Due to their highly distinctive morphology, flatfishes were previously treated as belonging to their own order, Pleuronectiformes.

An observation of a flatfish from the Bathyscaphe Trieste at the bottom of the Mariana Trench at a depth of almost 11 km (36,000 ft) has been questioned by fish experts, and recent authorities do not recognize it as valid.

[15] Among the deepwater species, Symphurus thermophilus lives congregating around "ponds" of sulphur at hydrothermal vents on the seafloor.

The primitive spiny turbots include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families.

[1] Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.

[1] In general, flatfishes rely on their camouflage for avoiding predators, but some have aposematic traits such as conspicuous eyespots (e.g., Microchirus ocellatus) and several small tropical species (at least Aseraggodes, Pardachirus and Zebrias) are poisonous.

[9][20][21] Juveniles of Soleichthys maculosus mimic toxic flatworms of the genus Pseudobiceros in both colours and swimming mode.

[1] Flatfishes range in size from Tarphops oligolepis, measuring about 4.5 cm (1.8 in) in length, and weighing 2 g (0.071 oz), to the Atlantic halibut, at 2.5 m (8.2 ft) and 316 kg (697 lb).

[1] The length of the planktonic stage varies between different types of flatfishes, but eventually they begin to metamorphose into the adult form.

The larva also loses its swim bladder and spines, and sinks to the bottom, laying its blind side on the underlying surface.

Richard Dawkins, in The Blind Watchmaker, explains the flatfishes' evolutionary history thus: ...bony fish as a rule have a marked tendency to be flattened in a vertical direction....

[32]The origin of the unusual morphology of flatfishes was enigmatic up to the 2000s, and early researchers suggested that it came about as a result of saltation rather than gradual evolution through natural selection, because a partially migrated eye were considered to have been maladaptive.

This started to change in 2008 with a study on the two fossil genera Amphistium and Heteronectes, dated to about 50 million years ago.

[33][34] The more recently described fossil genera Quasinectes[26] and Anorevus[27] have been proposed to show similar morphologies and have also been classified as "stem pleuronectiforms".

"[34] To explain the survival advantage of a partially migrated eye, it has been proposed that primitive flatfishes like Amphistium rested with the head propped up above the seafloor (a behaviour sometimes observed in modern flatfishes), enabling them to use their partially migrated eye to see things closer to the seafloor.