Why should we care about fossil leaves?

Fossil leaves from the Florissant fossil beds in Colorado (~ 34 Million years old).

Fossils of dinosaurs, trilobites, and wooly mammoths typically attract more public attention than fossil plants. Although they are not as eye-catching to most people, fossil plants are far more important than this lack of  interest suggests. They help reconstruct the morphology and evolution of long-extinct plant species (which are at the base of food chains, and thus affect whole ecosystems), and they are also among the most important sources of information for scientists trying to understand Earth’s past (paleo-) climates and environments. In recent decades, a growing interest in paleoclimate has accompanied the growing concern about climate change. In addition to documenting past climate change, paleoclimate data play an important role in testing climate models used for predicting future patterns of climate change.[1]

What makes plants such great indicators of former climate? Most plants are sessile, so they are dependent on the climatic conditions of their location. Like all organisms, plants have been forced to adapt to their surroundings to survive, and these adaptations may be recorded in their fossilized remains. Therefore, careful interpretation of a fossil plant provides clues as to in which terrestrial conditions that plant lived. Fossilized leaves are especially good at recording past climatic conditions. Because leaves are the primary photosynthetic organs of a plant, they are optimally adapted to environmental conditions and can react sensitively to environmental changes. Several methods have been developed to use the characteristics of fossil leaves to reconstruct paleoclimate.

Leaf size and shape

Illustration by Melissa McKee.

In most regions of the world, the proportion of woody, dicotyledon (or dicot) tree species with leaf teeth (serrated edges on the leaf margins), is inversely correlated with mean annual temperature. In addition, leaves from plants growing in cold climates are more likely to have larger and more numerous leaf teeth. Since leaf teeth can be observed and measured in leaf fossils, scientists have developed models to quantitatively reconstruct terrestrial paleotemperature from leaf teeth in fossils.[2] Models have also been developed to reconstruct other paleoclimate variables from the size and shape of fossilized leaves. For example, scientists can use the size of fossil leaves to infer past levels of precipitation, because larger leaves tend to be more prevalent in wetter climates.[3]

 Leaf cuticle

Another way to infer paleoclimate is by looking at the characteristics of the cuticle—the waxy, protective surface layer on the leaves of higher plants—of fossilized leaves. Scientists can look at the abundance of trichomes (leaf hairs) on the fossilized cuticle to infer water availability, because trichome density is often higher in plant species adapted to arid environments.[4]

Fossil leaf cuticle with visible stomata.

Stomata are small pores on plant surfaces surrounded by a pair of specialized guard cells that control gas exchange between the plant and atmosphere, influencing both photosynthesis and transpiration in the plant. Stomata on the cuticle of fossil leaves can be used to estimate atmospheric CO2 concentrations in Earth’s past. This is especially useful to scientists because there are no direct measurements of CO2 prior to the oldest ice cores, in which air bubbles are preserved (~1.5 million years old).[5] On the other hand, stomata have been around since about 400 million years ago[6] and, with some rare exceptions, found in all terrestrial plant groups. Plants change the number and/or size of stomata to optimize carbon uptake for photosynthesis, while simultaneously minimizing water loss.[7] Stomatal density (the number of stomata per unit area) and stomatal index (the percentage of epidermal cells that are stomata) are negatively correlated with atmospheric CO2 concentrations in many living plants; therefore, changes of stomatal density and stomatal index in fossil leaves are considered to represent changes of CO2 concentrations in the geological past.[8] Scientists have developed models to use the stomatal density or stomatal index of fossilized leaves to estimate the CO2 concentration of Earth’s atmospheric when that plant was alive.

Nearest Living Relative

The Joe Webb Peoples museum has hundreds of fossil leaves from the Florissant fossil beds in Colorado .

