No abstract

Belemnites (order Belemnitida), a very successful group of Mesozoic cephalopods, provide an important clue for understanding Mesozoic marine ecosystems and the origin of modern cephalopods. Following current hypotheses, belemnites originated in the earliest Jurassic (Hettangian, 201.6-197 Ma) with very small forms. According to this view their paleobiogeographic distribution was restricted to northern Europe until the Pliensbachian (190-183 Ma). The fossil record is, however, biased by the fact that all the previous studies on belemnites focused on Europe. Here we report two belemnite taxa from the Hettangian of Japan: a new species of the Sinobelemnitidae and a large taxon of the suborder Belemnitina. The Sinobelemnitidae, which may be included in the future in a new suborder, have also been recorded from the Triassic of China, specimens so far poorly understood. The presence of a very large rostrum attributed to the Belemnitina suggests in addition that a diverse belemnite fauna evolved earlier than previously thought. Our new fi ndings therefore (1) extend the origin of the belemnites back by ~33 m.y. into the Triassic, (2) suggest that this group did not necessarily originate in northern Europe, and (3) imply that belemnites survived the Triassic-Jurassic extinction, one of the fi ve big mass extinctions in the Phanerozoic. Since belemnites provided a considerable amount of food as prey, the origination of belemnites is probably an important event also for the evolution of their predators, such as marine reptiles and sharks.

By heating an aqueous solution of aspartic acid and urea, carbamylaspartic acid is first formed and then the molecule is cyclized to dihydroorotic acid (DHO) with loss of water. Irradiation of an aqueous solution of DHO with a tungsten lamp yields orotic acid by photo-dehydrogenation of the molecule. This pathway of orotic acid formation is quite similar to that of biosynthesis of the molecule.

Carbonate platforms consisting of hermatypic coral, rudists and larger foraminifers formed along the margin of the NE Japanese islands during the mid-Late Aptian. The geographical northern limit of the carbonate platforms extended to c . 36°N at that time, which is the highest latitude for coral and/or rudist formation in the North Pacific region. This geographical extension of the carbonate platform in the NW Pacific indicates extreme climatic warmth in the mid-latitudes and strong poleward heat transport systems. Contemporaneous warming has been detected in European and Australian regions, as represented by the migration of Tethyan fauna toward the boreal realm and the positive excursion of δ 13 C and negative excursion of δ 18 O in deep-sea carbonates. Our new constraint on the northern limit of ‘reef’ growth reinforces the view that an interval of greenhouse-like warmth punctuated the Late Aptian climate.

Guyots are large seamounts with a flat summit that is generally believed to form due to constructional biogenic and/or erosional processes during the formation of volcanic islands. However, despite their large abundance in the oceans, there are still very few direct constraints on the nature and formation of guyots, in particular those formed at high latitude that lack a thick cap of shallow-marine carbonate rocks. It is largely accepted based on geophysical constraints and surficial observations/sampling that the summit platform of these guyots is shaped by wave abrasion during post-volcanic subsidence of volcanic islands. Here we provide novel constraints on this hypothesis and the summit geology of guyots with a lithostratigraphic analysis of cores from three Louisville seamounts (South Pacific) collected during Expedition 330 of the Integrated Ocean Drilling Program (IODP). Thirteen lithofacies of sedimentary and volcanic deposits are described, which include facies not previously recognized on the top of guyots, and offer a new insight into the formation of high-latitude oceanic islands on a fastmoving plate. Our results reveal that the lithostratigraphy of Louisville seamounts preserves a very consistent record of the formation and drowning of volcanic islands, with from bottom to top: (i) volcaniclastic sequences with abundant lava-fed delta deposits, (ii) submarine to subaerial shield lava flows, (iii) post-volcanic shallow to deeper marine sedimentary rocks lacking thick reef deposits, (iv) post-erosional rejuvenated volcanic rocks, and (v) pelagic sediments. Recognition of erosional boundaries between subaerial lava flows and shallow-marine sedimentary rocks provides novel support for post volcanic wave planation of guyots. However, the summit geology of Louisville seamounts is dissimilar to that of high-latitude Hawaiian-Emperor guyots that have emplaced in a similar tectonic and environmental setting and that include thicker lava stacks with apparently little lava-fed delta deposits. To explain observed lithostratigraphic discrepancy we propose that Louisville seamounts represent a distinct type of intraplate ocean volcano characterized by formation of a smaller island, with a central shield volcano surrounded by extended shallow-marine shelves formed by lava-fed deltas. In this interpretation the summit platform of Louisville-type guyots results from early (syn-volcanic) subaerial to shallow-marine constructional volcanic processes and marine erosion, enhanced by later (post-volcanic) wave planation. This contrasts with larger Hawaiian edifices that are capped by thicker shield volcanoes, and that develop an extended wave planation surface during postvolcanic subsidence (in the absence of efficient coral growth). The difference between Hawaiian-and Louisville-type volcanic islands and guyots can be explained by contrasted dynamic disequilibrium between magmatic growth, erosion, and subsidence during the island-building stage. Unlike Hawaiian-type volcanoes, Louisville seamounts are characterized by a...