Flat clams represent a loosely defined group of thin-shelled, flat-valved bivalves. Their distribution and high concentration in some, especially Mesozoic, rocks have attracted interest, and several contrasting paleoecologic interpretations have been proposed to explain their occurrence. We present a comprehensive study of Posidonotis semiplicata (Hyatt), an Early Jurassic flat clam common in western North and South America. Posidonotis symmetrica (Hyatt), Posidonotis balteata (Crickmay), and Posidonotis cancellata (Leanza) are interpreted as subjective junior synonyms. In North America, P. semiplicata occurs in the upper Sinemurian to lower Toarcian of several Cordilleran allochthonous terranes. The South American record is restricted to upper Pliensbachian to lower Toarcian occurrences, suggesting that the species originated in the northeast Pacific in late Sinemurian time and spread to the southeast Pacific by the late Pliensbachian. The closely related Posidonotis dainellii (Losacco), sporadically known from the Tethys, is likely derived from P. semiplicata via migration through the Hispanic Corridor and subsequent geographic isolation. The functional morphology of P. semiplicata suggests an early ontogenic byssal attachment followed by a reclining mode of life with adaptation to soft substrates. The species is commonly found forming shell pavements in dark mudrocks with no, or very few, other benthic organisms. It provides an example of an epibenthic bivalve that favoured low-energy, dysaerobic environments, a niche preferentially exploited by flat clams.