Surfing in and on microbial mats: Oxygen-related behavior of a terminal Ediacaran bilaterian animal

Abstract

Geochemical evidence suggests that terminal Ediacaran (ca. 551–539 Ma) oceans experienced expansive anoxia and dynamic redox conditions, which are expected to have impacted animal distribution and behaviors. However, fossil evidence for oxygen-related behaviors of terminal Ediacaran animals is poorly documented. Here, we report a terminal Ediacaran trace fossil that records redox-regulated behaviors. This trace fossil, Yichnus levis new ichnogenus and new ichnospecies, consists of short and uniserially aligned segments of horizontal burrows that are closely associated with microbial mats. Thin-section analysis shows that the trace-making animal moved repeatedly in and out of microbial mats, with mat-burrowing intervals interspersed by epibenthic intermissions. This animal is hypothesized to have been a bilaterian exploring an oxygen oasis in microbial mats. Such intermittent burrowing behavior reflects challenging and dynamic redox conditions in both the water column and microbial mats, highlighting the close relationship between terminal Ediacaran animals and redox dynamics.

Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia)

“…The observed circular to oval structures filled with cements or grains (Figure 3a,b) are classic traces of burrowing organisms. Similar trace fossils are also known from the Shibantan Member (Xiao, Chen, Zhou, & Yuan, 2019; own observations). In both settings, these traces are horizontal and spatially associated with microbial mats (Table 2).…”

“…At both locations, sparry calcite overgrowth cements are restricted to coarse-grained layers, indicating that porosity is an important prerequisite for their formation.Notably, layers exhibiting sparry calcite overgrowth cements typically occur underneath very fine-grained allochthonous material or microbial mats(Figure 2f). It is therefore tempting to speculate that sedimentary sealing as well as microbial activity factored into the formation of the sparry calcite overgrowth cements.The observed circular to oval structures filled with cements or grains(Figure 3a,b)are classic traces of burrowing organisms.Similar trace fossils are also known from the Shibantan Member(Xiao, Chen, Zhou, & Yuan, 2019; own observations). In both settings, these traces are horizontal and spatially associated with microbial mats…”

The Ediacaran frondose fossilArboreafrom the Shibantan limestone of South China

“…These crinkled laminae have been interpreted as microbial mats (Chen et al, 2013), which have been hypothesized to be an important factor facilitating the preservation of soft-bodied Ediacaran fossils (Gehling, 1999; Callow and Brasier, 2009; Laflamme et al, 2011). A number of macrofossil taxa have been reported from the Shibantan Member, including classical Ediacara-type fossils (e.g., Rangea Gürich, 1929, Pteridinium Gürich, 1930, and Hiemalora Fedonkin, 1982; Chen et al, 2014), macroalgal fossils such as Vendotaenia Gnilovskaya, 1971 (Zhao et al, 1988), problematic fossils such as Yangtziramulus Shen et al, 2009 (Xiao et al, 2005) and Curviacus Shen et al, 2017, as well as remarkably diverse trace fossils that provide exciting opportunities to study early animal evolution (Zhao et al, 1988; Weber et al, 2007; Chen et al, 2013, 2018, 2019; Meyer et al, 2014; Xiao et al, 2019).…”

“…The Shibantan Member in South China and the Khatyspyt Formation in Arctic Siberia represent the only two carbonate successions that are known to host morphologically complex, soft-bodied Ediacara-type macrofossils (Duda et al, 2016), although dolostone of the Gametrail Formation in northwestern Canada also contains some simple discoidal Ediacara-type macrofossils (MacNaughton et al, 2000). The Shibantan limestone hosts a moderately diverse assemblage of Ediacaran fossils, including Pteridinium , Hiemalora , Rangea , and now Arborea , as well as the tubular fossil Wutubus Chen et al, 2014, the segmented and trilobate animal Yilingia Chen et al, 2019, and abundant trace fossils (Chen et al, 2013, 2018, 2019; Meyer et al, 2014; Xiao et al, 2019).…”

Diverse cuticular remains in Cambrian (Series 2) SSF assemblages from China and the pioneer metazoan colonization of offshore environments

“…Based on perceived differences in style of preservation, the 'Orsten'-Lagerstätten have been further sub-divided into an 'Orsten'-type preservation sensu stricto and sensu lato (Maas et al, 2006; or 'Orsten'-type and Doushantuo-type preservations, see Butterfield, 2003;Xiao et al, 2019;Bottjer et al, 2019). The majority of 'Orsten'-type fossils sensu stricto was obtained from Cambrian limestone nodules interbedded with rich black shales.…”