Masanori OKANISHI (Project Assistant Professor, Misaki Marine Biological Station)

Shota Mitsui (3rd year doctoral student, Tokyo University of Marine Science and Technology)

Ben Thuy (Curator, Natural History Museum, Luxembourg)

Key points of the presentation

• Fossils of the lateral arm plate of spider starfish ^{(Note 2)}, the first record in Japan, were discovered from the Middle Pleistocene ^{(Note 1)} stratum (Miyata Formation) distributed in the Miura Peninsula, Kanagawa Prefecture, Japan.
• Morphological studies of the fossilized lateral arm plate of a spider starfish have revealed that it is a fossil of a red spider starfish. This species is still widely found on sandy and muddy bottoms in the temperate zone south of Sagami Bay. Therefore, the occurrence of the fossil of this species suggests that the Miura Peninsula was such a temperate sea area at that time.
• This study pioneered the identification of the species ^{(Note 3)} and its usefulness for paleoenvironmental estimation of the lateral arm plate fossils of spider starfish, which have not received much attention in Japan until now. In the future, studies of lateral arm plate fossils are expected to contribute to the evolution of the diversity of spider starfish and to the elucidation of the formation history of the Japanese Islands.

Summary of Presentation

A research group led by Project Assistant Professor Masanori Okanishi at the Graduate School of Science, The University of Tokyo, has discovered fossilized lateral arm plates of the echinoderm ^(*4), a class of spider starfish, from the Miyata Formation (ca. 300,000- 450,000 years ago) on the Miura Peninsula, Kanagawa Prefecture. The body of arachnids is composed of several millimeters of calcium carbonate bone fragments, which are separated after death. The lateral arm plates are pieces of bone that cover the sides of the principle five arms of the spider starfish. Spider starfish species live in high densities in a variety of seafloor environments, depending on the species, and their fossils are useful as facies fossils ^{(Note 5)}. In Japan, studies have mainly focused on fossils of preserved whole bodies, and isolated bone fragments have not received much attention.

In this study, we focused on the lateral arm plate fossils of spider starfish for the first time in Japan. Detailed morphological comparisons revealed that the discovered fossil is the lateral arm plate of the present species of spider starfish.

This is the first time in the world that a fossil of a red spider starfish has been discovered. Based on the present habitat environment of this species, it is inferred that the Miura Peninsula at the time of deposition of the Miyata Formation was a continental shelf or shelf slope of a warm sea. Further studies on the fossil skeletons of spider starfish are expected to help elucidate the evolution of the diversity of spider starfish in Japan and the formation history of the Japanese archipelago.

Contents of Presentation

Background of Research
Spider starfish are marine invertebrates that live in a variety of environments, such as under rocks, in mud and other sediments, and on top of other animals, using their long, slender arms dexterously. The skeleton of spider starfish is composed of calcium carbonate bone fragments of a few millimeters in length. The morphology of these fragments varies from species to species, making them an important taxonomic trait. They dissociate postmortem and are buried in sediments, and are easily preserved as fossils, which have been used in Europe as facies fossils to infer paleoenvironmental conditions. In Japan, the study of spider starfish fossils has been conducted mainly using fossils of whole-body remains, while fossil bone fragments, which are thought to be more abundant, have hardly been known even in their occurrence.

Research Contents
A research group consisting of Assistant Professor Masanori Okanishi of the Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo, graduate student Shota Mitsui of the Graduate School of Marine Science and Technology, The University of Tokyo, and Curator Ben Thuy of the Natural History Museum of Luxembourg, has been studying the Miyata Formation (ca. 300,000 to 450,000 years ago) (Fig. 1).

Figure 1: Sampling of fossil bone fragments used in this study. One of the co-authors, Shota Mitsui, a graduate student of Tokyo University of Marine Science and Technology (Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology), is collecting sediment samples with a hammer and a hammer at an outcrop in Minamishitaura-cho, Miura City, Kanagawa Prefecture.

He took a handful of sand back to his laboratory, washed and sifted it, and continued to pick out small fossils. Detailed morphological comparisons with the lateral arm plates of extant spider starfish revealed that all of the fossils discovered were lateral arm plates of the red spider starfish Stegophiura sladeni (Figure 2).

Figure 2: Photograph of the dorsal surface of a Stegophiura sladeni obtained from Sagami Bay (A: photo by Hisanori Kozuka [Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo]) and a fossil of a minute lateral arm plate of this species obtained in this study (B).

The red spider starfish is still widely found on sandy and muddy bottoms from 40 m to 380 m depth in Sagami Bay and southward and in the Sea of Japan, but this is the first time in the world that it has been discovered as a fossil.

Fossil shells are also known from the Miyata Formation, and based on these fossils, it is presumed that this was a continental shelf paleoenvironment where both the Kuroshio and Oyashio currents flowed. Therefore, the production of fossils of this species is direct evidence that the Miura Peninsula at the time of deposition of the Miyata Formation was a continental shelf seafloor with sand and mud.

Social Significance and Future Plans
In this study, we excavated the first fossil lateral arm plates of spider starfish from Japan and identified the species, demonstrating their usefulness for paleoenvironmental estimation. More than 50 species of spider starfish fossils have been recognized from Japan so far, but this is a small number compared to the number of extant species (about 340). One of the reasons for this may be that previous studies have focused only on whole body fossils. Considering the diversity of extant and fossil species, it is highly likely that new fossil bone fragments of spider starfish will be discovered in marine strata in various regions of Japan. Using such fossil skeletons, we will be able to elucidate the regional and period composition of spider starfish assemblages, and thus the diversity of spider starfish species and their paleoenvironmental evolution along the coasts of Japan. This is an important finding that will provide new ideas about the origins of the Japanese archipelago and its marine biodiversity. In the future, we intend to conduct fieldwork not only in the Miyata Formation but also in other areas of Japan to extensively examine the spider starfish bone fragment fossils in various strata in order to clarify the diversity of spider starfish bone fragments in Japan and the past marine environment in more detail.

Journals

Journal name	Paleontological Research
Title of paper	Fossil lateral arm plates of Stegophiura sladeni (Echinodermata: Ophiuroidea: Ophiurida) from the Middle Pleistocene of Japan
Authors	Masanori Okanishi*, Shota Mitsui, Ben Thuy
DOI Number	10.2517/2020PR027
Abstract URL	https://doi.org/10.2517/2020PR027

Terminology

Note 1 Middle Pleistocene

A geologic age category, the period from 774,000 to 129,000 years ago. Known as the "Tibetan period. ↑up

Note 2: Lateral arm plate

A large number of bone fragments, ranging from a few millimeters to a few centimeters in length, that cover the sides of the arms of spider starfish. ↑up

Note 3 Identification

Determination of the taxonomic name of an organism. ↑up

Note 4 Echinoderms

A group of marine animals including sea urchins, sea cucumbers, starfish, etc. As a rule, they have star-shaped bodies. ↑up

Note 5: Facies fossil

Fossils that reveal the environment of the stratum in which they were produced at the time of deposition. ↑up