The blown-up guitarfish
final line drawing and partial shading in digital art; all artwork by Freddi Spindler
Apolithabatis seioma means exactly that: blasted guitarfish. The only known specimen of this new ray species was indeed discovered during a blasting operation. It is an extremely well-preserved female, recognisable by the absence of the claspers, the paired copulatory organs on the pelvic fins. Cartilaginous fish perform internal fertilisation, which is why complete skeletons can be immediately assigned to their sex.
The discovery of complete fossils is by no means a matter of course. This is because the skeletons of chondrichthyans are cartilaginous, not bony. After death, they disintegrate more easily into single parts and these skeletal elements dissolve. Even if they calcify during ageing, entire vertebral columns or even whole skeletons are very rare. The most common remains of cartilaginous fish are their teeth or the anatomically tooth-like skin scales.
Some complete rays from the Late Jurassic period can be found in the limestones of the Altmühl Valley in Bavaria. This is significant because rays are known only from the Jurassic or younger. We are therefore looking at an early chapter of their evolution, even though they had obviously already evolved into fully recognisable rays by then. They were embedded rather quickly in the limestones by a calcareous sediment on the sea floor, mainly favoured by the widespread absence of scavengers. This is because these fine-layered limestones formed under low-oxygen conditions, which would have meant suffocation for the usual consumers such as fish or crustaceans. The actual habitat of the rays was the even shallower mud plains between the reefs with plenty of oxygen.
Formerly, the Jurassic rays were classified as guitarfish, to which also modern rhinobatids belong. This seems logical, not least because they share the triangular head shape (in fact, the contour is determined more by the fin margin, i.e. the extremely enlarged pectoral fins). However, the first description of Apolithabatis and the analysis of its relationships shows that the Jurassic rays formed their own group, named Apolithabatiformes. This means, firstly, that they are not rays, at least not crown-group rays, but representatives of the stem group. And secondly, in many respects they represent the ancestral appearance of rays, i.e. the modern crown group, which are also known from fossils dating back to the Cretaceous period. The outward resemblance with the true guitarfish seems to be primordial.
This does not mean that Apolithabatiformes were ancestral rays in terms of their entire lineage. Those are still only documented by teeth and scales, without telling us when and how the typical body shape has been established, when the evolutionary line of rays had just split off from that of modern sharks. So there is still a lot of work to be done – even on Apolithabatis! Mapping the teeth and scales on this sensational fossil can help us understand the variation of scale types within the same body.
After all, Apolithabatis stands for a particular ecotype, namely the bottom-dwelling and bottom-feeding cartilaginous fish, which was already fully established by the Late Jurassic, survived the transition to the Cretaceous, then diversified even more and even survived the mass extinction at the end of the Cretaceous with a number of species.