''Occidens'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of stem tetrapod that lived during the earliest part of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

in what is now Northern Ireland. It is known from a single

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

, ''Occidens portlocki'', named in 2004 on the basis of a left lower jaw described by British geologist

Joseph Ellison Portlock Major-General Joseph Ellison Portlock (30 September 1794 – 14 February 1864) was born at Gosport and was a British geologist and soldier, the only son of Nathaniel Portlock, and a captain in the Royal Navy. Educated at Blundell's School and t ...

in 1843.

History

Portlock attributed the original fossil to the

lobe-finned fish Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...

Holoptychius ''Holoptychius'' (from el, όλος , 'whole' and el, πτυχή 'fold') is an extinction, extinct genus of porolepiform lobe-finned fish from the Middle Devonian to Carboniferous (Mississippian (geology), Mississippian) periods. It is known f ...

,'' and it was housed in the collections of the

British Geological Survey The British Geological Survey (BGS) is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS h ...

for over a century before being reevaluated. In 2004, it was redescribed by vertebrate paleontologists

Jenny Clack Jennifer Alice Clack, (''née'' Agnew; 3 November 1947 – 26 March 2020) was an English palaeontologist and evolutionary biologist. She specialised in the early evolution of tetrapods, specifically studying the "fish to tetrapod" transition: ...

and Per E. Ahlberg, who reclassified it as a new genus and species of early tetrapod. They established the genus name ''Occidens,'' referring to its presence west of better-known early tetrapod assemblages in Great Britain. The species name honors Portlock. The jaw likely comes from the Altagoan Formation and, based on an analysis of fossilized pollen, dates to the late

Tournaisian The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Ma. It is preceded by the Famennian (the uppermost stage ...

stage of the

Early Carboniferous Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

about 350 million years ago. The occurrence of ''Occidens'' in the Tournaisian makes it a critically important taxon because it lies within

Romer's gap Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...

, a time interval spanning most of the Early Carboniferous in which few tetrapod fossils are known. Romer's gap separates the first appearance of tetrapods in the

Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...

from the group's first evolutionary radiation toward the end of the Early Carboniferous. However, the relationship of ''Occidens'' to other early tetrapods both before and after the gap remain uncertain, which means that its context in tetrapod evolution remains unknown.

Description

Clack and Ahlberg noted several distinctive features of ''Occidens'', including a straight row of teeth along the

coronoid bones In human anatomy, the mandible's coronoid process (from Greek ''korōnē'', denoting something hooked) is a thin, triangular eminence, which is flattened from side to side and varies in shape and size. Its anterior border is convex and is continuou ...

on the inner surface of the lower jaw, an open groove for a

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

sense organ on the jaw's outer surface, and a stepped shape to the connection between the

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

and

angular bone The angular is a large bone in the lower jaw (mandible) of amphibians and reptiles (birds included), which is connected to all other lower jaw bones: the dentary (which is the entire lower jaw in mammals), the splenial, the suprangular, and the art ...

s. The jaw bone is deep, resembling those of ''

Crassigyrinus ''Crassigyrinus'' (from la, crassus, 'thick' and el, γυρίνος el, gyrínos, 'tadpole') is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous Limestone Coal Group of Scotland and possibly Greer, West Virginia. ...

'' and

whatcheeriid Whatcheeriidae is an extinct family of tetrapods which lived in the Mississippian sub-period, a subdivision of the Carboniferous period. It contains the genera ''Pederpes'', '' Whatcheeria'', and possibly ''Ossinodus''. Fossils of a possible w ...

s (which both occur in Romer's gap) in overall appearance.

Classification

In most

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

s produced by Clack and Ahlberg's 2004 analysis, ''Occidens'' fell near whatcheeriids and the Devonian taxon '' Tulerpeton'', being more

derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

than all other Devonian taxa and more basal than ''Crassigyrinus'' and the post-Romer's Gap taxa ''

Greererpeton ''Greererpeton burkemorani'' ("crawler from Greer, West Virginia") is an extinct genus of colosteid stem-tetrapods from the Early Carboniferous period (late Viséan) of North America. ''Greererpeton'' was first described by famed vertebrate pal ...

'' and '' Megalocephalus''. A 2008 phylogenetic analysis by paleontologists

Marcello Ruta Marcello Ruta is an Italian paleontologist. Ruta's research primarily has focused on the anatomy and evolutionary significance of Paleozoic tetrapods Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda () ...

and John Bolt found ''Occidens'' to be the closest relative of ''

Sigournea multidentata ''Sigournea'' is a genus of stem tetrapod from the Early Carboniferous. The genus contains only one species, the type species ''Sigournea multidentata'', which was named in 2006 by paleontologists John R. Bolt and R. Eric Lombard on the basis of ...

'', a species from the end of the gap found in Iowa, but could not determine where these two taxa fit relative to other Early Carboniferous tetrapods.

References

{{Taxonbar, from=Q18355912 Carboniferous tetrapods Prehistoric tetrapod genera Fossils of Ireland Fauna of Ireland