Paleontological Research

Small-sized ammonites assigned to the genus Tetragonites, whose ventral apertural margin projects forward in a broad convex arch, include two species, i.e., T. minimus and T. nanus sp. nov. These particular taxa occur only in the Upper Cretaceous (Turonian to Campanian) of Hokkaido, northern Japan. Tetragonites minimus occurs in the lower Turonian to the Santonian, and T. nanus, which occurs in the Coniacian to the middle Campanian, tends to be smaller in adult shell size than T. minimus. There is no significant bimodal distribution in the adult shell size of either species. The Tetragonites group characterized by a ventral projection on the apertural margin flourished mainly in the Tethyan realm during the late Aptian to middle Cenomanian time interval, but then the group mostly disappeared from the Tethyan realm, leaving only the Hokkaido occurrence of the present study. A working hypothesis can be formulated that Tethyan species such as T. spathi migrated to the Northwest Pacific region near the Cenomanian–Turonian boundary and gave rise to Turonian T. minumus and succeeding T. nanus during the Coniacian or possibly earlier. Tetragonites nanus is one of the smallest mature ammonoids known thus far.

Linkages between hatchling and adult sauropods concerning morphological and ontogenetic growth remain enigmatic, which is even more tenuous in early sauropods. However, discoveries from Southeast Asia could provide significant insights into these questions of developmental history with the recent discovery of a partial juvenile vertebral series from an early sauropod from the early Middle Jurassic Nam Phong Formation, Chaiyaphum province, northeastern Thailand. Here we present the anatomical description of preserved portions of the axial skeleton, including cervical and dorsal vertebrae. Our study demonstrates the presence of 1) incipient laminae and fossae on the centrum; 2) unfused neural arch (cervical and dorsal); and 3) well-marked vertebral lamination system, all of which are osteological hallmarks of an immature growth phase. Particularly, we note that in this hatchling-juvenile specimen, the vertebral lamination complex resembles an adult early sauropod's form, along with no true pleurocoel in the vertebral arches. The co-occurrence of these two characters would strongly indicate a non-eusauropod sauropod affinity. With the presence of the lamination pattern, and the temporal placement of the specimen in the early Middle Jurassic, our material mostly resembles the Tazoudasaurus, so we assign the material to the Vulcanodontidae. Further analysis of this hatchling-juvenile specimen will provide crucial insights into the ontogenetic developmental history and musculoskeletal function, successfully filling a current knowledge gap within early sauropods evolutionary studies.

Angiosperm pollen grains of the triprojectate and oculate groups are widely distributed in Upper Cretaceous strata of the Northern Hemisphere, being used as biostratigraphic markers from their high diversification rate. Most reports from northeastern Asia, however, concern terrestrial deposits, and the rich record existing in Japan from marine sequences, with a well-established bio-magnetostratigraphy, appears critical for correlating their range among the Circum Pacific region. Here we propose a revision of Japanese reports of these groups, constraining their diversity to 17 genera and 91 species. A selected array of 15 species of well-preserved triprojectate pollen was further obtained from an abundant sporo-pollen assemblage of a hadrosaurian dinosaur Kamuysaurus japonicus-bearing bone bed in the outer shelf deposits of the Upper Cretaceous Hakobuchi Formation, Yezo Group, recently discovered in the Hobetsu area of Mukawa Town in Hokkaido, Japan; the assemblage co-occurs with the ammonoid Pachydiscus (Neodesmoceras) japonicus belonging to the Nostoceras hetonaiense zone, supporting an early Maastrichtian age.

A new fossil species of the genus Ceratophyus Fischer von Waldheim, 1824 , was discovered in the Shiobara Group (Chibanian, ca. 0.3 Ma) of the Middle Pleistocene distributed in Nasushiobara City, Tochigi Prefecture, Japan. This fossil is female, large in size (ca. 25 mm), well preserved, and nearly completely preserved. The genus Ceratophyus is not distributed in modern Japan. This is the first discovery of a fossil of the genus Ceratophyus and the first report of a certain extinct species from the Shiobara Group. The genus Ceratophyus is now fragmentarily distributed in the Holarctic region of the world, but the discovery of the fossil suggests that the genus was widely distributed throughout the world during the Middle Pleistocene. Notably, the fossil was discovered by a student during a high school lesson from a rock brought into the classroom. We consider the examined fossil to be morphologically different from all known extant species of the genus, and therefore we describe it as Ceratophyus yatagaii sp. nov.

