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Middle East fossils push back origin of key plant groups millions of years


This fossilized twig of a podocarp conifer dates back to the Permian, much earlier than expected.

P. BLOMENKEMPER ET AL., SCIENCE, 362/1414 (2018)

By Elizabeth Pennisi

Paleobotanists exploring a site near the Dead Sea have unearthed a startling connection between today’s conifer forests in the Southern Hemisphere and an unimaginably distant time torn apart by a global cataclysm. Exquisitely preserved plant fossils show the podocarps, a group of ancient evergreens that includes the massive yellowwood of South Africa and the red pine of New Zealand, thrived in the Permian period, more than 250 million years ago. That’s tens of millions of years earlier than thought, and it shows that early podocarps survived the “great dying” at the end of the Permian, the worst mass extinction the planet has ever known.

Reported in this week’s issue of Science, the fossils push back the origins not just of podocarps, but also of groups of seed ferns and cycadlike plants. Beyond altering notions of plant evolution, the discoveries lend support to a 45-year-old idea that the tropics serve as a “cradle” of evolution. “This is an exciting paper,” says Douglas Soltis, a plant evolutionary biologist at the University of Florida (UF) in Gainesville. By revealing the richness of the Permian tropics, he adds, “The findings may also help researchers decide where to look for crucial fossil discoveries.”

During the Permian, from 299 million to 251 million years ago, Earth’s landmasses had merged to form a supercontinent, bringing a cooler, drier climate. Synapsids, thought to be ancient predecessors of mammals, and sauropsids, ancestors to reptiles and birds, roamed the landscape. Simple seed-bearing plants had already appeared on the scene. Family trees reconstructed from the genomes of living plants suggest more sophisticated plant groups might also have evolved during the Permian, but finding well-preserved plant fossils from that time has been difficult.

About 50 years ago, a German geologist described the Umm Irna formation, a series of sedimentary layers exposed along the Jordanian coast of the Dead Sea. Working at the site in the early 2000s, paleontologist Abdalla Abu Hamad, now with the University of Jordan in Amman, discovered some exquisitely preserved plants from Permian swamps and drier lowlands.

After moving to the University of Münster in Germany for a Ph.D., he teamed up with paleobotanists there to analyze hundreds of newly collected plant fossils, including leaves, stems, and reproductive organs. Many of the fossils preserve the ancient plants’ cuticle, a waxy surface layer that captures fine features, such as the leaf pores called stomata. That made it possible for the team to positively identify many of the plants.

Freed from rock by a strong acid, this fossilized frond preserves enough detail to identify it as a seed fern.

P. BLOMENKEMPER ET AL., SCIENCE, 362/1414 (2018)

“At first, we couldn’t really believe our eyes,” Benjamin Bomfleur, a study co-author at the University of Münster, recalls. Many were plants thought have gotten their start later in the Mesozoic, the period when dinosaurs ruled. Along with the podocarps, they identified corystosperms, seed ferns common in the dinosaur age but extinct now, and cycadlike Bennettitales, another extinct group that had flowerlike reproductive structures.

Such finds could help resolve an ongoing debate about why the tropics have more species than colder latitudes do. Some have suggested that species originate at many latitudes but are more likely to diversify in the tropics, with its longer growing seasons, higher rainfall and temperatures, and other features. But another theory proposes that most plant—and animal—species actually got their start near the equator, making the low latitudes an evolutionary “cradle” from which some species migrate north and south. The new work “supports the idea of the evolution cradle,” Bomfleur says. Philip Mannion, a paleontologist at Imperial College London agrees, but says the case is not fully settled. “Our sampling of the fossil record is extremely patchy throughout geological time and space,” he cautions.

It’s not clear how the newfound Permian plants made it through the great dying, a 100,000-year period when, for reasons that are still unclear, 90% of marine life and 70% of life on land disappeared. But their presence in the Permian raises the possibility that other plant groups thought to have later origins actually emerged then in the tropics, says UF plant evolutionary biologist Pamela Soltis. If these select plants survived the mass extinction, she says, “Perhaps the communities they supported may have been more stable as well.”

Source: Science Mag