Trinacromerum bentonianum
- "three headed femur from Benton Formation" (Montana)
- Mid-Cretaceous
- 18 feet long
- 1 - 2 tons

Trinacromerum was a pliosaur: a short-necked, big-headed, thick-bodied plesiosaur. Pliosaurs are among the largest marine predators to ever swim the oceans. Giants like Liopleruodon and Kronosaurus reached as much as 80 feet and weighed as much as 60 to 100 tons.

Pliosaurs most certainly preyed upon anything that they could catch and was big enough to be worth pursuing. Large squid, fish and other marine reptiles were almost certainly on the pliosaurs' menu, which really could eat anything in the oceans in wanted to eat.

Pliosaurs were the dominant big predator from the late Triassic through the Jurassic and to near the end of the Cretaceous. Late in the Cretaceous pliosaur numbers and diversity declined, and they seemed to be losing the evolutionary battle to mososaurs, which were rapidly increasing in numbers, size and diversity. Like all large marine reptiles living at that time (except turtles), the last pliosaurs were wiped out in the mass extinction at the end of the Cretaceous.

No one know exactly how pliosaurs--or plesiosaurs--swam simply because this is hard to figure out and there is nothing in the oceans today that swims on four large flippers. At present, the best we can figure is that they swam kind of like penguins, which swim in the water much like birds in the air with their wings. (This should not be surprising since penguins flippers evolved from wings.) This comparison with penguins is further suggestive as well: Given pliosaurs rear flippers are far behind the center of gravity, they may not have been for propulsion, but only for steering.

From the Dinosaur Walk Museum in Pigeon Forge, Tennessee.

~ harui