Just for Frills?

Decoding dinosaurs' cryptic anatomical features

It’s often easy to interpret a dinosaur’s distinctive characteristics. Tyrannosaurus rex almost assuredly used its banana-size teeth to crush bones and to rip flesh from a carcass. Immense sauropods such as Paralititan and Brachiosaurus probably used their remarkably long necks to browse on treetop vegetation. Features on other dinosaurs are more enigmatic.

FORM, NOT FUNCTION? Scientists suspect that the dorsal plates on stegosaurs were a means of species recognition. J. Tucciarone

Consider the plates on Stegosaurus and the domed skulls of pachycephalosaurs. Scientists have proposed various explanations for these and other bizarre body features of dinosaurs.

However, as modern paleontologists more closely scrutinize anatomical oddities of the dinosaur era, they’ve come to suspect that the same driving force underlies such features and much teen fashion—the need to be regarded as different.

Full plate

The 2-ton herbivore known as Stegosaurus stenops is readily identifiable by the two rows of fin-like plates running along its back and two pairs of 1-meter-long spikes jutting from its tail. Paleontologists have speculated that by swinging its tail side to side, S. stenops could use its spikes to inflict grievous injuries on its predators (SN: 10/26/02, p. 270: Available to subscribers at Stegosaur tails packed a punch). Some scientists propose that the creature’s dorsal plates had a defensive role, as well. The more these researchers look into S. stenops’ plates, however, the less clear their function becomes.

In S. stenops, the skin-covered dorsal plates measured up to 1 m long, says Russell P. Main of Harvard University. Although the structure within each plate included an outside layer of dense bone, the center was porous—an arrangement that wouldn’t have resisted crushing very well. If the plates weren’t armor, what other purpose might they have served? Because plate size and shape didn’t differ for males and females of the same species, the structures probably didn’t play a role in sexual display, Main notes.

Some paleontologists have proposed that the structures acted as heat-exchange devices that regulated the animal’s body temperature. However, there didn’t appear to be a lot of blood flow between the inside and outside of the plates, and there’s no evidence of elaborate networks of large blood vessels on the plates’ bony parts. Other dinosaurs of similar size and presumably with the same need to maintain a steady body temperature seem to have done fine without plates, notes Main.

The plates might have made a stegosaur appear large and invulnerable, but that argument doesn’t go very far because several stegosaur species had only small spikes or plates along their backs.

Main and his colleagues looked at the plate configurations on many different stegosaurs of several species. They found that over millions of years, evolution hadn’t produced bigger plates or longer spikes. The features just ended up looking different. That suggests that the plates’ and spikes’ main purposes were for species recognition, say Main and his colleagues in the spring Paleobiology.

Being easily recognizable would have been a plus for a small-brained dinosaur: At least six different types of stegosaurs with various arrangements of plates and spikes roamed what is now Montana about 150 million years ago.

Bone head

Pachycephalosaurs, in Greek, means thickheaded lizards. In the largest pachycephalosaurs, which ranged up to 5 m long, the top of the skull could be as thick as 25 centimeters, says Mark B. Goodwin, a paleontologist at the University of California, Berkeley.

In the 1970s, analyses of pachycephalosaur fossils revealed large numbers of bony fibers within the crown of the skull. One researcher suggested that those fibers would enable a skull to resist compression if the animal participated in head-to-head combat.

A variety of evidence undermines the notion of head-butting pachycephalosaurs, says Goodwin. For one thing, the creatures didn’t have shock-absorbing, air-filled chambers in their heads as today’s bighorn sheep do.

Microscopic analyses of fossils from pachycephalosaurs of various ages also counter the notion of head butting. Goodwin and his colleague Jack Horner of Montana State University in Bozeman found that the distinctive fibers appeared in the skulls of young pachycephalosaurs. In adults, which might be expected to head butt regularly in mate and territory disputes, the fibers had disappeared, Goodwin and Horner reported in the spring 2004 Paleobiology.

In their study, the researchers found that the skulls of adult pachycephalosaurs were covered with fossilized collagen fibers—a sign that the skull sported tissue, such as a fleshy comb. Because male and female pachycephalosaurs both had the decoration, it probably didn’t distinguish the genders. Instead, Goodwin and Horner suggest, their skull ornamentation gave pachycephalosaurs a way to recognize members of their own species.

After all, whether you’re a teen or a dinosaur, it’s important to know who’s on your turf.

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