Back to front stem tetrapods

Relatively complete, articulated (preserved in life position) fossil specimens are extremely precious items. But sometimes the fact that the specimen is articulated can mean that matrix (surrounding rock/soil/gravel) has to be left in place to ensure it holds together. This remaining matrix can occasionally obscure important morphological features on the bones of prehistoric creatures, leading to incorrect interpretations about the animals morphology.

Fortunately however, this is becoming a less frequent occurrence with increasing use of x-ray scanning technologies that allow palaeontologists to virtually strip away the surrounding rock and see the fossil as it actually is, and realise that previous interpretations may have been incorrect.

This is exactly what has happened in the case of the new paper by Pierce et al. in the prestigious journal Nature. To make matters even more interesting the taxon they were looking at was none other than Ichthyostega, the classic example of a stem tetrapod. More specifically, they have realised that the structure of the vertebrae are actually back-to-front compared to the traditional interpretation.

Using CT scanning, Pierce et al. have realised that what was thought to be the structure of Ichthyostega vertebrae (A & B) is incorrect. C & D show the actual structure of the vertebrae. Neural arches = pink; pleurocentra = yellow; intercentra = green; ribs = blue. Modified from Pierce et al. 2013.
Using CT scanning, Pierce et al. have realised that what was thought to be the structure of Ichthyostega vertebrae (A & B) is incorrect. C & D show the actual structure of the vertebrae. Neural arches = pink; pleurocentra = yellow; intercentra = green; ribs = blue. Modified from Pierce et al. 2013.

As you can see from parts A & B of the figure above, the traditional understanding of Ichthyostega vertebrae was that the paired pleurocentra sat behind the intercentrum with the neural arch on top (known as rhachitomous vertebrae). However when Pierce et al. looked at the CT scans they realised that everyone had been wrong all along. In fact the pleurocentra are actually fused to the intercentrum and are in front of them (reverse rhachitomous vertebrae)!

Intrigued by this finding, the team proceeded to look at two other stem tetrapods, Acanthostega and Pederpes. Both of these were also thought to possess the traditional rhachitomous vertebrae, but… you guessed it, they found that they both possessed the reverse rhachitomous vertebrae too! What this finding does is raise new questions about what we thought we knew about the early evolution of the tetrapod spine. It looks like the textbooks will have to be changed again, but that’s exactly the way it should be in science.

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