A Walk Through Time

All Creatures Great And Small

175 Million Years Ago

Huge dinosaurs rove mid-Jurassic Earth. Bigger is not necessarily better: larger life forms require more space and food, and have fewer offspring and fewer survival options in times of change. Microscopic "water bears," which survive today, appeared 200 million years before the huge dinosaurs. The Californian water bear loves to cling to moss and lichens with its tiny claws.

The dinosaur and the water bear are archetypes of animal mega-micro waverings. Getting smaller is not uncommon in evolution. The fossil record suggests that beings which "miniaturize" are those most likely to survive mass extinction crises.

Top: Sauropods, reaching 120 feet in length, are representative of the giant dinosaurs of the Jurassic Period. (Illustration by Jan Sovak)

Bottom; Tardigrades ("slow-step") or water bears, tolerate extremes. Water bears can survive almost total desiccation: rolling up into their tuns (mini-wine-cask shapes), they can hold out as long as 100 years awaiting water. They tolerate temperature ranges from 151 degrees C to -270 degrees C (almost absolute zero). They tolerate X-radiation: the lethal dose of X-rays for human beings is about 500 roentgens; for water bears, 570,000, making the bears of interest to scientists working on future space travel. They are tough, and many species can reproduce parthenogenetically (females hatching females on their own). (Photograph by Jerome Paulin © Mrs. Jerome Paulin)

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