Brief History of Dinosaurs

We arrived back in New York during the first week of December 1997. As detailed in our collecting agreements, Rodolfo and his museum in Plaza Huincul gave us permission to borrow several of the fossils we had collected so that Marilyn could prepare them at the Peabody Museum at Yale University and Luis could study them at the American Museum of Natural History. At the same time, Rodolfo and Sergio planned to prepare some of the other blocks of eggs at the Carmen Funes Museum. Our initial task was to compile and whittle down the list of possible victims.

As mentioned earlier, large, round eggs such as the ones we had found in Patagonia had been often identified as belonging to sauropods, and those from the end of the Cretaceous were usually attributed to titanosaurs. This preliminary identification had been based on circumstantial evidence, including the large size of the eggs, the occurrence of titanosaur fossils in the same rock layers as the eggs, and that these types of eggs are found only in deposits that contain skeletal remains of titanosaurs. However, sauropod dinosaurs had not previously been discovered inside any of the round eggs attributed to these dinosaurs, so we could not be certain of this identification. In fact, because of this very reason we could not be sure that sauropods laid eggs at all, an uncertainty that led some paleontologists to speculate that sauropods gave birth to live young.

Dinosaurs are a diverse group of animals. They dominated the continents for more than 150 million years, during which time dozens of groups arose and went extinct. Just about any large dinosaur that lived near the end of the Mesozoic had to be considered as a possible victim of the ancient catastrophe at Auca Mahuevo. The quickest way to compile our list of likely victims was to look back through the evolutionary history of dinosaurs and identify which groups lived in Patagonia at the end of the Mesozoic. This narrative will also provide those interested with a brief review of dinosaur evolution and the method that paleontologists use to reconstruct it.

Since the naming of the first dinosaur in 1824, several hundred kinds of dinosaurs have been discovered, and more are being found all the time. A primary goal of paleontology is to understand how different species are related to one another, in other words, to reconstruct the history of how they evolved. This pursuit is somewhat like studying the genealogy of a family tree. Contemporary paleontologists use a scientific method called cladistics to reconstruct the evolutionary history of extinct groups and draw family trees that link groups of ancient animals and plants. Willi Hennig, a German entomologist, introduced this analytical method in the 1950s, and subsequent researchers have refined it over the last forty years.

Cladistics is based on a fairly simple concept. Although life is diverse, we see a pattern in that diversity when we look for characteristics that are shared by different organisms. This pattern of characteristics can be used to arrange organisms into smaller groups contained within larger groups. The arrangement of groups within groups results from evolution, when descendants inherit new characteristics from their ancestors. By studying how these characteristics are distributed among different animals and plants, we can determine the order in which different groups evolved and thereby interpret the sequence of evolutionary history.

The evolutionary relationships among different groups of organisms can be shown on branching diagrams called cladograms. A clade is simply a group of organisms that includes the first member of the group, or the group's common ancestor, and all of its descendants. In essence, branches on the tree represent different clades of animals or plants, and the branching points on the tree represent common ancestors that possessed new evolutionary characteristics, which were inherited by the descendants of the common ancestor on higher branches of the tree.

For example, some characteristics are shared by a large number of animals. Fish, frogs, dinosaurs, and humans all have a backbone composed of vertebrae. Thus, they all belong to the group of animals called vertebrates, which constitutes a major limb on the family tree of animals. The backbone is thought to have evolved in the very first vertebrate, or the common ancestor of the group. Then, all of its descendants, including humans, inherited a backbone from that common ancestor. Other characteristics are shared by a smaller number of animals within the vertebrate group. For instance, frogs, dinosaurs, and humans have four limbs (arms and legs) with bony wrists, ankles, fingers, and toes, so these animals belong to a subgroup of vertebrates called tetrapods, which means "four-footed." Tetrapods represent a smaller branch on the limb of the tree that contains all the vertebrates. Again, four limbs originally evolved in the ancestor of all tetrapods, and all the descendants of that common ancestor, including humans, inherited some version of its four limbs.

Since having a backbone is more widespread among animals than having four limbs—for example, since fish have backbones but lack limbs—the backbone is thought to have evolved before the limbs did.

