Kenneth De Baets

Ammonoid Embryonic Development

A great number of new studies have been carried out on ammonoid embryonic development in the last two decades. We focus here on novel developments and interpretations in the description of the embryonic shell (including terminology, shape, size, ornamentation, microstructure, septa, siphuncle and muscle scars), the sequence of embryonic development, reproductive strategy and post-hatching mode of life, followed by conclusions and possible future areas of research.

Intraspecific variability through ontogeny in early ammonoids

Abstract Mollusks in general and ammonoids in particular are known to display a sometimes profound morphological intraspecific variability of their shell. Although this phenomenon is of greatest importance, it has rarely been investigated and quantified. It is especially crucial for taxonomy and incidentally for biodiversity analyses to account for it, because otherwise, the number of described species might exceed that of actual species within any group. Early ammonoids (Early Devonian, Paleozoic) typically suffer from this bias. For instance, most specimens from the same layer and the same region (e.g., the Erbenoceras beds of the Moroccan eastern Anti-Atlas studied here) differ morphologically from each other. Depending on the importance given to certain morphological characters, therefore, one could create a new species for almost every specimen. In this study, we measured nearly 100 such specimens from a restricted stratigraphic interval and quantified their intraspecific variability. There is a variable but strong overlap of the quantified shell characters at most ontogenetic stages, and only two species are here separated rather than the four previously recognized in Morocco. When ontogenetic trajectories of the Moroccan specimens are compared with coeval faunas from other regions (assigned to other species), a strong overlap between the morphospaces occupied by these taxa becomes apparent. The justification of some of these latter species is thus questionable even if their mean values in some conch parameters differ considerably from the mean values of the Moroccan species. Hence, the number of currently valid species of these loosely coiled early ammonoids is probably much too high. Extreme caution must therefore be taken when examining the diversity of groups in which the intraspecific variability is poorly known.

Devonian pearls and ammonoid-endoparasite co-evolution

Raised shell projections on the inner shell walls that form pits on the internal moulds of Devonian ammonoids have been known for several decades. New specimens from Morocco reveal novel details of these structures; most, if not all, of which consist of a capsule of ammonoid shell that covers tiny tubes attached to the outer (= lateral or ventral) shell wall from the inside. In accordance with comparable Recent occurrences of similar structures in molluscs, we use the term “pearls” for these structures and the pits they form on the internal moulds. The nature of these encapsulated tubes is described and discussed. Because of the presence of these tubes inside the pearls, pearl arrangement, and their similarity to Recent mollusc occurrences, the tubes are interpreted as traces of parasitoses. The pearls and pits were grouped into five types based on differences in morphology, size, and arrangement. Then, having used these traits to perform a simple cladistic analysis, the resulting cladogram was compared to the phylogeny of ammonoids. Based on this comparison, it appears likely that the parasites underwent a co-evolution with the ammonoids, which lasted 10 to 15 Ma. Patterns of evolutionary events include co-speciation, “drowning on arrival” (end of parasite lineage near base of a new host clade), and “missing the boat” (parasite lineage does not adapt to a new host clade, thus not evolving a new parasite clade). Because of the lack of fossilised soft tissue, only speculations can be made about the systematic affiliation of the parasites, their life-cycle, infection strategy, and ecological framework. Some co-occurring bivalves also have pits reminiscent to structures caused by trematodes in Recent forms. Based on the available information, the tubes are interpreted as artefacts of trematode infestations, which, if correct, would extend the fossil record of parasitic trematodes into the Early Devonian.

Red Devonian trilobites with green eyes from Morocco and the silicification of the trilobite exoskeleton

Latest Emsian (Early Devonian) sediments at the famous mud-mound- and trilobite-locality Hamar Laghdad (Tafilalt, Morocco) yielded some red-coloured remains of phacopid trilobites. Closer examination revealed that the eyes of these phacopids are often greenish in colour. EDX-analyses showed that the lenses retained their original calcitic composition, possibly greenish due to Fe- and Mn-impurities, while most of the exoskeleton was silicified. The silicified parts contain elevated concentrations of iron which causes the red colour. This phenomenon is explained by the porosity of the exoskeleton in contrast to the homogeneous and massive construction of the lenses and their Mg-content. These incompletely silicified trilobites enabled a reconstruction of the silicification process in trilobites. Their diagenetic alteration probably occurred as a result of events associated with the Cretaceous transgression.

