EVOLUTION OF THE WHALES
Christmas is almost here, so what better topic to post on than the evolution of the whales!?!?
65 million years ago the earth was devastated by a catastrophic meteor impact that resulted in the death of the dinosaurs. Mammals back then had been slowly evolving but remained small and mostly nocturnal, yet when the dinosaurs perished they quickly took over abandoned niches in the skies, land and water becoming one of the most incredible dynasties the world has ever seen.
Some of the most beautiful of this diverse group are the marine mammals, the cetaceans (whales, dolphins etc). The ancestor of the whales left life on land to make the oceans their home. With over 86 species existing today of whales, dolphins and other marine mammals, they make up the order Cetacea which include the magnificent blue whale reaching a whopping 30 metres in length to the little known porpoise species, Vaquita reaching only 4.5 metres long.
The evolution of the whales is documented in an incredibly rich fossil record dating back to over 50 million years ago…
Pakicetus, 50 million years ago
Pakicetus is regarded as the most basal (earliest) whale. Although Pakicetus primarily lived on land, it is the first of the land mammals to show significant developments towards a future in ruling the oceans. Pakicetus is known from only a few incomplete specimens found in Pakistan but it is predicted to have been about a metre in length. Interestingly, Pakicetus was an artiodactyl (or even-toed ungulate, an order which includes the giraffes, camels, pigs and cows), however, Pakicetus shows some defining characteristics of evolving for life in the water such as elongation of the skull and body and the teeth begin to lose the heterodontus nature. The eyes of Pakicetus were also high on its head suggesting a capability to hunt not only on land but in water too.
Ambulocetus, 49 million years ago
Ambulocetus literally means “walking whale” and shows more extreme divergence towards an aquatic lifestyle. Ambulocetus shows even greater elongation of the skull and simplification of its dental morphology. Unlike the marine reptiles of a bygone era, Ambulocetus would have swam through the water with vertical motion. The morphology of Ambulocetus’ inner ear is also similar to that of modern cetaceans meaning it could probably hear well underwater. Ambulocetus also shares some similarities with modern crocodiles such as high nostrils, pointed teeth and a long skull, making it likely that Ambulocetus was a deadly ambush predator, a far cry from its gentle giant descendants.
Rodhocetus, 46 million years ago
Rodhocetus fossils are also restricted to Pakistan and beautifully depict a familiar whale like skeleton with much shorter limbs and elongated hands and feet (that were most likely webbed). The nasal openings of Rodhocetus has also moved higher up the skull and closer to the eyes. Again, Rodhocetus shows specific morphologies that are characteristic of artiodactyls, they have a double-pulley astralagus (heel bone) found in all modern even toed ungulates.
Basilosaurus, 37 million years ago
Basilosaurus is probably the earliest skeleton that very closely resembles modern whales, the name means “king lizard” which is highly inaccurate but fits well when considering that Basilosaurus had a long and slender body that could reach an almighty 18 metres in length. Basilosaurus also shows an extraordinary reduction in limb size compared to its ancestors meaning it is in no way adapted to live on the land any longer. At the time of Basilosaurus’ existence it was one of the largest marine animals to have existed since the days of marine reptiles (such as Liopleurodon and Mosasaurs). The teeth of Basilosaurus had similar morphology to modern killer whales indicating they were highly active hunters.
The whales of today are some of the most remarkable creatures to have ever existed. We often stand and stare in awe at the immense sizes of prehistoric marine animals in museums and it is easy to forget that we are living at the same time of the largest animals to have ever existed, past of present, the whales. We then often neglect to appreciate how magnificent these creatures are. Sadly this has led to a massive depletion in their numbers and diversity due to pollution, fishing and hunting. The whales and all other cetaceans have some of the most wonderful social structures known in the animal kingdom as well as incredible intelligence. In the last 50 million years this order has conquered oceans across the world and delighted humans all over. Cetaceans are fast becoming more endangered and if we do not act, in years to come our descendants will wonder how their ancestors let these wonderful creatures slip through their fingers.