Thursday 18 July 2019

Sphenothallus sica: A branching Conularid from the Early Devonian of Brazil.

The Conularids are an enigmatic group of organisms with possible affinities to the Cnidarians, which first appeared very close to the beginning of the Cambrian (or possibly at the End of the Ediacaran), and disappeared in the Early Triassic. They form cones comprised of concentric disks stacked within one another; these disks have four-fold radial symmetry, as is seen in modern Jellyfish, and it has therefore been suggested that the Conularids may represent something similar to the Attached Medusa stage of the living Stalked Jellyfish, Stauromedusae, from which new free-swimming Jellyfish arise by budding off.

In a paper published in the journal Acta Palaeontologica Polonica on 3 April 2019, Heyo Van Iten of the Department of Geology at Hanover College, and the Department of Invertebrate Paleontology at the Cincinnati Museum Center, Juliana de Moraes Leme of the Department of Sedimentary and Environmental Geology at the University of São Paulo, Marcello Simões of the Department of Zoology at São Paulo State University, and Mario Cournoyer of the Musée de Paléontologie et de l’Évolution, describe a new species of Conularid from the Early Devonian Ponta Grossa Formation of Paraná State in Brazil.

The description of the new species is based upon specimens first described by the palaeontologist John Mason Clarke in 1903, and which he identified as a problematic fossil, and possibly a Hydroid, which he named Serpulites sica, and later redescibed by Friedrich Sommer in 1954 as an algae, which he named Euzebiola clarkei. Van Iten et al. accept Clarke's specific name, sica, but refer the specimen to the Conularid genus Sphenothallus, on the basis of that it apparently has a soft body covered by a tubular or steeply pyramidal organic or organo-phosphatic periderm with fine laminations, that this periderm ends in a sub-conical expansion, and exhibits a transverse internal wall.

Medusozoan Cnidarian Sphenothallus sica from the Early Devonian Ponta Grossa Formation, Paraná State, southern Brazil. The numbers 1–16 indicate the evident, marginal daughter tubes, while the letter P indicates the parent tube. (A) Reproduction of Clarke’s (1913) drawing. (B) Light photographs. (B₁), general view; 17?, the possible basal portion of a seventeenth daughter tube; (B₂), detail, the arrow indicates a short longitudinal cross section through one of the marginal thickenings. Also present, near this site, is apparent spalling of fine lamellae. Scale bars: (A), (B₁) 16 mm; (B₂) 4 mm. Van Iten et al. (2019).

The specimen comprises a partial branching periderm about 40 mm long and bearing at least 16 variably curved daughter tubes. Two of the daughter tubes originate at the widest point of the parent, while the remaining daughter tubes are arrayed in single file along each side. Both the daughter and parent tubes are narrowest at the base and widen towards the tips. This pattern of branching is similar to that seen in modern Cnidarians such as the Hydrozoan Eudendrium ramosum, or Scyphozoans such as Stephanoscyphus racemosus, which Van Iten et al. regard as evidence supporting the proposed relationship between the Conularids and the Cnidarians.

The extant Scyphazoan Nausitho punctata. Chapman & Werner (1972).

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