Colonization of the deep‐marine environment during the early Phanerozoic: the ichnofaunal record

Abstract

Temporal changes in the ichnogeneric diversity and ethological structure of 50 Cambrian to Carboniferous deep-marine ichnofaunal assemblages were quanti®ed. Assemblages of Cambrian age are distinctive: their diversity is lower, pascichnia and agrichnia relatively unimportant, and`shallow-marine' ichnotaxa more common, than in subsequent periods. There appear to be no environmental reasons that would have precluded more extensive colonization of the deep-marine environment during the Cambrian. The signi®cant restructuring of deep-marine communities at, or near, the Cambrian±Ordovician boundary is therefore attributed to competition for ecospace and/or resources within shallow-marine environments during the early Phanerozoic, as a result of which ichnotaxa, including examples of pascichnia and agrichnia, were displaced offshore. The Nereites ichnofacies, as traditionally understood, is thus characteristic of only deep-marine environments younger than the Cambrian.

The Trace-Fossil Record of Major Evolutionary Events

“…One would be to assess the trace-fossil record of the colonization of specifi c depositional settings, being the main focus on the reconstruction of major trends in global ichnodiversity. In fact, this approach has a relatively long history, starting with the colonization of the deep sea through geologic time due to pioneer papers by Seilacher (1974Seilacher ( , 1977 and Crimes (1974), continuing with more recent studies on the same topic (Orr 2001;Uchman 2003Uchman , 2004, as well as on the colonization of continental environments (Buatois and Mángano 1993;Buatois et al 1998), estuaries (Buatois et al 2005), and tidal fl ats (Mángano and Buatois 2015). Another approach would be to evaluate how the interactions between animals and substrates were shaped by major macroevolutionary events, such as the Cambrian explosion and the Permian mass extinction.…”

Bioturbation in aquatic environments: linking past and present

“…Restricted primarily to shallow marine environments during the Cambrian, complex, diverse bioturbation became prevalent in deep marine settings by the early Ordovician (Orr 2001). This offshore transition is attributed to an increasing competition for space and resources in shallow marine environments (Orr 2001) that led to the displacement of some groups of tracemakers (see also Crimes et al 1992, Crimes 2001.…”

“…This offshore transition is attributed to an increasing competition for space and resources in shallow marine environments (Orr 2001) that led to the displacement of some groups of tracemakers (see also Crimes et al 1992, Crimes 2001. Full details of the transition remain to be worked out, with deep marine strata of late Cambrian and early Ordovician age requiring further ichnological investigation, but the interpretation of its causes is generally accepted.…”

“…Boudreau (1994Boudreau ( , 1998 argued that the mean mixed depth in modern oceans is effectively independent of variations in water depth and sedimentation rate (but see Middelburg et al 1997). This was probably not the case in the early Palaeozoic, as deep marine environments in the Cambrian had lower ichnological diversity and more limited colonization than post-Cambrian equivalents (Orr 2001), but its applicability to later Palaeozoic, Mesozoic and Cenozoic strata has yet to be investigated. …”

Burgess shale−type biotas were not entirely burrowed away

“…The accepted explanation for the apparent closure of this taphonomic window after the Middle Cambrian is that BST biotas were burrowed away from the fossil record by increasing depth and intensity of bioturbation in muddy substrate environments; this reduced the likelihood of exceptional preservation (Allison and Briggs, 1993;Orr, 2001;Orr et al, 2003). In addition to direct physical disruption of carcasses by infaunal activity, enhanced sediment mixing led to greater rates of organic carbon oxida-tion, a deepened redox boundary in sediment pore waters (e.g., Thayer, 1983;Ziebis et al, 1996), and increased sulfate availability in the global ocean (Canfi eld and Farquhar, 2009), all of which would affect the preservation potential of labile tissues.…”