Cycads, with frond-like leaves and barrel-like trunks, look like palm trees, although they are not closely related. Cycads are ancient, first appearing fossilized in the early Permian (almost 300 million years ago). They became so abundant in the Mesozoic that it is called the age of cycads, reaching their heyday during the Jurassic. While cycad fossils tend to be plant fragments, paleobotanists have pieced together evidence that fossil cycads were similar to modern ones. Straight, thick trunks with scaly bark were topped by crowns of long, thin leaves. Tree forms as tall as 18 meters (60 feet) provided habitat for dinosaurs. However, their slow growth, thick leaves, tough construction, and toxic chemicals probably deterred many herbivorous dinosaurs from eating them. Cycads declined during the Cenozoic, perhaps among the many plant groups outcompeted by faster-growing flowering plants and conifers. Today, most cycads have a restricted geographic distribution, confined to one continent, or even to one region of a continent, in patches of what was once a more continuous distribution.

Flowering plants (or angiosperms) are the dominant group of plants today, but newcomers compared to others. The earliest, clear evidence of angiosperms is from the Cretaceous (about 100 million years ago). Classifying the earliest angiosperm fossils is difficult because they tend to be leaves and pollen, rather than flowers that would permit conclusive identification. Competing theories explain angiosperm origins: that they lived in disturbed areas along stream corridors from which they invaded lowland habitats; that they began as understory plants in dark forests; that they originated in coastal areas and moved inland; or that they started as aquatic plants. Questions about angiosperm origins led Charles Darwin to describe their origins as an abominable mystery. After they appeared on the scene, angiosperms gradually and then rapidly replaced conifers and seed ferns in ecosystems. Advantages may have been faster reproductive cycles, their intimate relationship with insects for pollination, large photosynthetic leaves, and improved systems to transport water and nutrients. Which combination of characteristics allowed angiosperms to become so successful is a question of continuing debate for paleobotanists.

The great biogeographer Wladimir Peter Koppen once said that plants are crystalized visible climate. He had studied the distribution of modern plants, but there is no reason to believe that ancient plants were not equally sensitive to climate. Indicators of paleoclimate, such as rainfall and surface temperature, can be found in the chemistry of fossil plants and the rocks that surround them. The form of the fossils themselves can also reveal a great deal about climate. For example, plants have tiny openings on their leaves (stomata) through which they absorb CO2 and release oxygen. More stomata occur in low CO2 atmospheres, and fewer in high CO2 environments. Some woody plants have growth rings, showing the alternation of favorable and unfavorable conditions. Leaf shapes can also act as thermometers. Leaves with serrated edges (toothed margins) are more common in cooler climates, whereas smooth-edged leaves dominate in warmer climates. By studying modern forests, and applying the findings to extinct plant communities, past climate conditions can be inferred. Changes in fossil plant assemblages mirror changes in global climate over time.