Pseudofossils are an interesting aspect of palaeontology that crop up from time to time and can make life both interesting and frustrating. I have already commented on the problems of identifying fossil bones in the field but these are a bit more specialised. Of course if you has as many different shapes as are available for all the fossils out there (think of the various shapes of the bones in just the human body, then add to that all other vertebrates and their variations, then add in shells, tracks, eggs and the rest) and the sheer number of rocks and pebbles that have odd shapes and can end up looking like fossils and you can probably see where this is going. Basically there are lots of bits of rock out there that are not fossils, but do look just like them.
This is generally only an issue for two reasons – it’s very annoying when you spot something in the field that looks like a fantastic find and turns out to be a lump of rock, and it’s even more annoying when someone brings you (by which I mean generally send you a photo) a ‘fossil’ that isn’t and then they get pissy / whiny / aggressive when you point out to them that it’s a lump of rock. I’ve not been on the receiving end of too much of the latter but I have heard plenty of it from colleagues and have stumbled through a couple of incidents on the web at various points.
To be fair, these things often look remarkably like fossils, and if they actually turn up in fossiliferous bed, it’s easy to understand why people think they are fossils. I’ve certainly seen a couple the required and double and then triple check to confirm that they aren’t bone (though some are obvious, do hunt down the ‘fossil mouse’ on display in the Natural History Museum in London). On that subject here is one that caught my eye in Mexico that I thought a few people might like, though perhaps only if you know what it looks like!
Here is the pseudofossil in question (above), which to many probably looks like the lump of brown rock that it actually is. However, it bears a remarkable similarity to the scapula (the shoulder blade) of various basal reptiles, with subtleties of form you don’t even find in some poor fakes or reconstructions. Just to prove it, here is a nice drawing of the scapula of Stenalorhynchus lifted and modified from the fantastic ‘Osteology of Reptiles’ by the great A.S. Romer. We are only interested in the bone in the upper middle of the drawing hopefully you can see just how similar these things are, or at least how easy it is to mistake one for the other at a quick glance at distance in the field.