Most of the Earth has always been covered by salt water. Starting from the late Cretaceous we begin to see the continents drifting into their modern positions. These geologic events had profound effects on global climate, which gradually became cooler and drier on average from the Eocene to the present day. Also, we see very early on how marine animals recovered following the devastation of the late Cretaceous and achieved its modern composition. Unlike terrestrial or freshwater animals, marine animals are able to achieve a truly worldwide distribution because there are no barriers to obstruct their expansion. As a result, many oceanic species are quite long-lived. The modern Great White Shark, for example, has existed for at least 16 million years, and its relative Megalodon was around for 28 million years.
|Tropical reefs like this one could be found in shallow|
waters all over the world during the late Paleocene.
|Ambulocetus natans, one of the first whales of the|
early Eocene. This species lived in both saltwater
and freshwater habitats.
|Dourodon atrox, a basilosaurid whale of the late Eocene. |
These were the first whales to have become fully incapable
of leaving the water, having rigid flippers and vestigal
Two significant tectonic events occurred during the Eocene; India collided with southern Asia around 45mya, and Arabia connected with west Asia 34mya around the Eocene/Oligocene boundary. As a result, the Tethys Sea had disappeared, possibly setting in motion the cooling trend that continued from the Oligocene to the Pleistocene.
|Reconstructed skeleton of Megalodon (Carcharocles |
megalodon) on display at the Calvert Marine Museum.
After evolving during the middle Oligocene, this giant
shark ruled the seas for almost 30 million years.
|Acrophoca longirostris, a long-bodied seal closely related|
to the extant Leopard Seal (Hydrurga leptonyx), and
possibly ancestral to it. It is known from late Miocene
rocks off the coast of South America.
|The Great White Shark (Carcharodon carharias) was the|
same 15mya as it is today. It evolved in response to
the adaptive radiation of pinnipeds at that time.
|The formation of the Panamanian Land Bridge 3.5mya led|
to changes in ocean circulation, which led to the formation
of the Greenland ice cap and the beginning of the glacial
activity that would characterize the Pleistocene.
|The Killer Whale (Orcinus orca) evolved during the Pliocene |
and survives virtually unchanged. Later by the end of the
Pleistocene it would become the top predator of the world's
surface waters following the extinction of Megalodon.
Starting in the middle Pliocene, northern and southern parts of the world began to experience more pronounced, seasonal drops in temperature. For the first time we see the evolution of truly cold-adapted land mammals such as bison, mammoths, and lynxes. Despite these events, the marine fauna remained largely unaffected. The majority of the animals around at the time would have been very familiar to us today with modern types of cetaceans, pinnipeds, sirenians, sharks, turtles, penguins, and sea birds.
|Polar Bears (Ursus maritimus) evolved from Brown Bears |
(Ursus arctos) during the Pleistocene and specialize
in hunting on the sea ice. During peak glacial conditions
this bear's range would extend as far south as
Harrison & Bryden. "Whales, Dolphins, and Porpoises". Facts on File, November 1988
Haines, Tim. "Walking With Prehistoric Beasts: A Prehistoric Safari". DK Publishing, November 2001
Hooker, J. J., "Tertiary to Present", pp. 459-465, Vol. 5. of Selley, Richard C., L Robin McCocks, and Ian R. Plimer, Encyclopedia of Geology, Oxford: Elsevier Limited, 2005. ISBN 0-12-636380-3
Barron, J, Bralower, T., Huber, M., Lyle, A., Ravelo, C., Rea, D., Wilson, P. (April 2008). "Pacific Ocean and Cenozoic Evolution of Climate". Reviews of Geophysics 46 (2): 1-47
- Reef system: Sean Conolly, 15 August 2006, Wikimedia Commons
- Coral Reef: Jim Maragous/U.S. Fish and Wildlife Service, 27 March 2011, Wikimedia Commons
- Ambulocetus skeleton: Ghedo,13 August 2011, Wikimedia Commons
- Dourodon skeleton: EveK, 2 September 2007, Wikimedia Commons
- Megalodon skeleton: Dr. Alton C. Dooley, Wikimedia Commons
- Great White Shark: Hermanus Backpackers, 10 March 2009, Wikimedia Commons
- Ice ridges of northern Alaska: Rear Admiral Harley D. Nygren, NOAA Corps, Spring 1949, Wikimedia Commons
- Killer Whales: Robert Pittman, 2005, Wikimedia Commons
- Polar Bear: Alan Wilson, 2007, Wikimedia Commons