# Ornithoprion

Ornithoprion is a genus of extinct cartilaginous fish in the family Caseodontidae. The only species, O. hertwigi, lived during the Moscovian stage of the Pennsylvanian, between 315.2 and 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 skull anatomy of eugeneodonts; a group which includes O. hertwigi and which were previously known primarily from isolated teeth. The genus name, which is derived from the ancient Greek órnith- meaning 'bird' and príōn meaning 'saw', was inspired by the animal's vaguely bird-like skull and the saw-like appearance of the lower teeth. The species name honors Oscar Hertwig.
Ornithoprion is unique among known eugeneodonts in possessing an extremely elongated structure connecting to the lower jaw, known as the mandibular rostrum, which was protected by a beak of fused bony scales and which the precise function of is not known. It inhabited shallow marine environments and coexisted with a variety of other cartilaginous fishes. The structure of Ornithoprion's teeth suggests that it was a durophage which hunted marine invertebrates, and bite marks and damage to its fossils indicate it was fed on by other carnivores. The total length is uncertain due to the rear portion of the body being missing in all known specimens, but the largest skulls are approximately 10 cm (3.9 in) in length.

## Discovery and naming

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 rocks which are part of the Linton Formation. A single specimen was also collected from the Logan Quarry in an exposure of the Staunton Formation, also in Indiana, 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 or Desmoinesian stage of the Pennsylvanian, which is part of the Carboniferous period and which lasted from 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 (now FMNH 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 shale. The captured images were subsequently printed and illustrated. The Staunton Formation specimen, FMNH PF-2656, was also cut into multiple cross-sections, with these being studied to reveal the internal anatomy of the dermal denticles and teeth. At the time of its discovery, Ornithoprion represented one of the best preserved eugeneodonts, and one of the few known from postcranial fossils.
The genus name may be literally translated as 'bird saw', and derives from the ancient Greek roots órnith- (ὄρνιθ-) meaning 'bird' and priōn (πρίων), meaning 'saw'. The suffix -prion is a common component in the names of eugeneodont fishes and is in reference to the saw-like form of their symphyseal (midline) tooth whorls,$^{:70–93}$ while ornithos in this case refers to the animal's uniquely bird-like skull.

## Description

O. hertwigi is known from multiple articulated but incomplete specimens, with none preserving skeletal material beyond the pectoral girdle. Most of these specimens are preserved in lateral view, and all, including the holotype, are heavily compressed. As in other eugeneodonts, the preserved portion of the skeleton was composed primarily of cartilage reinforced by mineralized prismatic tesserae; hexagonal structures which strengthen the cartilage and are also present in the skeletons of modern Elasmobranchs (sharks and rays). While the postcranial anatomy is incompletely known, it was likely similar to other caseodontids such as Fadenia and Romerodus, with a streamlined body, a homocercal (crescent-shaped) caudal fin, and reduced or absent pelvic fins. The presence or absence of pelvic claspers in eugeneodonts is unknown, and it is debated if they possessed them in life.$^{:167–169}$ The dorsal and pectoral fins of Ornithoprion are also unknown, but there is no indication they possessed fin spines like those seen in many other Carboniferous chondrichthyans. At least five branchial arches are present, although as with other eugeneodonts it remains unclear if these supported individual gill slits or an operculum (plate-like cover).$^{:143–144,167}$ The total body length is uncertain due to the incomplete nature of the fossils, although the largest skull measures approximately 10 cm (3.9 in). American illustrator Ray Troll has proposed a total length of 60 cm (2 ft), although his reconstruction gives the species an elongated, eel-like body, which is not phylogenetically supported.$^{:144}$ An estimate as high as 90 cm (3 ft) has been suggested by author Richard Ellis, based on an assumed skull length of 15 cm (6 in).

### Skull

The most diagnostic trait of Ornithoprion is the exaggerated rostrum extending from the lower jaw, which is nearly the length of the rest of the skull. The rostrum, as well as a corresponding section of the skull, was armored and reinforced by rods of bone, which appear to have been dermal structures formed separately from the underlying cartilage or tesserae. 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 of the rostrum, near the point of contact with the lower jaw. Below the whorl, a keel of cartilage protrudes from the bottom of the rostrum. 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 plates which support the rostrum and which attach to the palatoquadrates.
The cranium 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 this region of the skull is poorly preserved. The palatoquadrates, which typically form the upper jaw of living cartilaginous fish, are reduced and immobile, and articulate with the cranium in a greatly limited, modified autodiastylic (two jointed) manner. This reduced state is unique among cartilaginous fish, and differs greatly from that seen in other eugeneodonts such as Helicoprion, in which the palatoquadrates are large and specialized, and potentially Sarcoprion, which may have had them fused to the cranium or lost entirely. 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 cranial articulation. The remains of what Zangerl speculated to be part of the hyoid arch are also present along the back of the skull.

