Phylogenetic bracketing

Phylogenetic bracketing is a method of inference used in biological sciences. It is to infer the likelihood of unknown traits in organisms based on their position in a phylogenetic tree. One of the main applications of phylogenetic bracketing is on extinct animals, known from fossils.

Extant phylogenetic bracketing (EPB) is done by comparing an extinct animal to its nearest living relatives. For example, Tyrannosaurus, a theropod dinosaur, is bracketed by birds and crocodiles. A feature found in both birds and crocodiles would likely be present in Tyrannosaurus, such as the capability to lay an amniotic egg, whereas a feature both birds and crocodiles lack, such as hair, would probably not be present in Tyrannosaurus. Sometimes this approach is used for the reconstruction of ecological traits as well.

In the above example, concluding that Tyrannosaurus could lay amniotic eggs is known as a type I inference. In contrast, concluding that Tyrannosaurus had hair would be a type III inference. Type III inferences are only warranted if there is some positive evidence that the extinct creature possessed the trait. In between is a type II inference, where one of the extant relatives has the trait and the other does not. In the Tyrannosaurus example, endothermy would be such a feature.