UNIVERSITY OF MINNESOTA DEPARTMENT OF EARTH SCIENCES Dinosaur Designs: A Self-Guided Exploration of the Science Museum of Minnesota s Fossil Exhibits Name: Instructor:
Dinosaur Designs Exploration Page 2 Dinosaur Designs Logistics: This is an optional exercise that guides you through an exploration of fossil exhibits at the Science Museum of Minnesota in downtown St. Paul. The only things you must hand in are the completed on-site tour exercise (this packet) and your wristband from the museum. Science Museum of Minnesota The Science Museum is of Minnesota is on the south side of downtown St. Paul overlooking the Mississippi River at 120 West Kellogg Blvd., St. Paul, MN 55102 (651-221-9444). Directions to the museum are posted at: http://www.smm.org/visitorinfo/planyourtrip/directions/ This site also contains links to Google Maps if you are driving or Metro Transit if you are taking the bus. You can also get driving directions at http://www.mapquest.com/directions. For those of you using GPS, the museum is located at: 44 56' 34" North, 93 05' 54" West Parking: Parking is available beneath the Museum. Events at the Excel Energy Center also use this parking ramp, so if an event is taking place they may charge you a higher fee to enter. However, if you take your parking ticket to the admission desk, the museum will rebate part of your parking charge. Parking fees and updates are available at: http://www.smm.org/directions/parkingrates/ Parking meters in downtown St. Paul are not enforced on Sundays or on weekdays after 4:30 p.m. Regardless of where you park, you might want to check the following web sites to see if there are other events in downtown St. Paul that will affect traffic and parking. Excel Energy Center: http://www.xcelenergycenter.com/events/index.jsp Rivercentre: http://www.rivercentre.org/calendar/?month_choice=10&year_choice=2010 Ordway Theatre: http://www.ordway.org/calendar/ Museum hours - check museum website for updates, but currently: Sunday 9:30 a.m. 5 p.m. Monday Closed Tuesday & Wednesday 9:30 a.m. 5 p.m. Thursday through Saturday 9:30 a.m. 9:30 pm Admission Charges and Discount Vouchers for College Students Admission to the exhibit halls is normally $13, but if your institution arranges it ahead of time with the Museum, they should be able to get you vouchers for a reduced admission as college students. Note that the museum can be really busy and noisy on weekends or any day elementary schools are off. Things tend be much quieter during the weekdays or evening hours. Base image from Google maps. Copyright 2013 - all rights reserved KCK
Dinosaur Designs Exploration Page 3 Dinosaur Designs Tour Your tour begins in the Museum lobby entrance, even before you enter the ticket line. As you come in the main lobby doors, look up to see the skeleton mounted above you. Quetzalcoatlus was a late Cretaceous pterosaur (flying reptile). Pterosaurs were NOT dinosaurs, but another line of Mesozoic reptiles. However, Quetzalcoatlus earns a spot on the tour simply because it was the largest known animal to ever fly above the Earth s surface. As all vertebrate animals share a common skeletal design, the pattern of arm bones in Quetzalcoatlus is similar to your own, only the proportions differ. Like your own arm, Quetzalcoatlus wing consists of a single upper arm bone that connects to the shoulder girdle (shoulder blade and collar bone), and two lower arm bones between the elbow and wrist. Beyond the small wrist bones lie long slender bones that form the fingers, one of which is much larger than the other three. Which part of the forelimb comprises most of the animal s wing? Is it the upper arm bone, the paired lower arm bones, or the bones of the elongated finger? Remember this pattern, so you can compare it to the other flying animals you will see next. Note how Quetzalcoatlus ribs and chest bones are fused together to form a single solid surface. This surface is where the wing muscles attached to the body, so it needed to be strong to withstand the strain of beating wings. In life, the bones above you were hollow and air-filled, so despite its size Quetzalcoatlus may have weighed less than 250 pounds. After purchasing your tickets, enter the exhibit halls on the other side of the lobby. Upon entering, you will be on the uppermost floor of the three main exhibit levels. Take the stairs or elevator down to the middle (4 th ) floor. At the foot of the stairs turn left and enter the glass doors on your left, labeled Collections Gallery. These doors are by the entrance to the Body Worlds exhibit. Once in the Collections Gallery, walk past the tree trunk, gorilla and bear towards the Collector s Corner, then turn right and continue on to the crocodile skeleton. Mounted above the crocodile s jaws is a skeleton of Psuedodontron, one of the largest birds ever found (although a few others were larger). Unlike pterosaurs, birds are dinosaur descendants and many paleontologists consider them as flying dinosaurs! Almost 30 million years ago, Psuedodontron soared over southeastern North America. Look at the design of the wing and compare it to what you saw in Quetzalcoatlus. Both animals were active flying forms, not passive gliders, so they share many similar traits like hollow bones and a light design. However, they differed in the design of their wings. Both forms had a single upper arm bone and two lower arm bones above the digits, but note how the relative proportions of these bones differ from those in Quetzalcoatlus wing. Which part of the forelimb comprises most of this animal s wing? Is it the upper and lower arm bones, or the elongated finger bones? Have digits in the wing been lost or fused together, or are all five digits still clearly distinct? Is the chest area composed of discrete individual bones or have they been fused together? Does it appear that most of the muscles for the wing connect to the shoulders and back or to the chest area? Does this suggest that the wings up stroke or down stroke was more important in flight? (Note the size and depth of this main muscle attachment area to compare it to the next form on the tour.) Note that this bird had teeth! Living birds lack teeth but their ancestors had them.
