Exploring the aftermath of the Cambrian explosion: The evolutionary significance of marginal- to shallow-marine ichnofaunas of Jordan

摘要

The middle Cambrian Hanneh Member of the Burj Formation and the middle to upper Cambrian Umm Ishrin Formation of the Dead Sea area, Jordan, contain well-preserved and abundant ichnofaunas. Trace fossils are present in a wide variety of depositional environments, from tide-dominated shelf to prodelta, delta front and interdistributary-bay tidal flats. Twelve trace fossil assemblages have been identified within a number of sequence-stratigraphic settings, namely early transgressive subtidal dune and bar complex, late transgressive isolated dune patches and dune-field abandonment, highstand deltaic progradation, and forced-regressive deltaic progradation. These ichnofaunas provide information that helps us to understand the nature of Cambrian ecosystems in the aftermath of the Cambrian radiation. The Jordan ichnofaunas display evidence of firm ground conditions at or near the sea bottom. These include (1) well-preserved scratch marks in most Cruziana and Rusophycus ichnospecies, (2) adhering Gyrolithes polonicus burrows, and (3) unlined passively filled Diplocraterion isp. penetrating into underlying mudstone. Whereas the Diplocraterion isp. occurrences are a classic example of the Glossifungites ichnofacies linked to erosional exhumation, the firmground G. polonicus is associated to a maximum flooding surface with no erosion involved. The trace fossil suites dominated by trilobite structures are firmground examples of the Cruziana ichnofacies, raising issues with respect to the substrate affinities of this ichnofacies, which is typically considered indicative of softgrounds. Our study underscores evolutionary controls on this ichnofacies, suggesting that lower Paleozoic occurrences, which are dominated by trilobite trails and trackways, reflect firm substrates and younger ones represent the typical softground examples. The occurrence of abundant bilobate trace fossils, scratch marks and trackways in interdistributary-bay tidal flats within a braidplain delta complex in the Umm Ishrin Formation provides evidence that arthropods were able to foray into marginal-marine brackish-water settings during the Cambrian. Analysis of the tiering structure indicates relatively simple communities and limited use of the infaunal ecospace. Low-energy fully marine and marginal-marine ichnofabrics are dominated by shallow-tier deposit- and detritus-feeding trace fossils. Deep-tier structures of detritus feeders are present in ichnofabrics from fully marine low-energy settings, whereas deep-tier trace fossils of suspension feeders are dominant in high-energy subtidal zones.