre-Silurian history of the myriapod crown group has been recognized as an example of a general bias in the terrestrial fossil and rock records.…”

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

“…We thank Briggs et al (2019) for their Comment on our recent paper (Shillito and Davies, 2019). Our work reported a number of arthropod trackways from the Ordovician Borrowdale Volcanic Group (BVG), demonstrated that the trace fossils were hosted within very fine dacitic tuffs, and, coupling this with trackway morphology, proposed that the tracemakers had been lethally stressed by the presence of volcanic ash.…”

“…Our paper shows that the only sedimentological evidence at the BVG locality is for subaqueous deposition and, potentially, emergent substrates: not freshwater lacustrine conditions. Briggs et al contend that a lacustrine environment 'remains likely' because other parts of the 6-km-thick BVG appear to be non-marine in origin (but see the discussion in Shillito and Davies [2019]). They imply that the global singularity of a trace fossilbearing Ordovician lacustrine succession is not suspect because (1) there is a rarity of non-marine strata of this age globally.…”

A trace fossil made by a walking crayfish or crayfish-like arthropod from the Lower Jurassic Moenave Formation of southwestern Utah, USA

“…Perhaps more familiarly, walking tracks (repichnia) of arthropods have an extensive geological history, spanning from the Cambrian (and possibly latest Precambrian ( Chen et al, 2018 )) through the Holocene ( Eiseman & Charney, 2010 ). They constitute some of the earliest evidence of metazoan life venturing onto land (reviewed in Minter et al (2016a , 2016b) ) and are known from virtually every paleoenvironment, from near shore and shallow marine environments ( Collette, Hagadorn & Lacelle, 2010 ; MacNaughton et al, 2002 ; Pirrie, Feldmann & Buatois, 2004 ; Shillito & Davies, 2018 ; Trewin & McNamara, 1994 ) and, terrestrially, from proglacial systems ( Anderson, 1981 ; Lima, Minter & Netto, 2017 ; Lima et al, 2015 ; Uchman, Kazakauskas & Gaigalas, 2009 ; Walter, 1985 ) to desert ergs ( Gilmore, 1927 ; Good & Ekdale, 2014 ; Sadler, 1993 ).…”

“…Kouphichnium , are also present. Ichnospecies of Bifurculapes have been variably attributed to insects, possibly beetles, and to crustaceans ( Getty, 2016 , 2018 ; Hitchcock, 1858 , 1865 ); ichnospecies of Diplichnites have been attributed to myriapods ( Briggs, Rolfe & Brannan, 1979 ; Davis, Minter & Braddy, 2007 ; Pollard, Selden & Watts, 2008 ; Shillito & Davies, 2018 ), notostracans ( Lucas et al, 2006a ; Minter et al, 2007 ), and other arthropods ( Melchor & Cardonatto, 2014 ; Minter et al, 2007 ); and ichnospecies of Kouphichnium have been attributed to limulids ( Caster, 1944 ; King, Stimson & Lucas, 2019 ; Lomax & Racay, 2012 ).…”