The dentition of the extinct megamouth shark, Megachasma applegatei (Lamniformes: Megachasmidae), from southern California, USA, based on geometric morphometrics

Megachasma applegatei is an extinct megamouth shark (Lamniformes: Megachasmidae) commonly found in late Oligocene‒early Miocene marine deposits of the western USA, that is known only from isolated teeth exhibiting odontaspidid tooth design. In this study, we investigated the tooth morphometry of the extant megamouth shark (Megachasma pelagios) and smalltooth sandtiger (Odontaspis ferox: Odontaspididae) to aid in the reconstruction of the dentition of M. applegatei based on the tooth morphometry of 207 isolated fossil teeth from the lower Miocene Jewett Sand of southern California. Our landmark-based geometric morphometric analyses show that M. applegatei not only possesses a wider morphological range of teeth than M. pelagios, but also has morphological variation that can be corresponded to different tooth types in O. ferox, forming a unique heterodont dentition typical for macrophagous lamniform sharks known as the ‘lamnoid tooth pattern’. Therefore, our study suggests that the dentition of M. applegatei could have also exhibited the lamnoid tooth pattern. In order to reconstruct the dentition of M. applegatei, specific tooth specimens plotted on the morphospace of M. applegatei were selected by identifying teeth of specific tooth types in the corresponding morphospaces of M. pelagios and O. ferox. However, because the total number of teeth per each dental series cannot be ascertained, we generated three sets of reconstructed dentition for M. applegatei. The first set modeled the dentition of O. ferox, the second set representing an intermediate form between O. ferox and M. pelagios, and the third set mimicking the dentition of M. pelagios, with the assumption that the true dental pattern for M. applegatei lies somewhere between the first and third tooth sets, possibly close to the second set. This study represents the first case of using geometric morphometrics to reconstruct the dentition of an extinct shark.