Axelrodichthys

''Axelrodichthys'' is an extinct genus of mawsoniid coelacanth from the Cretaceous of Africa, North and South America, and Europe. Several species are known, the remains of which were discovered in the Lower Cretaceous (Aptian-Albian) of Brazil, North Africa, and possibly Mexico, as well as in the Upper Cretaceous of Morocco ( Cenomanian), Madagascar (Coniacian –Santonian) and France (Lower Campanian to Lower Maastrichtian). The ''Axelrodichthys'' of the Lower Cretaceous frequented both brackish and coastal marine waters (lagoon-coastal environment) while the most recent species lived exclusively in fresh waters (lakes and rivers). The French specimens are the last known fresh water coelacanths. Most of the species of this genus reached to in length. ''Axelrodichthys'' was named in 1986 by John G. Maisey in honor of the American ichthyologist Herbert R. Axelrod.

Description

Like its close relative ''Mawsonia'', ''Axelrodichthys'' is a coelacanth with an elongated, low, and wide skull, whose skull roof and cheek bones are characterized by strong ornamentation. It differs from ''Mawsonia'' mainly in its more elongated parietonasal shield, the development of the descending process of the supratemporal, and by the shape and arrangement of the cheek and lower jaw bones. On the latter, the two posterior branches of the dentary are similar in length in ''Axelrodichthys'' while the lower branch of the bifurcation is much longer than the upper in ''Mawsonia''. The contact area with the angular is also more extensive in ''Axelrodichthys''.

Taxonomy

Several species of ''Axelrodichthys'' have been described. The validity of some of them is discussed and other specimens are left in open nomenclature because they are represented by specimens too incomplete for specific determinations. The genus includes the following taxa:

* ''Axelrodichthys araripensis'' is the type species of the genus. It is also the best known species thanks to the discovery of numerous exceptionally preserved whole specimens in three dimensions in carbonate nodules. These fossils come from the Crato Formation, Aptian in age, and especially from the Romualdo Formation, whose age is generally considered Albian but which could date from the late Aptian. Both formations are located in the Araripe Basin in northeastern Brazil (Ceará, Pernambuco and Piauí states). It is possible that this species is also present in the Tlayúa Formation (late Albian) in the Puebla state in Mexico. However, the specimen mentioned in the scientific literature has never been described and unfortunately was subsequently lost.
* ''Axelrodichthys maiseyi'' comes from the mid-late Albian Codó Formation, located in the Grajaú Basin (Maranhão state) in northeastern Brazil. The species is named after John G. Maisey, the creator of the genus ''Axelrodichthys''. However, the status of ''A. maiseyi'' is discussed by some authors who doubt the interpretation of some anatomical structures and suggest revising this species.
* ''Axelrodichthys megadromos'' comes from various Upper Cretaceous geological formations of southern France ranging in age from the Lower Campanian to the Lower Maastrichtian. The species is named after the Greek ''megas'', large, and ''dromos'', driveway, and refers both to the arrival in Europe of this gondwanan taxon, and to the construction of a new motorway near the type locality. ''A. megadromos'' is represented by a partial skull and numerous isolated cranial bones from several sites of Provence (departments of Bouches-du-Rhône and Var) and Occitanie (Aude and Hérault