Decoupling of body-plan diversification and ecological structuring during the Ediacaran–Cambrian transition: evolutionary and geobiological feedbacks

Abstract

The rapid appearance of bilaterian clades at the beginning of the Phanerozoic is one of the most intriguing topics in macroevolution. However, the complex feedbacks between diversification and ecological interactions are still poorly understood. Here, we show that a systematic and comprehensive analysis of the trace-fossil record of the Ediacaran-Cambrian transition indicates that body-plan diversification and ecological structuring were decoupled. The appearance of a wide repertoire of behavioural strategies and body plans occurred by the Fortunian. However, a major shift in benthic ecological structure, recording the establishment of a suspension-feeder infauna, increased complexity of the trophic web, and coupling of benthos and plankton took place during Cambrian Stage 2. Both phases were accompanied by different styles of ecosystem engineering, but only the second one resulted in the establishment of the Phanerozoic-style ecology. In turn, the suspension-feeding infauna may have been the ecological drivers of a further diversification of deposit-feeding strategies by Cambrian Stage 3, favouring an ecological spillover scenario. Trace-fossil information strongly supports the Cambrian explosion, but allows for a short time of phylogenetic fuse during the terminal Ediacaran-Fortunian.

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

“…Extant meioendobenthic organisms are particularly important contributors to biogeochemical cycling, microbial ecology and ecosystem productivity, especially in muddy sediments 27,37 . Multiple studies discuss the trace fossil record of macrofaunal behaviour from the late Ediacaran onwards, its postulated impacts on sediment geochemistry and benthic ecology, and its role in ecosystem engineering and ecological escalation 1,2,8 . Constraining the deep time origins of a meiofaunal mode of life may be equally important for understanding the biological and chemical evolution of marine sedimentary environments.…”

“…Diverse ichnofossil assemblages in the earliest Cambrian place an important constraint on the tempo of bilaterian origins, as they indicate that some groups, including total group panarthropods and priapulid-like scalidophorans 2,9 , were globally distributed and abundant by this point. The major bilaterian divergences (that is, the protostome-deuterostome and ecdysozoan-lophotrochozoan divergences) must therefore pre-date the Ediacaran-Cambrian boundary.…”

“…he Lower Cambrian fossil record documents a major radiation of macroscopic animals (particularly bilaterian phyla), coupled with significant expansion of their behavioural interactions with substrates and other organisms 1,2 . However, a growing catalogue of evidence from body fossils, trace fossils, biomarkers and molecular clocks indicates a protracted Neoproterozoic history for the Metazoa, with the origin of animals significantly pre-dating the base of the Cambrian 3 .…”

“…Our understanding of early animal evolution is complemented by ichnological investigations of latest Ediacaran to Ordovician strata 1,2,8 . Diverse ichnofossil assemblages in the earliest Cambrian place an important constraint on the tempo of bilaterian origins, as they indicate that some groups, including total group panarthropods and priapulid-like scalidophorans 2,9 , were globally distributed and abundant by this point.…”

What sparked the Cambrian explosion?

“…And the transformation of the sea floor, says Narbonne, "may have been the most profound change in the history of life on Earth" 10,11 . The mat had previously covered the sea bed like a coating of plastic wrap, leaving the underlying sediments largely anoxic and off limits to animals.…”