Iniopteryx

Iniopteryx was described in 1973 by paleontologists Rainer Zangerl and Gerard Case as the type species of the family Iniopterygidae within the order Iniopterygiformes (then termed Iniopterygia). The genus is based on multiple well-preserved and articulated fossils from the Stark Shale and Wea Shale of Nebraska, and the Excello Shale and Logan Quarry Shale of Indiana. It is known from the Pennsylvanian sub-period of the Carboniferous period.{{Cite journal |last=Grogan |first=Eileen D. |last2=Lund |first2=Richard |date=2009 |title=Two new iniopterygians (Chondrichthyes) from the Mississippian (Serpukhovian) Bear Gulch Limestone of Montana with evidence of a new form of chondrichthyan neurocranium |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1463-6395.2008.00371.x |journal=Acta Zoologica |language=en |volume=90 |issue=s1 |pages=134–151 |doi=10.1111/j.1463-6395.2008.00371.x |issn=0001-7272}} The holotype of I. rushlaui, FMNH PF6678, is an articulated, nearly complete skeleton. The holotype of I. tedwhitei, FMNH PF7241, is also an articulated specimen, although the rear portion of the body is fragmentary.{{Cite book |last=Zangerl |first=Rainer |url=http://archive.org/details/iniopterygianewo06zang |title=Iniopterygia : a new order of Chondrichthyan fishes from the Pennsylvanian of North America |last2=Case |first2=Gerard Ramon |date=1973 |publisher=Chicago : Field Museum of Natural History |others=University of Illinois Urbana-Champaign |language=en}}{{Cite book |last=Bruner |first=John Clay |url=https://www.biodiversitylibrary.org/bibliography/3361 |title=A catalogue of type specimens of fossil fishes in the Field Museum of Natural History |last2=Bruner |first2=John Clay |last3=History |first3=Field Museum of Natural |date=1992 |publisher=Field Museum of Natural History |location=Chicago, Ill |doi=10.5962/bhl.title.3361}} Fossils of Iniopteryx rushlaui are noted to be significantly more common that I. tedwhitei. Many specimens of I. rushlaui were studied using radiographic imaging, although it was not possible to study specimens from locales in Nebraska using radiography due to the composition of the rock matrix.

The genus name, Iniopteryx, is derived from from the Greek roots iníon, meaning "nape", and pteryx, meaning "fin". This refers to the fish's enlarged, dorsally placed pectoral fins. I. rushlaui is named in honor of William Rushlau, while I. tedwhitei is named in honor of W. D. White. Both men were responsible for collecting large amounts of Iniopteryx material, and Rushlau was responsible for discovering the holotype specimen of I. rushlaui.

Articulated remains of Iniopteryx are known which are preserved in dorsoventral (above/below) and lateral (side) views, and often consist of nearly complete skeletons. Both male and female individuals are known, but males are significantly more common and better known. The largest specimens, PF6678 and PF6645, are between 30-35 centimeters in length. The internal skeleton of Iniopteryx was made up of calcified cartilage.

The skull anatomy of Iniopteryx was characterized as poorly known in its original description, as despite many skulls being known their means of preservation made them very difficult to study. In I. rushlaui, the braincase narrows towards the nasal openings, and concavities behind this region indicate where the orbits (eye sockets) were positioned. The eyes themselves were very large, and had a sclerotic ring made up of prisms of calcified cartilage. The skull greatly widens about halfway down its length, which is the point where the palatoquadrates (upper jaws) articulate with the Meckel's cartilages (lower jaws). The Meckel's cartilages are slender and do not fuse at the symphysis (midline). Zangerl and Case (1973) originally considered the jaw suspension to be holostylic (also termed autostylic), with the palatoquadrates entirely fused to the braincase. However, later research by paleontologist Barbara J. Stahl found that the palatoquadrates were not fused to the braincase, and that the jaw suspension was not holostylic. {{citation |last=Stahl |first=Barbara |title=Non-autostylic Pennsylvanian iniopterygian fishes |date=1980 |work=Paleontology |volume=23 |pages=315-324 |url=https://www.palass.org/publications/palaeontology-journal/archive/23/2/article_pp315-324 |publisher=The Paleontological Association}}{{cite book |last=Zangerl |first=Rainer |title=Chondrichthyes 1: Paleozoic Elasmobranchii |date=1981 |publisher=Friedrich Pfell |isbn=978-3899370454 |series=Handbook of Paleoichthyology |volume=3A |pages=32-45, 77-92}} Unfused palatoquadrates are also observed in other members of the family Iniopterygidae.{{Cite book |last=Zangerl |first=Rainer |url=https://www.biodiversitylibrary.org/bibliography/3273 |title=Cervifurca nasuta n. gen. et sp. : an interesting member of the Iniopterygidae (Subterbranchialia, Chondrichthyes) from the Pennsylvanian of Indiana, U.S.A |last2=Zangerl |first2=Rainer |last3=History |first3=Field Museum of Natural |date=1997 |publisher=Field Museum of Natural History |location=Chicago, Ill |doi=10.5962/bhl.title.3273}}