Fossil Represent Timeline
Genealogical relationships of the major groups of vertebrate animals. Birds arc included within dinosaurs.

This is why tetrapods are considered to represent a subgroup within vertebrates. In this example, the fossil record appears to confirm this idea, because the first known vertebrate lived about 500 million years ago, whereas the first known tetrapod appeared about 350 million years ago.

However, the fossil record does not always confirm the evolutionary sequence of characteristics suggested by looking at features shared by larger and smaller groups. This is not too surprising because only a small fraction of the organisms that once inhabited the earth are preserved as fossils, and many of those are yet to be discovered. How else could we explain why paleontologists discover new species of extinct dinosaurs and other organisms every year? Because the fossil record is so incomplete, most contemporary paleontologists prefer not to rely on the age of the fossils when reconstructing the family trees of ancient organisms. This task is more accurately achieved by comparing only the physical characteristics, such as backbones and limbs, preserved in the fossils.

Reconstructing the genealogy of plants and animals is often further complicated since the enormous array of characteristics found in these organisms—from physical features to molecular genes—did not evolve only once in a linear fashion. Similar characteristics have evolved in animals that are apparently not closely related. Among living animals, for example, wings evolved in insects, bats, and birds. A warm-blooded metabolism is found in mammals, birds, and tuna. Humans, birds, and kangaroos are all bipedal, meaning that they walk exclusively on their two hind legs. Other examples of features shared by distantly related groups abound.

An important component of cladistics involves sorting out whether these similar characteristics evolved from the same ancestor or different ancestors. To make this decision scientists rely on the principle of parsimony, often referred to as Occam's razor. Given alternative genealogical interpretations, the principle chooses the simplest explanation that accounts for the arrangement of groups on the family tree. In other words, it identifies the evolutionary sequence of events that is supported by the most pieces of evidence and contradicted by the fewest pieces of evidence. For example, there are two alternative interpretations involving the genealogy of bats: Are bats more closely related to birds or to mammals? Although bats and birds both have wings and are warm-blooded, a greater number of characteristics provide evidence that bats are more closely related to other mammals than they are to birds. Bats have hair, nurse their young with milk, have three bones inside their ears, and have teeth of different shapes like other mammals. Birds do not share these characteristics, and in fact, the wings of birds and bats are constructed quite differently. Whereas five fingers of the hand support the membrane that forms a bat's wing, the feathers of a bird's wing are attached to the arm and only one finger of the hand. This difference is interesting, but the number of similarities shared by bats and mammals is more important for interpreting the genealogy of bats. Using the principle of parsimony, we can see that most of the anatomical evidence suggests that bats are more closely related to mammals. Consequently, birds and bats are not as closely related, even though they both have wings and are warmblooded. In the end, cladistic methodology compares only similarities shared among different groups, then uses the principle of parsimony to choose the genealogical interpretation that is supported by the larger number of similarities.

The previous example may seem rather obvious, since most biology students have learned that bats are mammals and birds are not, but cladistics and the principle of parsimony have shed new light on other evolutionary puzzles that had previously proved more difficult to solve. One of these is the evolutionary relationship of birds to crocodiles, lizards, and other reptiles. At first glance, crocodiles look much more like lizards than birds. Crocodiles and lizards have a scaly, reptilelike body, whereas birds have a feathered body with wings. Yet, despite the obvious differences seen in crocodiles and birds, a closer examination reveals that crocodiles and birds share many more physical similarities than crocodiles and lizards do. For example, crocodiles and birds share a series of air holes in their middle ear that are not found in lizards. Birds and crocodiles also share a robust rib cage that is strengthened by short struts of cartilage called uncinate processes. In birds and some theropod dinosaurs, these uncinate processes ossify. Another feature found in crocodiles and birds is a muscular stomach or gizzard that processes food. Lizards do not have gizzards. Consequently, when we apply the principle of parsimony, crocodiles and birds are more closely related than crocodiles and lizards on the family tree of vertebrates. The similar reptilian appear ance of lizards and crocodiles is evolutionary misleading and is refuted by the careful scrutiny inherent in cladistics; so most evolutionary biologists now classify crocodiles and birds in the same evolutionary group (Archosauria), which is a subset of lizards and all other reptiles.

