The flatfish body plan, with its spectacular morphological specializations, has had a long and successful presence among marine teleost fish assemblages dating back at least to the Tertiary, more than 50 million years ago (mya). The oldest flatfish fossils are otoliths dating from early Eocene times (53-57 mya). Eobothus minimus, a representative of the bothoid lineage with uncertain affinities within the group, is the oldest known skeleton representative of the Pleuronectiformes, dating at least to the Lutetian (some 45 mya) in the Eocene. The oldest soles, Eobuglossus eocenicus and Turahbuglossus cuvillieri, both known from single specimens from the Upper Lutetian (Eocene) of Egypt, are early flatfish fossils that appear nearly identical to skeletons of recent soleids. The earliest bothid and pleuronectid fossils also are surprisingly "modern-looking" species dating to the Eocene. The appearance of representatives of different flatfish families in fossil deposits dating to about the same time period indicates that diversification of many of the major lineages of flatfishes took place in the distant past, earlier than 45 mya. The nearly simultaneous appearance of flatfish fossils representing different lineages and encompassing nearly all of the structural features and diversity of the order also may indicate that diversification of these lineages occurred suddenly.
It also is evident from these early fossils that anatomical specializations of flatfishes, including asymmetry of the skull, supracranial extension of the dorsal fin, and modifications of the caudal skeleton, occurred earlier than the period to which these fossil flatfishes belong. When flatfishes evolved and how rapidly they diversified are unresolved questions. Flatfish fossils are unknown from true freshwater sediments, which may indicate that the ancestor of this group was a marine fish. Because fossil flatfishes are relatively rare, our knowledge concerning their evolutionary history is still very incomplete.
Flatfishes have unique morphological specializations related to their asymmetry. Although earlier hypotheses proposed that flatfishes share a common ancestor with some as yet unidentified perciform group of symmetrical fishes, the origin and sister group of flatfishes are unknown. Interrelationships among flatfishes are not well resolved, and work continues toward understanding the evolutionary relationships of these interesting fishes. The order Pleuronectiformes is monophyletic, based on the presence of three derived characters: the ontogenetic migration of one of the eyes; the anterior position of the dorsal fin origin (overlapping the cranium); and the presence of a recessus orbitalis, a muscular, sac-like evagination in the membranous wall of the orbit that can be filled with fluid, causing protrusion of the eyes to a higher position above the surface of the head (and above the bottom when the fish is buried).
Many groups of flatfishes that traditionally were recognized as families and subfamilies do not seem to represent monophyletic groups. The lack of detailed phylogenetic studies for several pleuronectoid groups hinders understanding of the interrelationships of flatfishes even at the family level. Ongoing research using both morphological and molecular approaches is expected to provide interesting results on such interrelationships of pleuronectiform taxa. Changes in our understanding of higher relationships among these fishes can be expected as additional information is discovered.
Two major lineages of flatfishes are recognized: the Pset-toidei, made up of the family Psettodidae, and the Pleu-ronectoidei, containing all remaining flatfish groups. The Psettodids, or spiny-flounders, are a basal group of flatfishes hypothesized to be the sister group of the Pleuronectoidei. This suborder has one family with two species of Psettodes. These are relatively large flatfishes that do not feature strong
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