Phylogenetic bracketing

One of the main applications of phylogenetic bracketing is on extinct organisms, known only from fossils, going back to the last universal common ancestor (LUCA).

By considering the closest and second-closest well-known (usually extant) organisms, traits can be asserted with a fair degree of certainty, though the method is extremely sensitive to problems from convergent evolution.

However, since these forms of bracketing are also more generalized and may rely on inferring traits in extinct animals, they also offer lower explanatory power compared to the EPB.

[4] The extant phylogenetic bracket approach allows researchers to infer traits in extinct animals with varying levels of confidence.

[1] Level 1 — The inference of a character that leaves a bony signature on the skeleton in both members of the extant sister groups.

However the underlying pneumatic fossae, air sacs, in the bones of extant birds are remarkably similar to the cavities seen in the fossil vertebrae of Tyrannosaurus rex.

Level 3 — The inference of a character that leaves a bony signature on the skeleton but is not present in either extant sister group to the taxon in question.

Thus a level 3 inference receives no support from the extant phylogenetic bracket, but can still be used with confidence based on the merits of the fossil data itself.

[1] The Late Cretaceous Kryptobaatar and the extant monotremes (family Tachyglossidae and Ornithorhynchidae) all sport extratarsal spurs on their hind feet.

Greatly simplified, the phylogeny is as follows, with taxa known to have extratarsal spurs in bold:[6] Kryptobaatar Cimolomyidae Eobaataridae Ornithorhynchidae (platypus) Tachyglossidae (echidnas) Assuming that the Kryptobaatar and monotreme spurs are homologous, they were a feature of their mammalian last common ancestor, so we can tentatively conclude that they were present among the Early Cretaceous Eobaataridae—its descendants—as well.