Ichnofabrics Elucidate the Accumulation History of a Condensed Interval Containing a Vertically Emplaced Ichthyosaur Skull

A three-dimensionally preserved skull and parts of the postcranial skeleton of an ichthyosaur (Leptonectes) was found vertically oriented within on-average slowly deposited (0.5 m/My) Lower Jurassic shallow-water marls. The ichthyosaur sank headfirst into the seafloor because of its center of gravity, as anatomically similar comparably preserved specimens suggest. The skull penetrated into the soupy to soft substrate until the fins touched the seafloor. There is no evidence either for active penetration of the ichthyosaur during death agony or an acceleration by explosive release of sewer gas that would have pushed the skull into the substrate. Ichnofabrics and crosscutting relationships among trace fossils preserved therein allow analysis of stratigraphic completeness. In spite of on-average slow accumulation, the ichthyosaur-hosting sediments formed rapidly during three distinct but similar deposition–bioturbation phases. First, 10 to 15 cm of mud accumulated rapidly. Biodeformational structures subsequently produced therein imply a soupy consistency. As sedimentation slowed down, muds slightly dewatered and consolidated, as reflected by trace fossils with distinct outlines (Palaeophycus and Planolites, thereafter Thalassinoides and Chondrites). The contact with the overlying depositional interval is obliterated by biodeformational structures. Hence, the previously rapidly deposited mud must still have been soft. A short time after the third deposition–bioturbation phase, the ichthyosaur parts penetrated into the still-soft mud and started to be degraded microbially. Below the bioturbated zone, but before compaction, a concretion started to form around the ichthyosaur parts and led to their excellent preservation. During further burial, the skull-hosting concretion experienced differential compaction and moved downward relative to the underlying beds. The skull-hosting concretion penetrated through condensed deposits representing three ammonite zones. Restoring differential compaction, the initial porosity of the sediment can be estimated to have been > 70%. Compared to modern analogues, such muds are soft, as ichnofabrics imply.