A Guide to Mosasaur Scales

I've finally finished what is probably my most rigourous reconstruction yet. This is intended as a guide on how to depict the scales of mosasaurs, which I frequently see oversized in paleoart, and is primarily based on perhaps my favourite specimen, the LACM Platecarpus specimen*, as well as the skin impressions of Ectenosaurus and Tylosaurus. My depiction of Platecarpus itself here is based on the skeletal provided in Lindgren et al. (2010), and the close-ups of the scales of different areas of the body are based on the descriptions and measurements provided in the same paper, as well as on photographs of the specimen which were kindly provided to me by , who also helped me to get the soft tissue anatomy of the animal as accurate as possible (mainly in the position of the nostrils, the possible lack of external ear openings in mosasaurs, and the lack of a dorsal fin). All scales were drawn at 2x actual size (note to self: never set out on a project that requires the drawing of hexagonal scales again).


*Please note that this exact pattern of scalation cannot necessarily be extrapolated to all mosasaurs, and there is of course room for speculation. This is merely meant to get across that mosasaur scales are usually not uniform across the body, and to show their small size in comparison to the size of the animals.

The scales on the tip of the snout are subhexagonal in shape and do not overlap, and judging by photographs of the specimen they appear not to contact each other. The face is covered in surprisingly large thick rhomboidal scales (the shape seen on the entire body apart from the very tip of the snout), which may be specific to Platecarpus, however given mosasaurs seem to have been prone to face biting as seen in several specimens of larger mosasaurs like Tylosaurus, it is possible that they were much more widespread in the group and were an adaptation to deal with this behaviour.

Past the orbits, these scales shrink in size until the reach the dimensions of the overlapping scales that cover the trunk, which are extremely tiny and were likely difficult to make out as more than just a rough texture unless you were very close on the living animal. It is worth noting here that of all mosasaurs for which we have skin impressions, Platecarpus's scales are the largest in proportion to its body size, so those of most other mosasaurs were likely somewhat smaller in general (but only by a few millimetres) and this is the case in Ectenosaurus, Tylosaurus, and Plotosaurus. In several taxa, these trunk scales are keeled, however in Platecarpus itself they seem to lack keels on all areas of the body which are preserved. These types of scales continue down the tail until the fluke.

The paddle scales are similar to those on the trunk in shape and size, though they get progressively larger towards the distal ends of them. The scales on the upper lobe of the fluke seem to have been similar to this, but those on the lower lobe are unusually tall for their length (roughly 10x5 mm).

Most of what is seen in the skin impressions of Platecarpus is also seen in other mosasaurs, however, I have chosen to illustrate Platecarpus for this guide as it is the only one for which we have scale impressions from all the different areas of the body, and so it enabled me to provide much more detail on mosasaur scalation than any other species would. As a last note, you will notice that this Platecarpus does not have any external ear openings, unlike what is typically seen in mosasaur reconstructions. The reason for this is that many marine tetrapods lack external ear openings, as do all snakes, which are some of the closest relatives of mosasaurs. This is however speculation and we do not know one way or the other what mosasaur ears were like, so artists are free to choose which way to depict them. I hope you all like this piece, I am really quite happy with how it has turned out, and I hope it will be helpful to anyone who wishes to illustrate a mosasaur and needs info on what their scales were like .

Update: Did a quick shading job on the main illustration in Krita. Not entirely happy with it, but this is where it is for now and I think it does look better than it did beforehand; might improve on it at a later time.

References:
Lindgren J, Caldwell MW, Konishi T, Chiappe LM (2010) Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur. PLoS ONE 5(8): e11998.
Lindgren J, Everhart MJ, Caldwell MW (2011) Three-Dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae). PLoS ONE 6(11): e27343. doi:10.1371/journal.pone.0027343