Elpistostegalia is aclade containingPanderichthys and all more derivedtetrapodomorph taxa. The earliest elpistostegalians, combining fishlike andtetrapod-like characters, such asTiktaalik, are sometimes calledfishapods. Although historically Elpistostegalia (referred to asPanderichthyida) was considered anorder of prehistoriclobe-finned fishes, it wascladistically redefined to include tetrapods.[7]
A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators.[8] Several groups evolved to fill these niches, the most successful were the elpistiostegalians. In such environments, they would have been challenged by periodic oxygen deficiency.[9] In comparable modern aquatic environments like shalloweutrophic lakes and swampland, modernlungfish and some genera ofcatfish also rely on the more stable, atmospheric source of oxygen.[10][11]
Being shallow-water fishes, the elpistostegalians evolved many of the basic adaptions that later allowed the tetrapods to become terrestrial animals. The most important ones were the shift of main propulsion apparatus from the tail fin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen.[12] Both of these appear to be a direct result of moving to an inland freshwater mode of living.[13]
Fossils ofPanderichthys are dated to theGivetian (around 385 million years old) or theFrasnian (around 380–375 million years old[14]).[1] Thus, the oldest bone remains of elpistostegalians appear in the late Middle Devonian or in the early Late Devonian. However,a series of trace fossils from the early Middle Devonian of Poland suggests that tetrapod-limbed animals may have existed as early as in theEifelian, around 395 million years ago.[2]
The anal and posterior dorsal fin supported by a basal plate and three unjointed radials.
The pectoral fin skeleton is composed of bones homologous to the tetrapodhumerus,ulna, andradius, followed by a host of smaller bones anchoring the fin rays; the pelvic fin skeleton similarly hasfemur,tibia, andfibula.
The name, originally coined around thegenusElpistostege, later become a synonym for Panderichthyida.[12] In most analyses, the group as traditionally imagined is actually anevolutionary grade, the last "fishes" of the tetrapodstem line, though Chang and Yu (1997) treated them as the sisterclade toTetrapoda.[15][16] Elpistostegalia was re-defined as aclade containingPanderichthys and tetrapods.[7]
The 2020 study by Cloutieret al. revealed that the paired fins ofElpistostege contained boneshomologous to thephalanges (digit bones) of moderntetrapods. The analysis carried out in this study recoveredElpistostege as the sister taxon of all unequivocally digited vertebrates.[17]
^abLebedev, O. A.; Clément, G. (2018). "New tetrapodomorph vertebrates from the Yam-Tesovo locality (Amata Regional Stage, Middle–Upper Devonian) of Leningrad Region, northwestern Russia".Earth and Environmental Science Transactions of the Royal Society of Edinburgh.109 (1–2):61–73.doi:10.1017/S1755691018000907.
^Chang, M.-M.; Yu, X. (1997). "Reexamination of the relationship of Middle Devonian osteolepids–fossil characters and their interpretations".American Museum Novitates (3189):1–20.
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