|(Steve McConnell photo)|
Smallest fossil Triceratops yields clues to dinosaur growth
Cast on display in Valley Life Sciences Building is of a unique 'yearling' specimen
| 08 March 2006
With its big, hockey-puck-size eyes, shortened face, and nubby horns, it was probably as cute as a button - at least to its mother, a three-horned dinosaur called Triceratops that could weigh as much as 10 tons and had one of the largest skulls of any land animal on the planet.
Visitors to the Valley Life Sciences Building now can judge for themselves. A cast of the foot-long skull from the youngest Triceratops fossil ever found is on display in the building's Marion Koshland Bioscience and Natural Resources Library. The actual skull, also at Berkeley (though in fragments), is described by campus paleontologist Mark Goodwin in the March issue of the Journal of Vertebrate Paleontology.
Mounted in the library's entryway, the diminutive skull, likely from a year-old, three-foot-long baby, is dwarfed by the more-than-six-foot-long skull of a mature Triceratops. Standing menacingly outside the library's doors is a life-size cast of Triceratops' nemesis, Tyrannosaurus rex.
Despite the pup's size, its remains are telling Goodwin a lot about how dinosaurs grew, the purpose of their head ornaments, and the characteristics of their ancestors. In particular, since the horns and frill are present from a very early age, it is unlikely they were used exclusively for sexual display, he says, proposing that they were just as important for species recognition and visual communication.
Triceratops horridus was a strictly North American dinosaur, though ceratopsian relatives with different but equally formidable ornamentation roamed China and Mongolia during the Cretaceous period, 65 million to 144 million years ago. Adult Triceratops could be nearly 10 feet tall and 26 feet long, with a bony frill around the head that was as wide as 7 feet across. Two three-foot horns typically curved forward from the brow, while a third horn erupted from the nose above a narrow, horny beak.
(Photo by Mark Goodwin, UC Berkeley Museum of Paleontology)
The baby's skull, along with a few vertebrae, teeth, and bony tendons, were discovered by amateur fossil hunter Harley Garbani in 1997 in Montana's Hell Creek Formation, the source of many Triceratops and T. rex fossils. Garbani thought he'd found the skull of a dome-headed dinosaur, or pachycephalosaur, and sent Goodwin a photo of the bones he had reconstructed from hundreds of fragments. But Goodwin, recognizing the bones that make up the frill around the back of the head as those of a very young Triceratops, assembled the reconstructed bones into a skull and lower jaw that is missing only the nose and beak.
The fossil skull, about 67 or 68 million years old, has been a unique addition to the world's existing, mostly adult specimens of Triceratops. And the "yearling," as Goodwin calls it, fits perfectly into a study he is conducting with Jack Horner of Montana State University about the growth patterns of Triceratops and other dinosaurs.
Although Goodwin's conclusions about the lifelong growth of Triceratops will be published later this year, the baby skull offers its own insights. For one, the surface of the skull shows grooves where blood vessels used to be, evidently to nourish a fingernail-hard covering of keratin that was similar to the thicker layer that covered the adult skull. Such horny coverings are often brightly colored in the living descendants of dinosaurs - the birds - suggesting that adult Triceratops and their young may have been colorful, too.
In addition, the scalloped edges of the baby's frill became mere wavy edges in the adults, although the scallops foreshadowed the development of triangular scales along the edge of the adult frill, probably an attribute of sexual maturity, Goodwin says. The two brow horns started out straight and short in the baby - they're about an inch long - but ended up long and curved forward in the adult, while the nose horn became larger, like that of a rhinoceros, although it was made of bone in Triceratops.
The brain case of the baby also changed significantly, he says. Hidden beneath the bony frills of the skull, the hazelnut-size brain of the baby fit snugly within protective bones not yet fused, so as to allow further brain growth. In the adult, the brain, about the shape and size of a small sweet potato, was completely encased in fused bones. The relative position of the bones of the braincase as the animal matured recapitulates the cranial evolution of Triceratops from a more basal ancestor, such as Protoceratops.
"The baby skull shows us how the bones that make up the skull actually grew and fit together, because we see the sutures and sutural surfaces, which were completely obliterated in the adults," he says.
Because of the good condition of the bones, which show no gnawing, Goodwin thinks the baby died and the skull was buried before it could be scavenged or the bones eroded away along an ancient stream.
"It's an incredible specimen, with beautiful preservation," he says. Goodwin and Horner also have made casts of the skull for the American Museum of Natural History and for Montana's Museum of the Rockies.
Goodwin continues his excavations in Montana, concentrating on the dinosaurs of the Lower Hell Creek Formation that are slightly older than the T. rex and Triceratops fossils from the Upper Hell Creek Formation. His co-authors on the new paper are William Clemens, a Berkeley professor of integrative biology and Museum of Paleontology emeritus curator who opened up the Montana area for fossil exploration more than 30 years ago; field colleague Horner of the Museum of the Rockies at Montana State University in Bozeman; and Kevin Padian, Berkeley professor of integrative biology and Museum of Paleontology curator.