Welcome photo by SHINMURA Tatsuya・Ashoro Museum of Paleontology. The Image of the pliosaur attacking a polycotylid from Haboro, Hokkaido, Japan ©SHINMURA Tatsuya・Ashoro Museum of Paleontology
What's new
New Museum Exhibisions started on 15 September 2023
Site opening on 7 September 2023
On the Edge
We are pleased to present some new content, “Prehistoric Marine Reptiles”, in collaboration with Hokkaido University Museum. Dr. Echizenya, a researcher in the museum, will be the first to introduce content. Along with photos of replica fossils of extinct marine reptiles from the collections of the museum and the Faculty of Science, Hokkaido University, we can showcase the primitive marine reptiles that lived in the ancient sea. We would like to thank Mr. Tatsuya Shinmura, an assistant curator at the Ashoro Museum of Paleontology, for his excellent illustrations and computer graphics that help bring these ancient reptiles to life. We would like to express great thanks to him for his help.
The Prehistoric Marine Reptile content also introduces a surprising topic that makes us reconsider the ecosystem of the ancient sea. The Marine Reptile fossils have provided clues to solving the mysteries of microbial ecosystems, mainly chemosynthetic bacteria. The fact that the study of marine reptile fossils has provided insights into the great chain of life in the sea is a tribute to our valuable research.
The fossils of the marine reptiles can be seen in Hokkaido University Museum on our Sapporo campus. Please visit the museum and experience firsthand the wonder of ancient marine life.
FoM Editorial
7 September 2023 posted
The Earliest Ichthyosaur Utatsusaurus
The sea is housed many animals, some of which have returned to the sea from ancestors that lived on land. Current animals include cetaceans (whales and dolphins), sea cows (dugongs and manatees), pinnipeds (sea lions, seals, and walruses), sea otters, seabirds including penguins, sea turtles, and sea snakes. Many groups are extinct. Among mammals, Desmostylia (Desmostylus, etc.) were mammals with a hippopotamus-like appearance that fed on sea grass with teeth that looked like bundles of columns about 28-11 million years ago. Among reptiles, mosasaurs with fins on their limbs and tails emerged from a group of lizards similar to snakes dating back to about 98 million years ago during the Late Cretaceous and which became extinct at the same time as the dinosaurs. From the Jurassic to the Early Cretaceous, there were also Thalattosuchia (sea crocodiles) with fins on their limbs and tails.
Going back even further, to the beginning of the Triassic period before the appearance of the dinosaurs, various groups of animals made their way into the sea. These included nothosaurs, the ancestral group of the plesiosaurs that later flourished during the Jurassic and Cretaceous periods, and their close relatives, the Placodontia, which had turtle-like shells and ate shellfish and crustaceans with their plate-like teeth. Then there are the ichthyosaurs, in these marine animals ancestral to land animals, which became the first rulers of the sea.
Early ichthyosaurs were long bodied like mosasaurs and sea crocodiles, but the evolved form, represented by Ophthalmosaurus, resembled modern day dolphins. However, as mammals, dolphins evolved to have horizontally lying tail fins that oscillate up and down because their ancestors had a spine that oscillated up and down when they moved on land. In contrast, reptiles, the ancestors of ichthyosaurs, had integrated hips and tails and wagged their tails left and right when they moved, so ichthyosaurs evolved to have vertical tail fins that vibrate left and right, like fish.
Early ichthyosaurs and later ichthyosaurs are thought to differ not only in body shape but also in swimming style, often described as "eel-like" and "tuna-like" in extremely broad categories. In fact, early ichthyosaurs (and mosasaurs and sea crocodiles) were probably propelled by pushing water backward, using both whole body and tail fin movements like salmon (Hirata et al., 2002), rather than like eels. Evolved ichthyosaurs, similar to tuna and dolphins, are thought to have been propelled by using their tail fins to generate backward lift using the same principle as a screw.
The earliest ichthyosaur (approximately 250 million years ago) was Utatsusaurus, first discovered in Utatsu, Minamisanriku-cho, Miyagi Prefecture, Japan (Shikama et al., 1977). The specimen of Hokkaido University was found in Ogatsu, Ishinomaki (Minoura et al., 1993), and many other ichthyosaur fossils have been discovered in Miyagi Prefecture. Unlike the later form, which had smaller hindlimbs, the forelimbs and hindlimbs of Utatsuaurus were about the same size, and the bones that made up its limbs were relatively slender, unlike the later form, which had shorter bones making up its fins (Motani, et al., 1998), showing other traits of its land-walking ancestors, this is a valuable resource in the study of the advance of land animals into the sea.
