Synchrotron X‐ray imaging of a dichasium cupule of Castanopsis from Eocene Baltic amber

Abstract

Premise of the Study The Eocene Baltic amber deposit represents the largest accumulation of fossil resin worldwide, and hundreds of thousands of entrapped arthropods have been recovered. Although Baltic amber preserves delicate plant structures in high fidelity, angiosperms of the “Baltic amber forest” remain poorly studied. We describe a pistillate partial inflorescence of Castanopsis (Fagaceae), expanding the knowledge of Fagaceae diversity from Baltic amber. Methods The amber specimen was investigated using light microscopy and synchrotron‐radiation‐based X‐ray micro‐computed tomography (SRμCT). Key Results The partial inflorescence is a cymule, consisting of an involucre of scales that surround all four pistillate flowers, indicating a dichasium cupule. Subtending bracts are basally covered with peltate trichomes. Flowers possess an urecolate perianth of six nearly free lobes, 12 staminodia hidden by the perianth, and a tri‐locular ovary that is convex‐triangular in cross section. The exceptional three‐dimensional preservation suggests that the fossil belongs to the extant East Asian genus Castanopsis. The amber inclusion represents the first record of Castanopsis from Baltic amber and the first pistillate inflorescence of Fagaceae from Eurasia. Conclusions The partial female inflorescence reported here provides an important addition to acorns of Castanopsis described from middle Eocene strata of Europe. Furthermore, the intercontinental distribution of Castanopsis in the Eocene is confirmed. The amber fossil also broadens the picture of the Baltic amber source area, indicating oligotrophic, sandy, bog‐like habitats. Finally, this study underscores the great benefit of SRμCT as a powerful tool to investigate plant inclusions from amber in a nondestructive way.

Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests

“…Local Castanopsis associations with the same paleo-Antarctic lineages seen at Laguna del Hunco (as listed earlier), and usually with several of them at a time, are extensively documented in Malesia (1, 2, 23, 24, 32-37). Northern Hemisphere fossils assigned to Castanopsis are younger and never co-occur with PARLs (11,15,85). We find the marked spatial and temporal stability of the old southern associations, requiring a high degree of biome conservatism over time (31,53,54), to be especially compelling evidence of a direct connection from Laguna del Hunco to SE Asia for Castanopsis.…”

“…3). Among Laurasian fossils, Castanopsis kaulii, a spectacular pistillate inflorescence from late Eocene Baltic amber, is clearly consistent with the living genus (15); however, fruits that would confirm its relationships with extant lineages are not yet known. European fruit fossils referred to Castanopsis are nuts that are never found in cupules, making their systematic placement uncertain (85).…”

“…Among its 10 genera and >900 species, numerous Fagaceae are keystone species that define forest structure in biodiverse areas and produce abundant protein-rich, animaldispersed fruits (6,7). The family's fossil record is extensive but entirely restricted to the Northern Hemisphere (8)(9)(10)(11)(12)(13)(14)(15); likewise, biogeographic hypotheses for the living genera do not consider the Southern Hemisphere (16)(17)(18)(19)(20)(21)(22). The southernmost occurrences of Fagaceae are in New Guinea, where a few species of the castaneoid genera Castanopsis and Lithocarpus are abundant (1, 23,24).…”

The largest amber-preserved flower revisited

“…Although their exquisite preservation often allows assignment to genus or even species, most botanical amber inclusions are small in size. For example, inclusions of detached flowers from Baltic amber mostly range between a few millimeters and about 15 mm in size 7,18,19 (Supplementary Table S1), which is also the case for most flower inclusions from other amber deposits (Supplementary Table S1 and references therein). Here, we focus on a corolla inclusion with attached stamens from late Eocene Baltic amber measuring 28 mm in diameter.…”

New Fagaceous pollen taxa from the Miocene Søby flora of Denmark and their biogeographic implications

“…Ilex ) and include evergreen groups (many Castaneoideae), mixed evergreen/deciduous groups ( Quercus ), and purely deciduous groups ( Fagus , Castanea ). This vast ecological diversity evolved since the Late Cretaceous (Herendeen et al, 1995; Takahashi et al, 2008; Friis et al, 2011; Grímsson et al, 2016b; Sadowski et al, 2018). In addition to their modern diversity, the Fagaceae have a remarkably rich and well‐preserved fossil record throughout the Cenozoic (e.g., Mai, 1970, 1989; Crepet and Daghlian, 1980; Manchester and Crane, 1983; Kvaček and Walther, 1988, 1989a,b, 1991, 2010; Crepet, 1989; Crepet and Nixon, 1989a,b; Walther and Zetter, 1993; Mindell et al, 2007, 2009; Denk et al, 2010, 2012, 2017a, 2017b, 2017c, 2019a; Grímsson et al, 2015; Pavlyutkin, 2015; Grímsson, et al, 2016a, 2016b; Wilf et al, 2019) making them a key group for investigating the development of northern hemispheric forest ecosystems through time and to test responses of woody plants to major climatic changes during the past 60 million years.…”