1:45 PM - 3:15 PM
[O08-P15] The Ecology of Sphenoceramus Based on the Sedimentary Environment 85 Million Years Ago
Keywords:Sphenoseramus, deep-sea, low-oxygen, trace fossils, quadrats, Chi-square test
Background
On Maki Island in Amakusa City, fossils of the 85-million-year-old Himenura group can be found. There, there is a place where the seafloor at that time was raised to the surface due to crustal movement. Because of the many fossils of Sphenoceramus found there, it was named “The Wall of Sphenoceramus”. (pic 1)However there was never research about what kind of fossils there are, their distribution and so on. Therefore, we started our research to speculate on the ecology of Sphenoceramus and the environment at that time.
Research Purposes
1. Determine whether Sphenoceramus is native or non-native.
2. Case A: If Native: Speculate the environment at that time and the ecology of Sphenoceramus.
Case B: If Non-native: Speculate the environment at that time.
Research Methods
During the field survey, the wall is divided into sections with 1.5m x 1,5m quadrats(pic 1). Name the first level from the bottom as A, the second level as B, and again from left to right: 1, 2, and so on.
・On Research Purpose 1
The following colony identification method we have devised is used to confirm the presence or absence of colonies. If a colony is likely to be present, it is likely to be .
1First, we divide into groups k-means clustering(fig 1).
2Then, for groups containing 9 shells or more, we test the hypothesis: “these shells have no bias”, using the Chi-square test(fig 2).
In the case of group 2A, the probability of bias in shell direction was 10%. Thus, this group is thought to have a high possibility of accumulation, and not be a colony.(Consideration)
・On Research Purpose2
In both cases A and B, inferences are made based on the distribution of trace fossils, the fossils of Sphenoceramus, and rocks.
Result and Considerations
・On Research purpose 1
Since we found what we believe to be a 80% group, we believe that Sphenoceramus is likely to be native.
Therefore, we’ll now discuss research objective 2A, which is to speculate on the ecology of Sphenoceramus and the environment at that time. These 3 trace fossils were found at the wall of Sphenoceramus.
Pic 3 is considered to be Paschcinia because of its similarity in size and shape to Paschcinia from “the Japanese fossils collection 23”. From Obata’s research(fig 3), the environment in which Paschcinia are found is low-oxygen, and the wall of Sphenoceramus is considered to have been in a low-oxygen environment at that time.
The Paschcinia found on the wall is similar in size and shape to Cosmorhaphe, and from Noda’s research, Cosmorhaphe is a deep-sea trace fossil, the wall of Sphenoceramus is considered to have been in the deep sea at that time.
Regarding Pictures3.4.5, Cosmorhaphe, Spirorhaphe, and Lorezinia are classified as Zoophycos biofacies and Nereites biofacies based on Obata’s research, which is from the bathypelagic zones and abyssopelagic zone (Depth: 200 to 6000 m).
Based on the above, 85 million years ago (the first half of the Late Cretaceous) is considered to have been a low-oxygen environment.
From Tashiro’s research, it was reported that some groups of Sphenoceramus, for example, expanded their habitats to low-oxygen environments during the latter half of the Late Cretaceous. However, the wall of Sphenoceramus, which is thought to have been in a low-oxygen environment, is 85 million years old ( the first half of the Late Cretaceous), suggesting that it may have already adapted to a low-oxygen environment as early as 85 million years ago.
References
・Tashiro Masayuki et.al. 1992. Cretaceous Bivalves Occurrence and paleoecology of Sphenoceramus schmidti (Michael ):Japan
・Tsutsumi Yukiyasu et.al. 2018. Chalk Sandstones and Clastic Zircon U-Pb Ages in the Amakusa and Mifune Areas, Central Kyushu:Japan
・Tashiro Masayuki 1994 Colonies of inoceramus (cataceramus) bulticus Boehm and their conservation processes:Japan
・Kobatake Ikuo et.al. 1975. Age of the Cretaceous Choshi Group:Japan
・Obata et.al. 1999. Trace fossils discovered from the Jurassic and Cretaceous periods in the northeastern Kanto Mountains.:Japan
・Suzuki Akihiko et.al. 2014. Launched Shellfish of Yoron Island, Amami Islands:Japan
・Tashiro Masayuki. 1998. Deep-sea and paleoenvironmental evolution of bivalves during the Cretaceous:Japan
・Tanaka Keisaku & Katto Jiro. 1975. Japanese fossil collection 23: Cretaceous-Paleogene trace fossils:Japan
On Maki Island in Amakusa City, fossils of the 85-million-year-old Himenura group can be found. There, there is a place where the seafloor at that time was raised to the surface due to crustal movement. Because of the many fossils of Sphenoceramus found there, it was named “The Wall of Sphenoceramus”. (pic 1)However there was never research about what kind of fossils there are, their distribution and so on. Therefore, we started our research to speculate on the ecology of Sphenoceramus and the environment at that time.
