1:45 PM - 3:15 PM
[O11-P40] Identification of rhinoceros fossils from Minokamo City, Gifu Prefecture
Keywords:Rhinoceros, Miocene, Mizunami Group
Background
In 2006, a mandible and limb bones of a rhinoceros were discovered in the Nakamura Formation of the Mizunami Group in Minokamo City.
The rhinoceros species found in the Mizunami Group were previously identified as Chilotherium genus C. pugnator and C.? sp., but have been reidentified as the large species Brachypoterium? pugnator and the small species Plesiaceratherium sp. (Fukuchi & Kawai (2011)).Research was conducted with the aim of identifying which of two species the rhinoceros fossil (hereafter referred to as the Fujii specimen) found in Minokamo City, Gifu Prefecture, is.
Methods
Rhinoceros teeth have premolars P2 to P4 and molars M1 to M3. The lower jaw teeth of rhinoceros look like two inverted Ls connected together, and unlike the upper jaw teeth, there is little difference in shape between species. Therefore, based on the size difference between large and small species, the mesiodistal length and buccolingual width of the teeth were measured using calipers, compared, and identified. Where possible, we measured the data, and where impossible, we compared the data with values published in previous studies.
Results
1. Characteristics of the Fujii specimen
The right mandible measures 261 mm in anteroposterior length, 82 mm in dorsoventral height, and 40 mm in buccolingual width. Four teeth remain, P3 through M2. The crown height of P3 and P4 is low at 7-12 mm, and they are significantly worn. M1 is less worn, and M2 is chipped, but the fragments can be joined, and the enamel remains intact. The height of the occlusal surfaces from P3 through M1 is consistent, but the crown position of M2 is about 13 mm lower, and there is no root. Enamel material can be confirmed in the mandible below P3.
2. Identification by tooth size
The mesiodistal length of M1 of the Fujii specimen is 34.4 mm, which is close to the 37.5 mm of the type specimen of P. sp. and is about 1 cm shorter than the 43.9 - 51.9 mm of B.? pugnator. The buccolingual width of M1 is 22.6 mm, which is close to the 25.1 mm of the type specimen of P. sp. and is slightly narrower than the 27.8 - 33.8 mm of B.? pugnator. The mesiodistal length and buccolingual width of the other teeth are in harmony with the measurements of the specimens considered to be P. sp. and differ from the values of the specimens of B.? pugnator. When the occlusal surface size and enamel thickness of M1 and M2 of B.? pugnator were compared, it was confirmed that B.? pugnator was slightly larger and had thicker enamel.
3. X-ray CT images
X-ray CT images were taken and two teeth were confirmed inside the mandible below P3 and P4.
Discussion
The teeth identified as P3 and P4 in the Fujii specimen were significantly worn, and two teeth were identified in the mandible by X-rayCT images, so the two surface teeth are considered to be dP3 and dP4, and the two teeth in the mandible are considered to be P3 and P4. The less worn M1 had just erupted, and the unworn M2 had not yet erupted and the tooth root had not yet formed, so it is presumed that the Fujii specimen was a juvenile animal in the tooth-erupting stage. Comparing the Fujii specimen with the two specimens of P. sp. and B.? pugnator, the values of the Fujii specimen were in harmony with those of the small species P. sp. Since B.? pugnator was slightly larger in both mesiodistal length and buccolingual width, it is presumed that the Fujii specimen is a small species Plesiaceratherium sp.
Conclusion
Based on the condition of the teeth, the Fujii specimen is presumed to be a juvenile, small-sized Plesiaceratherium sp.
Acknowledge
For this study, we borrowed the rhinoceros fossils from Hideo Fujii. We received help from Yusuke Ando of the Mizunami Fossil Museum and Takayo Oumi of the Kani City History and Local History Museum in measuring the rhinoceros specimens, and from Soichiro Kawabe of Fukui Prefectural University in taking X-ray CT scans. We also received advice from Naoto Handa of the Lake Biwa Museum, Shiga Prefecture. We would like to express our gratitude.
References
Kani Town Board of Education (1977) Strata and fossils of Hiramaki Kani New Town Fossil Survey Report
Kouji Kawai (1996) On rhinoceros fossils discovered in the Nakamura Formation of the Mizunami Group Gifu Prefectural Museum Research Report No. 17
Ryo Fukuchi (2003) Terminology for the structure on the crown of rhinoceros teeth, Okayama University Earth Science Report
Fukuchi & Kawai (2011) Revision of fossil rhinoceroses from the Miocene Mizunami Group, Japan Paleontological Research, Vol. 15, no. 4
Handa Naoto (2018) Neogene-Miocene Rhinoceros Fossils in Japan (Mammalia, Perissodactyla): Current Status and Issues. Abstracts of the Annual Meeting of the Palaeontological Society of Japan.
