Aspidorhynchidae


Aspidorhynchidae, meaning "shield", and ῥύγχος 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:
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.