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Holostei is a group of Actinopterygii bony fish. It is divided into two major clades, the Halecomorphi, represented by the single living genus, Amia with two species, the ( Amia calva and Amia ocellicauda), as well as the Ginglymodi, the sole living representatives being the (Lepisosteidae), represented by seven living species in two genera ( Atractosteus, Lepisosteus). The earliest members of the clade, which are putative "Semionotiformes" such as Acentrophorus and Archaeolepidotus, are known from the Guadalupian to Lopingian and are among the earliest known Neopterygii.
Holostei was thought to be regarded as paraphyly. However, a recent study provided evidence that the Holostei are the closest living relatives of the Teleostei, both within the Neopterygii. This was found from the morphology of the Holostei, for example presence of a paired vomer. Holosteans are closer to teleosts than are the Chondrostei, the other group intermediate between teleosts and cartilaginous fish, which are regarded as (at the nearest) a sister group to the Neopterygii.
The spiracles of holosteans are reduced to vestigial remnants and the bones are lightly ossified. The thick scales of the gars are more primitive than those of the bowfin.
In the holosteans a primary pulmonoid (respiratory) swim bladder is still present, a trait that was independently lost in both chondrostei and teleostei, the only other two lineages of fish with a swim bladder (in some teleosts the swim bladder have since evolved to become secondarily respiratory again). Respiratory Biology of Animals: evolutionary and functional morphology
The gars have thick typical of whereas the bowfin has thin bony scales like the teleosts. The gars are therefore in this regard considered more primitive than the bowfin.
The name Holostei derives from the Greek language words holos, meaning whole, and osteon, meaning bone: a reference to their bony skeletons.
Ginglymodi comprises three orders: Lepisosteiformes, Semionotiformes and Kyphosichthyiformes. Lepisosteiformes includes 1 family, 2 genera, and 7 species that are commonly referred to as gars. Semionotiformes and Kyphosichthyiformes are extinct orders.
Halecomorphi contains the orders Parasemionotiformes, Panxianichthyiformes, Ionoscopiformes, and Amiiformes. In addition to many extinct species, Amiiformes includes only 1 extant taxon species that is commonly referred to as the bowfin. Parasemionotiformes, Panxianichthyiformes, and Ionoscopiformes have no living members.
Gars and bowfins are found in North America and in freshwater ecosystems. The differences in each can be spotted very easily from just looking at the fishes. The gars have elongated jaws with fanlike teeth, only 3 branchiostegal rays, and a small dorsal fin. Meanwhile the bowfins have a terminal mouth, 10–13 flattened branchiostegal rays, and a long dorsal fin.
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