The Viséan (Middle Mississippian) St. Louis Formation of the Illinois Basin of North America has yielded many ichthyodorulites (fish spines), few of which are known from articulated or associated remains. Five ichthyodorulites from a single bed in the St. Louis Formation of Putnam County, Indiana, USA are described—Spine 1: a fin spine resembling that of Echinochimaera meltoni Lund, 1977, provisionally referred to Echinochimaera and designated as Echinochimaera curva sp. nov., Spine 2: a curved, tapered spine having a flat proximal attachment area, Spine 3: a narrow, sharply-pointed, curved spine bearing distally-pointed denticles on both the concave and convex margins, Spine 4: a falciform dermal denticle, and Spine 5: an incomplete, tapered spine bearing three narrow prongs arranged slightly asymmetrically at the wider end. Other than the probable echinochimaerid fin spine, the affinities of these ichthyodorulites are obscure. Spine 2 might have menaspid affinities. Alternatively, it might be a fin spine similar to Physonemus. Spine 3 has characters in common with some menaspiform cephalic spines but might be a cephalic tenaculum (frontal clasper). Spine 4 resembles some traquairiid cephalic spines. Spine 5 might be an incomplete holocephalan frontal clasper. If Spine 1 does belong to an echinochimaerid, it would be the earliest known.
Wayne M. Itano (Thu,) studied this question.