Another way to use fossil plants to infer paleoclimate is to identify the nearest living relative of that fossil, of which the current climatic tolerances are used to infer past climate. For example, by analyzing fossilized plants from the Florissant fossil beds in Colorado (about 34 million years old), it was found that the fossilized plants most resemble modern deciduous forests of the eastern United States and the humid subtropical forests of central and northeastern Mexico.[9] In addition, the discovery of fossilized palm leaves as well as an analysis of fossil pollen and spores support that it was once a warm, relatively frost-free temperate climate[10], which is very different from Colorado’s climate today.

Illustration by Melissa McKee.


[1] MacLeod, N., and Steart, D., 2015, Automated leaf physiognomic character identification from digital images: Paleobiology, v. 41, no. 4, p. 528‐553.

[2] Royer, D. L., McElwain, J. C., Adams, J. M., and Wilf, P., 2008, Sensitivity of leaf size and shape to climate within Acer rubrum and Quercus kelloggii: New Phytologist, v. 179, p. 808‐817.

[3] Wilf, P., Wing, S. L., Greenwood, D. R., and Greenwood, C. L., 1998, Using fossil leaves as paleoprecipitation indicators: an Eocene example: Geology, v. 26, p. 203-206.

[4] Parrish, J. T., Daniel, I. L., Kennedy, E. M., and Spicer, R. A., 1998, Paleoclimatic significance of mid-Cretaceous floras from the middle Clarence Valley, New Zealand: Palaios, v. 13, p. 149-159.

[5] Fischer, H., Severinghaus, J., Brook, E., Wolff, E., Albert, M., Alemany, O., Arthern, R., Bentley, C., Blankenship, D., and Chappellaz, J., 2013, Where to find 1.5 million yr old ice for the IPICS” Oldest-Ice” ice core: Climate of the Past, v. 9, p. 2489-2505.

[6] Raven, J. A., 2002, Selection pressures on stomatal evolution: New Phytologist, v. 153, p. 371-386.

[7] Cowan, I. R., and Farquhar, G. D., 1977, Stomatal function in relation to leaf metabolism and environment, in Jennings, D. H., ed., Integration of Activity in the Higher Plant. Symposia of the Society for Experimental Biology: Cambridge, Cambridge University Press, p. 471-505.

[8] Beerling, D. J., and Royer, D. L., 2002, Fossil plants as indicators of the Phanerozoic global carbon cycle: Annual Review of Earth and Planetary Sciences, v. 30, p. 527-556.

[9] Boyle, B., Meyer, H. W., Enquist, B., and Salas, S., 2008, Higher taxa as paleoecological and paleoclimatic indicators: A search for the modern analog of the Florissant fossil flora: Geological Society of America Special Papers, v. 435, p. 33-51.

[10] Leopold, E.B. and Clay-Poole, S.T., 2001, Fossil leaf and pollen floras of Colorado compared: climatic implications. In Evanoff, E., Gregory-Wodzicki K.M. and Johnson, K.R. [Eds.] Fossil Flora and Stratigraphy of the Florissant Formation, Colorado: Proceedings of the Denver -Museum of Nature and Science, v. 4, p. 17-55.

Fossil Spotlight: Crawfordsville Crinoids

A crinoid fossil from Crawfordsville, Indiana in the Wesleyan University Joe Webb Peoples Museum (4th Floor Exley Science Center).

Crinoids are organisms that are neither abundant nor familiar to most people in today’s oceans. However, during most of the Paleozoic Era (from the Ordovician on) and in the early Mesozoic (Late Triassic through Jurassic), crinoids flourished in marine environments, carpeting the seafloor like a dense meadow of flowers. It is estimated that the number of extinct species of crinoids in the Paleozoic was 5 to 10 times greater than the 600 to 800 species that are currently living.[1]

Because of their “flower like” appearance, crinoids have been referred to as “sea lilies”, but do not be fooled: they are in fact animals. Crinoids belong to the phylum Echinodermata. Like other Echinoderms—sea stars, brittle stars, sea urchins, sand dollars, and sea cucumbers—crinoids have rough surfaces, five-sided symmetry, a calcium carbonate endoskeleton, a nervous system network without a central brain, and the ability to regenerate damaged body parts.[2]

