Ymeria

Ymeria Temporal range: Late Devonian
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	"Amphibia" (wide sense)
Genus:	†Ymeria Clack et al., 2012
Type species
†Y. denticulata Clack et al., 2012

Ymeria is an extinct genus of early tetrapod from the Devonian of Greenland. Of the two other genera of stem tetrapods from Greenland, Acanthostega and Ichthyostega, Ymeria is most closely related to Ichthyostega, though the single known specimen is smaller, the skull about 10 cm in length. A single interclavicule resemble that of Ichthyostega, an indication Ymeria may have resembled this genus in the post-cranial skeleton.^[1]

Discovery

Ymeria is known from a partial holotype skull including the lower jaws and palate, as well as impressions of the shoulder girdle. The holotype comes from the southern slope of Mt. Celsius on Ymer Island in northeast Greenland. Fossils of Devonian tetrapods like Ichthyostega have been known from Ymer Island since 1929. The skull of Ymeria found in 1947 by a team of paleontologists from Sweden and Denmark. It came from a talus slope or pile of rock fragments at the base of Mt. Celsius, encased in a pale red sandstone. The fossil's origin on the mountain has not been identified. There are four formations preserved at Mt. Celsius, all belonging to the larger Celsius Bjerg Group. Since the skull cannot be traced to any one of these formations, its exact age is uncertain.^[1]

Swedish paleontologist Erik Jarvik, a member of the 1947 expedition, could not assign the skull to either Ichthyostega or Acanthostega. In 1988, English paleontologist Jennifer Clack was the first to propose that the material represented a third type of tetrapod from Greenland, based mainly on the teeth. While the skull shape is closest to Ichthyostega, the teeth are smaller, more numerous and less curved, indicating the two exploited different foods. The surface ornamentation on the skull bones are less pronounced, indicating lighter dermal armour. The material was assigned to a new genus and species, Ymeria denticulata, in 2012. The genus is named after Ymer Island, while the specific name refers to the denticulated or bumpy surface of the lower jaw.^[1]

References