Image credits to Daniele Rios Boleeiro
Written by Mohd Amirul Faisal
The Triassic period marks the dawning age of the dinosaurs. After the catastrophic Permian Extinction that caused 95% of both terrestrial and marine organisms to become extinct around 250 million years ago, the Earth saw another major evolutionary landmark that leads to the emergence of land-dwelling reptiles aka the dinosaurs. However, similar events also occurred within the realms of the oceans as well. It is also where groups of marine reptiles begin to diversify. One of the most widely known examples is the Ichthyosaurs – an extinct group of aquatic reptiles that resembles modern-day porpoises in appearance and habits.
One particular species grew to huge proportions. The discovery of a single specimen that measures up to 21 meters sent shockwaves throughout the scientific community. It belongs to Shastasaurus which becomes the contender as the largest marine reptile in existence. Later phylogenetic studies confirm that Shonisaurus sikanniensis is more related to the Shonisaurus line than the Shastasaurus line.
Regardless, the Shonisaurus is still considered the largest animal that lived during the Triassic period and no Triassic land animal match the immense size of this early Ichthyosaur. But was it an apex predator?
Pangaea – a new beginning
A lot of us are familiar with the existence of a supercontinent called Pangaea. It is the same time when all tectonic plates collided together to form a supergigantic continent that would also create a vast ocean surrounded it. The Phantalassa superocean served as a significant natural habitat for many marine reptiles. This includes the Shonisaurus as well.
It was not until the late Triassic when the separation of Pangea is commenced which leads to the formation of two smaller supercontinents; Laurasia and Gondwana. Scientists speculate that seafloor spreading would be the primary factor that led to rifting events occurring within the initially formed supercontinent.
Oceans that are spreading at large distances made a suitable habitat for these early marine reptiles. The ocean climate was relatively warmer as there were no polar ice caps around. Survivors from the Great Permian extinction, the cephalopods such as squids and nautiloids to name a few, broadly contribute to the growing ecosystem. Predators like the Itchyosaurs became some of the dominant marine animals by the Mid-Triassic. The Shonisaurus is no different as they began to flourish from Mid-Triassic until the Late Triassic. Let us see some of the significant features that separate it from more evolved Itchyosaurs.
An early Itchyosaur with no dorsal fin
It is a massive beast. Capable of reaching enormous sizes similar to that of the baleen whales of today. The Shonisaurus received its name based on the mountainsides from its discovery site. A location that translates to “Shoshone Mountain Lizard“. Perhaps one of the earliest traits that you noticed when observing the vertebrae of the Shonisaurus would be its lack of dorsal fin. Unlike other ichthyosaurs, it has a much bulkier and streamlined body and the ordinary distinct tuna-like profile that is a trait that possesses other ichthyosaurs seems to be absent in the Shonisaurus.
Moving to its tail, as opposed to a vertical, crescent-like tail fin found in most ichthyosaurs species that roamed the seas during the Jurassic period, the Shonisaurus did not have any of those tail flukes. It only indicates that it might have swum in sideways motion like a sea snake rather than a dolphin. Speaking of these mammals, as you have known, it is not a shark nor even a whale. However, it regularly needs to go to the surface to breathe. On top of that, they also gave birth to live young.
Size comparison of the Shonisaurus popularis with a human being. The average length can reach up to 15 meters long. (Credits to: Chen Yu).
If we go down through the evolutionary line, the only significant trait brought upon to more evolved ichthyosaurs would be its massive eyes. Since all ichthyosaurs are predators, having a large pair of eyes is crucial to hunt down fast-moving prey. Since a significant part of its diet comprises an extinct group of squids called belemnites and other fishes that inhabit deeper waters.
Which brings us to the question: does it really capable of hunting fairly sized marine reptiles?
An apex predator or a gentle giant?
The common misconception is that when a large animal has small rows of teeth or no teeth entirely, many people recognised it as a gentle giant that only consumes microscopic animals and soft-bodied prey. That is true for the Shonisaurus a few years ago when analysis of its fossil specimens shows that its jaws are toothless. However, the diagram below might bring a different interpretation.
What you can see is a form of predation between another genus of ichthyosaurs called Guizhouichthyousaurus found in the fossil record of what is now Ladinian (Middle Triassic) Zhuganpo Member of the Falang Formation in Xingyi, Guizhou, southwestern China. The research (which you can refer to here) addressed how predation had begun to become widespread during the early parts of the Triassic when giant-sized marine reptiles such as the Guizhouichthyousaurus preyed upon average sized animals. Of course, most predatory reptiles need to be equipped with razor-sharp teeth to do so.
