Reconstructed a fossil aquatic plant from the Cretaceous found in Catalonia from its plant organs

‘Palaeonitella trifurcata’ is a new fossil species discovered and now reconstructed as if it were a puzzle

UB/DICYT Palaeonitella trifurcata is the name of a new fossil species of freshwater aquatic plant from the Lower Cretaceous that has been discovered and reconstructed by a team of geologists from the University of Barcelona. The reconstruction of the plant, dated between about 125 and 120 million years, has been made from plant organs found separately in a layer of calcareous rock in the Garraf Natural Park, within the municipality of Olivella (Barcelona).

This is one of the few fossil carophytes – multicellular algae that are considered the ancestors of vascular plants – that have been completely reconstructed. This reconstruction has been achieved by fitting together its various organs – the main stem, the branches and the fruiting bodies – as if they were the pieces of a puzzle. The work, published in the Review of Palaeobotany and Palynology magazine, is directed by Carles Martín-Closas, professor at the Faculty of Earth Sciences and the Biodiversity Research Institute (IRBio) of the UB. Experts from the Geomodels Research Institute of the UB and the Tubkal Ingeniería company also participate in the study.

Parataxonomy: a parallel classification

Plants from the past are often preserved in the fossil record with separate organs. That is, on the one hand, there are the leaves; and on the other, the fruits, pollen, wood, roots, etc. This has forced the scientific community to create different species taxa for each of these organs in a parallel taxonomy and nomenclature – parataxonomy – with the perspective of being able to relate the various components and thus reconstruct the entire plant. Only in a few cases, the process culminates with the complete reconstruction of the specimen, so it is usual to work with parataxons.

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The plant has been attributed to the new species Palaeonitella trifurcata and belongs to a group of carophytes that is rarely preserved in a fossil state. It is characterized by the presence of lateral branches with opposite trifurcations and another terminal trifurcation where the oospores are inserted.

«The genus Palaeonitella is a parataxon that has been used to classify the echorticate thalli — without bark — of fossil carophytes. Other species of the same morphogen are known from the Lower Cretaceous – specifically, in Cuenca – and from the Devonian period in Scotland. It has also been described in fossil carophytes from the Permian in Saudi Arabia. All these references belong to very different morphospecies, without any biological affinity for each other, ”explains Professor Carles Martín-Closas, from the Department of Earth and Ocean Dynamics at the UB.

“It is probable,” he continues, “that Palaeonitella trifurcata is a very ancient species of the clade currently represented by the genus Nitella. This would be indicated by characters such as the non-calcified oospore, the ecorticated thallus, and the repeated trifurcations in the vegetative structure. ‘

Why is it difficult to find Palaeonitella fossils in the Lower Cretaceous?

The conservation of carophytes in the fossil record depends on their calcification during the life cycle. Thus, if they do not calcify – as is the case with the current genus Nitella and its fossil ancestors, such as Palaeonitella trifurcata – fossilization only occurs in exceptional cases.

In the case of Palaeonitella trifurcata, the key to its conservation must be found in the tapestries of cyanobacteria that colonized this charophyte. The cyanobacteria formed a calcite crust on the plant – like the sarcophagus of a mummy – that protected the plant from post-mortem degradation.

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“However, the most exceptional thing about the studied fossil is not that it has been preserved to date, but that it has been possible to relate its organs to each other to reconstruct the entire plant”, highlights the researcher.

The study of the sedimentology and the microfossils associated with Palaeonitella trifurcata has made it possible to reconstruct a paleoenvironment with freshwater coastal lagoons, connected laterally with bays protected from marine or brackish salinity, similar to lagoons. In this habitat of the Lower Cretaceous, typical organisms of shallow waters lived, such as benthic foraminifera or chlorophytes of the dasicladales group, an order of the chlorophyll algae.

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