In general, crinoids have four main body parts. The first are food-gathering arms. The arms have tiny ‘hairs’ (tube-feet, part of a hollow, hydraulic system present in all echinoderms) that capture suspended food particles and direct them towards the mouth. The number of arms varies from five, common in primitive species, to as many as 200 in some living species, but the number of arms is always a multiple of five.[3] The arms project from the second main body part, the calyx, which encompasses the crinoid’s major soft-bodied tissues and organs.[4] The lower part of the calyx is made up of rigid, five-sided plates.[5] The calyx and the associated arms are collectively referred to as the crown. Next is the stem, which is composed of a series of flat discs that pile on one another like a stack of coins (with a hole in the center). These stem pieces come in a variety of shapes—round, pentagonal, star-shaped, or elliptical. The stem elevates the crinoid’s body into a more sediment free zone above the sea floor.[6] Lastly, the holdfast anchors the crinoid’s stem to the sea floor. The now-extinct crinoids of the Paleozoic were predominantly fixed by their stalk to the ocean floor, although some crinoids lived attached to driftwood floating in surface waters, but only about ten percent of crinoids living today are estimated to have stems.[7] Non stalked forms are called comatulids (or feather stars).

The parts of a typical crinoid with a stem from the Paleozoic. Illustration by Melissa McKee.

Wesleyan University’s Joe Webb Peoples Museum has an amazing collection of crinoids from Crawfordsville, Indiana, which were donated by Henry I. Nettleton, in the late 1800s. The crinoids from localities in and near Crawfordsville are world renowned for their amazing diversity, abundance, large size, preservation, and superb three-dimensional relief. In addition, the beautiful blue-gray colors of these fossils appeal widely to both fossil collectors and museum visitors.[8]

More examples of the crinoid fossils from Crawfordsville, in the Joe Webb Peoples Museum of Wesleyan University.

The preservation of the crinoids from Crawfordsville is very unusual. Crinoids are made up of multiple calcium carbonate plates held together by soft tissues, primarily ligaments. The ligaments are readily biodegradable. As a result, when crinoids die, their ligaments typically decompose within hours or a few days, leaving their plates to be easily scattered by currents or predators. Consequently, crinoid fossils are almost always found as what has become known as “crinoid hash”—scattered crinoid pieces. However, the Crawfordsville area had both shallow water conditions and an influx of silt from a neighboring delta. This caused the crinoids to be periodically buried alive by storm-generated slumping or silt flows. The crinoids were buried deep enough to avoid decomposition and predation, allowing for remarkable preservation.[9]

A sample of crinoid hash. Crinoid stem fragments are visible.

The Crawfordsville locality offers unprecedented insight into the ecology, morphology, and behavior of Mississippian-age (~ 350 million years) crinoid communities. Nearly half of the species found at Crawfordsville have at least one complete specimen, making it possible to determine the height of that species when it was alive. This helped scientists answer the question of how so many diverse crinoid species could thrive in such close proximity.[10] A clue is that different crinoid species are found with differing stem lengths, allowing each to find its own feeding niche in the water column. Additionally, even crinoids with stems of similar length can filter food particles of different sizes, as shown by different structure of their ‘arms’. In these ways, competition between the species was minimized and diversity could be maintained. This ecological phenomenon has become known as tiering, and is now widely recognized to have been an important aspect of the structure of benthic marine communities throughout much of the Phanerozoic[11], with tiering of suspension feeders most common in Silurian through Carboniferous and Late Triassic through Jurassic.[12] In today’s oceans, such highly diverse tiers of epifaunal organisms are no longer present, with too many active predators (fish, sea urchin) feeding in the water column.

