Bivalve mollusks and other shelled invertebrates of the sea floor also suffered greatly at the end of the Cretaceous. Rudists, bivalve mollusks and the dominant reef builders of the Cretaceous seas, declined sharply towards the end of the Period, and disappeared entirely at its close. Brachiopods, which are bivalved but not mollusks, also suffered greatly, but managed to survive in severely restricted numbers to the present.
Perhaps the most dramatic extinctions in the sea were among the nannoplankton, minute calcium-secreting algae, and the foraminiferans, calcium-secreting protozoans. Their abandoned shells piled up in immense thickness to form the great chalk cliffs that give the Cretaceous Period its name. ('Cretaceous' comes from the Latin word for 'chalk.') Marine sediments during the Cretaceous Period were comprised almost entirely of this chalk, with only a small percentage of clay particles. Sediments deposited immediately after the K/T boundary is dominated by clay particles, with only 20 to 40% being chalk. This clay layer, known as the "Fish Clay" in Europe, is widely accepted worldwide as the boundary between Cretaceous and Tertiary sediments. It ranges in thickness from less than one-half inch (~1 cm) to over three feet (~1 m) in thickness.
Thus the K/T boundary exhibits a drastic reduction in the abundance of calcium-secreting organisms. Assuming that the clay particles, derived from the erosion of nearby continents, continued at the same rate across the K/T boundary, this represents approximately a 97% reduction in the abundance of marine calcareous algae.
Many species of mammals also survived the extinction, as many mammalian orders also have Cretaceous representatives. Cretaceous mammals, however, tended to be quite small, and probably were predominantly nocturnal. Freshwater animals and the smaller terrestrial cold-blooded vertebrates, reptiles and amphibians, were largely unaffected by the K/T extinction.
Land plants were for the most part unaffected by the extinction event. One prominent plant community, however, was nearly obliterated at the end of the Cretaceous. This assemblage of predominantly by angiosperms (flowering plants) and conifers, is technically known as the Aquilapollenites botanical province, which flourished right up to the end of the Cretaceous Period in, among other places, Western North America. Western North America was separated from the rest of the continent throughout the Cretaceous by a body of water known as the Great Interior Seaway. The Aquilapollenites plant community occurred along the western margin of this seaway, the same location as some of the richest dinosaur finds in the world. These deposits are found from New Mexico through Wyoming, Montana, and Alberta. At the end of the Cretaceous Period in this region, above the Aquilapollenites sediments and the inevitable clay layer (which is reminiscent of the Fish Clay of Europe, and here one half (~1 cm) to over one inch, or ~3 cm thick) is found a layer of coal, which represents the remains of a fauna made up almost exclusively of ferns. After the coal layer, angiosperms return to the scene, but this time a different assemblage of species is found.
Even more, a really good extinction theory should explain more extinction events than just the K/T. The terminal Cretaceous event was just one of well over a dozen mass extinction events that have occurred over the last 600 million years -- and not the most recent nor the most severe. The most recent mass extinction, not counting extinctions attributable to mankind, occurred during the Miocene Epoch of the Tertiary Period, a mere 12 to 14 million years ago. This was a fairly mild extinction, as mass extinction events go.
The most severe mass extinction occurred during the Late Permian to the end of the Permian Period, between 248 and 256 million years ago. Where the terminal K/T event wiped out perhaps 20% to 25% of known species, the end Permian event eliminated a whopping 95% to 96% of known species (see Note 3.). This appears to have been a rather protracted extinction event, occurring in waves over a span of several million years. (The apparent suddenness of the K/T extinction contributes significantly to its mystery, but a number of researchers are questioning whether it was all that sudden after all. See Note 4.)
The second greatest mass extinction occurred near the end of the Ordovician Period, somewhere around 438 to 448 million years ago. Again, this appears to have been a more gradual extinction, wiping out over several million years an estimated 40% of the plant and animal population. The K/T event thus currently stands as the third most severe mass extinction known.
The longest interval between identified mass extinction events is on the order of 110 million years, between the Late Permian event and one at the Frasnian/Famennian boundary during the Late Devonian, around 367 or 368 million years ago. The Frasnian/Famennian event was probably the fourth worst extinction on record. The shortest interval between events, excluding the 12 to 14 million years since the last extinction, was around 24 or 26 million years, between the most recent Miocene event and one at the end of the Eocene, approximately 38 million years ago. Another 27 million years before that was the terminal Cretaceous event.
The Giant Meteor Impact Theory