Ornithoprion

''Ornithoprion'' is an extinct genus of cartilaginous fish. The only species, O. ''hertwigi'', lived during the Moscovian stage of the Pennsylvanian, between 315.2 and 307 million years ago, and is preserved in black shales from what is now 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 Paleozoic holocephalans. The classification of the genus has been the subject of debate due to its unique anatomy, but it is now placed in the order Eugeneodontiformes and the family Caseodontidae. ''Ornithoprion's'' genus name, which may be translated 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 species name honors Oscar Hertwig.

''Ornithoprion'' is unique among known eugeneodonts for the extremely long mandibular rostrum extending from the lower jaw, which was protected by a beak of fused bony scales and which the function of in life 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 shelled marine invertebrates, and bite marks and damage to its fossils indicate it was fed on by other carnivores. ''Ornithoprion'' was small relative to other members of its order, with a cranium length of up to and an estimated body length of up to approximately .

Discovery and naming

''Ornithoprion hertwigi'' was named and described in 1966 by paleontologist 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, and the preservation mode of the Illinois specimen has been described as pyritic. The Mecca and Logan Quarry material has been dated to the Moscovian (also called 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 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 fish fossils from the Mecca and Logan quarries, the studies 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'' represented one of the best preserved members of its family, and one of the few known from postcranial fossils alongside ''Fadenia'', '' Erikodus'', and what would later be described as '' Eugeneodus''. It also represented one of only a small number of then-known Paleozoic holocephalans known from endoskeletal fossils, and alongside the related ''Fadenia'' was the only one known to preserve the gills and hyomandibula.

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

Description

''Ornithoprion'' 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 flattened. The preserved portion of the skeleton was composed of cartilage reinforced by an outer coating of mineralized tesserae; hexagonal structures which strengthen the cartilage and are also present in the skeletons of modern cartilaginous fish. While the postcranial anatomy of ''Ornithoprion'' is incompletely known, other members of the family Caseodontidae are characterized by a streamlined body, a homocercal (crescent-shaped) caudal fin, and reduced or absent pelvic fins. Zangerl described ''O. hertwigi'' as "very small" in his description of the taxon, and author Richard Ellis suggested a total length of 90 cm (3 ft) based on an assumed skull length of less than 15 cm (6 in) in his 2003 book ''Aquagenesis: The Origin and Evolution of Life in the Sea.''

Skull

The lower jaw of ''Ornithoprion'' was roughly 1.3 times the length of the rest of the skull, and had a forward-facing projection termed the mandibular rostrum. While similar rostra are known in other eugeneodonts, 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 and spear-like. The rostrum articulated with the Meckel's cartilages (equivalent to the mandible), 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 thin, flattened cartilages which articulated with the palatoquadrates.

The palatoquadrates, which typically form the upper jaws in living cartilaginous fish, were reduced, immobile, and potentially fused partially with the cranium. This reduced state is unique among cartilaginous fish, and differs greatly from that seen in other eugeneodonts such as ''Helicoprion,'' in which the palatoquadrates were large and specialized, and potentially ''Fadenia'' and ''Sarcoprion'', which may have had them entirely fused to the cranium (termed holostyly) or completely lost. The condition in ''O. hertwigi'' most closely resembled that of other caseodonts such as '' Caseodus'' and ''Eugeneodus,'' although the degree of reduction is much greater in ''Ornithoprion''. The palatoquadrate articulated at the back of the neurocranium in a greatly limited, modified two jointed manner.

The neurocranium of ''O. hertwigi'' had a long, pointed snout and large, high-set eye sockets, which Zangerl likened to a bird's skull. 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 known. The brain was very small and was 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. The largest ''O. hertwigi'' cranium measures approximately 10 cm (3.9 in) in length.

Teeth

The lower dentition of ''O. hertwigi'' consisted of both multiple large tooth crowns extending from a connected base (or root) known as a tooth whorl, and tooth batteries (tightly stacked rows of crushing teeth) along the lateral sides of the whorl. The tooth whorl possessed up to seven broad, rounded, bulbous tooth crowns and was positioned along the midline of the jaw 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 of the whorl and the largest at the back. The crushing teeth were plate-like, flattened, rod-like, and possessed deep pits and grooves in their surface. They formed a flattened "tooth pavement" in life similar to that of many other Paleozoic cartilaginous fish, and the structure of these teeth was directly compared with those of the related '' Erikodus'' in Zangerl's 1966 description''.''

Both another battery of pavement teeth and larger, pointed V-shaped teeth formed the upper dentition. Zangerl, both in the taxon's initial description and in later works, suggested that these teeth were attached directly to the underside of the cranium, although it has alternatively been suggested they instead anchored to a fused, previously unrecognized portion of the palatoquadrate. These V-shaped upper teeth are thought to have formed another midline tooth row similar to that proposed in ''Sarcoprion'', although their precise arrangement in life is not known.

Based on thin sectioning, the teeth 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 (vitrodentin), but a small layer may have been present in life.

Postcranial skeleton

The known postcrania of ''Ornithoprion'' encompasses the frontmost portion of the skeleton. The left and right scapulocoradoids (pectoral girdle) were unfused and had a forward-angled scapular portion, a condition which differs from that in living chondrichthyans. Either five or six pairs of ceratobranchials (gill arches) were present behind the skull, with what has been tentatively identified as a " 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. While the pectoral fins of ''Ornithoprion'' are not known, paleontologist Svend Erik Bendix-Almgreen has suggested that they were likely greatly fused and different in morphology than those of other eugeneodonts based on the shape of the pectoral girdle. There is no indication the fins possessed defensive fin spines.

