UC Riverside

The radiation of burrowing metazoans in the early Phanerozoic is commonly assumed to have dramatically altered the properties of marine sediment. The advent of biogenically-mediated sediment mixing, by profoundly impacting nutrient burial fluxes, organic carbon remineralization, seafloor oxygenation and sediment ecology, likely instigated Phanerozoic biogeochemical cycling. However, the timing of the development of mixed seafloor sediment has, historically, not been well constrained. Mixing has commonly been assumed, in the absence of data, to occur at the Precambrian-Cambrian boundary with the appearance of the index fossil and three-dimensional burrow Treptichnus pedum (the &ldquoCambrian substrate revolution&rdquo or &ldquoagronomic revolution&rdquo). Likewise, instances of exceptional preservation--preservation of near-surface structures of high taphonomic fidelity, including bioglyphic detail--have previously been anecdotally reported from lower Paleozoic trace fossil assemblages worldwide. However, it has hitherto been unknown to what extent this exceptional preservation characterizes the lower Paleozoic record as a whole.

Herein I present new ichnological, stratigraphic and taphonomic data suggesting that, although significant developments in infaunalization-- paleobiologically complex animal-substrate interactions, particularly burrow construction--occurred during the early Paleozoic, mixing remained suppressed throughout this interval. I demonstrate that shelfal sediment in the earliest Cambrian was essentially unmixed and that, even as late as the middle Ordovician--over 75 million years after the Precambrian-Cambrian transition and well after the appearance and diversification of supposedly deposit-feeding trilobites, seafloor shelfal sediment remained largely unmixed on a global scale. Moreover, lower Paleozoic heterolithic shelfal successions are also characterized by widespread exceptional preservation of shallow-tier ichnological assemblages and bioglyphic structures, implying formation and preservation in a cohesive, i.e. unmixed substrate. Bed-sole assemblages are preserved through syndepositional casting rather than intrastratally. Well-mixed intervals are typically of limited spatial extent; macro-scale depositional fabrics are largely undisrupted. These findings challenge the current assumption that mixing occurred with the first appearance of three-dimensional burrows and that early Paleozoic infauna were efficient sediment mixers. The protracted development of the mixed layer holds important implications for exceptional preservation in both the trace and body fossil records, for the structure of lower Paleozoic benthic ecosystems, and for the advent and development of modern-style biogeochemical cycling.