Drawing a shark could be fun, so why not following a step-by-step tutorial for it? As many suggest, let`s start by deconstruct your soon to be shark in simple geometrical figures. A triangle can be the head, a dorsal fin or a tail. One or two rectangles for the body, and the general shape is pretty much already in front of you. It could happen that the result of your creative work led to an overall wacky sketch. No reason for stopping here or erasing everything: after all, the tree of life has seen sharks and akin of all kinds and shapes, so there is no reason to blame your drawing skills or your imagination. Let’s keep drawing. As you finish, your shark will probably look like a mix of gills, eyes, with some fins here and there. The next step would require passing from this postmodern art piece to a silhouette, in few words to create the actual shark. However, you can`t deny that the figure you drew is already pretty nice, maybe adding more would actually ruin it. So, you take your pencil, your marker or your preferred drawing instrument of choice, and you place it back to admire the masterpiece you created. Below is the one I have done, surely a weird composition of shapes (Fig. 1).
I drew this from memory, so please forgive me for any missing detail or any other error that I have done. At the same time, it`s also true that to some my sketch could be more or less easily recognizable. This group of sharks is in fact characterized by their overall peculiar anatomy (Fig. 2). So peculiar that such aspect is also partially highlighted in the name of the genus, Oxynotus, meaning “keeled back”. The body of this sharks is in fact laterally compressed, making it triangle shaped if seen from the front. Additionally, the English vernacular name of “rough sharks” refers to the prickly skin typical of the group. Together with the two tall dorsal fins, each having a defensive spine, the small mouth with enlarged lips, and the big nostrils, rough sharks are pretty distant from the typical idea of what people usually consider a shark to be.
Even to these days rough sharks are a mysterious group we know little of. All species reach a maximum size between 1 / 1.5 m. They have a deep-water lifestyle and can be found up to around 700m depth slowly moving over the seafloor to feed on small fish and invertebrates. In this rough sharks are however not so peculiar as Squaliformes, the order they are part of, has many representatives found in deep-water communities. Now that we have a general idea of what a rough shark is, here come the next two questions: when did they first appear? And where are they placed in the big Squaliform “family tree”?
For a long time, rough sharks were grouped inside Oxynotidae, one of the 7 distinct families collectively representing almost 27% of all modern shark species. Obtaining a clearer picture has however proved to be tricky as both molecular and, unsurprisingly, morphological analyses often led to inconsistent results. Only in 2015, Nicolas Straube and his colleagues managed to resolve this taxonomic dilemma. New molecular datasets were examined, and with that Squaliform sharks “lost” two families. Bramble sharks (family Echinorhinidae) were proved to be closer to angel sharks and sawsharks, thus excluded from Squaliformes. Rough sharks, once on their own, were found to be sleeper sharks (family Somniosidae) of which the Greenland shark (Somniosus microcephalus Bloch & J. G. Schneider, 1801) is one of the most iconic representatives (Fig. 3).
How did 4 of the 5 Squaliform families end up at ocean`s bottoms it`s equally complex. Molecular data support an Early Cretaceous ( Million years ago) origin of the group. However, the squaliform fossil record from this time originates from localities that originally were from shallow water environments. The colonization of deep-seas would have happened much later during the Late Cretaceous, and independently for each lineage. Major causes for these are to be found in oceanographic changes, something that has shaped Squaliform diversity up to what we can see today. However, this radical ecological shift could have also been facilitated by the sharks ‘ability of creating light, or in other words “bioluminescence”. While its origin and spread in the group is still unclear, bioluminescence plays an important role in intraspecific communication, camouflage and predatory strategies. As lantern sharks (family Etmopteridae) and kitefin sharks (family Dalatiidae) still retain the character, this could have been vastly lost in the other Squaliforms, rough sharks included.
Cover picture: Caribbean roughshark (Author: Nakedape13, used under CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0/)
References
- Straube, N., Li, C., Claes, J. M., Corrigan, S., & Naylor, G. J. P. (2015). Molecular phylogeny of Squaliformes and first occurrence of bioluminescence in sharks. BMC Evolutionary Biology, 15(1), 162. https://doi.org/10.1186/s12862-015-0446-6;
- Marion, A. F. P., Condamine, F. L., & Guinot, G. (2025). Bioluminescence and repeated deep-sea colonization shaped the diversification and body size evolution of squaliform sharks. Proceedings of the Royal Society B: Biological Sciences, 292(2042), 20242932. https://doi.org/10.1098/rspb.2024.2932;
- Compagno, L.J.V. (1984). FAO Species Catalogue. Vol. 4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 1 – Hexanchiformes to Lamniformes. FAO Fish. Synop. 125(4/1):1-249. Rome, FAO.
