Creature Feature 10

Hello, everyone! This week, I've decided to feature a somewhat misunderstood and very amazing squamate. It's time to meet the largest lizard currently known from Hell Creek - Palaeosaniwa canadensis!

Work-in-progress Palaeosaniwa model.

As stated above, Palaeosaniwa canadensis was a very large lizard, reaching an estimated 2 meters in length (Balsai, 2001). Unfortunately, not much is known about this amazing creature, though what is known is very fascinating.

There is one significant misconception about Palaeosaniwa that is relatively widespread - namely, that it was essentially a giant monitor lizard. However, this idea was based on the descriptions of isolated, fragmentary elements, and when more complete remains were found, they revealed that it was more closely related to Helodermatids than it was to Varanus (Balsai, 2001). Still, there are parts of the skull that are missing or poorly preserved, and as such there is still uncertainty as to how closely it resembled modern Heloderma. For example, the anterior end of the snout is missing and as such it is not known whether or not the snout was rounded or pointed (as seen in Heloderma and Varanus, respectively) (Balsai, 2016). One known divergence from Heloderma that is known based on the remains is the proportions of the parietals on Palaeosaniwa were more elongate and less robust, more closely resembling the state seen in Lanthonotus (Balsai, 2016).

Close-up of the head of Palaeosaniwa model. Note the more gradual slope of the snout compared to Heloderma.

Perhaps the most fascinating aspect of Palaeosaniwa is that it may offer insight into the evolution of venom in that group of animals (while originally placed within Monstersauria, that group has been shown in recent analyses (Reeder et al., 2015) to be polyphyletic, and as such I do not know what clade it would belong to). It is likely that Palaeosaniwa was a nest raider, preying on hatchlings and eggs of some of the dinosaurs living in its environment. This is hypothesized due to the similar behavior in its extant relatives, as well as Varanus niloticus, which have even been observed cooperating to raid Crocodylus niloticus nests. Additionally, remains assigned to Palaeosaniwa have been found associated with dinosaur nesting sites, further supporting this hypothesis.

Thus, it is possible that while modern Heloderma primarily use their venom for defensive purposes, it originally evolved as a mechanism for quickly dispatching of the hatchling dinosaurs, as well as offering some protection from their extremely massive and most likely very angry parents (Balsai, 2001). Unfortunately, remains of Palaeosaniwa are not sufficient to determine whether or not it was venomous with any great certainty (Balsai, 2016), and if it were there is no way (as far as I know) to determine the composition of the venom for determining its effects and relative toxicity. Only more detailed remains of the jaw and dentition will give us a better idea as to the validity of this hypothesis.

To sum up, Palaeosaniwa was a very large, possibly venomous lizard that likely preyed on vulnerable hatchlings and eggs of dinosaurs and other nesting animals in Hell Creek. When you play Dinosaur Battlegrounds, keep an eye on your nests in case you find an uninvited guest coming over for dinner!

Well, I hope you enjoyed this week's creature feature! I wanted to go very in-depth with this one, partially because the myth that Palaeosaniwa was a giant monitor lizard has been rather pervasive, and I want to make sure that people have the facts. I also want to give a shoutout to Michael Joseph Balsai, who was extremely helpful with my life-restoration of this species. Thanks to him, I have a better understanding of this amazing creature, and I hope that upon reading this article, you do too!

Acknowledgements:

Balsai, Michael Joseph. 2001. The Phylogenetic Position of Palaeosaniwa and the Early Evolution of the Platynotan (Varanoid) Anguimorphs. Univ. of Pennsylvania - Electronic Dissertations. Paper AAI3031637.

Balsai, Michael Joseph. Personal communication. 2016.
Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood Jr., Perry L.; Sites Jr., Jack W.; Wiens, John J. 2015. Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa. PLoS One 10 (3): e0118199.