20. Aug, 2017

The ancient whales: transitions from land to sea.

Once upon a time, long before humans made their entry in the wold,  large and strange  animals were swimming in our oceans. They had  sharp teeth,  a long tail and a kind of paddles at the side of their body looking like hands and feet. Some had  tiny hind limbs and flipper-shaped fore limbs, others large  feet and  a long tail like crocodiles. These strange and fierce looking creatures  were the ancestors of our modern whales. These extinct whales or Archaeoceti  (‘ancient whales’)  lived in an period stretching from  the  early Eocene to the late Oligocene, 55 to 23 millions of years ago. Archaeoceti possessed land and mammal characteristics, demonstrating the evolutionary transition from land to sea. Many of them were descendants  from even-hoofed land animals called Artiodactyla, with long skuls and carnivorous teeth  related to our hippos, dear and pigs. Whales move their tales up and down, which is much like the undulating movements  of the vertebral column of running four legged mammals living on land.

Varieties The ancient whales consisted of five families with many subspecies, some of which paleontologists managed  to reconstruct out of their  bony parts and skulls.  Initially many must have lived in shallow near-shore environments such as estuaries and bays. Some species still lived on land, others were amphibious. Examples are Pakicetus, an extinct genus of amphibious cetacean of the Pakicetidae family, that had long slender legs, a long narrow tail, and could reach the size of a modern wolf. Rodhocetus from the Protocetidae family (see the artist impression inserted above)  also  lived amphibiously, while Protocetus  from the same family must have been fully aquatic. Ambulocetus  from the  Ambulocetids family discovered in Pakistan was able to  walk on land, but probably spent most of  its time in  ancient estuaries with brackish water. An important tool for scientists to determine if a sea animal lived most of its time in fresh or salt water -which it also drunk- are the different ratios of oxygen isotopes in its bones.

Overselling of whale evolution?  Some opponents of the evolutionist view have argued that the ‘evolution of whales’ scenario could be  but a fairy tale devoid of any scientific evidence*. For example, Ambulocetus could also have been a four-footed animal, similar to that of common wolves, found in a region containing fossils of such terrestrial creatures as snails, tortoises or crocodiles.  In other words, it could  have been  an isolated species of a land animal, not an aquatic -or transition to aquatic- one. In order to suggest a transition from land to water artistic retouches have drawn webbings on its front feet, which are hard to find on the fossile's skeleton. It is true that it has been hard to derive from fossile bones of certain land animals specific characteristics that signal a transitional stage to sea mammals. One example is the presence of sound transmission structures found in the lower jaws. Another are isotopes of oxygen in the bones that reflect in what kind of watery environment the creature lived. But these -admittedly- vague indications will probably  never convince the rigid creationist. I personally  do not see a contradiction between evolution and  (divine) creation. Whats wrong with the view that evolution provides a scientific basis of the creation of species, divine or not divine? Certainly, even a divine job could never  have been done in seven days.

Fosssile hotspots have  been discovered at  various places of the World,  not only in  India, Pakistan and Egypt, but surprisingly also in coastal waters of North-West Europe.  In Dutch-Belgian coastal waters, for example, an unique cemetry of buried bones of long lost mammals has recently been discovered.  In this specific region old soil layers of different ages are found very close to the shallow seabed.  The findings included among others numerous fossilized shark teeth, the remains of walruses from the last Ice Age (116.000-12.000 years ago) and vertebrae of primal whales from the warm Eocene period (40 million years ago). The whale vertebrae possibly belonged  to the family of Protocetidae. The fact that the bones were mostly intact and the finding of both males, females and young animals suggest that these sea giants had formed a local colony in the cold climate of the last Ice Age. The landscape in the widened estuary mouth of the river de Schelde  was then a tundra  with  large grazers  like woolly mammoth, woolly rhino, hippo, giant deer, forest elephant and steppe wisent.  

Adaptations  Later species had long snouts, large eyes, and a nasal opening located further up the head than in earlier archaeocetes. Which suggests that  they could breathe with the head held horizontally, similar to modern cetaceans — a first step towards a blowhole. Another typical area  present in the skulls of ancient wales is the ear region, which is surrounded by a bony wall just like in the now  living whales.  In a much later stage of their evolution the ancient whales split up in two cetacean suborders,  the Odontoceti (the toothed whales) and Mysticeti  (the baleen whales). So the Odonteci kept their teeth and developed into our toothed whales  which  include the porpoises, dolphins, killer wales  and the sperm wales. The other half,  the Mysticeti, changed their teeth for hairy curtains or baleenes, like in the humpback and blue whales. Reflecting that they must have gradually switched to suction feeding. This meant a more effective way of feeding on and filtering out  the tiney preys and large amounts of krill that drifted  abundantly in the colder parts of the oceans. Probably this transition went via an intermittent stage  when these whales possessed teeth and baleens at the same time. Other non cetacean filter feeders like  the whale and basking sharks do not use baleens but filtering plates.


Source and links

Bajpai, S; Thewissen, JG; Sahni, A (2009). "The origin and early evolution of whales: macroevolution documented on the Indian subcontinent" (PDF). J Biosci34 (5): 673–86. OCLC 565869881PMID 20009264

Thewissen, J. G. M.; Hussain, S. T.; Arif, M. (1994). "Fossil evidence for the origin of aquatic locomotion in archaeocete whales". Science263(5144): 210–212. PMID 17839179doi:10.1126/science.263.5144.210.


Cooper, Lisa Noelle; Thewissen, J. G. M.; Hussain, S. T. (2009). "New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan". Journal of Vertebrate Paleontology. 94 (4): 1289–99. OCLC 506008976doi:10.1671/039.029.0423