Koshikibaatar

Koshikibaatar (lit.'Koshiki hero') is an extinct genus of cimolodontanmultituberculate mammal known from the Late Cretaceous (Campanian age) Himenoura Group of Japan. The genus contains a single species, Koshikibaatar kashimaensis, known from a partial lower jaw with teeth.

The Koshikibaatar fossil material was discovered in outcrops of unit U-III of the Himenoura Group on Shimokoshiki Island (part of the Koshiki Islands) in Kagoshima Prefecture of southwestern Japan. The specimen is housed in the Koshiki Museum, where it is permanently accessioned as specimen KSKM-V00001. The specimen consists of much of the right dentary (lower jaw bone) with four teeth preserved.^[1]

In 2026, Takuma Sera and colleagues describedKoshikibaatar kashimaensis as a new genus and species of multituberculate based on these fossil remains, establishing KSKM-V00001 as the holotype specimen. The generic name, Koshikibaatar, combines a reference to the Koshiki Islands, from which the holotype is known, with the Mongolian word baatarcode: mon promoted to code: mn , meaning 'hero' (a suffix often used in the genus names of Asian multituberculates). The specific name, kashimaensis, references the Kashima area of the island on which the holotype was found.^[1]

The K. kashimaensis holotype, KSKM-V00001, comprises most of the right dentary, missing the rear end, including the condyle (articular region for the cranium) and coronoid process. The incisor, third and fourth premolars, and first molar are preserved in the mandible. The second molar is not preserved, but an empty alveolus (tooth socket) is present. The height of the dentary near its midsection (below the first molar) is 9 millimetres (0.35 in).^[1]

Para probar las afinidades y relaciones de Koshikibaatar , Sera y colegas (2026) lo incluyeron en una versión actualizada de la matriz filogenética de Smith et al. (2021). ^{[ 2 ]} Recuperaron a Koshikibaatar como un miembro del clado multituberculado más derivado Cimolodonta , como taxón hermano de Cimolodon . Los investigadores propusieron que esta estrecha relación en el análisis puede deberse a caracteres compartidos que son en realidad plesiomórficos (ancestrales) a Cimolodonta, en lugar de únicos al grupo exclusivo formado por Koshikibaatar y Cimolodon . Como tal, consideraron que la ubicación de Koshikibaatar dentro de los cimolodontos era incierta, pero consideraron posible una estrecha afinidad con el clado más pequeño Ptilodontoidea (el grupo al que pertenece Cimolodon ). Su árbol de consenso de regla de mayoría del 50% (replicado en el cladograma a continuación) produjo resultados más resueltos que el árbol de consenso estricto, el cual presentó una politomía grande y mal resuelta . ^{[ 1 ]}