Gnathorhiza

Gnathorhiza is an extinct genus of prehistoric lobe-finned fish (lungfish) which lived from the Carboniferous period to the Early Triassic epoch.[1] It is the only known lungfish genus to have crossed the Permo-Triassic boundary. Several species have been described, ranging in size from 5 to 50 centimeters.[2]

Gnathorhiza
Restoration
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum:
Vertebrata
Class:
Sarcopterygii
Subclass:
Dipnoi
Family:
†Gnathorhizidae
Genus:
†Gnathorhiza

Cope, 1883
Type species
†Gnathorhiza serrata
Cope, 1883
Species
  • G. pusilla Cope, (1877)
  • G. serrata Cope, 1883
  • G. dikeloda Olson, 1951
  • G. noblensis Olson & Daly, 1972
  • G. bothrotreta Berman, 1976
  • G. bogdensis Minkh, 1977
  • G. lozovskii Minikh, 1977
  • G. otschevi Minikh, 1977
  • G. triassica Minikh, 1977
  • G. tatarica Minikh, 1989

Taxonomy

Gnathorhiza serrata was originally described by Edward Drinker Cope in 1883 based on fossilized teeth collected in Permian strata from Texas. Cope stated in the original description of the species that it may belong to the petalodont family, though he personally found that doubtful and thought the tooth roots were more like those of sharks.[3] Later authors would recognize Gnathorhiza to be a lungfish and more species would be assigned to the genus. In 1934, Romer and Smith assigned the genus to the family Lepidosirenidae on the basis that Gnathorhiza exhibited a similar shearing motion of the jaw to extant Lepidosiren and Protopterus.[4] In 1977, Gnathorhiza would be moved to a new family, Gnathorhizidae, which was thought to be the sister group to the extant Lepidosirenidae and Protopteridae 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]

Distribution and habitat

The numerous described species of Gnathorhiza have been found across the USA, Germany, Russia, Poland and Kazakhstan, with additional remains referred to Gnathorhiza sp. being known from the Gharif Formation of Oman and the Corumbatai and Rio do Rasto formations of Brazil.[2]

The oldest records of the genus come from Pennsylvanian-aged deposits in the United States, including freshwater deposits of the El Cobre Canyon Formation, estuarine strata in the Black Prince Limestone and marine strata in Kansas. During the early Permian, Gnathorhiza diversified into multiple species within North America, and also spread towards Germany and Oman. In the late Permian, the genus is recorded only in Russia and Brazil. Four species of Gnathorhiza are known from the Triassic, found in Russia, Poland and western Kazakhstan. The geologically youngest record of the species is from the uppermost Olenekian of Russia, where it is outnumbered by other lungfish genera (including Arganodus, Ceratodus and Ptychoceratodus) which may have replaced it.[2]

Paleobiology

Aestivation
Model of Gnathorhiza aestivating within a burrow, on display at the Cincinnati Museum Center

Fossilized remains of Gnathorhiza have been discovered within burrows at various localities, including the Wellington, Arroyo, and Matfield formations.[1][7][8] These burrows were likely constructed by the lungfish for aestivation, a behaviour observed in extant Lepidosiren and Protopterus lungfish, and the examined Gnathorhiza burrows are similar to those of extant lungfish. However, Gnathorhiza would have evolved this behavior independently from the extant taxa. In the Arroyo 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]

Euryhalinity

Because specimens have been found in Carboniferous and Permian strata representing freshwater, estuarine and marine environments, it is believed that Gnathorhiza was a euryhaline lungfish, capable of adapting to a wide range of salinities. 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 the Triassic period are all from freshwater deposits, suggesting the genus became strictly freshwater after the Permian–Triassic extinction event.[2]

References
  1. Carlson, Keith J. (1968). "The Skull Morphology and Estivation Burrows of the Permian Lungfish, Gnathorhiza Serrata". The Journal of Geology. 76 (6): 641–663. Bibcode:1968JG.....76..641C. doi:10.1086/627389. JSTOR 30066273. S2CID 129493320.
  2. Skrzycki, P. (2016-01-15). "The westernmost occurrence of Gnathorhiza in the Triassic, with a discussion of the stratigraphic and palaeogeographic distribution of the genus". Fossil Record. 19 (1): 17–29. doi:10.5194/fr-19-17-2016. ISSN 2193-0074.
  3. Cope, Edward Drinker (1883). Fourth Contribution to the History of the Permian Formation of Texas. Vol. 20. pp. 628–636.{{cite book}}: CS1 maint: date and year (link)
  4. Romer, Alfred S.; Smith, Homer J. (1934). "American Carboniferous Dipnoans". The Journal of Geology. 42 (7): 700–719. ISSN 0022-1376.
  5. Kemp, Anne; Cavin, Lionel; Guinot, Guillaume (2017-04-01). "Evolutionary history of lungfishes with a new phylogeny of post-Devonian genera". Palaeogeography, Palaeoclimatology, Palaeoecology. 471: 209–219. doi:10.1016/j.palaeo.2016.12.051.
  6. Brownstein, Chase Doran; Harrington, Richard C; Near, Thomas J. (2023-04-12). "The biogeography of extant lungfishes traces the breakup of Gondwana". Journal of Biogeography. 50 (7): 1191–1198. doi:10.1111/jbi.14609. ISSN 0305-0270.
  7. Dalquest, Walter W.; Kocurko, M. John; Grimes, John V. (November 1989). "Aspects of the postcranial skeleton of the Lower Permian lungfish, Gnathorhiza". Journal of Paleontology. 63 (6): 919–930. doi:10.1017/S0022336000036623. ISSN 0022-3360.
  8. McCahon, Thomas J.; Miller, Keith B. (2015-10-01). "Environmental significance of lungfish burrows (Gnathorhiza) within Lower Permian (Wolfcampian) paleosols of the US midcontinent". Palaeogeography, Palaeoclimatology, Palaeoecology. 435: 1–12. doi:10.1016/j.palaeo.2015.05.032. ISSN 0031-0182.
  9. Berman, David S. (1976). "Cranial Morphology of the Lower Permian Lungfish Gnathorhiza (Osteichthyes: Dipnoi)". Journal of Paleontology. 50 (6): 1020–1033. ISSN 0022-3360.


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