Ischyodus

{{Short description|Extinct genus of chimaera fish}}

{{Automatic taxobox

| fossil_range = {{fossilrange|176|23}}Middle Jurassic to Miocene

| image = Ischyodus quenstedti 45456 (cropped).jpg

| image_caption = Nearly complete specimen of Ischyodus quenstedti from the Solnhofen Limestone

| image2 = Ischyodus_quenstedti_or_egertoni.png

| image2_caption = Life restoration of Ischyodus quenstedti (sometimes considered a synonym of I. egertoni)

| taxon = Ischyodus

| authority = Egerton, 1843

| subdivision_ranks = Species

| subdivision = *Ischyodus bifurcatus

• Ischyodus brevirostris
• Ischyodus dolloi
• Ischyodus gubkini
• Ischyodus incisus
• Ischyodus latus
• Ischyodus lonzeensis
• Ischyodus minor
• Ischyodus mortoni
• Ischyodus planus
• Ischyodus rayhaasi
• Ischyodus thurmanni
• Ischyodus townsendi
• Ischyodus williamsae
• Ischyodus yanshini
• Ischyodus zinsmeisteri

}}

Ischyodus (from {{langx|el|ισχύς}} {{transliteration|el|ischýs}}, 'power' and {{langx|el|ὀδούς}} {{transliteration|el|odoús}} 'tooth') is an extinct genus of chimaera. It is the most diverse and long-lived chimaera genus, with over 39 species found worldwide spanning over 140 million years from the Middle Jurassic to the Miocene.{{Cite journal |last1=Johnson-Ransom |first1=Evan D. |last2=Popov |first2=Evgeny V. |last3=Deméré |first3=Thomas A. |last4=Shimada |first4=Kenshu |date=October 2018 |title=The Late Cretaceous Chimaeroid Fish, Ischyodus bifurcatus Case (Chondrichthyes: Holocephali), from California, USA, and Its Paleobiogeographical Significance |url=http://www.bioone.org/doi/10.2517/2018PR004 |journal=Paleontological Research |language=en |volume=22 |issue=4 |pages=364–372 |doi=10.2517/2018PR004 |s2cid=133941390 |issn=1342-8144|url-access=subscription }} Almost all species are only known from tooth plates, with the exception of the Jurassic species I. quenstedti.{{Cite journal |last1=Villalobos-Segura |first1=Eduardo |last2=Stumpf |first2=Sebastian |last3=Türtscher |first3=Julia |last4=Jambura |first4=Patrick L. |last5=Begat |first5=Arnaud |last6=López-Romero |first6=Faviel A. |last7=Fischer |first7=Jan |last8=Kriwet |first8=Jürgen |date=March 2023 |title=A Synoptic Review of the Cartilaginous Fishes (Chondrichthyes: Holocephali, Elasmobranchii) from the Upper Jurassic Konservat-Lagerstätten of Southern Germany: Taxonomy, Diversity, and Faunal Relationships |journal=Diversity |language=en |volume=15 |issue=3 |pages=386 |doi=10.3390/d15030386 |doi-access=free |issn=1424-2818 |pmc=7614348 |pmid=36950327}} Complete specimens of I. quenstedti from the Late Jurassic of Germany most closely resemble the genus Callorhinchus amongst living chimaera genera.Popov, E. V., Duffin, C. J., Tischlinger, H. and Atuchin, A., 2013: [https://www.researchgate.net/profile/Evgeny-Popov-3/publication/261178528_Reconstructions_of_the_German_Plattenkalk_Late_Jurassic_chimaeroid_fishes_Holocephali_Chimaeroidei_Poster/links/57ffde6c08ae6fc7fc6524d8/Reconstructions-of-the-German-Plattenkalk-Late-Jurassic-chimaeroid-fishes-Holocephali-Chimaeroidei-Poster.pdf Reconstructions of the German Plattenkalk (Late Jurassic) chimaeroid fishes (Holocephali, Chimaeroidei).] In, Schwarz, C. and Kriwet, J. eds., Sixth International Meeting on Mesozoic Fishes: Diversification and Diversity Patterns. Abstracts, p. 56. Verlag Friedrich Pfeil, Munich It is sometimes placed in the "Edaphodontidae", a unclearly defined group of chimaera with an uncertain position within the clade, while other authors place it into Callorhinchidae along with Callorhinchus,{{Cite journal |last1=Otero |first1=Rodrigo |last2=Figueroa Bravo |first2=Constanza |last3=Soto Huenchuman |first3=Paula |last4=Fernández-Collemann |first4=Sara |last5=Valenzuela Toro |first5=Ana |last6=Gutstein |first6=Carolina |date=2021 |title=First record of Ischyodus (Chondrichthyes, Holocephali) from the Upper Jurassic of southwestern Gondwana |journal=Acta Palaeontologica Polonica |volume=66 |doi=10.4202/app.00859.2020 |issn=0567-7920|doi-access=free }} a position that has been supported by at least some phylogenetic analyses.{{Cite journal |last1=Brownstein |first1=Chase D. |last2=Near |first2=Thomas J. |last3=Dearden |first3=Richard P. |date=October 2024 |title=The Palaeozoic assembly of the holocephalan body plan far preceded post-Cretaceous radiations into the ocean depths |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=291 |issue=2033 |doi=10.1098/rspb.2024.1824 |issn=0962-8452 |pmc=11521621 |pmid=39471859|pmc-embargo-date=October 30, 2025 }} It is suggested therefore to probably have had a similar ecology to living Callorhinchus.

Based on complete specimens from the Solnhofen Limestone of Germany, the total length is assumed to be 21 times larger than mesiodistal length of mandibular tooth plate. For example, I. bifurcatus with mandibular plate length of {{cvt|8.2|cm|order=flip}} possibly belongs {{cvt|1.72|m|order=flip}} long specimen.{{Cite journal |last1=Johnson-Ransom |first1=Evan D. |last2=Popov |first2=Evgeny V. |last3=Deméré |first3=Thomas A. |last4=Shimada |first4=Kenshu |date=2018-10-01 |title=The Late Cretaceous Chimaeroid Fish, Ischyodus bifurcatus Case (Chondrichthyes: Holocephali), from California, USA, and Its Paleobiogeographical Significance |url=https://www.researchgate.net/publication/328044465 |journal=Paleontological Research |language=English |volume=22 |issue=4 |pages=364–372 |doi=10.2517/2018PR004 |s2cid=133941390 |issn=1342-8144}} The front of the head of I. quenstedti has an elongate tapering snout, with the body having a long, unornamented dorsal fin spine, as well as a heterocercal tail fin.