The dentition consists of many small and simple denticles with conical, recurved crowns composed of orthodentine and somewhat wider bases. The undivided pulp cavity does not contain trabecular dentine, a form which normally appears at the base of chondrichthyan teeth.{{Cite book |last1 = Ginter |first1 = Michał |last2 = Hampe |first2 = Oliver |last3 = Duffin, Christopher

|date = 2010

|title = Chondrichthyes. Paleozoic Elasmobranchii : Teeth

|series = Handbook of Paleoichthyology. Volume 3D

|isbn = 978-3-89937-116-1 -}} The size and shape of these denticles varies considerably. Iniopteryx bears differently-shaped tooth whorls (fused arrangements of tooth crowns) at the symphyses of the palatoquadrates and the Meckel's cartilages. Successive teeth are unfused and significantly larger, indicating the rate of growth of these animals.

The gills of Iniopteryx were tightly packed and positioned beneath the skull. Three of the gill arches bear denticles, with the anteriormost arch bearing the smallest denticles and the posteriormost arch bearing the largest denticles. Iniopteryx display 6-10 rays of calcified cartilage protruding posteroventrally (backwards and towards the bottom) from the ventral (lower) side of the skull, which most likely represent hyoid radials associated with supporting a fleshy operculum.{{Cite book |last=Benton |first=Michael J. |title=Vertebrate Palaeontology |date=2015 |publisher=Wiley Blackwell |isbn=978-1-118-40684-7 |edition=4th |location=Oxford |pages=176-178}}

Similarly to the skull, the postcrania of I. rushlaui has been characterized as difficult to study. The vertebral column has at least 40 segments, about half of which compose the caudal peduncle and the caudal fin. As in modern chimaeroids, these vertebrae have no vertebral centra; however, unlike living chimaeras, Iniopteryx lacked a vertebral column made up of calcified rings, and had an entirely uncalcified notochord.

Each pectoral fin consists of a large basal plate which articulates with the scapulocoracoids (pectoral girdle) and supports at least eleven fin rays. The first fin ray is enlarged in both males and females, but is significantly larger in males and is covered by a sexually dimorphic "rasp" of large denticles. In I. rushlaui, these denticles resemble hooks, with recurved, forward-pointing crowns and deep, tubular bases, while in I. tedwhitei these denticles have straight, slender crowns and saddle-shaped bases. Behind the enlarged first ray are at least ten fin rays which slowly decrease in length and width towards the last. Two or three fin rays towards the posterior (rear) side of the fin bear rodlets of calcified cartilage that protrude at right angles from the fin rays. The pectoral fins articulate with the upper portion of the scapulocoracoid, giving the appearance that they protrude from the nape of the neck.

The pelvic girdle consists of two small, unfused cartilage elements and is unknown in any female specimens. Each cartilage connects to a basipterygium, which is a basal element that supports the pelvic fins. The basipterygium itself is a near-triangular plate that bears nine short pelvic fin rays that are followed by ceratotrichia (soft, ray-like cartilaginous structures which support the fins). The basipterygium of male Iniopteryx also supported a large, recurved denticle that acted as a tenaculum. The claspers of male Iniopteryx consists of a pair of long, cartilaginous rods that are followed by at least 15 short pieces of cartilage that taper off posteriorly.

Iniopteryx had a single dorsal fin located above the pelvic region, which consisted of about six fin rays that fuse dorsally to form a cartilaginous plate. The caudal (tail) fin is rounded and symmetrical (termed diphycercal), and is supported by approximately 15 fin rays forming the upper lobe and an equal number forming the lower lobe. Iniopteryx, like other iniopterygians, lacked an anal fin. The posterior region of the caudal peduncle seems to bear small fin rays that grade into the tail fin. The center of the posterior region of the caudal fin consists of an oval cartilaginous plate.

