54 New Approaches to Old Fossil Collections

Material collected during the early history of paleontology conserves deep time ecosystems and remains available for study. We invite presentations of new analyses and interpretations of well-studied collections in the context of the broader history of the discipline.

Sunday, 19 October 2025
1:30 PM - 5:30 PM
HBGCC, 303AB

Isotopic Insights into the Evolution of Plant-Microbial Nitrogen-Fixing Symbioses

Paper number: 54-6
Program: Technical Sessions
Time: 2:50 PM – 3:10 PM

Authors

Abstract

Nitrogen is an essential nutrient and rate-limiting reactant for terrestrial primary productivity. Among the many innovations plants employ to access N in the environment, plant-microbial nitrogen-fixing symbioses stand out as exceptional arrangements to assimilate nitrogen. Several vascular plant clades – spanning gymnosperms (cycads) and angiosperms (legumes, actinorhizal plants, alders, and Gunnera) – include taxa that forge symbiotic associations with nitrogen-fixing bacteria. The plants recruit bacteria to live in modified organs (e.g., root nodules, coralloid roots) and in turn these bacteria receive carbohydrates in exchange for fixed nitrogen. Such symbiosis is critical for the ecological viability of the host species, and further impacts ecosystems by bringing exogenous nitrogen into the ecosystem. For these reasons, there is great interest in studying the dynamics of symbiotic nitrogen fixation (SNF), so that we can predict how SNF might respond to climate change, or even harness it for agricultural purposes.

Despite this interest in SNF, direct evidence of its evolutionary history of is spotty.  Genetic mechanisms for symbiont recruitment are well-studied, but phylogenetic reconstructions of SNF capability (e.g., nodulation in legumes) leave multiple plausible histories, implicating multiple gains/losses of symbioses across clades containing nitrogen-fixing taxa. This is consistent with strong environmental pressures that select for or against SNF, but to date it has been difficult to identify such drivers of SNF evolution.

We have been developing a new geochemical approach to constrain SNF activity in deep time using nitrogen isotopic analysis of carbonaceous compression fossils. Because plants employing SNF tend to have atmospheric 15N/14N ratios, SNF can be identified using isotopic analysis of preserved leaf material. Comparisons to outgroup plants can determine whether isotopic signatures reflect soil or atmospheric sources. This directly constrains SNF activity in ancestral lineages of extant nitrogen-fixers.

In our first applications of this approach, we find that SNF may be much younger than the age of modern SNF clades. As most clearly demonstrated by cycads, N isotopes suggest a Cenozoic origin of SNF. Preliminary data from Paleocene/Eocene legumes and alders may also suggest later origins of SNF. Our work provides a potential path to quantitatively identify and constrain the timing of SNF origin among disparate plant groups. In this talk we will consider the limits of this proxy and avenues for future work.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412111

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