Aspidorhynchiformes

Aspidorhynchidae (from Neo-Latin "shield-snouts") is an extinct family of ray-finned fish from the Mesozoic Era. It is the only member of the monotypic order Aspidorhynchiformes. Members of the group are noted for their elongated, conical rostrums, of varying length, formed from fused premaxillae. The range of the group extends from the Middle Jurassic to the end of the Maastrichtian, with a potential record from the Late Paleocene. The family and order were described by Pieter Bleeker in 1859.

Taxonomy

Aspidorhynchiformes have generally been recovered as basal members of Teleosteomorpha, more closely related to teleosts than to Holostei. They have often considered to have a sister group relationship with Pachycormiformes, another group of basal teleosteomorphs. However, other studies instead suggest that they successively diverged after one another, with the aspidorhynchids actually being closer to the teleosts than the pachycormids.

Aspidorhynchiformes has one family, which is divided into at least two genera:
* Order †Aspidorhynchiformes Bleeker 1859 spidorhynchida; Aspidorhynchoidei Bleeker 1859** Family †Aspidorhynchidae Bleeker 1859 inctiferidae Silva Santos 1990; Diphyodontidae Jordan 1923*** Genus †'' Jonoichthys'' Gouiric-Cavalli 2015
*** Genus ?†'' Ophirachis'' Costa 1854
*** Genus †'' Platycerhynchus'' Costa 1864
*** Genus †'' Pseudovinctifer'' Arratia 2015
*** Genus †'' Richmondichthys'' Bartholomai 2004
*** Genus †'' Aspidorhynchus'' Agassiz 1833 non Fitzinger 1843
*** Genus †'' Belonostomus'' Agassiz 1834 'Dichelospondylus'' Costa 1856; ''Hemirhynchus">Dichelospondylus.html" ;"title="'Dichelospondylus">'Dichelospondylus'' Costa 1856; ''Hemirhynchus'' Kner 1867 non Agassiz 1844 non Hodgson 1843; ''Diphyodus'' Lambe 1902]
*** Genus †'' Vinctifer'' Jordan 1920

Fossils range have been found in the United States, France, Italy, Russia, Saskatchewan, Alberta, North Dakota, Montana, Wyoming, and Uzbekistan. These fossils range from 167.7 mya ('' Aspidorhynchus'') to 66 mya ('' Belonostomus longirostris'').

Evolutionary history

The earliest known remains of the group are known from the Middle Jurassic of Europe, in what was then the western Tethys Ocean, which was likely the centre of their initial diversification; during the Late Jurassic they dispersed to the waters around the Caribbean, South America and Antarctica. During the Cretaceous they dispersed worldwide. Some members, like '' Belonostomus'', attained a global distribution in both marine and freshwater habitats; others, like '' Vinctifer'', were restricted to the seas around Gondwana, while others like the giant '' Richmondichthys'' were restricted to small geographic regions such as the Eromanga Sea.

The youngest members of the group, belonging to ''Belonostomus'', went extinct at the end of the Maastrichtian during the K-Pg extinction. A specimen from the Late Paleocene of North Dakota suggests they may have persisted into the early Cenozoic, although this may have just been reworked from earlier formations.

Anatomy

The most distinctive feature of the Aspidorhynchiformes are the elongated, tube-like rostrums, which are formed from fused premaxillary bones. The scales are lepidosteoid, similar to those of gars.

Most aspidorhynchids were predatory fish, which is best exemplified by fossils of '' Aspidorhynchus'' from Germany that have been found entangled with those of the pterosaur ''Rhamphorhynchus'', which they appear to have attacked and died with. However, aspidorhynchids primarily fed on small fish and other vertebrates, with these attacks on pterosaurs appearing to be fatal mistakes. In contrast to other aspidorhynchids, the largest member of the family, the giant '' Richmondichthys'' from Australia, evolved clear adaptations for a planktivorous, filter-feeding lifestyle.

References

paleodb

†
Middle Jurassic first appearances
Maastrichtian extinctions
Taxa named by Pieter Bleeker

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