Preservation and possible distortion of the Ichthyostega material.
The apparently unique pattern of Ichthyostega’s vertebral column makes it important
to establish that the different neural arch morphologies are not simply the product of
post-mortem distortion. This requires a consideration of the geological history of the
deposit as well as detailed comparisons within and between specimens.
The Ichthyostega fossils occur in clastic sedimentary rocks, mostly siltstones with
some sand, from the Famennian of East Greenland. They are always threedimensional but usually flattened to a greater or lesser degree. The strata are nearhorizontal and show some faulting but little or no folding (pers. obs. PEA, JAC, HB).
As might be expected from this, there is no significant overall tectonic deformation of
the material: this is clearly demonstrated by the numerous Ichthyostega skulls, which,
with only one exception (ref. 5, pl. 6.1) show near-perfect bilateral symmetry
irrespective of whether they are dorsoventrally or laterally compressed5. There is thus
no reason to believe that the neural arch morphologies result from different distortions
caused by variable alignment relative to a universal plane of tectonic deformation.
Small-scale local deformation caused by sediment slumping or similar processes is
another possible cause of distortion: the likelihood of this has to be assessed on a
case-by-case basis. In relation to this we note that the neural spines of all our
specimens are preserved in immediate association with adjacent structures (neural
arch bases, rib heads, limb girdles) that show no sign of distortion beyond a degree of
vertical compression in some cases. For example, in the postsacral region of MGUH
VP 6054 (Figure 3 of paper), the transition from “postsacral fan” of broad triangular
neural spines to slender and anteriorly concave caudal neural spines takes place in
neural arches with neatly aligned, regularly spaced, and evidently undistorted bases,
closely associated with well-preserved ribs and an equally undistorted postiliac
process. Furthermore, neural arch morphology is reproducible (Supplementary Figure
1): neural arches from the same part of the column – easily determined even in
incomplete specimens due to the strongly regionalized ribs – show the same
morphology. The comparison between MGUH VP 6115 and 6139 is particularly
telling in this regard, as the former is substantially three-dimensional whereas the
latter is more strongly flattened and has a vertebral column that has been twisted and
partly disarticulated before fossilization (Figure 4 of paper). We might thus expect to
see different distortion-induced thoracic neural spine morphologies in these two
specimens, but in fact they are very similar (Supplementary Figure 1).
There are other factors, such as diagenetic processes, that can also distort bone
morphology and that we have not been in a position to investigate fully. However, the
available evidence strongly suggests that the observed differentiation of neural arch
morphology is real, not the product of post-mortem distortion.