The Pliocene Adventure -- Herbivores (Grazers) Part 2

Continued from Part 1

In going over these lists, it struck me that I was seeing three trends in the evolution of mammals from earlier eras through the Pliocene to later eras, down to modern times.

The first, obviously enough, was the megafaunal trend; the tendency to grow to larger sizes over time. One impetus for this was the extinction of the dinosaurs, which opened up all the megafaunal niches they had filled. Another was the presence of a great deal of lush vegetation, which could support large numbers of a wide variety of megafauna. A third impetus was the need for large bodies to fully digest plant material to extract as much nutrients as possible. While larger bodies require more energy, up to a certain point nutrient extraction can exceed nutrient use, so the largest size a species can maintain is that where nutrient extraction and use are balanced. A fourth impetus was the need to defend against predators that were also getting bigger and more powerful so as to successfully hunt larger prey.

It should be noted that the megafaunal trend was so successful that it survive the ice ages well into "modern" times (10,000 years ago or less). Why they nearly all died out at virtually the same time is still a mystery.

The second trend occurred because, since the end of the Mesozoic Era, the overall climate of the Earth had progressed towards cooler and drier conditions. This encouraged the spread of grasslands at the expense of forests. Though grasslands can support megafauna, the lack of cover meant that animals that could not rely on sheer size and strength to protect themselves had to develop a different way. As such, many species went in the opposite direction: not so much smaller but faster and more agile, so as to escape predators by running away instead of intimidating or fighting them. Even under these circumstances size could still be an advantage, but it is worth pointing out that while the megafauna had all died out by 7,000 years ago, the smaller more gracile runners survived and have thrived.

The third trend is more speculative, but as long as herbivores and carnivores remained big, they didn't have to be smart. Brain size, and presumably intelligence, had been increasing since the extinction of the dinosaurs, and large body size is not a definitive indication of intelligence, but fossil skulls indicate that during the Pliocene the larger animals tended to have smaller brains, whereas the smaller animals tended to have larger brains. This at least suggests the smaller animals were more intelligent. That makes some sense, in that the smaller animals must not only have to find food, but they must also avoid being eaten by bigger animals. An elephant or tiger can stand its ground; a horse or coyote can run. But a prairie dog or a ferret can do either of these and can also hide, depending upon whatever the situation calls for. This requires a level of scheming that implies higher intelligence. On top of that, the kinds of adaptations to the brain that permit animals to become fast, agile runners also most likely also promote and enhance greater intelligence.

It may be no coincidence that the animals that survive the extinction event at the end of the last ice age were probably smarter than the ones who didn't. By itself, intelligence may not have had any direct impact. However, there are only two ways animals can adapt to changing environmental conditions: by changing their bodies or their behaviors. Biological adaptation requires that the traits that make survival possible already be present, so the animal can take advantage of them. Behavioral adaptation, however, is more dynamic. It requires the ability to analyze a situation, understand the consequences of various possible actions, and choose which is the best. We humans are consciously aware of this process and have near total control over it. With animals it may be mostly or totally subconscious and thus beyond their control, but it still occurs, and it requires a level of intelligence that is very close to making conscious choices.

The point is, the megafauna may have lacked the biological traits to adapt and the intelligence to properly modify their behavior, and so went extinct, but the smaller animals and the runners probably did, especially the latter, and so survived. The irony is that a creature perfectly comfortable with harsh conditions may not be able to figure out how to live in a kinder, gentler world, especially if its food disappears and new predators show up, whereas an animal that just barely survived can now flourish because it can take advantage of all the new opportunities available.

Once again, the genus name for each creature is given in parentheses.

Llamas, Giant -- These Genera are extinct. Despite the fact that the llamas filled the gracile runner niche abandoned by the camels, they too evolved megafaunal forms, though they didn't reach the size of the giant camels. This most likely was in response to pressure exerted by the large and powerful predators that stalked the plain.

(Blancocamelus) -- This Genus had a similar build to Hemiauchenia, indicating that it too was a runner, but it was more than 50% larger; it stood 7 feet at the shoulder. In fact, there is sufficient evidence to suggest that this guy evolved from Hemiauchenia.

(Camelops) -- This Genus was a contemporary of Blancocamelus. It also stood about 7 feet tall, but it had a more robust body, with thicker legs, suggesting it was a slower runner, but a more powerful fighter.

TG & Differel note that the giant llamas, though grazers, are more generalized and eat whatever plants are available. As such, they spend time both on the grasslands and in the open-canopy savannas.

