Isotope Signature of Ammonoid Shells

Abstract

From the middle Paleozoic to the end of Mesozoic, ammonoids represent one of the major constituents of macro-organisms that are preserved as fossils in the epicontinental seas. Their rich fossil records and remarkable diversification of shell forms in space and time indicate that ammonoids are one of the best examples for understanding relationships between evolution of marine biota and changes in paleoenvironment and/or paleoceanography. The biotic and abiotic components in the ancient ocean are, obviously, closely connected through the chemical cycles and energy flow in the ancient eco-system. Therefore, for understanding the cause and/or effect of diversity change or extinction events in the ancient earth-and eco-system, identifying ecological niche, such as habitat, food habit, life history strategy, etc., of ammonoid species is extremely important.The mode of life of ammonoids has hitherto been investigated based mainly on functional morphology of their shells, as well as synpaleoecological analyses of fossil assemblages. The previous approaches on the basis of functional morphology are roughly divided into three aspects; (1) buoyancy of the animal including both a shell and a supposed soft part, (2) swimming and mobility and (3) depth habitat.Many workers (Trueman 1941;Heptonstall 1970;Ebel 1983;Saunders and Swan 1984;Saunders and Shapiro 1986;Ebel 1992;Shigeta 1993; and others) calculated density of living ammonoids based on shell thickness, volume ratio between a phragmocone and a body chamber, and the estimated density of the hypothetized soft part. Although there are some ambiguities, such as accuracy of shell thickness calculations based on theoretical models, quantity of cameral liquid in the phragmocone and the supposed soft part density, most previous studies, excluding Ebel (1983Ebel ( , 1992 and Shigeta (1993), estimated that living ammonoids had a density

Ammonoid Embryonic Development

“…However, there are numerous examples of mixed assemblages of juvenile and adult ammonoids from well-oxygenated environments in which very small juveniles (< 4 mm) are rare or absent, which suggest that newly hatched ammonoids might have lived in a different environment from that of the older juveniles and adults. This pattern could, however, be related to taphonomic processes, although isotope analyses through ontogeny are consistent with such a hypothesis for various Jurassic and Cretaceous ammonoids (Lukeneder et al 2010;Ritterbush et al 2014;Lukeneder 2015;Moriya 2015). The ecological differences between hatchlings and later ontogenetic stages might earn them the name of paralarvae (as it was introduced by Young and Harman 1988 for an early developmental stage of extant cephalopods that resides in the near-surface plankton and that differs from later stages in habit, habitat and, frequently, morphology).…”

“…They interpreted these as possibly reflecting mass mortality following reproduction (semelparity) as they found no evidence for post-mortem sorting (both small and large species are represented). Ontogenetic separation for at least some Jurassic and Cretaceous taxa is also suggested by oxygen isotope studies (Lukeneder et al 2010;Ritterbush et al 2014;Lukeneder 2015;Moriya 2015). However, the lack of reports of embryonic shells within these assemblages might also be related with collection biases, transport or diagenetic destruction of minute, fragile aragonitic shells Westermann 1996).…”

“…Nevertheless, a large range of interspecific variation in their reproductive strategies might have existed among different time frames and taxa. Our knowledge of post-hatching modes of life and reproductive strategies can only be expanded by combining studies on facies distribution (e.g., Westermann 1996), size class distribution (e.g., Bucher et al 1996;Walton et al 2010) and isotope analyses through ontogeny of various taxa and time frames (e.g., Lukeneder et al 2010;Lukeneder 2015;Moriya 2015) as well as the further discoveries of exceptionally preserved ammonoid eggs and adults (Etches et al 2009;Landman et al 2010;Klug et al 2012;Ritterbush et al 2014).…”

Paleobiogeography of Early Cretaceous Ammonoids

“…Besides the significance of overall temperature differences in their habitats, it is even more important to understand the ammonite lifestyle and reproduction strategies to correctly interpret paleobiogeographic affinities. The latter factor is very rarely addressed with regard to ammonites in general (Manger et al 1999;Mapes and Nützel 2009;reviewed in De Baets et al 2015), but there has been some progress made in reconstructing the ammonite lifestyle in recent years (Ritterbush et al 2014;Lukeneder 2015;Moriya 2015). But there is still much to be learned.…”

Refining the interpretation of oxygen isotope variability in free-swimming organisms

“…These model results also help to explain why in Nautilus, a known depth migrator, δ 18 O variance from bulk samples is low (Taylor and Ward, 1983;Landman et al, 1994;Moriya et al, 2003;Auclair et al, 2004;Ohno et al, 2014). Comparison of the slowly precipitating septa (Westermann et al, 2004) to shell wall that grows much faster (Martin et al, 1978) has recently been interpreted as a depth bias of septal growth (Moriya, 2015) or disequilibrium precipitation of septa (Ohno et al, 2014). Perhaps the time-averaging difference between samples, as modeled here, could explain this difference (Fig.…”