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25
Birds, Crocodiles, & Dinosaur Descendants
Which animals alive today are actually living relatives of the dinosaurs?

Here's an amazing truth: birds are actually living dinosaurs! Your learner will discover how modern birds inherited traits from their theropod dinosaur ancestors, and how understanding shared ancestry helps us see the connections between creatures that look totally different. They'll also learn about crocodilians, which survived the extinction event but took a very different evolutionary path. It's a fascinating lesson about evolution, adaptation, and how the past is still alive in the animals we see today.

S2S_Lesson25 by Selene

Key Ideas

  • Birds evolved from small theropod dinosaurs and share many traits with them.
  • Crocodilians are part of the archosaur group and survived by adapting as ambush predators.
  • Animals can look very different and still be related. Evolution can change body shape and behavior while preserving shared ancestry.
  • After the mass extinction, birds and reptiles filled new ecological niches.

Spines
  • DK's Science as You've Never Seen it Before
    • The Fossil Record pg. 136-137
✏️  Notebooking Activity
Draw a modern bird and a theropod dinosaur side-by-side. Label the traits they share (hollow bones, three-toed feet, egg-laying, feathers).

Vocabulary
  • Theropod — A group of bipedal dinosaurs (including T. rex) from which birds evolved; they had hollow bones and three-toed feet.
  • Feather — A complex keratin structure; originally evolved in theropod dinosaurs for insulation or display, later adapted for flight.
  • Common Ancestor — A species from which two or more different species both descended; birds and non-avian dinosaurs share a common ancestor.
  • Crocodilian — The group of large reptiles (crocodiles, alligators) that survived the K-Pg extinction due to their semi-aquatic lifestyle.
  • Hollow Bones — Lightweight bones with air pockets inside; a shared trait of theropod dinosaurs and birds that reduces weight.
  • Archosaur — The larger group that includes dinosaurs, birds, crocodilians, and pterosaurs — all descended from a common ancestor.
  • Lineage — A sequence of species each descended from the previous one, tracing an evolutionary family line over time.

Cosmic CalendarWhere we are: December 30 to 31
We cross from December 30th into the final day of the cosmic year. Read the script below before the lesson.
Read aloud: December 30th into December 31st on our Cosmic Calendar. We’re in the final stretch now. Not everything ended with the asteroid. Crocodilians survived, and they’ve changed remarkably little in millions of years. And the dinosaurs didn’t entirely disappear either, because birds are dinosaurs. Avian dinosaurs survived the extinction and went on to become one of the most diverse groups of animals on Earth today. This is an important thing to remember as we look at our calendar. Extinctions don’t wipe the slate completely clean. Survivors carry pieces of the past forward into the future. We’re almost at the end of the year. December 31st is here.

Timeline EntriesLabel the next page in your timeline “Crocodiles and Dinosaur Descendants.” The workbook prompt asks learners to draw a crocodilian lurking in a shallow river beside a colorful early bird perched on a branch, with the K-Pg boundary layer of ash visible in the rock cliff behind them.

Discussion Questions
  1. How are birds connected to dinosaurs? 
    Sample answer: Birds came from dinosaurs and are still living today.
  2. What is one way birds survive today? 
    Sample answer: They can fly, find food, and build nests.
  3. What evidence shows that birds evolved from theropod dinosaurs? 
    Sample answer: Fossils show feathers, similar bones, and shared traits like hollow bones and three-toed feet.
  4. Why were crocodilians able to survive the dinosaur extinction? 
    Sample answer: They lived in water, used little energy, and hunted as ambush predators.
Digging Deeper
  • How can animals with a common ancestor look very different today? 
    Sample answer: Natural selection changes traits over time as animals adapt to different environments.

SCIENTIST SPOTLIGHT: Charles DarwinCharles Darwin, a British naturalist, published On the Origin of Species in 1859, introducing the theory of evolution by natural selection. He proposed that small inherited variations, driven by survival pressures, lead to species change and branching into new forms. Darwin relied on observations from naturalists worldwide, including Indigenous guides and collectors whose contributions were often unrecorded. His theory underpins this lesson: surviving crocodilians and bird descendants are living examples of natural selection over millions of years. Though controversial in his time, Darwin’s theory remains a cornerstone of science.

Books:
Digging Deeper Activity:
Why are birds considered living dinosaurs? Research the skeletal features that birds share with theropod dinosaurs and find at least three specific examples. Then consider: what does natural selection have to do with why birds survived when non-avian dinosaurs did not?

Species to ResearchThis lesson explores the living descendants of the dinosaurs. Here are some great species for research:
  • Deinonychus— A feathered raptor closely related to modern birds; key to understanding the dinosaur-bird connection.
  • Gastornis— A giant, flightless, predatory bird from the Eocene; filled some of the ecological roles left by theropods.
  • Sarcosuchus— An enormous prehistoric crocodilian, sometimes called “SuperCroc,” from the Cretaceous.
  • Hesperornis— A large, diving, toothed bird from the Cretaceous that shows early bird evolution.
  • Kelenken— A massive “terror bird” (phorusrhacid) that dominated South America after the dinosaurs.

Sources and More Information:

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