Evolutionary Change
Animal species respond to environmental changes in a variety of ways. Simply put, some species die off, some adapt physically, and some move to a different habitat. On the next few pages are examples showing how three species responded to long-term environmental changes in the area around Agate Fossil Beds. The Stenomylus line died off; Miohippus’ evolutionary line remained a grazing animal but changed physically over the years, eventually becoming the modern horse; and the Palaeocastor line moved from land to water, gradually evolving into the beaver.
Each of these three animals is portrayed here with partial skeleton, musculature, and outer skin to help you see its general composition and to emphasize certain physical features that developed in the species over time. Paleontologists, of course, work this way. From fragments and bones they reconstruct full skeletons, and from surmises about muscular structure, often based on present-day animals, they project the appearance of the animal. The artist, in this case Jay Matternes, then brings together these bits of evidence to give us a picture of life long ago.
Stenomylus
A small, gazelle-like camel similar to the present-day gerenuk of Africa. Stenomylus is the second most common animal found in the fossil beds at Agate. Stenomylus had hard hooves like modern antelopes and deer, unlike modern camels which have flesh-padded feet adapted to desert terrain. The three-hued coat is inferred from the coat of the modern gazelle, a similar form in adaptation. Stenomylus’ evolutionary line eventually died out in North America at the end of the Pleistocene. No one knows why both camels and horses died out on this continent.
The distance between grid lines represents five centimeters.
1 The ears moved in a parallel fashion, not independently; the parallel movement is inferred from modern llamoids, to which Stenomylus is related.
2 Stenomylus’ musculature was adapted for high-speed running, similar to the present-day pronghorn.
3 The back structure suggests that Stenomylus would have made short, choppy leaps, not the graceful, arcing leaps of a modern impala.
4 Limbs were long in proportion to the body, allowing the animal to run with great speed.
5 Stenomylus had a hard, chitinous hoof, an adaptation for greater running speed, and for sure footing on rough terrain.
Miohippus
Over the last 60 million years the horses have evolved from small, terrier-sized animals to the diversity of size we know today, from the huge Clydesdales to the diminutive Shetland ponies. The three-toed early horse known as Miohippus was about the size of a sheep. The descendants of Miohippus apparently went in two directions in their evolution: One group continued to be forest-grazing, three-toed horses that eventually reached the size of modern horses but died out later. The other group, through such intermediate forms as Parahippus, became grassland forms that led eventually to the modern one-toed horses. Horses became extinct in North America at the end of the Pleistocene, but no one knows why. They continued to evolve on other continents and were re-introduced in historic times.
The distance between grid lines represents five centimeters.
1 The back was straighter and stiffer than in earlier horses, partly because of the increasing size of the animals and partly to allow sustained open-plains running.
2 Limbs were long in proportion to the body, an evolutionary trend in the horses for speed in open-plains running, rather than darting about in forests.
3 Most of the weight of the animal was on the middle toe, which has become a single toe in modern horses. This is an adaptation for endurance and stability in open grasslands.
4 The upright mane is a primitive horse characteristic; wild horses today have reverted to this trait.
5 The coat is shown as striped, a probable holdover from earlier horses that dwelled in forests, where a striped coat would provide camouflage.
6 A large, deep mandible supported teeth adapted to grazing and the grinding of grasses and other wild plants. The teeth were deep-rooted and continuously erupted as the surface was worn down by the grit and dirt that came with the large quantities of plant food consumed daily.
Palaeocastor
Palaeocastor was an ancient beaver whose mode of life was like that of a modern prairie dog—land-oriented instead of water-oriented. Palaeocastor was small, about 12 centimeters (5 inches) high, and about 30 centimeters (12 inches) long. Its fossilized spiral burrows, called Daemonelix, survive to tell us what its habitation was like, a feature unique to Palaeocastor among all the fossil beavers. The Daemonelix shown here dwarfs a member of Olaf A. Peterson’s field crew from the Carnegie Museum. The bones of a Palaeocastor and one of its predators were found at the bottom of one such burrow, helping to prove that Palaeocastor was responsible for making these corkscrew holes in the ground.