Dinosaur Designs Exploration Page 4 Exit the Collections Gallery, and go down the main stairs to the lower exhibit floor. Just ahead and to the left of the elevators is the skeleton of a horned dinosaur, Triceratops. This fall it is partially hidden behind a maze of wood-colored panels (Planet Earth Decision exhibit). As you reach the Triceratops, take a moment to consider the skeleton posed in flight above you and to the right of the Triceratops. The different wing patterns you saw in Quetzalcoatlus and Psuedodontron hold true for the all of the members of the pterosaur and bird groups. Although a sign identifies this creature, how can you tell from its wing design whether it is a bird (flying dinosaur) or a pterosaur (non-dinosaur reptile)? How does the size and depth of the main wing muscle attachment area compare to Psuedodontron s? Which animal appears to have had more powerful wing muscles? The weight of the rock burying the skeleton crushed the wing bones into the flattened shapes you see here. In life, the wing bones were round, hollow, and had walls less than a sixteenth of an inch thick! When asked, most people identify pterosaurs as flying dinosaurs and Hollywood movies, children s books and toy manufactures commonly support that misidentification. Far fewer correctly identify birds as dinosaurs. Skeletal designs though, clearly show relationships between the two lineages. Despite some differences due to fused digits, which features of their arm and digits suggest that birds share a common lineage with dinosaurs, while pterosaurs were a separate line of reptiles? Pterosaur, theropod and bird forelimbs (not shown to scale). Digit 1 corresponds to the thumb in human hands. Now walk around the Triceratops skeleton to examine it from different angles. Start with the skull. Triceratops lived at the end of the Cretaceous, after angiosperms (flowering plants) arose to dominate the landscape. Many features of its skull are adapted to acquiring or processing these higher-nutrient plants. The differences are dramatic when you compare the design of this animal s skull to that of one of the first dinosaurs (shown at right) which was a meat eater. Herrerasaurus teeth were simple blade-like sharp teeth that tore flesh without chewing. The margins of its jaws were smooth, lacking any indentations that would show the presence of cheeks or other structures to keep food in the mouth while chewing. However, plants are far more difficult to digest than meat, so the skulls of plant eaters often show adaptations that allow more extensive processing of food before swallowing it. Herrerasaurus skull Briefly describe some of the features of the Triceratops skull specifically tied to eating plants. How does the design and placement of the teeth differ from those of Herrerasaurus?
Dinosaur Designs Exploration Page 5 Is there any indication of recessed areas along the jaw that may indicate that Triceratops had some sort of cheek muscles or cheek-equivalent structures to hold food in its mouth while chewing? How has the front of the jaws been modified to crop plants? Although you can see the placement, size and shape of Triceratops teeth, it is more difficult to see that the teeth are fitted together to form a continuous slicing surface. As the teeth wore away, rows of erupting tooth buds in the jaw continually replaced them, so Triceratops never ran out of serviceable teeth. (It may be easier to see these tooth rows on the side of Triceratops closest to its flying friend.) Other parts of Triceratops skull served other uses. As an example, the horns were used for defense and in social contests (push the button on the Battle Scars panel to see the healed wound from another Triceratops). In the past there was some controversy about the frill at the back of the skull that covers the neck. Some people thought it was used as a shield to protect the neck, while others thought it was used as a surface area to shed body heat. Looking at the surface texture of the frill, what features suggest that the frill was used to shed heat rather than in defense? (You might want to wait to answer this question until you have seen the Stegosaurus and compared this frill to the Stegosaurus plates.) Another use of the frill was as a surface attachment area for large jaw muscles, seen here in a relative of Triceratops. Considering the size of its frill and the other jaw features you noted above, do you think Triceratops diet consisted of relatively soft plants or relatively tough woody vegetation? Front of Animal All dinosaurs had one of two hip designs (see figure at left). In lizard-hipped dinosaurs, one of the three bones of the hip (the pubis bone labeled at left) juts forward, while in bird-hipped dinosaurs this bone parallels and is sometimes fused to the other lower hip bone. Which of these hip designs did Triceratops have? What effect does this hip design have on the size of Triceratops gut area? Does it increase or decrease the area available to help break down and digest plants?
