田中 公教

Abstract The herbivorous dinosaur clade Ceratopsia flourished in the northern hemisphere during the Late Jurassic to Late Cretaceous. Previous palaeobiogeographic studies have suggested that their initial diversification occurred in Asia, with early‐branching neoceratopsians extending their geographical range to North America sometime during the Barremian to Albian. However, the specific timing and mode of their dispersal from Asia to North America remains unknown. Here we describe a new, early‐branching neoceratopsian, Sasayamagnomus saegusai gen. et sp. nov., from the Albian Ohyamashimo Formation in southwestern Japan, representing the easternmost fossil record of ceratopsians in Asia. Sasayamagnomus exhibits three diagnostic features in the jugal, squamosal and coracoid, respectively, and also has a unique combination of characters in the lacrimal. Our phylogenetic analysis indicates that Sasayamagnomus forms a clade with Aquilops americanus, one of the earliest neoceratopsians from North America, followed by the sister taxon Auroraceratops rugosus from China. The present time‐calibrated phylogenetic tree indicates that the immigration of neoceratopsians from Asia to North America occurred during the latest Aptian or early Albian, refining the previously suggested timeframe. This aligns with fragmentary neoceratopsian fossil records from the Lower Cretaceous of North America and the initial formation of the Bering land bridge. Furthermore, the simultaneous occurrence of global warming (which enabled the development of extensive forests in the Arctic region) and the emergence of the Bering land bridge during the Aptian–Albian, probably played a crucial role in facilitating the immigration of neoceratopsians from Asia to North America.

© 2020 Elsevier Ltd The Kamitaki Locality in the Albian Ohyamashimo Formation in the eastern Hyogo Prefecture of southwestern Japan is among the richest Lower Cretaceous fossil egg sites in the world. So far, eggshells of five different ootaxa, one ornithopod and four theropods, have been identified from the Kamitaki Bonebed Quarry. Although previous discoveries of egg remains were limited to isolated eggshell fragments, here we report on the discovery of a nearly complete egg, several partial eggs, as well as numerous eggshell fragments from horizons of the newly excavated Kamitaki Egg Quarry that are 5.5–6.75 m above the Kamitaki Bonebed Quarry. Taphonomical investigations indicate that the new site preserves the remnants of an in-situ nest of Himeoolithus murakamii oogen. et oosp. nov., mixed with scattered and isolated eggshell fragments of other non-avian theropod ootaxa, including Nipponoolithus ramosus, Prismatoolithus sp., and Subtiliolithus hyogoensis oosp. nov. The most abundant ootaxon at the quarry, Himeoolithus, is represented by four eggs and over 1300 scattered eggshell fragments. Himeoolithus is the smallest non-avian theropod egg known to date (9.9 g in estimated mass) and exhibits an unusually elongated shape (length:width ratio of 2.25) for such as small egg. The presence of six dinosaur ootaxa from the two quarries at the Kamitaki Locality reveals a hidden diversity of small dinosaurs, particularly non-avian theropods, in the Hyogo region and indicates the area was utilized for nesting by various small dinosaur species at the end of the Early Cretaceous.

© 2020 Elsevier Ltd Asian hesperornithiforms are extremely rare in contrast to North American records; thus, their diversity in Asia during the Cretaceous is unclear. Maastrichtian hesperornithiform materials have been reported from both fluvial and marine deposits in North America but only from fluvial deposits in Asia. Asian hesperornithiforms from Maastrichtian deposits have been considered as freshwater taxa because of their occurrence from fluvial sediments and their histological features. Here, we report the first hesperornithiform record from marine Maastrichtian deposits in Asia. It is represented by an isolated left tibiotarsus from the inter-arc basin deposit of the Kita-ama Formation (lower Maastrichtian), Izumi Group of southwest Japan. It has a shallow tibial incision, fibular crest extending to the mid shaft, and laterally angled lateral articular surface. Although its phylogenetic position within Hesperornithiformes is ambiguous, these characters are similar to non-hesperornithid hesperornithiforms. Unossified proximal and distal epiphyses indicate that this individual was immature. A remarkably thick cortical area of the tibiotarsus suggests that this hesperornithiform was a sea-dwelling bird and that the habitat of this group during the Maastrichtian extended to both terrestrial and marine environments in Asia and North America.