One dinosaur too many?

Some dinosaurs are going extinct — again. Their demise at the end of the Cretaceous Period, 65 million years ago, was bad enough. But now paleontologist Jack Horner has decided that some dinosaurs never existed at all. 

Horner, of the Museum of the Rockies in Bozeman, Montana, thinks that some dinosaurs are not distinct species, but rather juveniles or adults of other species. Though many others share his basic views, Horner has pioneered cutting-edge techniques to discover clues about dinosaur growth hidden within fossil bones. His work is changing how paleontologists study and classify dinosaurs. Yet not everyone agrees with all of his hypotheses, especially his startling contention that Torosaurus, a horned dinosaur with a large frill, is really an adult Triceratops.

Andrew Farke, a paleontologist at the Raymond M. Alf Museum of Paleontology in Claremont, California, says that “most folks right now are in agreement that [these dinosaurs] are probably different animals.”

The young of modern reptiles resemble miniature adults, but Horner thinks that dinosaurs grew differently, often looking very distinctive at each life stage. These differences between juvenile and adult forms can lead paleontologists to think that each life stage is a separate species, he argues. Horner says that paleontologists are often trained to look for such differences between fossils, rather than the similarities.

“When I pick up a specimen, the first thing that comes into my mind is how old is it?” and not “oh boy, this looks a lot different than everything else,” says Horner. He also points out that paleontologists like finding their own unique dinosaurs, which exacerbates the problem. Discovering a new species has been “like a currency in the field for decades,” agrees Mark Goodwin, a paleontologist from the University of California at Berkeley and co-author of several papers with Horner.

Many paleontologists, including Horner, noticed that some dinosaur groups include many species that are extremely similar. “That didn’t really jibe too well with what we know from modern ecosystems, where you usually don’t have 10 species … that are very similar to one another,” says Stephen Brusatte, a paleontologist at the American Museum of Natural History.

To better classify dinosaurs, Horner is cutting up fossils and looking at them under the microscope. He and his coworkers break a fossil bone into two pieces and remove a sample from the middle. That chunk is sliced very thin and placed under a microscope, revealing the inner structures preserved in the fossil.

This microscopic view can tell paleontologists how old a dinosaur was when it died. Young, still-growing dinosaurs have spongy bone with tiny holes. But as they age, their bones become more dense. By looking at the shapes and densities of fossilized bone cells, paleontologists can easily determine if the fossil was a baby, a juvenile or an adult.

This new information is helping Horner streamline dinosaurs from the end of the Cretaceous Period — just before dinosaurs went extinct. Horner condensed the 12 main dinosaurs from this period down to seven species. While paleontologists agree that we have too many dinosaur species, Horner’s work on Triceratops and Torosaurus has generated some intense but collegial debate.

Beloved by 10-year-olds, Triceratops has three prominent horns and a large, bony frill extending from the skull. Torosaurus shares this basic anatomy, but its frill is larger and has a gaping hole on either side. Last year, Horner and his graduate student John Scannella published a paper in the Journal of Vertebrate Paleontology arguing that Triceratops and Torosaurus are the same species. They proposed that Torosaurus is actually a full-grown Triceratops.

Scannella and Horner identified several Triceratops fossils with an area of thinning bone on either side of the frill. Microscopic study showed that parts of the frill were reabsorbing, creating thin areas that could eventually transform into the large holes present in the Torosaurus skulls. Scannella and Horner also argued that another species, Nedoceratops, represents the intermediary growth stage between Triceratops and Torosaurus. Thus in one paper, the pair reduced three dinosaur species into one.

Farke came to a different conclusion when analyzing the only Nedoceratops skull ever found. “I think it is highly unlikely that it is an intermediate life stage between Triceratops and Torosaurus,” Farke says. “Most people that work on ceratopsians… don’t agree with Scannella and Horner’s hypothesis.” Nedoceratops looks like a good intermediary at first, with the expected small frill holes. Yet rough bone texture suggests that this Nedoceratops fossil was an adult animal, not a mid-life intermediary, Farke says. Subtle differences in frill shape and the lack of a nose horn also led Farke to conclude that Nedoceratops is not an intermediary, but either a distinct species or an aberrant Triceratops.

Scannella and Horner countered with a second analysis of the Nedoceratops specimen late last year, sticking to their theory that it is a Triceratops–Torosaurus intermediary. Farke is not convinced. He says that they gathered some useful new information about the fossil, but there are still “major issues” with their theory.

Ongoing research by Nicholas Longrich, a paleontologist at Yale University, could further challenge Horner’s hypothesis. Longrich says that his analysis of a specimen from Yale’s Peabody Museum suggests that not all Torosaurus fossils are adults. Like Farke, Longrich is also skeptical about the lack of obvious transitional forms between Triceratops and Torosaurus. “We have a good hand,” Longrich says, but until the results are published “we can’t tell them what our cards are.”

Classification has challenged paleontologists for decades, and taxonomic changes are not unusual. As new fossils are excised from the earth, the dinosaur family tree branches out or is pruned to accommodate the new information. Horner has transformed this process by incorporating microscopic studies of dinosaur bones. Farke, though he disagrees with Horner about Triceratops and Torosaurus, thinks that Horner’s bone work is “really adding that extra piece to the toolbox.”

Time and new fossil discoveries will determine if Horner and his colleagues are correct. If a hypothesis is wrong, Goodwin says, it will be “easy to falsify.” Indeed, if future excavations find a juvenile Torosaurus, then Horner’s hypothesis could prove incorrect. If Horner is right, Torosaurus will cease to be a separate species, and our understanding of dinosaur growth will be definitively altered.

An earlier version of this story misspelled paleontologist John Scannella’s last name. The error was corrected at 11:30am, February 2, 2012.