Neoichnology II: Revenge of the Scorpion

While organizing my geo-picture collection, I ran across a Death Valley picture I hadn’t shared that ALSO tied in with the whole neoichnology trend from yesterday. The picture below is from the dune field in Death Valley, and shows a scorpion track left in a fine-grained sand substrate:


You can see the somewhat confusing scratch marks along either side of the trail; I guess having several pairs of legs skittering away all at once makes for some convoluted footprints. The central furrow in the trackway is from the tail. I’ve been told, though I’d have to look up a cite to be sure, that scorpions commonly only produce tail-dragging marks at night (or when it’s cool and shady), and will hold their tails up off the ground during the day. Thus, in SOME cases, you might be able to tell whether it’s night/day in the rock record depending on the scorpion trace fossil.

Seeing this picture again reminded me of a paper I had read a while back. Davis et al. (2007) wrote up a pretty nice summary paper of some experimental work on the neoichnology of some modern terrestrial arthropods. The point of their work was to investigate the effect of substrate conditions on both the morphology and taphonomy of the resultant traces. They used a variety of trace-making bugs (including Giant African Millipedes, Cockroaches, Tarantulas, Woodlice, and some Emperor Scorpions) to investigate the generalized range of bug bauplans, and they used a range of grain-sizes and moisture content to simulate varying substrate conditions, producing two taphoseries: A dry- to dampground series, meant to mimic fully subaerial conditions, and a soft- to firmground, meant to mimic a transitional state similar to a recently flooded overbank setting.

The approach used to produce the substrates was one I had never encountered in the literature before, and seemed fairly rigorous. For the subaerial setting, they simply sprayed an amount of water onto the substrate, and then the critter walked across it. For the transitional setting, though, they put 2 cm of sediment into the tray, removed it, filled the tray with 2.5 cm of water, and then sprinkled the sediment back into the tray. After allowing it to stand overnight, the siphoned off the water, and then proceeded to dump the animals into the experimental setup at regular intervals after the siphoning (0 mins, 60-75 min, 120-150 min).

The picture below is taken from Davis et al (2007; p. 292), and shows the dry to damp series for the Scorpion:


This picture is from Davis et al (2007; p. 293) and shows the soft- to firmground series:


The authors conclude that the increasing firmness of the substrate, related mostly to moisture content, exerted the largest control on the resultant morphologies. Overall, the authors saw a decrease in track width (or track row width) with increasing moisture. They also found, unsurprisingly, that big heavy animals make the best, most preservable trackways in these conditions.

The potential utility of the work is pretty interesting: maybe we could make interpretations of substrate moisture content, qualitatively at least, in some very special trackway settings, letting us get into some nitty gritty paleoenvironmental interpretations. Of course, all the old caveats would apply, least of which not being the fact that we don’t really KNOW how big the animal was or its specific physiology. Still, Davis et al. (2007) suggest a good starting point for this sort of work, and I think make a good case for the importance of careful neoichnological research and its potential impact to sedimentary geology.

WORKS CITED:

Davis, R.B., Minter, N.J., Braddy, S.J., 2007, The neoichnology of terrestrial arthropods: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 255, p. 284-307.

Burrowing Mechanics

It's International Trace Fossil Day (not really), so let's talk about bioturbation!

One of the perennial problems with animals burrowing into sediment has been the presumption that, energetically, it is way more costly than walking, running, swimming, or flying, especially when you are burrowing into cohesive sediment. As such, it has always been a little tricky to understand why any critter would be willing to expend so much energy in evolving into a benthic bioturbator. Additionally, part of the problem seems to have been that physically studying the energy use of a burrowing critter in situ is a bit tricky. As such, folks in the past used Newton's Third Law as an assumption, and always seemed to come up with burrowing as a very high cost (metabolically) biological activity.

However, some fairly recent studies by Dorgan et al. (2005) and Dorgan et al. (2007) have overturned some of these assumptions through novel and (to me, at least) insightful experimental design. To give away the punch line, burrowing is actually much easier than we had previously thought, since animals can use crack propagation to move through muddy sediments, expending much less energy than the previous models of whole-animal burrowing indicated.

First off, both of these papers point out that previous studies of animal burrowing always took place with the animal near a rigid wall (like the transparent edge of an aquarium); as such, the animal behavior exhibited was not, energetically, the same as moving through deformable sediment. In order to overcome this wall effect, the workers used a transparent gelatin as their muddy sediment analog, which has similar mechanical properties to marine muds. Also, and this is the slick part, gelatin is birefringent, meaning that the workers could look at it through polarized light and clearly track the deformation occurring around the critter AS IT HAPPENED. Nifty, huh? The picture below (and the caption) was seized from the Dorgan et al. (2005) paper.


