Australia: The Land Where Time Began

A biography of the Australian continent 

Valley of whales see Tethys Ocean

In this location in Egypt the fossils of 15 whale species have been found, some up to 10 m long. Their full skeletons, encased in lime-rich mudstone or siltstone preserved them where they died. There are also fossils of other animals such as dugongs, manatees and turtles, as well as a variety of small fish, sharks’ teeth and a number of invertebrates. There are fossil mangrove roots in the same layer, though they are more common just above the layer with the whale fossils. The author³ suggests this indicates that the early whales were near-shore feeders that fed in shallow water around coastlines. He also suggests they were the forerunners of the modern toothed whales that include dolphins and killer whales that fed mainly on fish and crustaceans. The baleen whales didn’t evolve until much later.

The finding of several proto-whale fossils in Egypt, and a few million years later in Pakistan, as well as from scattered finds from the northern and southern margins of the Tethys Ocean from about this time, their ancestry can be derived. Following the KT boundary there were significant numbers of ecological voids left empty. On land the mammals diversified explosively, and appear to have done so in more than 1 episodes of diversifcation, to fill many of these empty niches. In the oceans the radiations were slower to occur, the teleosts (bony fish) growing larger and moving up the food chain to join the more ancestral lineage of sharks.

The return of mammals to the sea took longer. The first mammals to arise included some bizarre and cumbersome forms, some of which resembled dinosaurs that had gone extinct not long before. Indricotherium was an enormous browser of tree tops, a member of the same family as rhinoceroses. These replaced the dinosaurian sauropods in Europe. Arsinoitherium was a browsing herbivore from Africa that had magnificent horns, that was very similar to the dinosaur Triceratops. Fossils of this animal were found in the Sahara, in rocks that were slightly older than those of the Valley of Whales, named after the ancient Egyptian queen Arsinoe. Pachyaena is a large hyena-bear from South Africa that is believed to have scavenged along the sea shore as the modern brown hyena (or strand wolf) does, is believed to be ancestral to the whales. Rapid, progressive evolution from these strand hunters to wading and walking whales, to paddle swimmers, and eventually fully marine mammals that swam with their tails and were streamlined, have been shown in the fossils of some of the early proto-whale fossils. As with the wales in Egypt and all whales since, these were true whales.

Baleen and echo location

The evolution of the whales began around the same time as our ancestral primates. There is another fossil site near Calvert, Maryland, USA, where the younger rocks and contain a diverse assemblage of marine mammals including toothed whales, early dolphins, baleen whales and some pinnipeds that were at a primitive stage of evolution (family that includes seals and sea lions). These have all been dated to the Miocene (Neogene age), and represent a snapshot of evolution about 20 My later than those in the deposits in the Valley of the Whales in Egypt.

By that time the Tethys Ocean had narrowed greatly, and the climate had cooled as it closed progressively, leading to a great radiation that was caused by the temperature decline in the ocean, that included adaptations to colder conditions in the polar regions, as well as to freshwater habitats of estuaries and rivers. There were also new types of food such as plankton.

The author³ suggests that at the present the baleen whales are possibly the ultimate in evolutionary perfection when it comes to harvesting the plankton of the oceans. The largest living animal on Earth is the blue whale that eats about 4 tonnes of plankton per day. It eats mostly krill, small shrimp-like animals that is a member of the zooplankton that feeds on the phytoplankton. When the blue whale opens its mouth all in its path enters and when it closes its mouth the water is filtered through the baleen that is made of keratin and is in the form of a tight mesh, the same protein in the nails and hair of other mammals). The krill and everything else that remains after the water has been forced through the baleen mesh is swallowed.

Humpback whales have a different method of feeding; they rise slowly beneath a rich patch of plankton while exhaling to form bubbles in a ring that tends to gather the krill into a dense mass that can be easily taken in a gulp. It is not known if the bubble net method of feeding on krill had been evolved by the Miocene.

The author³ suggests that the more sophisticated use of sound may have evolved during the Miocene. At the present it is the main retrieval system among dolphins and whales of the present. The technique of emitting high frequency sounds, in the form of audible clicks and ultrasonic vibrations of more than 100 KHz, then detecting the returning reflections of the sound has been evolved by dolphins. They form extremely accurate images of the sea around them, together with the distance, size and shape of objects in the water, and even the texture and density, the species of fish and their movements. The author³ suggests recycled compressed air may be used by dolphins to produce the clicks that are produced by the dolphin’s large forehead region, the received signal being received over a broader area around the sides of the head and lower jaw. The sound is better refined by asymmetry of the jaw. The complex array of signals is decoded with the aid of a gigantic auditory lobe in the brain. The echo-location capacity of the fossil genus Kentriodon, one of the dolphin-like species recovered from the deposits in Maryland was relatively rudimentary as indicated by the perfect symmetry of its lower skull region. Lineages arose from Kentriodon that eventually gave rise to dolphins of more modern in appearance, that had echo-location that was highly developed, and to porpoises, killer whales, belugas, and narwhals.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.