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Ornithoprion
Temporal range: Pennsylvanian (Moscovian), 315.2–307 Ma
Skeletal reconstruction of Ornithoprion, with known material represented in white and implied/suggested material represented in gray
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: †Eugeneodontida
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, 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 literally 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 10 centimetres (3.9 in) and an estimated body length 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). This description was based on material collected primarily from the Mecca Quarry of Indiana, in rocks which are part of the Linton Formation.[1][2][3] A single specimen was also collected from the Logan Quarry in an exposure of the Staunton Formation, also in Indiana,[1][4] 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.[1] The Mecca and Logan Quarry material has been dated to the Moscovian (also called Desmoinesian) stage of the Pennsylvanian,[4][5][6] which is part of the Carboniferous period and which lasted from 315.2 to 307 million years ago.[7] 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.[1][3] Multiple additional specimens have subsequently been assigned to Ornithoprion, including occurrences from the Excello Shale of Indiana.[2][8][9]

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][12] 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.[1][11][13] 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.[13]

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.[1][12][14]: 70–93  The species name, O. hertwigi, honors German zoologist Oscar Hertwig.[1]

Description

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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][5] 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 modern cartilaginous fish.[15][16][17] 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][18] 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 his 2003 book Aquagenesis: The Origin and Evolution of Life in the Sea.[12]

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,[19] and had a forward-facing projection termed the mandibular rostrum.[1][2][13] While similar rostra are known in other eugeneodonts, the structure was significantly longer in Ornithoprion,[20] and both the rostrum and a correlating section of the snout were uniquely armored by rods of bone embedded in the skin.[9][21][22] In life, the mandibular rostrum was likely to have been cylindrical in cross section and spear-like.[1] 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][9][21]

The palatoquadrates, which typically form the upper jaws in living cartilaginous fish, were reduced, immobile,[1] and potentially fused partially with the cranium.[13][21][23] This reduced state is unique among cartilaginous fish,[24] and differs greatly from that seen in other eugeneodonts such as Helicoprion, in which the palatoquadrates were large and specialized,[16][25] and potentially Fadenia and Sarcoprion, which may have had them entirely fused to the cranium (termed holostyly) or completely lost.[2][9][20] 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][13][20] The palatoquadrate articulated at the back of the neurocranium in a greatly limited, modified autodiastylic (two jointed) manner.[2][13][26]

The neurocranium of O. hertwigi had a long, pointed snout and large, high-set eye sockets, which Zangerl likened to a bird's skull.[1] 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 ventral (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.[1][13] The largest O. hertwigi cranium measures approximately 10 cm (3.9 in) in length.[21]

Teeth

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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,[1][9][26] and tooth batteries (tightly stacked rows of crushing teeth) along the lateral sides of the whorl.[9][13] 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.[2] 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.[1][2] The crushing teeth were plate-like, flattened, rod-like, and possessed deep pits and grooves in their surface.[9][12] They formed a flattened "tooth pavement" in life similar to that of many other Paleozoic cartilaginous fish,[2][9] and the structure of these teeth was directly compared with those of the related Erikodus in Zangerl's 1966 description.[1]

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,[1][9] although it has alternatively been suggested they instead anchored to a fused, previously unrecognized portion of the palatoquadrate.[13] These V-shaped upper teeth are thought to have formed another midline tooth row similar to that proposed in Sarcoprion,[2][21] although their precise arrangement in life is not known.[1]

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)[13][15] with an outer coating of orthodentin.[1][9][21] There is no indication of enameloid (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 anatomical clarity

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.[1][13][21] Either five or six pairs of ceratobranchials (gill arches) were present behind the skull,[13][18][21] with what has been tentatively identified as a "sternal cartilage" running beneath them.[1][21][27] This unpaired intercoracoidal cartilage has also been identified in living broadnose sevengill sharks,[28] as well as the extinct iniopterygians, the Jurassic chimaeriform Ischyodus,[27] and potentially the closely related Fadenia.[18] The function of this structure in Ornithoprion is unknown, although it is likely homologous 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.[13] There is no indication the fins possessed defensive fin spines.[2][13][21]

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][13] The spinal cord of Ornithoprion was sheathed by a soft, flexible notochord in life.[21]

