A Pleistocene Ecosystem
by Wesley Gordon
page 29



What was life like during mid-Pleistocene time? Even the refined list of fossil data does not help us to fully answer this question. The list tells us what kinds of organisms inhabited Irvington some 1.3 million years ago, and it shows how they might be classified. But what else do we know about the organisms? For example, what kinds of niches did they occupy? What were their relative sizes? What were their enemies, and how was protection against them achieved? What kinds of deer lived in this place: Caribou? Virginia deer? Mule deer? Elk? What was a seal doing among sabercats and other land mammals? Obviously, without answer to such questions, an ecosystem in the true sense of the word-with interacting plants and animals­­­-cannot be reconstructed.

Thus the refined list requires additional information before it can be interpreted. A description of each species is needed. But fossil specimens are not always complete enough to permit description of the whole animal. The fact prompts us to remember a very important point: Speculation about the past is very often based on knowledge of the present. Descriptions of present-day animals will tell us much about the animals that lived at Irvington during mid-Pleistocene time.

Figures below include descriptions of (1) present-day Irvington animals that resemble some of those from the fossil records, and (2) extinct animals whose fossils have provided sufficient evidence for description. With regard to the second item, the mere size of certain Irvington animals of long ago should affect your reconstruction of that ecosystem. For example, the Irvington bear, Arctodus, was truly a “monster”-much larger than any other bear, living or extinct. (This statement is made on the basis of discoveries to date; a bigger one might be found tomorrow.) Surely Arctodus occupied a different niche than would be the case if he had been the size of the small, modern honey bear of South America.

Western Toad (Bufo boreas)

Body, 2 2/5 to 4 5/8 inches; covered with rounded warts.  Found in open valleys, high mountains, wet meadows, and lake areas.  Inhabits the San Francisco Bay region.  Active at night.  Seeks shelter beneath logs, rocks, and boards and in burrows.  Breeds from January to July.  Spawns in water; eggs in long strings. 

Pacific Pond Turtle (Clemmy marmorata)

Shell, broad and short, average length, about 6 inches, maximum length, about 7 inches.  Found in the San Francisco Bay region.  Inhabits quiet ponds and streams; sometimes enters the sea.  Aquatic but basks on land; often seen on logs or rocks in or close to water.  Can be trapped, easily caught with hooks carrying minnows, earthworms or lumps of liver.  Diets chiefly on small animals.  Female lays eggs from about June to August (mostly mid-June to mid-July) in holes dug on shore; 5 to 11 eggs deposited at one time.  Hibernates in mud at the bottom of water during the winter months.

Harbor Seal (Phoca vitulina)

About 5 feet.  No external ears; no underfur.  Hind limbs (flippers) can not be brought forward, making it difficult to move on land.  Gets about by wriggling when not in the water.  Inhabits harbors and bays (San Francisco Bay).  Feeds on mollusks, crustaceans, and fish.  young seals (1 or 2 per litter) born in early spring.

California Ground Squirrel (Citellus beecheyi)

Body, 9 to 11 inches; tail, 5 to 9 inches.  Prefers open country and grasslands.  Found throughout the Bay area.  One litter (4 to 10 individuals) a year. Young born in spring.  Food consists of seeds, fruits, acorns, roots, bulbs, and meat.  Carries fleas which may transmit bubonic plague.

California Pocket Mouse (Perognathus californicus)

Only pocket mouse common in Bay area today.  Body, 3 1/5 to 3 1/2  inches; tail, 4 to 5 4/5 inches.  Forages at night for seeds and grain.  Carries food in fur-lined "pockets" (pouches), one on each side of its mouth; sotres food in small pits along passageways in burrow system.  Likes open and semiopen country and slopes covered with chaparral.  Produces 4 to 7 young in each litter; more than one litter a year.

California Vole (Microtus californicus)

Tunnel digger; needs an unobstructed entrance when scampering from its enemies (snake, hawk, owl, fox, coyote, bobcat, weasel, badger, and skunk).  Body, 4 3/4 to 5 2/3 inches; tail, 1 3/4 to 2 4/5 inches.  Ears nearly hidden by fur.  Prefers stream banks, grassy meadows, and salt marshes.  Ranges throughout the Bay area.  Produces 2 to 9 young in a litter; several litters a year.  Independent at the age of two weeks. 


