Presentation Information
[O12-P33]New Insights into Sphenoceramus naumanni Based on Bedding-Plane Paleoecosystem Analysis
*Sae Tanaka1 (1. Kumamoto Prefectural Amakusa High School)
Keywords:
Sphenoceramus naumanni,Santonian,Paleoecosystem,Deep Sea,Colony,Bedding-Plane
1. Background and Objectives
On Goshoura Island in Amakusa City, Kumamoto Prefecture, there is an exposure known as the "Sphenoceramus Wall" (Fig. 1), where a 35 m x 18 m bedding plane from the Santonian Stage of the Cretaceous is revealed. The Wall yields abundant fossils of Sphenoceramus sp. (including articulated shells) as well as numerous trace fossils, making it an outcrop that preserves the paleoecosystem of the time.
This study focuses on Sphenoceramus naumanni (Fig. 2) and aims to reconstruct the paleoecosystem by integrating biology, paleontology, statistics, and paleoenvironmental analysis. In part, new analytical methods were developed to support ecosystem reconstruction.
2. Data and Methods
2-1 Paleoenvironmental Reconstruction
Trace fossils discovered during fieldwork were identified. Cobbles from the Wall were also collected, chemically processed, and examined for microfossils.
2-2 Ecological Reconstruction
In the field, the distribution of S. naumanni was surveyed using the quadrat method, and shell length, shell height, umbo orientation, and rib morphology were recorded. A new method for identifying colonies on the bedding plane was also developed. Specifically, fossil positions were plotted as coordinate data, and the optimal number of groups (k) was determined using the elbow method and silhouette scores. Clustering was then performed using the k-means++ algorithm. Each group was tested for directional bias using a chi-square test, and shell orientations were visualized with radar charts to determine whether each group represented a colony. Using this method, colonies A and B in Fig. 3 were identified.
Ribs (Fig. 4) were examined following the growth-stage categories (alpha, beta, gamma, delta) defined by Matsuda & Ubukata (1999). Rib density was also calculated and compared with their results.
3. Results
3-1 Paleoenvironmental Reconstruction
Trace fossils identified include Cosmorhaphe, Spirorhaphe, and Lorenzinia (Fig. 5). Sponge spicules interpreted as belonging to hexactinellid sponges were found in the cobbles (Fig. 6).
3-2 Ecological Reconstruction
Seven colonies were identified in both areas A and B. Colony identification was also tested on modern deep-sea bivalves (Fig. 7), confirming the method's reliability. Comparison of shell length and shell height between A and B showed that individuals in A were larger.
Regarding rib growth stages, the beta stage was overwhelmingly dominant (Fig. 8). Compared with specimens described by Matsuda & Ubukata (1999), individuals from the Wall reached a greater shell height before the beta stage terminated and exhibited lower rib density (Fig. 9). Trace fossils were distributed densely near S. naumanni (Fig. 10).
4. Discussion
4-1 Paleoenvironmental Reconstruction
Based on Magdalena Lukowiak (2020) and Obata (1999), the environment at the time is inferred to have been a deep-sea setting at a depth of 1,000-6,000 m with low-oxygen conditions.
4-2 Ecological Reconstruction
Hosomi (1984) and Tagami (2002) indicate that juvenile bivalves possess high dispersal ability, and that larval inflow and outflow influence population persistence. Using this, the following scenario for colony development and decline is proposed:
Numerous larvae drifted outward from the original colony at A. These larvae settled in area B, forming a new colony. As larvae dispersed outward, the colony at A became depleted and declined.
Matsuda & Ubukata (1999) reported that their specimens inhabited shallow-marine environments. In contrast, the specimens in this study lived in deep-sea settings and show a high frequency of the beta stage. The beta ribs are interpreted as advantageous for life in deep water.
S. naumanni is thought to have originated in shallow seas before expanding into deeper environments. Based on this study, it is proposed that the species first adapted to deep water by developing coarser ribs (an expanded beta-rib stage), and later evolved more pronounced radial ribs, giving rise to S. nagaoi.
Because trace fossils were distributed close to S. naumanni, it is inferred that the organisms producing the traces inhabited areas where organic matter accumulated due to turbulence generated by the beta ribs of S. naumanni.
