Ornithoprion | Read 10 Min

Ornithoprion Temporal range: Pennsylvanian,

Skeletal reconstruction of Ornithoprion, with known material represented in white and implied/suggested material represented in gray
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Holocephali
Order:	†Eugeneodontiformes

Ornithoprion is an extinct genus of cartilaginous fish. The only known species, O. hertwigi, lived during the Moscovian stage of the Pennsylvanian subperiod, which spanned from 315 to 307 million years ago. Its fossils are preserved in black shales from what became the Midwestern United States. The study of Ornithoprion was performed primarily via x-ray imaging, and at the time of its discovery it represented one of the best known holocephalans of the Paleozoic era. The classification of the genus has been the subject of debate due to its unique anatomy; it is placed in the order Eugeneodontiformes and the family Caseodontidae. Ornithoprion's genus name, which may be translated from Greek as 'bird saw', was inspired by the animal's vaguely bird-like skull and the saw-like appearance of the teeth in the lower jaw, while the specific name honors German zoologist Oscar Hertwig.

Ornithoprion had a unique projection of its lower jaw termed the mandibular rostrum, which was covered by a beak of fused bony scales whose function is unknown. It inhabited shallow, seasonal marine and brackish water environments, alongside a variety of other cartilaginous fishes. The rounded shape of Ornithoprion's teeth suggests that it hunted hard-shelled invertebrates, and bite marks and damage to some of its fossils indicate that it was in turn fed on by other carnivores. Ornithoprion was small relative to other members of its order, with a cranium length of up to 10 cm (3.9 in) and an estimated body length of up to approximately 91 cm (36 in).

Discovery and naming

The genus Ornithoprion and its only species, O. hertwigi, were formally described in 1966 by paleontologist Rainer Zangerl. This description was based largely on seven fossils collected from the Mecca Quarry Shale of Parke County, Indiana, since the 1950s, in rocks which are part of the Linton Formation. A single specimen was also described from the Logan Quarry Shale of the Staunton Formation (alternatively the Tradewater Formation), also in Parke County, which along with the Mecca Quarry material is part of the collection of the Field Museum of Natural History. Another specimen from near Wilmington, Illinois, was also described, although the stratigraphic unit from which it originated is unknown, and it is part of a private collection. The specimens are all preserved in carbonaceous shale, and the preservation mode of the Illinois specimen has been described as pyritic (composed of the mineral pyrite). Specimen FMNH PF-2710 from the Mecca Quarry was designated as the holotype (the defining specimen of the species). Since Ornithoprion was named, additional fossil specimens from the Excello Shale and Carbondale Formation of Indiana, as well as material from Kansas, have been assigned. These specimens are housed in the collections of the American Museum of Natural History and the Natural History Museum, London.

Like many other fish fossils from the Mecca and Logan quarries, study of the holotype and paratypes of Ornithoprion were primarily performed by radiographic imaging. The specimens, which are extremely delicate, were not extracted from the surrounding rock matrix and were instead scanned via stereoscopic X-rays to study the hard parts of the body from within the shale. The Staunton Formation specimen, FMNH PF-2656, was also cut into multiple cross-sections, which allowed for study of the internal anatomy of the scales and teeth. At the time of its discovery, Ornithoprion was one of the few members of its order known from postcranial fossils, alongside the genera Fadenia, Erikodus, and what would later be described as Eugeneodus. It also represented one of only a small number of holocephalans from the Paleozoic era in which the endoskeleton was known, and alongside the related Fadenia was unique in preserving the gill arches in detail.

The genus name, Ornithoprion, translates literally from Greek as 'bird saw' and was given in reference to the saw-like row of teeth in the lower jaw and the animal's pointed, beaked skull. The specific name, hertwigi, honors German zoologist Oscar Hertwig.

