Ten Significant Finds in Paleontology Since SUE’s Unveiling

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Ten Significant Finds in Paleontology Since SUE’s Unveiling

CHICAGO - March 30, 2010 - SUE represents The Field Museum’s leadership in paleontology. Since the Museum’s founding in 1893, its scientists have made many breakthroughs in understanding the history of life on Earth. Here are 10 recent discoveries by Museum paleontologists:

Discovery of T. rex Growth Pattern and SUE’s Age

The evolution of Tyrannosaurus rex’s giant size is one of the most fascinating aspects of dinosaurs. The Field Museum announced a study in that demonstrated growth patterns of T. rex and three other species of tyrannosaur also revealed how old The Field Museum’s T. rex SUE was when she died. 

The Museum’s Curator of Dinosaurs Peter Makovicky and his colleague, Gregory Erickson of Florida State University made these breakthroughs by studying paper thin cross sections of tyrannosaur bones from twenty different specimens and counting the growth rings in them (much like counting rings in tree stumps.)  They calculated the corresponding body size from circumference measurements of the animal’s femur (large leg bone). By correlating these two sets of data they determined the growth curve. It has long been known that large weight-bearing dinosaur bones display growth lines, but they are hard to read because the bones’ marrow cavities expand and change shape as they grow, erasing some of the internal lines. This study, however, focused on smaller bones, including ribs and fibulas. These bones do not develop hollow cavities or remodel as they grow, so growth lines are clear when viewed under a microscope.

The researchers discovered that during the peak of its growth spurt, T. rex gained almost five pounds a day! The major growth spurt happened when the animal was 14-18 years old – a growth pattern that closely resembles that of human teenagers. By contrast other tyrannosaurs, grew only 15-25% as fast as T. rex.

The two scientists also determined that SUE lived to be 28 years old. While this may seem young by human standards, Sue was by far the oldest animal in the study. Because SUE’s skeleton shows signs of aging such as arthritis, and given the number of injuries and pathologies she suffered, scientists think that she was pretty close to the maximum age for her species.

Discovery of How Turtle Shell Evolved
 

With hard, bony shells to shelter and protect, turtles are unique and have long posed a mystery to scientists who wondered how such an elegant body structure came to be. Thanks to the work of a Field Museum scientist, we now have a clearer picture of how the turtle got its shell. 

Working with colleagues in China and Canada, Field Museum paleontologist Olivier Rieppel analyzed the world’s oldest turtle fossil, estimated to be 220 million years old. The ancient turtle, found in China, was dubbed Odontochelys semitestacea. Significant to scientists was the fact that Odontochelys has only a partial shell – making it an intermediate example of the evolutionary process. (A translation of the animal’s Latin name is “half-shelled turtle with teeth.”)

By studying Odontochelys, the team of scientists found evidence to support the notion that turtle shells are bony extensions of their backbones and ribs that expanded and grew together to form a hard, protective covering. They published their findings in 2008.  Prior to this discovery, some researchers theorized that turtle shells started as bony plates called osteoderms, which eventually fused to form a hard shell. Some of today’s reptiles such as crocodiles have skin with bony plates and this was also seen in dinosaurs. 

But the ancient turtle fossil has no osteoderms! Its partial shell extends from its backbone and it also shows a widening of ribs, leading scientists to believe this extension and widening of bones is how the turtle shell evolved.

The fossil did have a fully formed plastron – complete protection of its underside – just as turtles do today. This suggests that Odontochelys was a water-dweller whose swimming exposed its underside to predators.

Discovery of First Tree-Dwelling Vertebrate

 

In the Late Paleozoic (260 million years ago), long before dinosaurs dominated the Earth, ancient precursors to mammals took to the trees to feed on leaves and live high above predators that prowled the land. The elongated fingers, opposable “thumb,” and grasping tail of Suminia getmanovi demonstrate that this small plant-eater was the earliest known tree-climbing vertebrate (animal with a backbone).

Field Museum paleontologist Jörg Fröbisch and Robert Reisz of the University of Toronto published this breakthrough discovery in 2009. Their finding places vertebrates in trees far earlier than scientists previously thought.

Dr. Fröbisch studied several skulls and more than a dozen exceptionally well preserved, complete Suminia skeletons embedded in a single large block of red mudstone from central Russia. It’s rare to find fossils of several animals in a single block of stone and he was able to see examples of virtually every bone in Suminia’s body. 

The animal was relatively small, about 20 inches from its nose to the tip of its tail. The tree-climbing lifestyle of this Paleozoic relative of mammals is particularly important. For the first time, a vertebrate had access to new food resources high off the ground and also protection from ground-dwelling predators.

