Lecture 34—Mammals
This lecture is a true historical case study. It shows how the scientific process works—people trying to make sense out of the data and correcting mistakes as they go.
The story follows the discovery of the Australian platypus, and the tribulations it caused in the scientific community. The animal did not fit into the Linnaean classification scheme. It looked like a mammal but it seemed to lack the key characteristic of a mammal—mammary glands. Slowly over the decades, one by one, discoveries revealed they did have mammary glands but not nipples, milk oozed out of the skin. Biologists quarreled over how they gave birth and were astounded to find they laid eggs and their reproductive tract was like reptiles. Begrudgingly, they came to relaize that this animal and its relative the Spiny Echidna belonged in a new category of primitive mammal, the Monotremes. They were a perfect example of living transitional species, exactly what Darwin was looking for.
All of this happened as voyages of discovery by France, Dutch, Portuguese, Spain and England in the 15th-19th centuries found spectacular new plants and animals and revealed the great limitations of historical writings like the Bible. This, plus the Protestant Reformation and the rise of scientific reasoning deeply challenged the traditional creationist views. When Darwin offered a tightly reasoned alternate explanation of nature the world was never the same. Miracles were not needed to explain what fossils were, or how adaptations of organisms occurred, or the origin of species. An ever changing Tree of Life (phylogeny) view replaced the never-changing view of Aristotle’s, the ladder of life (Scala Naturae).
It is NOT necessary to memorize which scientist suggested which argument; it is the overall story line that is important.
It IS necessary that you learn this:
Class Mammalia
l Subclass Protheria (Egg laying mammals)
l Order Monotremata (Platypus & Echidna)
l Subclass Theria (Mammals bearing live young)
l Infraclass Metatheria
l Order Marsupialia (Pouched mammals)
l Infraclass Eutheria (Placental mammals)
This is the picture that you should have in mind when you think of mammals. They first appear in the fossil record about 200 MYA as a branch of the reptiles; the first mammals (Monotremes) still have clear reptile characteristics. Mammals then evolved into two major groups: Marsupials and Placental mammals which differ in their reproductive systems. Both groups specialized rapidly into the major taxa we see today.
Terms/Concepts to Define:
Mammal
Platypus
Cloaca
Double circulation
Viviparous
Oviparous
Ovoviviparous
Pices
Aves
Prototheria
Theria
Metatheria
Eutheria
Monotreme
Marsupial
Placental
Scala Naturae
Can you answer these questions?
1. List three characteristics of the Monotremata that suggests it is a transitional taxon between reptiles and traditional mammals.
2. The term cloaca is not used in human anatomy, why is this?
3. Describe the sequence in reproduction we see as we move from reptiles to modern mammals. Consider not only egg formation and birth but nutrition of the embryo.
4. Why is the Scala Naturae incompatible with Darwin’s view of life?
5. Draw a phylogeny showing reptiles, birds, monotremes, marsupials, and placental mammals.
6. Why did the voyages of discovery cause such difficulties with the creationist view of life?