100-million-year-old conifer tissues from the mid-Cretaceous amber of Charente (western France) revealed by synchrotron microtomography

The largest amber-preserved flower revisited

Amber exquisitely preserves the delicate organs of fossil flowers for millions of years. However, flower inclusions can be rare and usually do not exceed 10 mm in size. Here we report an exceptionally large flower from late Eocene Baltic amber, measuring 28 mm across, which is about three times as large as most floral inclusions. This fossil was described over 150 years ago as Stewartia kowalewskii (Theaceae) and has never been revised. The analysis of pollen extracted from the anthers of the flower inclusion, however, revealed strong affinities to Asian species of Symplocos (Symplocaceae), prompting the new combination Symplocos kowalewskii comb. nov. et emend. This fossil represents the first record of Symplocaceae from Baltic amber and supports affinities of its flora to evergreen broadleaved and mixed mesophytic forests of present-day East and Southeast Asia. The rarity of such large-sized flower inclusions is likely due to the size of the resin outpouring and its properties, which might affect the embedding of plant organs.

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

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.

Staminate flower of Prunus s. l. (Rosaceae) from Eocene Rovno amber (Ukraine)

The late Eocene ambers provide plethora of animal and plant fossils including well-preserved angiosperm flowers from the Baltic amber. The Rovno amber from NW Ukraine resembles in many aspects the Baltic amber; however, only fossilized animals and some bryophytes have yet been studied from the Rovno amber. We provide the first detailed description of an angiosperm flower from Rovno amber. The flower is staminate with conspicuous hypanthium, double pentamerous perianth and whorled androecium of 24 stamens much longer than the petals. Sepals are sparsely pubescent and petals are densely hirsute outside. The fossil shares important features with extant members of Prunus subgen. Padus s. l. (incl. Laurocerasus, Pygeum and Maddenia), especially with its evergreen paleotropical species. It is described here as a new species Prunus hirsutipetala D.D.Sokoloff, Remizowa et Nuraliev. Our study provides the first convincing record of fossil flowers of Rosaceae from Eocene of Europe and the earliest fossil flower of Prunus outside North America. Our record of a plant resembling extant tropical species supports palaeoentomological evidences for warm winters in northwestern Ukraine during the late Eocene, as well as suggesting a more significant role of tropical insects in Rovno amber than inferred from Baltic amber.

Plant Tissue Decay in Long-Term Experiments with Microbial Mats

The sequence of decay in fern pinnules was tracked using the species Davallia canariensis. Taphonomic alterations in the sediment–water interface (control tanks) and in subaqueous conditions with microbial mats were compared. The decay sequences were similar in control and mat tanks; in both cases, pinnules preserved the shape throughout the four-month experience. However, the quality and integrity of tissues were greater in mats. In control tanks, in which we detected anoxic and neutral acid conditions, the appearance of a fungal–bacterial biofilm promoted mechanical (cell breakage and tissue distortions) and geochemical changes (infrequent mineralizations) on the external and internal pinnule tissues. In mats, characterized by stable dissolved oxygen and basic pH, pinnules became progressively entombed. These settings, together with the products derived from mat metabolisms (exopolymeric substances, proteins, and rich-Ca nucleation), promoted the integrity of external and internal tissues, and favored massive and diverse mineralization processes. The experience validates that the patterns of taphonomic alterations may be applied in fossil plants.