Rebuilt a 129 million years old fern

In a recent study published this week in the journal PLoS ONE, paleobiologists at the Universidad Autónoma de Madrid (UAM), in collaboration with the Claude Bernard Lyon 1 University, describe for the first time the growth pattern of the Mesozoic fern Weichselia reticulata, a fossil plant from the exceptional site of Las Hoyas site, whose age is dated at 129 million years.

Plants are essential elements of the ecosystem’s landscape, and therefore, knowing their form and habits is essential to understand and reconstruct ecosystems from the past. However, fossil plants are rarely found complete and their reconstruction in many cases may be impossible. The new reconstruction of the fossil fern Weichselia reticulata is unique in that is based on parsing fragments of small leaves (18 and 30 cm) on the basis of the outstanding discovery in Las Hoyas of previously unknown large leaf fragments which were up to 2 meters long. This allowed the authors to apply a novel protocol based on measurements that provide new keys to address the growth and architecture of ferns. "This study not only provides information about the shape of the plant, but also about the way it grew and how it was in life. You can even infer their ecological adaptations or the type of environment they lived in," says Candela Blanco, lead author and last-year PhD student at the department of Paleontology at the UAM.

New methodologies for the study of fern leaves

The architecture of this plant was recognized as a complex leaf with a central head from which around 14 pinnae radiated. To determine its architecture, a previous model of the areas that make up the whole leaf was necessary, but unknown until the unprecedented discovery of the 2m leaves. The new fronds found at Las Hoyas helped to model the sequential changes that took place along the pinnae as they grew older. Twenty-eight leaf fragments were measured, obtaining more than one thousand measurements of each of their parts (secondary pinnae). This new protocol was applied to modern ferns to verify its validity and to compare the differences between Weichselia and modern ferns, as well as features stemming from distinct adaptations. In fact, it helped to elucidate whether this fern was arborescent, as it had previously been reported.

Elucidating growth, shape, and adaptations

This study is the first one dealing with growth in fossil plants from direct measurements. Such measurements along the leaf capture the degree and direction of maturity. The authors explain that "in the case of Weichselia, there would be two types of growth: from base to apex in the pinnae, and from apex to base in the complete leaf, formed by several pinnae. This discovery is the key that allows a correct reconstruction of the adult leaf, formerly reconstructed with pinnae of different sizes, and now we know that only the complete leaf was first asymmetrical in the younger stages". Moreover, the authors conclude that in Weichselia reticulata the leaves were thick and grew horizontally and in zig zag, a suite of unique traits suggesting an exposition to sunlight that could only be achieved if these ferns were the highest plants in the ecosystem.


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