Rock and life: Trace fossils
I know it sounds lame to bring out this excuse (yet again) … but it’s the truth! I have to hand in my dissertation to my committee in less than a week and simply don’t have the time for a real post. But…I couldn’t bear not contributing to this edition of The Accretionary Wedge (#3), which is about the intersection of geology and biology…of rock and life. It’s being hosted at the blog The Other 95%, check it out here.
I’d like to live beneath the dirt
A tiny space to move and breathe
Is all that I would ever need
lyrics from “Dirt” by Phish
For the rock and life edition of TAW, I’m gonna show you some photographs I’ve taken from my own field research showcasing trace fossils.
Trace fossils are not body fossils; they are simply tracks and traces of organisms preserved in the rock record. Human footprints in sand or mud, if lithified and preserved, are trace fossils. Most of my research is about the sedimentological characteristics and processes/mechanics of deposition, but the traces are there. In some cases, trace fossils are extremely useful for paleoenvironmental reconstruction or other paleoecological considerations.
Remember, these are not body fossils. What we are looking at is evidence of behavior of the organism Were they burrowing to escape something? Were they grazing for food?
Alright…here we go. Most of these are from Cretaceous turbidites in Patagonia. The captions are below the photo. If you click on them, you’ll get a slightly bigger version.
This is a bedding-plane view (i.e., looking down on top of the bed). Note the nice cross-cutting relationships of these various traces.
This is now looking at the cross section of a sandstone bed. This is a near vertical sand-filled tube with some internal structure. Ichnologists (those who study trace fossils) have given names to the myriad types of traces. This one is called Ophiomorpha. In modern settings, very similar burrows are made by a type of shrimp.
Another sand-filled vertical tube.
Another bedding-plane view of criss-crossing and branching traces.
This is a big one! Another Ophiomorpha (the divisions on the staff are 10 cm).
This is a mud-matrix conglomerate deposit. The light-colored, discontinuous vertical streaks are sand-filled burrows. Note how some are curving around the pebbles and cobbles (whatever the little buggers were could not have burrowed through them).
This one (only one not from Patagonia; is from Mississippian of New Mexico) is a bedding-plane view of some very intricate and beautiful traces called Zoophycus. Ichno-geeks interpret these as representing grazing traces.
Finally…this one looks boring because it is so burrowed that you cannot distinguish individual traces. It’s completely bioturbated (what a great word). A high degree of bioturbation is typical of a shallow marine setting where there is plenty of oxygen and food making burrowing creatures very happy.
That’s it! Rock and life!
The archive website for The Accretionary Wedge is here … please go and use it as a place to discuss future ideas and to organize hosting.
Must. Finish. Dissertation.