All amphibians, mammals and sauropsids (reptiles and birds) descend from the same lineage of sarcopterygian fish that conquered the terrestrial environment some 300 million years ago. However, more than 30 lineages of these tetrapods returned to conquer the aquatic environment later. Among the reptiles adapted to the aquatic environment we find examples such as turtles, crocodiles, ichthyosaurs, mosasaurs or snakes. Also some birds, such as penguins or albatrosses, and some mammals such as sirenians (dugongs and manatees) or cetaceans (whales, dolphins, narwhals, sperm whales…).
What are the adaptations of aquatic animals?
All these transitions were accompanied by their respective morphological and physiological changes to adapt to an aquatic life. Some of the most conspicuous adaptations are the development of fins, to be able to move more easily under water, or the development of the tail to propel itself with it. Also the displacement of the nostrils towards the posterior regions of the head, which allows them to take in air without having to take the entire head out of the water.
Many of these adaptations can be identified in fossil lineages. However, not always enough material is found to detect it. For example, the first remains of ichthyosaurs that were found were not enough to clarify what kind of animals they were. It was not until the discoveries of paleontologist Mary Anning , who found and excavated complete skeletal remains of these animals for years, that it was possible to conclude that ichthyosaurs were marine reptiles. The ichthyosaur remains found by Mary Anning left no room for doubt: its body is fusiform (like that of fish), with a long, toothed snout similar to that of dolphins, and with the bones of its limbs flattened to form fins.
Other times, even with sufficient skeletal remains, it is difficult to define the degree of specialization to the aquatic environment if there are ambiguous characters . Many aquatic taxa, such as hippopotamuses or early cetaceans, share a body plan very similar to that of terrestrial species. For example, Pakicetus , one of the earliest genera of cetaceans , show a skeleton that suggests they are wolf-sized land animals. But it is believed that they walked along the coast and fed on fish on the shore, and it is possible that they had a semi-aquatic lifestyle, but no consensus has been reached based on their body morphology. For this reason, it is important to have indicators from sources other than skeletal morphology , such as evaluating stomach content, performing isotopic studies or bone histology.
Where does Spinosaurus live?
Bone histology allows us, for example, to determine the density of the bones of current and extinct species. Aquatic species generally have denser bones, which helps them dive and stay underwater more easily. A new study, published in the prestigious journal Nature , collects bone density data from around 200 current and extinct species (including mammals and sauropsids) to compare with data from non-avian dinosaurs and concludes that spinosaurs show different degrees of adaptation to aquatic life : from wading species like a heron to species that would spend much of their time completely submerged like a hippopotamus. There is no current ecological analogue for these species, but Spinosaurus is described as not very hydrodynamic animals but with very dense bones (a density similar to that of penguins, alligators or seals), and they would move in a similar way to hippos but propelling with the tail.
This provides deeper insights into the behavior and ecology of non-avian dinosaurs , where until a few decades ago no strong evidence had been found that any lineage was aquatic, which is surprising considering the many times that lineages of other tetrapods have adapted to the marine environment. But, in addition, it confirms a suspicion since new skeletal remains of Spinosaurus aegyptiacus were discovered in 2014, which suggested that it could have an amphibian lifestyle . Prior to the discovery of these remains, spinosaurs were assumed to be terrestrial, bipedal theropods , and have even been so depicted in fiction, such as in Jurassic Park III . The paradigm shift in this emblematic species arouses a lot of interest both in the scientific community and in fans of the Jurassic Park sagas, and is a clear example of how science and knowledge advance despite preconceived ideas held for decades.
References:
Fabbri, M. et al. 2022. Subaqueous foraging among carnivorous dinosaurs. Nature. https://doi.org/10.1038/s41586-022-04528-0
Ibrahim, N. et al. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science 345, 1613–1616.
Kelley, NP & Pyenson, ND 2015. Evolutionary innovation and ecology in marine tetrapods from the Triassic to the Anthropocene. Science 348, aaa3716.