Understanding the genealogical relationships between ancient organisms is crucial for reconstructing the origin of living animals and plants, as well as for understanding the evolution of their anatomical, physiological, and behavioral systems. When looking at a particular characteristic, such as our grasping hand, the explanation of its evolutionary origin would be very different if we assumed that humans originated from bats rather than primates. If we originated from bats, our grasping hand would have had to evolve from a structure used for flying, which would be difficult to imagine, although not impossible. However, since many other characteristics of our skeleton indicate that humans evolved from other primates, which use their hands for holding on to branches and gathering food, the evolutionary origin of our grasping hands is easier to understand.

With this concept for reconstructing evolutionary history in mind, we can trace some of the steps that led to the evolution of the major groups of dinosaurs by working our way up the branches and branching points of the dinosaur family tree. The earliest known dinosaurs appear in the fossil record about 230 million years ago. Some of them, such as the herbivorous Pisanosaurus, and the carnivorous Herrerasaurus and Eoraptor, were found in northwest Argentina. The fact that these dinosaurs already had specialized anatomical features for eating plants and flesh, which distinguish them as members of specific dinosaur groups, indicates that there must have been earlier dinosaurs, although we have yet to find fossils of them. The first question that arises in looking at these and other dinosaurs is, what makes a dinosaur a dinosaur? In other words, what characteristic evolved in the common ancestor or very first dinosaur? To find out, we need to search for a characteristic that is found in all dinosaurs but not found in other reptiles, including turtles, lizards, snakes, and crocodiles.

The story of the evolution of dinosaurs basically revolves around locomotion. The common ancestor of dinosaurs possessed a hip structure different from that found in other reptiles. All dinosaurs have a hip socket, or acetabulum, that had a hole in the middle of it and a strongly developed ridge of bone along the top of the socket. As a result, dinosaurs inherited a vastly different posture and gait from their common ancestor.

To illustrate this, think of the way a lizard stands. Its hind limbs extend horizontally out from its hips before the lower hind limb bones reach vertically down to the ground. The limbs support the rest of the body in a sprawling posture, and when the lizard moves, its body basically makes an S-shaped motion. The acetabulum in a lizard is solid, with no hole in the middle, which makes sense structurally. Where the lizard's thighbone, or femur, meets the hip, a lot of force is generated by the muscles that pull the bone horizontally into the hip socket. The acetabulum is solid to help resist those horizontally directed forces, and no extra bone is needed along the upper margin to help provide support.

In contrast, dinosaurs have hind limbs that extend vertically down from the hips to the ground, resulting in a more upright or erect posture. Clear evidence for this is found in sequences of fossil foot-

Erect Posture
Figure contrasting sprawling posture in a turtle (left) with erect posture in a dinosaur (right).

Ornithischia

Saurischia

Dinosauria Genealogical relationships of dinosaurs.

prints called trackways, which show that when dinosaurs walked, the left and right footprints form a nearly straight line with one another. With this arrangement of the hind limbs, the femur exerted force in a much different direction where it fit into the acetabulum. The force generated by the dinosaur's weight was directed toward the upper margin of the hip socket rather than toward the center, with the bony ridge at the top of the acetabulum helping to counteract that force. And because no force was directed toward the center of the socket, a hole is found there. Dinosaurs were, thus, the first reptiles in which a fully erect stance evolved.

Two major groups within dinosaurs represent the two large limbs on the family tree of dinosaurs. One is called Ornithischia, which contains most of the groups of herbivorous dinosaurs, including armored dinosaurs, duckbills, horned dinosaurs, and dome-headed dinosaurs. The 230-million-year-old Pisanosaurus is the oldest known ornithischian dinosaur, and like all other early dinosaurs, Pisanosaurus walked on its two hind legs. This suggests that the common ancestor of dinosaurs must also have been bipedal. Even at this early date in dinosaur evolution, the plant-eating Pisanosaurus already exhibits the physical characteristics that are typically found in the group. Omithischians evolved from a common ancestor in which one of its hipbones, called the pubis, points toward the rear of the animal. In the common ancestor of all dinosaurs, the pubis pointed forward. It is not immediately obvious what purpose this change in direction served. Perhaps it provided extra support for the gut, which had to be large to process the enormous amount of vegetation that was required to nourish the herbivorous omithischians, but no one knows for sure.

