Crown group facts for kids

Two distinct crown groups (in red) are illustrated, connected by an ancestor (black circle). The two groups form a larger crown group (lilac).

Imagine a big family tree of living things! A crown group is like a special branch on this tree. It includes all the living species in a group, plus all their ancestors going back to their very first common ancestor. Think of it as a family that has members still alive today. This idea helps scientists study how different life forms are related.

Scientists use this term in cladistics and phylogenetics. These are ways to understand how different living things are connected through evolution. A crown group is a type of clade, which is a group of organisms that share a common ancestor. For a group to be a crown group, at least some of its members must still be alive today.

Who Came Up With This Idea?

The main idea behind crown groups came from a scientist named Willi Hennig. He was a pioneer in a field called cladistics, which studies how living things are related. However, he didn't use the exact term "crown group."

The terms "crown group" and "stem group" were actually created by R.P.S. Jefferies in 1979. Even though these terms were invented in the 1970s, they didn't become widely used until the 2000s.

Birds as an Example

Let's look at birds to understand crown groups better. Here's a simplified family tree for birds:

• Aves (This is the main group for all birds)
  • †Archaeopteryx and its extinct relatives
  • †Avisaurus and its extinct relatives
  • †Hesperornis and its extinct relatives
  • †Ichthyornis
  • Crown group, Neornithes (This includes all birds that are alive today)
    • Neoaves (Most modern birds and their extinct ancestors)
    • Paleognathae (Like ostriches and emus)
    • Galloanserae (Ducks, chickens, and their relatives)

In this example, Archaeopteryx and some other extinct groups are not part of the Neornithes crown group. This is because they branched off earlier in the bird family tree. They are older relatives that are no longer alive.

Different Crown Groups

You can define different crown groups depending on which living species you choose. For instance, Neoaves can also be considered a crown group. The same goes for Paleognathae (the group of ratites) and Galloanserae (the group of ducks and chickens).

Even bigger crown groups can be defined. For example, the entire phylum Chordata is a crown group. This group includes all vertebrates (animals with backbones) and some of their close relatives.

Stem Group

The stem group of birds would include all the earlier Aves (the main bird group) that are not part of the Neornithes crown group. These are the extinct relatives that branched off before the common ancestor of all living birds.

See also

In Spanish: Grupo terminal para niños