| Gnathorhiza | |
|---|---|
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| Restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Class: | Dipnoi |
| Order: | Ceratodontiformes |
| Family: | †Gnathorhizidae |
| Genus: | †Gnathorhiza Cope, 1883 |
| Type species | |
| †Gnathorhiza serrata Cope, 1883 | |
| Species | |
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Gnathorhiza is an extinctgenus ofprehistoriclobe-finned fish (lungfish) which lived from theCarboniferousperiod to theEarly Triassicepoch.[1] It is the only known lungfish genus to have crossed thePermo-Triassic boundary. Several species have been described, ranging in size from 5 to 50 centimeters.[2]
Gnathorhiza serrata was originally described byEdward Drinker Cope in 1883 based on fossilized teeth collected inPermian strata fromTexas. Cope stated in the original description of the species that it may belong to thepetalodont family, though he personally found that doubtful and thought the tooth roots were more like those ofsharks.[3] Later authors would recognizeGnathorhiza to be alungfish and more species would be assigned to the genus. In 1934,Romer and Smith assigned the genus to the familyLepidosirenidae on the basis thatGnathorhiza exhibited a similar shearing motion of the jaw to extantLepidosiren andProtopterus.[4] In 1977,Gnathorhiza would be moved to a new family,Gnathorhizidae, which was thought to be thesister group to the extantLepidosirenidae andProtopteridae based on morphological evidence.[5] More recent phylogenetic analyses however has recovered Gnathorhizidae as a basal family of lungfish, not closely related to any extant lungfish families.[6]
The numerous described species ofGnathorhiza have been found across theUSA,Germany,Russia,Poland andKazakhstan, with additional remains referred toGnathorhiza sp. being known from theGharif Formation ofOman and theCorumbatai andRio do Rasto formations ofBrazil.[2]
The oldest records of the genus come fromPennsylvanian-aged deposits in the United States, including freshwater deposits of theEl Cobre Canyon Formation,estuarine strata in theBlack Prince Limestone and marine strata in Kansas. During theearly Permian,Gnathorhiza diversified into multiple species within North America, and also spread towards Germany and Oman. In thelate Permian, the genus is recorded only in Russia and Brazil. Four species ofGnathorhiza are known from theTriassic, found in Russia, Poland and western Kazakhstan. The geologically youngest record of the species is from the uppermostOlenekian of Russia, where it is outnumbered by other lungfish genera (includingArganodus,Ceratodus andPtychoceratodus) which may have replaced it.[2]
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Fossilized remains ofGnathorhiza have been discovered withinburrows at various localities, including theWellington,Arroyo, andMatfield formations.[1][7][8] These burrows were likely constructed by the lungfish foraestivation, a behaviour observed in extantLepidosiren andProtopterus lungfish, and the examinedGnathorhiza burrows are similar to those of extant lungfish. However,Gnathorhiza would have evolved this behavior independently from the extant taxa. In theArroyo Formation, burrows ranged from 4 to 14 centimeters in diameter.[9] Judging from the lithology and form of the burrows,Gnathorhiza used its mouth to excavate them.[1] During aestivation, the fish would have stood within the burrow on its tail, which was stout and strong to support the animal in this position.[7] At some localities, intersecting burrow structure indicates repeated burrowing cycles, and that alternating wet and dry seasons were present.[1]
Because specimens have been found inCarboniferous andPermian strata representingfreshwater,estuarine andmarine environments, it is believed thatGnathorhiza was aeuryhaline lungfish, capable of adapting to a wide range ofsalinities. Furthermore, considering that the oldest known specimens of the genus originate from North America and that it had spread into Europe by the Permian period,Gnathorhiza may have crossed the sea to colonize Europe, further supporting the idea that it was salt-tolerant. However,Gnathorhiza specimens dating to theTriassic period are all from freshwater deposits, suggesting the genus became strictly freshwater after thePermian–Triassic extinction event.[2]
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