Research Purposes
1. Determine whether Sphenoceramus is native or non-native.
2. Case A: If Native: Speculate the environment at that time and the ecology of Sphenoceramus.
Case B: If Non-native: Speculate the environment at that time.
Research Methods
During the field survey, the wall is divided into sections with 1.5m x 1,5m quadrats(pic 1). Name the first level from the bottom as A, the second level as B, and again from left to right: 1, 2, and so on.
・On Research Purpose 1
The following colony identification method we have devised is used to confirm the presence or absence of colonies. If a colony is likely to be present, it is likely to be .
1First, we divide into groups k-means clustering(fig 1).
2Then, for groups containing 9 shells or more, we test the hypothesis: “these shells have no bias”, using the Chi-square test(fig 2).
In the case of group 2A, the probability of bias in shell direction was 10%. Thus, this group is thought to have a high possibility of accumulation, and not be a colony.(Consideration)
・On Research Purpose2
In both cases A and B, inferences are made based on the distribution of trace fossils, the fossils of Sphenoceramus, and rocks.
Result and Considerations
・On Research purpose 1
Since we found what we believe to be a 80% group, we believe that Sphenoceramus is likely to be native.
Therefore, we’ll now discuss research objective 2A, which is to speculate on the ecology of Sphenoceramus and the environment at that time. These 3 trace fossils were found at the wall of Sphenoceramus.
Pic 3 is considered to be Paschcinia because of its similarity in size and shape to Paschcinia from “the Japanese fossils collection 23”. From Obata’s research(fig 3), the environment in which Paschcinia are found is low-oxygen, and the wall of Sphenoceramus is considered to have been in a low-oxygen environment at that time.
The Paschcinia found on the wall is similar in size and shape to Cosmorhaphe, and from Noda’s research, Cosmorhaphe is a deep-sea trace fossil, the wall of Sphenoceramus is considered to have been in the deep sea at that time.
Regarding Pictures3.4.5, Cosmorhaphe, Spirorhaphe, and Lorezinia are classified as Zoophycos biofacies and Nereites biofacies based on Obata’s research, which is from the bathypelagic zones and abyssopelagic zone (Depth: 200 to 6000 m).
Based on the above, 85 million years ago (the first half of the Late Cretaceous) is considered to have been a low-oxygen environment.
From Tashiro’s research, it was reported that some groups of Sphenoceramus, for example, expanded their habitats to low-oxygen environments during the latter half of the Late Cretaceous. However, the wall of Sphenoceramus, which is thought to have been in a low-oxygen environment, is 85 million years old ( the first half of the Late Cretaceous), suggesting that it may have already adapted to a low-oxygen environment as early as 85 million years ago.
References
・Tashiro Masayuki et.al. 1992. Cretaceous Bivalves Occurrence and paleoecology of Sphenoceramus schmidti (Michael ):Japan
・Tsutsumi Yukiyasu et.al. 2018. Chalk Sandstones and Clastic Zircon U-Pb Ages in the Amakusa and Mifune Areas, Central Kyushu:Japan
・Tashiro Masayuki 1994 Colonies of inoceramus (cataceramus) bulticus Boehm and their conservation processes:Japan
・Kobatake Ikuo et.al. 1975. Age of the Cretaceous Choshi Group:Japan
・Obata et.al. 1999. Trace fossils discovered from the Jurassic and Cretaceous periods in the northeastern Kanto Mountains.:Japan
・Suzuki Akihiko et.al. 2014. Launched Shellfish of Yoron Island, Amami Islands:Japan
・Tashiro Masayuki. 1998. Deep-sea and paleoenvironmental evolution of bivalves during the Cretaceous:Japan
・Tanaka Keisaku & Katto Jiro. 1975. Japanese fossil collection 23: Cretaceous-Paleogene trace fossils:Japan