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| Burgessomedusa phasmiformis Moon, Caron & Moysiuk, 2023 Artwork by C. McCall. |
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: cnidarian, cambrian explosion, medusae
Systematic palaeontology
Phylum Cnidaria Verrill, 1865
Subphylum Medusozoa Peterson, 1979
Burgessomedusa phasmiformis gen. et sp. nov.
Diagnosis: Medusoid with a tetraradial bell-shaped umbrella with width reaching approximately 40% of umbrella height. Over 90 finger-like tentacles along the oral margin, reaching approximately 15% of umbrella height in length. Stomach cavity located at apex of the umbrella, occupying approximately 30% of the body area. Manubrium extending up to two-thirds the length of the umbrella. Gonads elongate and ovoid, occupying approximately 45% of the umbrella height, located along umbrella corners, but internally positioned approximately halfway between umbrella margins and manubrium.
Etymology: Genus is a compound name with Burgess referring to the locality, the Burgess Shale, and medusa (Latin) referring to the clade Medusozoa. Species is a compound name with phasma (Greek) and forma (Latin), in reference to the ghostly figure of the umbrella.
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| 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. |
Justin Moon, Jean-Bernard Caron and Joseph Moysiuk. 2023. A Macroscopic Free-swimming Medusa from the middle Cambrian Burgess Shale. Proc. R. Soc. B. 29020222490. DOI: 10.1098/rspb.2022.2490
https://phys.org/news/2023-08-oldest-species-jellyfish.html
