The Beloniformes is one of three orders within the series Atherinomorpha. One of the other two orders, the Atherini-formes, is thought by some ichthyologists to represent an unnatural grouping of several lineages, while others consider it monophyletic (a natural group). The other order, Cyprin-odontiformes, is the sister-group of the Beloniformes, as evidenced by numerous internal characters, including modifications to the gill arches and the bones surrounding the eyes. Both Cyprinodontiformes and Beloniformes are agreed to be monophyletic. Beloniformes themselves are united by derived internal characters of the gill arches rather than any conspicuous external morphological characters. Five families of fishes make up the order Beloniformes: Adrianichthyidae (rice-fishes), Belonidae (needlefishes), Scomberesocidae (sauries), Exocoetidae (flyingfishes), and Hemiramphidae (halfbeaks). Within these families are 38 genera and about 200 species, 51 of which are either brackish or freshwater, the remainder of which are marine.
The earliest known fossil Beloniformes are just over 50 million years old, and come from two sites: the exocoetids from Monte Bolca in Italy, and the hemiramphids from the Selsey formation in England. Beloniformes are broadly divided into two suborders, the Exocoetoidei (beaked forms: belonids, scomberesocids, exocoetids, and hemiramphids) and the Adrianichthyoidei (lacking a beak; adrianichthyids). Adri-anichthyids were traditionally included within the Cyprino-dontiformes until Rosen and Parenti argued for their inclusion within the Beloniformes in 1981, based mainly on characters of the gill arches and hyoid apparatus. Adri-anichthyids are now considered to be the sister lineage to the rest of the order, within which the sister groups Belonidae-Scomberesocidae and Hemiramphidae-Exocoetidae have been suggested.
Contrary to this scheme, a study based on morphology and molecules, published in 2000 by Lovejoy, places halfbeaks as the ancestral form among the beaked beloniforms. In Love-joy's tree, some halfbeaks are most closely related to needlefishes and sauries, while the marine halfbeak Hemiramphus is sister to flyingfishes. This result refutes an old hypothesis, based on the observation that needlefishes pass through a half-beak stage during their development, that halfbeaks derive from needlefishes via truncation of the development sequence.
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