The replica of Utatsusaurus from Ogatsu, Ishinomaki, Miyagi Prefecture, Japan on the lobby of grand floor, second building, Faculty of Science, Hokkaido University
ECHIZENYA Hiroki・The Hokkaido University Museum・Researcher
References
平田宏一・春海一佳・瀧本忠教・田村兼吉・牧野雅彦・児玉良明・冨田宏（2002），魚ロボットに関する基礎的研究．海上技術安全研究所報告，2 (3): p. 281-307．
箕浦名知男・小野慶一・鎌田耕太郎・加藤俊夫・高橋真千子・加藤誠・川上源太郎（1993），宮城県雄勝町下部三畳系産魚竜の発掘．地学研究，42: p. 215-232．
Motani, R., Minoura, N. and Ando, T. (1998), Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature, 393: p. 255-257. doi:10.1038/30473
Shikama, T., Kamei, T. and Murata, M. (1977), Early Triassic Ichthyosaurus, Utatsusaurus hataii Gen. et Sp. Nov., from the Kitakami Massif, Northeast Japan. Science Reports of the Tohoku University Second Series (Geology), 1977. 48(1–2): p. 77-97.
7 September 2023 posted
Saurian falls (Cretaceous plesiosaurid carcasses)
In the deepest parts of the sea, where light does not penetrate, there are creatures that live off whale carcasses. The whale carcass is eaten by deep-sea fish such as hagfishes, rattail fish, and large sleeper sharks, and the bones are further reduced by crustaceans, polychaetes, and mollusks such as snails. Eventually, lipids in the bones are broken down by microorganisms to produce methane and hydrogen sulfide, which are used as energy sources by a community of chemosynthetic bacteria as producers (Camps, 2016).
This community of organisms that appear during the decomposition process of whale remains is known as the fauna of whale falls. In addition to the high-density bacterial mats, there are creatures such as mussels and Calyptogena soyoae (deep-sea cold-seep clam) that symbiotically produce chemosynthetic bacteria on their body surfaces and inside their bodies, as well as Osedax japonicas (a species of tube worm), also known as zombie worms. Recent research seems to be revealing a succession of microorganisms that do not have symbiotic bacteria but live off the bacterial mats, snails and other organisms, and various other organisms that move up the food chain from these microorganisms.
In addition, other known communities of organisms that are producers of chemosynthetic bacteria are the ecosystems found around methane springs and hydrothermal vents. Although these ecosystems are known throughout the world, the mechanisms by which these organisms expanded their distribution areas were not well understood. However, the discovery of the whale falls in 1987 started a debate on whether whale falls were involved in the range expansion of these ecosystems, and which of these two types of communities was established first.
The problem with these discussions was that the ancestors of whales entered the ocean about 50 million years ago and did not become larger until much later. The ecosystems found around methane springs and hydrothermal vents were present in earlier periods, and the absence of whales in those periods proved a bottleneck in the discussion.
In 2008, a research group from the University of Tokyo and Hokkaido University reported the discovery of fossil snails of members of the family Provannidae and vetigastropod, about the size of a grain of rice, around a Late Cretaceous (approximately 90 million years ago) plesiosaur fossil discovered in Haboro Town, Hokkaido, Japan. These mollusks are similar to those found in methane spring sediments from the Cretaceous period and are thought to have fed on the bacterial mats that formed on the bones of the plesiosaur. The surface of the fossil is ragged, suggesting that a community of organisms that produced chemosynthetic bacteria existed in the environment associated with the decomposition of the remains of theplesiosaur. This was the world's first discovery of a "saurian falls" (Kaim et al., 2008).
Although plesiosaurs became extinct along with the dinosaurs at the end of the Cretaceous Period, there is one marine reptile that has survived to the present. That is the sea turtles; in 2017, a chemosynthetic assemblage of provannidsand thyasirids was reported from primitive Cretaceous leatherback turtle fossils (Jenkins et al., 2017).
Marine reptiles have been present since the beginning of the Triassic period (about 250 million years ago), and it is now believed that the remains of these marine reptiles have been closely involved in the evolution of chemosynthetic communities.
