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| Tarchia tumanovae Park, Lee, Kobayashi, Jacobs, Barsbold, Lee, Kim, Song & Polcyn, 2021 Artwork by Choi Yusik twitter.com/usikpaleoart |
Abstract
A new ankylosaurid dinosaur, Tarchia tumanovae sp. nov., has been recovered from the Upper Cretaceous Nemegt Formation of Mongolia. It includes a well-preserved skull, dorsal, sacral, caudal vertebrae, sixteen dorsal ribs, ilia, a partial ischium, free osteoderms, and a tail club. The squamosal horns of T. tumanovae are divided into two layers, the external dermal layer and the underlying squamosal horn proper. The irregular ventral margin of the base of the upper dermal layer may represent a resorption surface, suggesting that the squamosal horns of some ankylosaurids underwent extreme ontogenetic remodeling. Localized pathologies on the dorsosacral ribs and the tail provide evidence of agonistic behaviour. The tail club knob asymmetry of T. tumanovae resulted from restricted bone growth due to tail club strikes. Furthermore, T. tumanovae had an anteriorly protruded shovel-shaped beak, which is a morphological character of selective feeders. Ankylosaurid diets shifted from low-level bulk feeding to selective feeding during the Baruungoyot and the Nemegt “age” (middle Campanian-lower Maastrichtian). This ankylosaurid niche shifting might have been a response to habitat change and competition with other bulk-feeding herbivores.
Dinosauria Owen, 1842
Ankylosauridae Brown, 1908
Ankylosaurinae Nopcsa, 1918
Tarchia Maryañska, 1977
Type species. Tarchia kielanae Maryañska, 1977
Revised diagnosis. An ankylosaurid distinguished by having the following unique set of characters (autapomorphies with an asterisk): a narrow internarial bar of the premaxillae (shared with Tsagantegia) (ambiguous in Tarchia kielanae); large, rhomboidal loreal caputegulum with a laterally extended posterior keel (shared with Saichania) (ambiguous in T. kielanae); subrectangular frontal caputegulae (shared with Saichania); a “neck” present at the base of the quadratojugal horn (shared with Pinacosaurus mephistocephalus and Minotaurasaurus) (ambiguous in T. kielanae); sigmoidal and peaked anteromedial supraorbital caputegulum*; posterolateral supraorbital caputegulum with a rounded anterior surface, and a flat, anteriorly-inclined posterior surface*; anteromedially poorly defined postorbital fossa that medially reaches the lateral nuchal caputegulae*; occiput visible in dorsal view (shared with Minotaurasaurus and Zaraapelta); foramen magnum taller than wide*. Differs from Minotaurasaurus, Pinacosaurus grangeri, Saichania, and Zaraapelta in having no postocular caputegulae (ambiguous in T. kielanae) and a posteroventrally oriented occipital condyle. Differs from Minotaurasaurus, P. grangeri, and Zaraapelta in having confluent supraorbital horns. Differs from Minotaurasaurus and Saichania in having a relatively tall braincase. Differs from Minotaurasaurus and Zaraapelta in having a long nuchal crest. Differs from Minotaurasaurus in having relatively long paroccipital processes that laterally reach the squamosal horns. Differs from Saichania in having remodeled squamosal horns and anteroposteriorly short lateral nuchal caputegulae.
Tarchia tumanovae sp. nov.
Etymology. Named in honour of Tatiana Tumanova for her contributions toward the understanding of Mongolian ankylosaurs.
Holotype. MPC-D 100/1353, a well-preserved skull, dorsal, sacral, caudal vertebrae, sixteen dorsal ribs, ilia, a partial ischium, free osteoderms, and tail club.
Locality and horizon. Upper Cretaceous (upper Campanian-lower Maastrichtian) Nemegt Formation, Hermiin Tsav, southern Gobi Desert, Mongolia.
Diagnosis. An ankylosaurid distinguished by having the following unique set of characters: a single relatively bulbous internarial caputegulum that does not reach the rostral tip of beak*; a nasofrontal sagittal furrow with a weak Z-shaped offset (shared with Tarchia kielanae); lateral nuchal caputegulae taller laterally than medially (shared with Saichania); vomerine keel extends below the alveolar ridge (shared with Saichania). Differs from Minotaurasaurus, Pinacosaurus grangeri, T. kielanae, T. teresae, and Zaraapelta in having a moderate-sized basioccipital foramen. Differs from Minotaurasaurus, P. grangeri, Saichania, and Zaraapelta in having no postocular caputegulae and a posteroventrally oriented occipital condyle. Differs from Minotaurasaurus, P. grangeri, T. teresae, and Zaraapelta in having an anteriorly situated quadrate-quadratojugal region. Differs from Minotaurasaurus, P. grangeri, and Zaraapelta in having confluent supraorbital horns. Differs from P. grangeri, Saichania, and Zaraapelta in having a tall foramen magnum. Differs from Saichania, T. kielanae, and Zaraapelta in having unfused quadrate to the exoccipital area. Differs from Minotaurasaurus and Saichania in having a relatively tall braincase. Differs from Minotaurasaurus and Zaraapelta in having subrectangular frontal caputegulae and a long nuchal crest. Differs from Minotaurasaurus in having narrow narial caputegulae and long paroccipital processes that laterally reach the squamosal horns. Differs from Saichania in having remodeled squamosal horns, anteroposteriorly short lateral nuchal caputegulae, and occiput visible in dorsal view. Differs from T. teresae by having an interpterygoid vacuity visible in occipital view.
Jin-Young Park, Yuong-Nam Lee, Yoshitsugu Kobayashi, Louis L. Jacobs, Rinchen Barsbold, Hang-Jae Lee, Namsoo Kim, Kyo-Young Song and Michael J. Polcyn. 2021. A New Ankylosaurid from the Upper Cretaceous Nemegt Formation of Mongolia and Implications for Paleoecology of Armoured Dinosaurs. Scientific Reports. 11: 22928. DOI: 10.1038/s41598-021-02273-4
