Gerobatrachus
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''Gerobatrachus'' is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of amphibamid
temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order (biology), order of small to giant tetrapods—often considered Labyrinth ...
(represented by the
type species In International_Code_of_Zoological_Nomenclature, 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 spe ...
''Gerobatrachus hottoni'') that lived in the
Early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...
, 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 Lissamphibia (from Greek λισσός (lissós, "smooth") + ἀμφίβια (amphíbia), meaning "smooth amphibians") is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia ( ...
, the group that contains all modern amphibians including frogs, salamanders, and
caecilian Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...
s, or place modern amphibians far from ''Gerobatrachus'' within a group called
Lepospondyli Lepospondyli is a diverse clade of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco ('' Diplocaulus minimus''), lepospondyls lived from the Visean stage of the Early Carboniferous to the Ea ...
.


Description

The only known specimen of ''Gerobatrachus'' is a nearly complete skeleton (USNM 489135) about long, that is articulated, preserved in ventral view, missing only the stylopodia, zeugopodia, and ventral portions of the skull and pectoral girdle. It is preserved in red siltstone with only its underside exposed. Like other amphibolid 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 amphibolids. 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 optic notch, which is seen other amphibolids 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 Pedicellate teeth are a tooth morphology today unique to modern amphibians Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense ...
, 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 The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...
, 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 The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...
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 In anatomy, the palatine bones (; derived from the Latin ''palatum'') are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxilla, they comprise the hard palate. Stru ...
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 ''Amphibamus'' is a genus of amphibamid temnospondyl amphibians from the Carboniferous (middle Pennsylvanian) of North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemisph ...
'', ''
Doleserpeton ''Doleserpeton'' is an extinct, monospecific genus of dissorophoidean temnospondyl within the family (biology), family Amphibamidae that lived during the Upper Permian, 285 million years ago. ''Doleserpeton'' is represented by a single species, ' ...
'', ''
Eoscopus ''Eoscopus'' is an extinct genus of dissorophoidean euskelian temnospondyl in the family Micropholidae. It is known from Hamilton Quarry, a Late Carboniferous lagerstätte A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte ...
'', and ''
Platyrhinops ''Platyrhinops'' is an extinct genus amphibamid temnospondyl from the Late Carboniferous (late Westphalian stage) of Ohio and the Czech Republic The Czech Republic, also known as Czechia, and historically known as Bohemia, is a landloc ...
'' also have large otic notches; ''Amphibamus'', ''Doleserpeton'', and ''
Tersomius ''Tersomius'' is an extinct genus of dissorophoid temnospondyl within the family Micropholidae. It is known from the early Permian of North America (Oklahoma and Texas). History of study ''Tersomius'' was first named in 1910 by American pal ...
'' also have pedicellate teeth; and ''Amphibamus'' and ''Doleserpeton'' also have a wide parasphenoid plate.


Discovery

''Gerobatrachus hottoni'' was described for the first time on May 22, 2008 in the journal
Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
. The nearly complete
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
skeleton USNM 489135 was collected from a fossil locality known as Don's Dump Fish Quarry in Baylor County, Texas in 1995. It was then rediscovered in the collections of the
National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...
, Washington, D.C., in 2004. The genus name means "elder frog" in
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
, from (géros, "elder") and (batrakhos, "frog"). The species name honors Nicholas Hotton III, a paleontologist who had worked for the National Museum of Natural History.


Relationships

When ''Gerobatrachus'' was first described in 2008, it was incorporated into a
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis that found it to be the sister taxon or closest relative of Batrachia, an evolutionary group that includes living frogs and salamanders but not caecilians, which are the third major lineage of modern amphibians (the three main groups of modern amphibians are collectively known as lissamphibians). As was the case in some previous analyses, caecilians were found to be the descendants of a group of small amphibious Paleozoic tetrapods called Lepospondyli while frogs and salamanders had an independent origin within Temnospondyli. The phylogeny supported by the 2008 analysis has been called the "
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
hypothesis" and is one of three main hypotheses for lissamphibian origins. The other two hypotheses are the "lepospondyl hypothesis", which places all lissamphibians as the closest relatives of a group of lepospondyls called lysorophians, and the "temnospondyl hypothesis", which has all lissamphibians derived from amphibamid temnospondyls like ''Gerobatrachus''. Paleontologists David Marjanović and Michel Laurin published studies in 2008 and 2009 that supported the lepospondyl hypothesis. Their phylogenetic analyses confirmed that ''Gerobatrachus'' was an amphibamid temnospondyl, and since all modern amphibians nested within Lepospondyli, it was positioned far from the ancestry of frogs and salamanders. A 2012 study of the stem-
caecilian Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...
'' Eocaecilia'' found ''Gerobatrachus'' to group within Lissamphibia. In this phylogeny, ''Gerobatrachus'' is more closely related to frogs and salamanders than it is to caecilians, meaning that ''Gerobatrachus'' would have been a descendant of the last common ancestor of modern amphibians. Below is a
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
from the original 2008 phylogenetic analysis (left) and a cladogram from the 2012 analysis (right):


References

{{Taxonbar, from=Q132803 Amphibamids Dissorophidae Cisuralian temnospondyls of North America Permian geology of Texas Transitional fossils Fossil taxa described in 2008 Prehistoric amphibian genera