Figure 3.

Chordate phylogeny and a synopsis of telencephalon evolution.

A nested cladogram depicts the evolutionary relationships of the chordates and their relatives within the Bilateria, the group of all bilaterally symmetric animals. Red arrows and text identify common ancestors and approximate times of divergence in millions of years before the present (mya). Blue arrows and text identify the emergence of key innovations related to the evolution of the telencephalon and of the neocortex in particular. Blue text along the stems of the vertebrate groups identifies telencephalic morphological innovations of those lineages. Note that our placement of neocortical cell type origins with stem amniotes is a conservative one, based on clades for which extensive molecular data exist and are concordant with connections. See main text for discussion and references.