Nitrogen isotopes reveal independent origins of N2-fixing symbiosis in extant cycad lineages.

Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted populations in tropical and subtropical habitats and, with their conserved morphology, are often called 'living fossils.' All surviving taxa receive nitrogen from symbiotic N2-fixing cyanobacteria living in modified roots, suggesting an ancestral origin of this symbiosis. However, such an ancient acquisition is discordant with the abundance of cycads in Mesozoic fossil assemblages, as modern N2-fixing symbioses typically occur only in nutrient-poor habitats where advantageous for survival. Here, we use foliar nitrogen isotope ratios-a proxy for N2 fixation in modern plants-to probe the antiquity of the cycad-cyanobacterial symbiosis. We find that fossilized cycad leaves from two Cenozoic representatives of extant genera have nitrogen isotopic compositions consistent with microbial N2 fixation. In contrast, all extinct cycad genera have nitrogen isotope ratios that are indistinguishable from co-existing non-cycad plants and generally inconsistent with microbial N2 fixation, pointing to nitrogen assimilation from soils and not through symbiosis. This pattern indicates that, rather than being ancestral within cycads, N2-fixing symbiosis arose independently in the lineages leading to living cycads during or after the Jurassic. The preferential survival of these lineages may therefore reflect the effects of competition with angiosperms and Cenozoic climatic change.

Fossil callimothalloid fungi: Revised taxonomy, modern equivalents and palaeoecology.

This paper presents the reinterpretation of the taxonomic position of fossil epiphyllous callimothalloid fungi belonging to the fossil-genera Callimothallus (hitherto classified to the family Microthyriaceae) and Cribrites (?Microthyriales). These fungi thrived from the Late Cretaceous up to the Holocene. Investigation are based on collected material from the Oligocene of Hungary and the Miocene of Poland and on published data. For the common callimothalloid fungus Callimothallus pertusus, a new combination Neomycoleptodiscus pertusus is proposed. Callimothallus quilonensis is assigned to a new fossil-genus Muyocopromyces as a new combination Muyocopromyces quilonensis. We reconsider the fossil-species Ratnagiriathyrites hexagonalis as a younger synonym of the fossil-genus Cribrites. The geographical range of fossil callimothalloid fungi and their recent counterparts are discussed. Fossil Neomycoleptodiscus, Muyocopromyces, and Cribrites are proposed as reliable climate proxy of warm climate conditions in the past.

First cycad seedling foliage from the fossil record and inferences for the Cenozoic evolution of cycads.

The morphology of the early ontogenetic stages of cycad foliage may help resolve the relationships between extinct to extant cycad lineages. However, prior to this study, fossil evidence of cycad seedlings was not known. We describe a compression fossil of cycad eophylls with co-occurring fully developed leaves of adult specimens from the early Palaeocene ( ca 63.8 Ma) Castle Rock flora from the Denver Basin, CO, USA and assign it to the fossil genus Dioonopsis (Cycadales) based on leaf morphology and anatomy. The new fossil seedling foliage is particularly important because fully differentiated pinnate leaves of adult plants and the eophylls belong to the same species based on shared epidermal micromorphology, therefore, increasing the number of morphological characteristics that can be used to place Dioonopsis phylogenetically. Significantly, the seedling fossil has a basic foliage structure that is very similar to seedlings of extant cycads, which is consistent with a cycadalean affinity of Dioonopsis. Nevertheless, the set of morphological characters in the seedling and adult specimens of Dioonopsis suggests a distant relationship between Dioonopsis and extant Dioon. This indicates that extinct lineages of cycads were present and widespread during the early Cenozoic (Palaeogene) coupled with the subordinate role of extant genera in the Palaeogene fossil record of cycads.