My colleagues and I discovered a new species of extinct crocodile from a fossil found in the same rocks used to carve the Great Sphinx of Giza.
Author
- Philip Mannion
Professor of Palaeobiology, UCL
Today, there are only two species of the long-snouted gavial crocodile , both of which are endangered : Gavialis gangeticus and Tomistoma schlegelii. They are found only in freshwater environments on the Indian subcontinent and Southeast Asia.
However, the fossil record of gavials goes back 80 million years , to the time of the dinosaurs. It shows that they were once diverse, with more than 40 species known, and lived all over the globe. So how did a group of crocodiles that today cannot tolerate saltwater once live all over the world? Our discovery may help to solve this puzzle.
Although the distribution of the continents differed from today, most were already separated by oceans. My colleagues and I have been trying to understand how gavials achieved this cosmopolitan distribution, while also seeking to unlock what led to their current predicament.
Studying the anatomy of the skeletons of living and fossil gavials has given us a better understanding of the group's evolution in recent years. CT scans have enabled us to look inside their skulls , giving us a window into their brains and sensory systems.
This has revealed new anatomical information that helps differentiate species, and has allowed us to infer aspects of the biology and ecology of some of the extinct species .
For example, in an extinct group of crocodile relatives known to have lived entirely in the ocean, other researchers found thickening of the outer wall of their inner ear to cope with the higher pressures seen at ocean depths. My colleagues and I have found similar evidence for a marine lifestyle in some fossil gavials. This ability to live in the ocean was lost in living species, which are restricted to rivers and lakes.
Several of the gavial fossils we have been studying from Africa and Europe were first reported in the late 19th and early 20th centuries and they were often described as species of Tomistoma, i.e. closely related to Tomistoma schlegelii, one of the two living species of gavials. Our previous work and that of others has shown that some of these extinct species are not closely related to Tomistoma, with new names given for some of them . However, many of these historically described species remain neglected. Led by my former PhD student, Paul Burke, now a researcher at the Swedish Museum of Natural History in Stockholm, along with colleagues from Egypt and Germany, we re-examined one of these species in a new study.
In the 1920s, several fossils were collected from northeastern Egypt. These were found in a limestone rock, known as the Mokattam Formation , that had formed 45 million years ago in a shallow marine environment when sea levels were higher. This region was submerged at this time. These fossils, including a nearly complete skull, were described in 1927 as belonging to Tomistoma, because of their superficial resemblance to the living species, Tomistoma schlegelii.
However, aided by the use of CT scanning, our team was able to identify several anatomical differences, including the presence of raised rims around the eye sockets, which are absent in Tomistoma .
The Mokattam Formation already has a legacy: 4,500 years ago, these rocks were used to carve the Great Sphinx of Giza . The gavial skull shares more than this origin story though. Just as the Sphinx is famously missing the tip of its nose, the fossil is also missing the tip of its snout. This coincidence led us to name the new crocodile Herugavialis cairensis after the Egyptian falcon-headed god Heru (also known as Horus), who is often thought to be the inspiration for the iconic Sphinx.
Both northern Africa and southern Europe hosted a high diversity of gavials (and other crocodiles) during much of the last 80 million years, with these species thriving in the shallow, marine environments that covered much of the Mediterranean region at the time.
Unlike living gavials, it is likely that some of these species could cross oceans . This diversity appears to have persisted through the devastating mass extinction that wiped out the dinosaurs, and probably fluctuated during alternating warmer and cooler intervals over the next 60 million years, tied to higher and lower sea levels, respectively.
However, crocodiles disappeared from the Mediterranean region approximately 6 million years ago and never returned. Their demise coincides with an event known as the Messinian salinity crisis, when the Mediterranean essentially dried up as a result of being sealed off from the Atlantic Ocean. As the seawater became increasingly salty with the heightened evaporation of water, it would have become ever more intolerable to the animals inhabiting it, even those usually able to live in saltwater, leading to the loss of crocodiles and many other species.
The signature of this event can still be observed today, with the Mediterranean Sea still much saltier than the oceans, and crocodiles entirely absent from this region. The remaining living gavials live far away from the Mediterranean, but the loss of the adaptations that would allow them to live in saltwater means that they would not be able to survive there regardless.
Rapid and extreme changes in temperature, sea level, and aridification are critical components of the current climate emergency, and events such as the Messinian salinity crisis show that they can have a lasting impact on biodiversity, spanning millions of years.
Philip Mannion receives funding from The Royal Society and UK Research and Innovation.
