Phalarodon is an extinct genus of mixosaurid ichthyosaur known from the Middle Triassic (247.2-242.0 million years ago).[1] Its name is derived from the Greek φάλαρα (phálara) ("boss on the cheek plate of a helmet") and odon ("tooth"). The genus has had a tumultuous history since its classification in 1910, with different workers describing species under different genera or declaring the genus to be a nomen dubium. Currently three species are recognized, but more have been identified in the past.
The holotype of Phalarodon fraasi, which consisted of a skull and several jaw fragments, was collected by John Campbell Merriam from the West Humboldt Range of Nevada, and was subsequently described by Merriam in 1910.[2] Since then, the species has also been found in the Guizhou Province of China,[3] as well as the Botneheia Formation on the islands of Svalbard in Norway.[4] In 2004, a reanalysis of the available mixosaurid materials saw Phalarodon fraasi be merged into the genus Mixosaurus alongside several other species of mixosaurid ichthyosaur.[2]
Phalarodon atavus, from the Lower Muschelkalk of Althengstett, Germany, was originally described as Ichthyosaurus atavus. In 1998, reanalysis of the type material found it to be undiagnostic, resulting in a neotype being designated and subsequently described as a new genus, Contectopalatus.[5] Further analyses would see both the rejection of the neotype,[6] and the merging of both Phalarodon and Contectopalatus into the genus Mixosaurus.[2] This change in taxonomy would be later overturned, albeit with the merge of Phalarodon and Contectopalatus being maintained.[7] Additional specimens of Phalarodon atavus have been found in Middle Triassic Luoping Biota of Yunnan, China.[8]
Phalarodon callawayi, from the Favert Formation[9] in the Augusta Mountains of Nevada, was originally described as Mixosaurus callawayi[2] before being reclassified as a species of Phalarodon.[7] This species is also known from Svalbard and British Columbia.[2]
Additional species of Phalarodon have been identified throughout history,[1] but only the three listed here are currently considered valid.[8]
Phalarodon had a body plan typical of mixosaurs, being relatively elongate in comparison to later Jurassic ichthyosaur forms. Many specimens consist only of cranial material, meaning that many of the autapomorphies of Phalarodon are based on the morphology of the skull and jaw. These include the parietal forming a significant portion of the sagittal crest, the absence of a dental groove in the maxilla, and a nasal region with a pronounced narial shelf. Given that not all species possess the same degree of postcranial preservation, few characteristic from this region have been defined, but proposed defining features include an elongated humerus (not preserved in Phalarodon atavus) and fifth metacarpal notched along the anterior edge (not preserved in Phalarodon callawayi).[8]