''Sigournea'' is a
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 from 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 ...
. The genus contains only one species, the
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 ...
''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 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 follow ...
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 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 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 teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
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 epithelial ...
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 (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
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
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 () ...
published a phylogenetic analysis that did include ''Sigournea'', and found good support for ''S. multidentata'' grouping with ''Occidens portlocki
''Occidens'' is an extinct genus of stem tetrapod that lived during the earliest part of the Carboniferous in what is now Northern Ireland. It is known from a single type species, ''Occidens portlocki'', named in 2004 on the basis of a left lower ...
'', 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 stage ...
) of Northern Ireland.
References
{{Taxonbar, from=Q18358603
Carboniferous tetrapods
Prehistoric tetrapod genera
Paleozoic animals of North America