Yasuhisa Nakajima

Bone Inner Structure Suggests Increasing Aquatic Adaptations in Desmostylia (Mammalia, Afrotheria)

Background The paleoecology of desmostylians has been discussed controversially with a general consensus that desmostylians were aquatic or semi-aquatic to some extent. Bone microanatomy can be used as a powerful tool to infer habitat preference of extinct animals. However, bone microanatomical studies of desmostylians are extremely scarce. Methodology/Principal Findings We analyzed the histology and microanatomy of several desmostylians using thin-sections and CT scans of ribs, humeri, femora and vertebrae. Comparisons with extant mammals allowed us to better understand the mode of life and evolutionary history of these taxa. Desmostylian ribs and long bones generally lack a medullary cavity. This trait has been interpreted as an aquatic adaptation among amniotes. Behemotops and Paleoparadoxia show osteosclerosis (i.e. increase in bone compactness), and Ashoroa pachyosteosclerosis (i.e. combined increase in bone volume and compactness). Conversely, Desmostylus differs from these desmostylians in displaying an osteoporotic-like pattern. Conclusions/Significance In living taxa, bone mass increase provides hydrostatic buoyancy and body trim control suitable for poorly efficient swimmers, while wholly spongy bones are associated with hydrodynamic buoyancy control in active swimmers. Our study suggests that all desmostylians had achieved an essentially, if not exclusively, aquatic lifestyle. Behemotops, Paleoparadoxia and Ashoroa are interpreted as shallow water swimmers, either hovering slowly at a preferred depth, or walking on the bottom, and Desmostylus as a more active swimmer with a peculiar habitat and feeding strategy within Desmostylia. Therefore, desmostylians are, with cetaceans, the second mammal group showing a shift from bone mass increase to a spongy inner organization of bones in their evolutionary history.

Comparative Humeral Microanatomy of Terrestrial, Semiaquatic, and Aquatic Carnivorans using Micro-Focus CT Scan

Abstract. In order to investigate the relationship between limb bone internal structure and the aquatic adaptation, we examined humeri of multiple carnivoran species of various lifestyles (terrestrial, semiaquatic, and aquatic) using micro-focus CT scanner. The transverse section image passing through the inferred center of ossification is analyzed in order to estimate the structural remodeling in periosteal bone. Meanwhile, structural characteristics in sagittal section images of proximal end were described in order to assess the characteristics in endochondral bone. The degree of cancellousness around the center of ossification, which is interpreted as the amount of resorption in periosteal compact bone, is the lowest in the semiaquatic group and the highest in terrestrial groups. This suggests that the ratio of periosteal bone resorption in limb bones decreases in the first stage of the aquatic adaptation, possibly due to the less need for skeletal lightening in semiaquatic taxa than the terrestrial taxa, and then increases as the limb bones reduces their support function on land, presumably in relation to the deep diving. The bone trabecular architecture in humeral head is fine and well-oriented in terrestrial to semiaquatic carnivorans, while that of aquatic carnivorans is rather coarse and randomly oriented.

A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan

The first plesiosaur skeleton from the Triassic informs about the evolutionary success of four-flippered marine reptile. Secondary marine adaptation is a major pattern in amniote evolution, accompanied by specific bone histological adaptations. In the aftermath of the end-Permian extinction, diverse marine reptiles evolved early in the Triassic. Plesiosauria is the most diverse and one of the longest-lived clades of marine reptiles, but its bone histology is least known among the major marine amniote clades. Plesiosaurians had a unique and puzzling body plan, sporting four evenly shaped pointed flippers and (in most clades) a small head on a long, stiffened neck. The flippers were used as hydrofoils in underwater flight. A wide temporal, morphological, and morphometric gap separates plesiosaurians from their closest relatives (basal pistosaurs, Bobosaurus). For nearly two centuries, plesiosaurians were thought to appear suddenly in the earliest Jurassic after the end-Triassic extinctions. We describe the first Triassic plesiosaurian, from the Rhaetian of Germany, and compare its long bone histology to that of later plesiosaurians sampled for this study. The new taxon is recovered as a basal member of the Pliosauridae, revealing that diversification of plesiosaurians was a Triassic event and that several lineages must have crossed into the Jurassic. Plesiosaurian histology is strikingly uniform and different from stem sauropterygians. Histology suggests the concurrent evolution of fast growth and an elevated metabolic rate as an adaptation to cruising and efficient foraging in the open sea. The new specimen corroborates the hypothesis that open ocean life of plesiosaurians facilitated their survival of the end-Triassic extinctions.

