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Lestodon armatus Pleistocene Pampas Beds, La Paz, Uruguay One of the most bizarre examples of the gigantic critters that lived during the Pliocene and Pleistocene in South America, Lestodon armatus possessed the largest mouth of any Giant Ground Sloth in the fossil record – a feature amply illustrated by the present remarkable specimen. Up to 13 feet in length and weighing up to 3,000 pounds, adults probably had few natural predators. Length measures 24in; Height 18 1/2in, Width 12in Estimate: $50,000 - 60,000 (Bonhams)

Ursus spelaeus Pleistocene Romania Cave bears were larger than any bear alive today, standing over 10 feet tall. Ursus spelaeus lived 50,000 to 100,000 years ago in Romania and other parts of Northern Europe. The present complete skull of excellent quality is 19 inches long, displaying ivory colored bone and a complete set of perfect teeth, including 2.5-inch-long canines with lovely cream- to orange-hued enamel. Length measures 19in; Canine teeth measure 2 1/2 in Estimate: $6,000 - 7,500 (Bonhams)

Stegodon sp. Early Pleistocene Southeast Asia Stegodon specimens are exceedingly rare. A 1-2 million-year-old member of the Elephantidae subfamily, this genus is considered the ancestor of elephants, mastodons and mammoths. The genus Stegodon, from the Greek, stegos, meaning roof, and odon, meaning tooth (referring to the step ridges on the molar tooth) originated in the Late Miocene in Asia. Height of specimen measures 39in; length 87 1/2in; width 29in Estimate: $60,000 - 80,000 (Bonhams)

Coelodonta antiquitatis Pleistocene Jeneisey River, Krasnovarsk region, Southern Siberia, Russia Native to the northern steppes of Eurasia, the extinct woolly rhinoceros was one of the largest animals to inhabit the vast Arctic tundra of the Pleistocene, second in size only to the mastodon The woolly rhino, like the mastodon, exhibited a long shaggy coat. Other evolutionary attributes included small ears, short, thick legs and a stocky body. Its massive horns are thought to have been used to sweep away snow to forage for vegetation and possibly to attract mates. A member of the Pleistocene megafauna, this creature survived the last glacial period. Interestingly, its nasal horn was composed of keratin (the same material that composes human finger nails), not bone. Estimate: $70,000 - 90,000 (Bonhams)

Odobenus rosmarus rosmarus 20,000 years to recent Alaska Inclusive of the original lower jaw, this is a large and impressive fossilized walrus skull, exhibiting original tusks and showing excellent preservation. Mounted on a custom display stand. Measures 29 x 13 x 10in Estimate: $3,500 - 4,500 (Bonhams)

Paleokerabau bubalus Pleistocene Southeast Asia Measuring a magnificent eight feet on the curve, the horns on this keraba skull are awe-inspiring. This formidable species must have lived in an open grassy environment, as its breath-taking horns would have hindered movement in forested terrain. This magnificent fossil was found in four separate pieces that fit together perfectly. Considering the habits of its modern relative, the water buffalo, and the buffalo of Africa, it is likely that the kerabau would have frequented marshy areas during the heat of the day. Measures 66in from tip to tip Estimate: $30,000 - 40,000 (Bonhams)

Mammthus primigenius Pleistocene Indigirka Basin, Siberia, Russia Woolly mammoths were a very successful species; in parts of Siberia alone, their population density is estimated to have been 60 animals per hundred square kilometers– a similar distribution to the African Elephant of today. The present slender woolly mammoth tusk is nicely polished. Offered on a custom stand. Measures 49.5in; diameter 3in Estimate: $1,250 - 1,750 (Bonhams)

Tyrannosaurus rex Cretaceous Hell Creek Formation, Montana Measuring over two feet long on the curve, a metatarsal bone of a T. rex attests to the massive size of these famous Cretaceous carnivores. Length measures 25in Estimate: $1,200 - 1,500 (Bonhams)

Brachiosaurus Jurassic Northwest Africa Unlike other dinosaurs, and even other brachiosaurids, Brachiosaurus' front legs were longer than its hind legs, which provides an explanation for the stunning size of the present humerus (front upper leg bone) specimen. One of the tallest and largest dinosaurs yet found -- a sauropod grew up to 50 feet in height and 85 feet in length -- Brachiosaurus' giraffe-like stance was produced by the length of its front legs combined with the length of its neck. An herbivore, it probably ate vegetation from the tops of trees, using its chisel-shaped teeth. Measures 76in Estimate: $15,000 - 20,000 (Bonhams)

Tyrannosaurus rex Late Cretaceous Hell Creek Formation, Harding County, South Dakota The unusual appearance and exceedingly large size of this Tyrannosaurus rex tooth places it in a category of its own in terms of value and rarity. The presence of blue-gray and yellow dendrites is consistent with the result of the action of stomach acid leaching minerals from the tooth. Thus, the tooth is thought to have broken off and been swallowed, perhaps during combat. Collected in 1984 near the town of Buffalo, South Dakota on private ranch land, the 68-65-million-year-old specimen is most likely from the anterior maxilla (upper jaw). Length measures 4 3/8in; Width measures 1 5/8in; Thickness measures 1 1/8in Estimate: $12,000 - 15,000 (Bonhams)

Xiphactinus audax Cretaceous Niobrara Formation, Western Kansas While dinosaurs roamed the land masses, Xiphactinus audax -- the largest of all bony fish species -- menaced the intracontinental seaway that divided the western third of North America from the rest of the continent. The species' only enemies were the gigantic Mosasaurs and the sharks of the period. The massive skull and dagger-like teeth of the present specimen attest to its predatory habit. Length measures approximately 12 feet Estimate: $150,000 - 200,000 (Bonhams)

Trionyx sp. Late Cretaceous Lance Creek Formation, Wyoming Collected from a private ranch in Eastern Wyoming, this complete turtle shell was found in association with dinosaur bone in a sandstone sediment from the Late Cretaceous. Prepared free from matrix, the shell is from a soft shelled turtle that is distantly related to modern Trionichids. Turtle Shell measures 19.5 x 15in Estimate: $5,000 - 7,000 (Bonhams)