Trace fossils associated with Burgess Shale non-biomineralized carapaces: bringing taphonomic and ecological controls into focus

Abstract

The association of trace fossils and non-biomineralized carapaces has been reported from Cambrian Lagerstätten worldwide, but the abundance, ichnodiversity, taphonomy and ecological significance of such associations have yet to be fully investigated. Two main end-member hypotheses are explored based on the study of a relatively wide variety of trace fossils preserved associated to Tuzoia carapaces from the middle Cambrian Burgess Shale in British Columbia. In the ecological Tuzoia garden hypothesis, the bacterially enriched surface of carapaces provides opportunities for intricate ecologic interactions among trophic levels. In the taphonomic shielding hypothesis, the trace fossil–carapace association results from preferential preservation of traces as controlled by compaction independent of any association in life. In an attempt to better understand the role of the carapace as a medium for preservation of trace fossils and to evaluate the effects of mechanical stress related to burial, a numerical model was developed. Results indicate that the carapace can shield underlying sediment from mechanical stress for a finite time, differentially protecting trace fossils during the initial phase of burial and compaction. However, this taphonomic model alone fails to fully explain relatively high-density assemblages displaying a diversity of structures spatially confined within the perimeter of carapaces or branching patterns recording re-visitation.

A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources

“…The Cambrian colonization of infaunal and meiofaunal niches is thought to have been increasingly enabled by the oxygenation of marine sediments during the Cambrian agronomic revolution [47,53] and these niches were evidently well established by the mid-Cambrian in the Burgess Shale ecosystem and elsewhere [24]. The inferred ecology of Cambroraster indicates that the evolution of large nektobenthic consumers, alongside smaller carnivores like trilobites [54], occurred in tandem with the radiation of these prey, in line with hypotheses emphasizing the catalysing effects of escalation during this radiation [55].…”

“…figure 1g; electronic supplementary material, figure S4). These variations are interpreted to represent taphonomic artefacts mainly due to variations in the angle of burial and suggest that the H-element in particular was relatively flexible and easily deformed (compare also with similarly preserved carapaces of Tuzoia; [24]). The P-elements (figure 1g,l,j,m; electronic supplementary material, figures S1G,H and S5A,B) consist of a bulbous lenticular posterior portion and an elongate, curving, anterior neck section.…”

The rise and early evolution of animals: where do we stand from a trace-fossil perspective?

“…The underlying reasoning is that the trace-fossil record consists only of structures produced by the displacement of relatively large organisms ploughing the sea floor, and that tiny trace fossils (less than 1 mm), if produced, are not expected to survive the taphonomic barrier. However, the entrenched idea of non-preservability of small traces has been repeatedly falsified by the increasing number of reports of meiofaunal trace fossils in Phanerozoic siliciclastics and limestones [132,133,235,236], and even one putative record in the terminal Ediacaran [237]. In the absence of profuse and penetrative vertical bioturbation, and mediated by a microbially stabilized substrate, the anactualistic conditions for the preservation of tiny, including meiofauna-size, trace fossils are met (e.g.…”

“…Recently, the presence of grazing trails cutting through the external moulds and casts of several Ediacara body fossils prompted the interpretation that scavenging may have been involved [128]. The association of non-biomineralized skeletons and trace fossils is well known from Burgess Shale-type deposits [129][130][131][132][133], and has been interpreted as a record of the increasing heterogeneity of food resources on the Cambrian seabed [132]. Interestingly, Helminthoidichnites (an Ediacaran survivor) displays highdensity occurrences in Tuzoia and Arthroaspis carcasses.…”

“…Interestingly, Helminthoidichnites (an Ediacaran survivor) displays highdensity occurrences in Tuzoia and Arthroaspis carcasses. In royalsocietypublishing.org/journal/rsfs Interface Focus 10: 20190103 spite of this association, morphological features in Helminthoidichnites, such as the absence of active infill and curved to straight course, are suggestive of grazing, linked to targeting of bacteria hotspots, rather than scavenging in strict sense [132,133]. In this scenario, distribution in close proximity to body fossils may simply reflect bacterial enrichment (not necessarily implying feeding on decaying soft tissue).…”