Description

Drawing of a shark-like fish with an open mouth — Life reconstruction of *Ornithoprion*, with body proportions based on close relatives and with a hypothetical striped patterning

Ornithoprion is known from multiple articulated but incomplete skeletons, and none of the described specimens preserve the body past the pectoral girdle. Most of these are preserved on their side, and all, including the holotype, are flattened. Additional postcranial remains such as the tail are known, but these have not been prepared or described in detail. The internal skeleton of Ornithoprion was composed of the tissue cartilage and was reinforced by an outer coating of mineralized tesserae; prismatic structures which overlay the cartilage skeletons of chondrichthyans. Zangerl noted that Ornithoprion was "very small" in his description of the taxon, and in a 2003 book Richard Ellis suggested Ornithoprion had a total length of 90 cm (3 ft) based on an assumed skull length of less than 15 cm (6 in).

Skull

The lower jaw of Ornithoprion was roughly 1.3 times the length of the rest of the skull, and had a jointed, forward-facing projection termed the mandibular rostrum. While similar rostra are known in several related fish, the structure was significantly longer in Ornithoprion, and both the rostrum and a correlating section of the snout were uniquely armored by rods of bone embedded in the skin. In life, the mandibular rostrum was likely to have been cylindrical in cross section, with the exception of the rear portion which had a small bulging projection underlying a set of teeth. There is no indication that the mandibular rostrum contained sensory organs. The rostrum articulated with the Meckel's cartilages (equivalent to the mandible) at a flexible joint, and a flattened keel of cartilage protruded from the bottom of the rostrum near this point of contact. The Meckel's cartilages themselves consisted of a pair of broad, flattened, nearly immobile cartilages, which articulated with the palatoquadrates (equivalent to the upper jaws). Because of the shape of the Meckel's cartilages, the mouth was likely incapable of closing.

The palatoquadrates were greatly reduced, and potentially fused partially with the cranium. This differs greatly from related genera such as Helicoprion, in which the palatoquadrates were large and specialized, and potentially Fadenia, which may have had them entirely fused to the cranium or completely lost. The condition in Ornithoprion most closely resembled some other related genera such as Caseodus and Eugeneodus, although the degree of reduction is much greater in Ornithoprion. The palatoquadrate had two points of articulation with the cranium; one with a process which may have been the postorbital process at the back of the skull, and one beneath the eye socket.

The chondrocranium (or neurocranium) of Ornithoprion had a long, pointed snout and large, high-set eye sockets, which Zangerl compared to a bird's skull. An indentation set far forward on the snout is reported by Zangerl to have likely held the nasal capsule (skeletal support for the nostrils), although this region of the skull is poorly known. The brain was small and positioned along the lower surface of the neurocranium, but little else is known about the cranial nervous system. Processes on the back of the cranium that Zangerl speculated to be a fused hyoid arch are also known,, although the interpretation of these structures as part of a hyoid arch has subsequently been questioned by paleontologist Svend Erik Bendix-Almgreen. The largest Ornithoprion cranium measures approximately 10 cm (3.9 in) in length.

Teeth

The lower dentition of Ornithoprion consisted of multiple tooth crowns extending from a connected base (or root) along the midline of the lower jaw; an arrangement which resembled a saw and is referred to as a tooth whorl. Additional rows of tightly stacked, flattened teeth were also present, which were likely positioned along the lateral (side) surfaces of the jaws. The tooth whorl possessed up to seven broad, rounded, bulbous tooth crowns and was positioned near the point of contact between the Meckel's cartilages and the mandibular rostrum. The tooth crowns of the whorl varied in size, with the smallest teeth being situated at the front and the largest at the back. The flattened teeth elsewhere in the lower jaws were rod-like and had an uneven surface texture. They formed a brick-like arrangement termed a "tooth pavement" similar to that of many other Paleozoic cartilaginous fish.

Additional rows of pavement-forming teeth and larger, pointed V-shaped teeth formed the upper dentition of Ornithoprion. Zangerl, in both his initial description of the taxon and in later works, suggested that the upper teeth were associated directly with the underside of the cranium. Later works by Bendix-Almgreen and paleontologist Roger S. Miles have alternatively suggested that Ornithoprion's upper teeth instead attached to a previously unrecognized portion of the palatoquadrates that was fused to the cranium. The V-shaped teeth are thought to have formed a row along the midline of either the cranium or fused palatoquadrates.

