Stop being so picky, Goldilocks

[This post is about a new paper available freely here]

In 2012* with colleagues at Manchester we published a paper entitled ‘The ‘Goldilocks’ effect : preservation bias in vertebrate track assemblages.’ If I were to give you a simple one-liner for the paper, it would completely undermine the effort went into the paper…   that if sediment is too soft, you sink and can’t walk over it, and if sediment is too hard, you won’t leave footprints. This necessary ‘just right’ characteristic was what lead my co-author Lee Margetts, who was also one of my PhD supervisors, to coin the term ‘goldilocks effect’**. However, that one liner is a bit of an understatement; the paper used computer simulation (specifically, Finite Element Analysis) to simulate tracks of different shapes and sizes, and look at what could form in similar substrates. We found that on a homogeneous substrate, the range of sizes of possible track makers was surprisingly small. This provided a numerical, quantitative mechanism as to why some tracksites only preserve tracks from large animals, and others only tracks of small animals, and highlighted again that a tracksite is not necessarily a complete record of a fossil ecosystem.

The kicker though, is – ‘homogeneous substrates.’ Generally, substrates aren’t homogeneous, and even when they are, confining pressures at depth mean sometimes it takes more and more force to indent deeper.

At a conference in Oberkirchen, Germany, in 2011, I met Martin Bäker, a physicist from the Technische Universität Braunschweig (Martin has a great blog here, in German). Martin was not entirely convinced by the original Goldilocks paper, and felt materials would act differently to what we presented. Slightly worried I’d cocked up, and having a high regard of Martin’s expertise and knowledge, I suggested we work together to see if we could improve what had gone before.

And so we spent the next couple of years running more complex simulations. Martin enlisted his student Julian Hage to help run the simulations. We threw complex foot shapes out of the window and focused just on what changing soil properties and models had on how deep a round foot would sink. Loads of variables were incorporated to expand upon the simple von Mises model originally used.

It turned out great, in that we met somewhere in the middle – The goldilocks effect wasn’t necessarily as specific as originally presented, but there was still a pretty narrow range of foot pressures that could walk over any given substrate, even vertically heterogeneous substrates with firmer sub-surface layers. Part of that is down to the massive range of under-foot pressures dinosaurs as a group can produce.

 

Figure 6 from our new paper 'Mitigating The Goldilocks effect' showing how different animals can or can't leave tracks in various substrate models for our FEA simulations (See the full paper for details)
Figure 6 from our new paper ‘Mitigating The Goldilocks effect’ showing how different animals can or can’t leave tracks in various substrate models for our FEA simulations (See the full paper for details)

 

Anyway, it’s a fairly niche paper perhaps, so I won’t bang on about it too much here. Instead, go read the paper itself: http://rsos.royalsocietypublishing.org/content/1/3/140225

 

 

*The paper was available as advance online article from 2010
**This is not universally liked among ichnologists, which is absolutely fair enough. The reviews for this new paper are all open with the online article, and you can see for yourself.
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