Ammonoid Intraspecific Variability

Two main types of intraspecific variation can be distinguished in ammonoids, which are not mutually exclusive: continuous and discontinuous variation. Although many authors acknowledge or implicitly assume a large intraspecific variability is possible in shell shape, ornamentation and suture line, it has only been rarely studied quantitatively. Several potential biases need to be taken into account when studying intraspecific variation of fossil populations including paleoecological, taphonomic and collection biases. Intraspecific variation might be controlled both by genetic and environmental parameters, although both are difficult to separate in fossil samples. In ammonoids, a large part of intraspecific variation in morphology and size has been attributed to differences in growth rates and development. Taking intraspecific variation properly into account is not only of prime importance for taxonomy, but also for studies on biostratigraphy, paleobiogeography, ecology, paleobiology and evolution of ammonoids.

Tectonic blocks and molecular clocks

Evolutionary timescales have mainly used fossils for calibrating molecular clocks, though fossils only really provide minimum clade age constraints. In their place, phylogenetic trees can be calibrated by precisely dated geological events that have shaped biogeography. However, tectonic episodes are protracted, their role in vicariance is rarely justified, the biogeography of living clades and their antecedents may differ, and the impact of such events is contingent on ecology. Biogeographic calibrations are no panacea for the shortcomings of fossil calibrations, but their associated uncertainties can be accommodated. We provide examples of how biogeographic calibrations based on geological data can be established for the fragmentation of the Pangaean supercontinent: (i) for the uplift of the Isthmus of Panama, (ii) the separation of New Zealand from Gondwana, and (iii) for the opening of the Atlantic Ocean. Biogeographic and fossil calibrations are complementary, not competing, approaches to constraining molecular clock analyses, providing alternative constraints on the age of clades that are vital to avoiding circularity in investigating the role of biogeographic mechanisms in shaping modern biodiversity. This article is part of the themed issue ‘Dating species divergences using rocks and clocks’.

Anetoceratinae (Ammonoidea, Early Devonian) from the Eifel and Harz mountains (Germany), with a revision of their genera

“Cyrtoceratites arduennensis”, one of the stratigraphically oldest ammonoids and type species of the ambiguously defined genus Anetoceras, was first described from the Eifel Mountains. It has been misinterpreted several times and is here re-described, based on the holotype and additional material from the type area. Anetoceras arduennense (STEININGER, 1853) has a crioconical conch with whorls almost touching the preceding volution. The revision of the subfamily Anetoceratinae leads to the result that only five genera (Metabactrites, Borivites, Ivoites gen. nov. as a replacement name for Teneroceras, Anetoceras, and Erbenoceras) can be regarded valid, which probably represent a phyletic series.

Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence.

Novel fossil discoveries have contributed to our understanding of the evolutionary appearance of parasitism in flatworms. Furthermore, genetic analyses with greater coverage have shifted our views on the coevolution of parasitic flatworms and their hosts. The putative record of parasitic flatworms is consistent with extant host associations and so can be used to put constraints on the evolutionary origin of the parasites themselves. The future lies in new molecular clock analyses combined with additional discoveries of exceptionally preserved flatworms associated with hosts and coprolites. Besides direct evidence, the host fossil record and biogeography have the potential to constrain their evolutionary history, albeit with caution needed to avoid circularity, and a need for calibrations to be implemented in the most conservative way. This might result in imprecise, but accurate divergence estimates for the evolution of parasitic flatworms.

Parasites of Ammonoids

Extant cephalopods are commonly infested by parasites making it plausible that ammonoids also had parasites. We review the fossil evidence of parasitic infestations in ammonoids, which, due to low preservation potential of soft-tissues in ammonoids and parasites, are mainly recorded as shell pathologies on the external shell interpreted to be caused by parasites. Extant nautilids are of little help to interpret these structures as they only harbor parasitic copepods. Pathologies analogous to those found in ammonoids that are caused by parasites are common in extant bivalves and gastropods. The position of these features might reveal their parasitic nature. The restriction of these structures to certain ammonoid lineages suggests both the influence of phylogeny and the potential role of ecology (feeding, mode of life) on infestation risks. Other long-term associations with detriment to ammonoids such as epizoa and bioeroders as well as their possible confusion with parasitism are briefly discussed.

Ancestry, Origin and Early Evolution of Ammonoids

In order to put the origin of the Ammonoidea into the broader evolutionary context, we review the hypothesis on the origin of cephalopods in general, the origin of bactritids as well as the origin of bactritids with their respective Bauplan characters. We also list major morphological changes that occurred between the origin of cephalopods until the early evolution of ammonoids.