In 2006, a mandible and limb bones of a rhinoceros were discovered in the Nakamura Formation of the Mizunami Group in Minokamo City.
The rhinoceros species found in the Mizunami Group were previously identified as Chilotherium genus C. pugnator and C.? sp., but have been reidentified as the large species Brachypoterium? pugnator and the small species Plesiaceratherium sp. (Fukuchi & Kawai (2011)).Research was conducted with the aim of identifying which of two species the rhinoceros fossil (hereafter referred to as the Fujii specimen) found in Minokamo City, Gifu Prefecture, is.
Methods
Rhinoceros teeth have premolars P2 to P4 and molars M1 to M3. The lower jaw teeth of rhinoceros look like two inverted Ls connected together, and unlike the upper jaw teeth, there is little difference in shape between species. Therefore, based on the size difference between large and small species, the mesiodistal length and buccolingual width of the teeth were measured using calipers, compared, and identified. Where possible, we measured the data, and where impossible, we compared the data with values published in previous studies.
Results
1. Characteristics of the Fujii specimen
The right mandible measures 261 mm in anteroposterior length, 82 mm in dorsoventral height, and 40 mm in buccolingual width. Four teeth remain, P3 through M2. The crown height of P3 and P4 is low at 7-12 mm, and they are significantly worn. M1 is less worn, and M2 is chipped, but the fragments can be joined, and the enamel remains intact. The height of the occlusal surfaces from P3 through M1 is consistent, but the crown position of M2 is about 13 mm lower, and there is no root. Enamel material can be confirmed in the mandible below P3.
2. Identification by tooth size
The mesiodistal length of M1 of the Fujii specimen is 34.4 mm, which is close to the 37.5 mm of the type specimen of P. sp. and is about 1 cm shorter than the 43.9 - 51.9 mm of B.? pugnator. The buccolingual width of M1 is 22.6 mm, which is close to the 25.1 mm of the type specimen of P. sp. and is slightly narrower than the 27.8 - 33.8 mm of B.? pugnator. The mesiodistal length and buccolingual width of the other teeth are in harmony with the measurements of the specimens considered to be P. sp. and differ from the values of the specimens of B.? pugnator. When the occlusal surface size and enamel thickness of M1 and M2 of B.? pugnator were compared, it was confirmed that B.? pugnator was slightly larger and had thicker enamel.
3. X-ray CT images
X-ray CT images were taken and two teeth were confirmed inside the mandible below P3 and P4.
Discussion
The teeth identified as P3 and P4 in the Fujii specimen were significantly worn, and two teeth were identified in the mandible by X-rayCT images, so the two surface teeth are considered to be dP3 and dP4, and the two teeth in the mandible are considered to be P3 and P4. The less worn M1 had just erupted, and the unworn M2 had not yet erupted and the tooth root had not yet formed, so it is presumed that the Fujii specimen was a juvenile animal in the tooth-erupting stage. Comparing the Fujii specimen with the two specimens of P. sp. and B.? pugnator, the values of the Fujii specimen were in harmony with those of the small species P. sp. Since B.? pugnator was slightly larger in both mesiodistal length and buccolingual width, it is presumed that the Fujii specimen is a small species Plesiaceratherium sp.
Conclusion
Based on the condition of the teeth, the Fujii specimen is presumed to be a juvenile, small-sized Plesiaceratherium sp.
Acknowledge
For this study, we borrowed the rhinoceros fossils from Hideo Fujii. We received help from Yusuke Ando of the Mizunami Fossil Museum and Takayo Oumi of the Kani City History and Local History Museum in measuring the rhinoceros specimens, and from Soichiro Kawabe of Fukui Prefectural University in taking X-ray CT scans. We also received advice from Naoto Handa of the Lake Biwa Museum, Shiga Prefecture. We would like to express our gratitude.
References
Kani Town Board of Education (1977) Strata and fossils of Hiramaki Kani New Town Fossil Survey Report
Kouji Kawai (1996) On rhinoceros fossils discovered in the Nakamura Formation of the Mizunami Group Gifu Prefectural Museum Research Report No. 17
Ryo Fukuchi (2003) Terminology for the structure on the crown of rhinoceros teeth, Okayama University Earth Science Report
Fukuchi & Kawai (2011) Revision of fossil rhinoceroses from the Miocene Mizunami Group, Japan Paleontological Research, Vol. 15, no. 4
Handa Naoto (2018) Neogene-Miocene Rhinoceros Fossils in Japan (Mammalia, Perissodactyla): Current Status and Issues. Abstracts of the Annual Meeting of the Palaeontological Society of Japan.