Iniopteryx is the type genus of the Iniopterygiformes, a clade of chondrichthyans variously considered as stem-holocephalans{{Cite journal |last=Pradel |first=Alan |last2=Tafforeau |first2=Paul |last3=Janvier |first3 = Phillippe |date=2010 |title=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 |url=https://www.sciencedirect.com/science/article/pii/S1631068310000837/pdfft?md5=e1fee108afc63bbf12fa1f5457942152&pid=1-s2.0-S1631068310000837-main.pdf|journal=Comptes Rendus Palevol |language=en |volume=9 |issue=6-7 |pages=377–387 |doi=10.1016/j.crpv.2010.07.015 |issn=1631-0683}} or as stem-group chondrichthyans sister to the clade containing Elasmobranchii and Holocephali. Of these interpretations, the former is better supported in recent literature.{{Cite journal |last=Brownstein |first=Chase D. |last2=Near |first2=Thomas J. |last3=Dearden |first3=Richard P. |date=2024-10-30 |title=The Palaeozoic assembly of the holocephalan body plan far preceded post-Cretaceous radiations into the ocean depths |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2024.1824 |journal=Proceedings of the Royal Society B: Biological Sciences |volume=291 |issue=2033 |pages=20241824 |doi=10.1098/rspb.2024.1824 |pmc=11521621 |pmid=39471859}}{{Cite journal |last=Dearden |first=Richard P. |last2=Herrel |first2=Anthony |last3=Pradel |first3=Alan |date=2023-01-24 |title=Evidence for high-performance suction feeding in the Pennsylvanian stem-group holocephalan Iniopera |url=https://www.pnas.org/doi/10.1073/pnas.2207854119 |journal=Proceedings of the National Academy of Sciences |volume=120 |issue=4 |pages=e2207854119 |doi=10.1073/pnas.2207854119 |pmc=9942859 |pmid=36649436}} Within Iniopterygiformes, Iniopteryx is a member of the Iniopterygidae, characterized by a non-holostylic jaw suspension and Meckel's cartilages not fused at the symphysis. The internal phylogeny of the Iniopterygiformes according to Zangerl and Case (1973), Zangerl (1997), and Grogan and Lund (2009) is as follows.

Family Iniopterygidae:

• Iniopteryx
• Iniopteryx rushlaui Zangerl & Case, 1973
• Iniopteryx tedwhitei Zangerl & Case, 1973
• Promexyele
• Promexyele peyeri Zangerl & Case, 1973
• Promexyele bairdi Zangerl & Case, 1973
• Cervifurca
• Cervifurca nasuta Zangerl, 1981
• Rainerichthys
• Rainerichthys zangerli Grogan & Lund, 2009
• Papilionichthys
• Papilionichthys stahlae Grogan & Lund, 2009

Family Sibyrhynchidae:

• Sibyrhynchus
• Sibyrhynchus denisoni Zangerl & Case, 1973
• Iniopera
• Iniopera richardsoni Zangerl & Case, 1973
• Inioxyele
• Inioxyele whitei Zangerl & Case, 1973

The large size and position of the pectoral fins in Iniopteryx suggests that they were the primary means of locomotion. The pectoral fins likely would have moved vertically in a similar manner to swimming sea turtles or the flight stroke of a bird, while the caudal fin acted as a rudder and the dorsal and pelvic fins acted as stabilizers. The large, fused basal plate of the pectoral fin likely anchored large muscles that allowed both for a powerful downstroke and rotation of the fin during the downstroke and the upstroke. During slow movement, in contrast, Iniopteryx likely used its caudal fin for propulsion in a similar manner to most other fish.

The teeth of Iniopteryx are small and fragile, which suggests it fed on soft-bodied prey. A number of articulated specimens preserve a beige, fine-grained infill of the mineral hydroxyapatite in the stomach cavity. In some specimens (PF6672 and PF6646) this infill contains arthropod exoskeletons, conodonts, and plant matter, while in other specimens (PF6645), the infill contains only a blue-gray substance and a finely striated surface texture and darker, diagonal stripes. Zangerl and Case suggested that the blue-gray infills represents casts of the inner surface of the intestine and that the hydroxyapatite infills represent preserved gut contents.

