17:15 〜 18:30
[BPT02-P01] Preliminary reports on the ecology of Cretaceous pantrionychian turtles based on the carapace bone histology
キーワード：Pantrionychia、骨組織、生態、後期白亜紀、甲羅、カメ類
Pantrionychia is a group of non-marine, cryptodiran turtles that includes Nanhsiungchelyidae, Adocidae, and extant superfamily Trionychia (Trionychidae and Carettochelyidae). Pantrionychia originated in the Early Cretaceous of Asia and dispersed to North America during Cenomanian and Santonian. The Late Cretaceous turtle fauna in North America was thus likely influenced by the migration of Pantrionychia. Nevertheless of the importance, however, little is known about the ecology of Cretaceous Pantrionychia. Previous studies on bone/carapace morphology and/or taphonomic evidence suggest that extinct Adocidae was aquatic, whereas Nanhsiungchelyidae is still in debate between aquatic and fully terrestrial. Consequently, it is uncertain whether the clade Pantrionychia evolved from an aquatic or terrestrial form. The aim of this study therefore is to estimate their mode of lifestyles (i.e., terrestrial, freshwater, or marine) and discuss their evolutionally history.
The mode of lifestyle in tetrapods can be inferred based on bone histology. As a general trend, bone compactness is higher in freshwater animals than in terrestrial ones. Marine animals can acquire very high bone compactness (e.g., manatees) or very low (e.g., extant whales) according to animal’s swimming ecology. Such trends are also found in extant turtle limb bones, suggesting that the bone histology method can be applied for fossil Pantrionychia. Turtle limb bones, however, are relatively rare in the fossil record, comparing with abundant discoveries of carapace fragments. Thus, if the carapace histology reflects lifestyles, it can be an easy and powerful tool to infer the ecology of turtles.
In order to test whether the carapace bone histology can be used to infer the mode of lifestyles in turtles, we examined diverse extant taxa, including terrestrial, freshwater, and marine forms, before we applied the method to fossil taxa. Our results revealed that the carapace bone compactness is not necessarily correlated to their lifestyles, differing from previous speculations. However, we found that the size (cross-sectional area) of each intertrabecular space is correlated to the mode of lifestyles, at least between terrestrial and aquatic forms: the cross sectional area is significantly higher in terrestrial species than in aquatic species. The size of each intertrabecular space therefore can be an indicator of the lifestyles in turtles.
Based on the method above, we examined three fossil pantrionychian taxa discovered from the Late Cretaceous (Campanian) deposits in Alberta, Canada (Oldman and/or Dinosaur Park formations): Basilemys sp. (Nanhsiungchelyidae), Adocus sp. (Adocidae), and Aspideletoides sp. (Trionychidae). Our preliminary results revealed that Basilemys possesses a relatively large cross-sectional area of intertrabecular space, suggesting a terrestrial lifestyle. In contrast, Adocus and Aspideletoides show a relatively small cross-sectional area, suggesting an aquatic lifestyle. These results suggest that Pantrionychia changed their lifestyles though their evolutional history, which may be related to their radiation pattern.
The mode of lifestyle in tetrapods can be inferred based on bone histology. As a general trend, bone compactness is higher in freshwater animals than in terrestrial ones. Marine animals can acquire very high bone compactness (e.g., manatees) or very low (e.g., extant whales) according to animal’s swimming ecology. Such trends are also found in extant turtle limb bones, suggesting that the bone histology method can be applied for fossil Pantrionychia. Turtle limb bones, however, are relatively rare in the fossil record, comparing with abundant discoveries of carapace fragments. Thus, if the carapace histology reflects lifestyles, it can be an easy and powerful tool to infer the ecology of turtles.
In order to test whether the carapace bone histology can be used to infer the mode of lifestyles in turtles, we examined diverse extant taxa, including terrestrial, freshwater, and marine forms, before we applied the method to fossil taxa. Our results revealed that the carapace bone compactness is not necessarily correlated to their lifestyles, differing from previous speculations. However, we found that the size (cross-sectional area) of each intertrabecular space is correlated to the mode of lifestyles, at least between terrestrial and aquatic forms: the cross sectional area is significantly higher in terrestrial species than in aquatic species. The size of each intertrabecular space therefore can be an indicator of the lifestyles in turtles.
Based on the method above, we examined three fossil pantrionychian taxa discovered from the Late Cretaceous (Campanian) deposits in Alberta, Canada (Oldman and/or Dinosaur Park formations): Basilemys sp. (Nanhsiungchelyidae), Adocus sp. (Adocidae), and Aspideletoides sp. (Trionychidae). Our preliminary results revealed that Basilemys possesses a relatively large cross-sectional area of intertrabecular space, suggesting a terrestrial lifestyle. In contrast, Adocus and Aspideletoides show a relatively small cross-sectional area, suggesting an aquatic lifestyle. These results suggest that Pantrionychia changed their lifestyles though their evolutional history, which may be related to their radiation pattern.