Spinosaurs are strange dinosaurs, seemingly a mash-up of a crocodile's head on a dinosaur body with a distinctive sail on their backs (Hone and Holtz 2017). Even though its strange appearance has garnered much attention, there is still very little known about the animal (Hone and Holtz 2017).
Spinosaurs are thought to have roamed from the Jurassic and Cretaceous between 130-90 million years ago, with fossils largely being found in present day Morocco and Egypt (Hone and Holtz 2017). In fact, the first Spinosaur discovered was in Egypt which is the reason its latin name is Spinosaurus eagyptiacus (Hone and Holtz 2017).
One question that is actively debated is if Spinosaurus was an aquatic or terrestrial animal—and scientists have tried using some creative ways to try to find out.
One group of scientists used the amount of an oxygen isotope in the fossils to try to determine how much time Spinosaurus may have spent in the water (Amiot et al. 2020). This is because the amount of oxygen found in the fossils depends on how much time an animal spends in or out of the water while alive (Amiot et al. 2010).
As such, the scientists compared the level of oxygen isotopes in spinosaur teeth to that of crocodile teeth, other large therapod dinosaurs, and turtle shells that were found in the same location and geological time period (Amiot et al. 2010). The idea being that if the O2 content of spinosaur fossils was the same as known aquatic animals, it was also likely aquatic, but if the O2 content was the same as known terrestrial animals, then spinosaurs were likely also land based animals.
The results of the study indicated that the oxygen values in spinosaurs was 1.3% lower than that found in coexisting therapod teeth (indicating they were not purely land based animals), but was also generally quite different from the O2 values found in the teeth of crocodiles and turtles (which suggests they spent less time in the water than crocodiles and turtles) (Amiot et al. 2010). They also noted the amount of the oxygen isotope in the spinosaur fossils tended to vary depending on the location where the fossils were found (Amiot et al 2010). In some instances the amount of oxygen found in the fossils overlapped with crocodile and turtles that lived in the area and sometimes it overlapped with the therapod dinosaurs (Amiot et al. 2010).
What the authors took this to mean was that spinosaurs were actually more likely semi-aquatic animals that spent more or less time in/near the water depending on the circumstances (Amiot et al. 2010).
Further evidence for a semi-aquatic lifestyle can be found in the anatomy of the spinosurus skull. Arden et al. (2019) found that the eyes of Spinosaurus were located at the top of the skull which allowed for the eyes to be above the water when the animal was submerged—similar to hippos or crocodiles (Arden et al. 2019). They also noted that the olfactory bulb (part of brain involved in sense of smell) of Spinosaurus was similar to other animals that spend time near water (Arden et al. 2019).
Another team attempted to show that Spinosaurus eagyptiacus was not an aquatic animal. To do this, they first defined an "aquatic lifestyle" as one in which Spinosaurus walked on four legs on land, and dove and chased prey using its tail as a propulsion device when in the water (Sereno et al. 2002). They also said that if Spinosaurus eagyptiacus was truly living an aquatic lifestyle, its fossils should also be found near ancient deep water coasts and marine habitats (Sereno et al. 2022).
Then, Sereno et al. (2022) took CT scans of fossils of S. eagyptiacus and another Spinosaur called S. tenerensis in order to get a 3D reconstruction of the skeletons. With these CT scans, they found that many of the fossils had air spaces (which are commonly used in birds and other dinosaurs as a way to reduce the weight of the skeleton) (Sereno et al. 2022). The scientists then added muscle to the virtual skeletal reconstructions based on living reptiles. The scientists even attempted to add the trachea and lungs and made three different models based on three different ways in which the trachea and lungs could be adjusted/sized based on living squamates, crocodilians, and birds (Sereno et al. 2022). They then calculated the center of gravity and center of buoyancy for these three models in addition to calculating the estimated maximum force that the tail could generate (Sereno et al. 2022). The authors also examined where the fossils were distributed geographically to see whether or not the fossils were mostly located around ancient coastal areas (Sereno et al 2022).
In regards to the whether Spinosaurus walked on two or four legs, the authors found the center of mass of the animal to be in line with its hind legs. This indicates that Spinosaurus likely spent most of its time walking on two legs (Sereno et al. 2022)—if the center of mass was further forward, the Spinosaurus would tip forward and require walking on four legs. Furthermore, the authors found that the front feet of spinosaurs appear to be shaped for grasping prey rather than bearing weight which points away from spinosaurs being quadrupeds (Sereno et al. 2022).
In regards to the possibility of living an aquatic life, the authors found numerous indications that seemed to suggest that Spinosaurus did not live a completely aquatic life. Firstly, they mention that the spine of Spinosaurus and in particular the large spines on its back would not have been flexible enough to allow the animal to perform a dive from the surface of the water (Sereno et al. 2022). Secondly, they also found that the density of S. eagyptiacus was less than the density of water (Sereno et al. 2022). The density of water being 1000kg/m3 whereas the density of Spinosaurus was 833kg/m3 (Sereno et al. 2022). This is also much less than the density of crocodiles which is approximately 1080kg/m3 (Sereno et al. 2022). Therefore, it would be difficult for them to remain submerged.
Further evidence suggests that the speed at which Spinosaurus could swim was much less than that of animals known to be completely aquatic (Sereno et al. 2022). For example, the estimated swimming speed of S. eagyptiacus at the water surface was 0.8m/s (2.88km/hr) and underwater was 1.4m/s (5.04km/hr) (Sereno et al. 2022). This is in contrast to some modern day predators such as orcas and sharks which can achieve swimming speeds of between 10-33m/s (36km/hr-118.8km/hr) (Sereno et al. 2022). What this suggests is that its swimming speed was not condusive to an aquatic predator's lifestyle, implying that Spinosaurus was not a completely aquatic animal. Another interesting thing the scientists noted about Spinosaurus was that because the center of gravity and the center of buoyancy are in the same area of the dinosaur, it actually would have a tendency to roll over onto its side in deep water, and because of the way its arms and legs are arranged it likely would not have been able to right itself (Sereno et al. 2022).
Additionally, Sereno et al. (2022) note that the tail and spine of Spinosaurus were too stiff to allow for diving and for quick maneuvers in the water that are needed to catch prey. It is next noted that even Spinosaurus' arms and legs are too long to be beneficial for swimming. This was deduced by noting that among animals that are semiaquatic, such as crocodylians and turtles, the legs are reduced to minimize drag in the water whereas Spinosaurus' legs are even larger than those found on T. rex (Sereno et al. 2022).
In regards to the geographic distribution of Spinosaurus, the scientists noted that spinosaur fossils are often found in areas that used to be ancient deltas in the past (Sereno et al. 2022). This could point to spinosaurs living near large bodies of water. However, the authors also raise the possibility that spinosaur remains were actually being washed downstream to these deltas from farther inland (Sereno et al. 2022). As evidence, the scientists point to spinosaur fossils that were found deeper inland—again suggesting that Spinosaurus was not a largely aquatic animal (Sereno et al. 2022).
First published on: Sept. 12, 2023.