Tuesday, September 22, 2009

How Do We Know Fossils Were Once Living Organisms?

In the September issue of Geology there is a good give and take (open access) on the origin of early Neoproterozoic carbonate rock textures. The debate revolves around an interpretation made by Neuweiler et al. (2009) that certain cement filled cavity and mud textures in Neoproterozoic carbonate bioherms (carbonate mounds formed by aggregating and colonial organisms) look very similar to younger sponge replacement fabrics found in Palaeozoic and modern bioherms.

Even though there is no recognizable fossil sponge body parts in the studied Neoproterozoic carbonate (little Dal reefs, Canada)  the authors interpret the diagenetic fabric as indicative of a metazoan origin (they say they cannot conclusively link it to any specific sponge taxon) pushing in their view the geological evidence for multicellular animals to around 875 million years ago. This is about 200 million years older than what most scientists acknowledge. Noah Planavsky in the comment section disagrees about the metazoan origins of the texture and suggests that microbiota can also form similar fabrics.

I found the study interesting in itself. I have worked with some pretty complex diagenetic fabrics and it is always a challenge to tease out components that are purely inorganic from those that have a biogenic origin.

I feel though that a study like this fulfills another important role in science - it is of epistemological value. I used to help my PhD adviser with the paleontology exhibit on Science Day at the local mall in Tallahassee and we had people coming up to us and asking " ...but how do you know that this fossil was once a living creature...?"

A study like this tells us about the methodology and chains of reasoning scientists use to gather a body of knowledge about how fossils form. In this case the authors compared sponge remains from modern bioherms with older and older deposits. In the modern bioherms the sponge organism had died relatively recently and the organic tissue and other skeletal parts were still joined together as a coherent organism. Some organic material had degraded and in its place were tiny carbonate crystals. In some internal body cavities carbonate mud had accumulated forming a sort of a cast of the body part. This is a clear indication that as organic material degrades its place is taken up by inorganic material which retains the same shape as the organic matrix.

The researchers then went further back in time and looked at Cretaceous and Paleozoic rocks. In these samples there was no organic matter, that had decayed away long back,  but other typical sponge skeletal hard parts like spicules were still preserved and recognizable as sponge remains. The shape and form of the interior of the sponge the characteristic canal system was now completely filled by cement and mud. So although there was no coherent organism with linked body parts the association of spicules with a cement and mud filled connected cavity system which had a shape just like the canal system of the sponge gives us confidence that we are looking at sponge fossil fabrics.

One can then go one step further as the authors have done and interpret fabrics with characteristic shapes but no sponge remains (no spicules or anything) as having formed by the alteration of a large multicellular creature. That specific interpretation may be right or wrong in this case but that's the way scientists "know" that fossils were once living creatures. It's a good example to use to explain - How do we know what we know?

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