Seed-cone architecture of Pararaucaria utahensis Gee, Howell et Dayvault sp. nov. (Cheirolepidiaceae) from the Upper Jurassic Morrison Formation of Utah, USA.

PREMISE: Although Pararaucaria seed cones were first described 90 years ago, only in the last dozen years have we discovered the greater extent of the species diversity and paleogeographic range of the genus. Pararaucaria is now known across the mid-latitudes of the Lower Jurassic to Lower Cretaceous in the northern and southern hemispheres. Here we describe a new species, Pararaucaria utahensis Gee, Howell et Dayvault sp. nov. from the Upper Jurassic Morrison Formation in Utah, United States. METHODS: We studied the gross morphology of 12 permineralized cones and used micro-computed tomography (microCT) and three-dimensional visualization to investigate cone architecture and internal structure nondestructively. RESULTS: Imaging revealed a slender, cylindrical, woody cone axis with a small pith. The scale and bract traces separate near the axis, with the scale trace becoming flattened distally. Three parastichy spirals wrap around the cone, and each bears six pairs of seeds. The ontogenetic development of seeds and bract/scale complexes in this cone species is generally counterclockwise, from apex to base. In nearly all cases, there are two seeds per bract/scale complex. The cone peduncle of one cone bears small Brachyphyllum-type scale leaves. CONCLUSIONS: In addition to the formal description of a seventh global species of Pararaucaria and the second species in North America, our study of P. utahensis provides clear megafossil evidence for Cheirolepidiaceae in the Upper Jurassic Morrison Formation of western North America. However, the scanty evidence available for the cheirolepidiaceous seed cones in comparison to widespread pollen remains puzzling.