The type species was first described asSaurichthys mougeoti. Following a reinvestigation,Erik Stensiö concluded that this species cannot be ascribed toSaurichthys. He thus erected a new genus, which he named after his colleague Birger Sjöström, who had joined him on an expedition to the Arctic island of Spitsbergen (Svalbard) in 1915.[5]
In mostcladistic analyses,Birgeria and theSaurichthyiformes are recovered as each others' closest relatives.[9] Together, they are also often recovered as stemchondrosteans, closely related tosturgeons andpaddlefish (Acipenseriformes), with their exact relationship to each other and to sturgeons/paddlefish varying depending on the study.[10][11][12][13] However, other studies have suggested that they are not closely related to Acipenseriformes, and instead are part of the stem-group ofActinopterygii, and thus are not closely related to any living group of fish.[9][14]
A few species, such asBirgeria?costata orBirgeria?annulata, are only known from fragmentary material. Their affinity withBirgeria is uncertain. The type material ofBirgeria guizhouensis appears to be lost.[15] A jaw fragment from theLate Triassic of California, described asXenestes velox byDavid Starr Jordan, was tentatively synonymized withBirgeria. With about eight valid species,Birgeria was much less speciose thanSaurichthys.
†B. acuminata(Agassiz, 1843) - Middle Triassic (Anisian) of India, Late Triassic (Carnian toRhaetian) of Europe (Belgium, France, Germany, the Netherlands, Poland, Slovakia, Switzerland, and the United Kingdom)
†B. aldingeriSchwarz, 1970 - Early Triassic (Olenekian) of Norway
†B. americanaRomano et al., 2017 - Olenekian ofNevada, US
†B.? costata(Münster, 1839 ) - Middle Triassic (Anisian to Ladinian) of Germany
†B. groenlandicaStensiö, 1932 - Earliest Triassic (Induan) of Greenland
†B. guizhouensis?Liu et al, 2006 - Carnian of China
†B. liuiJin, 2001 - Ladinian to Carnian of China
†B. mougeoti(Agassiz, 1844) - Induan to Olenekian of France andSvalbard
The scale cover ofBirgeria is reduced. Most of the body is devoid of scales. Scales are only developed on the upper lobe of thecaudal fin and the hind portion of thecaudal peduncle. The scales are small, rhombic and lack a ganoine layer.
Theheterocercal tail fin is large and deeply forked. Thedorsal and anal fins are situated at the same level in the back of the body. The fin rays are segmented.
The eyes were located in the front of the skull. The jaws are long and thegape is large. The "parietals" (postparietals) are small and medially separated by the elongate "frontals" (parietals). Thepostrostral is large. The (rostro-)premaxilla is unpaired. Themaxilla is cleaver-shaped with a large postorbital blade. Two to three rows of conical teeth are present. The teeth normally show cutting edges. The preopercle is boomerang-shaped. The bones of thegill cover are small, often weaklyossified or not ossified at all.
Theaxial skeleton consists of ossifiedneural andhaemal arches, both of which may showspines, and additional supraneurals. Other elements are interpreted as parapophyses. Ossifiedcentra are missing.[15] The axial skeleton is regionalized, meaning that there are differences in bonemorphology between segments of the axial skeleton, although these differences are relatively subtle inBirgeria.[16]
Most species ofBirgeria grew over 1 metre (3.3 ft) in length, some even up to 2 metres (6.6 ft) or possibly more. Some of the largest species are theEarly TriassicBirgeria aldingeri (Spitsbergen) andBirgeria americana (Nevada). They were the first large-bodied predators after thePermian-Triassic mass extinction.[1]
A specimen ofBirgeria nielseni from Madagascar was described as supposedly carrying embryos whose bodies are covered with rhombic scales. However, this interpretation was later dismissed.[18] It is more likely that these "embryos" were actually preyed ray-fins, which would indicate that the diet ofBirgeria included small actinopterygians. UnlikeSaurichthys,Birgeria was probably notviviparous. This view is supported by the fact that fossils withcopulatory organs are yet unknown.
Based on its anatomical features,Birgeria is interpreted as apelagic, swift swimmer. Fossils are sparse, which supports the view that it lived offshore.
^abcRomano, C. & Brinkmann, W. (2009). "Reappraisal of the lower actinopterygianBirgeria stensioei ALDINGER, 1931 (Osteichthyes; Birgeriidae) from the Middle Triassic of Monte San Giorgio (Switzerland) and Besano (Italy)".Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen.252:17–31.doi:10.1127/0077-7749/2009/0252-0017.
^Nielsen, Eigil (1949). "Studies on Triassic fishes from East Greenland 2.Australosomus andBirgeria".Palaeozoologica Groenlandica.3:1–309..
^Stensiö, Erik (1919). "Einige Bemerkungen über die systematische Stellung vonSaurichthys mougeoti Agassiz".Senckenbergiana.1:177–181..
^abNi, P.; Tintori, A.; Sun, Z.; Lombardo, C. & Jiang, D. (2019). "Postcranial skeleton ofBirgeria liui (Osteichthyes, Actinopterygii) from the Longobardian (Ladinian, Middle Triassic) of Xingyi, Guizhou, South China".Swiss Journal of Geosciences.112 (2–3):307–324.doi:10.1007/s00015-018-0329-0.S2CID135305199.
^Bürgin, Toni (1990). "Reproduction in Middle Triassic actinopterygians; complex fin structures and evidence of viviparity in fossil fishes".Zoological Journal of the Linnean Society.100 (4):379–391.doi:10.1111/j.1096-3642.1990.tb01866.x.
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