''Trachymetopon'' is an extinct genus of

prehistoric Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins million years ago and the beginning of recorded history with the invention of writing systems. The use o ...

coelacanth Coelacanths ( ) are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia. As sarcopterygians, they are more closely related to lungfish and tetrapods (the terrestrial vertebrates including living amphibians, reptiles, bi ...

from the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

of Europe. Fossils have been found in the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...

Posidonia Shale The Posidonia Shale (, also called Schistes Bitumineux in Luxembourg) geologically known as the Sachrang Formation, is an Early Jurassic (Early to Late Toarcian) geological formation in Germany, northern Switzerland, northwestern Austria, souther ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

Marnes de Dives The Marnes de Dives is a geological formation in Normandy, France. It dates back to the upper part of the Callovian stage of the Middle Jurassic.Weishampel, David B; et al. (2004). "Dinosaur distribution (Middle Jurassic, Europe)." In: Weishampel ...

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

, and probably the

Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...

Kimmeridge Clay The Kimmeridge Clay is a sedimentary rock, sedimentary deposit of fossiliferous marine clay which is of Late Jurassic to lowermost Cretaceous age and occurs in southern and eastern England and in the North Sea. This rock formation (geology), form ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. Only one species has been named, ''Trachymetopon liassicum'', described by Henning in 1951 from an almost complete specimen found in the Lower

Toarcian The Toarcian is, in the International Commission on Stratigraphy, ICS' geologic timescale, an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic. It spans the time between 184.2 Megaannum, Ma (million ...

Ohmden Ohmden is a municipality in the district of Esslingen (district), Esslingen in Baden-Württemberg in Germany. Neighboring communities Neighboring municipalities are starting from North clockwise: Schlierbach (Göppingen), Schlierbach, Hattenhofe ...

Baden-Württemberg Baden-Württemberg ( ; ), commonly shortened to BW or BaWü, is a states of Germany, German state () in Southwest Germany, east of the Rhine, which forms the southern part of Germany's western border with France. With more than 11.07 million i ...

.Hennig, E. (1951). Trachymetopon liassicum, Ald., ein Reisen-Crossopterygier aus Schwäbischem Ober-Lias. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, Stuttgart, 94, 67-79. Another specimen is known from the same site, and two older specimens come from the

Sinemurian In the geologic timescale, the Sinemurian is an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic epoch (geology), Epoch or series (stratigraphy), Series. It spans the time between 199.5 ±0.3 annu ...

Holzmaden Holzmaden is a town in Baden-Württemberg, Germany that lies between Stuttgart and Ulm. Holzmaden is 4 km south-east from Kirchheim unter Teck and 19 km south-east of Esslingen am Neckar. The A 8 runs south from Holzmaden. The town a ...

. The holotype of this species is in length. A giant specimen of an undetermined species of ''Trachymetopon'' found at the Middle Jurassic (Late

Callovian In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 165.3 ± 1.1 Ma (million years ago) and 161.5 ± 1.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...

) Falaises des Vaches Noires of

Normandy Normandy (; or ) is a geographical and cultural region in northwestern Europe, roughly coextensive with the historical Duchy of Normandy. Normandy comprises Normandy (administrative region), mainland Normandy (a part of France) and insular N ...

. This specimen, composed of a 53 cm long

palatoquadrate In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel's cartilage. The palatoquadrate forms from splanchnocranium in various chordates including placoderms and acanthodians. See also * H ...

, belongs to an individual in length. A basisphenoid found in a museum in Switzerland that likely originates from the same locality probably belonged to an individual around long, making ''Trachymetopon'' the largest of all coelacanths alongside '' Mawsonia''. A study published in 2015 revealed that this coelacanth belongs to the

Mawsoniidae Mawsoniidae is an extinct family of prehistoric coelacanth fishes which lived during the Triassic to Cretaceous periods. Members of the family are distinguished from their sister group, the Latimeriidae (which contains the living coelacanths of t ...

. ''Trachymetopon'' is one of the few known mawsoniids to have been exclusively marine (with an indeterminate form from the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

), while most of the other members of the group have lived in

fresh Fresh may refer to: Arts, entertainment, and media Films and television * ''Fresh'' (1994 film), a crime film * ''Fresh'' (2009 film), a documentary film on sustainable agriculture * ''Fresh'' (2022 film), a thriller film *''Fresh with the Aust ...

and

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

waters.

Phylogeny

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis of the mawsoniids published in 2020 found a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...

grouping together ''Trachymetopon'' and the Cretaceous genera “'' Lualabaea''”, ''

Axelrodichthys ''Axelrodichthys'' is an extinct genus of mawsoniidae, mawsoniid coelacanth from the Cretaceous of Africa, North America, North and South America, and Europe. Several species are known, the remains of which were discovered in the Lower Cretaceous ...

'', and '' Mawsonia''. The genus “''Lualabaea''” could be congeneric with ''Axelrodichthys''.

References

Prehistoric lobe-finned fish genera Toarcian life Jurassic fish of Europe Jurassic Germany Fossils of Germany Posidonia Shale Mawsoniidae {{Jurassic-fish-stub