Armored dinosaurs represent one of the most spectacular branches on the ornithischian limb of the dinosaur family tree. Noteworthy members include the tanklike ankylosaurs and the spectacularly ornamented stegosaurs. Together, these two lineages make up the group called thyreophorans, which evolved from a common ancestor with a mosaic of bony armor that essentially covered the entire body. Even the earliest thyreophorans walked on all four legs, probably to better support the weight of their heavy, bulky body. The armor, which often included enormous spikes and plates, almost certainly played a role in protecting these dinosaurs from the predatory dinosaurs that roamed the environment. However, the armor may also have played a role in helping these dinosaurs recognize members of their own species and potential mates. Because we cannot observe the behavior of these animals in the wild, again, we simply cannot be certain.

The duckbills, horned dinosaurs, and dome-headed dinosaurs comprise a group called Cerapoda. Unlike the thyreophorans, most of these dinosaurs retained the hind-legged gait of their early ancestors. Only within the horned dinosaurs did a four-legged gait evolve, illustrating a common theme that often recurred in the evolution of large, heavy groups of dinosaurs. Cerapods evolved from a common ancestor that had an uneven covering of enamel on the inside and outside surfaces of their teeth, which, like other reptiles, they replaced continually throughout their life. In many cerapods, a complex mosaic of teeth grew one on top of the other in the jaws. Presumably, the uneven covering of enamel helped to maintain a rough, rasplike surface on top of the batteries of teeth to shred and crush the vegetation that they ate.

Duckbills, which are also called hadrosaurs, represent an extremely diverse and abundant lineage on the family tree of cerapods. They arose from a common ancestor in which the hingelike joint between the upper and lower jaws lay below the level of the tooth rows. This arrangement resulted in an extremely powerful crushing action as the chewing musculature clamped the jaws together. The lower position of the jaw joint, along with the rasplike chewing surface on top of the tooth rows, provided duckbills with an ability to grind up tough vegetation that was unmatched in other groups of dinosaurs. Undoubtedly, this specialized feeding adaptation played an important role in the evolutionary success of duckbills. In North America, these dinosaurs are among the most commonly found. Their fossil remains are sometimes preserved in large accumulations known as bone beds, which presumably represent death assemblages caused by the catastrophic demise of huge herds that contained individuals of all ages. Large herds of duckbills probably roamed the floodplains along major rivers that ran across the continent toward the end of the Mesozoic.

In spite of their distinctive jaw apparatus, however, duckbills are most commonly recognized by the elaborate bony crests that adorned the top of the skull in many species. The function of these crests has intrigued paleontologists for more than a century. Some have specu lated that the large air passages contained within the crests served as resonating chambers to amplify vocal calls made by the duckbills, while other paleontologists have suggested that the crests were used to identify other members of the same species and potential mates. Again, it is difficult to be certain since all the duckbills are extinct, and perhaps the crests were involved in both these activities.

The horned dinosaurs and dome-headed dinosaurs form a group within cerapods called the Marginocephalia. The name comes from the shelf of bone rimming the back of the skull, which was inherited from the common ancestor of the group. Evolution took this bony shelf and expanded it in two different directions in the horned dinosaurs and dome-headed dinosaurs. In horned dinosaurs, also known as ceratopsians, the bony shelf at the back of the skull expanded backward to form a shieldlike structure called a frill. Although the frill is relatively small in more primitive members of the group, such as the parrotlike Psittacosaurus and its distant relative Protoceratops, it became a large and elaborate structure in many later and larger members of the group, such as Triceratops and Styra-cosaurus. In dome-headed dinosaurs, also known as pachy-cephalosaurs, the bony shelf along the back of the skull expanded forward to form a thick, bony helmet on top of the skull. In some later members of the group, such as Pachycephalosaurus, this dome is about six inches thick. Furthermore, while horned dinosaurs developed a four-legged gait in the more advanced members of the group, such as Triceratops, the dome-headed dinosaurs retained the two-legged stance and gait of the earliest dinosaurs.

