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| Tyrannomimus fukuiensis Hattori, Shibata, Kawabe, Imai, Nishi & Azuma, 2023 |
Abstract
Ornithomimosauria consists of the ostrich-mimic dinosaurs, most of which showing cursorial adaptations, that often exhibit features indicative of herbivory. Recent discoveries have greatly improved our knowledge of their evolutionary history, including the divergence into Ornithomimidae and Deinocheiridae in the Early Cretaceous, but the early part of their history remains obscured because their fossil remains are scarce in the Aptian–Albian sediments. In recent years, many isolated ornithomimosaur remains have been recovered from the Aptian Kitadani Formation of Fukui, central Japan. These remains represent multiple individuals that share some morphological features common to them but unknown in other ornithomimosaurs, suggesting a monospecific accumulation of a new taxon. As a result of the description and phylogenetic analysis, the Kitadani ornithomimosaur is recovered as a new genus and species Tyrannomimus fukuiensis, the earliest definitive deinocheirid that complements our knowledge to understand the early evolutionary history of Ornithomimosauria. Due to its osteological similarity to Tyrannomimus, a taxon previously considered an early tyrannosauroid based on fragmentary specimens, namely Aviatyrannis jurassica, may represent the earliest ornithomimosaur from the Upper Jurassic of Europe, significantly expanding the temporal and biogeographic range of Ornithomimosauria. This finding fills a 20-million-year ghost lineage of Ornithomimosauria implied by the presence of the oldest fossil record of Maniraptora from the Middle Jurassic and is consistent with the hypothesis that their biogeographic range was widespread before the Pangaean breakup in the Kimmeridgian.
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| Locality, horizon, and overview of ornithomimosaur materials. Regional map for the location of Fukui in Japan, and the Kitadani Dinosaur Quarry in Fukui (a), stratigraphic section of the part of the Kitadani Formation in the Kitadani Dinosaur Quarry (b), photograph of the Kitadani Dinosaur Quarry in 2019 with an arrowhead indicating where the studied specimens were yielded (c), and overview of the ornithomimosaur materials (d). The map in (a) is modified from the one available at www.freemap.jp. Fossil specimens shown in (d) are not in the same scale while the bar is scaled for the paratype (FPDM-V-10295). The holotype (FPDM-V-11333) and paratype in (d) are colored in red anb blue, respectively. Right femur in (d) is mirrored from the left one (FPDM-V-11338). |
Systematic paleontology
Dinosauria Owen, 1842.
Theropoda Marsh, 1881.
Tetanurae Gauthier, 1986.
Coelurosauria von Huene, 1920.
Ornithomimosauria Barsbold, 1976.
Tyrannomimus fukuiensis gen. et sp. nov.
Etymology: The genus name is derived from its morphological resemblance with tyrannosauroids, in which the vertical ridge on the ilium has been regarded as a synapomorphy; the specific name is derived from Fukui, the prefecture where the type and referred specimens were found.
Diagnosis:
Based on the holotype, theropod dinosaur with following autapomorphies among ornithomimosaurs: deep dorsal tympanic recess with its bottom subdivided by an anteroposteriorly-oriented lamina; expanded spherical cavities within prezygocentrodiapophyseal, centrodiapophyseal and postzygocentrodiapophyseal fossae.
Based on the referred specimens, deep anterolateral pit on proximal part of humerus can be another autapomorphy among ornithomimosaurs. However, this feature should be confirmed by additional specimens including humeri and some elements shared by the holotype.
Tyrannomimus can be distinguished from penecontemporary Asian ornithomimosaurs by having the following features: from Harpymimus by having the deltopectoral crest shorter than the quarter of the humeral length, the muscle scar on the anterolateral margin of the deltopectoral crest, low and elongated ilium, and concave ventral margin of the postacetabular process; from Shenzhousaurus by having ventral grooves on anterior caudal centra, straighter manual unguals, and the lateral distal condyle of the femur extended further distally than the medial condyle that is flattened distally.
Holotype: FPDM-V-11311, a disarticulated but associated skeleton including two parts of the braincase, several dorsal, sacral and caudal vertebrae, and fragments of ilium.
Soki Hattori, Masateru Shibata, Soichiro Kawabe, Takuya Imai, Hiroshi Nishi and Yoichi Azuma. 2023. New Theropod Dinosaur from the Lower Cretaceous of Japan provides critical implications for the early Evolution of Ornithomimosaurs. Scientific Reports. 13: 13842. DOI: 10.1038/s41598-023-40804-3