The distance between grid lines represents five centimeters.
1 The powerful jaw and musculature allowed for grazing on grasses and other plants, as well as masticating. The teeth were deep-rooted and would continue to erupt as the surface was worn down.
2 The complex musculature supported the use of the forelimbs in burrowing. Palaeocastor had a collarbone or clavicle, like us, for greater agility in using the forelimbs.
3 The forelimbs were adapted to burrowing in the ground.
4 The tail is like that of a modern burrowing rodent, such as a muskrat, whereas the modern beaver has a different, very specialized tail.
A grazing animal is fairly easy to recognize, but how can we recognize a burrower? Some have radically adapted limbs and claws. Obvious cases are the common garden mole and the armadillo. The mole has powerful attachments for the muscles of the upper arm on the humerus, a bone so flattened that its width has come to match its length. Moles also have long, broad digging claws. The armadillo, which is also a digger but not a burrower in the same way a mole or a gopher is, has large curved claws for digging. Moles did start to become quite common in the Late Oligocene, so we can assume that a good burrowing environment was present.
Another group which became extraordinarily common in the Late Oligocene of western North America was that of the ancestral pocket gopher. Direct proof that this group actually burrowed does not exist, but the abundance of fossil gophers suggests that they might have lived underground in colonies.
A real surprise at Agate is the number of beaver burrows. The famous Daemonelix or “devil’s corkscrew” attests to the dense population of Palaeocastor. By that relatively advanced stage of beaver evolution, the animals might be expected to behave like the modern-day muskrat, perhaps digging dens along stream borders and spending some of their time in the water. The presence of skeletons in the spiral burrows, however, indicates that Palaeocastor was primarily a burrower, one which perhaps lived very much like our present-day prairie dog. Despite that, there is no apparent structural modification to indicate burrowing abilities.
Changing environmental conditions were pushing Palaeocastor toward extinction in the Early Miocene. The disappearance of that ancient beaver, while not unusual, presents a problem for the careless observer who might assume that ancient animals behaved like their modern counterparts. The burrowing beavers of Miocene Agate certainly have no modern counterparts.
While we can delineate in a general way the prehistoric life of Agate, we can’t describe the past in any detail. Plants most directly reflect the effects of climate—and plant fossils are absent at Agate. As the base of the food chain, plants carry the influences of climate on to the plant-eating animals. From the numerous animal fossils found at Agate we have learned most of what is known about the environment of that time. Sediments tell a good part of the story, and floras from other localities help, but much of Agate’s ancient ecology must be inferred from the bones.
Today, standing on the porch of the visitor center or walking along the path to University and Carnegie Hills, visitors find themselves in the midst of the shortgrass prairie. Five distinctive plant communities share this prairie, coexisting in a dynamic relationship which depends upon local climate variations.
Even to the untrained eye, it is evident that the basic short-grass pattern has been modified by the shape of the land and by the Niobrara River. In the stream valley, along the tributaries, and on shaded north-facing slopes, the shortgrass community is mixed with taller grasses. If a dry cycle began, the short grasses would take over the whole area by migrating downslope from the exposed prairies. Of interest is the fact that over-grazing by either domesticated or wild animals will have the same effect as a dry period in that taller grasses will be replaced by short ones.
Let’s examine the five communities present today so we can appreciate the complexity of relationships between living things and the earth upon which they depend.
First, we can begin in the Niobrara River itself. The river’s water-dwelling plant inhabitants include algae, which grow underwater.
Between the river and the dry ground is a second community—the marsh—which is often more wet than dry. The marsh has its own characteristic plant association. Most familiar are the cattails, mints, and willows, but just as important ecologically are arrowleaf, rush sedge, marshweed and blue verbena. These are moisture-loving plants that thrive on being thoroughly soaked during the wet part of the year.
