Osteology and systematic position of the Eocene salmonid Eosalmo driftwoodensis Wilson from western North America

The fossil salmonid dagger Eosalmo driftwoodensis was originally described from fragmentary specimens. Study of new material of this fossil species co nfirms that it is a stem-group salmonine, with a mixture of primitive and d erived salmonine features in its skull, but with its postcranial skeleton e ssentially of modern salmonine construction. Two autapomophies define the g enus dagger Eosalmo: a long anterodorsal process of the subopercle meeting the dorsal edge of the bone at an angle of about 60 degrees, and a thin der mal basihyal plate apparently lacking teeth. Its salmonine relationship is supported by eight derived features: (I) posterior part of frontal widely e xpanded above autosphenotic, (2) hyomandibular fossa on pterotic long, (3) posterior part of endopterygoid extending posteriorly and broadly overlappe d by both metapterygoid and quadrate, (4) premaxillary process of maxilla e xtending dorsally at an angle larger than 10 degrees, (5) infraorbitals 3 t o 5 narrow and covering less than anterior half of hyomandibula, (6) presen ce of suprapreopercle, (7) anterior end of preopercular canal on horizontal arm distinctly turning to anteroventral corner of preopercle, (8) first ur oneural amplified into large fan-shaped stegural, and (9) scales small, wit h more than two lateral line scales per vertebral centrum. Salmonidae are a monophyletic family defined by at least three synapomorphies: posterior su rface of epiotic with sulcus, peg-and-socket connection in caudal skeleton, and tetraploid karyotype. Within the Salmonidae, Thymallinae and Salmonina e form a clade based on features from premaxilla, supramaxilla, anguloartic ular, and supraorbital. (C) 1999 The Linnean Society of London.