California Mouse (Peromyscus californicus)

Body, 3 4/5 to 4 3/5 inches; tail 5 to 5 4/5 inches.  Large ears.  Prefers to live in chaparral-oak areas.  Common Bay region mammal.  Frequently found in the nests of wood rat.

Dusky-footed Wood Rat (Neotoma fuscipes)

Body, 7 3/5 to 9 inches; tail, 6 4/5 to 8 2/3 inches.  Large ears.  Common in Bay region.  Active at night.  Prefers dense chaparral or woods.  Builds stick-pile lodge on the ground or in tree.  Makes its nest of shredded bard and dry grass inside the lodge.  Nest may hold mice, lizards, salamanders, snails, and beetles.  Produces 3 to 4 young in each litter; more than one litter a year.  Food consists of acorns, seeds, leaves, grains, roots, and fruits.

Desert Cottontail (Sylvilagus auduboni)

Body, 12 to 15 inches; ear, 3 to 4 inches.  Inhabits burrows or shallow depressions; also hides in berry patches.  Ranges throughout the Bay area.  Produces 2 to 6 young in each litter; two or more litters a year.  Eats grasses, leaves, and fruits. 

Botta Pocket Gopher (Thomomys bottae)

Body 4 4/5 to 7 inches; tail, 2 to 3 3/4 inches.  Strong claws.  Powerful incisors penetrate hard soil, often hard-packet road beds.  Called the "little bulldozer' because chin and chest are used to push dirt along tunnel.  Carries food and nest material in fur-lined cheek pouches, one on each side of its mouth.  Nests in deep burrows.  Ranges throughout the Bay area.  Usually one litter (1 to 13 individuals) a year.  Eats green stalks, grains, roots, bulbs, and tubers. 

Coyote (Canis latrans)

Swift runner; often hunts in teams.  Body, 32 to 37 inches; tail, 11 to 16 inches; height at shoulder, 23 to 26 inches.  Common in Bay region.  Likes open country.  One litter (3 to 9 individuals) a year.  Eats rodents, rabbits, and carrion.

Gray Fox (Urocyon cinereoargenteus)

Keen sense of smell and hearing.  Body, 21 to 29 inches; tall, 11 to 16 inches; height at shoulder, 14 to 15 inches.  Lives primarily in chaparral or chaparral-oak regions.  Ranges throughout the Bay area.  Dens in crevices or under large rocks.  One litter (2 to 5 individuals) a year.  Eats fruits, insects, rodents, and rabbits.

Badger (Taxidea taxus)

Body 18 to 22 inches; tail, 4 to 6 inches.  Likes open country.  Digs easily in hard soil.  Makes burrows or dens deep underground.  Ranges throughout the Bay area.  One litter (1 to 4 individuals) a year.  Eats reptiles, rabbits, and rodents.  Leaves trail of potholes and piled earth when digging for food.

Black-tailed Deer (Odocoileus hemionus columbianus)

Body, 58 inches; tail 5 1/2 inches, all black on upper surface; height at shoulder, 3 feet.  Prefers forests, woodland, and chaparral.  Ranges throughout the Bay area.  Young (1 to 3 individuals) born in spring or early summer.  Eats plants.

Ground Sloth (Paramylondon harlani)

Common throughout North America during the Pleistocene.  Height at shoulder, about 4 feet; massive and powerful claws.  Moved slowly and with great difficulty.  Sabercat, dire wolf, and probably bear were its enimies.

Irvington Mammoth (Mammuthus columbi)

Large and heavy; height at shoulder, about 10 1/2 feet; molars, high-crowned with many ridges.  Abundant during the Pleistocene.  Related to mastodon.  "Mammoth" is a term general applied to extinct elephants.

Irvington Mastodon (Mammut americanus)

Smaller than the mammoth.  Height at shoulder, from 6 to 9 feet; teeth, low-crowned with few ridges; tucks and lower jaw, whort-as in modern elephant.

Irvington Sabercat (Smilodon californicus)

Larger than the modern tiger; short tail; strong forelegs with powerful claws.  Lower canines, small; upper canines-long, curved sabers-used for stabbing thick-skinned prey (probably mammoths, mastodons, and ground sloths).  Lower jaw opened to form a right angle-90 degrees.


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