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Mosasaur sclerotic rings

Ben Creisler

New in PLoS ONE:

Momo Yamashita , Takuya Konishi & Tamaki Sato (2015)
Sclerotic Rings in Mosasaurs (Squamata: Mosasauridae): Structures and
Taxonomic Diversity. PLoS ONE 10(2): e0117079

Mosasaurs (Squamata: Mosasauridae) were a highly diverse, globally
distributed group of aquatic lizards in the Late Cretaceous (98–66
million years ago) that exhibited a high degree of adaptation to life
in water. To date, despite their rich fossil record, the anatomy of
complete mosasaur sclerotic rings, embedded in the sclera of the
eyeball, has not been thoroughly investigated. We here describe and
compare sclerotic rings of four mosasaur genera, Tylosaurus,
Platecarpus, Clidastes, and Mosasaurus, for the first time. Two
specimens of Tylosaurus and Platecarpus share an exact scleral ossicle
arrangement, excepting the missing portion in the specimen of
Platecarpus. Furthermore, the exact arrangement and the total count of
14 ossicles per ring are shared between Tylosaurus and numerous living
terrestrial lizard taxa, pertaining to both Iguania and Scleroglossa.
In contrast, two species of Mosasaurus share the identical count of 12
ossicles and the arrangement with each other, while no living lizard
taxa share exactly the same arrangement. Such a mosaic distribution of
these traits both among squamates globally and among obligatorily
aquatic mosasaurs specifically suggests that neither the ossicle count
nor their arrangement played major roles in the aquatic adaptation in
mosasaur eyes. All the mosasaur sclerotic rings examined consistently
exhibit aperture eccentricity and the scleral ossicles with gently
convex outer side. Hitherto unknown to any squamate taxa, one specimen
of Platecarpus unexpectedly shows a raised, concentric band of
roughened surface on the inner surface of the sclerotic ring. It is
possible that one or both of these latter features may have related to
adaptation towards aquatic vision in mosasaurs, but further
quantitative study of extant reptilian clades containing both
terrestrial and aquatic taxa is critical and necessary in order to
understand possible adaptive significances of such osteological