''Sigournea'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

stem tetrapod The Stem Tetrapoda are a cladistically defined group, consisting of all animals more closely related to extant four-legged vertebrates than to their closest extant relatives (the lungfish), but excluding the crown group Tetrapoda. They are thus ...

from the

Early Carboniferous Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

. The genus contains only one species, the

type species In International_Code_of_Zoological_Nomenclature, 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 spe ...

''Sigournea multidentata'', which was named in 2006 by paleontologists John R. Bolt and R. Eric Lombard on the basis of a single lower jaw from Iowa. The jaw came from a fissure-fill deposit of the

St. Louis Limestone The St. Louis Limestone is a large geologic formation covering a wide area of the midwest of the United States. It is named after an exposure at St. Louis, Missouri. It consists of sedimentary limestone with scattered chert beds, including the ...

that was exposed in a quarry near the town of Sigourney and dates to the

Viséan The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from to Ma. It follows ...

stage, making it approximately 335 million years old. Bolt and Lombard named the genus after Sigourney. The species name ''multidentata'' alludes to the many teeth preserved in the jaw. The jaw, which is housed in the

Field Museum The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educationa ...

and cataloged as FM PR 1820, curves strongly downward but was probably straight to begin with, having been deformed by the process of fossilization after the individual died. Rooted in the

dentary bone In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

along the outermost edge of the jaw are 88 small, pointed marginal teeth. An additional row of even smaller teeth runs along the coronoids, three bones positioned lengthwise along the lower boundary of the dentary on the inner surface of the lower jaw. Bolt and Lombard were able to classify ''Sigournea'' as an early member of Tetrapoda based on the presence of bone surfaces covered in pits and ridges, a single row of dentary teeth, a jaw joint that faces upward, and 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 epithelia ...

along the outer surface of the jaw, and on the absence of teeth on the prearticular bone or enlarged fangs on the coronoids. ''Sigournea'' differs from other stem tetrapods in having several holes within a depression called the exomeckelian fenestra on the inner surface of the jaw. The closest relatives of ''Sigournea'' within Tetrapoda are unknown. Bolt and Lombard did not include it in a

phylogenetic analysis 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 data ...

in their 2006 description because they thought the single known jawbone did not possess enough unique anatomical characteristics to elucidate its evolutionary relationships. However, in 2008, Bolt and his colleague Marcello Ruta published a phylogenetic analysis that did include ''Sigournea'', and found good support for ''S. multidentata'' grouping with '' Occidens portlocki'', a species of stem tetrapod named in 2004 from the earliest Carboniferous (

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 st ...

) of Northern Ireland. Milner ''et al.'' (2009) suggested that ''Sigournea'' may be a close relative of the slightly younger baphetoid tetrapod ''

Spathicephalus ''Spathicephalus'' is an extinct genus of stem tetrapods (early four-limbed vertebrates) that lived during the middle of the Carboniferous Period. The genus includes two species: the type species ''S. mirus'' from Scotland, which is known from tw ...

'' from Scotland and Nova Scotia based on the presence of small, closely packed marginal teeth in both taxa. However, their proposition was tentative because they did not include ''Sigournea'' in their phylogenetic analysis.

References

{{Taxonbar, from=Q18358603 Carboniferous tetrapods Prehistoric tetrapod genera Paleozoic animals of North America