Flowering plants, or angiosperms, are the dominant form of plant life on Earth today, but their origins trace back to the age of dinosaurs. The Cretaceous period, from 145 to 66 million years ago, is when these plants first appeared and began to diversify. This era witnessed the evolutionary debut of the flower, a structure that would alter the course of life on land. Understanding the first flowers reveals a turning point in biological history.
The Emergence of Flowering Plants
The first definitive evidence of flowering plants dates to the Early Cretaceous, around 130 to 140 million years ago. This initial evidence comes not from preserved blossoms but from microscopic, single-grooved pollen grains found in rock layers. Following these clues, the fossil record begins to include impressions of leaves and eventually the preserved structures of flowers themselves.
This relatively abrupt appearance and rapid diversification puzzled Charles Darwin, who famously described it as an “abominable mystery.” Before angiosperms, Mesozoic landscapes were dominated by other plants, including:
- Conifers
- Cycads
- Ginkgoes
- Ferns
The sudden rise of a new and complex plant group was a profound evolutionary event.
Profile of an Early Flower
The earliest Cretaceous flowers were small, structurally simple, and often adapted to aquatic or semi-aquatic environments. These pioneering forms frequently lacked distinct petals and sepals, instead featuring a more generalized set of leaf-like structures called tepals. This suggests some of the earliest angiosperm evolution may have occurred in aquatic habitats.
Two of the most informative fossils are Archaefructus from China and Montsechia vidalii from Spain. Dated between 125 and 130 million years old, Montsechia is considered one of the oldest known flowering plants. It was a freshwater aquatic plant that resembled a modern hornwort and had no obvious petals or nectar-producing structures. Its status as a flowering plant is confirmed by its single, enclosed seed—the defining feature of an angiosperm.
Similarly, Archaefructus was an aquatic plant that lacked petals and sepals, possessing reproductive organs on an elongated stem. The existence of these submerged, unassuming plants challenges the traditional image of the first flower.
Cretaceous Floral Expansion
After their initial appearance, flowering plants underwent significant diversification that accelerated through the middle and Late Cretaceous. This expansion is documented by an increase in the variety of pollen found in sediment layers from this period. For instance, tricolpate pollen, a type with three grooves distinctive of a major group of angiosperms called eudicots, first appeared and spread globally during this time.
This evolutionary burst also involved a great expansion in morphological diversity. Research suggests the variety of floral structures during the Early Cretaceous was high, with early flowers displaying more variation in their forms than the average flower does today. As they spread geographically, angiosperms moved from being a rare component of ecosystems to a dominant one, becoming the most abundant group of land plants by the end of the Cretaceous.
Reshaping Ancient Ecosystems
The spread of flowering plants altered Cretaceous ecosystems and terrestrial food webs. A primary change was the development of new relationships between plants and animals, particularly insects. The evolution of flowers with features like nectar led to the co-evolution of specialized pollinators. Fossil evidence, such as a 99-million-year-old beetle preserved in amber and covered in pollen, provides a direct window into these ancient interactions.
This dynamic spurred the diversification of insect groups, including early bees, wasps, and flies that evolved alongside the flowers they fed on. While the impact on large herbivores is less certain, there is evidence that some dinosaurs began to consume these new plants. Fossilized droppings have been found containing angiosperm fragments, and an ankylosaur fossil was discovered with angiosperm fruit in its gut. However, other studies suggest many large dinosaurs continued to feed on ferns and conifers.
The proliferation of angiosperms provided new food sources at the base of the food chain and contributed to major shifts in plant communities across the globe, setting the stage for modern biomes.