Flowers turned to animals for pollination during ancient warming

Fifty-six million years ago, Earth warmed fast and ecosystems reshuffled. During the Paleocene-Eocene Thermal Maximum (PETM), global temperatures rose by roughly five degrees Celsius and stayed elevated for well over 100,000 years.

In a new study, experts have reconstructed how flowers and their pollinators responded, showing a swift move toward animal pollination in Wyoming’s Bighorn Basin. The study infers pollination modes directly from the fossil record of pollen.

Ancient warming changed the land

The pace of the global warming event is a big part of the story. On land in the northern Rockies, the climate turned warmer and seasonally drier. Paleosols from the basin document transient drying that altered floodplains and soils.

Earlier palynological work in the same region already showed large, rapid floral turnover across the PETM.

The researchers documented significant shifts in plant communities during the carbon isotope excursion (CIE) and the period that followed.

Evidence of plant pollination changes

The team approached the question using palynology, the study of spores and pollen preserved in sediments.

The researchers examined pollen preserved as clumps, the pollination mode of nearest living relatives (NLRs), and the morphological variety of pollen types across the interval.

Pollen clumps matter because sticky, aggregated grains tend to reflect animal transport rather than wind. When clumps become more common in the same kind of sediments, it flags a shift toward animal pollination, not a quirk of preservation.

Nearest living relatives add a second line of evidence. If a fossil pollen type’s closest modern relatives are pollinated by insects or vertebrates, it strongly hints that the fossil plant was too.

A third line looks at angiosperm pollen disparity, the spread of shapes, apertures, and surface textures. Higher disparity often tracks animal pollination, where more specialized interactions are common.

“These suggest animal pollination became more common and wind pollination less common during the PETM,” wrote Vera A. Korasidis of the University of Melbourne. The synthesis points the same direction. 

Animals supported plant range shifts

During the hot, dry interval, plant immigrants included subtropical lineages that still thrive in today’s warmer climates, such as palms and relatives of silk-cotton trees.

As temperatures rose and rainfall patterns changed, species ranges expanded or contracted, reshaping the region’s flora.

Fossil insect data from the early Eocene are scarce in North America, but amber and leaf damage records confirm that major pollinator groups like bees, beetles, and flies were already diverse.

These insects likely played a role in moving pollen during the PETM, mirroring the shift in floral composition. Their rise aligns with evidence of increased insect herbivory on fossil leaves, suggesting larger and more complex insect communities in warming landscapes.

Small mammals also mattered. Early primates and marsupials arriving in the Bighorn Basin may have supplemented insect pollination by interacting with flowers and dispersing seeds.

Their presence adds another layer to how animal activity supported plant range shifts and helped new plant communities establish in hotter, drier conditions.

Consequences of plant pollination changes

The rise of animal pollination during the PETM meant that food webs adjusted in ways beyond just flowers and insects. Animal-pollinated plants often produce more nectar, fruits, and fleshy seeds, creating fresh resources for vertebrates.

This would have strengthened links between plants, pollinators, and fruit-eating animals, tying community survival more tightly to pollinator presence.

At the same time, the decline of wind-pollinated trees would have reshaped forests. Canopies became more open, floodplains dried, and the local vegetation mosaic shifted.

These landscape changes set the stage for new combinations of species to co-exist, with cascading effects on the mammals and insects that relied on them.

Broader implications of the study

Ancient ecosystems changed dramatically, yet many interactions endured. That speed gap matters when thinking about pollinators.

Reviews of the PETM emphasize that while climate and carbon cycles shifted strongly, only a few groups suffered major extinctions, and rate of change is a key reason.

The pollen evidence from the Bighorn Basin reminds us that interactions can reorganize quickly when climates flip to hotter and drier. It also shows that plants and pollinators can track each other across the map when given time and space.

The practical takeaway is simple. If today’s changes outrun the ability of species to move and adapt, the partnerships that make seeds, fruits, and ecosystems may not keep up.

The study is published in the journal Paleobiology.

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