### Teeth

The lower teeth of O. hertwigi consist of both multiple large tooth crowns extending from a connected root, known as a tooth whorl, and tooth batteries (tightly stacked rows of crushing teeth) down the length of the lower jaw. The crushing teeth are plate-like, flattened, rectangular, and possessed deep pits and grooves in their surface. They formed a flattened "tooth pavement" in life; likely used for processing shelled prey and similar to that of many other Paleozoic chondrichthyans. The structure of these pavement teeth was directly compared with those of Erikodus, a related genus, in Zangerl's 1966 description. The lower tooth whorl, a characteristic trait of the eugeneodonts, possessed up to seven broad, rounded, bulbous tooth crowns and was positioned along the midline of the body. 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 both another battery of pavement teeth and larger, pointed V-shaped teeth were attached directly to the underside of the cranium. These V-shaped teeth are thought to have formed another symphyseal (midline) tooth whorl similar to that proposed in Sarcoprion, although their precise arrangement in life is not known. Based on thin sectioning, the teeth of Ornithoprion were composed primarily of trabecular dentin (a spongy form of dentin present in holocephalan fishes) with an outer coating of orthodentin. There is no indication of enameloid (vitrodentin), but a small layer may have been present in life.

### Postcranial skeleton

The known postcrania of Ornithoprion encompass the frontmost portion of the skeleton. The left and right scapulocoradoids (pectoral girdle) are not fused to each other, unlike living chondrichthyans, and had a forward-angled scapular portion. Either five or six pairs of ceratobranchials (gill arches) are present, with what Zangerl tentatively identifies as sternal cartilage running beneath them. This unpaired intercoracoidal cartilage has also been identified in living broadnose sevengill sharks, as well as the extinct iniopterygians, the Jurassic chimaeriform Ischyodus, and potentially the closely related 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 are not preserved and were likely uncalcified, 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. These specialized neural arches may be an adaptation associated with the morphology or function of the animal's pointed skull and rostrum.

### Dermal denticles

Unlike living chimaeras, in which dermal denticles (also called placoid scales) are only present in isolated regions, the known body of Ornithoprion was completely covered in tiny, rounded, tooth-like denticles similar to those of sharks. These denticles possessed a pulp cavity, were predominantly made up of orthodentin, 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 a form of dentin, and many denticles form fused, compound structures. These compound denticles, termed "polydontode scales", share a single mushroom-shaped base with multiple crowns and pulp cavities emerging from it, and in O. hertwigi may consist of more than seven crowns. Similar polydontode scales are known to occur in the related Sarcoprion and potentially Helicoprion, although it has not been noted that these 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 denticles and armor composed of dentin. Extremely small denticles were also present in the mouth and throat, which were exclusively composed of orthodentin.
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 polydontode scales. 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 evolved independently in Ornithoprion, he honors Hertwig in the name of the type species.

## Classification

Though often referred to as sharks in both formal and informal texts, the eugeneodonts are likely only distant relatives of living sharks. When first described, Ornithoprion was placed as a member of the family Edestidae within the order (sometimes class) Bradyodonti, although at the time Zangerl considered it to potentially be an elasmobranch instead. In the 1971 edition of Paleozoic Fishes, researcher R. S. Miles considered the genus to be of uncertain position within Chondrichthyes, and tentatively placed it within Holocephali. He suggested that its similarities to edestids might be the result of convergent evolution (independent evolution) due to differences in the anatomy of the gills, the tooth histology, and the palatoquadrates. In 1981, Zangerl considered O. hertwigi as a member of Caseodontidae, as part of the larger superfamily Caseodontoidea and the newly established order Eugeneodontida, in light of the numerous new taxa and characteristics that had been observed since Ornithoprion's original description. In this publication, however, he considered all newly christened eugeneodonts to be members of the Elasmobranchii rather than the traditionally assumed Holocephali or Bradyodonti. During the 1990s edestid specialist Svend Erik Bendix-Almgreen, in his personal communications with artist Ray Troll, expressed belief that the features used to unite the Eugeneodontida may be the result of convergent evolution and that the group as defined by Zangerl was likely polyphyletic (not a natural group).$^{:108–109}$ Bendix-Almgreen had expressed a similar attitude in earlier publications throughout the 1960s and 1970s, in these cases supported primarily by the apparent presence or absence of enameloid or vitrodentin between different edestid taxa, or differences in the features of their skulls.
Today, while Zangerl's classification of eugeneodonts as elasmobranchs is no longer supported, his erected suborders and families within the group remain in use. Eugeneodontida is regarded as a monophyletic group on the stem of subclass Holocephali (sometimes defined as the more broadly-encompassing Euchondrocephali) and as part of the class Chondrichthyes. Discrepancies in tooth histology previously used to argue against their close relation of different eugeneodonts has been alternatively explained by different members' variable rates of tooth replacement or wear. The only extant members of the Euchondrocephali are three families of chimaeras, all of which are highly specialized deep-water fish and are not closely comparable to eugeneodonts in anatomy or lifestyle. In the absence of living analogues, the higher level interrelationships between extinct members of the subclass remain enigmatic. Eugeneodontia has been suggested to be most closely related to (or potentially descended from) the Orodontiformes, a group of Devonian-Carboniferous euchondrocephs with similar dentition.
The skull and vertebral morphology of Ornithoprion is very different from that of other known eugeneodonts, and key elements of the postcranial skeleton are missing. Its classification within the Caseodontidae is based on the bulbous, rounded nature of its tooth crowns and reduction of its palatoquadrates, features which are also found in related genera such as Caseodus. Below is a cladogram as illustrated by Zangerl (1981) based on morphological traits, which places Ornithoprion as a basal member of a clade also containing Erikodus and Fadenia.

