Foliage and Fruits of Early Poplars (Salicaceae: Populus) from the Eocene of Utah, Colorado, and Wyoming

A new species of poplar is recognized based on abundant specimens from the early Middle Eocene Parachute Creek Member of the Green River Formation in eastern Utah and western Colorado and compared with two other contemporary species. A rare twig bearing both leaves and fruits serves as a Rosetta stone, linking the vegetative and reproductive structures that formerly were only known from dispersed organs. Fruit and foliage characters distinguish Populus tidwellii sp. n. from Populus cinnamomoides (Lesquereux) MacGinitie (typified on specimens from Green River Station, WY), to which the isolated leaves had formerly been attributed. In addition, new data from fruits and foliage confirm that there were two distinct sympatric species of Populus in the Parachute Creek flora (P. tidwellii and Populus wilmattae). These species, together with Pseudosalix and Salix, provide our earliest record of diverse Salicaceae. An interesting feature of P. tidwellii is that the inflorescence is terminal in contrast to extant species of Populus, in which the catkins are axillary and borne on wood of the previous season. In addition, the infructescence shows lateral branching near its base, distinguishing it from the simple racemes of extant species of the genus.