Pampathere (Holmesina) -- This Genus is extinct. They belonged to the same Order as the Armadillos and the Glyptodonts. They closely resembled Armadillos, but were herbivores instead of insectivores. They were also larger; Holmesina was 6.5 feet long and weighed 500 pounds. As such, they are often, if erroneously, referred to as giant armadillos. Holmesina preferred humid lowlands and tended to eat a wider variety of less course vegetation than the other Genera of its Family.

TG & Differel find only a few Pampatheres on the Denver plain, and most make their homes close to water sources. One lives close to their cave, and makes a daily circuit from a small open copse of trees to the beaver pond and back.

Pika (Ochotona) -- This is the contemporary Genus, and the Pliocene species are virtually identical to the modern species. These are small mouse-like mammals with no tails that are closely related to hares. They prefer cooler environments, but can be found in mountains and grasslands. They eat a wide variety of green plants, from which they obtain water, but they will also drink from available local sources. They deliberately cache food in haypiles as a food source for winter. They prefer to nest within existing spaces inside rock piles, but they can burrow if necessary.

TG & Differel did not expect to find pikas on the Denver plain, but one day they catch one in a trap set on the Aurora ridge. At first they fear they have killed the only one, but Kitty (who often goes exploring at night) reports that she found a colony living almost at the top of the ridge, where the rocks are broken the most and provide many places to nest. They only come out at night and do not descend the slope, so TG/Differel never see them.

Prairie Dog (Cynomys) -- This is the contemporary Genus, and the Pliocene species are virtually identical to the modern species. They are instantly recognizable to Denver natives like Eile and Sunny, even compared to other ground squirrels. Other than marmots they are the largest ground squirrels on the Denver plain, ranging from 1-1.5 feet long. They eat selectively, preferring grass over forbs, but they will change their diet as needed to meet changing conditions. They will also eat insects, larva, and the scat of large grazing herbivores. They can obtain water from vegetation, but they often live close to water. They prefer grasslands, but will live in open-canopy savannas as long as the trees are fairly sparse. They can serve as a keystone species, in that they enhance the biodiversity of most grasslands they inhabit.

Eile and Sunny would have been very disappointed if there hadn't been prairie dogs in the mid-Pliocene. However, they and Differel note that the colonies, while fairly large, are all located near the beaver ponds, the three rivers, and the numerous streams.

Pronghorn -- These Genera are extinct. These guys closely resemble African antelopes due to convergent evolution, but they are not themselves true antelopes. They fill a similar ecological niche, but receive some competition from horses and (in the Pliocene) gazelle-horses and antelope goats. They prefer open grasslands, but can be found in scrubland and sparse open-canopy savannas. They eat a wide variety of vegetation, such as grass, forbs, and shrubs, including plants that are toxic to other animals. Females form herds, while males are solitary. They are the fastest land mammals in the Western Hemisphere, and few creatures can catch them in a running race. Most predators rely on stalking and ambush, or take fawns.

Though they have different sizes and body builds, pronghorn Genera differ most in the size and shape of the horns.

(Capromeryx) -- These guys were fairly similar to modern pronghorns, but only weighed 100 pounds (modern pronghorns average 155 pounds) and were more gracile. They had horns with two small prongs of asymmetrical height that were straight and closely spaced. Later species during the Pleistocene would get even smaller. They preferred open-canopy savannas that were more dense, especially those with some underbrush.

(Sphenophalos) -- These guys were much smaller than modern pronghorns, weighing only 35 pounds. They had horns with a single core that branched into two small prongs at the top. They preferred denser open-canopy savannas and closed canopy woodlots and riparian zones, especially areas with dense undergrowth where they could hide.

(Tetrameryx) -- These guys were also smaller than modern pronghorns, weighing only 125 pounds, but they were more heavily built. They had horns with two angled asymmetrical prongs, a short one in front and a long one in back. In profile they look like check marks. They preferred sparser open-canopy savannas and grasslands

TG & Differel note that, after horses and llamas, Capromeryx and Tetrameryx together are the third most common running herbivores on the Denver plain. They also speculate that their speed has prompted the evolution of lighter, more gracile predators as opposed to stronger, heavier megafauna-killers.

Rabbits -- Modern Colorado has only one Genus; in the mid-Pliocene, it had ten. Though the surviving Genus is fairly adaptable, and can be found in a variety of habitats from forest to desert, it was less common in the mid-Pliocene, being one of the most recent, if not the most recent, Genus to appear. As such, most of the habitats it would eventually occupy were already filled.