By Melissa McKee

[1] Moore, R.C. and C. Teichert, eds, 1978. Introduction. In: Treatise on Invertebrate Paleontology, Part T, Echinodermata 2, Vol. 1, T7-T9. R.C. Moore and C. Teichert, eds. Geological Society of America and University of Kansas.

[2] Morgan, W. W., 2014, Collector’s Guide to Crawfordsville Crinoids, Schiffer Publishing, Limited.

[3] Brosius, L., 2005, Fossil Crinoids: Kansas Geological Society.

[4] Morgan, W. W., 2014, Collector’s Guide to Crawfordsville Crinoids, Schiffer Publishing, Limited.

[5] Brosius, L., 2005, Fossil Crinoids: Kansas Geological Society.

[6] Morgan, W. W., 2014, Collector’s Guide to Crawfordsville Crinoids, Schiffer Publishing, Limited.

[7] Moore, R.C. and C. Teichert, eds, 1978. Introduction. In: Treatise on Invertebrate Paleontology, Part T, Echinodermata 2, Vol. 1, T7-T9. R.C. Moore and C. Teichert, eds. Geological Society of America and University of Kansas.

[8] Morgan, W. W., 2014, Collector’s Guide to Crawfordsville Crinoids, Schiffer Publishing, Limited.

[9] Ibid.

[10] Ibid.

[11] Ausich, W., 1999, Lower Mississippian Edwardsville Formation at Crawfordsville, Indiana, USA: Fossil crinoids. Edited by H. Hess, WI Ausich, CE Brett, and MJ Simms. Cambridge University Press, Cambridge, UK, p. 145-154.

[12] Ausich, W. I. and Bottjer, D. J., 1982. Tiering in suspension feeding communities on soft substrata throughout the Phanerozoic. Science, 216: 173-174.

Getting to know Greg, Wesleyan University’s own taxidermic American bison

You may have wandered around the 4th floor of Exley Science Center, looking for your class in room 405, and caught a glance of a bison. And if you had just awoken from a short sleep the night before, you may have even thought that this bison was one of your sleep deprivation-induced hallucinations. Fear not: there really is a taxidermic bison on the 4th floor of Exley in Wesleyan’s Joe Webb Peoples Museum of Natural History, and he goes by the name Greg! In 1875, Georg Brown Goode, an ichthyologist and Wesleyan alum (year of 1870), received Greg (who had been stuffed in Carson City, NV) from John Wallace. G. Brown Goode was the son-in-law of Orange Judd, who funded the building of Judd Hall in which the Wesleyan Museum was originally based, and he was the first Curator of the Wesleyan Museum at its establishment in 1871.

You can visit Greg at Wesleyan University’s Joe Webb Peoples Museum of Natural History.

Greg at his former residence in the Wesleyan Museum in Judd Hall back in the summer of 1957. Image from Wesleyan University Library, Special Collections & Archives.

Greg being moved after the dissolution of Wesleyan Museum in Judd Hall,  1957.

Image from Wesleyan University Library, Special Collections & Archives.

Greg is an American bison (Bison bison), the U.S. national mammal and the largest terrestrial animal in North America. Male bison weigh up to 2,000 pounds, whereas females weigh up to 1,000 pounds.[1] Despite their size, bison can run at speeds over 30 miles per hour—which is why Yellowstone National Park warns visitors to stay at least 25 yards away from all wild bison.[2] In his old age, Greg no longer quite reaches those speeds, so feel free to get a bit closer! During Greg’s time at Wesleyan, American Bison went from becoming virtually extinct in the wild to having their population rebound in what is considered one of the greatest conservation success stories of all time.

