Aquatic Theropod.
Some time ago I proposed that Tyrannosaurus rex was aquatic, based on the following features:
- High set eyes
- Heavy brow ridges to protect eyes from biting insects
- Dentition intermediate between a saltwater crocodile and an orca
- Short front legs to allow manoeuvering close to the bank
- Deep pelvis to allow powerful hind legs to silently move forward to spring into action (like a flea)
- Tapered head, narrower at the front to hide closer underwater to the prey on the bank
- Rapid growth possible only by eating fish and other underwater creatures
- Claws to dig in to soft mud while accelerating out of the water
- Great sense of smell like a shark and crocodile
- Limited stamina compared to quadrupedal prey on land
- Ease of hiding under water vs difficulty of hiding in vegetation
Back then, I stopped short of suggesting that other theropods were also aquatic.
Now, I’m just starting to read a recent book about dinosaurs “The world of dinosaurs – The definitive illustrated collection” by the American Museum of Natural History. I’m only up to the second dinosaur in the book, but already the evidence pointing to theropods being aquatic is far stronger than I was aware.
Coelophysis bauri – Late Triassic, three times as ancient as T rex. This theropod seems less adapted for underwater living than T rex. It has high set eyes, heavy brow ridges, short front legs and tapered head, but not to the same extent as T rex. The dentition is more like that of a fish-eater than T. rex.
One piece of evidence in favour of Coelophysis bauri being aquatic is that it ate crocodile-line archosaurs.
The death of the creatures is almost certainly due to the evaporation of lakes. Coelophysis skeletons show features of desiccation, with the necks pulled back. Most of the specimens were completely articulated, and specimens of all ages were found. This suggests that Coelophysis was not just aquatic, but obligate aquatic, it couldn’t abandon its lake even to save its life.
Another piece of evidence in favour of Coelophysis bauri being aquatic is the Ghost Ranch assemblage. In terrestrial ecosystems today, herbivores vastly outnumber carnivores. But at Ghost Ranch, almost all of the animals found are Coelophysis. The few other specimens include crocodile-line archosaurs, fishes, drepanosaurs and non-dinosaurian dinosauromorphs. This is consistent with an aquatic ecosystem but not a terrestrial ecosystem.
Ghost Ranch is not unusual in having an assemblage dominated by carnivores. Several dinosaur-dominated communities show the same pattern.
Dilophasaurus wetherilli – Early Jurassic. This theropod also has high set eyes, heavy brow ridges, short front legs, and a dentition somewhat similar to crocodiles.
It has two head crests. “Head crests are ubiquitous features of dinosaurs in general, and many theropods in particular, and evolved independently in several different groups of dinosaurs. The crests of Dilophasaurus and the tyrannosaur Guanlong are paper thin.” They are connected to the nasal passages. These head crests make sense if their purpose is buoyancy. Being on top of the skull they trap air even if the head is submerged. Being paper thin makes them good floats to counteract the weight of the skull. It could even be that head crests allow a dinosaur to sleep underwater without any risk of drowning.
In other evidence for an aquatic lifestyle, Dilophasaurus trackways preserve the posture of the animal resting. The resting depression is on an incline with the head oriented uphill. This is the same orientation I suggested for T. rex hunting, being prepared to sprint up the bank after prey at a moments notice.
Not specifically in favour of an aquatic lifestyle for Dilophasaurus, but interesting none the less. There are a few trackways with furrows from the tail. The furrows are not continuous, so there is no evidence that they dragged their tail like a browsing kangaroo. This is rare with other dinosaurs, who held their tails horizontal. Having a tail drag in the mud would not be good for running after prey.
