A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale

Abstract

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian–Cambrian transition.

The Cambrian Odaraia alata and the colonization of nektonic suspension-feeding niches by early mandibulates

“…Zooplankton communities were already diverse by the middle Cambrian [3,43,74], but the interconnections between benthos and plankton at that time are not fully understood, yet. It is possible, then, that several Cambrian groups were already establishing pelagic communities that were connecting food webs across the water column, including arthropods [75], chaetognaths, ctenophores [76] and cnidarians [77]. Today, suspension feeders play a critical role in connecting the energy flux at different levels of the water column, especially in larger-bodied taxa that enrich the water column through faecal pellets or food clustering [78], a role that could have been performed by large odaraiids and suspension-feeding radiodonts in the Cambrian.…”

Self-Organisation of Prediction Models

“…"Lower animals often possess a richer embodiment of their activity system as compared to a poor perception system" (von Uexküll 1973 [33]: p. 161). For example, comb jellies are the oldest known animals with a nerve net (Schultz et al, 2023;Moon et al, 2023) [34,35]. Likely, such first nervous systems had developed for active body motion to be performed synchronously in space and time, coordinated by symbolic nerve pulses, yet, assumingly, without the reception of external signals and lacking any symbolic processing of such perception.…”