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, which may have been an adaptation associated with the morphology or function of the animal's pointed skull and rostrum. The spinal cord of ''Ornithoprion'' was sheathed by a soft, flexible notochord in life.

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 referred to as lepidomoria. These 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 as in other holocephalans, and many denticles form fused, compound structures. These compound denticles, termed " polyodontode scales", share a single mushroom-shaped base with multiple crowns and pulp cavities emerging from it, and in ''O. hertwigi'' may have more than seven crowns. Similar polyodontode scales are known to occur in the related ''Sarcoprion'' and potentially ''Helicoprion.'' Extremely small denticles were also present in the mouth and throat, which were exclusively composed of orthodentin.

In his 1966 description, Zangerl speculates that the reinforcing "beak" of bony rods present on the snout and 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 referred to as a shark in both formal and informal texts, ''Ornithoprion'' is only a distant relative of living sharks (Selachii). When first described, ''Ornithoprion'' was placed as a member of the family Edestidae, which was traditionally considered a member of the order (sometimes class) Bradyodonti. In his description of ''O. hertwigi'', however, Zangerl suggested that it and other edestids were more likely elasmobranchs. 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 ''Ornithoprion'''s similarities with edestids may be the result of convergent evolution (meaning they developed independently) due to differences in the anatomy of the gills, the tooth histology, and the palatoquadrates. Svend Erik Bendix-Almgreen similarly expressed belief that the features used to unite the edestids may be convergent, and that the group was likely polyphyletic (not a natural group). His conclusions were supported primarily by the apparent presence or absence of enameloid or vitrodentin 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 publications, both Bendix-Almgreen and paleontologist Colin Patterson also considered the features of ''Ornithoprion'' inconsistent with classification as a bradyodont, which at that point was also doubted to be a natural group.

In 1981, Zangerl considered ''O. hertwigi'' a member of the new family Caseodontidae, as part of the larger superfamily Caseodontoidea and the newly established order Eugeneodontiformes (then Eugeneodontida), in light of the numerous new taxa and characteristics that had been observed since ''Ornithoprion's'' original description. In this publication he again classified the eugeneodonts as members of Elasmobranchii rather than the traditionally assumed Holocephali or Bradyodonti. While Zangerl's classification of eugeneodonts as elasmobranchs has been refuted by later publications, his erected suborders and families within the group remain in use. Eugeneodontiformes is regarded as a monophyletic group in the subclass Holocephali (sometimes defined as the more broadly-encompassing Euchondrocephali), and discrepancies in tooth histology previously used to argue against their close relation has been alternatively explained by different members' rates of tooth replacement or wear. The only extant members of Holocephali 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.

The skull and vertebral morphology of ''Ornithoprion hertwigi'' is very different from that of other known eugeneodonts, and key elements of the postcranial morphology are unknown. ''Ornithoprion'''s classification within the Caseodontidae is based on the bulbous, rounded nature of its tooth crowns and the reduction of its palatoquadrates, features which are also found in 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 the Late Permian ''Erikodus'' and ''Fadenia''.

Paleoecology and paleobiology

Habitat and ecological niche

The Mecca, Logan, and Excello shales all represented marine depositional environments, and all preserve a diverse assemblage of species. In a 1963 publication, researchers Rainer Zangerl and Eugene S. Richardson Jr. 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 overlaid drowned forests and are a transgression of a marine environment over a terrestrial one (known as a cyclothem). The rich, black, fissile shale which encases the fossils indicates large 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.

Slabs of shale containing ''Ornithoprion'' fossils sometimes also preserve the remains of other animals, 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 chondrichthyans such as ''Orodus'', '' Denaea'', ''Cobelodus'', ''Symmorium'', and several members of Iniopterygiformes. The Logan Quarry was inhabited by, in addition to many chondrichthyans, an unnamed chondrost-like actinopterygian (ray-finned fish) with a similar elongated rostrum. 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'', 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. The skulls of several ''Ornithoprion'' specimens also display small crushed or missing chunks, which Zangerl proposed to have resulted from other fishes biting them and fracturing the cartilage.

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 tooth whorl and the immobile upper jaws were likely adaptations for crushing shelled invertebrates, 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 uncertain. In 1966, Zangerl proposed that the structure may have been used to disturb or probe sediment while hunting for prey living on or in the seabed, and potentially to fling dislodged prey into the water column. He notes that this possible feeding mode is entirely speculative, although later works agree with the conclusion that the rostra of caseodonts could have been used to dislodge brachiopods. There is no indication that the mandibular rostrum contained sensory organs. Some features of the animal's skull, such as the armor and articulation of the upper and lower jaws, were suggested by Zangerl to be shock-absorbing adaptations, although he considered it unlikely that the rostrum was used as a weapon. The mandibular rostrum of ''Ornithoprion'' was compared with those of the unrelated extinct bony fish ''Saurodon'' and ''Saurocephalus'', in which the function is also not known.

See also

*''Alienacanthus,'' an unrelated arthrodire with a similarly elongated mandibular rostrum
*List of prehistoric cartilaginous fish genera

Notes

References

External links

Original description published in Fieldiana Geology
provided by the Biodiversity Heritage Library
{{Portal bar, paleontology, fish, sharks

Carboniferous fish of North America
Carboniferous cartilaginous fish
Monotypic prehistoric cartilaginous fish genera
Fossil taxa described in 1966
Pennsylvanian fish of North America
Moscovian life
Carboniferous extinctions
Eugeneodontida
Edestidae
Caseodontidae
Pennsylvanian extinctions
Fossils of the United States