The axial skeleton of Acipenseriformes is only partially ossified, with the majority of the bones being replaced with cartilage. The notochord, usually only found in fish embryos, is unconstricted and retained throughout life.[6] The premaxilla and maxilla bones of the skull present in other vertebrates have been lost. While larvae and early juvenile acipenseriforms have teeth, the adults are toothless, or nearly so. The infraorbital nerve is carried by a series of separate canals, rather than being within the circumorbital bones. The palatoquadrate bones of the skull possess a cartilaginous symphysis (joint), and also have a broad autopalatine plate, as well as a narrow palatoquadrate bridge, and a quadrate flange. The quadratojugal bone is three-pointed (triradiate), and the dentition on the gill-arch is confined to the upper part of the first arch and to only the first and second hypobranchials.[5] Members of Acipenseriformes retain the ability to sense electric fields (electroreception) using structures called ampullae. This ability was present in the last common ancestor of all living jawed fish, but was lost in the ancestor of neopterygian fish.[7] All acipenseriforms probably possessed barbels like modern sturgeon (which have four) and paddlefish (which have two).[5]
Evolutionary history
Acipenseriforms are assumed to have evolved from a "palaeoniscoid" ancestor. Their closest relatives within the "palaeoniscoids" are uncertain and contested.[5] The ancestors of Acipenseriformes are thought to have split from those of other living fish around the Carboniferous period (360–300 million years ago).[8] The last common ancestor of Acipenseriformes underwent a whole genome duplication event suggested to have occurred around 242–255 million years ago, with the genome subsequently undergoing rediploidization, both before the split between sturgeons and paddlefish, and separately in both lineages after the split.[9]
Eochondrosteus from the Early Triassic (252–247 million years ago) of China has been suggested by some authors to be the oldest acipenseriform.[10] The oldest unambiguous members of the order are the Chondrosteidae, a group of large fish found in marine deposits from the Early Jurassic (201–175 million years ago) of Europe, which already have reduced ossification of the skeleton.[11] The Peipiaosteidae are known from Middle Jurassic-Early Cretaceous freshwater deposits in Asia.[12] The estimated time of the divergence between sturgeons and paddlefish varies. An estimate based on 30 protein-encoding nuclear markers suggest 204.1 million years ago, research on mitochondrial genomes suggest 155.2 million years ago, and Bayesian dating based on the combined matrix of molecular (mitogenomes) and morphological characters set the divergence to 162 (195–137) million years ago.[13]
The oldest known paddlefish is Protopsephurus from the Early Cretaceous of China around 120 million years ago,[14] while the earliest known sturgeons appear in the Late Cretaceous in North America and Asia, around 100–95 million years ago.[15]
A study published in 2020 reported a successful hybridization between a Russian sturgeon (Acipenser gueldenstaedtii) and an American paddlefish (Polyodon spathula), indicating that the two species can breed with one another despite their lineages having been separated for hundreds of millions of years. This has marked the first successful hybridization between members of Acipenseridae and Polyodontidae.[18]
^Venkatesh, B. (December 2003). "Evolution and diversity of fish genomes". Current Opinion in Genetics & Development. 13 (6): 588–592. doi:10.1016/j.gde.2003.09.001. PMID14638319.
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