Cenozoic are “eras,” and each era comprises several “periods”; the
Mesozoic, for example, spans the Triassic, the Jurassic, and the
Cretaceous.) The fossils from the three eras were so different that Phillips
thought they represented distinct acts of creation.
This sketch by John Phillips shows the diversity of life expanding and contracting.
Lyell was well aware of these breaks in the fossil record. In the third
volume of the Principles of Geology, he noted a “chasm” between the plants
and animals found in rocks from the late Cretaceous period and those
found directly above, at the start of the Tertiary period (which is now
technically known as the beginning of the Paleogene). For instance, late
Cretaceous deposits contained the remains of numerous species of
belemnites—squid-like creatures that left behind fossils shaped like bullet
casings. But belemnite fossils were never found in more recent deposits.
The same pattern held for ammonites, and for rudist bivalves—mollusks
that formed immense reefs. (Rudists have been described as oysters
pretending to be corals.) To Lyell, it was simply impossible, or
“unphilosophical,” to imagine that this “chasm” represented what it
seemed to—sudden and dramatic global change. So, in a rather neat bit of
circular reasoning, he asserted that the faunal gap was just a gap in the
fossil record. After comparing the life forms on both sides of the supposed