Medusozoa Peterson, 1979

Iten, Heyo Van, Leme, Juliana De Moraes, Simões, Marcello G. & Cournoyer, Mario, 2019, Clonal colony in the Early Devonian cnidarian Sphenothallus from Brazil, Acta Palaeontologica Polonica 64 (2), pp. 409-416 : 412

Subphylum Medusozoa Peterson, 1979

Class, order, and family uncertain Genus Sphenothallus Hall, 1847

Type species: Sphenothallus angustifolius Hall, 1847 , New York, Middle Ordovician .

Remarks.— Specimens belonging in the genus Sphenothallus generally consist of a single, slender, gently tapered hollow tube constructed of numerous, very thin (approximately 1–2 μm) lamellae that parallel the exterior surface of the tube (e.g., Van Iten et al. 1992, 2002; Muscente and Xiao 2015; Vinn and Kirsimäe 2015). Most specimens contain substantial amounts of the mineral apatite, but others are largely or entirely organic (see Muscente and Xiao 2015 and references cited therein). In transverse cross-sections the tube is sub-elliptical (except near the apical end, where it is more or less circular), with the total thickness of the tube increasing to its maximum at the ends points of the tube’s greatest diameter, thus forming two continuous, marginal thickenings having a crescent-shaped transverse cross-section. The relatively thin, frequently crumpled walls between the two thickenings are referred to here as faces (though without implying homology with the faces of conulariids). The pair of marginal thickenings, which owing to compaction are readily discernable as raised, levee-like berms or trough-like external molds, partially distinguishes Sphenothallus from all other tubular fossils such as Byronia Matthew, 1899 , Torellela Linnarson, 1871 and Annulitubus Vinn, Zabini, Sene-Silva, Kirsimäe, and Susan-Marcos, 2016 ( Zhu et al. 2000; Vinn et al. 2016; Landing et al. 2018). The wide or oral end of the tube is open, and the oral margin of each face is gently curved. The narrow, aboral (apical) end of the tube exhibits a small, sub-conical expansion floored by a thin membrane cemented in life to hard substrates including hardgrounds ( Bodenbender et al. 1989) and shell material. Finally, the soft body of Sphenothallus , preserved in pyrite in certain specimens from the Lower Devonian Hunsrück Slate, Germany ( Fauchald et al. 1986), includes a manubrium-like central mouth structure flanked by a pair of elongate tentacles situated at the end points of the greatest diameter of the tube.