Kazunori Miyata

Anchitherium (Mammalia, Perissodactyla, Equidae) from the Early Miocene Hiramaki Formation, Gifu Prefecture, Japan, and its Implication for the Early Diversification of Asian Anchitherium

Abstract An Anchitherium specimen with a nearly complete series of the upper cheek teeth (P2–M3), from the upper part (ca.17–18 Ma) of the Hiramaki Formation, Kani Basin, Gifu Prefecture, Japan, was previously referred to Anchitherium “hypohippoides” nomen dubium. Despite that it provides one of the best examples of significant dental characters of Asian Anchitherium, it has remained undescribed and unprepared until recently. Although a paucity of materials from Asia makes the taxonomy of Asian Anchitherium difficult to assign, comparison showed that the specimen should be reassigned to Anchitherium aff. A. gobiense; it differs from A. aurelianense and is rather similar to A. gobiense from China by virtue of large size and expanded hypostyles. The Japanese Anchitherium also shows distinct features including straight (flattened) ectolophs with narrow mesostyles, rudimentary crochets, and enamel protuberances at the lingual mouth of the median valleys. This combination of accessory features has not been known in Asian Anchitherium and seems to be rarely observed among the diversified European species. The existence of Japanese Anchitherium implies early species diversification in East Asia that predates a greater diversification in Europe associated with the Middle Miocene Climatic Optimum and supports paleogeographical and paleozoological connection to the Asian mainland under a warm and humid climate prior to the formation of the Japanese archipelago (ca. 16.5 Ma).

New material of Asian Trogosus (Tillodontia, Mammalia) from the Akasaki Formation, Kumamoto Prefecture, Japan

Abstract A new specimen of Trogosus from the Akasaki Formation, Amakusa City in Kumamoto Prefecture, Japan, is represented by 12 teeth, mandibular fragments, and cervical vertebral remains of an old individual. This is the most informative specimen of Trogosus from Asia. Comparisons with all known species of Trogosus indicate that the specimen is comparable to the largest species, Trogosus latidens, which has been recorded only from the Bridgerian North American Land Mammal Age. In addition to few diagnostic features of T. latidens except for the large size, the poor preservation of the new specimen makes its taxonomic identification at the species level debatable. However, the presence of the Japanese Trogosus further supports the intercontinental diversity and distribution of the genus via Beringia.

New species of Trogosus (Tillodontia, Mammalia) from the Green River Basin, Wyoming, U.S.A.

Abstract A new species, Trogosus gazini, is represented by eleven upper teeth (i2, P3-M3), eleven lower teeth (i2, i3, c1, and p3–m3), and some fragmentary postcrania of a single individual (USNM 364762), from the Blacks Fork Member of the Bridger Formation, Green River Basin, Wyoming, U.S.A. The new species has unique dental characters with a diminutive parastyle on the P3 and the asymmetrical pattern of the P4 lingual crests (reduced preprotocrista and expanded postprotocrista), in addition to having brachydont molars with dominant shearing wear facets. A phylogenetic analysis based on dental morphology places T. gazini as the sister taxon to either Trogosus latidens or Tillodon fodiens. The result also produces a solution for previously unresolved phylogenetic relationships among Kuanchuanius, Trogosus, and Tillodon. Although the result requires more morphological data to enhance the credibility of relationships among trogosine tillodont species, Trogosus gazini may represents a transitional form to Tillodon fodiens, which is most likely a descendant from a paraphyletic Trogosus with some autapomorphies. On the contrary, Kuanchuanius is allocated as a sister taxon to the Trogosus + Tillodon clade.

New Material of a ‘Short-Faced’ Trogosus (Mammalia, Tillodontia) from the Delmar Formation (Bridgerian), San Diego County, California, U.S.A.

