|An artistic reconstruction of the floating ammonite leaving behind the drag mark.|
Illustration: James McKay
Trackways and tracemakers preserved together in the fossil record are rare. However, the co-occurrence of a drag mark, together with the dead animal that produced it, is exceptional. Here, we describe an 8.5 m long ammonite drag mark complete with the preserved ammonite shell (Subplanites rueppellianus) at its end. Previously recorded examples preserve ammonites with drag marks of < 1 m. The specimen was recovered from a quarry near Solnhofen, southern Germany. The drag mark consists of continuous parallel ridges and furrows produced by the ribs of the ammonite shell as it drifted just above the sediment surface, and does not reflect behaviour of the living animal.
|Fig 2. Various Plattenkalk localities of the Franconian and Swabian Alb.|
|Fig 1. MCFO 0492, the entire drag mark created by the drifting shell of a dead ammonite (Subplanites rueppellianus), with close-up of several portions. A. The first portion of the drag mark clearly showing two prominent ridges. B. Drag mark showing two prominent ridges with additional faint ridges. C. Drag mark showing four prominent ridges and a gentle curve. D. Drag mark showing numerous prominent ridges, along with the ammonite.|
Large scale measures 1 m. Small scales measure 10 cm.DOI: 10.1371/journal.pone.0175426
|Fig 3. The ammonite Subplanites rueppellianus, the producer of the drag mark (MCFO 0492). Note the touch down mark which changes the orientation (and number) of the ridges in the substrate, anteroventral to the ammonite. Scale measures 5 cm.|
This exceptionally long fossil was produced by an ammonite shell post-mortem. The shell must have been partially buoyant, firstly because only a small portion of the shell contacts the substrate over the length of the mark, and secondly to be moved by a current that was gentle enough not to disturb the surrounding sediment. It is likely that the ammonite was losing buoyancy over the length of the drag mark, which resulted in eventual loss of all buoyancy and the ammonite falling on its side.
The drag mark of the studied specimen does not represent a mortichnion because it was not created by the animal when alive. Rather, this structure should more correctly be considered a tool mark. As such, behaviour must not be inferred from the drag mark of specimens such as MCFO 0492, and they have to be interpreted as non-biogenic structures produced by physical means. MCFO 0492 represents the hitherto longest fossil drag mark created by a dead animal, complete with the animal preserved at the end.
Dean R. Lomax, Peter L. Falkingham, Günter Schweigert and Alejandro P. Jiménez. 2017. An 8.5 m Long Ammonite Drag Mark from the Upper Jurassic Solnhofen Lithographic Limestones, Germany. PLoS ONE. 12(5): e0175426. DOI: 10.1371/journal.pone.0175426
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