Occidens portlocki
Occidens is an extinct genus of stem tetrapod from the Early Carboniferous Altagoan Formation of Northern Ireland. It is known from a single type species, Occidens portlocki, named in 2004 on the basis of a left lower jaw described by British geologist Joseph Ellison Portlock in 1843.
History
The holotype, a left lower jaw, was discovered in Northern Ireland in 1843 by Joseph Ellison Portlock, who attributed the specimen to the lobe-finned fish Holoptychius. It was housed in the collections of the British Geological Survey for over a century before being reevaluated. In 2004, it was redescribed by vertebrate paleontologists Jenny Clack 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 stage of the Early Carboniferous 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, 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 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.