August 12, 2012
Mini paleo lesson for the nerdly reader!
Note – spotty interwebs and the site was down for maintenence yesterday evening. This is from Saturday night.
Photos coming but maybe after dinner tonight.
As I said earlier, in the sea, little animals make rocks. Limestone is a rock usually made chiefly from the shells of little boneless animals on the seafloor. Clams, snails, corals, and less famous critters like crinoids, bryozoans, and brachiopods. We have many tasks when we approach these in the field.
Paleontologists first typically want to know what kinds (species, if they can be identified that specifically) of animals were living at a certain time. Old-school paleontologists approach this by making a “range chart”; determining how long each given animal group lived. Based on this body of work, over 100 years old, I can say, “Oh, I found Psiloceras tilmani, a coiled ammonite [like a squid] shell. These rocks are from the beginning of the Jurassic.”
My approach is very different, and serves very different purposes. I am an ecologist. I want to know, after this mass extinction, what were conditions like for animals? I want to know about abundance – how many? And when I ask “what type of animal?” what I mean is “did animals burrow? Did animals live on the seabed? Did animals live together or keep their own territories separately?” Usually I go to places where other geologists have already made a list of the species of animals present, but these other important observations are left for me.
Ecology isn’t complete without a sense of the environment in which the animals lived. Was it a fast-moving shallow sea coast, by the beach? Was it a quiet deep setting with little flurries of mud and the occasional storm-generated dump of shells?
In this picture, in the center, is a small white clam – the kind that burrows into the seafloor. It was found by our driver, Raul! Surrounding it is “matrix”; the sand or gunk on the seafloor, and other “clasts”; the big chunks. Oh, and a TON of living lichen, a fungal/algal association that makes it really tricky to see fossils some times. This layer of rock is about 8 inches thick, full of small pieces of busted-up clam and scallop shells, and is a distinctive laterally consistent layer surrounded above and below by finer, muddier material. This information fits a “storm bed”; when there’s a storm, waves in the shallows bust up shells and smear them out into deeper water. It’s a great snapshot of animals that lived nearby, but it has important caveats.