Oldest Cretaceous latimeriid elucidates cranial evolution in derived and extant coelacanths (Actinistia, Latimeriidae)

The fossil record of coelacanths (Actinistia) is diminished by several nominal gaps that obscure vital information pertaining to the clade's evolutionary history. Latimeriidae, the family that includes the extant coelacanth Latimeria, in addition to the Cenozoic, has an outstanding missing gap of 50 myr during the Mesozoic, with no records of this family from the Lower Cretaceous. Upper Cretaceous latimeriids are morphologically derived and show close anatomical similarities with Latimeria, although an understanding of their macroevolutionary trends, timings of key character acquisitions, and gross patterns in their skeletal evolution are largely obscured by a lack of diagnosed Lower Cretaceous taxa. Using x-ray computed tomography (XCT), we here report the first diagnostic latimeriid from the Lower Cretaceous: Macropoma gombessae sp. nov. based on an articulated cranium and associated fragments of the post-cranial skeleton from the Albian Gault Formation of southern England. The new species demonstrates a drastic shift in dermal ornamentation between the Lower and Upper Cretaceous, including an unusual narrowing and change in placement of the supraorbital sensory canal within the supraorbitals, and increased reduction in the dentary leading towards the morphologies present in Latimeria. Phylogenetic analysis resolves the new species as sister to Macropoma, and together are sister to a clade containing Swenzia and Latimeria. We additionally reappraise ‘Macropoma’ willemoesii (Upper Jurassic, Germany) as not belonging to Macropoma, indicating M. gombessae as the stratigraphically oldest species of this historic and evolutionarily novel genus.