The second major limb at the base of the family tree contains the saurischians, who evolved from a common ancestor that had a hand capable of grasping. The thumb was slightly offset from the rest of the fingers. The bony structure of the saurischian's grasping hand differs slightly from the structure of our hand, but the basic result was the same. Within saurischians, we find two large branches on the evolutionary tree. One contains all the giant, long-necked, four-legged herbivores called sauropods. The other contains all the two-legged, carnivorous theropods, including their descendants, birds.

Theropods evolved from a common ancestor that had only three fully developed toes on the hind feet, with the central toe being the longest. The common ancestor of theropods gave rise to many

Theropod Family Tree

branches near the base of this limb on the dinosaur family tree. For the most part, these branches contain relatively primitive, meat-eating dinosaurs. Some of these are among the oldest dinosaurs that we know about, such as Herrerasaurus and Eoraptor, which were collected from rocks in northwest Argentina that are about 230 million years old. Another branch near the base of the limb represents the abelisaurs, including the Argentine Carnotaurus. This fearsome predator lived later in the Mesozoic era, about 75-80 million years ago, although the lineage leading to this imposing branch of dinosaurs must extend back millions of years before this. As mentioned earlier, Carnotaurus was unusual in that it had short arms and sported two prominent bony horns above its eyes. We will delve into more details regarding abelisaurs later in our story.

The other larger branch of theropods contains a group called tetanurans. These dinosaurs evolved from a common ancestor that possessed collarbones that were fused together like the wishbone of birds, as well as a hand that had no more than three fingers. Branches near the base of the tetanuran limb of the tree include the giant meat-eaters Allosaurus and Giganotosaurus. Giganotosaurus, one of the largest of all the known carnivorous dinosaurs, had been found in the same province of Argentina where our expedition conducted its exploration, and our colleague Rodolfo had been instrumental in collecting and describing the massive skeleton.

Another branch within tetanurans contains smaller meat-eaters, such as Velociraptor and Compsognathus of Jurassic Park fame, as well as birds and the infamous 'Yyrannosaurus. This group contains all dinosaurs called coelurosaurs, who evolved from a common ancestor that had an extra hole in the snout for reducing the weight of its skull and an elongated ilium, the main bone that forms the top part of the hip. The first coelurosaur also had elongated arms, although this feature was later modified in many of its descendants, including Tyrannosaurus and the kiwi. For those descendants that retained long arms, these appendages may well have been an advantage in capturing prey.

Spectacular new discoveries in China have shown that primitive coelurosaurs were covered with downy structures that are interpreted to be the evolutionary precursors of the vaned feathers found in birds. These discoveries suggest that most, if not all, coelurosaurs

The Patagonian theropod Gigano-tosaurus is among the largest carnivorous dinosaurs. Our crew discovered the remains of some of its relatives at Auca Mahuevo.

were feathered; even the colossal Tyrannosaurus must have had a feathered body at some early stage of its life. However, scientists do not believe that adult tyrannosaurs were feathered because the combination of this insulating covering and their large size might have posed a disadvantage in regulating the animal's body temperature.

One branch within coelurosaurs leads to the "bird-mimic" dinosaurs called ornithomimids. Ornithomimids have a skeleton that looks superficially like that of an ostrich, which explains the derivation of their name. That these dinosaurs had long legs with shortened toes, which would have reduced the amount of friction with the ground, suggests that ornithomimids were swift runners. Most ornithomimids lack teeth in their jaws, which has led to some questions about what these animals ate. Recent discoveries of stones called gastroliths inside the stomach area of some ornithomimid skeletons suggest that these dinosaurs may have eaten plants because similar stones are commonly found in the gizzard of herbivorous birds and other plant-eating dinosaurs. Yet gastroliths are also known from other theropods that clearly had a carnivorous diet.

The other branch within coelurosaurs leads to birds and small, meat-eating dinosaurs, such as Velociraptor. These evolved from a common ancestor that possessed a crescent-shaped bone in their wrist called the semilunate, because it is shaped like a crescent moon, and hips in which the pubic bone pointed toward the rear of the animal. The shape of the semilunate bone allows the wings to be folded back against the body in the distinctive position that we see in birds today. The presence of this bone in Velociraptor and its relatives indicates that these dinosaurs were also able to fold their arms back against their body in the same way that birds do. These dinosaurs form the group called maniraptors, named for their extremely enlarged and elongated hands, which were certainly formidable predatory weapons.