Based on thin sectioning, the tooth crowns of Ornithoprion are thought to have been 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 (also called vitrodentin), but a thin layer may have been present in life.

Postcranial skeleton

Either five or six pairs of gill arches were present in Ornithoprion, and positioned behind the arches were the scapulocoracoids (pectoral girdles). The paired left and right scapulocoracoids were unfused and had forward-angled scapular portions, as well as an unpaired cartilaginous structure which has been tentatively identified as sternal cartilage running beneath their coracoidal portions. A similar intercoracoidal cartilage has also been identified in living broadnose sevengill sharks, as well as the extinct iniopterygians and the chimaeriform Ischyodus. The function of this structure in Ornithoprion is unknown, and it may have been homologous with similar, albeit paired cartilage structures known in other eugeneodonts. While the pectoral fins of Ornithoprion are not known, Bendix-Almgreen suggested that they were likely anatomically unique based on the shape of the animal's pectoral girdle. There is no indication that the fins supported defensive fin spines.

The vertebral centra of Ornithoprion are not preserved and were uncalcified in life, although a series of cartilaginous structures are present along the path of the vertebral column. Zangerl suggested that these elements, which he described as large and leaf-shaped, represented the neural arches of the animal's vertebrae, and he further proposed that their shape may have been an adaptation associated with the unique morphology of the animal's skull. The spinal cord of Ornithoprion was sheathed by a flexible notochord.

Dermal denticles

The known portion of Ornithoprion's body was completely covered in tiny, tooth-like dermal denticles with rounded crowns. These possessed a pulp cavity, were predominantly made up of orthodentin, and grew from proportionally large, flattened bases. Many of these denticles formed fused, compound structures termed polyodontode scales, which shared a single mushroom-shaped base with upwards of seven crowns emerging from it. Similar polyodontode scales occurred in the related Sarcoprion, and potentially also in Helicoprion. However, the bases of the denticles in Ornithoprion may have uniquely been composed of bone, rather than a form of dentin like in other cartilaginous fish. Extremely small denticles were also present in the mouth and throat of Ornithoprion, which also had crowns composed of orthodentin and bases of bone.

In his 1966 description, Zangerl speculates that the reinforcing bony rods present on the snout and mandibular rostrum were formed by the compounding and fusion of polyodontode scales. He likens this phenomenon to that proposed by Oscar Hertwig as an explanation for the origin of vertebrate dermal armor, although Zangerl acknowledges that this adaptation evolved independently in Ornithoprion.

Classification

Though sometimes referred to as a shark, Ornithoprion is only a distant relative of living sharks (Selachii). When first described, the genus was classified in the family Edestidae, which itself was traditionally considered a member of the order (sometimes class) Bradyodonti or the equivalent subclass Holocephali. In his description of the genus, however, Zangerl classified it and other edestids as elasmobranchs. In the 1971 edition of Paleozoic Fishes, Roger S. Miles considered the genus to be of uncertain position within Chondrichthyes, and only tentatively placed it within Holocephali. He suggested that Ornithoprion's similarities with edestids may be the result of convergent evolution (meaning they developed independently) because of differences in the anatomy of the gills, the tooth histology, and the palatoquadrates. In several works Bendix-Almgreen similarly expressed belief that the features used to unite the edestids may be convergent, and that the family was likely polyphyletic (not a natural group). His conclusions were supported primarily by the apparent presence or absence of enameloid between different edestid taxa, and differences in the features of their skulls. He considered Ornithoprion to be possibly related only to several genera traditionally classified as edestids from the Late Permian of Greenland, although only very distantly. In two 1968 works, both Bendix-Almgreen and paleontologist Colin Patterson considered the features of Ornithoprion inconsistent with a member of Holocephali. In contrast, paleontologist Richard Lund asserted in a 1977 paper that edestids (including Ornithoprion) were indeed primitive relatives of holocephalans, and were part of a larger group he termed Paraselachii.