Among Zangerl and Case (1973)'s original sample of 56 specimens of Iniopteryx rushlaui, only seven were females. Zangerl and Case theorized that this may have been caused by an increased rate of predation among the females, as the males could have discouraged predators from attacking them by holding their large, denticle-bearing fins at a right angle.

The Stark Shale, where the type specimen of I. rushlaui originated, and the Wea Shale, where the type specimen of I. tedwhitei originated, were deposited in deep-water, offshore environments with calm, anoxic bottom waters caused by a thermocline (temperature gradient) and halocline (salinity gradient).{{Cite journal |last=Schnetz |first=Lisa |last2=Butler |first2=Richard J. |last3=Coates |first3=Michael I. |last4=Sansom |first4=Ivan J. |date=2024 |title=The skeletal completeness of the Palaeozoic chondrichthyan fossil record |url=https://royalsocietypublishing.org/doi/10.1098/rsos.231451 |journal=Royal Society Open Science |language=en |volume=11 |issue=1 |pages=14–17 |doi=10.1098/rsos.231451 |issn=2054-5703 |pmc=10827434 |pmid=38298400}}{{Cite journal |last1=Mapes |first1=Royal H. |last2=Doguzhaeva |first2=Larisa A. |date=2018 |title=New Pennsylvanian coleoids (Cephalopoda) from Nebraska and Iowa, USA |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/new-pennsylvanian-coleoids-cephalopoda-from-nebraska-and-iowa-usa/7AF391EB293CB7CF716DD33C211A8C47 |journal=Journal of Paleontology |language= |volume=92 |issue=2 |pages=146–156 |doi=10.1017/jpa.2017.79 |bibcode=2018JPal...92..146M |issn=0022-3360}}{{Cite journal |last=Heckel |first=Phillip H. |date=1977 |title=Origin of Phosphatic Black Shale Facies in Pennsylvanian Cyclothems of Mid-Continent North America |url=http://search.datapages.com/data/doi/10.1306/C1EA43C4-16C9-11D7-8645000102C1865D |journal=AAPG Bulletin |language=en |volume=61 |doi=10.1306/C1EA43C4-16C9-11D7-8645000102C1865D |issn=0149-1423}} These sites are considered konservat-lagerstätten, meaning that fossils collected from the site are exceptionally well preserved. In addition to Iniopteryx, a wide array of chondrichthyans are known from the sites, including other iniopterygians such as Promexyele and Iniopera, eugeneodonts (whorl-toothed sharks) such as Romerodus, Gilliodus, and Agassizodus,{{Cite book |last=Case |first=Gerard R. |url=https://archive.org/details/pictorialguideto0000case |title=A pictorial guide to fossils |date=1982 |publisher=Van Nostrand Reinhold |isbn=978-0-442-22651-0 |edition=1st |location=New York |publication-date=1982 |pages=}} the ctenacanthiform Heslerodus,{{Cite web |title=Collection: Hansen Quarry, Quarry 6, Stark Shale, Papillion Sarpy County, Nebraska, USA |url=https://www.mindat.org/paleo_collection.php?col=132117 |access-date=1 October 2024 |website=mindat.org}} the symmoriiform Cobelodus, and the enigmatic dermal denticles of Listracanthus. Other organisms, including conodonts, palaeoniscoid bony fish, crinoids, jellyfish medusae, brachiopods, bryozoans, bivalves, gastropods,{{Cite journal |last=Schram |first=Fredrick R. |date=1 January 1984 |title=Frederick R. Schram; Upper Pennsylvanian arthropods from black shales of Iowa and Nebraska |url=https://www.jstor.org/stable/1304745 |journal=Journal of Paleontology |volume=1 |pages=197–209 |jstor=1304745 }} coleoids and ammonoid cephalopods, and the tyrannophontid mantis shrimps Tyrannophontes and Gorgonophontes are also known from the Stark and Wea shales.{{Cite journal |last=Bicknell |first=Russell D.C. |last2=Smith |first2=Patrick M. |last3=Klompmaker |first3=Adiël A. |last4=Hegna |first4=Thomas A. |date=2024-11-06 |title=A New Archaeostomatopod from the Pennsylvanian Wea Shale Member, Nebraska |url=https://bioone.org/journals/american-museum-novitates/volume-2024/issue-4028/4028.1/A-New-Archaeostomatopod-from-the-Pennsylvanian-Wea-Shale-Member-Nebraska/10.1206/4028.1.full |journal=American Museum Novitates |volume=2024 |issue=4028 |pages=1-25 |doi=10.1206/4028.1 |issn=0003-0082 |doi-access=free}}

