During the Late Cretaceous period, the oceans were dominated by mosasaurs, a group of massive marine reptiles. These animals were apex predators, with large jaws and limbs that had evolved into paddle-like flippers for steering. For a long time, their appearance, particularly the tail, was a subject of speculation. How they moved to achieve their predatory status was a long-standing question in paleontology.
Shifting Scientific Perspectives
Early interpretations of mosasaur anatomy were based on relatives like modern monitor lizards and sea snakes. For decades, scientists reconstructed them with long, tapering tails that moved in a simple, side-to-side motion. This model depicted them as inefficient swimmers that relied on ambush rather than speed. This view was shaped by fossil evidence of vertebral columns that straightened toward the tip, offering no indication of a specialized fluke.
This long-held view was overturned by fossil discoveries that included the rare preservation of soft tissues. A specimen of the mosasaur Prognathodon, described in 2013, provided the first clear evidence of a sophisticated tail structure. This fossil preserved the outline of a large, crescent-shaped tail fluke, similar in form to that of a modern shark. The discovery altered the understanding of how these animals lived and hunted.
The fossil impression showed a two-lobed fin at the end of the tail, resolving hypotheses based only on skeletal comparisons. The existence of this fluke indicated that mosasaurs were not the sluggish, eel-like swimmers once imagined, but possessed a swimming apparatus built for speed. Older depictions in popular culture often reflect the outdated serpentine tail, which has been replaced by this new, evidence-based reconstruction.
The Structure of the Mosasaur Tail Fluke
The mosasaur tail fluke was composed entirely of soft tissue, lacking an internal bony skeleton. Its shape was hypocercal, with two lobes: a large, fleshy lower lobe that provided the main propulsive surface and a smaller upper lobe. This anatomical arrangement is convergent with that of sharks, representing a similar solution for moving powerfully through water. The fluke’s existence was confirmed by fossils where skin impressions were preserved.
Support for this soft tissue fluke came from a modification in the animal’s skeleton. The final tail vertebrae exhibited a downward kink, which served as the anchor point for the large lower lobe of the fluke. This bony deflection provided the rigid support needed to handle the immense forces generated during swimming. Without this feature, the soft fluke would have lacked the structural integrity to be an effective propeller.
The vertebral column provided the core framework, while the fluke, formed from skin and connective tissues, created the hydrodynamic surface. This combination allowed for a tail that was both powerful and flexible, completing the picture of a creature adapted to a fully aquatic existence.
Function in Swimming and Hunting
The shark-like tail fluke provided an efficient method of propulsion known as carangiform or thunniform locomotion. This swimming style concentrates movement in the rear portion of the body and the tail fin. The rest of the body remains relatively rigid, which reduces drag as the animal moves through the water, a contrast to the eel-like swimming previously assumed.
Propulsion was generated by powerful, side-to-side strokes of the large tail fluke. The broad surface of the lower lobe pushed against the water, creating forward thrust with each beat. This method allowed for rapid acceleration and maintaining high cruising speeds, with power comparable to that of large sharks.
This advanced swimming capability was directly linked to its hunting strategy. The mosasaur needed to catch swift-moving prey, such as large fish, cephalopods, and other marine reptiles. The ability to produce sudden bursts of speed was a significant advantage, allowing it to ambush and overpower its targets effectively.