What is a crown group?
Q: What is a crown group?
A: A crown group is a clade of living species and their ancestors back to the most recent common ancestor. It includes at least some members that have survived to the present day.
Q: Who invented the term "crown group"?
A: The terms "crown group" and "stem group" were invented by R.P.S. Jefferies in 1979.
Q: How was the idea of a crown group developed?
A: The idea of a crown group was developed by Willi Hennig, the founder of phylogenetic systematics, but he did not use this term.
Q: Are all birds included in the crown group Neornithes?
A: No, Archaeopteryx and some other extinct groups are not included in the crown group Neornithes as they diverged earlier than other birds included in this clade.
Q: Can bigger crown groups be defined?
A: Yes, bigger crown groups can also be defined such as the phylum Chordata which includes all animals with spinal cords or nerve cords such as fish, amphibians, reptiles, birds and mammals.
Q: Is there any difference between how a stem and a crown group are defined?
A: Yes, while both stem and crown groups include living species and their ancestors back to their most recent common ancestor, how they are defined differs slightly; one definition for defining a stem or crowmgroup stresses the need for members of that clade to have its characteristic synapomorphy (a shared derived character).