The worm (Nereis virens) moved through the gelatin by exerting a dorsoventral force against the walls of its burrow, resulting in an oblate hemispherical crack; stresses are concentrated at the tip of this crack, and exceed the critical stress needed for the crack to propagate. In otherwords, the worm creates a wedge-driven fracture (like an axe being struck into a log), rather than actively excavating a vacuity in the sediment. The picture below, also from the Dorgan et al. (2005) paper, explains the whole process visually.



The results of this study match (or exceed) modeled critical intensity stress values for sediment, suggesting that crack propagation processes are a viable burrowing mechanism. It's a pretty slick study, I think, and shows how some of the benthos may do their burrowin' in the sediment.

WORKS CITED:

Dorgan, K.M., Jumars, P.A., Johnson, B., Baudreau, B.P., and Landis, E., 2005, Burrow extension by crack propagation: Nature, v. 433, p. 475.

Dorgan, K.M., Arwade, S.R., and Jumars, P.A., 2007, Burrowing in marine muds by crack propagation: kinematics and forces: The Journal of Experimental Biology, v. 210, p. 4198 - 4212.

Hippo-turbation

When I started this blog, waaaaaaay back in February, I did a post about the Okavango delta, a geomorphically/stratigraphically contentious little piece of real estate in Botswana. A comment under that post, by Brian of Clastic Detritus fame, mentioned his curiosity regarding any studies that had looked at the sedimentological and geomorphic effects of some of the charismatic megafauna in the region. Well, I recently ran across a paper that addresses just that, so I thought I’d share (there’s a full citation at the end of the post, and if anyone actually wants the paper, I’d be happy to e-mail them a pdf).

McCarthy et al. (1998) state that hippopotamus in the Okavango play an important part in the geomorphology of the distributary system, primarily for the following reasons:

1) Hippos are big

2) In order to get that big, Hippos eat a lot of grass, which they prefer to get in the form of short grasses that they keep cropped by their constant grazing.

3) Hippos are gregarious, so they’re a bunch of them in any one spot

4) Hippos hang out near channel, preferentially.

That’s a pretty comprehensive list of hippo characteristics, if you ask me. Because of these attributes, Hippos have the tendency to move around in search of food, forging paths through the vegetation that are maintained by continuous hippo traffic.

The authors point out that there are lots of big grass eating critters running around in groups in the Okavango (such as Elephants or Water Buffalo). The important difference between the trails of these critters and the trails of Hippos lies in their different ecologies. Elephants and Buffalo are primarily trying to get from island to island, so their paths through the swamps and backwaters of the Okavango tend to cut perpendicular to the regional gradient. Hippos like to stick close to the water, and so they tend to produce trails that parallel their favorite trunk channels and therefore, run parallel to the regional slope. The picture below is from page 49 of the Mcarthy et al. (1998) paper, and shows the short, parallel trails of more terrestrial vertebrates being cut be long trails running from left to right; these trails are hippo paths.


Because of the parallel-to-regional-slope nature of the hippo trails, the authors posit that these artificial channels, kept free of vegetation by hippo activity, could play an important role in channel avulsion. Their evidence here is a little sparse, though they cite a historical avulsion in the delta that they infer to be hippo-mediated, at least in part.

In addition to channels, Hippos also tend to produce cleared paths in the lakes and quiet-water regions of the Okavango. The picture below, form page 51 of McCarthy et al. (1998), shows a hippo track in a dry lakebed in the delta region.


It would be nice, of course, if someone would go through and survey in the cross-sections of these hippo-channels, both within the course of a single season as well as across multiple seasons. Detailed sedimentology in the channels would also be a nice touch, particularly if you could demonstrate a hippo channel getting utilized as a major sediment routing pathway in the delta region.

And I think it’s good to keep in mind the important lesson of this sort of work: the word Hippo-turbation is hilarious. And at the end of the day, hilarious jargon is all that really matters.

McCarthy, T.S., Ellery, W.N., and Bloem, A., 1998, Some observations on the geomorphological impact of hippopotamus (Hippopotamus amphibius L.) in the Okavango Delta, Botswana: African Journal of Ecology, v. 36, p. 44-56.