The reptile fauna

A first synthetic overview of the reptilian fauna shows that its composition is globally consistent with that of other Late Jurassic Lagerstätten of western Europe, which have been deposited in similar palaeoenvironments (Canjuers, Cerin, Crayssac, Solnhofen, Solothurn) (Billon-Bruyat, 2003; Billon-Bruyat & Marty, 2004). Three major groups have not been found as yet: lepidosaurs, ichthyopterygians, sauropterygians; the absence of ichthyopterygian and sauropterygian remains for the moment is probably due to their general rarity (Fig.1).

Fig.1: Diagram of the composition (family level) of the major groups of reptiles in the latest Jurassic and earliest  Cretaceous coastal marine environments of western Europe (from Billon-Bruyat, 2003). © Palaeontogy A16

The abundance of theropod and sauropod dinosaur trackways is a particular feature of the Late Kimmeridgian fauna of Porrentruy compared to the other Late Kimmeridgian sites. Dinosaur footprints are abundant, exhibit a large range of footprint size classes and trackway patterns and configurations, and they occur at three time intervals on 30 levels within biolaminites. This supports the hypothesis that these dinosaurs could habitually enter into coastal marine environments, including small to large carnivores and large-bodied herbivores (Billon-Bruyat & Marty, 2004).

Fig.2: This reconstruction shows the composition of the Late Kimmeridgian coastal reptile fauna as inferred from the discoveries of the Palaeontology A16. Illustration by Tayfun Yilmaz.

Besides scattered vertebrate remains of turtles and crocodilians, the shell beds of the Banné Member consist of abundant invertebrates, notably molluscans. Most of the shells are highly encrusted and bored, and many bivalve shells are in life position. This points to an environment with a very low-energy and sedimentation rate. It is suggested that the community succession was for a great part controlled through (par-autochthonous) shell accumulation (taphonomic feedback of Kidwell, 1991). The Palaeontology A16 has already acquired a huge collection of invertebrates from the Banné Member, and a collaboration has been launched with the “Institut für Paläontologie” of the University of Würzburg (Germany). The aim of this project is a re-description of this classical Kimmeridgian bivalve fauna. Further, two master theses (Hicks, 2006; Richardt, 2006) of the same institute have analysed the evolution of the faunal associations using a bulk sampling method. Another master thesis (Silvan Thüring, University of Basel) analyses the palaeoecology of shell beds on an isolated bloc.

Omission surfaces

The term “omission surface” was coined by Heim (1924, 1934). Omission surfaces are discontinuity surfaces of the most minor nature, which mark temporary halts in deposition (time of non-deposition/sedimentation), but involve little or no erosion (Bromley, 1975). However, in a broader sense any discontinuity surface like hardgrounds or bed surfaces with dinosaur tracks and/or desiccation cracks are omission surfaces. Often, on carbonate platforms such surfaces mark a transition from shallower to significantly deeper water facies. They reflect reactions of the sedimentary system to rapid and drastic environmental changes. They record the most important times during platform evolution, the highest dynamics in environmental change respectively (Hillgärtner, 1998). In fact, on shallow carbonate platforms, the sedimentary record represents only a small part of the geological time and many elementary sequences have not been deposited at all or are only found reworked (Strasser et al., 1999). Moreover, it is difficult to estimate the duration of an omission surface. It can probably take from a few days to hundreds to several thousands of years.

Omission surfaces as the only record of time of non-deposition allow the formation and preservation of in situ ichnofabrics (Photo 1) and faunal communities, unique opportunities for the study of community palaeoecology and the reconstruction of the depositional environments. In the case of dinosaur track-bearing surfaces (Photo 2), they are the only witness of time of prolonged terrestrial settings on a carbonate platform. In this case, they are the key to the partial reconstruction of a terrestrial ecosystem, which otherwise is not documented.

Photo 1: Hardground 2000 at the Courtedoux—Tchâfouè site. The surfaces is highly encrusted (oysters and serpulids) and bored (Trypanites sp., Gastrochaenolites sp.), and shows a pronounced palaeorelief. Scale bar on the right is in cm.

Photo 2: Omission surface with left pes/manus footprints of a small sauropod dinosaur. Note the well-marked displacement rims in front of the semi-circular manus print. On the same surface wave ripples and desiccation cracks can be observed as well. Footprints have been coloured. Scale bar is 20 cm.