5. Future Perspectives
Future work will include expanding the survey area, conducting turbulence experiments using beta-rib models, and determining water depth using microfossils, thereby advancing the reconstruction of the paleoecosystem.
On Goshoura Island in Amakusa City, Kumamoto Prefecture, there is an exposure known as the "Sphenoceramus Wall" (Fig. 1), where a 35 m x 18 m bedding plane from the Santonian Stage of the Cretaceous is revealed. The Wall yields abundant fossils of Sphenoceramus sp. (including articulated shells) as well as numerous trace fossils, making it an outcrop that preserves the paleoecosystem of the time.
This study focuses on Sphenoceramus naumanni (Fig. 2) and aims to reconstruct the paleoecosystem by integrating biology, paleontology, statistics, and paleoenvironmental analysis. In part, new analytical methods were developed to support ecosystem reconstruction.
2. Data and Methods
2-1 Paleoenvironmental Reconstruction
Trace fossils discovered during fieldwork were identified. Cobbles from the Wall were also collected, chemically processed, and examined for microfossils.
2-2 Ecological Reconstruction
In the field, the distribution of S. naumanni was surveyed using the quadrat method, and shell length, shell height, umbo orientation, and rib morphology were recorded. A new method for identifying colonies on the bedding plane was also developed. Specifically, fossil positions were plotted as coordinate data, and the optimal number of groups (k) was determined using the elbow method and silhouette scores. Clustering was then performed using the k-means++ algorithm. Each group was tested for directional bias using a chi-square test, and shell orientations were visualized with radar charts to determine whether each group represented a colony. Using this method, colonies A and B in Fig. 3 were identified.
Ribs (Fig. 4) were examined following the growth-stage categories (alpha, beta, gamma, delta) defined by Matsuda & Ubukata (1999). Rib density was also calculated and compared with their results.
3. Results
3-1 Paleoenvironmental Reconstruction
Trace fossils identified include Cosmorhaphe, Spirorhaphe, and Lorenzinia (Fig. 5). Sponge spicules interpreted as belonging to hexactinellid sponges were found in the cobbles (Fig. 6).
3-2 Ecological Reconstruction
Seven colonies were identified in both areas A and B. Colony identification was also tested on modern deep-sea bivalves (Fig. 7), confirming the method's reliability. Comparison of shell length and shell height between A and B showed that individuals in A were larger.
Regarding rib growth stages, the beta stage was overwhelmingly dominant (Fig. 8). Compared with specimens described by Matsuda & Ubukata (1999), individuals from the Wall reached a greater shell height before the beta stage terminated and exhibited lower rib density (Fig. 9). Trace fossils were distributed densely near S. naumanni (Fig. 10).
4. Discussion
4-1 Paleoenvironmental Reconstruction
Based on Magdalena Lukowiak (2020) and Obata (1999), the environment at the time is inferred to have been a deep-sea setting at a depth of 1,000-6,000 m with low-oxygen conditions.
4-2 Ecological Reconstruction
Hosomi (1984) and Tagami (2002) indicate that juvenile bivalves possess high dispersal ability, and that larval inflow and outflow influence population persistence. Using this, the following scenario for colony development and decline is proposed:
Numerous larvae drifted outward from the original colony at A. These larvae settled in area B, forming a new colony. As larvae dispersed outward, the colony at A became depleted and declined.
Matsuda & Ubukata (1999) reported that their specimens inhabited shallow-marine environments. In contrast, the specimens in this study lived in deep-sea settings and show a high frequency of the beta stage. The beta ribs are interpreted as advantageous for life in deep water.
S. naumanni is thought to have originated in shallow seas before expanding into deeper environments. Based on this study, it is proposed that the species first adapted to deep water by developing coarser ribs (an expanded beta-rib stage), and later evolved more pronounced radial ribs, giving rise to S. nagaoi.
Because trace fossils were distributed close to S. naumanni, it is inferred that the organisms producing the traces inhabited areas where organic matter accumulated due to turbulence generated by the beta ribs of S. naumanni.
5. Future Perspectives
Future work will include expanding the survey area, conducting turbulence experiments using beta-rib models, and determining water depth using microfossils, thereby advancing the reconstruction of the paleoecosystem.