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Ornithoprion
Temporal range: Pennsylvanian, Moscovian
Skeletal reconstruction of Ornithoprion, with known material represented in white and implied/suggested material represented in gray
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: †Eugeneodontiformes
Family: †Caseodontidae
Genus: †Ornithoprion
Zangerl, 1966
Type species
†Ornithoprion hertwigi
Zangerl, 1966

Ornithoprion is an extinct genus of cartilaginous fish. The only species, O. hertwigi, lived during the Moscovian stage of the Pennsylvanian, which spanned from 315 to 307 million years ago. Its fossils are 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 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 species name honors German zoologist Oscar Hertwig.

Ornithoprion is unique among known eugeneodonts for the extremely long mandibular rostrum extending from the lower jaw, which was covered 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 hunted hard-shelled 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 10 centimetres (3.9 in) and an estimated body length of up to approximately 91 centimetres (36 in).

Discovery and naming

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Stratigraphy of the Illinois Basin, where O. hertwigi's fossils have been found

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).[1]: 1  This description was based on material collected primarily from the Mecca Quarry of Parke County, Indiana, in rocks which are part of the Linton Formation.[1]: 10–11 [2][3] A single specimen was also described from the Logan Quarry in an exposure of the Staunton Formation, also in Parke County,[1]: 10–11 [4][note 1] and another from near Wilmington, Illinois that is part of a private collection. The specimens are all preserved in carbonaceous black shale, and the preservation mode of the Illinois specimen has been described as pyritic.[1]: 1-11  The Mecca and Logan Quarry material has been dated to the Moscovian stage[note 2] of the Pennsylvanian subperiod,[7][8] which itself is part of the Carboniferous period and which spanned from 315 to 307 million years ago.[6][9] The precise age and locality of the Illinois specimen is unknown. Nine specimens of Ornithoprion were initially described,[1]: 11  with specimen FMNH PF-2710 from the Mecca Quarry being designated as the holotype (defining specimen of the taxon).[3] Additional material has subsequently been assigned to the genus, including occurrences from the Excello Shale of Indiana.[2][7]

Like many other fish fossils from the Mecca and Logan quarries,[4][10][11] 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.[1]: 1-6 [5] 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.[1]: 3  At the time of its discovery Ornithoprion was one of the few members of its order known from postcranial fossils, alongside Fadenia, Erikodus, and what would later be described as Eugeneodus.[11][12][note 3] It also represented one of only a small number of holocephalans from the Paleozoic in which the endoskeleton was known, and alongside the related Fadenia was the only one known to preserve the gills.[12]

The genus name, Ornithoprion, translates literally from Greek 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.[5] The species name, O. hertwigi, honors German zoologist Oscar Hertwig.[1]: 10 

Description

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Life reconstruction of O. hertwigi, with body proportions based on related caseodontids and a hypothetical striped patterning

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.[1][3] The preserved portion of the skeleton was composed of cartilage reinforced by an outer coating of mineralized tesserae;[1] hexagonal structures which strengthen the cartilage and are also present in the skeletons of other cartilaginous fish.[13][14][15] 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.[2][16] Zangerl described O. hertwigi as "very small" in his description of the taxon,[1] 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 a 2003 book.[5]

Skull

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Restored skull of Ornithoprion hertwigi scaled to the largest figured specimen. Thin lines along the rostra represent rods of armor
Restored skull of the distantly related Helicoprion davidsii, displaying well-developed palatoquadrates and a lack of a mandibular rostrum

The lower jaw of Ornithoprion was roughly 1.3 times the length of the rest of the skull,[8] and had a forward-facing projection termed the mandibular rostrum.[1][2][12] While similar rostra are known in other eugeneodonts, the structure was significantly longer in Ornithoprion,[17] and both the rostrum and a correlating section of the snout were uniquely armored by rods of bone embedded in the skin.[7][18][19] In life, the mandibular rostrum was likely to have been cylindrical in cross section and spear-like, with the exception of the rear portion which had a bulge that supported the teeth.[1]: 18-19  There is no indication that the mandibular rostrum contained sensory organs.[4] 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.[2][7][18]

The palatoquadrates, which typically form the upper jaws in living cartilaginous fish, were reduced, immobile,[1] and potentially fused partially with the cranium.[12][18][20] This reduced state is unique among cartilaginous fish,[21] and differs greatly from that seen in other eugeneodonts such as Helicoprion, in which the palatoquadrates were large and specialized,[14][22] and potentially Fadenia and Sarcoprion, which may have had them entirely fused to the cranium (termed holostyly) or completely lost.[2][7][17] 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.[2][12][17] The palatoquadrate articulated at the back of the neurocranium in a greatly limited, modified two jointed manner.[2][12][23]

The neurocranium of O. hertwigi had a long, pointed snout and large, high-set eye sockets,[18] 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 noted to be poorly known.[1]: 12  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.[12] Processes on the back of the cranium that Zangerl speculated to be a fused hyoid arch are also known,[1]: 12, 16 , although the interpretation of these structures as part of a hyoid arch has subsequntly been questioned.[12] The largest O. hertwigi cranium measures approximately 10 cm (3.9 in) in length.[18]

