Exploring the Aftermath of the Cambrian Explosion - The Evolutionary Significance of Marginal- to Shallow Inchnofaunas of Jordan

Australia: The Land Where Time Began

Exploring the Aftermath of the Cambrian Explosion - The Evolutionary Significance of Marginal- to Shallow Inchnofaunas of Jordan

Abundant, well-preserved Inchnofaunas have been recovered from the Hanneh Member of the Burj Formation, dated to the Middle Cambrian, and the Umm Ishrin Formation, Middle to Upper Cambrian, the Dead Sea area, Jordan. A wide variety of depositional environments, from tide-dominated shelf to prodelta, delta front and interdistributary-bay tidal flats, contain trace fossils. According to the authors¹ 12 trace fossil assemblages have been identified within a number of sequence stratigraphic settings, early transgressive subtidal dune and bar complex, late transgressive isolated dune patches and dune-field abandonment, high stand deltaic progradation, and forced-regressive deltaic progradation. Information is provided by these Inchnofaunas that helps with the understanding of the nature of ecosystems in the Cambrian following the Cambrian Radiation. Firm ground conditions at or near the seafloor are indicated by the evidence of the Jordan Inchnofaunas, which include well-preserved scratch marks in most Cruziana and Rusophycus ichnospecies, adhering Gyrolithes polonicus burrows, and passively filled, unlined Diplocraterion isp. which penetrate into the underlying mudstone. The firmground G. polonicus is associated to a maximum flooding surface that has no erosion involved, though the occurrences of Diplocraterion isp. are classic examples of the Glossifunglites ichnofacies that are linked to erosional exhumation. Trace fossil suites that are dominated by trilobite structures are firmground examples of the Cruziana ichnofacies, which raises issues with respect to the affinities of this ichnofacies to the substrate, which is usually considered to be indicative of softgrounds. According to the authors¹ their study underscores evolutionary controls on this ichnofacies, which suggests that occurrences from the Lower Palaeozoic, dominated by trilobite trails and trackways, reflect firm substrates while the younger ones represent typical softground examples. The presence of abundant bilobate trace fossils, such as trackways and scratch marks, in interdistributary-bay tidal flats within a braidplain delta complex in the Umm Ishrin Formation provides evidence that during the Cambrian arthropods were capable of foraying into marginal-marine brackish water environments. Communities that were relatively simple and limited use of infaunal ecospace are indicated by analysis of the tiering structure. Shallow-tier deposit- and detritus-feeding trace fossils dominate the low-energy fully marine and marginal-marine ichnofabrics. Ichnofabrics from fully-marine low-energy environments contain deep-tier structures of detritus feeders, but in high-energy subtidal zones deep-tier trace fossils of suspension feeders are dominant.