The shape of features involved in key biological functions, such as teeth in nutrition, can provide insights into ecological processes even in ancient time, by linking the occupation of the morphological space (disparity) to the occupation of the ecological space. Investigating disparity in radiating groups may provide insights into the ecological diversification underlying evolution of morphological diversity. Actinopterygian fishes initiated their radiation in the Devonian, a period characterized by the diversification of marine ecosystem. Although a former morpho-functional analysis of jaw shape concluded to conservative and poorly diversified morphologies in this early part of their history, fish tooth disparity evidenced here an unsuspected diversity of possible functional significance in the pivotal period of the Late Devonian (Famennian). All teeth being caniniforms, some were stocky and robust, in agreement with expectations for active generalist predators. More surprisingly, elongated teeth also occurred at the beginning of Famennian. Their needle-like shape challenges morpho-functional interpretations by making them fragile in response to bending or torsion. The occurrence of both types of fish teeth during the beginning of the Famennian points to a discrete but real increase in disparity, thus testifying a first burst of feeding specialization despite overall conservative jaw morphology. The disappearance of these needle-like teeth in the Late Famennian might have been related to a relay in dental diversity with abundant co-occurring groups, namely conodonts and chondrichthyans (sharks).