The first temnospondyl amphibian from Japan

Staatliches Museum fur Naturkunde Rosenstein 1, 70191 Stuttgart, Germany,¨schoch.smns@naturkundemuseum-bw.deTemnospondyls are the most diverse, speciose, and long-livedgroup of early tetrapods. Whereas their first record in the latePaleozoic is restricted to Euramerica, during the early Meso-zoic they reached an almost worldwide distribution. The large,crocodiliform stereospondyls in particular experienced a phaseof rapid dispersal across vast areas of the Pangean superconti-nent (Schoch, 2000). Rich deposits have been reported from theEarly and Middle Triassic of Arizona, Argentina, Brazil, Cen-tral Europe, European Russia, South Africa, India, Madagascar,and Australia (Schoch and Milner, 2000). However, in East Asiatheir remains are still very rare and utterly fragmentary (Lucasand Hunt, 1993; Liu and Wang, 2005). At the same time, the pa-leogeography of the eastern Pangean margin is still rather poorlyunderstood, which hampers a clear picture of migratory patternsof aquatic tetrapods during the early Mesozoic. In this paper, wereport the occurrence of a capitosauroid taxon in the Lower Tri-assic of northeastern Honshu, Japan. This is not only the firsttemnospondyl from the Japanese islands but also forms the firstnon-marinevertebrate fromthe Triassicof Japan, which suggestsa migration of aquatic tetrapods across the land bridge betweenthe North China block and the ancient Japanese islands in pre-Jurassic time.SYSTEMATIC PALEONTOLOGYTEMNOSPONDYLI Zittel, 1887–1890 (sensu Milner, 1990)STEREOSPONDYLI Watson, 1919 (sensu Milner, 1994)CAPITOSAUROIDEA Watson, 1919 (sensu Schoch andMilner, 2000)

Morphological and histological evidence for the oldest known softshell turtles from Japan

ABSTRACT Herein we report morphologically and histologically diagnostic trionychid specimens from the Barremian—Aptian (129.4–113.0 Ma) of Japan. One specimen (FPDM-V9487) is an associated skeleton that consists of a scapula, a humerus, an ischium, and hypoplastra. The limb and girdle bones are similar in morphology to those of modern trionychids. The hypoplastron also resembles that of modern trionychids; however, it is distinctive in that it completely lacks callosities. The other three new specimens are fragmentary costals that show a reduction in the shell bones and an absence of scute sulci. Two of the newly reported costals and FPDM-V0127 were examined histologically. Bone fiber bundles organized in a plywood-like structure, which is unique to trionychids, were identified. Fossil occurrence data indicate that morphologically and histologically typical trionychids already inhabited the coastal region of Asia (e.g., western Japan) as early as the Aptian. In contrast, the Hauterivian—Aptian stem trionychid Kappachelys okurai from Japan does not show a plywood-like shell microstructure, suggesting that K. okurai could be the earliest-branching taxon of known stem trionychids or a carettochelyid. Our paleobiogeographic compilation suggests that the spread of wetlands in the Northern Hemisphere and the high global temperature during the middle Cretaceous (Aptian—Turonian, 125.0–89.8 Ma) might have contributed to the dispersal of trionychids from Asia, allowing for the establishment of this modern freshwater reptilian fauna.

An Early Triassic Ichthyopterygian Fossil from the Osawa Formation in Minamisanriku Town, Miyagi Prefecture, Japan

Abstract. The ichthyopterygian Utatsusaurus hataii Shikama et al. 1978 is the only valid reptilian taxon known from the Lower Triassic Osawa Formation in Minamisanriku Town, Miyagi Prefecture, which records the recovery of the marine ecosystem shortly after the end-Permian mass extinction. In this paper, we describe a fragmentary specimen of an indeterminate ichthyopterygian which is distinguished from Utatsusaurus hataii based on rib morphology. The discovery of a previously unknown ichthyopterygian implies that the taxonomic diversity of the reptilian fauna of this formation is higher than previously assumed.

