The fossil record of ginglymodians goes back at least to the Anisianstage of the Triassicperiod, over 240 million years ago.[3]Eosemionotus is one of the earliest ginglymodians. Acentrophorus, another taxon from the Middle and Late Permian, and Paracentrophorus from the Early Triassicepoch, could be even earlier members of the group. Ginglymodi was diverse and widespread during the Mesozoicera, but they represent a depauperate lineage today. The group first evolved in marine environments, but several lineages made separate transitions into freshwater environments.[4] The basal ginglymodian order Kyphosichthyiformes is known from a few genera from the Triassic of China, many of which have deep bodies.[5]
Ginglymodi underwent substantial diversification during the Late Triassic and the Late Jurassic, with the Lepisosteiformes and Semionotiformes having likely diverged during the Middle Triassic.[6] Early non-gar ginglymodians of all groups, but especially early lepisosteiforms, show heavy morphological convergence with modern cypriniforms (carp and relatives), suggesting that they may have had a similar ecological niche. Notably, molecular evidence suggests that the cypriniforms may have originated and diversified around the same time and place (Late Jurassic and Early Cretaceous Southeast Asia) that the similar freshwater ginglymodians showed major diversification.[7] Eventually, some ginglymodians evolved a more predatory lifestyle, with the earliest known true gars from the Late Jurassic.[8] Ginglymodians underwent a major decline during the mid-Cretaceous, eventually leaving gars as the only surviving members of the group.[6] Gar fossils have been found on all continents except Australia and Antarctica. Only seven species exist today, distributed in the freshwater systems of North America.[8]
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