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| Hypnovenator matsubaraetoheorum Kubota, Kobayashi & Ikeda, 2024 |
Abstract
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.
Keywords: Arctometatarsus, Geometric morphometric analysis, Sleeping posture, Theropoda, Troodontidae, Troodontinae,
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| Hypnovenator matsubaraetoheorum gen. et sp. nov. Blocks including the forelimb, knee (A), and heel (B). (C) Reconstructed skeleton. Red and yellow colors show the confirmed and probable positions of the recovered elements, respectively (Courtesy of Genya Masukawa). (D) Left manus in medial view. (E) Left manual phalanx I-1 in dorsal view. (F) Removed fragmentary left manual phalanx II-3 (manual ungual phalanx II) for preparing the left manus. (G) Cross-section of the bent right ankle. (H) Left metatarsus in posterior view. Abbreviations: as, astragalus; dp, depression; fe, femur; fi, fibula; hu, humerus; mc I, metacarpal I; mc II, metacarpal II; mc III, metacarpal III; mp I-1, manual phalanx I-1; mp I-2, manual phalanx I-2 (manual ungual phalanx I); mp II-1, manual phalanx II-1; mp II-3, manual phalanx II-3 (manual ungual phalanx II); mp III-3, manual phalanx III-3; mp III-4, manual phalanx III-4 (manual ungual phalanx III); mr, medial ridge; mt II, metatarsal II; mt III, metatarsal III; mt IV, metatarsal IV; mt V, metatarsal V; ra, radius; ti, tibia; ul, ulna. Almost all elements are from the left side. Abbreviations for elements from the right side added ‘(r)’ at the end. |
Systematic paleontology
Theropoda
Coelurosauria
Troodontidae
Hypnovenator matsubaraetoheorum gen. et sp. nov.
Diagnosis: A troodontid with the following unique characters: a pair of proximodistally extended depressions on the proximodorsal surface of manual phalanx I-1; long dorsal and ventral proximal lips on manual phalanx III-2 for tight articulation with phalanx III-1; a proximodistally longitudinal medial ridge on the anterior surface of the femur proximal to the medial condyle; and distorted distal condyles with a widely convex distoventral margin on pedal phalanx III-3. Additionally, it is characterized by the following combination of two features: the thickest portion near the middle portion of the distal end of the ulna, and an angle of less than 11 degrees between the anterior edge of the cnemial crest and the anterior edge of the tibial shaft.
Locality and horizon: Hyogo Prefectural Tamba Namikimichi Central Park at Nishikosa, Tambasasayama City, Hyogo Prefecture, Japan; the early to middle Albian (112.1–106.4 Ma26) Ohyamashimo Formation of the Sasayama Group.
Etymology: The genus name derives from “hypno”, refers to “sleep” in ancient Greek, and “venator”, means “hunter” in Latin. The specific name, “matsubaraetoheorum”, honors Mrs. Kaoru Matsubara and Takaharu Ohe, who are the first discoverers of a block including a part of Hypnovenator matsubaraetoheorum holotype specimen.
Katsuhiro Kubota, Yoshitsugu Kobayashi and 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: doi.org/10.1038/s41598-024-66815-2
