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Gerobatrachus
''Gerobatrachus'' is an extinct genus of amphibamid temnospondyl (represented by the type species ''Gerobatrachus hottoni'') that lived in the Early Permian, approximately 290 million years ago (Ma), in the area that is now Baylor County, Texas. When it was first described in 2008, ''Gerobatrachus'' was announced to be the closest relative of Batrachia, the group that includes modern frogs and salamanders. It possesses a mixture of characteristics from both groups, including a large frog-like head and a salamander-like tail. These features have led to it being dubbed a frogamander by the press. Some more recent studies place ''Gerobatrachus'' as the closest relative of Lissamphibia, the group that contains all modern amphibians including frogs, salamanders, and caecilians, or place modern amphibians far from ''Gerobatrachus'' within a group called Lepospondyli.
==Description==
The only known specimen of ''Gerobatrachus'' is a nearly complete skeleton about long. It is preserved in red siltstone with only its underside exposed. Like other amphibamid temnospondyls, ''Gerobatrachus'' has a rounded and flattened head, well-developed limbs, and a small tail. Its vertebral column is somewhat shorter than those of related amphibamids. The large, round head and shortened vertebral column are features ''Gerobatrachus'' shares in common with frogs and the early salamander ''Karaurus''.〔(''Science Daily'', "Ancient Amphibian: Debate Over Origin Of Frogs And Salamanders Settled With Discovery Of Missing Link" ) Accessed 22 May 2008.〕 ''Gerobatrachus'' also has a large embayment at the back of the skull called an otic notch, which is seen other amphibamids and in frogs and supports the tympanum, an eardrum-like structure used in hearing.〔
Many finer details of the skull link ''Gerobatrachus'' with modern amphibians. ''Gerobatrachus'' has a row of very small pedicellate teeth, a feature shared with modern amphibians. Pedicellate teeth are characterized by two layers of hardened dentine, one at the tooth base and one at the tooth tip, surrounding a middle layer of softer, uncalcified dentine. The palate, which in other amphibians bears many distinguishing characteristics, is poorly preserved in ''Gerobatrachus''. The preserved portion lacks the large palatal teeth commonly found in other temnospondyls. The vomer bone is much shorter in length than it is in other amphibamids, but similar in proportion to living amphibians. As in most frogs, the palatine bone forms a narrow strip along the side of the palate. ''Gerobatrachus'' possesses another modern amphibian characteristic at the back of the skull, a widened bone called the parasphenoid basal plate. Many of the features that link ''Gerobatrachus'' with modern amphibians are also seen in other amphibamids. For example, the amphibamids ''Amphibamus'', ''Doleserpeton'', ''Eoscopus'', and ''Platyrhinops'' also have large otic notches; ''Amphibamus'', ''Doleserpeton'', and ''Tersomius'' also have pedicellate teeth; and ''Amphibamus'' and ''Doleserpeton'' also have a wide parasphenoid plate.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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