Eriptychius, Shenacanthus, Rainerichthys and Papilionichthys

Eriptychius americanus (Walcott 1892, Bryant 1936; Dearden et al 2023, earliest Late Ordovician) is one known from scattered bones here put together from µCT scans from Dearden et al 2023 based on the bauplan of Early Devonian Arctolepis. The multiple mouth parts (blue) move very little, as shown in the animation of a related taxon.

Rhinopteraspis animation demonstrates the primitive action of the oral elements, as in Eriptychius.

Shenacanthus vermiformis (Zhu et al 2022, Early Silurian, est 3cm long) is the earliest known of the jawed placoderms and the basalmost member of the spiny sharks (Osteichthys). Ozarcus is a much larger and much later sister from the Late Mississippian. Claspers are present.

Rainerichthys zangerli (Grogan and Lund 2009; Mississipian, 320mya; 20 cm long) is newly reconstructed here. The original high-set pectoral fin is here interpreted as a set of closely kept branchial arches derived rom the evolutin of the tabular and splitting of the placoderm thoracic armor. The thorny portion is the former dorsal carapace. The actual pectoral fins (cyan) appear to have been overlooked originally. This taxon appears to be more closely related to Shenacanthus than to iniopterygians.

Papilionichthys stahlae (Grogan and Lund 2009; Mississipian, 320mya; 9 cm long) has a laterally compressed skull, a large orbit and short rostrum. Again, the 'high-set pectoral fins' are interpreted here as branchial arches derived from the breakup of ancestral placoderm armor.

Bryant WL 1936. A Study of the Oldest Known Vertebrates, Astraspis and Eriptychius. Proceedings of the American Philosophical Society 76 (4): 409-427.American Philosophical Society.

Dearden RP et al 2023. The oldest three-dimensionally preserved vertebrate neurocranium. Nature https://doi.org/10.1038/s41586-023-06538-y

Grogan ED and Lund R 2009. Two new iniopterygians (Chondrichthyes) from the
Mississippian (Serpukhovian) Bear Gulch Limestone of Montana with evidence of a new form of chondrichthyan neurocranium. Acta Zoologica (Stockholm) 90 (Suppl. 1): 134–151.

Hu Y, Lu J and Young 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

Walcott CD 1892. Preliminary notes on the discovery of a vertebrate fauna in Silurian (Ordovician) strata. Geological Society of America Bulletin 3:153-172.

Zhu Y-A et al (10 co-authors) 2022. The oldest complete jawed vertebrates from the early Silurian of China. Nature 609:954–958. online

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