As odd as it may seem, birds are maniraptors and, therefore, dinosaurs. They belong on the same branch of the dinosaur family tree that contains Velociraptor. Earlier, when we introduced the fundamentals of cladistic methodology and the principle of parsimony, we downplayed the role of physical differences and emphasized the role of similarities in reconstructing the evolutionary relationships of organisms. The same concepts must be applied to reconstruct the evolutionary relationships between birds and other dinosaurs.

Superficially, Velociraptor and other well-known maniraptors may appear very different from living birds. While the fearsome star of Jurassic Park possessed large claws and sharp teeth, birds lack both these features. Yet, a closer examination of the skeletons in birds and Velociraptor reveals numerous similarities, and these become even clearer when one compares the skeleton of Velociraptor with those of ancient Mesozoic birds.

Birds not only share the unique structure of the hips and wrists found in other maniraptors, but also the long arms of coelurosaurs, the three-fingered hands of tetanurans, the three-toed feet of theropods, and the perforated hip socket of dinosaurs. In addition, they have feathers, like their most immediate forerunners among the manirap-tors and coelurosaurs. In the last few years, spectacular discoveries from the northeastern corner of China have shown that the body of maniraptors was also covered with feathers. A close cousin of Veloci-raptor, the Chinese maniraptor called Sinornithosaurus had two-to-three-inch-long downy feathers covering its skin. The fact that feathers, long thought to be a characteristic unique to birds, have been found on fossil skeletons of other maniraptoran dinosaurs has dealt the final blow to paleontologists who doubted that birds evolved from dinosaurs. Today, we can declare that birds are dinosaurs with the same degree of confidence that we can say that humans are primates.

Birds experienced a long and complex evolutionary history, most of which was played out in the Mesozoic era. The earliest known bird is Archaeopteryx, which lived about 150 million years ago in the late Jurassic period. Exquisite fossils of this crow-sized bird were first found in the limestone quarries of southern Germany in the mid-1800s. The skeleton of Archaeopteryx still had a very dinosaurian appearance, with a long tail, sharp teeth, and powerful claws on its wings, although its plumage was fully modern. Within 20 million years after Archaeopteryx lived, birds had already evolved with a more typical, stumpy tail and wings that gave them a more modern appearance, though birds would retain their teeth throughout the Mesozoic.

Another early branch on the family tree of birds contains all Enantiornithes, who were sophisticated fliers, even though the first known members of the group lived as early as 125 million years ago. It is rather amazing to realize that even that far back in time, birds were able to perform the same aerodynamic feats that they delight us with today. The common ancestor of Enantiornithes and modern birds possessed a small tuft of feathers that attaches to the first finger, or thumb, of the hand. This "bastard wing" was an important aerodynamic innovation and somewhat comparable to the flaps of an airplane. The fine-tuning of flight capabilities led to a large radiation of birds. Enantiornithes quickly adapted to life in the water, in the air, across more open terrain, and even in deserts. In fact, the primary initial goal of our expedition was to find fossils of this primitive group of birds.

The other major branch within saurischians includes the giant plant-eating dinosaurs called sauropodomorphs, which arose from a common ancestor with a relatively long neck and small head. This group contains the largest animals ever to have walked on land, and walk they did. Their remains have been recovered from every continent.

The earliest member of the group is Saturnalia, a 230-million-year-old dinosaur from the late Triassic of Brazil, who had a gracile body that was about five feet long with a small head and a long neck. A more advanced and better known primitive sauropodomorph is the

Genealogical relationships of sauropods.

W Neosauropods JP Eusauropods Sauropods

Genealogical relationships of sauropods.

The dicraeosaurid sauropod Amar-gasaurus lived in Patagonia during the early Cretaceous period. Its back was lined with two rows of tall spines that projected from each of its backbones.

fifteen-foot-long Plateosaurus, which possessed a grasping hand that it inherited from the common ancestor of all saurischians. Its hind limbs were much longer and stronger than its arms, and it is clear from the proportions of these limbs that Plateosaurus, like the smaller Saturnalia, walked only on its hind legs.

