Crown group

In phylogenetics, the crown group is a way to group living things together. It includes all the living members of a group, their most recent common ancestor, and every descendant of that ancestor. This helps scientists understand how different species are related to each other and to their ancient relatives. For example, birds, known as Aves, can be thought of as a crown group. This group includes the ancestor of all modern birds and all the birds that came after that ancestor, both alive today and those that have lived and gone extinct.

The idea of a crown group was first developed by Willi Hennig, who created the study of phylogenetic systematics. The words "crown" and "stem" to describe these groups were first used by R. P. S. Jefferies in 1979. Even though the idea was created in the 1970s, it didn’t become commonly used until the year 2000, when Graham Budd and Sören Jensen brought it back into discussions among scientists.

Contents of the crown group

A crown group includes all the living members of a species, their most recent common ancestor, and all descendants of that ancestor—even if they are no longer alive. For example, when we look at birds, the crown group of birds includes the most recent common ancestor of all living birds and every descendant from that ancestor, whether they are still around today or not. This means that extinct birds like the dodo or the great auk are part of the bird crown group because they come from the same common ancestor as today’s birds.

In some diagrams, the group labeled “Aves” shows the crown group of birds. It includes the most recent common ancestor of all living birds and everyone descended from them. Some older groups, like Archaeopteryx, are not part of this crown group because they come from earlier ancestors. This way of defining groups can be different from older ways people used to classify animals, which sometimes included fossils that were not directly related to today’s species.

Other groups under the crown group concept

The idea of using family trees to group living things leads to different ways to describe groups that include fossils. This has created many special words to talk about different parts of these family trees compared to living organisms.

Pan-group

A pan-group or total group includes the crown group and all creatures more closely related to it than to any other living creatures. Imagine a tree where the crown group is the living part; the pan-group adds all branches back to where it splits from the closest living branch. For example, Pan-Aves includes today’s birds and all ancient creatures more closely linked to birds than to crocodilians. This includes early birds like Archaeopteryx, along with dinosaurs and pterosaurs.

Stem groups

A stem group includes ancient relatives of a crown group, going back to just before the last common ancestor of the crown group and its closest living relatives. All members of a stem group are extinct. Stem groups help us understand ancient creatures that are related to living groups but are not part of them.

Plesion-group

The term plesion group has different meanings. It can mean a group close to another group on the family tree, or a group defined by ancient traits. It usually refers to a side branch that split off earlier than the group being studied.

Palaeontological significance of stem and crown groups

Placing fossils in the right order in a stem group helps us understand when important features developed in a group of organisms. This lets scientists connect fossils to living animals, even if the fossils do not have all the features of today's animals. These fossils have been very useful for studying how tetrapods, mammals, and animals first appeared.

Using the stem group idea also changed how scientists think about animals from the Burgess Shale. By grouping them with living animal groups instead of separate groups, it might help explain the Cambrian explosion without needing unusual ideas about evolution. However, this idea does not solve all the challenges in understanding big changes in evolution during that time. Focusing too much on stem groups might make it harder to recognize new animal groups.

Stem groups in systematics

Scientists sometimes use the word "stem" to talk about groups of animals that came before the modern ones. For example, they might say "Stem-Aves" to mean animals that are related to birds but lived long ago. The group we call "birds" today doesn’t get a special prefix. Not all scientists agree on this idea, but some still use it to study ancient animals.