Thanks to Monday’s article on the unusual African mosasaur Goronyosaurus, I will admit that I was – quite seriously – considering doing a ‘mosasaur week’, perhaps even a ‘weird mosasaur week’. Alas, I have not had the time. However: brand-new in the journals is Johan Lindgren et al.’s article on the skin of a Plotosaurus bennisoni specimen from the Upper Cretaceous Moreno Formation of central California (Lindgren et al. 2009) [adjacent Plotosaurus life restoration from wikipedia]. The specimen was collected by Anthony Fiorillo in 1993 and belongs to an individual that was about 6.4 m long when complete. The most interesting thing about it is its skin. Let’s see why.
Plotosaurus has already been the subject of much attention recently, as new work has shown that it had a deep, relatively stiff thorax and (apparently) a specialised, semilunate tail (Lindgren et al. 2007). Incidentally, Plotosaurus and Goronyosaurus are apparently sister-taxa within the mosasaurid clade Plotosaurini (Conrad 2008). Preserved in association with the specimen’s bones are patches of skin: as you can see from these images (Fig. 1 from Lindgren et al. 2009), they are beautifully preserved, though don’t be fooled into thinking that you’re looking at a colour pattern (instead, the dark colour represents preserved organic matter) [note that (d) shows the scales of an extant skink for comparison].
Mosasaur skin is actually not a new discovery: Snow reported it in 1878, Samuel Williston illustrated it in his 1914 book Water Reptiles of the Past and Present [see image below], and various workers have drawn attention to it in specimens of the derived mosasaurid mosasaurs Tylosaurus and Platecarpus* (Snow 1878, Martin & Rothschild 1989). Preserved skin has also been reported in basal mosasaurs like Vallecillosaurus from Mexico (Smith & Buchy 2008). Having said all that, it’s rarely discussed in the literature and all too few people know about it. The new skin specimen is preserved three-dimensionally. You might predict that derived mosasaurs, being fully aquatic and strongly streamlined, would have smooth, scale-less skin. Though there is still a great deal of uncertainly, a naked, scale-less epidermis has also been suggested for other Mesozoic marine reptiles, like ichthyosaurs and plesiosaurs. Ichthyosaurs apparently did lack scales, but we’re still waiting for more information on plesiosaurs (apparently, the New Zealand cryptocleidoid Kaiwhekea includes skin impressions [Arthur Cruickshank pers. comm.], but I don’t think these have ever been published). In fact some mosasaur workers have indeed suggested that at least some mosasaurs were naked-skinned, simply because it has been assumed that an absence of scales might improve hydrodynamic efficiency. Well, forget it: it seems that all mosasaurs were scaly.
* Lindgren et al. (2009) state that the Plotosaurus skin is ‘the first documented squamation in a mosasaurine mosasaur’ and also ‘the first record of skin in an advanced member of the Mosasauroidea’. I’m somewhat confused by the second of these claims, given that skin preservation has previously been reported in taxa that are indeed ‘advanced’ within Mosasauroidea.
The skin in the new Plotosaurus is covered in scales, though the scales are tiny for the size of the animal (approximately 2 mm x 2 mm). They’re rhomboidal, and arranged in an alternating, overlapping pattern. What’s perhaps most interesting is that the scales have keeled surfaces: a central keel, running along the long axis, is flanked on either side by lower keels. Longitudinal grooves separate all of the keels (Lindgren et al. 2009). Keels have been mentioned on mosasaur scales before, but I don’t think that multiple keels have been noted. Some of the Plotosaurus scales lack any of this surface ornament, and the authors suggest that these scales might have belonged to the animal’s ventral surface.
