Featured Article Summary #111

Ornithoprion is an extinct group of cartilaginous fish, known from a single species called O. hertwigi. It lived during the Moscovian stage of the Pennsylvanian period, and its fossils have been found in black shale deposits in the Midwestern United States. Scientists have mainly studied it using x-ray imaging, and when it was first discovered it was among the best known holocephalan fishes from the Paleozoic. Its unusual body structure caused debate about where it belonged, but it is now classified in the order Eugeneodontiformes and the family Caseodontidae. Ornithoprion had a distinctive lower-jaw projection covered by fused bony scales, and its name means “bird saw,” referring to its bird-like skull and saw-shaped teeth. Its rounded teeth suggest it ate hard-shelled animals, while fossil bite damage shows it may also have been preyed on, and it was relatively small (up to about 10 cm for the skull and up to about 91 cm for the body). Ornithoprion, with its single species O. hertwigi, was formally described in 1966 using several fossils found in Indiana’s Mecca Quarry Shale and a specimen from the Logan Quarry Shale. More specimens have since been assigned from other Indiana formations and Kansas, with the holotype (FMNH PF-2710) and other material studied mainly through X-ray scans and, for one specimen, cross-sections to examine internal structures. The name Ornithoprion (“bird saw”) refers to the saw-like teeth and beaked skull shape, while hertwigi honors German zoologist Oscar Hertwig. Ornithoprion is known from several incomplete skeletons, none of which preserve the body beyond the pectoral girdle. Some postcranial parts like the tail exist but haven’t been described in detail, and the specimens are flattened and often preserved on their side. Its internal skeleton was made of cartilage reinforced with mineralized tesserae like those found in some related animals, and it was described as very small; one estimate suggests it may have been about 90 cm long. Ornithoprion’s lower jaw was about 1.3 times longer than the rest of the skull and formed a long, forward-projecting, bony-armored “mandibular rostrum,” which likely had a mostly cylindrical shape. Its jaw parts were connected by flexible cartilage joints, and the mouth was probably unable to fully close. The upper-jaw bones (palatoquadrates) were strongly reduced and may have partly fused with the skull, while the rest of the head had a long pointed snout and high, bird-like eye sockets; the nasal region is uncertain, and the brain was small and low in the skull. Ornithoprion had complex teeth in its lower jaw, including a saw-like “tooth whorl” made of up to seven rounded tooth crowns along the midline, plus other tightly packed flattened teeth that formed a “tooth pavement” on the sides. The upper teeth likely lined the underside of the skull or attached to fused parts of the jaw (the palatoquadrates), and included both pavement teeth and larger V-shaped teeth. Their tooth cores were mostly spongy trabecular dentin covered by orthodentin, with no clear sign of enamel-like enameloid.