Hypnovenator, new troodontid from Early Cretaceous of Japan + hadrosauroid vertebra from Late Cretaceous of Japan

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

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Hypnovenator matsubaraetoheorum gen. et sp. nov.

Katsuhiro Kubota, Yoshitsugu Kobayashi & Tadahiro Ikeda (2024)
Early Cretaceous troodontine troodontid (Dinosauria: Theropoda) from the Ohyamashimo Formation of Japan reveals the early evolution of Troodontinae
Scientific Reports 14: 16392
doi: https://doi.org/10.1038/s41598-024-66815-2
https://www.nature.com/articles/s41598-024-66815-2


A new troodontid dinosaur, Hypnovenator matsubaraetoheorum gen. et sp. nov., is described based on an articulated postcranial skeleton recovered from the fluvial deposits of the Albian Ohyamashimo Formation of the Sasayama Group in Tambasasayama City, Hyogo Prefecture, Japan. Hypnovenator is distinguished from other troodontids by four autapomorphies and a combination of additional features. Our phylogenetic analysis positions Hypnovenator as the oldest and one of the most basal troodontines, forming a clade with Gobivenator mongoliensis. The discovery of Hypnovenator suggests that small-bodied maniraptorans with a sleeping posture were common not only in environments with volcanic and eolian events or alluvial systems but also in fluvial systems. Geometric morphometric analysis of manual ungual phalanges shows that manual ungual phalanges I and III of Hypnovenator exhibit considerable morphological variation but are functionally similar, which differs from those of non-troodontine troodontids, reflecting the transition of manual motion within Troodontinae. Hypnovenator also has mosaic features in the pes related to cursoriality. This study reveals that asymmetrical arctometatarsus occurred by the Albian, and some morphological changes, such as shorter digit IV than digit III and non-ungual phalanges of digits III with roller joints and digit IV with weakly ginglymoid articulation, arose during the early Late Cretaceous.

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Shoji Hayashi, Yasuhisa Nakajima, Yoshihiro Tanaka, Benjamin T. Breeden, Yoshihiro Kanazawa & Chinzorig Tsogtbaatar (2024)
A Hadrosauroid Vertebra from the Upper Cretaceous Izumi Group, Kagawa Prefecture, Japan
Paleontological Research 28(4): 442-451
doi: https://doi.org/10.2517/pr230027
https://bioone.org/journals/paleontological-research/volume-28/issue-4/PR230027/A-Hadrosauroid-Vertebra-from-the-Upper-Cretaceous-Izumi-Group-Kagawa/10.2517/PR230027.short


Of the four main Japanese islands, Late Cretaceous dinosaur fossils have been discovered in Hokkaido, Honshu, and Kyushu. Here, we report the first Late Cretaceous dinosaur from the remaining main island Shikoku, an isolated dorsal vertebra from the upper Campanian Hiketa Formation (Izumi Group) in Sanuki, Kagawa Prefecture. The vertebra is incomplete, comprising a robust centrum with a subrectangular profile that is generally characteristic of dinosaurs. Histological examination of the centrum reveals moderately packed trabecular bone and the complete absence of pneumatic structures, which precludes referral of the specimen to Saurischia or Pterosauria. Instead, we interpret it as a hadrosauriform ornithischian based on the slightly expanded anterior articular surface of the centrum; heart-shaped outline of the intervertebral surface, resulting from its dorsally wide and ventrally narrow shape; ventral keel; and neural canal partially embedded into the centrum. As the ventral positioning of the neural canal is common to Bactrosaurus and higher taxa within Hadrosauriformes, we tentatively refer the specimen to Hadrosauroidea. This discovery provides additional evidence that hadrosauriforms had dispersed into present-day Japan by the Campanian and highlights the potential of the Hiketa Formation to provide additional evidence to improve our understanding of the dinosaur diversity at the extreme eastern continental margin of Eurasia.

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