Advances in the reconstruction of the landscapes of the past

The fossil record of plants presents a series of characteristics that make it difficult to understand the plant landscape to which they belonged. On the one hand, the plants are generally fragmented in the form of leaves or loose branches, making it difficult to know what the entire plant looks like. On the other hand, many times these fragments have been transported to the place where they become fossilized by the wind or water currents, losing information about their place of origin.

Now, Candela Blanco-Moreno, Hugo Martín-Abad and Ángela D. Buscalioni, from the Autonomous University of Madrid (UAM) in Spain, have proposed a new method to infer the transport distance of leaves from their origin to their place of preservation. , taking into account the type of environment (lake or river) in which these leaves were deposited.

The new methodology combines, on the one hand, current knowledge about the way in which leaves are transported by water currents, and on the other, data collected from 3,338 leaf fragments of the fossil fern Weichselia reticulata, belonging to 25 Lower Cretaceous locations ( between 140 and 100 million years ago) from Europe and Africa.

“To minimize the bias of the material studied, we decided to focus on a single species, and thus avoid differences in the size of the fragments that were related to the shape or size of the original leaf”, explain the authors of the study.

Previous studies on the transport of current leaves show that they have a different buoyancy capacity in water depending on their size: the smallest fragments soak up and sink quickly, while the largest ones take longer to completely soak up and can float for longer. weather.

The results obtained by the UAM team show that, by studying the range of sizes of fragments found in different samples (different deposits on a world scale or differentiated areas within a deposit), it is possible to infer, roughly, the distance traveled by the leaf fragments.

Fossil leaf fragments. (Image: Blanco et al. 2022)

“The presence of small fragments in the sample suggests that they come from a nearby area, while the exclusive presence of large fragments suggests that they have been transported over a great distance,” the researchers specify.

Another result that the team highlights is the inference of the environment in which the fossils were deposited from the diversity in size of the fragments. “For this part, only remains of burned leaves were studied, since the flotation capacity is different for charred leaves, dry leaves and fresh leaves.”

In aquatic systems, because streams cause fragment size selection, it is possible to infer the type of aquatic system where deposition occurred from an analysis of fragment size variability.

“The less diverse the size of the fragments, the greater the current present in the environment where they were deposited: in fluvial environments the fragments are of little different size, while in lacustrine environments the fragments are of very different sizes,” the researchers detail. study authors.

“This fragment size analysis -they add- can be applied to plants of any group and any geological age, so we hope it will be very useful for the interpretation of past landscapes in general.”

As the team itself clarifies, by focusing on a specific species, this work also contributes to the knowledge of the diversity of its habitat. The Weichselia reticulata fern is very common and abundant in many Cretaceous paleontological sites in Europe and Africa, making it especially interesting to better understand its contribution to the landscape.

The size ranges of the leaf fragments of this species suggest that this fern lived relatively close to the river beds and lake areas where it was preserved: small fragments are found in fluvial environments, and fragments of very varied sizes in the quietest environments.

Finally, the proportion of burned fragments with respect to fragments with other types of preservation is very high, suggesting that fires often broke out in their natural habitat.

The study is titled “Quantitative plant taphonomy: the cosmopolitan Mesozoic fern Weichselia reticulata as a case study”. And it has been published in the scholarly journal Palaeontology. (Source: UAM)