Rigby unearths remains of two (maybe three) T. Rex

by Kim McDonald

FORT PECK, Mont.— A team of students and volunteers working in northeastern Montana has unearthed the remains of what appear to be two, or possibly three, specimens of Tyrannosaurus rex, a rare find that could shed new light on the lives and habits of this fearsome dinosaur.p. The first of the gigantic creatures was discovered a year ago on a ranch near the Fort Peck Dam, east of here, in a region where the famed fossil hunter Barnum Brown discovered two of the first-known specimens of T. rex at the turn of the century. The second dinosaur and the remains of a much smaller carnivorous dinosaur were discovered this summer on an exposed ridge that, 66 million years ago, was a riverbed into which the bones apparently had washed and been trapped.p. “We know that there are at least two, because we have duplicate elements of the same bone in the skeleton,” said J. Keith Rigby, Jr., who led the team, in an interview here during the last week of his summer excavation. “And there are about a half dozen pieces of a third carnivorous dinosaur that is probably either a juvenile T. rex or a middle-sized carnivore like Albertosaurus,” a smaller cousin of T. rex.p. Mr. Rigby, a professor of engineering and geological sciences at the University of Notre Dame, said one of the dinosaurs has an exceptionally large pubis, a part of the pelvic bone, that suggests it is one of the biggest T. rex skeletons ever found, perhaps reaching 60 feet from head to tail. The other specimen is from an average-size T. rex, about 45 feet in length.p. Mr. Rigby had planned not to disclose details of the find — which was made by Louis Tremblay, a summer volunteer who teaches earth sciences at Avon High School, near Hartford, Conn. — until he could excavate and fully examine the remains. But he, Notre Dame, and the Earthwatch Institute — a non-profit organization in Watertown, Mass., that provides him with volunteers and research support — decided to announce the discovery last September. The decision was made after poachers used a backhoe to remove two-thirds of the left side of the skull of what was then thought to be one specimen of a large tyrannosaur, both of its lower jawbones, and other pieces of its skeleton (The Chronicle, September 26, 1997).p. The fossil fragments were recovered by agents of the Federal Bureau of Investigation and are now in the hands of local prosecutors, who are building a legal case against the poachers. To prevent further looting, armed security guards patrolled the site this summer. Meanwhile, the rest of the fossils — some in cabinets and others still embedded in large, plaster-covered blocks of sediment — are being safeguarded in a converted warehouse by a $30,000 security system.p. Mr. Rigby’s conclusion that the remains are from two or possibly three specimens of Tyrannosaurus rex was based on his examination of fossils that were removed from those blocks over the past three months by Mr. Tremblay and other volunteers. The fossils include two pairs of left and right ischia — the rear third of the pelvis — from the two larger dinosaurs and the back end of a skull and some toe bones from the small dinosaur.p. In addition, he said that this summer’s excavation appeared to have yielded from 60 to 80 per cent of one of the dinosaur’s fossil remains, “which would put it up there with probably the top four or five T. rex skeletons of all time in terms of completeness.”p. Although the Notre Dame professor has not formally announced his most recent finds, some details have begun to circulate among North American authorities on carnivorous theropod dinosaurs.p. “It’s pretty impressive,” said Philip J. Currie, curator of dinosaurs and birds at the Royal Tyrrell Museum, in Drumheller, Alberta.p. “It probably is three T. rexes, which is really cool,” said Peter L. Larson, a commercial collector who excavated “Sue,” the largest and most complete skeleton of a T. rex yet found.p. He said that because fewer than two dozen skeletons of the ferocious-looking dinosaur are known — each of which has yielded more information about its anatomy — the discovery of two or more specimens was a scientifically noteworthy achievement that should greatly improve knowledge of the creatures.p. “The more material we have, the better picture we’ll have of its anatomy,” said Mr. Larson, president of the Black Hills Institute of Geological Research, in Hill City, S.D. “There are always surprises. The real learning happens in the lab, when you start looking at these things closely.”p. He said the excavation of two or three T. rexes in one location may lend support to the emerging idea that the carnivorous dinosaurs lived in “family groups and were not lone hunters or scavengers.”p. “Finding a third individual would really give some indication of