Chaetognatha Leuckart, 1854

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 416

Phylum Chaetognatha Leuckart, 1854 Class, order, and family uncertain

Remarks.―Two orders, Protoconodontida and Pseudoconodontida are proposed by Landing (1995) as histologically distinct phyletic/taxonomic groups to encompass organo-phosphatic spiniform sclerites described herein. The or- der Protoconodontida Landing, 1995, with such forms as Amphigeisina danica ( Poulsen, 1966) , Gapparodus bisulcatus ( Müller, 1959) , and Hagionella cultrata ( Missarzhevsky, 1977) among others, includes, according to Landing (1995), sclerites that grew by basal-internal accretion of microcrystalline laminae. By contrast, the order Pseudoconodontida Landing, 1995 , is characterised by fibers aligned subparallel with the surface of the wall, such as those in Protohertzina Missarzhevsky, 1973 . A fibrous structure of the wall (arranged in growth lamellae) was later reported from G. bisulcatus and H. cultrata ( Kouchinsky et al. 2011, 2015a) and contrasts with the lamellar microcrystalline structure of the wall in A. danica (see Kouchinsky et al. 2011 and herein). Pseudoconodontida originally refer to the same group (sensu Landing 1995) other histologically and morphologically different body-surface sclerites (not grasping spines) of problematic Fomitchella (see Kouchinsky et al. 2017) and Rhombocorniculum (see e.g., Kouchinsky et al. 2015a). It appears therefore that the orders Protoconodontida and Pseudoconodontida are neither histologically distinct nor monophyletic and cannot be used herein in their original formal meaning.