So, the scales possessed parallel grooves and keels on their surfaces, all of which were aligned with the body’s long axis. The most interesting thing about Lindgren et al.’s new paper is that they note the similarity this has with similar ridge and groove systems seen on the skin surfaces of certain other aquatic vertebrates. They note that the dermal denticles of pelagic sharks [like those shown in the adjacent image] possess a superficially similar ornamentation, and that – in the sharks – this seems to have a hydrodynamic role: by ‘channelling’ the boundary layer of liquid across the animal’s epidermis, the ridges and grooves reduce microturbulence and hence lower overall drag. Some work (which I haven’t seen) demonstrates this experimentally (Raschi & Tabit 1992). I’ve also heard of work on the cetacean epidermis which, similarly, demonstrates that microscopic, sagitally aligned ridges and grooves have an identical, drag-reducing function. I heard about this research on TV years ago and have been unable to discover whether it’s been published or not. The relevant TV programme (which would have been screened in the early or mid 1990s) explained how efforts were now underway to use this system on military submarines. Does anyone know any more about any of this? There is, at least, some published research showing that cetacean skin is microscopically grooved (Purves 1963 and references therein) [dermal ridge pattern in a dolphin shown below, from Purves (1963)].
The obvious inference to make from the parallel grooves and ridges on the tiny Plotosaurus scales, then, is that they served an identical hydrodynamic function (Lindgren et al. 2009). Perhaps the next step is to test this by making models and testing them in flume tanks (real or virtual). What improvement or advantage might drag-reducing scales make on a mosasaur, if any? And would the structures really enhance performance as their supposed analogues do in sharks?
Indeed, we may be jumping ahead too far, and Lindgren et al. (2009) also note that the keels on the scales may have reduced shininess, and hence helped mosasaurs to conceal themselves from prey (and from other predators). They further note that scales may also have helped protect these animals from predators, rivals and ectoparasites. An anti-parasite function is not obvious to me, perhaps it’s something to do with the regularity with which scales can be shed? [Platecarpus image below from wikipedia].
There is one final confound to all this, and this is the phylogenetic perspective: given that keeled scales are ancestral for anguimorphs (the major squamate clade to which mosasaurs and other platynotans belong), are mosasaur scales, in fact, nothing special? And what about the complicating factor that unkeeled scales have been reported from one basal mosasaur? Lindgren et al. (2009) provide answers on this: it seems that multiple keels are derived relative to the primitive anguimorph condition (in which case, mosasaur scales might be highly specialised). Meanwhile, the unkeeled scales reported in one basal mosasaur (Vallecillosaurus) come from the animal’s ventral surface and hence do not demonstrate that keeled scales were definitely absent in this taxon.
So, it’s an interesting paper that proposes an interesting idea. As for me: having just written an article of over 1000 words, I have failed failed failed again.
For a previous article on the skin of Mesozoic marine reptiles see The skin of ichthyosaurs.
Refs – –
Conrad, J. L. 2008. Phylogeny and systematics of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 1-182.
Lindgren, J., Jagt, J. W. M. & Caldwell, M. W. 2007. A fishy mosasaur: the axial skeleton of Plotosaurus (Reptilia, Squamata) reassessed. Lethaia 40, 153-160.
– ., Alwmark, C., Caldwell, M. W. & Fiorillo, A. R. 2009. Skin of the Cretaceous mosasaur Plotosaurus: implications for aquatic adaptations in giant marine reptiles. Biology Letters doi:10.1098/rsbl.2009.0097
Martin, L. D. & Rothschild, B. M. 1989. Paleopathology and diving mosasaurs. American Scientist 77, 460-467.
Purves, P. E. 1963. Locomotion in whales. Nature 197, 334-337.
Raschi, W. & Tabit, C. 1992. Functional aspects of placoid scales: a review and update. Australian Journal of Marine and Freshwater Research 43, 123-147.
Smith, K. T. & Buchy, M.-C. 2008. A new aigialosaur (Squamata: Anguimorpha) with soft tissue remains from the Upper Cretaceous of Nuevo León, Mexico. Journal of Vertebrate Paleontology 28, 85-94.
Snow, F. H. 1878. On the dermal covering of a mosasauroid reptile. Transactions of the Kansas Academy of Science 6, 54-58.