Rapetosaurus

The discovery of the Rapetosaurus, known by the single species Rapetosaurus krausei (pronounced rah-PAY-too-SORE-us KROW-sie, and meaning "Krause's mischevious giant lizard") marks the first time a titanosaur has been recovered with an almost perfectly intact skeleton, complete with a matching skull. It has helped sort out some sticky century-old classification issues among this large group of sauropod dinosaurs, and provides a good baseline for the reconstruction of titanosaurs known only from partial fossilized remains.

The discovery was published in 1991 by Kristina Curry Rogers and Katherine A. Forster in the scientific journal Nature. The nearly complete skeleton is that of a juvenile, and partial remains from three other individuals were also recovered.

Description

The Rapetosaurus was a fairly typical titanosaur, with a short and slender tail, a very long neck, and a huge, elephant-like body. Its head resembles the head of a diplodocid, with a long, narrow snout, and nostrils on the top of its skull. It was an herbivore, and its small pencil-like teeth were good for ripping the leaves off trees, but not for chewing.

It was fairly modest in size, for a titanosaur. The juvenile specimen measured 8 meters (26 feet) from head to tail, and "probably weighed about as much as an elephant", according to Kristina Curry Rogers. An adult would be about twice as long, or 15 meters (50 feet) in length — which is still less than half the length of its gigantic kin, like the Argentinosaurus and the Paralititan.

Range

Most of the other sauropods, like the Brachiosaurus and Diplodocus, became extinct in the early Cretaceous. The titanosaurs were the one exception. They evolved on the southern supercontinent of Gondwana, and grew in numbers and diversity until they became the dominant herbivore of the late Cretaceous. Their reign was cut short by the Cretaceous-Tertiary extinction event, which killed almost all the dinosaurs about 65 million years ago.

The Madagascar dig

"Only a few of the tail bones were missing."

—Kristina Curry Rogers

The dig uncovered a partial skull (UA 8698, the holotype specimen), another partial skull, a juvenile skeleton missing only a few tail vertebrae, and an unrelated vertebra. The juvenile skeleton, in particular, is the most complete titanosaur skeleton ever recovered, and the only one with a head still attached to the body.

The fossilized remains were found in the Mahajanga basin in northwest Madagascar, not far from the port city of Mahajanga. They were recovered from a layer of sandstone known as the Anembalemba Member, which is part of the Maevarano Formation. The rock formation has been dated to the Maastrichtian stage of the late Cretaceous, which means the fossilized bones are about 70 million years old.

They were found by a field team from the State University of New York (SUNY) at Stony Brook, with the assistance of the local Universite d'Antananarivo. The team leader, David Krause, has been excavating fossils from the site since 1993.

A treasure-trove of bone

"We dug into the hillside, and the more you dug, the more bones we found".

—Kristina Curry Rogers

The Madadgascar location has produced a large number of significant paleontological discoveries for Krause and his team. As well as dinosaurs, fossils of fishes, frogs, turtles, snakes, crocodiles, birds, and mammals have been unearthed. Significant finds include:

• The skull of the Majungatholus, a large carnivorous theropod like the Tyrannosaurus, was discovered in 1996. It is similar to species found in India and Argentina, which indicate that land bridges between the fragments of the former supercontinent of Gondwana still existed in the late Cretaceous, far later than was previously believed. The most likely occurrence was a land bridge allowing animals to cross from South America to Antarctica, and then up to India and Madagascar. (See also Polar dinosaurs in Australia.)
• Majungatholus fossils have also been discovered with teeth marks that clearly come from the same species, making it the first dinosaur known to have practiced cannibalism.
• Masiakasaurus is a new species of raptor, with very unusual teeth that stick straight out from its jaw.
• A single, 70 million year old marsupial tooth. Madagascar was separated by water when the marsupials first evolved in the northern hemisphere, and there are no current species of marsupial on the island, which has revived the idea that colonies of animals might have somehow crossed vast stretches of water.

