Velokovski's close approach of the protoplanet Venus (He correctly predicted both the temperature and composition of Venus’ atmosphere before any probes were sent. The prevailing view at that time was that Venus was a warmer version of Earth).
The Northern Ice Cap had massive freshwater lakes - think the Great Lakes super sized - which collapsed into the warmer oceans sending water vapor into the stratosphere, which then fell back to Earth supercooled, flash freezing the beasts.
Theories abound, but none can be verified, currently.
from “Earth in Upheaval” (p 3)
High in the north above Siberia, six hundred miles inside the Polar Circle, in the Arctic Ocean, lie the Liakhov Islands. Liakhov was a hunter who, in the days of Catherine II, ventured to these islands and brought back the report that they abounded in mammoths’ bones. “Such was the enormous quantity of mammoths’ remains that it seemed . . . that the island was actually composed of the bones and tusks of elephants, cemented together by icy sand.” 3
The New Siberian Islands, discovered in 1805 and 1806, as well as the islands of Stolbovoi and Belkov to the west, present the same picture. “The soil of these desolate islands is absolutely packed full of the bones of elephants and rhinoceroses in astonishing numbers.” 4