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24
Mammals Take the Stage
What advantages helped mammals rise after the dinosaurs disappeared?

With the dinosaurs gone, mammals inherited the Earth! But they didn't start out as the massive creatures we might imagine. Early mammals were small, diverse, and adaptable. Your learner will discover three wildly different ways mammals reproduce (eggs, pouches, and live babies), and learn how their flexibility and small size gave them a huge advantage in the recovering world. This is the beginning of the mammalian era that would eventually lead to humans.

S2S_Lesson24 by Selene

Key Ideas

  • Mammals diversified rapidly after the Cretaceous–Paleogene extinction because ecological niches left empty by dinosaurs became available.
  • Three main types of mammals evolved: placentals (live birth), marsupials (pouches), and monotremes (egg-laying).
  • Small size and flexibility in diet, reproduction, and habitat gave mammals a survival advantage in post-extinction ecosystems.
  • Mammals started living in many different places and eating many kinds of food.


Spines
  • DK's Science as You've Never Seen it Before: Producing Young pg. 176-177
  • Visual Timelines: Life on Earth pg. 48-49 & 89-105 (will be gone over multiple weeks)
     
✏️  Notebooking Activity
Create a chart showing the three types of mammal reproduction: egg-laying (monotremes), pouch-bearing (marsupials), and live birth (placentals). Draw an example of each and write 1-2 sentences explaining how each strategy helped mammals survive after the dinosaurs.

Vocabulary
  • Mammal — A warm-blooded vertebrate with hair or fur that typically gives birth to live young and nurses them with milk.
  • Placental Mammal — A mammal whose young develop inside the mother's uterus; the most common type of mammal today.
  • Marsupial — A mammal whose young are born early and continue developing in a pouch; examples include kangaroos and opossums.
  • Monotreme — An egg-laying mammal; the only living examples are the platypus and echidnas.
  • Cenozoic Era — The current era of Earth's history (66 million years ago to present), sometimes called the 'Age of Mammals.'
  • Ecological Niche — The role an organism occupies in its environment; empty niches after dinosaur extinction let mammals diversify rapidly.
  • Diversification — The rapid evolution of many new species from a common ancestor, triggered by extinction of competitors or new environments.

Cosmic CalendarWhere we are: December 30
Still December 30th. The world has changed completely since this morning. Read the script below before the lesson.

Read aloud: Still December 30th on our Cosmic Calendar, but the world looks completely different now than it did this morning. The non-avian dinosaurs are gone. And the mammals that survived, small, adaptable, warm-blooded creatures that they were, are beginning to spread into every habitat that opened up. Mammals have actually been around since December 26th on our calendar, but they were small and in the shadow of the dinosaurs. Now, with the dinosaurs gone, they have the whole world to explore. It’s still December 30th. We have one day left on this calendar. The mammals are just getting started.

Timeline EntriesLabel the next page in your timeline “Mammals Take the Stage: 66 Million Years Ago”. The workbook prompt asks learners to draw a Paleocene forest floor with small, fuzzy early mammals (in all different shapes) exploring the ferns and fallen logs left behind by the dinosaurs.

Discussion Questions
  1. What happened to mammals after the dinosaurs disappeared?
    Sample answer: They started to grow, change, and live in more places.
  2. Why was being small helpful for mammals?
    Sample answer: Small animals could hide and survive better when the environment changed.
  3. Can mammals lay eggs or do they always have live babies
    Sample answer: Some lay eggs (like monotremes), some carry babies in pouches (marsupials), and some have live babies (placentals).
Digging Deep
  1. Why did mammals diversify after the dinosaurs went extinct
    Sample answer: Many ecological niches were empty, so mammals could adapt to new environments and foods.
  2. How did mammals eventually take over ecosystems after the extinction?
    Sample answer: By filling empty niches, evolving new traits, and reproducing successfully, mammals became dominant land animals.

Species to ResearchThis lesson covers the rise of mammals after the extinction. Here are some early mammals learners can choose to research:
  • Purgatorius— One of the earliest known primates; a tiny, shrew-like creature from the earliest Paleocene.
  • Uintatherium— A large, bizarre-looking early herbivore with bony protrusions on its skull.
  • Pakicetus — An early ancestor of whales that still walked on land; a fascinating transitional species.
  • Hyracotherium (Eohippus)— The earliest ancestor of horses; about the size of a small dog.
  • Mesonyx— A carnivorous hoofed mammal that filled predator roles in the early Eocene.

SCIENTIST SPOTLIGHT: Zofia Kielan-JaworowskaZofia Kielan-Jaworowska was a Polish paleontologist who led a series of expeditions to the Gobi Desert in Mongolia during the 1960s and 1970s and discovered some of the most complete and important early mammal fossils ever found. She worked under the constraints of Communist Poland and led her teams into remote and physically demanding terrain at a time when women rarely led major scientific expeditions anywhere in the world. The skulls and skeletons she brought back transformed what scientists knew about Mesozoic mammals, the small, mostly nocturnal creatures that lived in the shadow of the dinosaurs and eventually gave rise to all living mammals. She is widely regarded as the founder of modern Mesozoic mammal research.

Videos:
Book:
Digging Deeper Activity:
What did early Mesozoic mammals look like and how did they live? Research what the fossil evidence tells us about their behavior, diet, and senses. Why were small size and nocturnal habits likely advantages for mammals living alongside dinosaurs?

Sources and More Information:

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