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

Ammonoid Intraspecific Variability

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 comparison yields information on the locomotor muscular systems and organs as well as possible modes of propulsion while the shape and physics of ammonoid shells provide information on buoyancy, shell orientation, drag, added mass, cost of transportation and thus on limits of acceleration and swimming speed. On these grounds, we conclude that ammonoid swimming is comparable to that of Recent nautilids and sepiids in terms of speed and energy consumption, although some ammonoids might have been slower swimmers than nautilids