New species provide missing links in evolutionary history of horned dinosaurs
DICKINSON — Badlands Dinosaur Museum's Denver Fowler, Ph.D., and Dickinson State University's Elizabeth Freedman Fowler, Ph.D., recently completed research that describes two new transitional species of dinosaur that fill gaps in the lineage of the horned dinosaur Pentaceratops.
Horned dinosaurs, called Ceratopsidae, include species such as Triceratops from the Late Cretaceous Period of North America. These dinosaurs are famous for their large skulls which feature huge horns and a decorated bony frill that extends from the back of their head. Scientists like the Fowlers use differences in the shapes of these features to tell one species from another.
The missing link
“They (the new dinosaurs) show a link between two different dinosaurs that we didn’t previously realize were linked. They show that basically Pentaceratops that lived in New Mexico is actually the ancestor of Anchiceratops that is known from southern Canada,” Fowler said.
Prior to this research, it wasn’t clear where Anchiceratops came from. The frill of the Pentaceratops and the Anchiceratops are different. In the former, there’s a depression in the center, forming a sort of V shape. In the latter, the depression is completely closed in. These newly discovered dinosaurs, named Navajoceratops sullivani and Terminocavus sealeyi, show how the frill changed over time.
“This depression in the frill gets deeper and deeper until it starts to close in on itself, and it’s completely closed by (Anchiceratops),” Fowler said.
“I was working in New Mexico … back in 2002, and we found the fossilized remains of one of these … and it looked very similar to Pentaceratops," Fowler said. "It’s from the same area as Pentaceratops is from, but it’s ever so slightly younger. It came from rocks about 200,000 years younger than Pentaceratops rocks. They look slightly different.”
Two other transitional species of dinosaur were discovered, as well. One is in Kansas with a student and fits into the timeline just before the Navajoceratops. The other was too incomplete to model but would fit in just after Terminocavus.
Not long after the fossils were discovered, Fowler found a paper from 1998 that suggested Pentaceratops could be an ancestor of Anchiceratops and thought that what he had found could be transitional species between the two. The answer would be years in the making.
Dating the fossils
To tell if the timeline fit, the Fowlers had to first determine the age of the dinosaurs, for which they used radiometric data, which is examining radioactive minerals inside ancient volcanic ashes.
"When these dinosaurs were living in North America, there were big volcanoes going off ... through the Rockies," Fowler said. "When a volcano erupts, it spews out ash, and the ash lies on the ground. Usually it gets washed away and destroyed, but every now and then a little bit of ash is preserved. If you find some ash, you can sample it, take it to the lab and measure the amount of uranium and lead in that ash in certain crystals or you can look at another element called argon."
Radioactive elements decay over time, and uranium specifically decays into lead, a process that takes a specific amount of time.
"You can count up the amount of uranium in your crystal and figure out how long ago it was 100%. The key thing is the volcano ... when the volcano erupts and lava solidifies, that's the point it's 100%. The uranium is locked into the crystal then, so we can look inside the crystal and say that should have been 100% uranium, but now it's 50% uranium and 50% lead," Fowler said.
Volcanic ash was found near the fossils and used to date them.
An evolutionary split
In their research, the Fowlers also proposed the foundation for an evolutionary splitting event that provides an explanation for the apparent differences between the northern and southern dinosaur faunas.
When those dinosaurs were alive, North Dakota was underwater in what was known as the Western Interior Seaway, which divided North America into eastern and western subcontinents. A short period of high sea level pushed the edge of the sea very close to the Rocky Mountains. For hundreds of miles across what is now central Utah to southern Alberta, the land would have been very narrow.
"You really would have had this mountain front with the sea right up to it … and that cut off dinosaurs that lived in the south from the ones that lived in the north … There was this group living in Alberta, there was a group living in what is now New Mexico and southern Utah, and they couldn’t mix for like a million years or so; they were separated. It causes different species to arise," Denver said.
When the sea level went down again, the northern and southern species were once again able to mingle.
Testing the hypothesis
To test the hypothesis, Freedman Fowler performed two forms of computer analysis — morphometric analysis and cladistic analysis.
Describing the former, she said, "You take the shape of an object and you digitize it, and then the computer compares the shapes of the different objects and shows you which ones are more similar than others."
Describing the latter, she said, "We manually describe like ... this has a spike here, and it's a really big one. Then if another species has a really small spike there, it would be coded differently ... It's a way of boiling down all the differences that we see between species into just basic zeros and ones, yes or no characteristics, and then the computer finds the ones that have the most characteristics in common, and they're presumably more closely related, so it starts creating this hypothetical evolutionary tree of the different species and what their relationships are."
Both fossils were found in New Mexico in the San Juan Basin of New Mexico. The Navajoceratops was found by Robert Sullivan, Ph.D., and is now located in the State Museum of Pennsylvania. Paul Sealey found the Terminocavus fossil, which is now located at the New Mexico Museum of Natural History and Science.