A unique cache of plant fossils from volcanic deposits in New Mexico contradicts the common narrative that flowering plants were minor players in Earth's forests until dinosaurs disappeared 66 million years ago.
Based on an analysis of large seeds buried under volcanic ash about 74.6 million years ago — nearly 10 million years before a catastrophic asteroid impact wiped out the dinosaurs — UC Berkeley paleobotanists reconstruct a thriving, mature forest dominated by flowering plants, many of which produced relatively large and fleshy fruits, or diaspores.
The discovery calls into question the view that flowering plants, or angiosperms, only took over the planet in the wake of the dinosaurs' extinction, when the proliferation of mammals like rodents and bats made it energetically favorable for plants to produce large, fleshy fruits that mammals could help disperse. After the extinction, angiosperms dominated the world, forming dense forests from pole to pole under greenhouse climates where modern groups of fruit- and seed-eating birds and mammals, such as bats, rodents and primates, radiated and consumed the large diaspores.
"Our results show that, at least in some hot and humid environments during the Late Cretaceous, well before the extinction boundary by 10 million years, angiosperms were already investing more resources into individual diaspores and forming dense forests," said the lead author and UC Berkeley doctoral student Jaemin Lee.
Flowering plants arose about 135 million years ago in the Early Cretaceous and were initially small, weedy and inconspicuous, producing small seeds that dispersed unassisted or with a bit of wind. The story goes that by the Late Cretaceous they had already diversified their size, leaves and flowers, but in the shadow of the dinosaurs, the way they dispersed their seeds did not change.
Contradicting that scenario, the ancient fossilized forest includes large-trunked flowering trees, such as laurel relatives and palms, and a great diversity of other flowering plants, growing alongside more ancient lineages of ferns and redwoods. Unlike other Cretaceous floras where angiosperm diaspore size, on average, were comparable to a poppy seed, the average diaspore size in this fossil forest is comparable to a large blueberry, showing over a hundredfold increase in volume.
This may not sound that big, Lee said, but the large fruits we eat today are the result of centuries of selective breeding. Wild watermelons, for example, were only 5 centimeters (2 inches) wide.
According to Cindy Looy , a Berkeley professor of integrative biology and curator in the UC Museum of Paleontology, the New Mexico site is unique in capturing an ancient environment at a single moment in time, when an ashfall buried an inland forest. Most fossil plant sites consist of material that ended up in lake, river or coastal sediments, which are conducive to fossilization but often represent a mashup of material from different times and habitats.
"This ash came down within days, because ash doesn't stay in the air very long. It's really a snapshot in time," she said. "At the base of the solidified ash layer you can still find ground cover plants. And then a little bit higher up you just see leaves in all kinds of orientations because they were brought down by the ash."
"You can think of it as like a botanical Pompeii, where ashfall deposits preserve everything in position and we can reconstruct the forest structure" Lee said. "These diaspores are preserved together with various leaves and flowers, brought from the canopy down to the forest floor, by the ashfall."
The team that consists of Looy, Lee, former doctoral student Dori Contreras, and their colleagues will publish their study in the xx issue of the journal Science.
Fossil-bearing tuff
Flowering plants, or angiosperms, now dominate Earth's flora and comprise all the food we eat, from staple grains and spices to squashes and avocados. What we refer to as fruits, grains, and nuts, biologists call diaspores: the seed(s) and associated structures that help the seeds disperse. Their size gives an idea of the ecological strategies of plants. Numerous, tiny poppy seeds, for example, are dispersed unassisted or by wind after they are released. On the other hand, producers of many large fleshy fruits, like peaches, invest a huge amount of resources per dispersed seed and often require large animals, such as humans, to spread them.
Today, angiosperm diaspore size ranges from tiny, dust-like orchid seeds that are only a few micrometers across and lack nutrient reserves, to the giant double coconut, a palm fruit that weighs up to 55 pounds. The greater the investment from the parent plant, the higher the chance of seedling survival and dispersal.
The solidified ash deposit, referred to as Dori's tuff, is about three-quarters of a mile (1.2 km) long and part of the Jose Creek Formation in New Mexico, not far from Truth or Consequences. When the ash was deposited after a nearby volcanic eruption, the site was about 200 kilometers west of the coast of the Western Interior Seaway, which at the time divided eastern from western North America.
The forest at the time of its inundation by ash was situated in the mid-latitudes, though the Earth was much warmer then and the site more closely resembled a tropical forest, Lee said.
While dinosaurs — including a large Tyranosaurus species — have been found in the area, Dori's tuff is best known for its abundant fossilized plants. Contreras, during her time as a doctoral student with Looy, conducted extensive excavations at two dozen quarry sites within the tuff, digging up thousands of fossilized leaves, fruits and flowers. Now a curator at the Perot Museum of Nature and Science in Dallas, Contreras is finalizing her analysis of the leaves, many from plants that are now extinct.
Co-author Garland Upchurch of the University of Colorado Museum of Natural History in Boulder has also excavated at the site and is leading an analysis of the wood fossils, which include some of the largest Cretaceous angiosperm trunks known to date. Lee, who studies animal-plant interactions, focused on the diaspores in these deposits.
Other fossil sites from the Late Cretaceous led paleobotanists to believe that many angiosperms were still low growing, formed open vegetation, and produced small seeds that were dispersed without assistance. One theory was that dinosaur disturbances, like trampling, kept angiosperms from forming dense forests and suppressed the evolution of mammals that eat and disperse diaspores.
Instead, Lee and others found that the tuff contained large fruits from flowering trees — many of which formed the canopy, based on the remains of fossilized trunks — growing alongside mature conifers, including redwoods..
"This is the first record of pretty sizable fruits and seeds at the assemblage level—with a total of nearly 80 distinct types including several forms reaching about an inch in length — in the Cretaceous. This suggests that plant-animal interactions and the formation of angiosperm-dominated dense forests likely evolved before the end-Cretaceous extinction and subsequent ecological restructuring," Lee said.
"That animals were eating large fleshy diaspores during that time is not a surprise because other seed plants, such as ginkgos, were already producing them and had been for a very long time," Looy said. "This fossil flora suggests that these animals were already moving over to eating bigger seeds produced by angiosperms 75 million years ago. This is a surprise, because people thought they didn't exist yet. And here they are."
"We still don't know what drove the initial rise in angiosperm diaspore size," Lee added. "It was probably multifaceted ecological factors, and different groups of angiosperms may have developed larger diaspores for different reasons. But at least now we know that it wasn't the end-Cretaceous extinction and the following emergence of more modern groups of frugivores that led to the diversification of angiosperm reproductive strategies. It coincided with the broader Late Cretaceous ecological radiation of flowering plants. This gives us a new view of the evolutionary ecology of angiosperms that represent 90% of today's land plants, and their potential ecological interactions with animals before the age of the mammals."
"This forest is the earliest known angiosperm-dominated forest with much larger diaspores and one of the most diverse Cretaceous leaf flora ever described," Lee said. "The instantaneous preservation of everything, just with the minimal transport from the forest canopy to the bottom — really enabled the reconstruction of landscape in high detail. It's bringing more light into the complexity of ecological interactions in groups that we no longer have."
James Saulsbury of the University of Kansas in Lawrence, a former Berkeley undergrad, is also a co-author of the paper. The research is funded by the National Science Foundation (DEB 1655973 and 16655985) and the UC Museum of Paleontology.
