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Ornithoprion ("bird saw") is a monotypic genus of extinct eugeneodont holocephalan in the family Caseodontidae. The type and only species, O''. hertwigi'', lived during the Moscovian stage of the Carboniferous, between 315.2 to 307 million years ago, and is known from black shale deposits in what is now the Midwestern United States. The discovery and description of Ornithoprion, performed primarily via radiography, helped clarify the cranial anatomy of the eugeneodonts, a group which were previously known primarily from isolated tooth whorls. The genus derives its name from the elongate, bill-like rostrum and large eyes, which vaguely resemble the features of a bird, while the species name honors Oscar Hertwig. It is known from specimens preserving the skull and anterior portion of the body.

Ornithoprion is unique among known eugeneodonts in possessing an extension of the lower jaw, known as the mandibular rostrum, which was protected by a beak of dermal armor and which the function of is not known. It inhabited shallow, warm marine environments and coexisted with a variety of other cartilaginous fishes. The structure of its teeth suggest Ornithoprion was a durophage which hunted marine invertebrates, and in turn bite marks and damage to its fossils indicate it was fed on by other carnivores. The total length and weight of the species has not been estimated, but the largest skulls are approximately 10 cm (4 in) in length.

Discovery and specimens
The genus and species were named and described in 1966, by geologist Rainer Zangerl, in a paper published by the Field Museum of Natural History (then the Chicago Museum of Natural History). This description was based on material collected primarily from the Mecca Quarry of Indiana, in black shale which is part of the Linton Formation. A single specimen was also collected from the Logan Quarry in an exposure of the Staunton Formation, and another from a coal mine near Wilmington, Illinois. All specimens are preserved in organic black shales, with the Illinois specimen being described as pyritic. The Mecca and Logan Quarry material has been dated to the Moscovian stage of the Carboniferous (Desmoinesian stage in American regional stratigraphy), which is part of the Pennsylvanian subperiod, and consists of a timeframe of 315.2 to 307 million years ago. The precise age and locality of the Illinois specimen is unknown due to it being held in a private collection. Nine specimens were initially described, with CNHM PF-2710 from the Mecca quarry being designated as the holotype. Multiple additional specimens have subsequently been assigned to Ornithoprion, including occurrences from the Excello Shale of Indiana.

Like many other chondrichthyans from the Mecca and Logan quarries, the studies of the holotype and paratypes of Ornithoprion were primarily performed by radiographic imaging. The specimens were not extracted from the surrounding matrix, but were instead stereoscopically scanned via X-rays, with the calcified elements of their skeletons appearing clearly against the rock. The captured images were subsequently printed and illustrated. The Staunton Formation specimen, CNHM PF-2656, was also cut into multiple cross-sections, with these being ground down to reveal the internal anatomy of the placoid scales and teeth. At the time of its discovery, Ornithoprion represented one of the best preserved eugeneodonts, and one of only a small few known from body fossils.

Description
O. hertwigi is known from multiple articulated but incomplete specimens, with none preserving skeletal material beyond the pectoral girdle. Most of this material is preserved in lateral view, and all, including the holotype, is heavily compressed. The preserved portion of the skeleton was composed primarily of cartilage reinforced by prismatic tesserae, structures which are present in the skeletons of modern Elasmobranchs but which are not apparent in living chimaeras. These tesserae are also known from other members of Eugeneodontida and are not unique to Ornithoprion. While the postcranial anatomy is incompletely known, it was likely similar to other caseodontids such as Fadenia and Romerodus, with a streamline body, a homocercal caudal fin, and reduced or absent pelvic fins. The dorsal and pectoral fins of Ornithoprion are also unknown, but there is no indication they possessed fin spines like many other Carboniferous chondrichthyans. At least five branchial arches are present, although it is unknown if these supported individual gill slits or an operculum. The total body length is uncertain due to the incomplete nature of the fossils, although the largest skull measures approximately 10 cm (4 in).

