The general aim of this thesis is to improve our knowledge of the evolutionary history of coelacanth (Actinistia), a group of sarcopterygian fishes considered to be the emblematic example of a "living fossil" due to their low morphological diversity. For such purpose, this work focuses on the descriptions of new coelacanth species, especially with aberrant morphology, and on a re-evaluation of the phylogeny of this group.

The main coelacanth species studied here come from the Middle Triassic of Switzerland. These latter species form a new, short and geographically confined clade of anatomically diverse taxa, restricted to the Western Tethys between 242 and 241 million years ago.

This new data show that, after a rapid morphological evolution, coelacanths have experienced several phases of diversification and bursts of morphological disparity, especially after the Permian-Triassic Mass Extinction, rather than a near stasis during their long evolutionary history of 420 million years.