Why do Ankylosaurus have spikes?

While nodosaurs share the armor and tank-like build of Ankylosaurus, they did not have the dinosaur&#;s clubbed tail. In , the unearthing of a remarkably well-preserved nodosaur fossil&#;one of the best ever found&#;offered new insight into the nodosaur&#;s pigmentation, which may have served as camouflage, as well as the size and shape of its armor. The discovery of a fossilized ball of digested plant matter in this animal&#;s stomach also confirmed what researchers had suspected about armored dinosaurs: They preferred to graze on ferns and other low-growing plants.

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Ankylosaurus had a narrow beak at the end of its skull to help it strip leaves from plants. It also had small, leaf-shaped teeth, which may have been useful for gnashing small fruits or invertebrates, and a large gut for digesting the large quantities of plant matter necessary to sustain its formidable body.

Evolving fossil knowledge

Though Ankylosaurus is among the most famous dinosaurs, it is also one of the most puzzling. Unlike its nodosaur relatives, scientists have only unearthed a handful of Ankylosaurus fossil fragments, and with each new discovery, our understanding of the species grows. Paleontologists have rearranged the available fossils time and again, with the most recent proposed body shape suggesting it may have been longer and thinner than previously thought, and equipped with a denser coat of spikes.

In , a team of paleontologists redefined Ankylosaurus once again, revealing its large body size, small teeth, and nostrils that were placed to the side of its head rather than in the front of its snout. Its displaced nostrils might signal an evolutionary change in its diet or sense of smell. But it will take more research&#;and the discovery of more specimens&#;to better understand this iconic dinosaur.

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Often compared to an army tank or bus, Ankylosaurus was a heavily armored dinosaur with a large club-like protrusion at the end of its tail. Ankylosaurus means "fused lizard" in Greek, and it was given that name because bones in its skull and other parts of its body were fused, making the dinosaur extremely rugged. Ankylosaurus lived in the late Cretaceous Period, about 65.5 million to 66.8 million years ago, and roamed the Western United States and Alberta, Canada.

While this herbivorous dinosaur was a massive animal, a re-examination of its fossils in by armored dinosaur expert Kenneth Carpenter downsized it a bit. The largest Ankylosaurus specimen ever found was 20.5 feet (6.25 meters) long, 5.6 feet (1.7 m) tall at the hips and 4.9 feet (1.5 m) wide, according to the study, published in the Canadian Journal of Earth Sciences. It likely weighed up to 4 tons (3.6 metric tons).

The top of the dinosaur was almost completely covered with thick armor consisting of massive knobs and oval plates of bone, known as osteoderms or scutes, which are also common on crocodiles, armadillos and some lizards. "They are bones that form within the skin, just like crocodiles," Carpenter, who's the director of the USU Eastern Prehistoric Museum in Utah, told Live Science.

The osteoderms of all ankylosaurs (dinosaurs in Ankylosaurus' suborder, Ankylosauria) were composed of an thin outer cortical or compact bone and an thick inner cancellous bone (spongy, porous bone), according to an analysis Carpenter and his colleagues published in the journal Acta Palaeontologica Polonica in . The osteoderms were probably covered with skin and keratin, the fibrous protein that makes up hair and nails in people.

The plates, which varied in size, were aligned in regular horizontal rows down the dinosaur's neck, back and hips. There were also smaller plates or other similar features protecting the areas between the larger plates, and there may have also been smaller plates on its tail and limbs. The animal's biggest cluster of armor was in its neck region, Carpenter said.

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Along with its armored plating, Ankylosaurus had two rows of spikes along its body. Additionally, its head was long and low, with prominent horns projecting back and to the side and plates protecting its eyes. [Related: Ancient Armored Fish Was Toothy, Too]

Speaking of spikes, a bizarre-looking ankylosaur, described May 10, , in the journal Royal Society Open Science, had such an uncanny resemblance to the spiky-faced "Ghostbusters" monster Zuul that paleontologists named it Zuul crurivastator (CRUR-uh-vass-TATE-or). The species name means "destroyer of shins" in Latin, referring to the ankylosaur's 10-foot-long (3 meters) tail, which was tipped with a club that was likely used to swipe at the legs of predators. The 75-million-year-old remains of this beast were uncovered in in Montana's Judith River Formation. Despite its ferocious appearance &#; several rows of bony spikes covered its 20-foot-long (6 m) body &#; Z. crurivastator was a plant eater, researchers said. 

An ankylosaur discovered in Montana in looks eerily like Zuul, a fictional monster from the movie "Ghostbusters."

