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

  • ️Sun Jan 01 2023

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

Justin Moon et al. Proc Biol Sci. 2023.

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.

Keywords: cambrian explosion; cnidarian; medusae.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1.
Figure 1.

Size variations and general morpho-anatomical details of Burgessomedusa phasmiformis gen. et sp. nov. (a) Holotype ROMIP65781.1 (close-up in figure 2a). (b) ROMIP65782.2–3, with putative gonads (close-up in figure 2b). (c), ROMIP65783.1, with putative gonads. (d) ROMIP65784, with putative stomach cavity. e,f, specimens with putative gonads ROMIP65785 (e), ROMIP65786 (f). (g) ROMIP65787, with a contracted umbrella. (h) ROMIP65788, with putative gonads (close-up in figure 2e). (i) ROMIP65114.1–3. (j) ROMIP65789. (k) ROMIP65790.1–2. Abbreviations: bm, bell margin; go, gonads; man, manubrium; st, stomach cavity; ten, tentacles. Scale = 2 cm.

Figure 2.
Figure 2.

Morphological details of Burgessomedusa phasmiformis gen. et sp. nov. (a) Close-up of stomach cavity, manubrium, and gonads, ROMIP65781.1. (b) Close-up of tentacles ROMIP65782.2. (c,d) specimen showing disarticulated tentacles (close up in d), ROMIP65791. (e) Close-up of tentacles showing equidistant interspaces, ROMIP65788. (f,g) ROMIP65792, with short tentacles (close up in g) placed under the oral umbrella margin. (h,i), ROMIP65793, with tentacle remnants (close up in i). (j) ROMIP65794, specimen with irregular umbrella margin. (k) ROMIP65795.1, specimen showing tetraradial symmetry. All abbreviations are as in figure 1. Scales = 1 cm.

Figure 3.
Figure 3.

Phylogenetic position of Burgessomedusa phasmiformis gen. et sp. nov. Bayesian phylogenetic analysis (347 characters, 106 taxa, Mkv + Γ model) showing the position of Burgessomedusa phasmiformis gen. et sp. nov. within Medusozoa. Numbers indicate the posterior probabilities; scale bar indicates the average number of substitutions per site. Nodes with 100% posterior probability are not labelled. Red lines indicate alternative placements for Burgessomedusa.

Figure 4.
Figure 4.

Medusozoan umbrella morphospace based on umbrella diameter and height. Fineness ratio is umbrella height divided by the diameter. Colour based on the medusozoan class: light purple = Hydrozoa; black = Cubozoa; grey green = Scyphozoa; purple = Burgessomedusa phasmiformis gen. et sp. nov. Data on swimming umbrella dimensions from [27].

Figure 5.
Figure 5.

Life reconstruction showing a cluster of Burgessomedusa phasmiformis gen. et sp. nov. swimming above the benthos. This reconstruction is based on the Raymond Quarry Burgess Shale community with clusters of Vauxia sponges represented in the foreground. Artwork by C. McCall.

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