Hemipristis tanakai, a new species of snaggletooth shark (Order Carcharhiniformes, Family Hemigaleidae), is erected based on the fossil tooth specimens recovered from the Lower Oligocene of the Yamaga Formation of the Ashiya Group in Kyushu, Japan. This species was once proposed by Naoyuki Kuga in his dissertation in 1985, based on a single tooth; however, it has been considered unavailable because of its failure to meet the criteria of the International Code of Zoological Nomenclature. We reviewed this specimen, together with some additional specimens, and confirmed that it was distinguishable from existing congeners. This species is informative for the evolution of Hemipristis because it fills the gap in tooth morphology between Paleogene and Neogene species.

Tanabeceras nakagawaense sp. nov. is described from the lower part of the middle Cenomanian (Cunningtoniceras takahashii Zone, Upper Cretaceous) in the Nakagawa area, Hokkaido, northern Japan. This new taxon is characterized by having the most compressed shell among all species assigned to Tanabeceras, and it also represents the youngest taxon of the genus. The stratigraphic distribution of Tanabeceras in Hokkaido suggests that the genus migrated from the Tethyan realm to the Northwest Pacific region during late Albian, evolved and radiated regionally during the late Albian to early middle Cenomanian, and then became extinct during late middle Cenomanian time.

Two new diatom species, Praestephanos tokaiensis sp. nov. and P. miensis sp. nov. are described from lacustrine deposits in the Pliocene Ueno Formation of the Kobiwako Group in Iga City, Mie Prefecture, Japan. The stratigraphic ranges of P. tokaiensis and P. miensis are approximately 3.9–3.8 Ma and 3.8–3.7 Ma, respectively. The morphology of these species is mainly characterized by a slightly doubly undulating central area; this feature is not found in previously described Praestephanos species or similar Stephanodiscus species. The stratigraphic distribution and morphology of the two new species suggest that P. tokaiensis and P. miensis have an ancestor–descendant relationship.

This paper describes two brachiopod species, Cyrtospirifer cf. procumbens Simorin and Tylothyris sp., from the Upper Devonian Naidaijin Formation of the Chunoboridake area in the Kurosegawa Belt of Kyushu, southwestern Japan. The age of the small brachiopod fauna is assigned to the Late Devonian (probably Famennian). In terms of palaeobiogeography, the Chunoboridake fauna has a certain affinity with the Famennian brachiopod fauna of northwestern China (Xinjiang). Proto-Japan was probably located north of the North China Block during the Late Devonian.

This paper presents the first analytical examination of the evidence of growth and aging in pycnodonts. Preliminarily, the author discusses the difficulties of evaluating auxological phenomena in animals that do not have current descendants and in which the anatomical traits typical of juveniles persist in adults (neoteny), as an evolutionary strategy. The author collected biometric data on juvenile and corresponding adult subjects of only 15 pycnodont species. Despite this scarcity of species, the author demonstrates that there does not seem to exist a single pattern of somatic growth. However, at least two characters (one skeletal and one dental) seem common to all the examined species: (i) the relative dimensions of the orbit decrease in a constant, sometimes conspicuous manner with respect to the total body dimensions, and (ii) the width of the interdental diastemas is noticeably reduced during growth. At least two phenomena seem to be present also in aging: (i) bone hypotrophy (detectable only through radiological means) and (ii) the depletion of ameloblastic activity leading to the cessation of polyphyodontia. However, the inability to replace worn teeth may have played a role in causing natural death in aged pycnodonts by interfering with their ability to feed themselves.

A new fossil species of the genus Archaeromma Yoshimoto, 1975 was discovered in Iwaki amber from the Tamayama Formation (Late Cretaceous Coniacian) in Iwaki City, Fukushima Prefecture, Japan. The genus Archaeromma belongs to the family Mymarommatidae, and the members are called false fairy wasps because of their small bodies; only 10 species have been described from Cretaceous amber (Albian to Campanian). In this paper, we have described the new fossil species as Archaeromma chisatoi sp. nov. To the best of our knowledge, this is the second report of a fossil of the genus Archaeromma from Japan; the first one was found in Kuji amber, Iwate Prefecture. However, this is the first report of a fossil species from the Coniacian age and the first paleontological description from Iwaki amber.