97% of all animals, marine and land, which have ever existed, are not found as fossils. The Leviathon in my mind is an animal that is among the 97% who left no fossils. I will quote something from my blog:
Colin Tudge observes,
" Logic dictates, too, that the oldest known fossils cannot possibly be the oldest representatives of their kind. Fossilization is a rare event, after all; and when animals first appear, they are rare. The earliest fossil bones are therefore likely to date from a time when their erstwhile owners were already common. Logic similarly dictates that if an animal is particularly unlikely to form fossils–as primates seem to be–then paleontologists are particularly unlikely to find the very earliest types. In fact, this logic can be translated into a mathematical formula (see Robert D. Martin, ““Primate Origins: Plugging the Gaps,”” Nature, May 20, 1993, pp 223-234). The fewer fossils there are (relative to the calculated number of extinct species), the older the group is liable to be, relative to the number of fossils found. " 2
So, how rare is the fossilization of a species ? To answer that, we must ask look at how many species alive today are found as fossils. Consider what Foote et al,
" The number of living species that have been described is about 1.5 million…If we focus on the paleontologically important groups, present-day diversity is about 180,000 species. …Suppose we assume that the present-day level of diversity was attained immediately at the beginning of the Cambrian Period and has been maintained since then. Then 25 percent of 180,000 species, or 45,000 species became extinct and were replaced by new species every million years. In rough terms, the Phanerozoic is 550 million years log. this leads to an estimate that there have been 180,000+(45,000 X 550) or about 25 million species. Comparing this with the 300,000 described fossil species implies that between 1 percent and 2 percent of species are known as fossils. " 3
So,98-99% of species were never fosilized.