Opisthomyzon

{{Short description|Extinct genus of fishes}}

{{Speciesbox

| fossil_range = {{fossilrange|33.9|30|Early Oligocene}}

| image = Echeneis glaronensis.JPG

| image_caption = Fossil specimen, Paleontology Museum of Zurich

| display_parents = 3

| taxon = Opisthomyzon glaronensis

| grandparent_authority = Berg, 1940

| parent_authority = Cope, 1889

| authority = (Wettstein, 1886)

| synonyms = * †Uropteryx elongatus Agassiz, 1835 (name only)

• †Echeneis glaronensis Wettstein, 1886

}}

Opisthomyzon is an extinct genus of stem group-remora that lived during the Oligocene epoch.{{cite journal | last = Sepkoski | first = Jack | title = A compendium of fossil marine animal genera | journal = Bulletins of American Paleontology | volume = 364 |page=560 | year = 2002 | url = http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=611&rank=class | accessdate = 2009-02-27 }}{{Cite web |title=PBDB Taxon |url=https://paleobiodb.org/classic/basicTaxonInfo?taxon_no=36103 |access-date=2025-04-11 |website=paleobiodb.org}} It contains a single species, O. glaronensis, from the Early Oligocene-aged Matt Formation of Switzerland. It is the only member of the family Opisthomyzonidae.{{Cite journal |last=Friedman |first=Matt |last2=Johanson |first2=Zerina |last3=Harrington |first3=Richard C. |last4=Near |first4=Thomas J. |last5=Graham |first5=Mark R. |date=2013-09-07 |title=An early fossil remora (Echeneoidea) reveals the evolutionary assembly of the adhesion disc |url=https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2013.1200 |journal=Proceedings of the Royal Society B: Biological Sciences |volume=280 |issue=1766 |pages=20131200 |doi=10.1098/rspb.2013.1200 |pmc=3730593 |pmid=23864599}}

It is the oldest known remora-like fish known from the fossil record, providing important evidence about the evolution of this family and its distinctive morphological features. It had already developed the iconic adhesive disk, further affirming its close relationship to extant remoras, but is presently retained in its own family due to its distinctive morphological features from them. For example, the adhesive disk was in a postcranial position extending onto the back, rather than directly on the head as with modern remoras.{{Cite journal |last=Friedman |first=Matt |last2=Johanson |first2=Zerina |last3=Harrington |first3=Richard C. |last4=Near |first4=Thomas J. |last5=Graham |first5=Mark R. |date=2014-06-07 |title=On fossils, phylogenies and sequences of evolutionary change |url=https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2014.0115 |journal=Proceedings of the Royal Society B: Biological Sciences |volume=281 |issue=1784 |pages=20140115 |doi=10.1098/rspb.2014.0115 |pmc=4043088 |pmid=24759860}}

The specimen was originally named as Uropteryx elongatus by Agassiz (1835). However, this name was done without a proper description and is thus a nomen nudum. Wettstein (1886) described it among the extant remoras as Echeneis glaronensis, but Cope (1889) placed it in its own genus, Opisthomyzon. Friedman & Johanson (2012) erroneously revived U. elongatus for the genus, but Friedman et al (2013) reversed this decision after noting the former name's status as a nomen nudum.{{Cite journal |last=Friedman |first=Matt |last2=Johanson |first2=Zerina |date=2012 |title=† Opisthomyzon glaronensis (Wettstein, 1886) (Acanthomorpha, †Opisthomyzonidae), a junior synonym of † Uropteryx elongatus Agassiz, 1844 |url=http://www.tandfonline.com/doi/abs/10.1080/02724634.2012.684817 |journal=Journal of Vertebrate Paleontology |language=en |volume=32 |issue=5 |pages=1202–1206 |doi=10.1080/02724634.2012.684817 |issn=0272-4634|url-access=subscription }}