(Image credit: Brian Boyle/Copyright Royal Ontario Museum; Columbia Pictures Corporation)

However, even without these defenses, Ankylosaurus would have been difficult for predators to kill. "You have to understand that they had a very rotund body and were wider than they were deep," Carpenter said. "It would be difficult even without the armor to get a purchase on it because its body is relatively flat."

Aside from its armor, another defining characteristic of Ankylosaurus was its tail club. The tail contained vertebrae that were woven together to form a stiff rod at the base of the club at the end. The stiff tail likely evolved before the knob, according to a study in the Journal of Anatomy. Scientists have proposed several different hypotheses for the purpose of the dinosaur's club, according to a study in the journal PLOS ONE. For instance, the tail may have been used for display purposes, such as for attracting mates. Alternatively, it may have been used for combat between two Ankylosaurus, such as over territory or mates. [Related: How Armored Dinosaur Got Its Bone-Bashing Tail]

Whatever the case, it seems the dinosaur did sometimes use its tail as a weapon. "In two fossil specimens, the tail clubs show damage," Carpenter said. "It looks like they definitely struck something hard." What's more, the PLOS ONE study concluded that that the huge tail could easily have broken the bones of most of its predators.

Ankylosaurus moved on all four limbs, and its hind limbs were slightly longer than its forelimbs. Though there are insufficient foot fossils to establish whether Ankylosaurus had toes, it's believed the dinosaurs likely had five toes on each foot like other ankylosaurs.

What did Ankylosaurus eat?

Ankylosaurus grazed on low-lying plants. The dinosaur's triangular skull was wider than it was long and had a narrow beak at the end to aid in stripping leaves from plants. Its small leaf-shaped teeth were not designed to break up large plants and it had no grinding teeth. A broadness to part of its ribcage suggests Ankylosaurus had some sort of fermentation digestive system to break down the massive amounts of un-chewed plants it ate, according to Carpenter's analysis.

Ankylosaurs had a complex nasal passage and a large cavity volume for the olfactory region of their skulls. The looping nasal cavity probably didn't improve smell much and instead was important for things like regulating temperature, according to a study in the Journal of Anatomy. However, ankylosaurs seemed to have had a large olfactory bulb (brain structure involved in the sense of smell), so the dinosaurs likely had a strong sense of smell to help seek out food and avoid predators, the study suggested.

Research presented in November at the annual Society for Vertebrate Paleontology in Berlin further suggests that ankylosaurs' nasal passages helped keep their brains cool.

Learn about the plated Cretaceous-era dinosaur Ankylosaurus.

(Image credit: Ross Toro, Livescience contributor)

Fossil finds

A team led by American paleontologist Barnum Brown discovered the first Ankylosaurus fossil &#; which included the top of a skull, vertebrae, ribs, a shoulder girdle piece and armor &#; in the Hell Creek Formation of Montana in .

Six years later, Brown unearthed Ankylosaurus osteoderms, which he initially thought belonged to another type of dinosaur. He unearthed his third set of Ankylosaurus remains in Alberta in when on an expedition at the Scollard Formation &#; it included ribs, limb bones, armor, a complete skull and the first known tail club. All three of Brown's finds are housed at New York's American Museum of Natural History. In , Charles M. Sternberg discovered the largest known Ankylosaurus skull.

No complete Ankylosaurus skeleton has been unearthed to date. And aside from the isolated bones, armor and teeth, only three major specimens of Ankylosaurus have been unearthed.

"This raises an interesting question: Why is it so rare?" Carpenter said. One possibility, he said, is that the dinosaur lived upland in environments away from the rivers and swamps that are conducive to fossilization. Or, they may not have been all that common in the ecosystem at the time. "We just don't know why they are so rare," he said.

Kim Ann Zimmermann contributed to this article.

Related pages

More dinosaurs:

Time periods

Precambrian: Facts About the Beginning of Time

Paleozoic Era: Facts & Information

Mesozoic Era: Age of the Dinosaurs

Cenozoic Era: Facts About Climate, Animals & Plants

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Additional resources

• Dinosaur Provincial Park, in Alberta, Canada, which "contains some of the most important fossil specimens discovered from the 'Age of Dinosaurs' period of Earth's history,"is a UNESCO Heritage Site.
• "Ankylosaurus Fights Back" is an interactive book app from the Smithsonian Institution for children ages 3 to 8. It teaches literacy skills and provides fascinating information on the prehistoric world.
• Read Barnum Brown's original article, "The Ankylosauridae, a New Family of Armored Dinosaurs From the Upper Cretaceous," in the Bulletin of the American Museum of Natural History.