Gobi rangelands are fragile and easily destroyed by overgrazing, which results in expansion of the true desert, a stony waste where not even Bactrian camels can survive. The arid conditions in the Gobi are attributed to the rain shadow effect caused by the Himalayas. Before the Himalayas were formed by the collision of the Indo-Australian plate with the Eurasian plate 10 million years ago, Mongolia was a flourishing habitat for major fauna but still somewhat arid and cold due to distance from sources of evaporation. Sea turtle and mollusk fossils have been found in the Gobi, apart from well-known dinosaur fossils. Tadpole shrimps ("Lepidurus mongolicus") are still found in the Gobi today. The eastern part of Mongolia including the Onon and Kherlen rivers and Lake Buir form part of the Amur river basin draining to the Pacific Ocean. It hosts some unique species like the Eastern brook lamprey, Daurian crayfish ("cambaroides dauricus") and Daurian pearl oyster ("dahurinaia dahurica") in the Onon/Kherlen rivers as well as Siberian prawn ("exopalaemon modestus") in Lake Buir.