Implications

"What's been particularly frustrating to paleontologists who study these animals is that we haven't had a clue what a titanosaur skull looks like".

—Kristina Curry Rogers

The titanosaurs are the largest group of sauropods, but they are very poorly represented in the fossil record. Other groups of sauropods, even small families like the brachiosaurids, are known from more complete remains. Until the Rapetosaurus, each of the 30 or so genera were represented by just a few bones, a partial skeleton, or a skull. The first titanosaur, discovered in 1887, is still only known from a partial skull.

This has made it difficult to determine not just the relationship between different genera of titanosaur, but even how the titanosaurs are related to other, higher-level groups like the macronarians (the group of "big nosed" sauropods, which include the titanosaurs, the nemegosaurids, and the brachiosaurids). The whole taxon has been used as a dumping ground, with many genera labeled as incertae sedis (belonging to an unknown group), because not enough is known about them to classify them any further.

The Diplodocus-like skull has demonstrated that titanosaur skulls vary more than was previously believed. Most paleontologists believed that titanosaurs had boxy skulls with the nostrils on the side, like the Camarasaurus, but the Rapteosaurus has a long, low skull, with the nostrils on the top, like a Diplodocus. This has allowed genera known only Diplodocus-like skulls (like the Quaesitosaurus; and the Nemegtosaurus and the rest of the nemegtosaurids) to be positively classified as macronarians instead of diplodocidimorphs.

Analysis of the rest of the skull and the body has also confirmed what was only previous speculated: That titanosaurs are most closely related to the brachiosaurids. "The discovery of this dinosaur is particularly exciting because it confirms a close relationship between the titanosaurs and brachiosaurs, something that could only be surmised previously," according to Rich Lane, of the National Science Foundation.

A complete skeleton can also serve as a baseline when reconstructing other titanosaurs from limited remains. This is the basis for new, revised estimates of the size of the super-giant titanosaurids.

Classification

The new species, Rapetosaurus krausei, was described in the August 2, 2001 issue of the scientific journal Nature by Kristina Curry Rogers, then a graduate student under Catherine Forster, and now employed by the Science Museum of Minnesota and Macalester College, in St. Paul, Minnesota; and Catherine A. Forester, an Associate Professor at the Department of Anatomical Sciences of the State University at Stony Brook, in Stony Brook, New York.

The Rapetosaurus is a member of the Nemegtosauridae family, which is within the unranked Titanosauria taxon.

Etymology

"Rapeto is the name of a mischievous, mythical giant in Malagasy folklore, rather like our Paul Bunyan"

—Kristina Curry Rogers

The genera name Rapteosaurus is dervived from Rapeto (see quote above), and saurus, which is Greek for lizard. The species epithet, krausei, is named after the team leader of the expedition, David W. Krause.

References

• T. Mike Keesey's cladogram at the Dinosauricon. (See also cladistics.)
• Kristina Curry Rogers, and Catherine A. Forster. 2004. "The skull of Rapetosaurus krausei (Sauropoda: Titanosauria) from the Late Cretaceous of Madagascar". Journal of Vertebrate Paleontology, 24(1), pages 121–144. (Abstracts at BioOne and DinoData, with DinoData including additional analysis.)
• Kristina Curry Rogers, and Catherine A. Forster. August 2, 2001. "The last of the dinosaur titans: a new sauropod from Madagascar". Nature 412, pages 530–534. (Abstract here.)

External links

• Paleontology News — Rapetosaurus from the Science Museum of Minnesota.
• Stony Brook Paleontologists Discover New Dinosaur and Name It in Honor of One of Their Own from the University of Stony Brook.
• New Dinosaur Species Found from Time for Kids Online.
• Skeleton of New Dinosaur "Titan" Found in Madagascar from National Geographic.
• Dino skull fills knowledge gap from the BBC.
• New Madagascar Dinosaur Discoveries from the T. Rex Museum.
• A press release, from the National Science Foundation, including a reconstruction (illustration).