History:
to ~150M BC

Paleo-Ecology

The very first grasses have been documented to occur 83 to 55 million years before present (MYBP). These dates have been derived using molecular dating techniques, macrofossil evidence and/or fossilized pollen.

Diagram of grasslands in Cenozoic Period
A scenario for the evolution of grassland ecosystems during the Cenozoic Period in arid to subhumid climatic regions during the past 56 million years. Adapted from Retallack (1997). Note however, that recent research suggests grasslands may have developed during the Cretaceous Period, 145 to 66 MYBP (Prasad et al. 2005).

Development of the Grassland Biome

For many years, researchers thought that grassland biomes evolved ~ 30 MYBP in response to mammalian grazing (Wigand 2007). Evidence for this scenario was that grasses first began to diversify during this period: the fossil record showed rare fossils and pollen ~35 MYBP and molecular dating suggested an earlier diversification around 55 MYBP. Both of these estimates put development of the grassland biome well after dinosaurs had gone extinct (65 million years ago (Kellog 2001).

Special silicaceous structures in grass cells, called phytoliths, were hypothesized to be adaptations to avoid grazers.

Recently, however, analysis of fossilized dinosaur scat (called coprolites) tells a different story. Researchers looked for grass in 67-65 million year old coprolites. They not only found grass, but a diversity of grass types (Prasad, et al. 2005), indicating that grass diversification had occurred much earlier that previously believed.

They proposed that grasses were diversifying as much as 80 MYBP, even before India had lost its connections with other southern continents. Most dinosaurs did not have dental features (such as grinding cheek teeth) to suggest they specialized on grasses – except for one group called sudamericid gondwanatherians that lived in South America, Madagascar, and India. This group had teeth suited for handling abrasive materials, such as grass.

Black and white diagram of the mandible teeth of a Gondwanatheria Sudamerica
Partial mandible of a sudamericid gondwanatherian, which emergeed ~80 MYBP and were among the first mammals to have teeth specialized for grass-eating. Wighart V. Koenigswald, Francisco Goin, and Rosendo Pascual 2014, Wikipedia.

While researchers previously believed that phytoliths—hard, microscopic structures found in some plant tissues—had evolved as a defense to mammalian grazing, some now believe they may have evolved in response to Cretaceous dinosaurs or insects (or have some other origin all together) (Prasad, et al. 2005).

Cretaceous Diorama
Cretaceous Diorama, Dinosaur Hall (Cretacious Period 145 to 66 MYBP), National Museum of Natural History. Alfred F. Harrell 2004, Wikimedia Commons.
Reconstruction of Earth in late Cretaceous Period
Global paleogeographic reconstructed approximation of the Earth in the late Cretaceous period ~80–90 million years ago. Ron Blakey 2008, Wikimedia Commons.

Development of Grasslands and Coastal Prairies in California

Grasses first appeared in California during the Pliocene (5.3- 2.58 MYBP).  The grasslands were generally mesic (moderately moist) and supported at least 19 species of plant-eating megafauna, as diverse as that of East Africa today (Wagner 1989).  The broad plains of the wet California coast started to buckle and lift to form the coast range during the late Pliocene (1.6 to 5 MYBP).

A green plain with yellow flowers running above cliffs that lead down to the ocean.
Coastal terrace prairie at Fort Ross State Historic Park, Sonoma County, 2016.

The Pleistocene (2.58-0.012 MYBP) was a time of intense environmental change for California grasslands:

These dramatic changes created new regional, local and microclimate conditions. Many herbaceous (non-woody) annual and perennial plants in California evolved in the last 1 to 3 million years in response to these climatic and geological changes (Dallman 1998; Raven and Axelrod 1995; Solbrig, et al. 1977).

Coastal prairie plants evolved with the grazing, browsing, and trampling of large mammals (megafauna) during the Rancholabrean Period, from 300,000 to 11,000 years ago (Edwards 1992; Wagner 1989).

The Rancholabrean Age

The Rancholabrean Age is from 300,000 to 11,000 years ago and is named for the fossils found in the Rancho La Brea Tar Pits. It is one of nineteen time intervals in the “North American Mammal Ages” set by paleobiologists beginning 66.5 million years ago, and it ended with the demise of megafauna such as mammoths and bison. Some paleontologists place the beginning of the Rancholabrean at 150,000 years ago marked by the arrival of Bison in North American from Asia. Mammoths occupying the North Coast of California during the Pleistocene destroyed forests helping to convert them to grasslands; their wallowing behavior may have even created some of California’s vernal pools (Parkman 2006; Parkman c2011).

The dawning of the Holocene (12,000 YBP) brought yet another change: the arrival of humans and the end to an ecosystem dominated by grazing megafauna. The broad scale extinctions of these species have generally been attributed to hunting  by humans, but the development of the Mediterranean climate may also have contributed to their demise (Edwards 2007).

California Grasslands Timeline
Approximate timeline for major events in grassland evolution in North America and California.

Megafauna of Coastal Prairies and Grasslands (10,000 bp to 1.6 mybp)

“The diversity of carnivores and scavengers suggests, moreover, that the objects of their mutual alimentary interest—the megaherbivores—were not sparsely distributed in the landscape. They were abundant, and they were intensely active” (Edwards 1995).

Mural of Rancho la Brea Tar Pool
Rancho la Brea Tar Pool. One sloth (Mylodon, now Paramylodon) trapped, two guarding against Sabre Tooth. Condors waiting on McNabb's cypress. In the rear a pool which has yielded much elephant material. San Gabriel range with Mt. Lowe center and Mt. Wilson at right of erect sloth. Old Baldy at right. Rendered by Charles R. Knight, a 9' by 12' mural in the Hall of the Age of Man at the American Museum of Natural History in New York City. Wikimedia Commons.

It is hard to imagine the sheer number of plant-eating animals needed to support the carnivores that roamed California’s grasslands for prey. The size of the grazing/browsing populations must have had an immense impact on California grassland flora and ecosystem processes.  By the late Pleistocene (10,000 BP to 1.6 MYBP), California grasslands supported one of the greatest wildlife assemblages on the Earth. The diversity and abundance of pre-historic grazers, browsers, predators and scavengers may be one of the greatest in the world exceeding that of East Africa (Edwards 2007).

The following list is a sampling of the grazers, browsers, carnivores and scavengers of Pleistocene grasslands (Edwards 1992; Edwards 2007; Parkman 2006; Wagner 1989). Many have been documented in Sonoma and Marin Counties (UCMP 2006). Species that are still extant are marked with an asterisk.

Grazers and Browsers

Note: Underlined Species are still extant today

Columbian mammoth
Skeleton of a Columbian mammoth, Mammuthus columbi, in the George C. Page Museum at the La Brea Tar Pits, Los Angeles. Wolfman SF. 2009, Wikipedia Commons.

Carnivores and Scavengers

Merriam’s terratorn
Merriam’s terratorn, Terratornis merriami. Nobu Tamura 2007, Wikimedia Commons

Small Carnivores and Scavengers Present Since Rancholabrean Times