Back Photo: The fossil snail of member of the family Provannidae from Haboro, Hokkaido, found during the preparation of the plesiosaur fossil (please watch before and after removing the fossil snail block)
ECHIZENYA Hiroki・The Hokkaido University Museum・Researcher
References
Camps, M. A. (2016), What lies beyond the death of a whale? All you need is Biology.
Jenkins, R. G., Kaim, A., Sato, K., Moriya, K., Hikida, Y. and Hirayama, R. (2017), Discovery of chemosynthesis-based association on the Cretaceous basal leatherback sea turtle from Japan. Acta Palaeontologica Polonica, 62(4): p. 683-690.
Kaim, A., Kobayashi, Y., Echizenya, H., Jenkins, R. G., and Tanabe, K. (2008), Chemosynthesis-based associations on Cretaceous plesiosaurid carcasses. Acta Palaeontologica Polonica, 53(1): p. 97-104.
Back Photo: Plesiosaur fossil (brown) after removing the fossil snail block.
7 September 2023 posted
The plesiosaurs were marine reptiles that appeared in the Late Triassic and became extinct along with the dinosaurs at the end of the Cretaceous. Contrary to the Japanese name of plesiosaur "Kubi-naga-ryu (long-necked dragons)", there were short-necked as well as long-necked groups, but both groups had limbs that were modified into fins.
Unlike the limb fins of ichthyosaurs and mosasaurs, which were probably used primarily as rudders, the fins of the plesiosaurs are lightly curved and tapered, resembling the front fins of a sea turtle. Like sea turtles, sea lions, and penguins, it is thought that the plesiosaurs propelled itself by opening its fins from side to side and flapping them to generate backward lift, using the same principle as a screw. What makes them different from extant animals is that the plesiosaurs also flapped their posterior fins in the same manner as their anterior fins. It is doubtful that they simply doubled their efficiency, but it is thought that they had excellent swimming ability.
Three lineages of plesiosaurs lived in the Cretaceous: the long-necked Elasmosauridae, the short-necked and relatively small Polycotylidae, and the large but short-necked Pliosauridae. If the polycotylids were compared to sea lions, the pliosaurs were ferocious creatures that could be compared to killer whales.
The family Pliosauridae is a group of large-bodied creatures characterized by their huge skulls and teeth and short neck, with some known to have reached over 10 meters in length during the Late Jurassic and Early Cretaceous periods. Unlike the other two groups, which survived until the end of the Cretaceous Period, the Pliosauridae are thought to have become extinct by the middle of the Late Cretaceous Period at the latest. Despite probably being the apex marine predator of the Jurassic and Cretaceous, the extinction of the Pliosauridae has not been well studied.
Late Cretaceous period pliosaurid fossils, discovered in four locations in Hokkaido (Haboro Town, Nakagawa Town, Obira Town, and Mikasa City) and housed in a museum for many years, were described and reported in 2023 (Sato, et al., 2023). Fossils discovered in Haboro in 2000 and collected by Hokkaido University included a skull, teeth, vertebrae, ribs, and girdles. The specimen represents a large individual whose skull size approaches that of a species from North America. The maximum length of the entire skull, including the mandible, is estimated to be around 1.7 meters, and the total length is estimated to be around 7-8 meters.
The Cenomanian-Turonian boundary of the first half of the Late Cretaceous (ca. 94 million years ago) has long been recognized as a period of major changes in biota, with major changes in marine environments associated with marine anoxic events in many parts of the world. Of the major groups of Cretaceous marine reptiles, the ichthyosaurs became extinct during the Cenomanian, and the diversification of the Mosasauroidea occurred between the Cenomanian and the following Turonian, as if they were replacing the ichthyosaurs.
It has been thought that the extinction of the Pliosauridae may have occurred around this time, but the age of this fossil is approximately 2-3 million years younger than the Cenomanian-Turonian boundary, indicating that all three Late Cretaceous lineages of the plesiosaurs survived the environmental changes at the end of the Cenomanian period (approximately 94 million years ago).
Back Photo: Preserved parts of the pliosaur fossil from Haboro, Hokkaido. ©SHINMURA Tatsuya・Ashoro Museum of Paleontology
ECHIZENYA Hiroki・The Hokkaido University Museum・Researcher
References
7 September 2023 posted
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