2018-04-20 00:34:26
More than a century ago, around the middle of the century XIX, an amateur naturalist found a fossilized log that had been unearthed by a flood in Schoharie Creek near the town of Gilboa, about 150 kilometers north of New York City, in the Catskill Mountains. It was a sandstone cast of a tree stump from the Devonian period. The fossil was sent to McGill University in Canada, where it was described by Canadian paleontologist John W. Dawson.
In the 1920s, New York City, in need of water, planned to build a dam at Gilboa. The population had to be moved, and the New York State Museum conducted an exhaustive search that brought to light some fifty fossil stumps up to two meters in diameter.
385 million years ago, in the middle of the Devonian, the region was a plain on the coast of an inland sea, in the tropical zone of the southern hemisphere. There grew the forest of which the Gilboa fossils were part, the oldest we know of. There were still a few million years to go before the first amphibians came out of the sea, but the trees were already receiving visits from arachnids, myriapods and other invertebrates. The climate was seasonal, with periods of drought followed by heavy monsoon rains. When a flood uprooted or buried a tree, the plant died and rotted, and the sand that filled the hole it left ended up turning to rock. This is how these fossils were formed. In some cases, in addition to the base of the trunk, the roots have also been preserved: The trunk sinks underground in the shape of an inverted cone, from which numerous small roots emerge perpendicularly, helping to maintain the tree’s verticality.
In 1926, American paleontologist Winifred Goldring identified the Gilboa fossil stumps as seed ferns, naming them Eospermatopteris, “dawn seed fern.” Goldring was famous for the dioramas of living things from the past that she built at the New York State Museum; one of the most famous precisely reconstructs the Gilboa Fossil Forest.
In 2005, an international group of paleontologists from the State Museum of New York and the universities of Binghamton (New York) and Cardiff, in the United Kingdom, found an intact, complete specimen of a petrified tree in the vicinity of the original site, which for The first time he kept, in addition to the stump of the trunk, the cup. Superficially resembling a palm, the tree, more than twenty feet tall, branches at its top into a tuft of leafless fronds, resembling long, cylindrical brushes, like those used to clean bottles. From the study of this specimen, published in 2007, it is concluded that Eospermatopteris is the same species as Wattieza, another fossil tree described in 1968 from remains found in Belgium.
Wattieza is not a seed fern. It belongs to the class of cladoxylopsids, the first arboreal plants, which are considered ancestors of ferns and horsetails.
Wattieza’s fronds, more than five inches thick, are helically attached to the trunk, drooping, replaced by new ones higher up, as the tree grows. Thus, the bark was marked with scars similar to those of current palm trees. There were probably sterile fronds, in the center of the crown, dedicated to photosynthesis, and fertile, spore-producing fronds, concentrated on the outside of the crown. Since there were no leaves, the shade was scant; the soil, continually fertilized by fallen fronds, was very rich.
Until very recently we knew nothing about the internal structure of these primitive trees; the preserved fossils were no more than molds left behind by the decaying tree. But as of 2012, silicified fossil trunks of another cladoxylopsid have been found in northwest China, which they have named Xinicaulis lignescens, in which the organic tissue is gradually replaced by silicates so that the original structure is preserved. sometimes down to the cellular level. Thus we have been able to know how these trees grew. And we have been surprised: its growth was more complex than that of current trees.
In modern trees, the growth in thickness is produced from the cambium, a layer of tissue located under the bark, which each year produces two layers of cells: the xylem, towards the interior, which forms the rings that allow us to know the age of a tree, and that conducts the crude sap, formed by water and mineral salts, from the root upwards; and the phloem, towards the outside, the tissue in charge of transporting the elaborated sap, which contains the nutrients, from the leaves downwards.
But in the cladoxylopsids it was not like that. The trunk of these trees is a hollow cylinder, whose wall, a few centimeters thick, is formed by hundreds of cylindrical xylems, each one with its cambium layer, and connected by horizontal branches. Each xylem grows independently; as a consequence, the horizontal branches tear and must be rebuilt later. On the other hand, a hollow tree does not support weight as well as a solid one, so the weight of the tree ends up deforming its base, which widens into a bulbous shape.
By the end of the Devonian, forests, which had already spread throughout the world, lowered the global temperature thanks to the massive absorption of carbon dioxide. And they became a new ecosystem that allowed the appearance of new animal species.
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