Fossil are Rare

Probably only a fraction of the vertebrate animals that once lived in California (or anyplace else) have been discovered in fossil form. There are many reasons for this. The major requirement for bone fossilization is quick burial. Quick burial reduces the oxygen supply needed by bone-destroying bacteria. It also prevents scavenging birds, mammals, and insects from getting at the remains. Animals could have been buried quickly in the past by landslides along deep channels or gullies, by drifting sands at the bases of cliffs, by quicksands, or by other natural phenomena.

But it is unlikely that all of the Irvington animals were buried immediately after death. And even when quick burial and fossilization occurred, the remains could have been destroyed. Soil acids could have done this. A good example is the large end of a mammoth’s femur taken from the Irvington deposits. This specimen weighed less than a pound, even though it was about 5 inches in diameter. Normally, a fossilized fragment this size would weigh at least 4 pounds. Mineral matter (probably calcium carbonate) that once filled the holes in the cellular structure of the specimen had been dissolved by soil acids and carried away by percolating waters. A smaller bone that had been exposed to such erosion probably would have completely disintegrated before fossil hunters arrived on the sense. Note the broken Irvington bone in Figure 45. Had it not been discovered when it was, soil acids probably would have destroyed it completely before too long. In such cases as these, the acid is aided by tree or other plant roots that penetrate the bones. The bones crack and eventually break into small fragments, which are then more vulnerable to acids.