Osteohistology of the Early Triassic Ichthyopterygian Reptile Utatsusaurus hataii: Implications for Early Ichthyosaur Biology

Ichthyosaurs were highly adapted to a marine lifestyle, as shown by their fish-like body shape and their assumed active swimming abilities and high metabolic rates. However, the processes of adaptation to an aquatic life in the early stages of this lineage remain poorly understood. Here, we present the first osteohistological data concerning the most basal ichthyopterygian yet known, Utatsusaurus hataii, from the Lower Triassic of Japan. The cancellous bone structure suggests adaptation to active swimming in an open marine environment. Moreover, the possible occurrence of rapidly deposited bone tissue, in a fibrolamellar complex, suggests a higher metabolic rate than in modern poikilothermic reptiles, and therefore a trend toward homeothermy. This basal ichthyosaur, with its elongate body, was already more adapted to an aquatic lifestyle than expected from its morphology, and the process of adaptation to a marine lifestyle was already well advanced by the Early Triassic.

Structural, functional, and physiological signals in ichthyosaur vertebral centrum microanatomy and histology

ABSTRACT The first tuna-shaped amniotes evolved among ichthyosaurs, but this group exhibits in fact a wide diversity of morphologies and swimming modes. The histology and microanatomical features of vertebral centra of a diversity of ichthyosaur taxa from most basal to highly derived illustrating this variability were analyzed. The occurrence of unusual parallel fibered bone with platings of true parallel-fibered bone confirms high growth rate in all these taxa. Ichthyosaur vertebrae, which are deeply amphicoelous, show a limited endosteal territory associated with a limited growth in length. No bone mass increase nor decrease occurs. The vertebral centrum is spongious, and two microtypes are observed in the periosteal territory, with different degrees of organization of the trabecular network. The microtypes appear to be associated with the shape of the vertebral centrum, the organization of the spongiosa becoming homogeneous in the disk-shaped centra of cymbospondylids and Neoichthyosauria, rather than much more heterogeneous in spool-shaped centra of primitive Triassic forms. As opposed to what was previously suggested in other amniotes, the main switch in microanatomical organization appears thus to be correlated to the acquisition of deeply amphicoelous disk-like vertebral centra rather than to a shift in swimming mode from long and slender-bodied anguilliform swimmers to thunniform swimmers.

Sphenodus (Chondrichthyes, Neoselachii) from the Upper Cretaceous in Nakagawa Town, Hokkaido, Japan

Abstract. Eight specimens of Sphenodus have been collected from the Upper Cretaceous, Coniacian of Nakagawa Town in Hokkaido, northern Japan. They are referred to as S. cf. lundgreni and Sphenodus spp. 1 and 2, and described in detail. Nearly complete specimens of Sphenodus were previously unknown from the Cretaceous in the Pacific region, and the Nakagawa specimens are the first to exhibit distinct root morphologies, which allow comparison at the species level. A review of the occurrences of this genus reveals that their distribution concentrates in the mid- to high palaeolatitude regions. This distributional pattern of Sphenodus may represent its preference for colder water and possibly explains the extinction of this genus across the Paleocene—Eocene boundary, when the thermal maximum began.

Discovery of the Early Spathian (Late Olenekian, Early Triassic) Ammonoid Tirolites in the Hiraiso Formation, South Kitakami Belt, Northeast Japan

Abstract. Until now, the Hiraiso Formation was considered to be entirely of Smithian (early Olenekian, Early Triassic) age, but the discovery of the ammonoid Tirolites cf. ussuriensis in the lower part of the formation on the rocky coast between the Maekawara and Akaushi ports, Motoyoshi Town, Kesennuma City, in the South Kitakami Belt, Northeast Japan, establishes that this portion is actually equivalent to the upper part of the lower Spathian Tirolites-Amphistephanus Zone (upper Olenekian). The fauna of the Hiraiso Formation, which includes the large bivalve Eumorphotis iwanowi and crinoid ossicles, very much resembles the Tirolites ussuriensis beds in South Primorye, Russian Far East, suggesting a wide distribution of this shallow marine fauna in the low to middle northern latitudes on the western side of the Panthalassa during the early Spathian. Because pre-Spathian deposits contain only small-sized species of Eumorphotis and very rare crinoid ossicles, the appearance of large species of Eumorphotis and the flourishing of crinoids in the shallow marine environment during the early Spathian suggest that the Smithian-Spathian boundary marks a major change in the marine ecosystem.