Other Transitional Fossils The Evidence



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Other Transitional Fossils - The Evidence
You just made predictions about some transitional forms; now let's look at the evidence.
Birds and reptiles
If birds and reptiles have a common ancestor, you might expect to find some intermediates. Birds don't fossilize too well, as their bones are very light and fragile, but there are some very famous fossils of an organism called Archaeopteryx lithographica. These date to about 150 million years ago (abbreviated MYA from here on).
a. Compsognathus,  a small dinosaur

b. Archaeopteryx

c. Gallus (modern chicken)

 

Illustration from Strickberger, Evolution





Dimorphodon was a pterosaur, a flying reptile at the same time as dinosaurs.  Note that its hind limb is not as bird-like as those of the dinosaur Compsognathus or Archaeopteryx


Skull of Archaeopteryx - note the teeth and lower jaw bones.



Ornitholestes and Deinonychus were dinosaurs.




Photo of one of the best fossils

Note feather imprints.

Is Archaeopteryx intermediate between reptiles and birds?  Explain your answer.

Please write your answer on the separate answer sheet.

Whales and land mammals:
You might note that the major difference between whales and the land mammals is that whales have no hind limbs. Actually, they have tiny pelvic bones and some portion of a femur, but nothing that protrudes from the body. Presumably, if they evolved from land mammals, there should be some evidence of gradual reduction of hind limbs in their ancestors.

Some whale and whale - like fossils, including Ambulocetus natans,  an organism with a whale-like skull and teeth, and functional hind limbs.






Basilosaurus isis, a fossil whale with small but complete hind limbs.

Do the whale fossils shown here support the hypothesis that whales evolved from land mammals? Explain your answer.

Please write your answer on the separate answer sheet.


Horses and mammals with 5 toes:
Horses have only a single functional toe on each foot, terminating in a mass of fingernail tissue known as a hoof. If they evolved from 5-toed ancestors, we should expect to see some intermediate forms with gradual reduction in the other toes.




Approximate dates for fossil genera mentioned here: 

Hyracotherium, Early Eocene (55 MYA)

Mesohippus, Mid-Oligocene (30 MYA)

Miohippus, Late Oligocene - Early Miocene (25  MYA)

Parahippus,
Early - Mid Miocene (20 MYA)

Pliohippus, Late Miocene (15 MYA)

Equus, (modern horse) Quaternary (2 MYA to present)



Do the horse fossils shown support the hypothesis that horses are related to other mammals with 5 toes? Explain your answer.

Please write your answer on the separate answer sheet.

Fish and amphibians:

Fish are the first vertebrates to appear in the fossil record; the first land vertebrate is the fossil amphibian Ichthyostega.  If amphibians came from fish ancestors, we should see some intermediates indicating the development of feet. 





Eusthenopteron (a lobe-finned fish) 
Devonian (ca. 380 MYA)

Hind limb of Eusthenopteron





Ichthyostega (early amphibian)
Late Devonian (ca. 360 MYA)

Hind limb of Ichthyostega



Do the fossils shown support the hypothesis that amphibians evolved from fish? Explain your answer.

Please write your answer on the separate answer sheet.

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