22 TRILOBITE TALES | FEBRUARY 2020 PETALOUS OHIOENSIS, THE MOST COMMON SHARK TOOTH FOUND AT MCCOY , COLORADO, AND THE SEARCH FOR ITS LOST HOLOTYPE by Wayne Itano Part 1: History of Petalodus ohioensis from its discovery to the early twentieth century Introduction Petalodus ohioensis is a “shark,” the teeth of which are commonly found in marine sediments of Pennsylvanian to Lower Permian age. I place the word “shark” inside quotes, since it would be more proper to call Petalodus a “chondrichthyan.” Chondrichthyans are a group of fishes having cartilaginous skeletons that includes not only modern sharks, but also modern chimararoids (e.g. ratfish) and many extinct lineages. Petalodus is not necessarily more closely related to modern sharks than to modern chimaeroids. Teeth of Petalodus ohioensis are by far the most common “shark” teeth found at McCoy, Colorado, the location of frequent WIPS field trips. A good nontechnical article on Petalodus was written recently by John A. Harper and is available online (Harper, 2018). I have recently written some articles on Petalodus, including Petalodus ohioensis, jointly with Dr. Ken Carpenter of the Prehistoric Museum in Price, Utah, and the Natural History Museum of the University of Colorado (Carpenter & Itano, 2018, 2019). I started this collaboration after noticing his article about Petalodus teeth from Utah (Carpenter & Ottinger, 2018) and realizing that I had something to contribute. Petalodus teeth are unlike any modern shark teeth. They have been called “spade-shaped” (Harper, 2018). Viewed from the inner (lingual) or outer (labial) sides, the crown has a low triangular shape. The labial face is convex; the lingual face is concave. Where the two faces come together, they form a sharp cutting edge. Figure 1 shows a particularly large Petalodus ohioensis tooth from Texas, with the labial side exposed. On both the labial and lingual faces, near the base of the crown, all Petalodus teeth display parallel ridges. These ridges are shown in Figure 2, on a tooth of P. ohioensis from Kansas. Figure 1. (left) Tooth of Petalodus ohioensis, with the labial (outer) side exposed. Note the long root. From Harpersville Formation, Upper Pennsylvanian, Coleman County, Texas. WMI 2108. 1 cm scale. Figure 2. (above) (A) Labial (outer) and (B) lingual (inner) views of a Petalodus ohioensis tooth from the Raytown Limestone, Pennsylvanian (Missourian), Lexena, Johnson County, Kansas. Arrows point to rows of ridges near base of crown. WMI 2113. 1 cm scales. Photos: Wayne Itano