Like the closely related hares, and the ground squirrels, rabbits served as major prey for a wide variety of predators, some of whom evolved to take advantage of their presence. Though grazers, they use hindgut fermentation, so they chew fecal pellets to recycle partially digested food. Most prehistoric rabbits more closely resembled hares, suggesting that hare morphology is the ancestral form, whereas modern rabbit morphology is a more recent adaptation.

(Alilepus) -- This Genus is extinct. It was fairly common and widespread, found in Europe and Asia as well as North America. It was closely related to a megafaunal form of rabbit in Europe, but was itself normal size.

(Aluralagus) -- This Genus is extinct.

(Aztlanolagus) -- This Genus is extinct. It was named the Azatlan rabbit after the mythical homeland of the Aztecs. It was smaller than modern rabbits.

(Hypolagus) -- This Genus is extinct. This was perhaps the most common and widespread rabbit during the mid-Pliocene. It was similar in size and appearance to modern rabbits, highly diverse, it lived in a wide range of habitats, and could be considered the quintessential rabbit of that era.

(Lepoides) -- This Genus is extinct. It more closely resembled a jackrabbit than a true rabbit, suggesting it had a similar lifestyle.

(Nekrolagus) -- This Genus is extinct. It was possibly the ancestor of Sylvilagus (see below). It was fairly widespread and common.

(Notolagus) -- This Genus is extinct. It was called the southern rabbit because it mostly lived in Mexico and the southwest United States. It was smaller than the modern rabbit.

(Pewelagus) -- This Genus is extinct. Like Hypolagus, it was common and widespread.

(Pratilepus) -- This Genus is extinct. It was called the meadow rabit becsause its fossils are found in meadow-like environments. It is possibly the ancestor of Aluralagus (see above). Like Hypolagus and Pewelagus, it was widespread and common.

(Sylvilagus) -- This is a contemporary Genus, and the Pliocene species are virtually identical to the modern species. It was called the cottontail rabbit because its white fluffy tail resembles a cotton ball. It has replaced Hypolagus as the quintessential rabbit of Colorado, and as the other Genera went extinct it diversified to fill the empty niches. Though the earlier species looked more primitive than the modern species, they still looked more like rabbits than hares.

TG & Differel note that rabbits are not as plentiful as hares or ground squirrels, and at first they assume they are hares until they catch one and analyze its DNA. The familiar cottontails are restricted to closed canopy woodlots and riparian zones, while the smaller rabbits live on and near the Aurora ridge. The rest distribute themselves across the Denver plain, inhabiting the grasslands and the open-canopy savannahs. TG/Differel sometimes trap some to supplement their rations.

Sloths, Ground (Glossotherium) -- This Genus is extinct. The third of three ground sloth Genera in the mid-Pliocene of North America, it was 13 feet long and weighed over a ton. It tended to be wider and more robust than the other two Genera. Its teeth suggested it was a grazer and it may have used foregut fermentation, but it probably ate other types of foliage as well.

TG & Differel soon learn why it was named the "tongue beast", when they find a family group living close to their home cave. They have long sinuous tongues that they use to grab and hold grass for biting, and to strip leaves, shoots, and twigs off shrubs. They use their claws to catch and pull tree branches down to their mouths. They are invulnerable to all but the largest of predators, who usually go after the young ones in any event.

Zebra -- These Species are extinct. The earliest members of the Genus Equus closely resembled zebra's, particularly Grévy's zebra. Most paleontologists even belief it had some striping. They tended to be smaller and slighter than modern zebras, but they probably lived a a similar lifestyle.

(Equus fromanius) -- This guy was probably a direct descendent of E. simplicidens (see below). It was taller and stockier, with longer legs, but still smaller and slighter than modern zebras. Otherwise, it closely resembled E. simplicidens, and probably also had striping.

(Equus simplicidens) -- Also known as the Hagerman horse and the American zebra, it is the oldest known species of the Genus Equus in the fossil record. It stood about 4 feet high at the shoulder, with a stocky build, straight shoulder, and thick neck like a modern zebra. The environment in which it lived was grasslands and floodplains. It was probably the ancestor of E. fromanius, E. idahoensis, and E. scotti (see above).

When TG & Differel first see these guys on the plain, they wonder if they may have been sent to Africa by mistake, but a quick check of the star positions confirm they are in Colorado. They had assumed that they would see some creatures that resembled the living animals of Africa, but they did not expect to see anything that was practically identical. They note that the herds confine themselves to the grasslands, the sparse open-canopy savannas, and the scrubland at the base of the Aurora ridge.