Marine extinctions and recovery

In the seas, several major Tertiary biotic events stand out. The major extinction event at the boundary between the Mesozoic and Cenozoic eras, 66 million years ago, affected not only the dinosaurs of the terrestrial environments but also large marine reptiles, marine invertebrate faunas (rudists, belemnites, ammonites, bivalves), planktonic protozoans (foraminiferans), and phytoplankton. The recovery of biological diversity after this event took hundreds of thousands to millions of years, depending on the group. At the boundary between the Paleocene and the Eocene, between 30 and 50 percent of all species of deep-sea benthic foraminiferans became extinct in a sudden event associated with the warming of the deep oceans. The present-day fauna of the deep, cold oceans (the so-called psychrosphere) evolved in the latest part of the Eocene about 35 million years ago. This was concomitant with a significant cooling of oceanic deep waters of some 3–5 °C (5.4–9 °F). The transition between the Eocene and Oligocene was also marked by several extinction events among marine faunas. The closure of the Tethys seaway in the late Early Miocene about 15 million years ago resulted in the disappearance of many of the larger tropical foraminiferans called nummulitids (large lens-shaped foraminiferans) whose habitat ranged from Indonesia to Spain and as far north as Paris and London. Although the descendants of nummulitids can be found today in the Indo-Pacific region, they show much less diversity.

The marine faunas of the eastern Pacific and western Atlantic region were similar throughout the Tertiary until about 3–5.5 million years ago. The elevation of the Central American isthmus at that time created a land barrier between the two regions that during the Tertiary resulted in the isolation of one fauna from another and differentiation (that is, “provincialization”) between the groups. In addition, the presence of the isthmus may have led to environmental changes in the western Atlantic that caused high rates of extinction in old species and the origination of new ones.

Radiation of invertebrates

In the oceans, patterns of evolution that had begun during the Cretaceous Period continued and in some cases accelerated during the Tertiary. These include the evolutionary radiation of crabs, bony fish, snails, and clams. An increase in predation may have been an important driving force of evolution in the sea during this time (see community ecology). Many groups of clams and snails, for example, show increased adaptations for resisting predators during the Tertiary. Episodes of rapid diversification also occurred in many groups of clams and snails during the Eocene Epoch and at the Miocene-Pliocene boundary. Following the extinction of the reef-building rudists (large bivalve mollusks) at the end of the Cretaceous, reef-building corals had recovered by the Eocene, and their low-latitude continuous stratigraphic record is taken as an indicator of the persistence of the tropical realm.

Large marine animals

Cetaceans (whales and their relatives) first appeared in the early Eocene, about 51 million years ago, and are thought to have evolved from early artiodactyls (a group of hoofed mammals possessing an even number of toes). Whale evolution accelerated during the Oligocene and Miocene, and this is probably associated with an increase in oceanic productivity. Other new marine forms that emerged in late Paleogene seas were the penguins, a group of swimming birds, and the pinnipeds (a group of mammals that includes seals, sea lions, and walruses). The largest marine carnivore of the period was the megalodon (Carcharocles megalodon), a shark that lived from the middle Miocene to the late Pliocene and reached lengths of at least 16 metres (about 50 feet).

Foraminiferans

Foraminiferans, especially those belonging to superfamily Globigerinacea, also evolved rapidly and dispersed widely during the Tertiary Period. Consequently, they have proved to be extremely useful as indicators in efforts to correlate oceanic sediments and uplifted marine strata at global and regional scales. Differential rates of evolution within separate groups of foraminiferans increase the utility of some forms in delineating stratigraphic zones, a step in the process of correlating rocks of similar age. For example, conical species of Morozovella and Globorotalia are often used to correlate rock strata across vast geographies because they have wide stratigraphic ranges that vary from one to five million years.

The nummulitids were a group of large lens-shaped foraminiferans that inhabited the bottoms of shallow-water tropical marine realms. They had complex labyrinthine interiors and internal structural supports to strengthen their adaptation to life in high-energy environments. Nummulitids also received nourishment from single-celled symbiotic algae (tiny photosynthetic dinoflagellates) they housed within their bodies. Nummulitids of the genus Nummulites grew to substantial size (up to 150 mm [6 inches] in diameter), and they occurred in massive numbers during a major transgression taking place during the middle of the Eocene Epoch. This transgression produced high sea levels and formed extensive limestone deposits in Egypt, which produced the blocks from which the pyramids were built. Nummulites lived throughout the Eurasian-Tethyan faunal province from the later part of the Paleocene Epoch to the early Oligocene, but it did not reach the Western Hemisphere. Following the extinction of Nummulites, other larger foraminiferans, the miogypsinids and lepidocyclinids, flourished.