Amane Tajika

• Dr. sc. nat.
• American Museum of Natural History

Predator–prey relationships are considered a major driver for the evolution of organisms, and thus contributed to shaping morphology, ecology, and diversity. During the Late Cretaceous of North America, ammonoid cephalopods were one of the most abundant and diverse marine invertebrates. Despite frequent reports of shell breakage in ammonoids, littl...

Understanding the mechanism of selective extinction is of utmost importance to predict the impact of current anthropogenic environmental changes on the ecosystem. The Cretaceous-Paleogene (K-Pg) mass extinction event has attracted the attention of both the general public and researchers due to its selective nature. In the marine realm, ammonoids an...

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Understanding the mechanism of selective extinction is important in predicting the impact of anthropogenic environmental changes on current ecosystems. The selective extinction of externally shelled cephalopods at the Cretaceous-Paleogene (K-Pg) mass extinction event (am­monoids versus nautiloids) is often studied, but its mechanism is still debate...

The impact of increasing atmospheric CO2 and the resulting decreasing pH of seawater are in the focus of current environmental research. These factors cause problems for marine calcifiers such as reduced calcification rates and the dissolution of calcareous skeletons. While the impact on recent organisms is well established, little is known about l...

Hatching size has been considered of great importance in the evolution of externally shelled cephalopods. However, our knowledge of how hatching size varies in response to biotic and abiotic factors is largely lacking. We present a comprehensive overview of hatching size in all known species of modern nautilids (225 specimens, representing eight sp...

In the Alpstein massif of north-eastern Switzerland, a complete succession of uppermost Hauterivian to uppermost Barremian condensed hemipelagic sediments crops out. This succession is known as Tierwis Formation, comprising in ascending order, the Altmann and Drusberg members. The sedimentary succession bears a number of fossiliferous glauconite- o...

Modern nautilids (Nautilus and Allonautilus) have often been studied by paleontologists to better understand the anatomy and ecology of fossil relatives. Because direct observations of these animals are difficult, the analysis of light stable isotopes (C, O) preserved in their shells has been employed to reveal their habitat and life history. We ai...

We document an upper upper Albian (Mortoniceras rostratum Zone) cephalopod assemblage from Clansayes (Drôme, south-eastern France). Although fossils are rare in local exposures and in the single sampled level, a decade of intensive fossil collecting yielded 290 ammonite and 5 nautilid specimens. In total, we describe 1 spe- cies of nautilid and 24...

Nautilid, coleoid and ammonite cephalopods preserving jaws and soft tissue remains are moderately common in the extremely fossiliferous Konservat-Lagerstätte of the Hadjoula, Haqel and Sahel Aalma region, Lebanon. We assume that hundreds of cephalopod fossils from this region with soft-tissues lie in collections worldwide. Here, we describe two spe...

Assessing the taxonomic importance of the suture line in shelled cephalopods is a key to better understanding the diversity of this group in Earth history. Because fossils are subject to taphonomic artifacts, an in-depth knowledge of well-preserved modern organisms is needed as an important reference. Here, we examine the suture line morphology of...

The Aitamir Formation, situated in the Koppeh Dagh Basin in the northeast of Iran, is known for its well-exposed Albian-to-Cenomanian succession. Although geologists previously documented a number of macro-and microfossils, no nautilids had been discovered until now to our knowledge. Here, we present lower Albian and middle Cenoma-nian nautilids fr...

The Salazac locality (Gard, southeastern France) is renowned for the richness of its cephalopod fauna (especially ammonites) from the Mortoniceras fallax Zone (uppermost Albian, Lower Cretaceous). However, most ammonite species have paradoxically been scarcely illustrated up to now. Furthermore, the rare assessments of ammonite taxonomic diversity...

The magnitude and ontogenetic patterns of intraspecific variation can provide important insights into the evolution and development of organisms. Understanding the intraspecific variation of organisms is also a key to correctly pursuing studies in major fields of palaeontology. However, intraspecific variation has been largely overlooked in ectococ...

Recent advancements in tomographic techniques allow for detailed morphological analysis of various organisms, which has proved difficult in the past. However, the time and cost required for the post-processing of highly resolved tomographic data are considerable. Cephalopods are an ideal group to study ontogeny using tomography as the entire life h...

