Cell type profiling in salamanders identifies innovations in vertebrate forebrain evolution

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these novelties arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. In contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex, yet shares similarities with entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the sauropsid DVR, and the mammalian six-layered neocortex.