Skull and teeth
The most diagnostic trait of Ornithoprion is the exaggerated rostrum extending from the mandible, which is nearly the length of the rest of the skull. The rostrum, as well as a correlating section of the upper jaw, was armored and reinforced by rods of bone, which appear to have been dermal structures formed separately from the underlying cartilage. The mandibular rostrum is connected to the Meckel's cartilage (lower jaw) by an unfused joint, with a single whorl of teeth positioned along the midline near the base of the rostrum where it meets the jaw. A keel of cartilage extends off of the rostrum from below, correlating with the position of the whorl. The tip of the rostrum is extremely elongated, and according to Zangerl's description was likely to have been cylindrical in cross section and spear-like in life. There is no evidence that sensory structures were present within the rostrum. The Meckel's cartilage consists of a pair of thin, flattened cartilage rods which support the rostrum and which attach to the palatoquadrate. The chondrocranium is long and pointed and possesses large eye sockets, a condition which vaguely resembles the skull of a bird and inspired the name of the genus. An indentation set far forward on the snout is reported by Zangerl to have likely held the nasal capsule, although it is unpreserved. The palatoquadrate, which typically forms the upper jaw of living cartilaginous fish, is reduced and immobile, and articulates with the chondrocranium in an autodiastylic, unfused manner. The reduced state differs greatly from that of the eugeneodonts Helicoprion and Edestus, in which the palatoquadrates are large and specialized, and potentially from Sarcoprion, which may have them fused to the cranium. The palatoquadrate seen in Ornithoprion is most similar to that seen in other caseodonts such as Caseodus, which typically share its thin, band-like shape and limited articulation. The remains of what Zangerl speculated to be part of the hyoid arch are also present along the back of the skull.

The lower teeth of O. hertwigi consist of both multiple large tooth crowns extending from a single root, known as a tooth whorl, and of pavement teeth. The pavement teeth, which are plate-like, flattened, and rectangular, were present on both the upper and lower jaw and possessed deep pits and grooves in their surface. The structure of these teeth was compared with the tooth pavement of Erikodus, a related genus, in Zangerl's 1966 description. The lower symphyseal whorl, a characteristic trait of the eugeneodonts, possess up to seven broad, rounded tooth crowns, with the largest figured whorls spanning approximately 1 cm (.39 in) in length. The tooth crowns on the lower whorl vary in size, with the smallest teeth being situated at the front of the whorl and the largest at the back. It is thought that V-shaped teeth were present in another symphyseal row attached directly to the chondrocranium. These teeth, the largest figured being approximately 4 mm (.16 in) in length, were significantly different than those of the lower jaw, and the two likely contacted each other during feeding. Zangerl noted, however, that due to the nature of the material's preservation, the precise arrangement of O. hertwigi 's dentition is unknown.

Postcranial skeleton
The known postcrania of Ornithoprion encompasses the anterior-most portion of the skeleton. The scapulocoradoids are unfused, unlike living chondrichthyans, with a forward-angled scapular portion. Either five or six pairs of ceratobranchials are present, with what Zangerl tentatively identifies as sternal cartilage running beneath them. This unpaired intercoracoidal cartilage has also be identified in living broadnose sevengill sharks, as well as the extinct Iniopterygians, the Jurassic Chimaeriform Ischyodus, and potentially Fadenia. The function of this structure in Ornithoprion is unknown, although it is likely homologous to similar, paired cartilage structures observed in other extinct chondrichthyans.

The vertebral centra of Ornithoprion were likely uncalcified and are unpreserved, although a series of diamond-shaped cartilage structures are present along the expected path of the vertebral column. These cartilage structures may represent heavily modified neural arches, the anatomy of which is unique to O. hertwigi. It is possible these structures are an adaptation to the morphology or function of the animal's skull.

Dermal denticles
Unlike living chimaeras, in which scales are only present in isolated regions, the known body of Ornithoprion was completely covered in tiny denticles similar to those of sharks. These denticles possessed a pulp cavity and enameloid, and grew from a flattened base, much like those of modern cartilaginous fish. However, the bases of the denticles may have been composed of bone, rather than dentin,  and many denticles form fused, compound, polydontode scales. These compound denticles share a single base with multiple crowns and pulp cavities emerging from it, and in O. hertwigi may consist of more than seven crowns sharing a single base. Similar polydontode scales are known to occur in the related Sarcoprion and potentially Helicoprion, although these are not noted to contain bone. If Zangerl's interpretation of the dermal structures of O. hertwigi is correct, it would represent a rare example of bone among extinct euchondrocephalans, which otherwise have scales and armor composed of dentin.

In his original 1966 description, Zangerl speculates that the reinforcing "beak" of bony rods present on the snout and rostrum were formed by the compounding of these polydontous denticles. He likens this phenomenon to that proposed by Oscar Hertwig as an explanation for the origin of vertebrate dermal armor, and although Zangerl acknowledges that this adaptation is almost definitely convergent in Ornithoprion, he honors Hertwig's hypothesis in the name of the type species.

