Hettangian (Early Jurassic) plant fossils from Puale Bay (Peninsular terrane, Alaska)

Abstract

Middle Hettangian (Early Jurassic) plant macrofossils from the Kamishak Formation at Puale Bay, Alaska occur mainly as leaves and leafy shoots found together with ammonites that allow precise biostratigraphic age assignment. This new locality is the first in the Jurassic of Alaska where the plant material shows preserved cuticle. Four species of three genera are identified representing three different gymnosperm groups: Otozamites mimetes and O. tenuatus (Bennettitales), Brachyphyllum crucis (Coniferales), and Sagenopteris pualensis Barbacka n. sp. (Pteridospermophyta) which is described as new. Their macro- and micromorphology show xeromorphic characters that suggest either an arid environment or a habitat with soil of increased salinity. The latter inference is preferred because it is consistent with conditions expected on an oceanic island, the mode of preservation, and accompanying marine fossils. Adaptation to coastal habitats, possibly occupied by a halophytic, mangrove-type vegetation, is implied. The new species is thus far only known from Alaska whereas the three other taxa from Puale Bay also occur at a few localities of the Middle Jurassic in the UK but they are not known from any other areas. Paleoclimatic and paleogeographic implications of the Puale Bay flora help constrain models for the tectonic evolution of the Peninsular terrane. The paleobotanic evidence favors a low-latitude setting for the Early Jurassic Talkeetna volcanic island arc to which the Kamishak Formation belongs. Plant dispersal requires proximity of the Peninsular terrane to cratonic North America at least by Middle Jurassic time.