## Paleoecology and paleobiology
### Habitat and ecological niche

The Mecca, Logan, and Excello shales all represent marine depositional environments, and all preserve a diverse assemblage of species. In a 1963 publication, researchers Rainer Zangerl and Eugene Richardson proposed that the Mecca and Logan sites were extremely shallow habitats, likely less than a meter (3.3 ft) of water, with small, isolated deeper areas. The presence of peat and coal indicates that the deposits overlay drowned forests and are a recent transgression of a marine environment over a terrestrial one (known as a cyclothem). The rich, black, fissile shale which encases the fossils indicates massive amounts of decaying organic material such as algae were present, which led to anoxic conditions and formed organic mud. Zangerl and Richardson also suggest that 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 conditions for preservation. The Logan and Mecca environment likely only existed for a brief period, with overlying invertebrate communities and limestone deposits indicating that deeper water eventually flooded the region and created a more stable habitat. The presence of larger fish and cephalopods at the Logan Quarry site may suggest somewhat deeper waters. Some subsequent authors have suggested that these shales were in fact formed in deep-water environments with anoxic mud bottoms, similar to the conditions seen in many other fossiliferous midwestern shales, although other later authors have treated the conditions that formed the Mecca and Logan sites as distinct from those that formed deep-sea shales such as the Stark Shale and continued to accept a shallow water environment.
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 fauna, which includes both the Mecca and Logan Quarry sites, also preserves an assemblage of conodonts, palaeoniscoids, brachiopods, orthocones, and cartilaginous fishes such as Orodus, Denaea, Cobelodus, Symmorium, and other holocephalans, including members of Iniopterygii. The Logan Quarry was inhabited by, in addition to many chondrichthyans, an unnamed chondrost-like actinopterygian with a similarly elongated rostrum, which was entirely unrelated to Ornithoprion. Invertebrates such as brachiopods and ammonoids are known from the Excello Shale, as are a wide variety of cartilaginous fishes including Listracanthus, Caseodus, Edestus, and Stethacanthus, all of which were roughly contemporaneous with Ornithoprion.

#### Evidence of predation

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 has been suggested that the unpreserved rear halves of the animals may have been severed by predation attempts. In a 2019 publication, author Wayne Itano suggests that the bisected fish remains of the Mecca and Logan shales, such as the known fossils of O. hertwigi, may be the result of a novel feeding method displayed by the contemporaneous and much larger Edestus, which cut prey in half in a manner similar to modern sawfish. The skulls of several Ornithoprion specimens also display crushed or missing chunks, which are proposed to have resulted from other fishes biting them and fracturing the cartilage. Coprolites and fossilized pellets containing fish remains are often found in the same formations as Ornithoprion and were likely produced by larger predators.

#### Diet and proposed feeding methods

Similar to many of its close relatives, Ornithoprion is believed to have been a durophage that fed on benthic invertebrates. The rounded, bulbous crowns of the lower symphyseal whorl, as well as the immobile upper jaw, appear to be adaptations for crushing, and the remains of brachiopod shells are known from the stomach of the related Fadenia.
The mandibular rostrum is believed to have been utilized in feeding, although the exact mechanism is unclear. Zangerl (1966) proposes that it was used to disturb or probe sediment while hunting for prey living on or within the seabed, and potentially flinging dislodged prey into the water column. He notes that this possible feeding mode is entirely speculative, although a later study agrees with the conclusion that the rostra of caseodonts could have been used to dislodge brachiopods. The anatomy of the rostrum is inconsistent with its use as a sensory structure due to the lack of grooves and pits to house electroreceptors. Some 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 he considers it unlikely that the rostrum was used as a weapon. The mandibular rostrum of Ornithoprion has been compared with those of the unrelated extinct bony fish Saurodon and Saurocephalus, in which the function is also not confidently known.

### Use in reconstructing Helicoprion

Because of its phylogenetic proximity, Ornithoprion has been used as a basis for restoring the anatomy and physiology of other eugeneodonts. Ray Troll, an Alaskan illustrator, has cited the taxon as one of his references while reconstructing the potential close relative and more widely publicized genus Helicoprion.$^{:144,151}$ Both murals and a life-sized model of Helicoprion davidsii, designed by Troll and displayed at the Idaho Museum of Natural History, directly reference features of O. hertwigi such as gill anatomy.$^{:151}$ The genus is also cited as one of the taxa which indicated the position of Helicoprion's whorl in the lower jaw prior to descriptions of its own fossils becoming available. Edestid researcher Bendix-Almgreen had, however, criticized the use of caseodonts as the basis for Helicoprion in Troll's art, as he believed they did not represent close phylogenetic or ecological analogues.$^{:108,109}$