What does it mean for the diet of the Shonisaurus? A piece of new supporting evidence suggests that Shonisaurus might be akin to a whale-sized crocodile instead of a gentle baleen whale swimming through the waters trapping any microscopic animals as previously thought. It recently gets a makeover when examination of its jaws featuring miniature pointed teeth that are acutely rooted. These teeth serve as tools to consume a colossal amount of its favourite prey including squid and occasionally fish. A compelling discovery were parts of coelacanths that appear as fossilized remains inside its digestive system.
These type of prey also associate with deep water settings. It means that the Shonisaurus identical to the sperm whale can dive hundreds of meters below to hunt for them and might also encounter heavier and far more dangerous nautiloids. As for marine reptiles, however, apparently, we have yet to discover remains of other marine reptiles inside the stomachs of the Shonisaurus. But it would not be surprising if we did base on its serrated rows of teeth.
Shonisaurus vs Nothosaurus – who would win?
Artist’s illustration of a Nothosaurus chasing a Mixosaurus while at the same time, a Shonisaurus is about to clench its jaws on this early ancestor of giant plesiosaurs. (Credits to Kuzim).
The Nothosaurus is another aquatic reptile that coexists with the Shonisaurus for around 50 million years until the Late Triassic. It may be smaller compared to the star of today’s article. However, its jaws coupled with several teeth indicate it’s an animal with a carnivorous appetite. Reaching only up to 4-5 meters long, you would think it is no match for the Shonisaurus. Palaeontologists have discovered fossil specimens containing juvenile placodonts and other marine reptiles inside the digestive systems of smaller species of nothosaurus.
What if both are to be engaged in a battle among each other? The nothosaurus is an agile predator and having a smaller size provides an advantage for it to attack vulnerable parts of the Shonisaurus, before attempting to land the final blow. Nevertheless, the Shonisaurus has been proven to dive hundreds and thousands of meters below for a few hours.
It means that if the nothosaurus ended up between the shonisaurus jaws, it might try to drown it. Although it is no doubt that the nothosaurus is an elegant swimmer evidenced by its two nostrils, it is not well adapted to the surrounding marine life compared to later plesiosaurs and pliosaurs. The winner? It depends on how long they can withstand receiving attacks from each other. As long as the nothosaurus can avoid the shonisaurus attempts to launch an attack, it will keep slicing small chunks of meat until the animal becomes wounded and tired.
Some of the largest marine reptiles did not originate from the Jurassic nor the Cretaceous period. They first emerged from the earliest age of the dinosaurs. These massive ichthyosaurs can reach magnitudes of the largest toothed carnivores on earth; the notorious sperm whale. The quest to search for a marine reptile that dwarfs the blue whale has yet to be solved.
References:-
- Black, R. (2021). Shonisaurus Gets a Makeover | Hakai Magazine. Retrieved from https://www.hakaimagazine.com/news/shonisaurus-gets-a-makeover/
- Cheng Ji, Da-Yong Jiang, Ryosuke Motani, Wei-Cheng Hao, Zuo-Yu Sun & Tao Cai (2013) A new juvenile specimen of Guanlingsaurus (Ichthyosauria, Shastasauridae) from the Upper Triassic of southwestern China, Journal of Vertebrate Paleontology, 33:2, 340-348, DOI: 10.1080/02724634.2013.723082
- DiPiazza, C. (2017). Shonisaurus: Beast of the Week. Retrieved 27 May 2021, from http://prehistoricbeastoftheweek.blogspot.com/2017/01/shonisaurus-beast-of-week.html
- Jiang, D., Motani, R., Tintori, A., Rieppel, O., Ji, C., & Zhou, M. et al. (2020). Evidence Supporting Predation of 4-m Marine Reptile by Triassic Megapredator. Iscience, 23(9), 1-17. doi: 10.1016/j.isci.2020.101347
- McMenamin, Mark & Hussey, Meghan & Orr, Lydia. (2016). ICHTHYOSAUR COPROLITE WITH NAUTILOID: NEW DATA ON THE DIET OF SHONISAURUS. 10.1130/abs/2016AM-284943.
- Strauss, Bob. (2020, August 25). Nothosaurus. Retrieved from https://www.thoughtco.com/nothosaurus-1091514
Et illuminare 