GYRACANTHID MATERIAL FROM THE MISSISSIPPIAN (NAMURIAN) OF KENTUCKY AND THE EVOLUTIONARY RELATIONSHIPS OF GYRACANTHIDAE

Pectoral fin spine material of a gyracanthid acanthodian has been recovered from the Hancock County Locality in northern Kentucky (Mississippian: Namurian). The material consists of two pectoral fin spines: an incomplete spine measuring 268 mm and a complete spine of 608 mm. The spines are laterally flattened and display proximal to distal curvature. The insertion base extends on the posterior edge most of the length of the spine, where a posterior groove is present. This region is unornamented, however, striations run parallel to the long axis of the spine. Ornamented ridges extend from the insertion area at an angle of 35° on both sides of the spine, meeting at an uneven chevron on the anterior leading edge. The ornamented ridges originate at an angle of 30° to the long axis proximally, and become more parallel distally. The distance between the ridges decreases distally, with 6 ridges/cm at the base and 8 ridges/cm at the distal tip. A phylogenetic analysis was conducted using 24 characters and 12 taxa to determine placement of these spines within Gyracanthidae. Taxa used for the phylogenetic reconstruction include Gyracanthus, Gyracanthides and Anklycanthus species with Luposyrus pygmaeus as the outgroup. Searches were conducted on three data sets. The first included all characters and taxa, the second just the pectoral spine characters, and the third excluded problematic taxa, but included shoulder characters. One relationship is common to all three analyses, the sister relationship between Gyracanthus filius and Gyracanthus youngi, with the Iowa gyracanthid as the sister-taxon to that pair. Ankylacanthus incurvus is consistently resolved at the base of the most parsimonious trees, supporting a primitive relationship to the rest of the taxa. The Hancock gyracanthid as Gyracanthides is strongly supported, however, Gyracanthides is consistently recovered as polyphyletic when Gyracanthides murrayi is included in the analyses. The lack of resolution and abundance of most parsimonious trees produced in these analyses suggest that pectoral spine and girdle morphology may be insufficient to resolve relationships among gyracanthids.