A new Carboniferous coelacanth illuminates the evolution of the actinistian hyobranchial skeleton

The hyobranchial skeleton plays a fundamental role in breathing and feeding in gnathostomes (jawed vertebrates). Nevertheless, despite a relatively rich fossil record spanning more than 400 million years, our knowledge of the anatomy and evolution of the hyoid and branchial arches of coelacanths is ­limited due to the extremely delicate and poorly ossified small elements forming the arches, which are rarely preserved. Comparative data are mainly available from the extant Latimeria chalumnae Smith, 1939, which represents a crucial landmark for all morphological descriptions of extinct coelacanth taxa. Here we introduce a new genus and species, Aemilia stellata n. gen., n. sp., from the Late Carboniferous (Pennsylvanian) of Texas, United States. The use of micro-computed tomography (μCT) has revealed in exquisite detail the complete series of the hyoid and branchial arches as well as part of the braincase, dermal skull roof and cheek, and pectoral girdle. Our new data shed light on the organisation of the skull in coelacanths and allow us to identify new trends in the evolution of the neurocranium and hyobranchial skeleton. A phylogenetic analysis of actinistian interrelationships reconstructs Aemilia stellata n. gen., n. sp. as the oldest representative of a large Mesozoic radiation of coelacanths, which extends its roots into the Carboniferous. Based on the new topology, coelacanth systematics are reviewed, and we propose a new phylogenetic definition of the order Coelacanthiformes. By its unexpected combination of features, Aemilia stellata n. gen., n. sp. bridges the gap between Devonian and post-Carboniferous coelacanths and reveals that the hyobranchial skeleton displays a certain degree of evolutionary plasticity in the traditionally considered “static” coelacanth anatomy.