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Birgeria is a genus of carnivorous marine Actinopterygii from the Triassic period. Birgeria had a global distribution, with fossil known from Madagascar, Spitsbergen, Germany, Switzerland, Italy, Slovenia, China, Russia, Canada and Nevada, United States. The oldest fossils are from Induan aged beds of the Wordie Creek Formation of East Greenland.. Birgeria existed throughout the entire Triassic period, from the very beginning just after the Permian-Triassic mass extinction, up to the very end with its extinction during the Triassic-Jurassic mass extinction.
The type species was first described as Saurichthys mougeoti. Following a reinvestigation, Erik Stensiö concluded that this species cannot be ascribed to Saurichthys. 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..
In most cladistics, Birgeria and the Saurichthyiformes are recovered as each others' closest relatives. Together, they are also often recovered as stem Chondrostei, closely related to sturgeons and paddlefish (Acipenseriformes), with their exact relationship to each other and to sturgeons/paddlefish varying depending on the study. However, other studies have suggested that they are not closely related to Acipenseriformes, and instead are part of the stem-group of Actinopterygii, and thus are not closely related to any living group of fish.
A few species, such as Birgeria? costata or Birgeria? annulata, are only known from fragmentary material. Their affinity with Birgeria is uncertain. The type material of Birgeria guizhouensis appears to be lost. A jaw fragment from the Late Triassic of California, described as Xenestes velox by David Starr Jordan, was tentatively synonymized with Birgeria. With about eight valid species, Birgeria was much less speciose than Saurichthys.
The following species are known:
The heterocercal tail fin is large and deeply forked. The fish fin 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 the gape is large. The "parietals" () are small and medially separated by the elongate "frontals" (parietal bone). The postrostral is large. The (rostro-)premaxilla is unpaired. The maxilla 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 the gill cover are small, often weakly ossified or not ossified at all.
The axial skeleton consists of ossified neural arches and haemal arches, both of which may show neural spines, and additional supraneurals. Other elements are interpreted as parapophyses. Ossified centra are missing. The axial skeleton is regionalized, meaning that there are differences in bone morphology between segments of the axial skeleton, although these differences are relatively subtle in Birgeria.
Most species of Birgeria grew over in length, some even up to or possibly more. Some of the largest species are the Early Triassic Birgeria aldingeri (Spitsbergen) and Birgeria americana (Nevada). They were the first large-bodied predators after the Permian-Triassic mass extinction.
A specimen of Birgeria nielseni from Madagascar was described as supposedly carrying embryos whose bodies are covered with rhombic scales. However, this interpretation was later dismissed. It is more likely that these "embryos" were actually preyed ray-fins, which would indicate that the diet of Birgeria included small actinopterygians. Unlike Saurichthys, Birgeria was probably not viviparous. This view is supported by the fact that fossils with sex organ are yet unknown.
Based on its anatomical features, Birgeria is interpreted as a pelagic, swift swimmer. Fossils are sparse, which supports the view that it lived offshore.
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