- Slides: 32
Whale Evolution Life Science: Natural World
Cetacean, which literally translates to large sea mammal, includes whales, dolphins, and porpoises.
Mesonychids Mesonychia ("Middle Claws") are an extinct order of medium to large-sized carnivorous mammals that were closely related to cetaceans (dolphins and whales). They first appeared in the Early Paleocene (67 -55 mya).
Pakicetus is a genus of extinct cetaceans found in the early Eocene (55 -34 mya) of Pakistan. Complete skeletons were discovered in 2001, revealing that Pakicetus was primarily a land animal, about the size of a wolf, and very similar in form to the related mesonychids.
Pakicetus The first fossil, a lone skull, was thought to be a mesonychid, but Gingerich and Russell recognized it as an early cetacean from characteristic features of the inner ear, found only in cetaceans. This suggests that it is a transitional species between extinct land mammals and modern whales.
Ambulocetus ("walking whale") was an early cetacean that could walk as well as swim. It lived during early Eocene about 46 million years ago. It is a transitional fossil that shows how whales evolved from land-living mammals. It was clearly amphibious, as its back legs are better adapted for swimming than for walking on land. Chemical analysis of its teeth shows that it was able to move between salt and fresh water.
Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the transition from land to sea that whales went through. The first fossils of this species were found in Balochistan Province, Pakistan in 2001 by Philip Gingerich. These organisms lived approximately 43 million years ago and contain a very characteristic whale ear.
Basilosaurus was a cetacean that lived from 40 to 34 million years ago. Fossil remains were first discovered in the southern United States (Louisiana), and were initially believed to be some sort of reptilian sea monster. These were enormous creatures up to 60 feet long. They had a tailfluke, complete hind legs, and mobile knees.
Albert Koch's 114 -foot long “Sea Serpent” skeleton from 1845. Later was properly identified as the Eocene whale known as Basilosaurus.
Dorudon was a genus of ancient cetacean that lived alongside Basilosaurus 40 to 35 million years ago, in the Eocene. They were about five meters (16 feet) long and were most likely carnivorous, feeding on small fish and mollusks. Dorudontids lived in warm seas around the world, fossils have been found in North America as well as in Egypt where the Tethys Sea once existed.
Aetiocetus is an extinct genus of baleen whale from the Oligocene (about 24 mya). Found in Oregon, Aetiocetus is believed to be a transitional fossil between Pakicetus and the modern beluga whale.
Nostril Comparison Intermediate
Whale Discovery: Discussion Questions 1. Which typical whale traits were the earliest to appear? Early whale teeth, then early whale-type ear bones; skulls becoming long and narrow; shorter legs with big feet.
Whale Discovery: Discussion Questions 2. Which whale traits evolved much later? Nostrils shifted back toward top of skull; hind legs reduced, then eliminated; forelimbs become paddle like; strong swimming tail with flukes form.
Whale Discovery: Discussion Questions 3. What age sediments, and in what region of the world, would you search now to get the fossils which would shed more light on whale origins, and what specific traits would you expect to find? 40 -50 million year old sediments in the Pakistan region. Traits showing increasing adaptations to marine life over time.
Whale Discovery: Discussion Questions 4. Does Ambulocetus seem to fit the “predicted traits” for an intermediate between mesonychids and Rodhocetus? Yes, Ambulocetus does fit well into the sequence, showing transitional features.
Whale Discovery: Discussion Questions 5. Notice the reconstruction of Pakicetus, showing it as a four-legged animal. What evidence, if any, would suggest such a reconstruction? What additional evidence would give you greater confidence in that reconstruction? Ear bones more like mammals; Chemical analysis indicates dependence on freshwater. There were no complete fossils discovered until 2001, and these confirm the presence of four terrestrial legs, with hooves.
Whale Discovery: Discussion Questions 6. As each new “intermediate fossil” was found, filling a “gap”, how many new gaps were formed? Two new gaps were formed, one above, and one below the new find.
Whale Discovery: Discussion Questions 7. Can we make predictions about past events? Why? Yes. Based on existing fossils and assumed evolutionary trends, knowledge of anatomy, geography, and geological history.
Whale Discovery: Discussion Questions 8. Explain why the absence of transitional (intermediate) fossils is not a fair argument against evolution. They may not have been found yet; transition may have been fairly rapid, leaving few fossils; many new transitional fossils have been found in recent years.
Whale Discovery: Discussion Questions 9. Why is it very unlikely that these fossils of early whale evolution are the direct ancestors of whales? How are they probably related to those direct ancestors? What is wrong with the popular “missing link” concept of evolution. As in living species today, there are many different species in the same families (i. e. cat family). They are much more likely to be close cousins to any direct ancestors. “Missing link” suggests a simplistic linear evolutionary chain of life, rather than the more realistic multi-branched tree-of-evolution concept.
Whale Discovery: Discussion Questions 10. Several species of modern whales have welldeveloped rear limbs while embryos. As the embryo continues to mature these limbs atrophy (shrink) and become nonfunctional. Why do you suppose this happens? Genes for leg formation are still in the whale genome (as expected from a terrestrial ancestry), but are turned off during later development, since legs were selected against in the marine environment.
Whale Discovery: Discussion Questions 11. Summarize what you have learned about the process of science in this lesson Testing hypotheses about whale origins (evolution), both in form and in geography, includes the making predictions of expected traits, age and locations, looking for expected fossils, and finding them, which confirms the hypothesis.
Whale Discovery: Discussion Questions 12. Summarize what you have learned about the process of evolution in this lesson. Fossils were discovered which show traits intermediate between terrestrial mammals and whales, with a continuum of earlier and later features, which is consistent with the idea that whales have evolved from earlier non-whales, gradually acquiring features which enable them to survive better in a deep ocean environment.