9:45 AM - 10:00 AM
[BPT07-04] Environmental change and morphological variability in mid-Cretaceous planktic foraminifer, Muricohedbergella delrioensis
Keywords:Cretaceous, Planktic foraminifer, Morphological evolution
We here examined morphological variability, the maximum diameter and the total number of chambers, of the mid-Cretaceous planktic foraminifers, Muricohedbergella delrioensis, across the MCE. The specimens were obtained from ODP Site 1258 at Demerara Rise, equatorial Atlantic. Carbon and oxygen isotopic data from the same samples we utilized, published by Moriya et al. (2007), enabled us to compare our results on morphological variability with those isotopic records. The maximum diameter becomes smaller in the sequence younger than the MCE, which is accompanied by decrease in the total number of chambers within a single individual. According to the modern cultivation experiments, the maximum dimeter and the total number of chambers become greater with increase in feeding frequency (Bé et al., 1981). These results indicate that M. delrioensis older than the MCE might have more feeding opportunity than those younger than the MCE. Additionally, we analyzed a coiling (sinistral/dextral) ratio of M. delrioensis. The coiling ratio shows a significant correlation with the oxygen isotopic composition, hence temperature. The correlation between the coiling ratio of M. delrioensis and temperature has also been reported by Desmares et al. (2016). Our results reconfirm their implication. Considering modern examples showing genetic discrepancy between sinistral and dextral morphotypes in the species having a correlation between a coiling ratio and ambient temperature (Darling et al., 2006), the coiling direction of M. delrioensis might indicate existence of potential cryptic (sub)species.
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