Beyond the marsh on the valley floor is a third community. Here the water table (the top of the saturated soil and rock zone) is close enough to land surface that the plants can easily send their roots down into the saturated zone. Here, in what the plant ecologists call the “sub-irrigated floor plain” we find a mid-grass community. Eighty-five percent of the vegetation is slender wheatgrass. Its wheat-like heads may, under favorable conditions, grow to a height of one meter (three feet). At Agate it is seldom over knee high. Kentucky bluegrass takes care of another 10 percent of the plant population. Imported from Europe as a pasture grass in the 1600’s, it spread so rapidly that it often beat the settlers onto new land as they moved westward. The remaining five percent includes imported redtop and such native grasses as switchgrass, foxtail barley, little bluestem, prairie cordgrass, and inland saltgrass. Wildflowers such as Flodmon thistle, yarrow, heath aster, salsify, and blue-eyed grass complete the community.
Moving farther away from the stream, we rise up onto terraces within the valley. These terraces represent levels where the stream paused in its downcutting and cut sideways for awhile. At a drier level, on deep, well-drained sandy soils, they support the fourth or mixed-grass community.
No exotics have yet appeared in this plant community. The grasses include prairie sandreed, sand bluestem, blue grama, needle-and-thread grass, and Indian ricegrass. Wildflowers include the prominent phlox, penstemon, and lupine. Unwelcome (to man and his grazing animals) is Astragalus, the selenium-concentrating plant better known as loco weed. The brittle prickly pear and spiderwort cactus are found here too.
At higher levels in the terrace community, slightly steeper slopes and shallower soils cause some change in this mixed-grass assemblage. Here the dominant grasses are little bluestem, threadleaf sedge, needle-and-thread grass, and blue grama. Lupine disappears, and common pricklypear becomes the only cactus. In this community is found the yucca, its flowers a beautiful soft yellow in season and its spiny leaves painful at any time of the year. Avoid this plant; yucca spines break off under the skin and soon cause irritating festers. The yucca moth, often seen flying around the yucca seed pods, lays eggs in the plant’s lemon-sized fruits. Inside the fruit are long rows of flattened, wedge-shaped seeds. When the yucca moth eggs hatch into caterpillars, they eat their way through the seeds, killing them. On the other hand it is the yucca moth with its long tongue that is solely responsible for pollinating the yucca flower! If you find a yucca fruit in early summer, you can (elsewhere than in the park) slice through it and see the caterpillars at work.
On the high bluffs and overgrazed terraces is the fifth community, the short grass. This community too can be divided into two slightly different parts. The bluffs support blue grama grass, needle-and-thread grass, and Sandberg blue grass. Flowers and shrubs include Eriogonum, brittle pricklypear cactus, pepperweed, penstemon, broom snakeweed, fringed sagewort, and yucca. The other part of this community, the overgrazed terraces, have threadleaf sedge, needle-and-thread grass, and blue grama. Except for the familiar penstemon, all the flowers are restricted to this community. Gronwell, menzania, and bee plant are indicators of overgrazing.
Certain cyclical variations are characteristic of these plant communities. First, the shortgrass and mixed-grass areas ebb and flow with changing moisture conditions from year to year. Second, grass populations change with the seasons. Cool-season grasses (foxtail barley, Indian rice grass, Kentucky bluegrass, needle-and-thread grass, Sandberg blue grass, and slender wheatgrass) flourish during spring and fall. During the warm summer the blue grama, inland saltgrass, little bluestem, prairie cordgrass, prairie sandreed, and switchgrass predominate. This natural adaptation to seasonal conditions uses the greatest potential of the growing season and at the same time provides species that will flourish in both wet and dry cycles.
After reading this last section, you might look back at the section on Early Miocene ecology. Comparison reveals that a great deal of information can be obtained by examining living plants. In contrast, the lack of fossil flora from the Early Miocene at Agate has resulted in a scarcity of ecological information from that early epoch. Scientists begin their reasoning by such comparisons; you can begin your own exploration of the past in the same way.