Dinosaur Designs Exploration Page 6 Triceratops frill and hips also play a role in an ongoing controversy over its lifestyle. Triceratops had a very large, powerful hip for an animal of its size with over a dozen sacral vertebrae fused together at the hip (in the mount, these lack white spacers between them). This created an even more powerful hip than you will see in the much larger Diplodocus later on the tour. Typically, an animal with such a powerful hip uses it to either run from predators or wrestle with members of its own species to establish social dominance. Let s consider other features to decide which option seems more likely here. Hollywood movies and artists often assume that Triceratops used its horns and frill to protect itself from predators like Tyrannosaurus rex. However, most living animals with horns and antlers primarily use those features to establish social dominance between members of their own species. Most fast-running animals also share a distinctive leg design. The upper leg bone is shorter than, or equal in length to, the two lower leg bones, while the ankle bones are elongated and the digits are long and slender. Finally, animals that rely on speed typically have erect, vertical limbs so their legs move in line with their body. Taken all together, what do the relative limb bone proportions and limb stance suggest about Triceratops lifestyle? Does Triceratops have the typical limb pattern and stance of an animal built for speed or do the limbs proportions and stance instead suggest its large hip helped it win shoving or wrestling social dominance contests with other Triceratops? Briefly explain which features support your answer. If you crouch down, you can see another area where Triceratops vertebrae were fused for greater strength. The three neck vertebrae closest to the skull were fused together to support its massive head. Ceratopians, like Triceratops, had the largest skulls of any land animals (including elephants). The whole skull attached to the spine with a simple ball and socket joint, which allowed it to rotate its head to strike predators with its horns or to push rivals in social dominance contests. In figure, skull and neck are pulled slightly apart to show neck joint. To the left of the Triceratops there is an exhibit with pathologic evidence of injuries and disease that you might find interesting. As you stand looking at Triceratops there should be a large glass case behind you that holds a Tyrannosaurus rex skull. After making the skull talk (you know you want to make the jaw move, so go ahead), look at the cast of the complete tooth (with root) mounted by the lever that moves the jaw. For years, people have debated whether Tyrannosaurus rex was a fearsome predator or the world s largest known scavenger. Consider this as you examine the tooth cast, as well as the teeth along the jaw. Approximately how much of the whole tooth was exposed above the gum line? (Circle one.) 1/10 1/3 1/2 2/3 9/10 Deeply rooted teeth are well designed to deal with struggling prey; scavengers do not need strongly mounted teeth, so what does this suggest about Tyrannosaurus rex? Most predatory dinosaurs have relatively thin, dagger-like teeth, well designed to slice into prey. Look at the shape of the T. rex tooth. Is it shaped more like a flat knife blade or a round spear point? If most predator dinosaurs had dagger-like teeth, does the T. rex tooth fit this pattern? (Can you begin to see why folks argue both ways over whether T. rex was a predator or scavenger?)
