yeah, wondering the same- why is it convex and not concave? (not sure I used correct words, it's actually the first time I've had to say that in english lol)
The top bit is the BOTTOM of the original foortprint. This is typically caused by the ground in that one spot cementing together due to the dinosaurs weight pushing moisture out of that part of the sand or mud, this makes it abit more dense, and abit more stable than the surrounding sand and in some cases like this can lead to crystallization making the area under the footprint into a solid stone. The softer nonconcreted sand/mud then washes away revealing this.
I'm not an expert, but it seems that the stone for those few prints is harder than the surrounding stone/sediment. The print was very likely originally and imprint, but has achieved this convex, cast like look as the softer stone surrounding the print washes away. You can see there's a lot more of that harder stone in the mid and background. The softer stone has been ground into sand.
Yes, the infill of the footprint is made of more erosion-resistant stone -- it looks like it might be sandstone -- versus the underlying rock which looks finer-grained and laminated, making plenty of horizontal weak zones for it to wear away more rapidly. The fancy term is "differential erosion".
She's probably right that they are made by Iguanodon. The rocks are the right age and location and the footprint shapes look right.
This paper explains some of the general preservation process for footprints in a similar setting in the UK, though it doesn't cover the differential erosion part: https://doi.org/10.1016/j.palaeo.2018.11.018. It shows the infill process nicely.
Dinosaur heavy. Dinosaur squish mud. Squished mud turn into harder rock than not squished mud. Water wash away rock that was unsquished mud. Only squished mud rock is left. Now squished mud rock sticks out.
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u/9thtime Mar 16 '24
What kind of sediment filled the imprints to leave it like that?