The Giant Cretaceous Coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus dobiei Schwimmer, Stewart & Williams, 1994, and Its Bearing on Latimerioidei Interrelationships
Résumé
We present a redescription of Megalocoelacanthus dobiei, a giant fossil coelacanth from Upper Cretaceous strata of North
America. Megalocoelacanthus has been previously described on the basis of composite material that consisted of isolated
elements. Consequently, many aspects of its anatomy have remained unknown as well as its phylogenetic relationships with
other coelacanths. Previous studies have suggested that Megalocoelacanthus is closer to Latimeria and Macropoma than to
Mawsonia. However, this assumption was based only on the overall similarity of few anatomical features, rather than on a
phylogenetic character analysis. A new, and outstandingly preserved specimen from the Niobrara Formation in Kansas
allows the detailed description of the skull of Megalocoelacanthus and elucidation of its phylogenetic relationships with
other coelacanths. Although strongly flattened, the skull and jaws are well preserved and show many derived features that
are shared with Latimeriidae such as Latimeria, Macropoma and Libys. Notably, the parietonasal shield is narrow and flanked
by very large, continuous vacuities forming the supraorbital sensory line canal. Such an unusual morphology is also known
in Libys. Some other features of Megalocoelacanthus, such as its large size and the absence of teeth are shared with the
mawsoniid genera Mawsonia and Axelrodichthys. Our cladistic analysis supports the sister-group relationship of
Megalocoelacanthus and Libys within Latimeriidae. This topology suggests that toothless, large-sized coelacanths evolved
independently in both Latimeriidae and Mawsoniidae during the Mesozoic. Based on previous topologies and on ours, we
then review the high-level taxonomy of Latimerioidei and propose new systematic phylogenetic definitions.