cetacean systematics

 
 
Ambulocetus - "Walking whale"
 

                                                                                                                                                     Thewissen
 
 
                          This page will by no means contain all there is to know about cetacean systematics. However, I wrote
                  a paper for one of my classes that addressed two questions:
 
                          (1) are cetaceans the product of a mesonychid condylarth, and thus, the closest extant relative (CER)
                          to artiodactyls (even-toed ungulates), or are they more closely related to hippopotamid artiodactyls?
 
                          (2) is the sperm whale, an odontocete (toothed-whale) more closely related to other toothed whales,
                          or to the baleen (mysticete) whales?
 
 
Question #1
                  What are mesonychid condylarths?
 
                          The mesonychids are an extinct group of large terrestrial mammals that lived during the Paleocene
                          and Eocene. Thus, about 65-55 million years ago.
 
                          This group is an interesting mix of herbivores, carnivores, and omnivores, and some were even thought
                          to feed on fish.
 
                          Their fossils are sometimes found deposited in estuaries and lagoons, suggesting that they might be
                          the ancestors of the modern day cetaceans.
 
 
                  What does closest extant relative (CER) mean?
 
                          This term means the closest living relative. Thus, the animals linked by this term shared an ancestor at
                          some point during their history. When I say that cetaceans are the CER to artiodactyls, I mean that these
                          mammals shared an ancestor within the mesonychian group.  
 
                  What are artiodactyls (even-toed ungulates)?
 
                          Artiodactyls are animals like cows, pigs, hippos, deer, camels, and giraffes.
 
 

 
 
                          Although the fossil record is not complete, it does suggest that cetaceans are the progeny of a
                  mesonychid condylarth, and that artiodactyls and cetaceans share a common ancestor. However, the
                  recent molecular phylogenies support a sister relationship between cetaceans and hippopotamid artio-
                  dactyls. Until the gaps in the fossil record are filled and/or molecular techniques become more rigorous,
                  I am only confident in concluding that artiodactyls and cetaceans are closely related.
 
                          Since I have written my paper, more fossils have come to light, and some researchers have made
                  stronger conclusions than I felt it was possible to make. See the links below to find out more information
                  about these new discoveries and conclusions.
 
        "Bones boost whale-hippo link" - Science Now Article
 
        "When is a whale a whale?" - "Vital Articles on Science/Creation" Article
 
 
Question #2
                  What is an odontocete?
 
                          Odontocete refers to the toothed whales, animals ranging in size from the sperm whale to the
                          harbor porpoise.
 
                          This group includes the familar bottlenose dolphin and killer whale (Orca).
 
 
       
 
 
                  What is a mysticete?
 
                          Mysticete refers to the baleen whales, animals ranging in size from the humpback whale to the
                          largest animal on earth, the blue whale.
 
                          These animals have baleen, also known as "whale bone." Baleen is composed of keratin, the same
                          material that makes up your fingernails. It is arranged in huge plates that hang from the whales upper
                          jaws. The plates are frayed at the ends so that they can act as a sieve, which the whales use to
                          filter feed.
 
 

 
 
                          The fossil record is strong in its support of a close relationship of the sperm whale to the other
                          odontocete cetaceans. This conclusion is also supported by morphological and some molecular
                          evidence. The studies supporting odontocete paraphyly, although interesting, do not have the
                          strength of the fossil record behind them. Thus, I will conclude that sperm whales are more close-
                          ly related to toothed whales, making the clade Odontocete monophyletic.
 
                  What does paraphyletic mean?
 
                          Paraphyletic is the term applied to a group of organisms that contains an ancestor and only
                          some of its descendents. In this case, if the sperm whale was more closely related to the baleen
                          whales, it would suggest that the mysticete arose from within the odontocete. Thus, the current
                          clade recognized as Odontocete would be paraphyletic.
 
                  What does monophyletic mean?
 
                          Monophyletic is the term applied to a group of organisms that contains an ancestor and all
                          of its descendents. If the sperm whale is more closely related to the other odontocete whales
                          then the clade Odontocete would be monophyletic.
 
 
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