Fossils upend long-standing theory about angiosperm evolution
Fruit salad may have been on the menu of some dinosaurs.
More than 74 million years ago, there was a richer garden of fruit and seed plants than scientists thought. Fossil analysis suggests that large forest trees spread winged seeds and fed animals fleshy blueberry-sized fruits long before the end of the reign of dinosaursreport the researchers on June 25 in Science.
These results shake up our understanding of the evolution of flowering plants, say paleoecologist Jaemin Lee of the University of California, Berkeley and his colleagues. The traditional view was that flowering plants, called angiosperms, primarily used wind and other inanimate means to disperse seeds until the late Cretaceous. It wasn’t until an asteroid hit Earth 66 million years ago, causing a mass extinction that killed off all non-avian dinosaurs and ushered in the age of mammals, that angiosperms have hit their stride. In this new world, plants developed a larger and more complex repertoire of seeds and fruits that were often eaten and carried away by animals.
But an analysis of fossil diaspores – structures that include fruits and seeds – from the Late Cretaceous calls this account into question. Lee’s team examined 450 fossils discovered in a layer of ancient volcanic ash in south-central New Mexico between 1992 and 2016. The diaspores took on nearly 80 different shapes. Some had winged shapes while more than a third were fleshy like berries. The largest ones were about the size of a small date.
“There are rocks with a group of large diaspores preserved together,” says Lee. “They remind me of punnets of grapes or pistachios.”
Until now, the diversity of diaspore types before the extinction was thought to be low, says Brian Atkinson, a paleobiologist at the University of Kansas in Lawrence who was not involved in the work. And mentions of large, fleshy fruits were rare. “This study suggests that larger diaspores were certainly well established before the end of the Cretaceous,” says Atkinson. “This is a very important discovery.”
Today, animals such as birds, mammals and reptiles regularly eat the fruits of flowering plants, excreting or discarding the remaining seeds. The similarities between these ancient diaspores and modern diaspores strongly imply that Cretaceous animals, perhaps some extinct pterosaurs and rodent-like mammals, would have enjoyed the fruits, Lee and his colleagues think. Cretaceous birds and dinosaurs, known to have fed on diaspores from other plant groups, such as conifers, may have switched to angiosperms when their fruits became available.
Fossilized, diaspore-speckled droppings from the Late Cretaceous discovered in previous work suggest that vertebrates ingested diaspores. But it is difficult to know which animals the droppings belonged to. “Associating droppings with their extinct producers can be difficult,” says Lee.
Overall, the results align with other models of angiosperm evolution in the Cretaceous, says paleobotanist Selena Smith of the University of Michigan in Ann Arbor. Plants were rapidly evolving in terms of leaves, growth forms, and overall size. “It makes perfect sense that reproductive structures evolved in the same way during the Cretaceous, as plants became more efficient and more specialized,” says Smith.
These plants eventually evolved into the dense angiosperm-dominated forests we know today. They simply seem to have gotten their start at a time when the local wildlife was very different. The next step, Atkinson says, is to sample other fossil sites dating to the Late Cretaceous. This would confirm that these fruits filled prehistoric forests around the world.
