Abstract

A new gyracanthid, Gyracanthus? jasperi, is described based on partially articulated and isolated elements
from the Lower Carboniferous (Mississippian) lower Waugh Member of the Ste Genevieve Formation, at the early
tetrapod site near Delta, Iowa, U.S.A. Pectoral fin spines of the new gyracanthid are characterized by the insertion on the
fin spine being a half to one-third the length of the spine and maximum length:depth ratio 10:1. Ornament ridges on the
pectoral fin spine converge at ≥90° along a low leading edge ridge, and ridge tubercles are oriented oblique to the ridge;
the distal end of one pectoral is described using micro-computed tomography. The scapulocoracoids have an anteriorly
curving bulbous apex, and a base length that is 35–40% of pectoral spine length. The procoracoid is associated with an
unornamented ventral plate. A possible isolated dorsal fin spine is more symmetrical and less worn than the pectoral fin
spines. Pelvic spines are dorsoventrally flattened, with oblique ridges meeting at approximately 100° proximally and 80°
distally, along a smooth leading edge ridge. Scales, found on the partial articulated specimen and as isolated remains, are
polyodontode with a flat base. Adults of the new taxon are estimated to have been about one meter long. Deposition was
in debris flows, within a sinkhole lake most likely removed from the edge of the ocean. Other Carboniferous gyracanthids
from North America are reviewed.