The Chaetognaths, or Arrow Worms, are an enigmatic group of marine invertebrates, with a bodyplan which is not close to that of any other group. They are pelagic ambush predators, with torpedo-like bodies and large external jaw-structures comprising two prominent bundles of anterior grasping spines and associated teeth. Attempts to determine how Chaetognaths are related to other animals using molecular methods have met with limited success; Chaetognaths have consistently been recovered as members of the Protostomes (the group that includes Arthropods, Molluscs, Annelids and the Lophophorate groups), with some studies suggesting that they occupy a basal position within the Protostomes (i.e. they branched off from the rest of the Protostomes before all other groups diverged from one-another, closer to the base of the family tree), or that they occupy a basal position within the lophotrochozoans (Molluscs, Annelids and Lophophorate phyla). To make matters worse, the Chaetognaths have almost no fossil record, with only one fossil species described, Protosagitta spinose, from the Early Cambrian Chengjiang Lagerstätte of Yunnan Province, which appears to be essentially similar to living Chaetognaths.
In a paper published in the journal Palaeontology on 1 September 2017, Degan Shu of the Early Life Institute, Department of Geology, and State Key Laboratory of Continental Dynamics at Northwest University, Simon Conway Morris of the Department of Earth Sciences at the University of Cambridge, Jian Han, also of the Early Life Institute, Department of Geology, and State Key Laboratory of Continental Dynamics at Northwest University, Jennifer Hoyal Cuthill, also of the Department of Earth Sciences at the University of Cambridge, and of the Earth-Life Science Institute at the Tokyo Institute of Technology, Zhifei Zhang and Meirong Cheng, again of the Early Life Institute, Department of Geology, and State Key Laboratory of Continental Dynamics at Northwest University, and Hai Huang of the College of Petroleum Engineering at Xi’an Shiyou University, describe a second species of Chaetognath from the Chengjiang Lagerstätte, which may shed more light on the origin of the group.
The new species is named Ankalodous sericus, where ‘Ankalodous’ means ‘bundles of teeth’ and ‘sericus’ means ‘silk’, a reference to the medieval Silk Road, connecting China to the Mediterranean region, which started at Xi’an in Shaanxi Province, the location of both Northwest and Xi’an Shiyou universities. The species is described from a number of specimens collected from outcrops of the Qiongzhusi Formation in Jingning and Haikou counties of Yunnan Province. Only the jaws of these specimens are preserved, nothing of the rest of the animals, but the preserved parts conform to the Chaetognath pattern, of opposed pairs of bundles of bristles. However, unlike modern Chaetognaths, Ankalodous sericus appears to have not just one pair of opposable jaw bundles but a whole series.
Multi-jawed Chaetognath Ankalodous sericus. (A)-(B) Specimen and interpretative drawing; bilaterally symmetrical assemblage of grasping spines, five bundles (A-E) on either side (L, left; R, right) of midline; up to six spines per bundle; some spines (Lc6, Rc2, 3) show basal apertures. (C)–(D) Part and interpretative drawing; assemblage of grasping spines; three bundles (A–C) from left (L) and right (R) sides, latter rotated; details from counterpart incorporated by reversal; cross-hatched areas oxides (originally pyrite), mostly as infills of internal cavities of spines. (E)–(F) Assemblage of grasping spines and interpretative drawing; assemblage of grasping spines is bilateral but right-hand array (RA–RC) somewhat displaced relative to left-hand array (LA–LC). (G)–(H) Part and interpretative drawing; left (L) and right (R) assemblages of grasping spines, separated by thick layer of sediment; left assemblage comprises five (A–E) bundles of spines, with up to six spines per bundle; right assemblage somewhat less clear, but at least four bundles discernible; drawing is combination of part and by reversal counter-part. In all drawings stippled areas represent sediment, hachured lines breaks in slope with hachures down-slope. All scale bars represent 1 mm. Shu et al. (2017).
Shu et al. suggest that the arrangement seen in Ankalodous sericus, with several pairs of opposed bristle-bundles, may have been used as part of an ambush strategy by a buried benthic animal rather than a free swimming one, with an attack something like that of an Ant-lion.
Reconstruction of Ankalodous sericus. Animal depicted here with feeding apparatus open prior to prey capture. Position of the plate-like structure suggests that it was located on a mouth-cone or similar protrusion. Details of region posterior to feeding apparatus are hypothetical. Shu et al. (2017).
Shu et al. observe that the arrangement of bristles in Ankalodous sericus resembles that seen in an enigmatic group of early Cambrian fossils called Protoconodonts, suggesting that these may have been related to Chaetognaths. They suggest that the group could have evolved from a primitive Lophotrochozoan ancestor with a complex jaw arrangement that has become simplified as an adaptation to a pelagic lifestyle, and thatbthis might imply a close relationship with the Gnathifera, a subdivision of the Lophotrochozoans which includes the Gnathostomulids, Micrognathozoans, Rotifers and Acanthocephalans, which are united by their complex jaw apparatuses.
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