ATMOSPHERE - Not all fossils are bones. (Or shells. Or teeth.) Most of us would agree that mammoth tusks and Stegosaurus spikes are pretty darn cool. And yet, the fossil record is not limited to old body parts. "Fossils" are defined as any "naturally preserved remains or traces of [life forms] that existed in the geologic past."
If you'll excuse the pun, that covers a lot of ground. A fossil can take the form of a footprint, a leaf impression or a filled-in tunnel left behind by prehistoric land beavers.
One of the strangest fossils ever discovered is actually a cave. About 15 million years ago, in eastern Washington state, a volcanic fissure eruption sent lava streaming into a shallow river or lake where a rhino happened to be wallowing.
A layer of basaltic rock formed around the beast, preserving the outline of its (well-cooked) body. For millions of years, this rhino-shaped hole in the earth lay hidden in the cliffs of Washington's Grant County, near Blue Lake, a popular hiking destination.
By the 1930s, erosion had worn a hole into one end of the subterranean creature mold, exposing it to the open air. Here's the story of how the "Blue Lake Rhino Cave" came to be — and how four Seattle rock hounds accidentally discovered it.
Only five rhino species are alive today, and none of the living five species are indigenous to North or South America. However, from about 40 to 70 million years ago, rhinos were common in North America. Some — like the barrel-chested Teleoceras — were hippo-like, semiaquatic animals. Others had wicked tusks instead of the nasal horns we see in their modern-day counterparts.
Paleontologists think the Blue Lake Rhino Cave likely formed around the corpse of a Diceratherium. This type of rhino was sexually dimorphic, meaning that males and females looked visibly different from one another. While female Diceratherium were hornless, each adult male had a pair of small horns sitting side-by-side near the tip of his snout.
The dimensions of the Blue Lake cave tell us that the Diceratherium who left it behind was about 8 feet (2.4 meters) long from snout to rear and stood a little less than 4 feet (1.2 meters) tall at the shoulder. In life, the animal probably weighed 1 ton (0.9 metric tons) or so.
Nobody knows if the creature had already died when it became entombed. However, judging by the contours of the mold, it seems the body was rather bloated. This could indicate that decomposition was already setting in. Also, the legs are pointed skyward, telling us the rhino may have been floating on its back in a state of rigor mortis.
The cave's walls are made of 15-million-year-old pillow basalt, a kind of igneous rock that normally forms when lava contacts cold water and rapidly cools down. So dead or alive, the Diceratherium must've been hanging out in a body of water during a volcanic eruption. Then the lava came pouring in.
Lava can hit temperatures of more than 1,600 degrees Fahrenheit (900 degrees Celsius). Ordinarily, this ultra-hot material would've burned right through the beast's skin, flesh and bone. But instead, the cold water converted the molten rock into a tightly packed layer of hardening pillow basalt.
The corpse eventually rotted away and most of its bones disappeared. Yet the mold that enveloped the body stayed largely intact.[HWS]