| Ymeria Temporal range:Late Devonian | |
|---|---|
 | |
| Holotype specimen,Zoological Museum of Copenhagen | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Stegocephali |
| Genus: | †Ymeria Clacket al.,2012 |
| Type species | |
| †Ymeria denticulata Clacket al., 2012 | |
Ymeria is an extinctgenus of earlystem tetrapod from theDevonian ofGreenland. Of the two other genera ofstem tetrapods from Greenland,Acanthostega andIchthyostega,Ymeria is most closely related toIchthyostega, though the single known specimen is smaller, the skull about 10 cm in length. A singleinterclavicle resembles that ofIchthyostega, an indicationYmeria may have resembled this genus in the post-cranial skeleton.[1]
Ymeria is primarily known from a partialholotype skull including the lower jaws andpalate, as well as impressions of the shoulder girdle. The holotype comes from the southern slope of Mt. Celsius onYmer Island in northeast Greenland. Fossils of Devonian tetrapods likeIchthyostega have been known from Ymer Island since 1929. The skull ofYmeria found in 1947 by a team of paleontologists from Sweden and Denmark. It came from atalus slope or pile of rock fragments at the base ofMount Celsius, encased in a pale redsandstone. The fossil's origin on the mountain has not been identified. There are four formations preserved at Mt. Celsius, all belonging to the largerCelsius Bjerg Group. Since the skull cannot be traced to any one of these formations, its exact age is uncertain.[1]
Swedish paleontologistErik Jarvik, a member of the 1947 expedition, could not assign the skull to eitherIchthyostega orAcanthostega. In 1988, English paleontologistJennifer 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 toIchthyostega, 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]
The cranium of the holotype only preserves a few fragmentary bones. Thepremaxillae (toothed bones at the tip of the snout) were present and seemingly bore indentations at the front for a median rostral. A small bone is preserved behind the right premaxilla. This bone may be a lateral rostral due to possessing a lateral line, but its shape is more similar to a tectal. Rostrals and tectals are small skull bones scattered around the snout, which are present intetrapodomorph fish but lost in true tetrapods. Bothmaxillae (toothed bones at the side of the snout) are well-preserved, but not particularly specialized. The premaxillae and maxilla bear numerous tapering teeth, with about 11 on each premaxilla and up to 24 on each maxilla. This is slightly more thanIchthyostega (which has 8–10 on the premaxilla and 16–23 on the maxilla), andYmeria further differs by having the largest teeth be slightly further forwards in the snout. Bones of thepalate (roof of the mouth), such as thevomers,palatines, ectopterygoids, andpterygoids, are poorly preserved, but similar to those of other Devonian stem-tetrapods (in terms of both shape and dentition) when visible. A sliver of bone near the cheek region may represent a branchial element (gill bone). Preserved fragments of the shoulder girdle resemble those ofIchthyostega, such as smoothclavicles and a pointed rear stalk of theinterclavicle.[1]
The lower jaws were thick and well-preserved. Their external bone texturing consisted of indistinct and shallow pits and grooves. This contrasts withIchthyostega andAcanthostega, which have much more pronounced systems of ridges and pits. The jaw also has a (mostly) openlateral line groove on its outer surface. Like other early tetrapods and their close relatives, there were two main rows of teeth on each lower jaw. The outer (marginal) tooth row was present solely on the dentary bone, which was narrow and had alternating regions of light and absent texturing. It had at most 33 teeth, including a symphysial fang (an enlarged tooth near the chin) which was only slightly larger than the other dentary teeth. The inner tooth row stretched along four plate-like bones: the parasymphysial plate and three coronoid bones. The parasymphysial plate has a large tooth at its front edge, followed by a smaller tooth and a diastema (toothless area), similar toIchthyostega. About 22 teeth were present in a mostly unbroken row along the coronoids. The largest coronoid teeth were present at the front of the first coronoid, the middle of the second, and (to a lesser extent) about a third the way down the rear coronoid. These large teeth are the same size as the dentary teeth. In contrast, even the largest coronoid teeth ofIchthyostega are much smaller than the dentary teeth. Another unique feature ofYmeria is the presence of a large patch of tiny tooth-like denticles on the prearticular bone, which lies directly under the inner main tooth row. This patch of denticles is unknown inIchthyostega or any of its relatives. All ofYmeria's teeth (on both the top and bottom jaws) were sharp but conical, in contrast to the recurved teeth ofIchthyostega.[1]
| |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
.jpg)
.jpg)