Dinosaur Designs Exploration Page 7 Besides its teeth, the predator versus scavenger controversy also involves T. rex s eyes and nose. Those who view T. rex as a giant turkey vulture argue that its large nose suggests T. rex was a scavenger who relied on smell rather than sight to find decaying carcasses. But eye placement is also an important consideration. If the eyes are placed so their lines of sight overlap (stereoscopic vision), an animal has depth perception. Non-flying scavengers do not need depth perception as their prey is already dead. If you stand in front of T-rex s skull, can you see both of its eyes? What does this suggest about a predator versus scavenger lifestyle? Walk toward the Paleontology Hall, keeping to the right of the staircase and the free standing exhibit panels. Look up to see the skeleton of a Mosasaurus mounted above the Step Back 70 Million Years sign. Imagine swimming across Kansas at a time when these characters, along with the large fish in the wall display ahead of you, roamed the waters! Like pterosaurs, Mososaurus is often misidentified as being a dinosaur. Mosasaurus sharp teeth identify it as a predator, but also hold clues to the identity of its favorite prey. The most abundant prey animals in Cretaceous seas were fish and ammonites. Ammonites were closely related to squid and octopi, but had hard shells. Most fish eaters have sharp slender teeth that curve towards the back of the mouth to trap and hold slippery fish. In contrast, predators that prey on shelled animals have thicker, spear-like, upright teeth that are capable of easily puncturing shells without breaking. Based on the design of its teeth, did Mosasaurus most likely rely primarily on a fish diet or is it more likely that ammonites made up much of its diet? Dinosaurs arose from a line with an erect stance (one with the limbs directly under the body as in most mammals), other reptile lines often had sprawled stances, with the limbs jutting out to the sides (like lizards). Animals with sprawled stances keep the same side-to-side backbone motion that their ancestors used to swim with, but they cannot easily run and breathe at the same time. A lizard can run quickly, but only over short distances before it must stop to catch its breadth. In contrast, animals with erect stances can run and breathe at the same time so they have greater running endurance. However, their spines need to move in a vertical plane, rather than a horizontal plane. So how does this tie to Mosasaurus? Sprawling Stance (crocodiles & lizards) Erect Stance (dinosaurs & modern mammals) Mosasaurus obviously used its tail to propel it through the water, but did its tail move from side to side or in an up and down motion? The difference between the two styles of swimming is a crucial clue as to the animal s ancestry. Animals that rely on side-to-side swimming tend to have pronounced lateral process on the vertebrae close to the hips (lower back and upper tail) for side muscles, while the end of the tail is taller than it is wide to catch the water and push the animal forward. In contrast, animals that swim with an up-and-down motion have little use for lateral processes and their tails tend to be much broader than they are tall. Looking at the design of Mosasaurus tail and spine, is it more likely that Mosasaurus swam with an up-and-down tail motion or a side-to-side tail motion? Does its tail and spine design suggest that Mosasaurus ancestors most likely had an erect stance (like mammals or dinosaurs) or a sprawled stance (like lizards)?
Dinosaur Designs Exploration Page 8 Continue on to the skeleton of the Stegosaurus (plated dinosaur) just below the large Diplodocus (long-necked sauropod dinosaur). Examine the Stegosaurus skeleton from all sides, as well as the single plate displayed in the glass case. As with the frill of the Triceratops, some people have thought that Stegosaurus plates were used for defense, while others argued that their primary role was shedding heat. Looking at the surface of the plate, what features of the plate suggest that its purpose was primarily as a heat-shedding device? Why would the presence of these features make the plate poorly suited to fend off a predator s teeth? (If you postponed the Triceratops frill question earlier, now would be the time to do it.) The display text notes that the plates could also be used to warm the animal up, an idea that dates to when we thought dinosaurs were cold-blooded. Now most workers believe dinosaurs were more likely warm-blooded. For a warm-blooded animal with a body this size and shape, which is likely to be more important, warming up in order to move more quickly or shedding heat to avoid heatstroke? Now look at the design and proportions of Stegosaurus upper and lower leg bones, as well as the shape of their anklebones and digits. Does this animal appear to be one designed for rapid running or not? Which features support your answer? Stegosaurus long tail is a legacy of it having a bipedal ancestor who used its long tail to counterbalance its body weight over its hips, in order to move on two legs. When its descendants (the stegosaurs) became larger and adopted a quadrupedal stance, this long tail could be adapted for uses other than balance. If Stegosaurus ancestors were bipedal though, it might be worth considering if Stegosaurus occasionally adopted a bipedal gait as well. Look at the design of the hands (forelimbs) and feet (rear limbs). Could Stegosaurus use its hands to grasp and manipulate things, or was their sole purpose walking? Which features or characteristics of the hand design supports your interpretation?