ABSTRACT The ‘Swamis Point local fauna,’ recovered from the Delmar Formation in coastal San Diego County, California, U.S.A., represents the first documented Bridgerian vertebrate assemblage in this region. The Delmar Formation is chronologically constrained (ca. 48-47 Ma) on the basis of paleomagnetic polarities, the superposition of marine strata containing nannofossils of the CP 12b subzone, and occurrences of the Bridgerian index taxa Trogosus castoridens and Hyrachyus modestus. The specimen of Trogosus (SDSNH 40819) is assigned here to the genotypic species, T. castoridens, hitherto known only from incomplete dentaries from the middle Bridgerian (Br2) Bridger Formation, Green River Basin, Wyoming. The present specimen is represented by a skull with diagnostically short rostrum, incomplete stylohyoids, posterior parts of both dentaries, and cervical vertebrae from a single individual. This is one of the most anatomically informative specimens of Trogosus and for the first time provides details of the basicranial region in the order Tillodontia. A well-preserved occipitomastoid process is clearly composed of the mastoid process of the petrosal and the paracondylar process of the exoccipital. Cranial comparison reveals greater similarity with T. hyracoides from the Bridger Formation than with T. grangeri and T. hillsii from the Huerfano Basin, Colorado. The results of a phylogenetic analysis suggest that the short-faced condition evolved at least twice in species of Trogosus and leads to recognition of two sympatric trogosine species pairs: T. hyracoides + T. castoridens from the Green River Basin and T. grangeri + T. hillsii from from the Huerfano Basin.

New mammalian specimens from the Lower Cretaceous Kitadani Formation, Tetori Group, Fukui, Japan

New specimens of two non-therian (non-tribosphenidan) mammals are described from the Lower Cretaceous Kitadani Formation in the upper part of the Tetori Group, Katsuyama, Fukui Prefecture, Japan. Despite their poor preservation, these specimens represent undescribed species from Japan, suggesting additional mammalian diversity in the Tetori Group. Previously, four mammal taxa had been formally described from this rock unit: a spalacotheriid ‘symmetrodont’, two eobaatarid multituberculates and a eutriconodont mammal. One of the new specimens is a damaged left p4 of a ‘plagiaulacidan’ grade multituberculate assignable to the family Eobaataridae. The Kitadani eobaatarid is a large species distinguished from the two previously described eobaatarids, Tedoribaatar and Hakusanobaatar, which are known from the stratigraphically lower Kuwajima Formation of the Tetori Group, Hakusan, Ishikawa Prefecture, Japan. It also differs from Sinobaatar, Heishanobaatar and Liaobaatar described from the Early Cretaceous of China. Another specimen, a partial right dentary with a faint Meckelian groove, is assigned to a eutriconodontan that is larger than and morphologically distinguishable from the eutriconodont Hakusanodon from the Kuwajima Formation. The Kitadani eutriconodontan is potentially related to the family Triconodontidae. The additional specimens from the Kitadani Formation shed new light on Early Cretaceous mammalian faunal change and dispersal in East Asia.

The most basal ankylosaurine dinosaur from the Albian–Cenomanian of China, with implications for the evolution of the tail club

The tail club knob is a highly specialized structure thought to characterize a subgroup of the ankylosaurine ankylosaurians, and the oldest documented tail club knob in the fossil record occurred in the Campanian ankylosaurine Pinacosaurus. Here we report a new ankylosaurid Jinyunpelta sinensis, gen. et sp. nov., from the Albian–Cenomanian Liangtoutang Formation, Jinyun County, Zhejiang, China. This is the first definitive and the best preserved ankylosaurid dinosaur ever found in southern China. Jinyunpelta possesses unique cranial features differs from other ankylosaurs including two paranasal apertures level with and posterior to the external naris, a triangular fossa on the anterodorsal edge of the maxilla, an antorbital fossa in the junction between the maxilla, lacrimal and jugal, and an anterior process of the prearticular that lies ventral to the splenial. Our phylogenetic analysis suggests Jinyunpelta as the most basal ankylosaurine dinosaur. Jinyunpelta has a tail club with interlocking caudal vertebrae and a well-developed tail club knob, it represents the oldest and the most basal ankylosaurian known to have a well-developed tail club knob. The new discovery thus demonstrates that a large and highly modified tail club evolved at the base of the ankylosaurine ankylosaurs at least about 100 million years ago.

LA-ICP-MS Zircon U—Pb Dating of Acidic Tuff from the Eocene Akasaki Formation, Amakusa, Kumamoto Prefecture, Western Kyushu, Japan: Implications for the Age of the Land Mammal Fauna in Japan

Abstract. We obtained zircon U—Pb ages using LA-ICP-MS for an acidic tuff bed in the upper part of the Akasaki Formation, a basal unit of the Eocene sequence in the Amakusa area, Kumamoto Prefecture, Japan. The U—Pb age of 49.1±0.4 Ma (95% confidence interval) suggests a late early Eocene age. Our result is important for biochronological assessment of the land mammal fauna of the Akasaki Formation. The new age date supports the correlation of the Akasaki mammal fauna with the Bridgerian North American Land Mammal Age.