Dinosaur hip groups
Use the traditional dinosaur groups Saurischia (lizard-hipped, including theropods and sauropods) and Ornithischia (bird-hipped, including Triceratops and Stegosaurus). Explain why one hip feature alone may not identify a group.
What a learner can do afterwards
- Name the two major dinosaur groups: Saurischia and Ornithischia
- Explain that the traditional group names refer to hip structure, but a backward-pointing pubis alone does not identify a dinosaur's group
- Correctly classify at least two dinosaurs into each group (e.g. T. rex = Saurischia, Triceratops = Ornithischia)
1 · Read
You have met T. rex, Brachiosaurus, Triceratops and Stegosaurus. Naming identifies a dinosaur. Classification puts dinosaurs into groups. A group can contain smaller groups. Traditional dinosaur classification uses two groups: Saurischia, meaning 'lizard-hipped,' and Ornithischia, meaning 'bird-hipped.' Their names refer to hip bones, including a bone called the pubis. Food and body size do not define these groups.
Saurischia includes theropods, such as T. rex, and sauropods, such as Brachiosaurus. T. rex ate meat, while Brachiosaurus ate plants. Many saurischians had a pubis that pointed forward. Different foods did not put these two dinosaurs in different hip groups.
Ornithischia includes Triceratops and Stegosaurus. In these dinosaurs, part of the pubis points backward. This feature gave the group its name, 'bird-hipped.' The name does not mean that these dinosaurs were birds.
Living birds are dinosaurs descended from theropods in Saurischia. Some saurischians also had a backward-pointing pubis. So a backward-pointing pubis alone does not prove a dinosaur belongs to Ornithischia. Scientists compare several bones and other features to study relationships.
Saurischia includes T. rex and Brachiosaurus. Ornithischia includes Triceratops and Stegosaurus. A backward-pointing pubis alone cannot identify the group.
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What the hip-group names tell us
Saurischia means 'lizard-hipped.' Ornithischia means 'bird-hipped.' These traditional names describe features of dinosaur hips. They do not tell us what the animals ate.
Brachiosaurus and Triceratops both ate plants, yet they belong to different groups. Brachiosaurus is a saurischian, while Triceratops is an ornithischian. T. rex ate meat and belongs to Saurischia with Brachiosaurus.
The bird name needs care too. Living birds are dinosaurs descended from theropods in Saurischia. They did not descend from Triceratops or Stegosaurus in Ornithischia. A name's meaning and an animal's ancestry are different pieces of information.
From a fossil clue to a conclusion
Imagine that a fossil preserves a pubis, one of the hip bones. You can see that it points backward. That is an observation. Naming the dinosaur's group is a conclusion that needs evidence.
Ornithischians had a backward-pointing part of the pubis. Some saurischians also had a backward-pointing pubis. That single observation fits more than one group, so it does not settle the question.
Scientists compare several features to study relationships. An ancestor is an earlier animal from which later animals descended. An evolutionary family tree shows proposed relationships through shared ancestors. One matching feature does not always mean that two animals belong on the same branch.
If other useful bones are missing, the fossil's group may remain uncertain. Recording that uncertainty is part of describing what the evidence can tell us.
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