Grasses and animals: from Cretaceous traces to grasslands

Grass now covers vast areas of Earth, feeds grazing mammals and supports agriculture. That modern importance can make it seem as if grasses appeared together with open plains. The fossil record tells a different story. The grass family existed during the Cretaceous, but early grasses were components of diverse habitats. Extensive grasslands and the animals specialised for grazing them became widespread much later.

How fossils identify an ancient grass

Leaves and stems of grasses are delicate and rarely fossilise as recognisable plants. Researchers therefore use phytoliths: microscopic silica bodies formed within plant tissues. Their shapes can help identify plant groups, although they do not always identify a species and can be affected by preservation and mixing.

Prasad and colleagues reported grass-like phytoliths in Late Cretaceous coprolites from India, dated to roughly 66 million years ago. Some samples were attributed to large herbivorous dinosaurs. The phytolith shapes indicated several grass lineages, including forms related to groups that later used different photosynthetic pathways. This is direct evidence that grasses existed and were eaten by some dinosaurs. It does not show that grasses were abundant across the landscape or that dinosaurs grazed like modern cattle.

Plants can be old before their habitats dominate

A plant lineage may persist as scattered species in forests, wetlands or river margins for millions of years before becoming ecologically dominant. Cretaceous grasses likely grew among other vegetation, and their early distribution remains difficult to reconstruct because macrofossils are scarce. A phytolith in a coprolite establishes a dietary encounter, not the vegetation cover of an entire continent.

Open grasslands expanded in stages through the Cenozoic. Climate, atmospheric carbon dioxide, fire, soil development, rainfall patterns and herbivore activity varied by region. The replacement of woodland by grass-dominated ecosystems was not a synchronous global event with one cause or one date.

C4 grasses and the later grazing world

Some grasses use the C4 photosynthetic pathway, an adaptation that can improve carbon fixation under hot, bright and seasonally dry conditions. C4 grasses arose before they became dominant over large areas. Carbon-isotope records from soils and herbivore teeth show that C4 ecosystems expanded substantially in many regions during the Miocene, especially from about 8 to 3 million years ago, with timing differing among continents.

That expansion coincided with changes in climate and animal communities, but coincidence alone does not establish a single cause. Lower atmospheric carbon dioxide can favour C4 plants relative to C3 plants under some conditions. Seasonal drought, fire and grazing can also shape grassland structure. Local records are needed to determine which factors mattered in a particular basin.

Animals responded on different schedules

Grazing mammals evolved teeth with high crowns and complex enamel folds that resist abrasion from tough vegetation and grit. Such traits appeared in different lineages at different times. Some herbivores browsed leaves and fruit; others mixed browsing with grazing. A tooth suited to abrasive food can reflect diet and feeding height, but it does not identify a grassland by itself.

Dinosaurs lived through the early history of grasses, but the evidence does not support a Cretaceous world of endless prairies. The coprolite phytoliths show that at least some dinosaurs consumed grass. They do not establish herd grazing, open savannas or a revolution in dinosaur diets. The major ecological transformation associated with extensive grasslands belongs to the later Cenozoic.

A long transition rather than a single revolution

Grasses connect plant evolution to changes in soil, fire, climate and herbivore anatomy. Their story is not a simple replacement of forests by plains. It is a long, uneven interaction between a diverse plant family and changing environments, with local ecosystems following different paths.

The key distinction is between the first evidence for grasses and the much later spread of grass-dominated landscapes. Keeping those events separate lets the fossil record show both what early dinosaurs ate and how much later ecosystems changed.