Teeth

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The lower dentition of O. hertwigi consisted of multiple large tooth crowns extending from a connected base (or root) along the midline of the lower jaw in an arrangement which resembled a saw, and additional rows of tightly stacked crushing teeth on either side of the midline teeth.[1][5][7] The midline tooth row 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 midline teeth varied in size, with the smallest teeth being situated at the front and the largest at the back.[1]: 24  The crushing teeth were flattened, rod-like, and possessed deep pits and grooves in their surface.[7][5] They formed a "tooth pavement" in life similar to that of many other Paleozoic cartilaginous fish,[2][7] and the structure of these teeth was directly compared with those of the related Erikodus in Zangerl's 1966 description.[1]: 33 

Additional rows of crushing teeth and larger, pointed V-shaped teeth formed the upper dentition of Ornithoprion. 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,[1][2][7] although it has alternatively been suggested they instead attached to a previously unrecognized portion of the palatoquadrate that was fused to the cranium.[12][18] The V-shaped teeth are thought to have formed a row along the midline of the upper jaw.[1]: 22-23 [18]

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)[12][13] with an outer coating of orthodentin.[1][7][18] There is no indication of enameloid (also called vitrodentin), but a small layer may have been present in life.[1][2]

Postcranial skeleton

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Illustrated diagram of the holotype, with damaged or poorly defined anatomy represented by dashed lines. Disarticulated teeth and denticles have been omitted for clarity

The left and right scapulocoradoids (pectoral girdles) of Ornithoprion were unfused and had a forward-angled scapular portion.[1]: 21 [12][18] Either five or six pairs of ceratobranchials (gill arches) were present behind the skull,[12][16][18] with what has been tentatively identified as a "sternal cartilage" running beneath them.[1]: 21-22 [18][24] This unpaired intercoracoidal cartilage has also been identified in living broadnose sevengill sharks,[25] as well as the extinct iniopterygians, the Jurassic chimaeriform Ischyodus,[24] and potentially the closely related Fadenia.[16] The function of this structure in Ornithoprion is unknown, although it is likely equivalent to similar, paired cartilage structures observed in other extinct chondrichthyans.[2] 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.[12] There is no indication the fins possessed defensive fin spines.[2][12][18]

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.[1][2][12] The spinal cord of Ornithoprion was sheathed by a soft, flexible notochord in life.[18]

Dermal denticles

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Unlike living chimaeras, in which dermal denticles (also called placoid scales) are only present in isolated regions,[13][26] the known body of Ornithoprion was completely covered in tiny, rounded,[27] tooth-like denticles referred to as lepidomoria.[1][12] 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 like in other cartilaginous fish.[1][18][19] Many denticles form fused, compound structures termed "polyodontode scales", share a single mushroom-shaped base with multiple crowns and pulp cavities emerging from it, and may have more than seven crowns.[1][2] Similar polyodontode scales are known to occur in the related Sarcoprion and potentially Helicoprion.[27] Extremely small denticles were also present in the mouth and throat.[1]

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.[1]

Classification

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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.[12][27][28] In his description of O. hertwigi, however, Zangerl suggested that it and other edestids were more likely elasmobranchs.[1] 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 the subclass 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.[18] 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).[2][29][30]: 108–109  His conclusions were supported primarily by the apparent presence or absence of enameloid or vitrodentin between different edestid taxa,[2][31] and differences in the features of their skulls.[29] 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.[12][29] In two 1968 publications, both Bendix-Almgreen and paleontologist Colin Patterson also considered the features of Ornithoprion inconsistent with classification as a relative of chimaeras.[12][28]

Eugeneodontida (=Eugeneodontiformes)
Cladogram modified from figure 102 of Zangerl (1981)[2]

In a 1981 publication, 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. 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 and Erikodus.[2][7][26] In this publication he again classified the eugeneodonts as members of Elasmobranchii rather than the traditionally assumed Holocephali or Bradyodonti.[2] While Zangerl's classification of eugeneodonts as elasmobranchs has been refuted by later publications,[14][22][23] his erected suborders and families within the group remain in use.[7][16][26] Eugeneodontiformes is regarded as a monophyletic group in the subclass Holocephali (sometimes defined as the more broadly-encompassing Euchondrocephali),[7][26][32][note 4] 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.[2] The only extant members of Holocephali are three families of chimaeras,[26] all of which are highly specialized deep-water fish and are not closely comparable to eugeneodonts in anatomy or lifestyle.[30]: 40 

Paleoecology and paleobiology

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Paleoenvironment

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The Mecca, Logan, and Excello shales all represented marine depositional environments, and preserve a diverse assemblage of species.[34][35][36] 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.[10][11][37] 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 shale which encases the fossils indicates large amounts of decaying organic material such as algae were present, which led to anoxic (without oxygen) 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.[10][38][39] 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.[37] The presence of larger fish and cephalopods at the Logan Quarry site may suggest somewhat deeper waters.[10][37] 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,[40][41] 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 depositional environment.[37][42]

