New Features of Cyclocarya brownii Manchester & Dilcher from the Late Paleocene of North Dakota, USA

Abstract

Premise of research. Fossil infructescences of the late Paleocene Cyclocarya brownii (Juglandaceae) and winged fruits with newly recognized anatomical structure are described from Almont and Beicegel Creek, North Dakota. These fossils demonstrate that C. brownii has many similarities to extant Cyclocarya paliurus but differs in several morphological and anatomical features. Comparisons of the fossil record of Cyclocarya document the transition of characters in the evolution of the oldest extant genus of Juglandaceae. Methodology. Fossils were digitally imaged with reflected light microscopy (LM). Some specimens were embedded in Ward’s Bio-Plastic synthetic resin, sectioned into wafers, mounted on microscope slides, and imaged with reflected LM. Permineralized specimens from the Beicegel Creek site were prepared with the cellulose acetate peel technique, mounted on microscope slides, and imaged with transmitted LM. Extant fruits of C. paliurus were photographed from freshly collected material. Pivotal results. Infructescences of C. brownii differ from those of modern C. paliurus in several ways. The fossils are racemes with crowded fruits borne on elongate pedicels. In contrast, extant C. paliurus has spikes bearing fewer fruits per infructescence (10 vs. 23). Fruit wall anatomy is similar to that of extant and other known fossil species of Cyclocarya but is more complex and has a distinctive idioblast layer not present in extant Cyclocarya. Fossil pollen is triporate in contrast to the tetraporate pollen in extant Cyclocarya. As in extant Cyclocarya, pollen is borne both in staminate catkins and occasionally in stamens attached to fruits. We document a fossil fruit with stamens bearing pollen. This feature, sporadic in extant Cyclocarya, was also present in the Paleogene. Conclusions. Late Paleocene C. brownii differs in comparison with extant C. paliurus and related fossil forms in infructescence architecture, fruit size and symmetry, fruit wall anatomy, and pollen aperture number. Trends in character evolution from the Paleogene to the present day include (1) changes from helically arranged, densely distributed pedicellate fruits to sessile fruits borne singly along a slender axis, (2) transition from pyramidal to round nutlets with (3) equatorial versus basal wing attachment, (4) a change in pollen aperture number from three to four, and (5) simplification of the fruit wall in extant Cyclocarya.