Classification
Though often referred to as sharks in both formal and informal texts, the eugeneodonts are only very distant relatives of living sharks. In its initial description, Ornithoprion was placed as a member of Edestidae under the order (sometimes class) Bradyodonti. In the 1971 edition of Paleozoic Fishes, researcher R. S. Miles considered the genus to be of uncertain position with Chondrichthyes, although still tentatively within Holocephali. He regarded its similarities with the edestids to potentially be the result of convergence, due to differences in the anatomy of the gills, teeth and palatoquadrate, although subsequent publications have not followed this hypothesis. In 1981, Zangerl placed O. hertwigi as a member of the Caseodontidae, as part of a larger superfamily Caseodontoidea and order Eugeneodontida, in light of the numerous new taxa and characteristics that had been observed since Ornithoprion's original description. This classification is still followed today, with Eugeneodontida itself typically being regarded as a stem group of the subclass Holocephali or Euchondrocephali and as part of a monophyletic Chondrichthyes. Euchondrocephali is today only represented by the chimaeras, and the higher level interrelationships between extinct members of the subclass remain enigmatic.

The skull and vertebral morphology of Ornithoprion is wildly different from that of other known eugeneodonts, and key elements of the postcranial skeleton are missing. Its assignment among the Caseodontidae is based on similarities in dentition and similar reduction of the palatoquadrate to that seen in genera such as Caseodus. Below is the complete cladogram provided by Zangerl (1981) based on morphological analysis.

Paleobiology and paleoecology
The Mecca, Logan, and Excello shales all represent marine depositional environments, and all preserve a diverse assemblage of species. The Mecca and Logan Quarries represent extremely shallow habitats, likely less than a meter (3.3 ft) of water, with small, isolated deeper areas. The presence of peat and wood indicate that the sediment of the environment was made up of drowned forest, and the rich, black shale indicates massive amounts of decaying organic material, likely algae, was present, which formed organic mud. There is evidence of water levels lowering significantly during the dry season, often isolating fishes into small saltwater ponds or "fish traps" and creating ideal environments for preservation. The Logan and Mecca depositional environment likely only existed for a brief interval, with overlying invertebrate communities and limestone deposits indicating deeper water eventually flooded the region and created more stable conditions. The conditions of the Logan Quarry shale were likely similar, although the presence of larger fish and cephalopods suggests somewhat deeper waters.

Ornithoprion fossils often contain a number of other remains in the same shale slabs, although in different bedding planes and not directly associated. These include isolated spines and denticles from acanthodians, Listracanthus, and Petrodus. The Mecca Quarry also preserves an assemblage of conodonts, palaeoniscoids, brachiopods, orthocones, and cartilaginous fish body fossils. The Logan Quarry was also home to an unnamed chondrost-like actinopterygian with a similarly elongated rostrum, which was entirely unrelated to Ornithoprion.

Numerous specimens of O. hertwigi show damage which Zangerl interprets as feeding traces left by predators or scavengers. Portions of the skeleton are often broken, maimed or missing, and it is postulated that the unpreserved rear halves of the animals may have been severed by predation attempts. The skull of the holotype and several assigned specimens have crushed or missing chunks, proposed to have resulted from other fishes biting or mouthing. Coprolites and fossilized pellets containing fish remains are often found in the same formations as Ornithoprion, which were likely produced by its predators. Like many of its close relatives, Ornithoprion itself is believed to have been a durophage which fed on benthic invertebrates, due to its dentition and the presence of crushed brachiopod shells known from the stomach of the related Fadenia.

The function of O. hertwigi's armored rostrum remains unclear. Zangerl (1966) proposes its use was to disturb or probe sediment while hunting for prey living on or within the seabed, as well as potentially flinging dislodged prey into the water column, but notes this proposal is entirely speculative. The anatomy of the mandibular rostrum is inconsistent with its use as a sensory structure, and features of the animal's skull, such as the armor and articulation of the upper and lower jaws, are suggested by Zangerl to be shock-absorbing adaptations, although use of the rostrum as a weapon is considered by him to be unlikely. The mandibular rostrum of Ornithoprion is said to most closely resemble those of the extinct actinopterygians Saurodon and Saurocephalus, in which the function is also not confidently known.