Scientists estimate that there were 30 to 60 million bison living in the continent when the first European settlers arrived in North America, ranging all the way from northern Canada to northern Mexico, and from western New York to eastern Washington.[3] Bison were commonly referred to as buffalo by European explorers due to their perceived resemblance of Asian and African buffalo. As Euro-Americans began to settle westward, they changed the Bison’s native grassland habitat through plowing and farming, as well as introducing domestic cattle, which brought diseases and competition for grazing. Farmers and ranchers began killing bison to make room for their animals. In addition, as native American tribes acquired horses and guns, they began to kill bison in larger numbers than before. Some U.S. soldiers even killed bison to spite their native American enemies who depended on the animals for food and clothing. Western railroads greatly accelerated the decimation of the American bison by bringing hunters who would shoot the animals out of the open windows of moving trains. The bison were not only killed for sport, but also for their skin, bones (used in making fertilizer)[4], and tongues (a culinary delicacy).[5]

Had it not been for a few private individuals and government action, the American bison would be extinct today. In 1884 there were only 325 out of the many millions bison left in the wild, including 25 in Yellowstone National Park. Congress finally tasked the U.S. Army with enforcing laws prohibiting the killing of any birds or animals in Yellowstone.[6] Congress’s efforts proved successful—as of July 2015, Yellowstone’s bison population is estimated at 4,900 individuals. Private organizations also helped save the bison population. In 1905, Teddy Roosevelt formed the American Bison Society with zoologist William Hornaday, with the aim to start a breeding program at the New York City Zoo (Bronx Zoo today). By 1913, the American Bison Society had enough bison to restore a free-ranging herd to Wind Cave National Park in South Dakota, and this herd have helped reestablish other herds across the United States and Mexico.[7] Restoring bison herds is not only an enormous victory for the U.S. national mammal, but also for the whole grassland ecosystem, because the unique spatial and temporal complexities of bison grazing are critical to the successful maintenance of biotic diversity in grasslands.[8]

Bison in private herds in part account for the rebound in the bison population of North America. In the 1870s, when the bison population was dwindling, people began to realize that owning bison could be profitable. Ranchers started collecting the few remaining bison scattered across the prairies to breed them in private herds.[9] It is estimated that by the year 2000 at least 250,000 bison were living in private herds, and 92,000 of these bison were raised for meat. Bison can process North American grasses more efficiently than cattle, and their meat contains less fat and cholesterol, making it an attractive option for human carnivores.[10]

According to Dr. James Derr, Professor of veterinary pathobiology, most bison alive today are genetically different from their wild ancestors. In the late 19th and early 20th centuries, the ranchers who owned much of the remaining bison population bred their bison with cattle to try to create better animals for meat. It is believed that only about 1.6 percent of today’s bison population is not hybridized.[11] Wesleyan’s Greg dates back to the time severely declining numbers of bison, and represents the original American bison before hybridization.

Some of Greg’s favorite puns. Illustrations by Melissa McKee.

By Melissa McKee

For more information check out


[1] U.S. Department of the Interior, 15 facts about our national mammal: the American bison.

[2] Portman, J., 2011, The great American bison: PBS.

[3] Portman, J., 2011, The great American bison: PBS.

[4] U.S. Fish & Wildlife Service, 2014, Time line of the American bison: National Bison Range Wildlife Refuge Complex.

[5] Winkler, P., 2014, In the beginning, bison: Smithsonian National Zoological Park Conservation Biology Institute.

[6] U.S. Fish & Wildlife Service, 2014, Time line of the American bison: National Bison Range Wildlife Refuge Complex.

[7] U.S. Department of the Interior, 15 facts about our national mammal: the American bison.

[8] Knapp, A. K., Blair, J. M., Briggs, J. M., Collins, S. L., Hartnett, D. C., Johnson, L. C., and Towne, E. G., 1999, The keystone role of bison in North American tallgrass prairie: Bison increase habitat heterogeneity and alter a broad array of plant, community, and ecosystem processes: BioScience, v. 49, p. 39-50.

[9] Polziehn, R. O., Strobeck, C., Sheraton, J., and Beech, R., 1995, Bovine mtDNA discovered in North American bison populations: Conservation Biology, v. 9, p. 1638-1643.

[10] Portman, J., 2011, The great American bison: PBS.

[11] Portman, J., 2011, The great American bison: PBS.