An Upper Carboniferous eurypterid trackway from Mostyn, Wales (original) (raw)
Since 2002, dinosaur tracks are excavated on the future course of the “Transjurane” highway (Canton Jura, Jura Mountains, northwestern Switzerland) (Marty et al., 2003). At the Combe Ronde tracksite (municipality of Chevenez) excavations have been concluded in 2005. Here, dinosaur tracks have been uncovered within a 65 cm thick sequence of inter- to supratidal calcareous laminites. These have been deposited during the Late Jurassic (approximately at the transition from the Early to the Late Kimmeridgian) on a carbonate platform, which was located at the Northern Tethys passive margin. At the Combe Ronde tracksite 1167 dinosaur tracks have been documented on 8 successive surfaces. Most of the tracks are true tracks. However, some of the surfaces also exhibited undertracks (sensu Lockley, 1991) and overtracks, sometimes associated with each other and/or with true tracks on a single bedding plane. This indicates peculiar substrate and preservational conditions of the tracks. Thus, an identification of true tracks, undertracks and overtracks was only possible because all tracks were excavated and documented level-by-level. Together with the analysis of other sedimentological features such as desiccation cracks or ripple marks, this enabled to distinguish “true” palaeosurfaces (sensu Smith, 1993) from bedding planes. In addition to that, several tracks extracted in limestone blocks were cut into serial sections and facilitated the study of track formation and taphonomic history (Marty, 2005). This is a key point for consistent ichnotaxonomy and palaeoecological interpretations. The main track level, situated at the base of the laminite sequence, was uncovered on a total surface of 570 m2. It yielded the most diverse ichnocoenosis with two trackways of tiny sauropods (pes footprint length (FL)<25 cm), 12 trackways of medium-sized sauropods (25<FL<40 cm), one trackway of a minute theropod (FL<10 cm), 41 trackways of small theropods (10<FL<25 cm), and two trackways of medium-sized theropods (25<FL<30 cm). The main track level is considered a geotope (Marty et al., 2004) and will be protected for posterity. This will be realized by means of a bridge specifically built for this purpose. Within the overlying laminites, only tracks of mostly medium-sized sauropods have been discovered. 12 trackways were identified, but most of the tracks are not arranged in trackways, as several surfaces were subjected to relatively intense dinoturbation (Lockley and Conrad, 1989).