PER AHLBERG, from the Evolutionary Biology Centre at the Uppsala University, Sweden, has two principal fields of research: Palaeozoic vertebrate evolution with focus on the origin of tetrapods - the morphological transition from fish to tetrapod - and the relationship of molecular and developmental patterning to morphological evolution. Specifically he is looking into the evolution of muscles and bones in vertebrate heads, necks and limbs, bringing together molecular techniques such as transgenic cell lineage labeling with palaeontological data in a single analysis. He has been invited by James Sharpe (CRG).

Abstract:
Facial anatomy differs fundamentally between extant jawless and jawed vertebrates (cyclostomes and gnathostomes). Cyclostomes such as lampreys and hagfishes have a median nasohypophysial duct; gnathostomes such as zebrafish and humans have separate nasal sacs opening externally, and a palatal hypophysis. Premandibular neural crest cells migrate forwards either side of the nasohypophysial placode to form the upper lip in cyclostomes, but between the hypophysial and nasal placodes to form the trabecular region in gnathostomes (1). In cyclostomes the forebrain is much shorter than in gnathostomes and the hypophysis is relatively anterior.
A series of fossils belonging to the gnathostome stem group bridges the gap between these facial architectures and provides a surprisingly detailed account of the evolutionary and developmental transformation of the vertebrate face. The most basal of these is the galeaspid Shuyu, a 430 million year old jawless stem gnathostome (2). Shuyu has a nasohypophysial duct, short forebrain, and anteriorly oriented hypophysis, but the nasal sacs and hypophysis are separated by a rudimentary trabecula. The placoderm Romundina, a 415 million year old jawed stem gnathostome, represents a more advanced transitional step. Its cranial cavity is similar to that of Shuyu, with an anteriorly directed hypophysis and very short telencephalon. The trabecular region is exceptionally long and wide, with proportions similar to a cyclostome or galeaspid upper lip, whereas the nasal capsule (demarcated by a fissure) is small and located far behind the tip of the snout. We interpret these features as uniquely primitive among jawed vertebrates. In slightly more advanced placoderms like Dicksonosteus (3), the trabecular region is shortened anteriorly so that the nasal capsule becomes terminal: a spatialrelationship that is retained in all later jawed vertebrates.
We suggest that during the evolution of the gnathostome face, separation of the nasal and hypophysial placodes was followed by loss of the nasohypophysial duct, then by anterior shortening of the trabecular region which placed the nasal capsules in a rostral position, and finally by a lengthening of the entire preorbital face (including the forebrain) without further change in the relative proportions of its components.

(1) Oisi, Y. et al. Nature 493, 175-180 (2012).
(2) Gai, Z. et al. Nature 476, 324-327 (2011).
(3) Goujet, D., Éditions du Centre national de la recherche scientifique, 284 pp (1984).