Dinosaur Designs Exploration Page 9 Look at the design of Stegosaurus skull and mentally compare it to that of Triceratops. Are there any large features that could have served as attachment surfaces for jaw muscles (like Triceratops frill)? Did Stegosaurus have a large sharp shearing beak, like Triceratops? Overall, does the design of Stegosaurus skull suggest that its diet consisted of relatively soft plants or relatively tough woody vegetation? Front of Animal Which type of hip design did Stegosaurus have and how did that affect the size of its gut area? Compare the Stegosaurus back vertebrae to those of its tail. The back vertebrae are taller, so the ribs attach to the vertebrae well above the spine s base (compare the cross-sections of a theropod and a stegosaur shown below). In addition, Stegosaurus ribs had a sharp bend so they arched out away from the vertebrae. Similar features also occurred in Triceratops. Along with the hip design, what impact do these modifications of the vertebrae and ribs have on the size of Stegosaurus gut area? Do they reduce or enlarge it? Do the much longer rear legs complement this impact or offset it? A B Cross-sections through the rib cages of a typical theropod (A) and Stegosaurus (B). Above is a larger image of a Stegosaurus back vertebra. Meat-eating theropod dinosaurs did not need these adaptations, yet if you walk back to Triceratops, you will find that it had similar hip, vertebra and leg designs. So why would plant-eating dinosaurs benefit more from these adaptations than meat-eating dinosaurs?
Dinosaur Designs Exploration Page 10 Next, examine the skeleton of Diplodocus, the long-necked dinosaur that dominates the entrance to the Paleontology Hall. Look at the skull (you may have to move back a bit to do this). Are the points of the teeth sharp or rounded? Do you think they are designed to cut and shear vegetation or simply rake vegetation off into the mouth? Is there any evidence of recessed areas along the jaw line for cheek muscles or beak structures? Based on this and the design of its teeth, is it likely that Diplodocus did a lot of food processing in its mouth before swallowing its food? Then look at the top of the vertebrae bones that make up the creature s neck and upper spine. If you were to run your finger down your backbone or a friend s backbone (do not try this with random museum visitors) you would feel a line of bumps that marks the top of your vertebrae, but each bone has a single bump. Diplodocus, though, had two spines on the top of each vertebra forming a notch between them (see image at right). In life, this notch held large ligaments that stretched from the back of the animal s skull, down its neck, to its back. These ligaments helped hold the skull and neck up, like a cable strung across a bridge. As you walk towards the rear of the animal, do the tail vertebrae closest to the hips exhibit similar features? In other words, did the tail counterbalance the weight of the neck or did these ligaments only extend to the animal s back? Look at the rest of the tail vertebrae. Does it appear that the animal s tail was flexible, or does it show evidence of being fused and rigid? If it was flexible, what use might this creature have for a flexible tail? If it was rigid, what use might this creature have for a rigid tail? Despite being a plant-eater, Diplodocus had a lizard-hip rather than a bird-hip. Its immense size allowed it to have a large gut even without a bird-hip design. Although Diplodocus did not have the same hip design as either Stegosaurus or Triceratops, what similarities does its skeleton share with those animals that helped create a larger gut area? Diplodocus lived long before flowering plants arose, so the plants it fed on were not very nutritious. Its immense size and large gut was a simple approach to dealing with a low-nutrient diet. However, while a large gut helps to digest plants, it also makes it more difficult for large animals to avoid overheating. Plates and frills helped some large dinosaurs shed heat, but Diplodocus did not have any of these features. What features of its body greatly increased Diplodocus surface area to help it shed heat?
