EvoDevo of key anatomical novelties of the Fish-Tetrapod transition

Elegant palaeontological work over the past two decades by Clack, Ahlberg and others has established a sequence of pivotal character transformations that happened before and around the fish-tetrapod transition in the Devonian.

These involve the emergence of a choana from a primary osteichthyan palate (justifying 'Choanata' monophyly, the transformation of the crown gnathostome suspensorial hyomandibula into a bone of hearing (columella/stapes) and a functional tongue as key tetrapod novelties.

The processes and causes by which such essential and profound evolutionary transformations came about in the first place at the ontogenetic level have so far remained entirely obscure.

The Koentges lab has recently discovered that these disparate features are linked in early ontogeny by virtue of discrete novel neural crest cell populations involved in the morphogenesis of these structures. Changes in such population is hypothesized to affect jointly middle ear, tongue and choanal structures.

The fact that these diverse structure share same cellular origins also raises doubts about the required independence of cranial characters usually used in sarcopterygian phylogenetic tree reconstructions. We will test tree topologies in relation to such dependencies.

To understand the emergence of the choana from (presumably) the second/posterior osteichthyan nostril we will be tracing in live tissues genetically labelled neural crest cell migrations in several model systems as we posit and have evidence that evolutionary changes in the migratory characteristics of these cell populations are responsible for character polarities of anatomical features in question(choanal position, tongue emergence and hyomandibular structure/positioning).

By applying the extant phylogenetic bracket we will be in a position to reconstruct, at single cellular resolution and with genetic precision, larval embryonic processes that first emerged in the Devonian.