Reconstructing the physiology of extinct organisms is key to understanding mechanisms of selective extinction during biotic crises. Soft tissues of extinct organisms are rarely preserved and, therefore, a proxy for physiological aspects is needed. Here, we examine whether cephalopod conchs yield information about their physiology by assessing how t...

In the Alpstein massif (cantons of Appenzell Auserrhoden, Appenzell Innerrhoden and St. Gallen, Switzerland), a complete succession of uppermost Hauterivian to lower upper Barremian condensed hemipelagic series known as Tierwis Formation crops out. The Tierwis Formation, which records several periods of major palaeoenvironmental change, comprises s...

The Alpstein (northeastern Switzerland) has yielded a relatively high diversity of Cretaceous macrofossils. Here, new discoveries of invertebrate fossils from a new locality of the early to late Albian age in Semelenberg (northeastern Alpstein, canton St. Gallen) are documented. In spite of the very small size of these samples and the 1-m-thick, ve...

Im Kapitel "Ammoniten (Ammonoidea)" des Buches "Fossilien im Alpstein - Kreide und Eozän der Nordostschweiz" (Kürsteiner Peter und Klug Christian 2018) wird eingegangen auf: Anatomie, Ursprung und Evolution, Dimorphismus, Lebensweise, Aussterben, Klassifikation. Im Hauptteil des Kapitels werden die verschiedenen Ammoniten-Arten - rund 80 Taxa - auf...

Intraspecific variation of organisms is of great importance to correctly carry out taxonomic work, which is a prerequisite for key disciplines in paleontology such as community paleoecology, biostratigraphy, and biogeography. However, intraspecific variation is rarely studied in ectocochleate cephalopods (ammonoids and nautiloids), for which an exc...

Owing to their great diversity and abundance, ammonites and belemnites represented key elements in Mesozoic food webs. Because of their extreme ontogenetic size increase by up to three orders of magnitude, their position in the food webs likely changed during ontogeny. Here, we reconstruct the number of eggs laid by large adult females of these cep...

Studies of global palaeoecology through time usually ignore regional details. Such regional studies on palaeoecology are required to better understand both regional- and global-scale palaeoecolgical changes. We analyzed the palaeoecolgy of a Cretaceous sedimentary sequence in the Alpstein (cantons of Appenzell Ausserrhoden, Appenzell Innerrhoden an...

Palaeodiversity, palaeoecology and organismic evolution are essential disciplines in palaeontology. Although major trends and changes in biodiversity through the Phanerozoic are more or less well known, much more work is needed to understand ecological details as well as all the involved biases linked with biodiversity analyses. As far as evolution...

The Alpstein (cantons of Appenzell Ausserrhoden, Appenzell Innerrhoden and St. Gallen, northeastern Switzerland) has been of great interest for geologists over the last decades because of its excellent outcrops. However, there was no comprehensive overview over its Cretaceous fossil content. Here, we describe the cephalopod associations, which are...

Nautilus remains of great interest to palaeontologists after a long history of actualistic comparisons and speculations on aspects of the palaeoecology of fossil cephalopods, which are otherwise impossible to assess. Although a large amount of work has been dedicated to Nautilus ecology, conch geometry and volumes of shell parts and chambers have b...

Most ammonoids display a distinctly different morphology when they are adults or subadults. Depending on the taxon, these mature modifications may comprise changes in coiling, changes in ornamentation, and conspicuous changes of the terminal aperture. These mature modifications permit, at least when a combination of which occurs, to identify adult...

Because ammonoids have never been observed swimming, there is no alternative to seeking indirect indications of the locomotory abilities of ammonoids. This approach is based on actualistic comparisons with the closest relatives of ammonoids, the Coleoidea and the Nautilida, and on the geometrical and physical properties of the shell. Anatomical com...

Molluscs such as ammonoids record their growth in their accretionary shells, making them ideal for the study of evolutionary changes in ontogeny through time. Standard methods usually focus on two-dimensional data and do not quantify empirical changes in shell and chamber volumes through ontogeny, which can possibly be important to disentangle phyl...

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Tajika, A. & Wani, R. 2011: Intraspecific variation of hatchling size in Late Cretaceous ammonoids from Hokkaido, Japan: implication for planktic duration at early ontogenetic stage. Lethaia, Vol. 44, pp. 287–298. Intraspecific variations of the early shell dimensions (ammonitella and protoconch diameters) of two Late Cretaceous (earliest Campanian...