SYNCHROTRON X-RAY MICROTOMOGRAPHY: AN INSIGHT INTO UPPER TRIASSIC CONODONT ULTRASTRUCTURE

In order to study the ultrastructure and the ontogenetic processes of Late Triassic conodont P1 elements, we performed synchrotron x-ray microtomographies on 13 conodonts from the Pizzo Mondello section (Sicily, Italy; GSSP candidate for the Norian), providing internal tomographic images of Late Triassic conodonts for the first time. The analyses were performed at the Swiss Light Source of the Paul Scherrer Institut (Zurich, Switzerland), with the scanned conodonts belonging to 5 genera and 12 species (all of them phylogenetically related) and ranging from the Upper Carnian to the Lower Norian. The scanned specimens have two CAI values (1.5 and 7) and have different degrees of preservation. The synchrotron x-ray microscopy is a non-invasive tool and it has the great advantage to examine an almost unlimited number of sections per conodont that can be oriented in all the directions, allowing investigations of the inside of an element from all perspectives. The different kind of tissues and the internal ultrastructure are distinguishable at different levels in most parts of the scanned conodonts. Growth lines are visible in all parts of the platforms, providing valuable data concerning the growth processes of the P1 elements. In particular, it is possible to observe in detail the ontogenetic processes of some diagnostic morphological characters of the platforms, allowing us to distinguish homologies from analogies and thus, to unravel evolutionary convergences that, otherwise, could not be distinguished with other analytical techniques. X-ray synchrotron analysis is thus a reliable tool to investigate the internal structure of conodonts and to refine preexisting phylogenetic models

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