Dinosaur Tail Evolution: What Came First, Knob or Handle?

Scientists at North Carolina State University have traced the evolution of the Ankylosaur's clubbed tail to discover the age old question about the dinosaur: what came first, the club or the handle?

Ankylosaurus, the "armored lizard" of the late Cretaceous period (65.5 million to 66.8 million years ago), were armored with tough hides and bone-like "scales", and a famed tail club that would be used as a weapon for defense from predators. However, there were many types of ankylosaurian dinosaurs, some of which lived in the Jurassic period 145 million years ago, without the clubbed tail.

According to the study, the ankylosaur's tail is made up of a handle and a knob. The knob is made of osteoderms, a type of bone unique to armored dinosaurs, and the handle is simply the lower portion of the tail, which supports the knob.

Ankylosaur's tails needed to be stiff in order to be able to support the weight of a knob and swing it effectively. To get the stiffness required, the vertebrae along the tail had to become less flexible as it evolved, or knob's weight could tear muscle or dislocate vertebrae in the animal's tail, according to Phys.

Victoria Arbour, a postdoctoral researcher at NC State, and the North Carolina Museum of Natural Sciences, compared the Jurassic and the Cretaceous ankylosaurs to look into how the dinosaur developed the unique tail. During her research, she discovered that there are three ways in which the tail could have evolved.

"The knob could have evolved first, in which case you'd see ankylosaurids with osteoderms enveloping the end of the tail, but with the tail remaining flexible. The handle could have evolved first, meaning you would see early ankylosaurids with overlapping or fused tail vertebrae. Or the knob and handle could have evolved in tandem, in which case you'd see ankylosaurids with both structures, but there could have been other differences like shorter handles or smaller knobs," Arbour said, according to Eurekalert.

Arbour believes that the tails formed the handles first, as akylosaurs in the early Cretaceous period began to show fused vertebrae toward the end of their tails. This stiffening reduced flexibility toward the end of the tail, but was building up stability in order to support the soon-to-evolve heavy knob. The knobs began ot appear in the late Cretaceous.

"While it's possible that some of the species could still have developed the handle and knob in tandem, it seems most likely that the tail stiffened prior to the growth of the osteoderm knob, in order to maximize the tail's effectiveness as a weapon," Arbour said.

In her study, Arbour compared Liaoningosaurus, which lived 122 million years ago with a flexible tail; Gobisaurus, which lived 90 million years ago with a stiff tail but had no knob; and Pinacosaurus, which lived 75 million years ago and had a complete clubbed tail.

Many recognize the Ankylosaurus from the "Jurassic Park" film series, where it was made to be very popular.

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