A Phalarodon fraasi (Ichthyosauria: Mixosauridae) from the Middle Triassic of Svalbard

Abstract

A new specimen of Phalarodon from the Middle Triassic Botneheia Formation on Svalbard, although not complete, is well preserved with both cranial and postcranial elements. Uncompressed preservation of the rostrum reveals the dental groove divided by alveolar bone between the teeth. The right opisthotic has a thin and plate-like paroccipital process, a structure which has only been observed in Mixosaurus cornalianus among mixosaurids. The rare preservation of the atlas-axis complex, being nearly complete and articulated, is very similar to that of Phalarodon callawayi. The interclavicle, although not complete, is preserved with detailed structures which have previously not been described in Mixosauridae. Contrary to earlier studies, the shoulder girdle, with an articulated scapula and coracoid, reveals a coracoid facet on the scapula. Using comparative morphological and phylogenetic analyses, similarities between the new specimen and the known species of Mixosauridae are addressed. We assign the new specimen to Phalarodon fraasi. With its uncompressed preservation, the new specimen contributes to the knowledge of our morphology of the species. This study suggests a reassignment of the specimen PMO 219.250 to the genus Mixosaurus, and opens up for the possibility of Mixosaurus on Svalbard. This would imply a wider geographical distribution of the genus than previously recognised. Additionally, it would prove the coexistence of Mixosaurus and Phalarodon in a shared habitat at the same point in time, a hypothesis that is strengthened by the differences in dentition which reflects separate feeding habits, and thus different niches.

Exceptional X-Ray contrast: Radiography imaging of a Middle Triassic mixosaurid from Svalbard

“…PMO 219.250 was initially mechanically prepared in an external laboratory [ 27 ], with experience on similar material. Although chemical preparation has not been attempted on PMO 219.250, methodology using 10% acetic acid was tested on mixosaurid material from the same formation in Økland, Delsett [ 22 ]. This had limited success on specimens embedded in shale matrix.…”

“…The two genera of mixosaurids are differentiated largely based off differences in dentition: Phalarodon– sports an ankylosed thecodont and usually heterodont dentition with bulbous crushing teeth posteriorly and conical teeth anteriorly; Mixosaurus has conical teeth set in grooves (subthecodont), tooth size may increase slightly posteriorly [ 19 , 20 ]. However, the number of valid taxa has been the subject of debate in the literature, with multiple taxa being reassigned to different genera [ 16 , 18 , 21 , 22 ]. Currently seven species are broadly accepted: Mixosaurus cornalianus , Mixosaurus kuhnschyderi , Mixosaurus xindianensis , Phalarodon atavus , Phalarodon callawayi and Phalarodon fraasi [ 22 , 23 ].…”

“…However, the number of valid taxa has been the subject of debate in the literature, with multiple taxa being reassigned to different genera [ 16 , 18 , 21 , 22 ]. Currently seven species are broadly accepted: Mixosaurus cornalianus , Mixosaurus kuhnschyderi , Mixosaurus xindianensis , Phalarodon atavus , Phalarodon callawayi and Phalarodon fraasi [ 22 , 23 ]. An additional species Barracudasauroides panxianensis , has either been seen as an outgroup to Mixosaurus and Phalarodon [ 24 , 25 ] or incorporated into the genus Mixosaurus [ 26 ].…”

“…An additional species Barracudasauroides panxianensis , has either been seen as an outgroup to Mixosaurus and Phalarodon [ 24 , 25 ] or incorporated into the genus Mixosaurus [ 26 ]. Currently only Phalarodon fraasi and Phalarodon callawayi have been confirmed from the Middle Triassic of Svalbard [ 15 , 18 , 22 , 23 , 27 ].…”

“…Initially, PMO 219.250 was tentatively referred to Phalarodon , based on having a pronounced narial shelf and maxillary dentition set in sockets [ 27 ]. During re-examination of the specimen in Økland, Delsett [ 22 ], this referral was questioned due to the placement of PMO 219.250 in Mixosaurus by phylogenetic analysis. However, this was weakly supported, forming a polytomy with the rest of Mixosaurus .…”