One of my favourite mosasaurs is the unusual Goronyosaurus nigeriensis (Swinton, 1930) from the Maastrichtian of Nigeria and Niger. Mosasaurs are Cretaceous marine lizards, (probably) closely related to gila monsters and kin (the monstersaurians) and monitor lizards and kin (the goannasaurians), and well known for evolving gigantic size (>12 m in some taxa), flippers, and paddle-like tails. Goronyosaurus is cool because, with increasing knowledge, it has become weirder and weirder and weirder…
When William Swinton first described and named Goronyosaurus (as Mosasaurus nigeriensis), it was assumed to be essentially similar to the better-known mosasaurs of Europe and North America. Azzaroli et al. (1972) were the first to realise that it was actually an aberrant mosasaur, and they reconstructed it with atypically massive cheek bones and a blunt, rounded snout tip. They also showed that the maxillae (and thus the maxillary toothrows) extended posteroventral to the orbits and that the teeth were set on bony pedicels like those seen in some crocodiles. These features and others led them to coin the new generic name Goronyosaurus. Soliar (1988) contested some of these details and produced a less aberrant-looking reconstruction, but he still concluded that the maxillary toothrows extended posteroventral to the orbits.
Lingham-Soliar (1991, 1999) – note the name change – looked anew at Goronyosaurus and now produced a reconstruction more similar to the Azzaroli et al. one than the Soliar 1988 one. He argued that, during jaw occlusion, the teeth fitted into interdental pits on the opposing jaws, that Goronyosaurus had proportionally small orbits for a mosasaur, and that it had an unusually high number of foramina on some of its skull bones. Lingham-Soliar (1991, 1999) concluded that Goronyosaurus might have had a highly sensitive snout and that it was perhaps specialised for ‘hunting in the quiet murky waters of sheltered bays and estuaries and in preying upon hidden young of marine fauna including perhaps those of other mosasaurs … Further, the very small eyes of Goronyosaurus (relatively perhaps the smallest in the Mosasauridae) suggest that Goronyosaurus did not rely heavily on vision’ (Lingham-Soliar 1991, pp. 667-668). This inspired me to produce the above sketch. Goronyosaurus is shown as a specialised, mud-grubbing mosasaur that has small eyes and relies on enhanced tactile abilities to find prey in the sediment. See below for a rather more satisfying illustration (produced by Dmitry Bogdanov, available via wikipedia).
More recently, it’s been discovered that Goronyosaurus had particularly long teeth in the premaxilla and at the anterior end of the dentary (Lingham-Soliar 2002) though, by ‘particularly long’ I mean that they were only slightly longer than the longest teeth present elsewhere in the toothrow [see image above, from Lingham-Soliar (2002). Scale bar in A and B = 25 mm, in C = 100 mm]. These long, reinforced anterior teeth may have made this mosasaur better at rapid grabbing of evasive, slippery food items.
Tongues and… venom glands?
The speculations made about the sensory abilities of Goronyosaurus have yet to be subjected to proper scrutiny and some of them can be doubted. Furthermore, whatever Goronyosaurus‘s visual and tactile abilities were like, we must remember that mosasaurs were almost certainly equipped with forked tongues, and hence would have had an efficient sense of vomeronasal chemoreception as well (Schulp et al. 2005).
Based on the probable position of mosasaurs within Platynota (the monstersaurian-goannasaurian clade), Schulp et al. (2005) made inferences about the mosasaur tongue, concluding that the bilobed and protrusible foretongue was relatively short, and that the hindtongue was robust, covered in the papillae and mucocytes typical for the squamate tongue, and still with an important role in food transport and swallowing. The mosasaur tongue, they argued, was therefore similar to that of the extant platynotans Heloderma and Lanthanotus [Lanthanotus tongue shown here, from Schulp et al. (2005). Note the bilobed foretongue and papillose hindtongue. Anterior is at the top]. A long and highly specialised forked tongue, like that seen in monitors and snakes, was thought unlikely (incidentally, the long tongues of monitors and snakes are thought to have evolved independently). However, worth noting here is that things change according to the favoured phylogeny. What if mosasaurs are more closely related to snakes that they are to other platynotans, as favoured by some authors?
This leads on to another inference we might make from phylogeny. Were mosasaurs venomous? Until recently we would have said ‘no’, given that mosasaurs lack the dental and osteological correlates of venomosity seen in venomous platynotans like the helodermatids (gila monster and beaded lizard). However, we now know that venomosity is somewhat more widespread in lizards than previously thought, and that venomous mandibular glands evolved at the base of Platynota at least (Fry et al. 2005). If mosasaurs are part of this clade, and they almost certainly are (Conrad 2008), then, yes, venomosity can be assumed to be primitive for Mosasauria. But did mosasaurs retain their venom glands, or lose them during their evolution? We don’t know, but it makes for an interesting speculation.
Refs – –
Azzaroli, A., De Guili, C. & Torre, D. 1972. An aberrant mosasaur from the Upper Cretaceous of noth western Nigeria. Accademia nazionale dei Lincei. Rendiconti. Classe di Scienze fisiche matematiche e naturali 52, 53-56.
Conrad, J. L. 2008. Phylogeny and systematics of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 1-182.
Fry, B. G., Vidal, N., Norman, J. A., Vonk, F. J., Scheib, H., Ramjan, S. F. R., Kuruppu, S., Fung, K., Hedges, S. B., Richardson, M. K., Hodgson, W. C., Ignjatovic, V., Summerhayes, R. & Kochva, E. 2005. Early evolution of the venom system in lizards and snakes. Nature 439, 584-588.
Lingham-Soliar, T. 1991. Mosasaurs from the Upper Cretaceous of Niger. Palaeontology 34, 653-670.
– . 1999. Functional analysis of the skull of Goronyosaurus nigeriensis (Squamata: Mosasauridae) and its bearing on the predatory behaviour of this enigmatic taxon. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 213, 355-374.
– . 2002. First occurrence of premaxillary caniniform teeth in the Varanoidea: presence in the extinct mosasaur Goronyosaurus (Squamata: Mosasauridae) and its functional and paleoecological implications. Lethaia 35, 187-190.
Schulp, A. S., Mulder, E. W. A. & Schwenk, K. 2005. Did mosasaurs have forked tongues? Netherlands Journal of Geosciences – Geologie en Mijnbouw 84, 359-371.
Soliar, T. 1988. The mosasaur Goronyosaurus from the Upper Cretaceous of Sokoto State, Nigeria. Palaeontology 31, 747-762.