Paleontology basics: calcium carbonate outlasts chitin

I have no academic background in paleontology so I found this simple fact fascinating:

Trilobite populations were once thought to have composed the majority of aquatic life during the Cambrian, due to the fact that their calcium carbonate-rich shells were more easily preserved than those of other species which had purely chitinous shells.
---- Ward, Peter (2006). Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere

Considering the fact that chitin is a polysaccharide, a long-chain polymer derived from glucose, I can certainly understand why it would break-down more easily than a calcium carbonate shell.

I realize that this is no doubt a yawn-inducing post for many PS participants—but for this has-been linguist, it sure helps explain the “Cambrian explosion.” If someone produced “fast fact flashcards” for paleontology basics, this kind of concise explanation would be a big help for amateurs like me.


Your mention of paleontology flash cards reminded me of this. I was at a conference in Colorado earlier this year and the USGS had a table. They gave away these neat bookmarks showing the divisions of geologic time. I thought it was a great idea to have all of that on a bookmark. I picked up a few. Here is the pdf


A basic, and I mean a basic understanding of taphonomy answers a lot of arguments and questions regarding the Cambrian radiation.


My favorite lack of fossil argument comes from Stephen Meyer. Basically it’s, “we have fossilized sponge embryos in the Precambrian. If they can fossilize then everything else can. Where are the fossils?”
Think of the absurdities this leads to. I’ve worked on mosasaurs that are missing some verts. Now the other verts fossilized. Why didn’t these? Am I supposed to conclude this mosasaur had less verts than others?

They also seem to think you can just go anywhere and put a shovel in the ground and find fossils. Have to find the right rocks. And you have to find them before the fossils are eroded away. Hope the landowner lets you get on their property to look (do you know how many discoveries haven’t been made because landowners wont let us on?).

Some fossil sites are in the middle of dangerous areas and you can’t really access them. Finding fossils is very very very hard.


Currently working on a pterosaur here. Try to imagine what it’s like after it rains there. So much wash out. Then we have to read the slopes correctly to locate the fragments

Edit: all those things you see sitting on the surface are ghost shrimp burrows


@T.j_Runyon, would you have an approximate number of “sponge embryo fossils” that are documented? My conjecture is that there are very few (and likely a fluke, even with silica or calcium carbonate structure), but Meyer et al can still claim “we have sponge embryo fossils!” Meyer proceeds to assume that since sponge embryo fossils exist, then the current fossil record we have is near-complete, thus supporting the claim that major groups of animals “popped up” with virtually no fossil record. I think this assumption is woefully short of accurate, though. Would you agree?

Yep. If the fossil record was complete or even nearly complete we wouldn’t be discovering the amount of new species we do each year

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As far as the amount of embryos goes I’m not too sure but I can find out

It probably isn’t worth the effort. I was just curious if the case is as I suspect - deep conclusions based on a minimum of evidence.


Meyer is probably referring to the Doushantuo fossils, which may be embryonic sponges. They’re preserved as phosphate (apatite) cells with none of the original material, probably the result of bacterial action. Of course that doesn’t fit his thesis; phosphatized fossils require very special and rare circumstances, and the Doushantuo only preserves very small organisms.

I thought you made a very important point here:

You say: “If tiny little bacteria can be preserved, reasons Meyer, then no large animals should remain unpreserved. Can anyone be this naive about taphonomy? I suppose so. But different taphonomic conditions preserve different things; what preserves bacteria doesn’t necessarily preserve animals, and vice versa”


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