A quarter-billion years ago, life on earth almost died out. As the continents collided and formed a single great land mass – the supercontinent of Pangaea – the global climate changed. It became drier, especially in the interior. Around 96 percent of all the species thronging the seas and 70 percent of the animals on land died off in what has become known as the Great Permian Extinction. The ferns and mosses that had blanketed the land shriveled up, though some survived, and still do, in isolated pockets.
Seed plants had begun to evolve during the Devonian, before the Permian. But to the astonishment of the paleo set, sediment from the Permian period found in Jordan, by the Dead Sea of all places, shows that if anything, it was during that blasted time that new groups of seed plants were evolving; three of them, in fact, one being the earliest known pine trees. The other two later went extinct.
Plants and the Permian extinction
The Permian Extinction is believed to have stretched through about 100,000 years, and to have been the most extreme extinction event the planet has known. Paleontologists largely agree on that. They don't agree on much else, such as what caused it, though recent thinking has been less oriented towards a single event such as a comet impact, and more towards problems in pulses, including the fact that the creation of the supercontinent led to climatic extremes.
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We are not clear on how many plant species met their maker during the Permian Extinction.
"Some estimates say 70 to 80 percent, but I do not know how reliable these numbers are," Prof. Hans Kerp of the Munster University, a coauthor of a paper on the Permian Extinction with Abdalla Abu Hamad, William A. DiMichele and Benjamin Bomfleur, told Haaretz.
Plants in the tropical regions seem to have been devastated, though in the far north (the boreal realm), the effect was less intense.
No plant group went completely extinct as the global climate changed from a greenhouse world in the Permian to a hothouse world in the Triassic (the first period of the Mesozoic), Kerp says. The ferns and mosses survived somewhere, it seems at higher latitudes where the effects of large extinction events were usually less severe.
Gymnosperm seed plants could happily proliferate in the Triassic; among other things, they have internal vascular systems for water transport, so were better adapted to the drier habitats that had become common. The surprise was finding gymnosperm development during the Permian in a tropical area that featured seasonality. Sediments from the Permian had been largely devoid of fossils, Kerp explains.
The climate on that Jordanian coast a quarter-billion years ago was monsoonal, with a long dry season and a short wet season, Kerp says. "This is indicated by the presence of fossil soils, typically with calcarious layers and so-called iron pisoliths, which usually formed in regions with such a climate."
The typical Permian vegetation of ferns, giant horsetails, mosses and their ilk need constant humidity to survive, so they wouldn't have done well in this fickle environment. But the very environmental stresses – the seasonality itself, wet winter, long dry summer – could have driven plant evolution, says the team, Kerp, Abu Hamad, DiMichele and Bomfleur.
Stressed environments prone to disturbances, such as flash floods, erosion and the like forces the life forms to adapt and evolve, or die out, Kerp points out.
In short, the findings indicate that drought-prone tropical habitats like the one found in Jordan served as evolutionary cradles for early plant diversification.
One of the fossil plants found in Jordan's Permian layers is none other than the fathers of pine trees, the Podocarpaceae. They turn out to have begun to evolve well over a quarter-billion years ago – and to have survived the great Permian wipeout, say the scientists after unexpectedly finding fossil twigs from these trees in the Jordanian rocks.
The other two groups predated and then survived the Permian extinction, but eventually died off. One was the Corystospermaceae, a group of seed plants that went extinct some 150 million years ago. The other is the Bennettitales, a "peculiar lineage of extinct seed plants with flower-like reproductive organs," explains Dr. Christina Heimken of the Westfalische Wilhelms-Universitat Munster.
Truth be told, Heimkin says, the paleobotanisns have known that Corystospermaceae evolved in the Permian, and existed where Jordan is today, for about ten years. They had found leaves. Now they have also found the reproductive organs characteristic to this group. Mazal tov.
The discovery of the Jordanian cache of Permian fossils is even more extraordinary because it wasn't the type of area prone to sedimentation. In any case, these were not the first seed plants, but were new groups of seed plants that would become dominant during the Mesozoic.
"Jordan is probably not the area where these plants evolved. This could have happened everywhere where such conditions existed, meaning, a terrestrial region in the equatorial region with a seasonally dry climate," Kerp spells out. "But the floras from Jordan are unique, because no similar floras are known so far. They are very rich and varied. Large specimens are preserved (with leaves up to 60 centimeters long), and last but not least, there is the excellent preservation of the material with superbly preserved cuticles. These are the best preserved cuticles I have seen in my nearly 40-year long career."