5:15 PM - 6:45 PM
[BPT03-P04] Taphonomical process of Akishima Kujira (Eschrichtius akishimaensis Kimura et al., 2018) inferred from preservation of the fossil rib: a preliminary report.
Keywords:whale fossil, whale-fall community, Kazusa Group, Pleistocene, Tamagawa River, Tokyo
The "Akishima Kujira" (Eschrichtius akishimaensis Kimura et al., 2018) is an extinct species of the family of Eschrichtidae that was discoverd on August 20, 1961 from Lower Pleistocene (Kazusa Group Komiya formation) distributed in the Tama River bed in Akishima City, Tokyo, Japan. The holotype of E. akishimaensis (storage number: GMNH-PV3210) has nearly 90 % of skeleton fossil, and is extremely important for understanding the evolutionary history of the genus Eschrichtius, of which only survived as a single modern genus and species.
The holotype of E. akishimaensis has almost entire skeleton fossil preserved, which indicates that a "whale fall" (e. g., Smith, 1989; Smith, 1992; Fujiwara, 2002; Fujiwara and Kawato, 2010; Smith et al., 2015), it is an important specimen for discussing changes in biodiversity from the establishment of benthic communities. Although, Takakuwa et al. (2021) described trace fossil of Blanoidea and considered that fossil shark teeth associated with the skeleton of E. akishimaensis have a predetor-prey relationship between shark and whale, they did not provide microbial activities on the skeleton after death.
In this study, we prepare thin sections (storage number: LMA-FT-1a) from same specimen (storage number: LMA-F7-3) of the holotype and descript fossil preservation and fossil micro-boring. Specimen of the LMA-F7-3 corresponds to the area near the median of the left ninth fossil rib (LR-9) of the holotype specimen, and the cross section is connected to the distal part of rib (LR-9-2). This study obeserved thin section using an image obtained by scaning the entire thin section using a polarizing plate, a stereomicroscope, and polarzing microscope.
As a result of observation, although this thin section has a micro-defective part on the surface of the fossil rib, abundant tubular micro-borings occur external part of compact bone. These micro-borings show diameter size of approximately 4-5 micrometer and have a straight of slightly curved tunnels that they are formed by not bifurcating. Micro-borings were mainly observed on the posterior and medial side of fossil rib, and carbonate crystallization was also observed in some areas where the perfomation marks were concentrated. The morphology and occurrence of these destruction pattern of compact bone is called "Wedl tunneling" (e. g., Davis, 1997; Turner-walker, 2008; Jans, 2008; Danise et al., 2012), and ascribed to the aciton of cyanobacteria.
These results are considered that taphonomic process of the individual for below. 1. After the Akishima Kujira sink to shallow sea area as lower shoreface and begin to be buried in sediment. 2. A part of bones were directly exposed to seawater, suggesting that bacterial mats were colonized on the bone surfaces. 3. Macro-sized behthos such as curstaceans and polychaetes had not established skeleton of the "Akishima Kujira" , before the individual is completely buried in sediment between about six month to two years.
In the future, by observing the thin sections using SEM-EDS, Raman spectroscopy, etc, we will survey the classitification of microbiota colonized by bone fossils by looking at the morphology of micro-bioerosions three dimensionally through observations such as microtomography.
Institutional abbreviations.-GMNH, Gumma Museum of National history, Gumma, Japan; LMA, Local Museum of Akishima city, Tokyo, Japan.
The holotype of E. akishimaensis has almost entire skeleton fossil preserved, which indicates that a "whale fall" (e. g., Smith, 1989; Smith, 1992; Fujiwara, 2002; Fujiwara and Kawato, 2010; Smith et al., 2015), it is an important specimen for discussing changes in biodiversity from the establishment of benthic communities. Although, Takakuwa et al. (2021) described trace fossil of Blanoidea and considered that fossil shark teeth associated with the skeleton of E. akishimaensis have a predetor-prey relationship between shark and whale, they did not provide microbial activities on the skeleton after death.
In this study, we prepare thin sections (storage number: LMA-FT-1a) from same specimen (storage number: LMA-F7-3) of the holotype and descript fossil preservation and fossil micro-boring. Specimen of the LMA-F7-3 corresponds to the area near the median of the left ninth fossil rib (LR-9) of the holotype specimen, and the cross section is connected to the distal part of rib (LR-9-2). This study obeserved thin section using an image obtained by scaning the entire thin section using a polarizing plate, a stereomicroscope, and polarzing microscope.
As a result of observation, although this thin section has a micro-defective part on the surface of the fossil rib, abundant tubular micro-borings occur external part of compact bone. These micro-borings show diameter size of approximately 4-5 micrometer and have a straight of slightly curved tunnels that they are formed by not bifurcating. Micro-borings were mainly observed on the posterior and medial side of fossil rib, and carbonate crystallization was also observed in some areas where the perfomation marks were concentrated. The morphology and occurrence of these destruction pattern of compact bone is called "Wedl tunneling" (e. g., Davis, 1997; Turner-walker, 2008; Jans, 2008; Danise et al., 2012), and ascribed to the aciton of cyanobacteria.
These results are considered that taphonomic process of the individual for below. 1. After the Akishima Kujira sink to shallow sea area as lower shoreface and begin to be buried in sediment. 2. A part of bones were directly exposed to seawater, suggesting that bacterial mats were colonized on the bone surfaces. 3. Macro-sized behthos such as curstaceans and polychaetes had not established skeleton of the "Akishima Kujira" , before the individual is completely buried in sediment between about six month to two years.
In the future, by observing the thin sections using SEM-EDS, Raman spectroscopy, etc, we will survey the classitification of microbiota colonized by bone fossils by looking at the morphology of micro-bioerosions three dimensionally through observations such as microtomography.
Institutional abbreviations.-GMNH, Gumma Museum of National history, Gumma, Japan; LMA, Local Museum of Akishima city, Tokyo, Japan.