family life,” he said.p. The larger and medium-sized T. rexes from the Montana excavation may also help paleontologists finally determine whether large T. rexes — which have anatomical differences from the medium-sized creatures — are females, as Mr. Larson believes, or are a distinct subspecies.p. Mr. Rigby’s discovery is not the first time specimens of the dinosaur have been found together.p. During his 1990 excavation of Sue, Mr. Larson unearthed in the same location several fossilized bones from another, smaller T. rex, a sub-adult T. rex, and a baby T. rex, leading him to suspect that he had discovered members of a family (The Chronicle, September 15, 1995).p. He said he also believed that a juvenile T. rex had been found with a large T. rex in the 1960s by an amateur collector in the Fort Peck area. (The smaller specimen was catalogued as another species of theropod dinosaur at the Los Angeles County Museum of Natural History, but Mr. Larson said, based on his examination of the material, that it was misclassified.)p. The experts are not unanimous in their views of those discoveries, however. Kenneth Carpenter, a paleontologist at the Denver Museum of Natural History, said the deposit of fossils from different T. rexes had probably been the result of bones’ being washed together or “chance” occurrences. “It’s pure speculation,” he said, to conclude that the dinosaurs had come from the same family or hunted together. “It may make a good story, but there’s no evidence for it.”p. However, Mr. Currie, of the Royal Tyrrell Museum, said he had irrefutable evidence that some of T. rex’s smaller cousins lived or hunted in groups. Last summer, he and his team located the site in Alberta’s Dinosaur Provincial Park where Barnum Brown in 1910 reported the discovery of a mass grave of Albertosaurus dinosaurs.p. He said his team’s excavation of the site thus far had shown that nine albertosaurs were buried in one spot, with no other dinosaur fossils mixed in — proof that the carnivores had lived or hunted together. Because calculations suggest that albertosaurs, like tyrannosaurs, made up only about 5 per cent of the large-animal fauna at the end of the Cretaceous period, he explained, finding so many albertosaurs buried together indicates that the deposit could not have been the result of their skeletons’ randomly washing down the same hole. “The probability of finding even two together would be one in 400,” he said.p. Mr. Larson pointed out that like wolves, which hunt large animals traveling in herds, T. rex and other carnivorous dinosaurs would have had an advantage hunting in packs to aid them in bringing down large plant-eating dinosaurs.p. Mr. Rigby said the discovery of so many tyrannosaurs, like his own, in ancient riverbeds also suggested that the dinosaurs lived and hunted in riparian environments, hiding behind trees and probably ambushing their prey.p. He said one of the T. rexes from his excavation has interesting features that may provide clues about how it hunted and lived: broken ribs that healed before its death, abscesses around its teeth, fragments of vertebral bone that are as thin as construction paper, and bones that show arrested development and a secondary growth of marrow.p. “Does this represent where the dinosaur nearly died of starvation and then all of a sudden got healthy again?” he asked. “We picture these animals as being such mighty, tyrannical giants, but as we learn more about them, we see that they were also vulnerable. They’re prone to injury, they have diseases.”p. He said he also had found evidence that one of the T. rexes was partially eaten by another. He drew that conclusion from scratches on the fossil and a collapsed piece of bone, “where a tooth came across it and essentially shoved the bone in.” As for the bones themselves, they’re “exceptionally well preserved,” he said. “They’re not even fossilized in the classic sense.”p. Over the whine of dental drills, which his volunteers used to remove the debris covering the dinosaur’s bones, Mr. Rigby added that although he initially suspected that the two or three T. rex specimens in his excavation were from the same family, he had been unable to conclude so. The reason, he said, is that they had been retrieved from three adjacent, geologically distinct deposits that probably buried the bones “weeks to months” apart.p. But he said he and his co-workers expected in subsequent summer digs to retrieve many more pieces to their paleontological puzzles, which they hope will become the centerpieces of a museum scheduled for completion in the Fort Peck area in 2005.p. “We’ve only gone through about half of that ridge, and there are much larger ridges that are producing bones in the immediate vicinity,” he said. “So we’re not done, by any means.”

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