Monday, April 13, 2009

Trampled under Hoof

Last Weekend, I was recruited as driver for a student field trip. Location & Time: Jura(-ssic) / Western Switzerland. In fact a great place for ichnologists: the first visited outcrop is one of the best track sites currently exposed worldwide. It is located just a few km in the north of Courtedoux. There are hundreds of imprints of walking sauropods, some theropod tracks as well as thousands of invertebrate traces. This trampled ground is found within the Reuchenette Formation which roughly represents the Kimmeridgian (middle Late Jurassic) of the Jura Mountains. Let me introduce some significant features of this truly remarkable occurrence.

Fig. 1: This photo (click on picture to enlarge) covers roughly a third of the tracksite. You can see the dish-like structures forming trails which are trackable for more than 100 m. These imprints (ichnogenus Brontopodus) are referred to large sauropods like Diplodocus or Apatosaurus. With some empirical formulas it is possible to assess several parameters such as shoulder height, pace and spacing etc. Suggestions about the size of the trackmakers inferred from these trackways range up to 35 m. If correct, this is truly a pathway of titans.

Fig. 2: People for scale

Fig. 3: A more detailed view of individual imprints.

The “crater”-like appearance of the imprints (fig. 3) is due to the impact of the foot on this slippery and readily deformable substrate. These creatures weighted certainly more than 10 tons. Actually it is quite mysterious that these large creatures did not stick in the mud. My two suggestions solving this riddle.

(1) These structures represent already more or less deep undertracks of traces that were produced at some levels above. While walking over a muddy substrate, a creature usually does not only leave an imprint at the very surface, it furthermore obliterates older strata beneath. In fact, numerous trackways do occur quite frequently as undertrack preservation. The problem is that at this locality analogue structures are not observed in slightly higher levels, which however might be due to subsequent erosion.

(2) The subsurface substrate was already somewhat consolidated and, thus, provided solid ground.

There is evidence to favour this one and it comes from the underlying sediment and the invertebrate traces:

Fig. 4: Desiccation cracks and invertebrate burrows in the underlying bedding plane. The burrows are in fact present below this surface. What you see here is just the "halo".

On fig. 4, you can see the bedding plane beneath the track-site levels and it reveals something special. I never saw such beautifully preserved and exposed desiccation cracks and invertebrate burrows of the ichnogenus Thalassinoides. The cracks prove that this surface was fully emerged and got dry. Enough time for calcareous mud to transfrom into a firm substrate. In the next step, the area became flooded again and a new layer of mud was deposited in the intertidal or even supratidal zone of a shallow sea. Now, the Dinosaurs strolled along the Jurassic beach leaving nothing but their massive foot steps in slippery mudflat deposits.

Fig. 5: Thalassinoides isp. It looks as if it stopped digging at the tripple junction.

Last question: When did the burrows form? First, clearly after desiccation. Fig. 4 and 5 show that they are not affected by the cracks and second, after the Dinosaurs passing. At this site, there is no evidence for Dinos trampling crustacean burrows. Thalassinoides are trace fossils usually indicating fully marine conditions. I think they were created later when this area became a seafloor once again. These burrows were open (fig. 6) which is another hint that the desiccated level was firm. The crustaceans dug down into the firm substrate and created these burrows.

Fig. 6: Thalassinoides isp. with passive fill.

I think it is odd in a way that two different structures occuring in one bed chronologically bracket an event preserved in the bed above.

Fig. 7: A scenic interpretation by Alain Bénéteau. (click for enlarge)

To close this post you may listen to Trampled under Hoof from Mastodon.

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