A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs

A specimen of a halisaurine mosasaur is reported from Japan for the first time, closing the pre-existing biogeographical gap between the Middle East and the eastern Pacific. <i>Phosphorosaurus ponpetelegans</i> sp. nov., from the lowermost Maastrichtian of Hokkaido, has been assigned to the genus <i>Phosphorosaurus</i> for sharing the following suite of major cranial characters with <i>P</i>. <i>ortliebi</i>, the type species from Belgium: apex of posterodorsal triangular plateau on frontal reaching level of interorbital constriction; frontal lateral border forming a step-like junction between interorbital and preorbital segments of frontal; preorbital segment of frontal sloping anteroventrally; and stapedial meatus parallel-sided in latero-medial view. Potential autapomorphies that distinguish <i>P</i>. <i>ponpetelegans</i> from other members of Halisaurinae include: postorbitofrontal jugal process elongate and stalk-like, projecting laterally; this process distally bearing a ventrally facing depression for jugal articulation; and lateral surangular–articular suture angular rather than round. The long and laterally projecting jugal processes, when combined with a depressed as well as narrow snout, provide compelling evidence for well-developed binocular vision for the new mosasaur, with an estimated binocular field of view (BFoV) of 35°. This value is unusually high for non-ophidian squamates that typically exhibit a BFoV of 10—20°, and is higher than those of other measured mosasaur taxa by at least 5°. Among colubrid snakes, nocturnal species exhibit greater BFoV than diurnal ones in both arboreal and terrestrial taxa. Known also from the Maastrichtian of Hokkaido are fossils of lantern fish (myctophids) and 10-armed cephalopods (coleoids), both of which are typically bioluminescent today. It is hence proposed that the exceptionally large, forward-facing eyes of <i>P</i>. <i>ponpetelegans</i> may well have been a special adaptation for a nocturnal lifestyle, where they hunted small, bioluminescent prey at night while avoiding direct competition with larger, more piscine mosasaurine taxa such as <i>Mosasaurus hobetsuensis</i> that co-existed with <i>P</i>. <i>ponpetelegans</i>. http://zoobank.org/urn:lsid:zoobank.org:pub:3EF73BE7-B3FE-4441-BA5E-29C5D5FCCBC9

首次报道了产自日本的海神龙亚科（Halisaurinae）沧龙类标本，填补了此前中东与东太平洋之间的生物地理空白。
*Phosphorosaurus ponpetelegans* sp. nov.（新种）产自北海道下马斯特里赫特阶，因其与产自比利时的模式种*P. ortliebi*共享以下一系列关键头骨特征，被归入磷龙属（*Phosphorosaurus*）：额骨后背侧三角隆起的顶端达到眶间收缩位置；额骨侧缘在额骨眶前段与眶后段之间形成阶梯状衔接；额骨眶前段向前腹侧倾斜；以及镫骨孔（stapedial meatus）在内外侧观下呈平行侧边形态。
该新种与其他海神龙亚科成员的潜在自有衍征包括：眶后额骨的轭突细长呈茎状，向外侧突出；该突起远端带有一处朝向腹侧的凹陷，用于与轭骨形成关节连接；以及外侧上隅骨-关节骨缝合线呈角状而非圆形。
修长且向外侧突出的轭突，结合凹陷狭窄的吻部，为该新沧龙类具备发达的双眼视觉提供了有力佐证，其估算双眼视野（binocular field of view, BFoV）可达35°。该数值在典型双眼视野为10°~20°的非蛇类有鳞目（non-ophidian squamates）类群中异常偏高，且比其他已测量的沧龙类类群至少高出5%。在游蛇科蛇类（colubrid snakes）中，无论是树栖还是陆栖类群，夜行性物种的双眼视野均大于昼行性物种。
同样产自北海道马斯特里赫特阶的化石还包括灯笼鱼（myctophids，灯笼鱼科）以及十腕头足类（coleoids，鞘亚纲）的化石，这两类类群现今大多具备生物发光能力。
因此推测，*P. ponpetelegans*异常宽大且朝向前方的眼部，很可能是对夜行性生活方式的特殊适应：它们可在夜间捕猎小型发光猎物，同时避免与同域共存的、体型更大且更偏向鱼类食性的沧龙亚科类群（如*Mosasaurus hobetsuensis*）产生直接竞争。
http://zoobank.org/urn:lsid:zoobank.org:pub:3EF73BE7-B3FE-4441-BA5E-29C5D5FCCBC9