Dermal denticles

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Unlike living chimaeras, in which dermal denticles (also called placoid scales) are only present in isolated regions,[15][29] the known body of Ornithoprion was completely covered in tiny, rounded,[30] tooth-like denticles referred to as lepidomoria.[1][13] 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,[1][21][22] 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.[1][2] Similar polyodontode scales are known to occur in the related Sarcoprion and potentially Helicoprion.[30] Extremely small denticles were also present in the mouth and throat, which were exclusively composed of orthodentin.[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.[13][15][30] In his description of O. hertwigi, however, Zangerl suggested that it and other edestids were more likely elasmobranchs.[1][31] 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.[21] 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][14]: 108–109  His conclusions were supported primarily by the apparent presence or absence of enameloid or vitrodentin between different edestid taxa,[2][32] and differences in the features of their skulls.[33] 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.[13][33] 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.[13]

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.[2] While Zangerl's classification of eugeneodonts as elasmobranchs has been refuted by later publications,[16][25][26] his erected suborders and families within the group remain in use.[9][18][29] Eugeneodontiformes is regarded as a monophyletic group in the subclass Holocephali (sometimes defined as the more broadly-encompassing Euchondrocephali),[9][29][34] 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, all of which are highly specialized deep-water fish and are not closely comparable to eugeneodonts in anatomy or lifestyle.[14] In the absence of living analogues, the higher level interrelationships between extinct members of the subclass remain enigmatic.[35]

The skull and vertebral morphology of Ornithoprion hertwigi is very different from that of other known eugeneodonts,[2][18] and key elements of the postcranial morphology are unknown.[1] 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.[2][9][29] 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.[2]

Paleoecology and paleobiology

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Habitat and ecological niche

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The Mecca, Logan, and Excello shales all represented marine depositional environments, and all preserve a diverse assemblage of species.[10][36][37] 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.[11][38] 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.[10][39][40] 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.[38] The presence of larger fish and cephalopods at the Logan Quarry site may suggest somewhat deeper waters.[10][38] 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,[41][42] 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.[38][43]

The holotype of Orodus greggi, discovered at the 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,[38] palaeoniscoids, brachiopods, orthocones, and chondrichthyans such as Orodus, Denaea, Cobelodus, Symmorium, and several members of Iniopterygiformes.[36][10] The Logan Quarry was inhabited by, in addition to many chondrichthyans, an unnamed chondrost-like actinopterygian (ray-finned fish) with a similar elongated rostrum.[4][44] 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.[37]

Evidence of predation

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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 has been suggested that the unpreserved rear halves of the animals may have been severed by predation attempts.[1][45] 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 proposed feeding methods

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

Similar to many of its close relatives,[20][46][47] 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,[1][23] and the remains of brachiopod shells are known from the stomach of the related Fadenia.[47] 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][12][47] 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 dislodge brachiopods.[21][47] There is no indication that the mandibular rostrum contained sensory organs.[4] 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.[1]

Use in reconstructing Helicoprion

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Because of its phylogenetic proximity, Ornithoprion has been used as a basis for restoring the anatomy and physiology of larger eugeneodonts that are known only from fossilized teeth or jaws. 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.[14]: 144, 151 [16] Both murals and a life-sized model of H. davidsii, designed by Troll and displayed at the Idaho Museum of Natural History,[48] directly reference features of O. hertwigi such as gill anatomy.[14]: 151 [16] Edestid researcher Svend Erik Bendix-Almgreen had, however, criticized the use of caseodonts as Troll's basis for reconstructing Helicoprion, as he believed they did not represent close phylogenetic or ecological analogues.[14]: 108, 109 

