Anagaudryceras limatum (Yabe, 1903)

Wani, Ryoji, 2007, Differential preservation of the Upper Cretaceous ammonoid Anagaudryceras limatum with corrugated shell in central Hokkaido, Japan, Acta Palaeontologica Polonica 52 (1), pp. 77-84 : 82-83

Abundance of Anagaudryceras limatum

The selective destruction of Anagaudryceras limatum possibly affected the fidelity of species composition and relative abundance of the fossil assemblage in the study area. However, the analysis of the individual numbers in each fragmentation category suggests that the effect of this selective destruction on the fidelity is negligible: overlapping numbers of the fragmentation categories c and d, which complement each other morphologically, are few in the studied fossil assemblage (only five of 73 specimens; Table 1). The balance (68 specimens), probably the true relative abundance of A. limatum in the fossil assemblage, still exceeds the individuals of the other associated species ( Table 1; see also Matsumoto et al. 1981, for contemporaneous faunal compositions of the shallower environments, Tanabe et al. 1978, for upper Turonian compositions from the subordinate strata in the adjacent areas, and Kawabe 2003 and Tanabe 1979, for compositions of the upper Albian–Cenomanian and middle Turonian in the Oyubari area, respectively). Molluscan death assemblages in marine systems consistently show strong fidelity to relative abundances in the live community ( Kidwell and Flessa 1996, and references therein), however those in ammonoids have been uncertain. This study quantitatively illustrates a similar high fidelity also in ammonoids, suggesting that there was an Anagaudryceras −dominant ammonoid community in the late Turonian in the study area. Similar dominance of A. limatum in the late Turonian has been also reported in the adjacent area with offshore facies of central Hokkaido ( Kurihara and Hirano 2003), which clearly contrasts with Anagaudryceras −impoverished communities in the contemporaneous shallower environment ( Matsumoto et al. 1981). This suggests that the ammonoid communities during the late Turonian were laterally different, depending on environment. This is in contrast to those in the upper Albian– Cenomanian of central Hokkaido, where the relative faunal compositions of ammonoids were similar from inshore to offshore facies (Kawabe 2003). Furthermore, the scarcity of intact shells less than 100 mm in diameter ( Fig. 4B ) is probably not due to the difference in their taphonomy but in their paleoecology, because the other ammonoids of similar size are abundant. This scarcity might be related to the life cycle of A. limatum (e.g., migration through growth, reproductive behaviour at adult stage), although this is difficult to argue based only on this study, and therefore should be investigated together with other approaches (e.g., isotopic analysis of shells).

Destructive processes of disarticulation, fragmentation, and corrosion affected different skeletal types in different ways ( Brett 1990). Differential durability of invertebrate skeletal materials to destructive processes has been recognized ( Chave 1964), which is informative as qualitative indicators of physical environmental parameters ( Brett and Baird 1986). Therefore, the factors of selective destruction should be considered for improved paleoecological interpretation. The taphonomic bias similar to those in A. limatum of this study might therefore be presumed for other ammonoids with ornamented body chambers (e.g., Cretaceous acanthoceratids), although taphonomic bias varies widely in taxonomic groups, tectonic settings, geological time, geological evolution of Earth’s surface, and the cumulative effects of age ( Kidwell 2001).

Figures

Fig. 4. Comparison of taphonomic attributes between Anagaudryceras limatum (Yabe, 1903) and other planispiral ammonoids. A. Fragmentation patterns. Numbers around pie diagrams signify number of individuals in each category. B. Size−distribution patterns and mean shell diameters of specimens whose diameters can be measured. Categories: a, intact; b, phragmocones with distorted body chamber; c, phragmocones lacking body chambers; d, isolated body chamber parts; e, indeterminate.