The study also provides the first evidence in the fossil record of food partitioning between small climbing and large ground-dwelling plant-eaters. Earlier terrestrial vertebrate communities did not have this hierarchy, but instead were composed of various-sized predators and relatively few plant-eaters.

Discovery of Two New Dinosaurs from China

During the summers of 2006 and 2007, an international team of researchers from China and the United States, including The Field Museum’s Curator of Dinosaurs Peter Makovicky, excavated a treasure trove of dinosaur skeletons in the Gobi Desert in China.  Two of their discoveries represented new species of theropod dinosaurs.

One was an early relative of T. rex named Xiongguanlong baimoensis (shong-GWAN-long  by-mo-En-sis) that would have stood about five feet tall at the hip and weighted close to 600 pounds. (Sue, The Field Museum’s famous T. rex, is 14 feet tall at the hips and weighed between six and seven tons.)

Xiongguanlong represents a “missing link” in the fossil record of tyrannosaur dinosaurs. Large tyrannosaurs, like T. rex, lived at the end of the age of dinosaurs and have been known to science for over 100 years. Until recently, there was a huge gap between early and late chapters of tyrannosaur history. Xiongguanlong is significant because it sheds light on the missing 40 to 50 million years of tyrannosaur evolution.   The team of scientists working in China also discovered a remarkable theropod named Beishanlong grandis (bay-SHAN-long gran-DIS). It is a new species of ornithomimosaur, or ostrich-mimic dinosaur. With an estimated body mass of almost 1,400 pounds, Beishanlong is one of the largest ornithomimosaurs yet described. By studying growth rings in the animal’s bones, scientists determined it was not yet fully grown when it died at age 14. In the future, scientists might find an even larger, adult specimen! 

Beishanlong was equipped with hand claws up to six inches in length and powerful forelimbs that could not be elevated much but may have been used for digging or raking the ground. Although the skull was not preserved, Beishanlong likely had a beak like other ostrich-mimic dinosaurs, and may have been herbivorous, despite its meat-eating origins.

Discovery: Ancient Snakes with Legs Were Similar to Modern Snakes

In 2000, Field Museum paleontologist Olivier Rieppel studied a new species of an ancient snake with well-developed hind limbs. The new snake, named Haasiophis terrasanctus, was the second limbed snake to come from an area in the Middle East which millions of years ago had an environment similar to a reef in the Bahamas.

By studying the well preserved 95-million-year-old fossil, Dr. Rieppel and his colleagues determined that this new species and the previously discovered limbed snake were close relatives. The team found that these animals were not primitive ancestors of today’s snakes but were similar to modern boas and pythons, with jaws that could be nearly unhinged to allow feeding on prey larger than the diameter of their own heads.

But an important question remains unanswered: Why do these two ancient snake species have stubby hind limbs?  Dr. Rieppel notes, “Since our fossil record of snakes is very poor, we can’t exclude the possibility that limbs in snakes were lost not just once in the beginning of their evolution, but several times throughout their history.”  The legs seen in the two species of fossil snakes are too small in relation to their whole bodies to provide locomotion. Interestingly, modern pythons have a rudimentary hindlimb which males use during mating.  It’s possible that these ancient snakes used their legs in a similar way.

Discovery of Oldest Raptor in South America
 

In 2004, a team of Argentine and American paleontologists including Peter Makovicky, curator of dinosaurs at The Field Museum, discovered a 90-million-year-old dinosaur about 700 miles southwest of Buenos Aires in the Patagonia region of Argentina. The new dinosaur, named Buitreraptor gonzalezorum belongs to a group of swift-running, birdlike dinosaurs called dromaeosaurs (the group includes Velociraptor). The new find was significant because it demonstrated that dromaeosaurs originated much earlier than previously thought.

During the Late and Middle Jurassic, the Earth’s one giant supercontinent split into two land masses. Laurasia, composed of what today is North America, Asia, and Europe, drifted north; Gondwana, composed of today’s Southern Hemisphere and India, drifted south. Until recently, dromaeosaur fossils had been found only in the rocks of Asia and North America. Buitreraptor (bwee-tree-rap-tor) provided the first definitive evidence that dromaeosaurs also lived in South America. Scientists concluded that this group of dinosaurs must have originated during the time when all the continents were assembled in a single land mass – as far back as 180 million years ago. 

Buitreraptor has birdlike features – a huge, hollow wishbone, long, wing-like forelimbs, and a bird-like pelvis – that provide more evidence linking dinosaurs to birds. It’s about the size of a very large rooster, but with a long head and very long tail. With adaptations such as a long, slender snout and small, widely-spaced teeth, Buitreraptor may have hunted small prey, such as snakes, mammals, and lizards.

