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.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Great photos. I’m intrigued by that massive Ophiomorpha though – why would something want to burrow so deeply into the sediment? Or was there a high sedimentation rate?
Chris, they be just giant tubeworms, like Riftia pachyptila at hydrothermal vents which grow to more 2 meters and live in chitin tubes. There is a growing literature on fossil tubeworms from fossil vent, seep and whale fall sites from the work of Crispin Little and others. Riftia’s body also doesn’t span the entire tube, it forms septa, or membranes as it adds on to the length of the tube.
I’ve always wondered that myself. The beds with the long, vertical Ophiomorpha burrows are single turbidite sedimentation units (i.e., product of one turbidity-current flow event), so the sed rate for that particular bed is very high.
But, to answer your question…I don’t know why a critter would go so deep. There must be some good eatin’ down there (?). I will consult my ichno-geek colleague and report back.
KevinZ…one of the characteristics of the Ophiomorpha trace fossil is that it has a ‘lining’ (usually a finer grain-sized) on the interior walls of the burrow and sometimes an internal structure. The second photo from top shows this nicely…there’s almost a spiral pattern. So, the thought is that animals smaller than the resulting burrow make their way down and line the burrow w/ their fecal matter to give it some structural integrity.
In the modern, studies have shown burrows similar to these (and in similar depositional environments) made almost exclusively by a type of shrimp (callianassid shrimp [see here]). But, the problem we always face is just because that’s what we see now, is that how it was then?
Could it be a worm burrow? Possibly.
When it comes to traces, since they are merely recording behavior, you can have one organism make multiple traces, and you can have one trace made by multiple organisms. There are certainly some common patterns, but there is not a one-to-one correlation between animal and trace.
Love your photos, here and on Flickr. It stands to reason that you’d have good shots of all the sedimentary structures. I have a copy of Ricci Lucchi’s “Sedimentographica” that I keep meaning to study so I can start recognizing these things in the field.
Andrew…I have tons more of fundamental sed structures. Someday I’ll take the time to upload them :)
I’d love to see your sedimentary structure pictures as I am always looking for good photographic examples.
Good luck with your dissertation by the way. I will defend mine on Tuesday. Older, part time student and it took way too much time.
dmonte…good luck to you as well…for now, to see some photos and posts about sed structures, just click on that category title towards the bottom of the sidebar on right
perhaps someday I will organize a separate page on here about that
hi,
I am Abhi form India and trying to some work on trace fossils which is well preserved in my study area like Ophiomorpha,Thalassinoides. i am very much intrested doing some creative work on that so plz help me regarding this. plz send some infmormation or any new topic related to trace fossil on which i can work.
thanks,
regards
Abhi
email Id: geoabhi@rediffmail.com
Abhi … search for papers/books by George Pemberton of University of Alberta in Canada … he has worked on and taught classes about trace fossils for many decades.