The holotype of Orodus greggi, discovered in Logan Quarry,[2] on display at the Field Museum of Natural History

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.[1] The Mecca fauna, which includes both the Mecca and Logan Quarry sites, also preserves an assemblage of conodonts, palaeoniscoids, brachiopods, orthocones,[10][37] larger chondrichthyans such as Edestus, Orodus, Cobelodus, Symmorium, and Stethacanthus, and several members of Iniopterygiformes.[2][34][35] The Logan Quarry was inhabited by, in addition to chondrichthyans and palaeoniscoids, an unnamed chondrost-like actinopterygian (ray-finned fish) with an elongated rostrum somewhat similar to that of Ornithoprion.[4][43] Invertebrates such as brachiopods and ammonoids are known from the Excello Shale, as are a wide variety of cartilaginous fish which were roughly contemporaneous with Ornithoprion.[36]

Predators and scavengers

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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 suggested that the unpreserved rear halves of the animals may have been severed by predation attempts.[1][35] 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.[1]

Diet and feeding

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The skull of the Cretaceous actinopterygian Saurodon, which Rainer Zangerl considers the closest morphological analog for the rostrum of Ornithoprion[1]

Similar to many of its close relatives,[17][44][45] 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 prey.[1][18][20] 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.[1][5][18] He notes that this possible feeding mode is entirely speculative,[1] although later works agree with the conclusion that the rostra of caseodonts could have been used to probe sediment or dislodge shelled invertebrates such as brachiopods.[18][45] 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 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 unknown.[1]

See also

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Notes

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  1. ^ Richard Ellis mistakenly suggests that three Ornithoprion skulls have been described from Logan Quarry[5]
  2. ^ Zangerl used the Westphalian European regional stage to refer to the Mecca and Logan Quarry remains, and identified the Illinois specimen as being from the Desmoinesian North American regional stage.[1]: 10-11  Both of these are contemporaneous with the Moscovian[6]
  3. ^ Prior to its description in 1981,[2] Eugeneodus was referred to as "Agassizodus sp."[11]
  4. ^ A 2009 phylogenetic analysis by Richard Lund and Eileen Grogan recovered Ornithoprion as an early-diverging elasmobranch, but the authors attribute this result to the limited data available for the taxon[33]

References

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  1. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao Zangerl, Rainer (17 March 1966). "A new shark of the family Edestidae, Ornithoprion hertwigi, from the Pennsylvanian Mecca and Logan quarry shales of Indiana". Fieldiana Geology. 16. Chicago Field Museum of Natural History: 1–42. doi:10.5962/bhl.title.5302.
  2. ^ a b c d e f g h i j k l m n o p q r s t u v w x Zangerl, Rainer (1981). Chondrichthyes 1: Paleozoic Elasmobranchii (Handbook of Paleoichthyology). Friedrich Pfell (published January 1, 1981). pp. 2–3, 79–92. ISBN 978-3-89937-045-4.
  3. ^ a b c Bruner, John Clay (1992). "A catalogue of type specimens of fossil fishes in the Field Museum of Natural History". Fieldiana Geology. 23. Chicago: Field Museum of Natural History: 27. doi:10.5962/bhl.title.3361.
  4. ^ a b c d Poplin, Cécile M. (1978). "An actinopterygian with a long rostrum from the Pennsylvanian of Logan Quarry, Indiana". Journal of Paleontology. 52 (3): 524–531. ISSN 0022-3360. JSTOR 1303953.
  5. ^ a b c d e f g Ellis, Richard (2003). Aquagenesis: the origin and evolution of life in the sea. New York, N.Y: Penguin. pp. 118–121. ISBN 978-0-14-200156-1.
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  11. ^ a b c d Zangerl, Rainer (1995). "The problem of vast numbers of cladodont shark denticles in the Pennsylvanian Excello Shale of Pike County, Indiana". Journal of Paleontology. 69 (3): 556–563. Bibcode:1995JPal...69..556Z. doi:10.1017/S0022336000034922. ISSN 0022-3360.
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  22. ^ a b Ramsay, Jason B.; Wilga, Cheryl D.; Tapanila, Leif; Pruitt, Jesse; Pradel, Alan; Schlader, Robert; Didier, Dominique A. (2014-01-18). "Eating with a saw for a jaw: Functional morphology of the jaws and tooth-whorl in Helicoprion davisii". Journal of Morphology. 276 (1): 47–64. doi:10.1002/jmor.20319. ISSN 0362-2525. PMID 25181366.
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  24. ^ a b Pradel, Alan; Tafforeau, Paul; Janvier, Philippe (2010-03-04). "Study of the pectoral girdle and fins of the Late Carboniferous sibyrhynchid iniopterygians (Vertebrata, Chondrichthyes, Iniopterygia) from Kansas and Oklahoma (USA) by means of microtomography, with comments on iniopterygian relationships". Comptes Rendus Palevol. 9 (6–7): 377–387. Bibcode:2010CRPal...9..377P. doi:10.1016/j.crpv.2010.07.015. ISSN 1631-0683.
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