See also

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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. Retrieved 14 March 2020.
  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 y Zangerl, Rainer (1981). Chondrichthyes 1: Paleozoic Elasmobranchii (Handbook of Paleoichthyology). Friedrich Pfell (published January 1, 1981). pp. 2–3, 74–94. ISBN 978-3899370454.
  3. ^ a b Bruner, John Clay.; Field Museum of Natural History. (1992). A catalogue of type specimens of fossil fishes in the Field Museum of Natural History / John Clay Bruner --. Chicago, Ill.: Field Museum of Natural History. p. 27. doi:10.5962/bhl.title.3361.
  4. ^ a b c d e 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 "PBDB Taxon". The Paleobiology Database.
  6. ^ "International Commission on Stratigraphy Subcommission on Carboniferous Stratigraphy". carboniferous.stratigraphy.org. Retrieved 2024-04-23.
  7. ^ "Ornithoprion hertwigi". Mindat.org. Retrieved 23 April 2024.
  8. ^ "Ornithoprion Zangerl, 1966". www.gbif.org. Retrieved 2024-04-25.
  9. ^ a b c d e f g h i j k l m Ginter, Michał; Hampe, Oliver; Duffin, Christopher J. (2010). Handbook of paleoichthyology: teeth. München: F. Pfeil. pp. 117–120. ISBN 978-3-89937-116-1.
  10. ^ a b c d e Zangerl, Rainer; Richardson, Eugene S. (1963). The paleoecological history of two Pennsylvanian black shales. Fieldiana. Vol. 4. Chicago: Chicago Natural History Museum. doi:10.5962/bhl.title.7199.
  11. ^ a b c 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.
  12. ^ a b c d e 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.
  13. ^ a b c d e f g h i j k l m n o p q r s Orvig, Tor; Nobel Symposium, eds. (1968). Current problems of lower vertebrote phylogeny: proceedings of the 4. Nobel Symposium held in June 1967 at the Swedish Museum of Natural History (Naturhistoriska riksmuseet) in Stockholm. New York: Interscience Publ. [u.a.] pp. 153–205. ISBN 978-0-471-65713-2.
  14. ^ a b c d e f Ewing, Susan (2017). Resurrecting the shark: a scientific obsession and the mavericks who solved the mystery of a 270-million-year-old fossil (Ebook ed.). New York: Pegasus Books. ISBN 978-1-68177-343-8. OCLC 951925606.
  15. ^ a b c d Patterson, C. (1965-06-10). "The phylogeny of the chimaeroids". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 249 (757): 101–219. Bibcode:1965RSPTB.249..101P. doi:10.1098/rstb.1965.0010. ISSN 2054-0280.
  16. ^ a b c d e Tapanila, Leif; Pruitt, Jesse; Pradel, Alan; Wilga, Cheryl D.; Ramsay, Jason B.; Schlader, Robert; Didier, Dominique A. (2013-04-23). "Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion". Biology Letters. 9 (2). doi:10.1098/rsbl.2013.0057. ISSN 1744-9561. PMC 3639784. PMID 23445952.
  17. ^ Pears, Jacob B.; Johanson, Zerina; Trinajstic, Kate; Dean, Mason N.; Boisvert, Catherine A. (2020-11-26). "Mineralization of the Callorhinchus Vertebral Column (Holocephali; Chondrichthyes)". Frontiers in Genetics. 11. doi:10.3389/fgene.2020.571694. ISSN 1664-8021. PMC 7732695. PMID 33329708.
  18. ^ a b c d e Mutter, Raoul J.; Neuman, Andrew G. (2008-01-01). "New eugeneodontid sharks from the Lower Triassic Sulphur Mountain Formation of Western Canada". Geological Society, London, Special Publications. 295 (1): 9–41. Bibcode:2008GSLSP.295....9M. doi:10.1144/SP295.3. ISSN 0305-8719.
  19. ^ a b Jobbins, Melina; Rücklin, Martin; Sánchez Villagra, Marcelo R.; Lelièvre, Hervé; Grogan, Eileen; Szrek, Piotr; Klug, Christian (2024-01-31). "Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution". Royal Society Open Science. 11 (1): 231747. doi:10.1098/rsos.231747. PMC 10827443. PMID 38298398.
  20. ^ a b c d Mutter, Raoul J.; Neuman, Andrew G. (2008-06-10). "Jaws and dentition in an Early Triassic, 3-dimensionally preserved eugeneodontid skull (Chondrichthyes)". Acta Geologica Polonica. 58 (2): 223–227.
  21. ^ a b c d e f g h i j k l m n Moy-Thomas, J. A. (1971), "Subclass Chondrichthyes. Infraclass Holocephali", Palaeozoic Fishes, Boston, MA: Springer US, pp. 226–245, doi:10.1007/978-1-4684-6465-8_10, ISBN 978-1-4684-6467-2, retrieved 2024-07-05
  22. ^ a b HALL, BRIAN K. (1975). "Evolutionary Consequences of Skeletal Differentiation". American Zoologist. 15 (2): 340. doi:10.1093/icb/15.2.329. ISSN 0003-1569.
  23. ^ a b Dearden, Richard P.; Herrel, Anthony; Pradel, Alan (2023-01-24). "Evidence for high-performance suction feeding in the Pennsylvanian stem-group holocephalan Iniopera". Proceedings of the National Academy of Sciences. 120 (4): e2207854119. doi:10.1073/pnas.2207854119. PMC 9942859. PMID 36649436.
  24. ^ Grogan, Eileen D.; Lund, Richard; Didier, Dominique (1999). "Description of the chimaerid jaw and its phylogenetic origins". Journal of Morphology. 239 (1): 45–59. doi:10.1002/(SICI)1097-4687(199901)239:1<45::AID-JMOR3>3.0.CO;2-S. ISSN 0362-2525.
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