Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil

Abstract

The evolutionary events during the Ediacaran-Cambrian transition (~541 Myr ago) are unparalleled in Earth history. The fossil record suggests that most extant animal phyla appeared in a geologically brief interval, with the oldest unequivocal bilaterian body fossils found in the Early Cambrian. Molecular clocks and biomarkers provide independent estimates for the timing of animal origins, and both suggest a cryptic Neoproterozoic history for Metazoa that extends considerably beyond the Cambrian fossil record. We report an assemblage of ichnofossils from Ediacaran-Cambrian siltstones in Brazil, alongside U-Pb radioisotopic dates that constrain the age of the oldest specimens to 555-542 Myr. X-ray microtomography reveals three-dimensionally preserved traces ranging from 50 to 600 μ m in diameter, indicative of small-bodied, meiofaunal tracemakers. Burrow morphologies suggest they were created by a nematoid-like organism that used undulating locomotion to move through the sediment. This assemblage demonstrates animal-sediment interactions in the latest Ediacaran period, and provides the oldest known fossil evidence for meiofaunal bilaterians. Our discovery highlights meiofaunal ichnofossils as a hitherto unexplored window for tracking animal evolution in deep time, and reveals that both meiofaunal and macrofaunal bilaterians began to explore infaunal niches during the late Ediacaran. NATuRE ECoLoGy & EvoLuTIoN Articles Nature ecology & evolutioNsupport for these suggestions is limited to purported body fossils of sponges 19 and demosponge biomarkers 20 . A considerable gap therefore remains between the fossil record of the late Ediacaran and molecular clock estimates for deep splits in the animal tree, for example the origin of Metazoa and Eumetazoa 3 . Assuming that contemporary molecular clock analyses yield accurate, if imprecise 18 , node ages for animal divergences, a small body size and concomitant limited fossilization potential 21 could reconcile these discordant records of animal evolution (but see ref. 22 ).The small body size of the ancestral bilaterian is supported by recent phylogenomic analyses of deep animal relationships, with acoel flatworms and xenoturbellids (Xenacoelomorpha) being a sister group to all remaining bilaterians (Nephrozoa) 23 , and smallbodied spiralian taxa (the 'Platyzoa') recognized as a paraphyletic grade with respect to macroscopic trochozoans 24 . This suggests that early bilaterians and spiralians were small bodied, possibly meiofaunal, and moved using ciliary gliding.Meiofauna comprises all organisms between 32 and 1,000 μ m in size that inhabit pore-water-rich sediments in freshwater to deepmarine environments 25 . Modern meiofaunal communities include animals, foraminifera and some ciliates, and contribute significantly to sediment bioturbation and bioirrigation 26,27 . The meiofauna can be divided into permanent members (that is, animals with organisms of a small size adapted and restricted to the meiofaunal, interstitial realm) and temporary meiofauna (for example, the ...

Early fossil record of Euarthropoda and the Cambrian Explosion

“…The absence of animals from Ediacaran age rocks has been explained by either poor fossilization at this time (8) or by being too small to fossilize (e.g., the meiofauna of ref. 5) (88). Hypotheses that regard Precambrian preservation as insufficient to preserve euarthropods can no longer be sustained, given the abundant lagerstätten from the Ediacaran Period.…”

Permian macroburrows as microhabitats for meiofauna organisms: an ancient behaviour common in extant organisms

“…; Parry et al . ; Tarhan ). Although the petite size of some Palaeozoic burrows has been attributed to meiofauna nematodes (Parry et al .…”

“…Although the petite size of some Palaeozoic burrows has been attributed to meiofauna nematodes (Parry et al . ; Tarhan ), the record of burrowing activity attributed directly to meiofaunal nematodes in pre‐Mesozoic rocks is scarce until now. Thus, the scarcity of anatomical evidence of fossil nematodes bodies makes trace fossils a primary source of information on their evolution.…”

Field workshop on the Ediacaran Corumbá Group of southwestern Brazil

“…Of great interest is the occurrence of at least two conglomerate intervals near the base of the Tamengo Formation, and the possible presence of a depositional break between the Tamengo and Guaicurus formations. The Guaicurus Formation at Laginha contains branching algal fossils, vendotaenids, as well as entangled filamentous structures interpreted as meiofaunal trace fossils (Parry et al, 2017). Participants also discussed the placement of the Ediacaran-Cambrian boundary and whether the Guaicurus Formation is Ediacaran or Cambrian in age.…”

“…Additionally, Aspidella-like discoid fossils were also discovered. Importantly, several ash beds from the Corumbá Group have been recently dated, giving tight age constraints on the fossiliferous Tamengo and Guaicurus formations (Parry et al, 2017).…”