Dinosaur Designs Exploration Page 11 Finally, compare the design of Diplodocus feet to those of Stegosaurus. Although both have roughly the same number of bones, Diplodocus feet are more vertically-aligned and the animal is actually walking on tip-toe. Most people intuitively grasp that short, stocky bones are well designed to carry weight, but it is more counterintuitive to realize that walking on tiptoe is an even better way for a really immense animal to carry its mass. Modern elephants have the same basic foot design as Diplodocus and fast running animals, like horses, that deal with high stresses also run on tiptoe. Why would walking on tip-toe be better for a really immense animal like Diplodocus, than having a foot design like Stegosaurus? (Consider how stress affects the toes as the animal moves forward.) Stegosaurus hand (big animal) Diplodocus hand (immense animal) Note that the Diplodocus foot still retained one large claw. As falls can be fatal for large animals, the claw on the inside of the foot might have kept Diplodocus from slipping as it crossed muddy surfaces. Be sure to check out the small mammal beneath Diplodocus. While not all Mesozoic mammals were this small, only a few of them grew to be as large as a small pig. Behind the Diplodocus there is a display with an Allosaurus skeleton and two Camptosaurus skeletons (one of which appears to be napping). Personally, I like that this Allosaurus is standing in an older, tail-dragging pose. Although we now believe Allosaurus walked with its tail extended to balance its body, this display reminds us of how our interpretations have changed over the years. It will be interesting to see how our current ideas evolve as we continue to learn more about these creatures. If you are a bit disappointed in the size of this meat-eater, realize that you are looking at an adolescent Allosaurus; the adults would probably have twice the size of this youngster. Still, its skull is beautifully designed for the kill, with a light-weight, flexible design that could resist the stress of struggling prey without breaking, stereoscopic vision, and a full complement of sharp, deeply rooted teeth to match its formidable claws. If the skull and forelimbs were missing, which features of the skeleton s hips, legs and chest area would still suggest that this was a meat-eating predator, rather than a plant-eater? (Compare the design of these features to those of the Stegosaurus or Diplodocus behind you.) Behind the Allosaurus and Camptosaurus display, beneath the wall painting are two display cases with dinosaur eggs. Although the eggs on the right are labeled as Protoceratops eggs, this is probably a misidentification. We now have fossils of Oviraptors that died, trapped by sandstorms or collapsing sand dunes, while sitting in brooding positions over similar egg clutches. In fact, we have even discovered embryos of Oviraptors within similar eggs, so these are most likely Oviraptor eggs. On the left are four sauropod eggs, a visual reminder that the immense Diplodocus behind you started life as an infant that could fit comfortably in such an egg. Although we do not know for sure how long it took Diplodocus to grow up, how do the apparent growth rates of dinosaurs compare with your own? (See exhibit information for this one.)
Dinosaur Designs Exploration Page 12 The small Compsognathus skeleton in the glass case to the left of the wall painting is one of my favorite dinosaur displays. Although this was a Jurassic animal, it is close in design and size to the ancestors of all dinosaurs. As you look into its eyes, you are seeing a blueprint for animal design that successfully dominated the land for over 150 million years and evolved into all the variety of dinosaurs you have seen so far. An erect stance (vertical legs, rather than sprawling legs) gave this animal greater running endurance and its long tail counterbalanced the body so it could run on its back legs (bipedal stance). Only two animal groups, dinosaurs (including birds) and humans, have become fully bipedal and both times it has proven to be very successful, but why? What advantage would having its hands free provide a small predator? An erect stance and bipedal gait can also be very advantageous for a plant-eating dinosaur, so early herbivorous dinosaurs had very similar designs to this one, just with some adaptations for plant eating. If you wanted to morph this animal into a small, bipedal herbivore, what changes would you make to the skull to make it better adapted for life as a plant eater? Since your new plant eater has to escape swift predators, you will want to keep its legs and tail as they are, but how else might you change the rest of the body (below the neck) to help your new herbivore better digest plant material? By now, you are probably tired of questions, so just enjoy the rest of the exhibits and take some time to see the other floors as well. Before leaving the Paleontology Hall though, be sure to see two last displays. Just behind the Diplodocus is a display of two skeletons that look like a giant armadillo and a small dinosaur. These are a glyptodont and Paraphysornis (a large flightless bird). Both lived in South America nearly 10 million years ago. When North and South America joined roughly 3 million years ago, the animal communities from both continents encountered one another and ended up competing for the same resources. In general, the North American community came out on top, simply because it had been in continual contact and competition with Eurasian and African lines, so North American animals had to be very good at what they did. The animals here, along with opossums and giant ground sloths, represent some of the few South American lines that survived the competition and expanded their range north. The glyptodonts survived because few predators could tackle them. Paraphysornis survived by following the same lifestyle of its dinosaur ancestors. However, note how the proportions of its leg bones differ from those of the Allosaurus behind you. Flightless birds have to sacrifice most of their upper leg for balance, to center their weight over their knees rather than their hips. This is necessary because their flying ancestors lost the nice heavy tails that dinosaurs used to balance their bodies over their hips. A claw from a giant ground sloth is in a glass case on the display railing. Because of their immense size, adult giant ground sloths had no natural predators until humans showed up. Although this one came from North Carolina, giant ground sloths made it as far as St. Paul s parks, which must have made picnicking in Como Park a few thousand years ago much more interesting. At the back of the hall, to the right of the staff counter in the reading area, there is a cave bear skull mounted in a glass case. As you look at this skull, can you see how skulls of this animal found in caves across Europe might have led to the legends of dragons? Enjoy the rest of the museum and remember to keep your wristband to turn in with the assignment!