In contrast, the Logan Quarry and the Excello Shale were deposited in a highly productive, shallow-water environment with seasonal changes in rainfall, warm temperatures, anoxic bottom water, and abundant organic material in the form of both dense mats of floating marine vegetation and abundant land plant remains. {{citation |last=Rexroad |first=Carl B. |last2=Wade |first2=Jan A.|last3=Merrill |first3=Glen K.|last4=Brown |first4=Lewis M.|last5=Padgett |first5=Penny|title=Conodont biostratigraphy and depositional environments of the Mecca Quarry Shale Member and the Velpen Limestone Member of the Linton Formation (Pennsylvanian, Desmoinesian) in the eastern part of the Illinois Basin, USA |date=2001 |work=Indiana Geological Survey Special Report 63 |url=https://scholarworks.iu.edu/dspace/items/0eac60fb-c5f3-431c-a292-1ed7fba83224}}{{citation |last=Ece |first=Omer Isik |title=Organic Maturation and Paleoceanographic/Paleogeographic Implications of the Desmoinesian Cyclothemic Excello Black Shale of the Midcontinent, USA |date=1989 |url=https://archives.datapages.com/data/ocgs/data/012/012001/0273.htm}}{{citation |last=Zangerl |first=Rainer |title=The problem of vast numbers of cladodont shark denticles in the Pennsylvanian Excello Shale of Pike County, Indiana |date=1995 | journal = Journal of Paleontology | volume = 69| issue = 3| pages = 556-563| publisher = Paleontological Society |url=https://www.jstor.org/stable/pdf/1306327.pdf}}{{cite book |last=Zangerl |first=Rainer |url=https://www.biodiversitylibrary.org/bibliography/7199 |title=The paleoecological history of two Pennsylvanian black shales |last2=Richardson |first2=Eugene S. |date=1963 |publisher=Chicago Natural History Museum |series=Fieldiana. Geology memoirs |doi=10.5962/bhl.title.7199}} In these localities, Iniopteryx would have similarly coexisted with other chondrichthyans, including other iniopterygians such as Promexyele, Cervifurca, Sibyrhynchus, and Iniopera, the small eugeneodonts Caseodus and Ornithoprion, the symmoriiforms Denaea and Stethacanthulus, and Petrodus and Listracanthus denticles. Palaeoniscoid and chondrostean bony fish,{{Cite journal |last=Stack |first=Jack |last2=Hodnett |first2=John-Paul |last3=Lucas |first3=Spencer G |last4=Sallan |first4=Lauren |date=2021-01-09 |title=Tanyrhinichthys mcallisteri , a long-rostrumed Pennsylvanian ray-finned fish (Actinopterygii) and the simultaneous appearance of novel ecomorphologies in Late Palaeozoic fishes |url=https://academic.oup.com/zoolinnean/article/191/2/347/5860401 |journal=Zoological Journal of the Linnean Society |language=en |volume=191 |issue=2 |pages=347–374 |doi=10.1093/zoolinnean/zlaa044 |issn=0024-4082}}{{Cite journal |last=Poplin |first=Cécile M. |date=1978 |title=An Actinopterygian with a Long Rostrum from the Pennsylvanian of Logan Quarry, Indiana |url=https://www.jstor.org/stable/1303953 |journal=Journal of Paleontology |volume=52 |issue=3 |pages=524–531 |issn=0022-3360}} the acanthodian Acanthodes, a rhipidistian lobe-finned fish, and conodonts made up the remainder of the vertebrate community, and nautiloids, ammonoids, gastropods, bivalves, arthropods, crinoids, annelids, bryozoans, brachiopods, the rugose coral Lophophyllidium proliferum, and sponges made up the invertebrate community. Iniopteryx and other iniopterygians may have been open-ocean visitors to these basin environments, rather than full-time residents.

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{{Portal|Paleontology}}

• [https://www.biodiversitylibrary.org/item/25090#page/13/mode/1up Description of Iniopteryx and the order Iniopterygia], Biodiversity Heritage Library

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Category:Iniopterygiformes