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The neurocranium of sarcopterygian fishes was originally divided into an anterior (ethmosphenoid) and posterior (otoccipital) portion by an intracranial joint, and underwent major changes in its overall geometry before fusing into a single unit in lungfishes and early tetrapods.sup.1. Although the pattern of these changes is well-documented, the developmental mechanisms that underpin variation in the form of the neurocranium and its associated soft tissues during the evolution of sarcopterygian fishes remain poorly understood. The coelacanth Latimeria is the only known living vertebrate that retains an intracranial joint.sup.2,3. Despite its importance for understanding neurocranial evolution, the development of the neurocranium of this ovoviviparous fish remains unknown. Here we investigate the ontogeny of the neurocranium and brain in Latimeria chalumnae using conventional and synchrotron X-ray micro-computed tomography as well as magnetic resonance imaging, performed on an extensive growth series for this species. We describe the neurocranium at the earliest developmental stage known for Latimeria, as well as the major changes that the neurocranium undergoes during ontogeny. Changes in the neurocranium are associated with an extreme reduction in the relative size of the brain along with an enlargement of the notochord. The development of the notochord appears to have a major effect on the surrounding cranial components, and might underpin the formation of the intracranial joint. Our results shed light on the interplay between the neurocranium and its adjacent soft tissues during development in Latimeria, and provide insights into the developmental mechanisms that are likely to have underpinned the evolution of neurocranial diversity in sarcopterygian fishes. Micro-computed tomography and magnetic resonance imaging of a growth series of the coelacanth Latimeria chalumnae traces the ontogeny of the brain and neurocranium, which sheds light on neurocranial evolution in sarcopterygian fishes., Author(s): Hugo Dutel [sup.1] [sup.2] , Manon Galland [sup.3] , Paul Tafforeau [sup.4] , John A. Long [sup.5] , Michael J. Fagan [sup.1] , Philippe Janvier [sup.6] , Anthony Herrel [...]

The evolution of the mammalian jaw is one of the most important innovations in vertebrate history, and underpins the exceptional radiation and diversification of mammals over the last 220 million years.sup.1,2. In particular, the transformation of the mandible into a single tooth-bearing bone and the emergence of a novel jaw joint--while incorporating some of the ancestral jaw bones into the mammalian middle ear--is often cited as a classic example of the repurposing of morphological structures.sup.3,4. Although it is remarkably well-documented in the fossil record, the evolution of the mammalian jaw still poses the paradox of how the bones of the ancestral jaw joint could function both as a joint hinge for powerful load-bearing mastication and as a mandibular middle ear that was delicate enough for hearing. Here we use digital reconstructions, computational modelling and biomechanical analyses to demonstrate that the miniaturization of the early mammalian jaw was the primary driver for the transformation of the jaw joint. We show that there is no evidence for a concurrent reduction in jaw-joint stress and increase in bite force in key non-mammaliaform taxa in the cynodont-mammaliaform transition, as previously thought.sup.5-8. Although a shift in the recruitment of the jaw musculature occurred during the evolution of modern mammals, the optimization of mandibular function to increase bite force while reducing joint loads did not occur until after the emergence of the neomorphic mammalian jaw joint. This suggests that miniaturization provided a selective regime for the evolution of the mammalian jaw joint, followed by the integration of the postdentary bones into the mammalian middle ear.Biomechanical analyses of mammaliaform and cynodont fossils demonstrate that miniaturization drove the evolutionary transformation of the mammalian jaw, which preceded the optimization of bite force-to-joint load in the mandible, Author(s): Stephan Lautenschlager [sup.1] [sup.2] , Pamela G. Gill [sup.1] [sup.3] , Zhe-Xi Luo [sup.4] , Michael J. Fagan [sup.5] , Emily J. Rayfield [sup.1] Author Affiliations:(1) School of Earth [...]