A reconstruction of this unusual dinosaur – the most complete small theropod (carnivorous dinosaur) ever discovered in South America – is now on display Evolving Planet, a permanent Field Museum exhibition.

Discovery that Rise of Dinosaurs not as Rapid as First Believed

A team of paleontologists including Nate Smith, a graduate student working for The Field Museum, discovered fossils at Ghost Ranch, New Mexico that provided evidence showing dinosaurs and their ancestors coexisted for 15–20 million years or more. Previously, scientists thought that dinosaur precursors disappeared long before dinosaurs appeared, possibly because they were out-competed for resources. The team published its findings in the journal Science in 2007.

The fossils found at Ghost Ranch (a place made famous through the paintings of Georgia O’Keefe) provided paleontologists significant new information about the evolution of dinosaur precursors, their transition into true dinosaurs, and how dinosaurs diversified.

Dinosaurs and many other animals including mammals, lizards, crocodiles, turtles, and frogs, arose in the Late Triassic (235-200 million years ago) but it was only after the end–Triassic mass extinction (200 million years ago) that dinosaurs dominated the planet and all their predecessors vanished.

The team working in New Mexico found fossils of both early dinosaurs and dinosaur precursors, as well as bones of crocodile relatives, fish, and amphibians, all dating around 215 million years ago. These included a new species of small dinosaur precursor the team named Dromomeron romeri in 2007, and a new species of meat-eating dinosaur the team named Tawa hallae in a paper in Science in 2009. One of the additional dinosaur precursors found was a relatively large plant-eater that was quadrupedal (walked on four feet). Until recently, scientists thought all dinosaur precursors were probably small, carnivorous animals that walked on two feet.

Discovery of Giant “Reaper” Dinosaur in Utah Desert
 

In 2009, Field Museum paleontologist Lindsay Zanno was the principle author of a study that reported the discovery of a new species of the bizarre and enigmatic dinosaurs known as therizinosaurs (“reaper-lizards”). The fossilized remains of the new dinosaur, dubbed Nothronychus graffami (no-thrown-EYE-kus GRA-fam-eye), are the most complete remains of a large-bodied therizinosaur yet discovered worldwide. Only three species of this rare type of dinosaur have been found in North America to date.

Therizinosaurs are intriguing because they belong to a group of dinosaurs that includes such legendary meat-eaters Tyrannosaurus rex and Velociraptor but scientists think therizinosaurs were plant-eaters, not predators. Advanced therizinosaurs, like N. graffami, had odd bodies. Their small heads had a beak and tiny, leaf-shaped teeth. The dinosaur had a long neck, an enormous, barrel-shaped gut, stumpy legs, and a short tail, but most impressive were its enlarged scythe-shaped hand claws almost a foot in length!  The team doesn’t know why Nothronychus has such enormous claws on its hands but speculate they may have been used in feeding, for predator intimidation, or for sexual display.

Scientists found N. graffami while excavating plesiosaurs and other sea-dwelling animals in marine rocks deposited in southern Utah. No dinosaurs are known to have lived in the water, so encountering one was the last thing the paleontologists expected. The burial ground of the nearly complete skeleton was at least 60 miles from the ancient shoreline.  How the dinosaur carcass made it so far out to see relatively intact when the waters were teeming with predators and scavengers remains a mystery.            

Discovery of a Profound Ecological Change in World’s Oceans
 

The Earth experienced its biggest mass extinction about 250 million years ago, an event that wiped out an estimated 95 percent of marine species and 70 percent of land species. In 2006, two Field Museum paleontologists, Peter Wagner and Scott Lidgard, along with former University of Chicago colleague Matt Kosnik, published a paper detailing how this mass extinction did more than eliminate species: it fundamentally changed the basic ecology of the world’s oceans.

After the mass extinction, ecologically simple marine communities were largely displaced by complex communities. This apparently abrupt shift set a new pattern that has continued ever since. It reflects the current dominance of higher-metabolism, mobile organisms (such as snails, crabs, burrowing clams, and sea urchins) that actually go out and find their own food and the decreased diversity of older groups of low-metabolism, stationary organisms (such as lamp shells and sea lilies) that filter nutrients from the water. After the mass extinction, the complex communities outnumbered the simple communities nearly two to one.

This striking change escaped detection until recently because previous research relied on single numbers – such as the number of species alive at one particular time or the distribution of species in a local community – to track the diversity of marine life. In the new research, however, scientists examined the relative abundance of marine life forms in communities over the past 540 million years.

One reason they were able to do this is because they tapped the new Paleobiology Database, a huge repository of fossil occurrence data. The result is the first broad objective measurement of changes in the complexity of marine ecology during the Phanerozoic era (the half-billion-year period in which multi-celled organisms have lived on Earth).