田中 公教

© 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.

© 2019 Fiorillo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. While there are now numerous records of dinosaurs from Cretaceous rocks around the state of Alaska, very few fossil records of terrestrial vertebrates are known from the Mesozoic rocks of the southwestern part of the state. Here we report the new discovery of extensive occurrences of dinosaur tracks from Aniakchak National Monument of the Alaska Peninsula. These tracks are in the Late Cretaceous (Maastrichtian) Chignik Formation, a cyclic sequence of rocks, approximately 500-600 m thick, representing shallow marine to nearshore marine environments in the lower part and continental alluvial coastal plain environments in the upper part of the section. These rocks are part of the Peninsular Terrane and paleomagnetic reconstructions based on the volcanic rocks of this terrane suggest that the Chignik Formation was deposited at approximately its current latitude which is almost 57° N. Recent field work in Aniakchak National Monument has revealed over 75 new track sites, dramatically increasing the dinosaur record from the Alaska Peninsula. Most of the combined record of tracks can be attributed to hadrosaurs, the plant-eating duck-billed dinosaurs. Tracks range in size from those made by full-grown adults to juveniles. Other tracks can be attributed to armored dinosaurs, meat-eating dinosaurs, and two kinds of fossil birds. The track size of the predatory dinosaur suggests a body approximately 6-7 m long, about the estimated size of the North Slope tyrannosaurid Nanuqsaurus. The larger bird tracks resemble Magnoavipes denaliensis previously described from Denali National Park, while the smaller bird tracks were made by a bird about the size of a modern Willet. Previous interdisciplinary sedimentologic and paleontologic work in the correlative and well-known dinosaur bonebeds of the Prince Creek Formation 1400km-1500km further north in Alaska suggested that high-latitude hadrosaurs preferred distal coastal plain or lower delta plain habitats. The ichnological record being uncovered in the Chignik Formation of southwestern Alaska is showing that the hadrosaur tracks here were also made in distal coastal and delta plain conditions. This similarity may corroborate the habitat preference model for Cretaceous high-latitude dinosaurs proposed for the data gathered from the Prince Creek Formation, and may indicate that at least Beringian hadrosaurids had similar habitat preferences regardless of latitude.