The main branch on the sauropodomorph limb of the dinosaur's family tree contains all the enormous sauropods. In these animals, all four limbs were strongly developed like the columns of a building, indicating that they walked on four legs. The bones of their wrists and ankles were greatly reduced in number, presumably an adaptation for supporting their heavy weight. Fossilized trackways confirm their quadrupedal mode of locomotion. The four-legged gait of sauropods represents another example of the recurrent evolution of this type of locomotion in large dinosaurs, probably to help support the weight of their colossal bodies.

Within sauropods, there are several branches on the evolutionary tree, although scientists are still debating how the different groups are related to one another. These groups include diplodocids, dicraeo-saurids, camarasaurids, brachiosaurids, and titanosaurs. Diplodocids are exemplified by Diplodocus, the extremely long-necked, whip-tailed sauropod of the late Jurassic in North America. Although one of the longest dinosaurs ever discovered, it probably did not weigh as much as some of its more robust sauropod relatives. Its teeth are elongated, peglike or pencil-like structures that were probably used to strip vegetation off branches.

The dicraeosaurids include Dicraeosaurus, a relatively small, short-necked sauropod from the late Jurassic of Tanzania, and Amar-gasaurus, a Patagonian form that lived in the early Cretaceous. Dicraeosaurids evolved from a common ancestor with tall spines on top of its vertebrae, especially in the hip region, and peglike teeth similar to those of diplodocids. Most paleontologists agree that dicraeosaurids are very closely related to diplodocids and believe that their peglike teeth evolved in the common ancestor of these two groups.

Brachiosaurids have longer front legs than hind legs, a posture present in their common ancestor that made them some of the tallest dinosaurs ever found. The immense Brachiosaurus was discovered in the late Jurassic of Tanzania by a German expedition at

Titanosaurs Skeletons
The skeleton of the small titanosaur Saltasaurus was protected by large bony scutes imbedded in its skin.

the beginning of the twentieth century. This dinosaur is probably the tallest of all, with an extremely long neck extending above its tall shoulders.

Titanosaurs range from medium to gigantic in size. The titanosaur Argentinosaurus, with its 100-ton, 120-foot-long body, is the largest dinosaur ever found. It may well have shaken the Patagonian landscape with every step it took. Titanosaurs are the most diverse group of sauropods; their skeletons are known from South America, North America, Africa, Europe, and Asia. Most lived during the Cretaceous period, although the earliest known form is Janenschia, from the late Jurassic of Tanzania, and more than a dozen species inhabited South America. Titanosaurs evolved from a common ancestor that had small oval air holes in their vertebrae, an important adaptation for lightening their heavy skeletons. There were as many as thirteen vertebrae in the neck, twelve in the trunk, six in the sacrum, and more than thirty in the highly flexible tail. The spines on top of the neck and trunk vertebrae were simple, instead of being split as in diplodocids. Advanced titanosaurs had tail vertebrae with a concave front surface, and their teeth resemble the pencil-like teeth of diplodocids. The front legs of these animals were shorter than the hind legs. One of their most remarkable characteristics is the bony covering of armor formed by thousands of small, rounded lumps and a few larger plates, although it is unclear that this formidable armor developed in all members of the titanosaur lineage.

This abbreviated cladogram of dinosaurs provided us with a list of potential victims that could have perished inside the dinosaur eggs at Auca Mahuevo. Our next job was to begin eliminating candidates by determining what kinds of dinosaurs had previously been found in Patagonian rocks, especially ones that were from about the same age as the dinosaurs collected at Auca Mahuevo.

Many diverse lineages of dinosaurs have been discovered in Patagonia's vast Mesozoic rock layers, extending from the late Triassic up to the very end of the Cretaceous, a period of more than 130 million years. In the past, only a handful of discoveries of omithischians had been made, but recent findings are demonstrating that the history of omithischians in Patagonia is much richer than previously thought. All Patagonian omithischians are restricted to the Cretaceous. Although specimens of ceratopsians, stegosaurs, and ankylosaurs are known from

Omithischian

With an enormous body supported by its pillar-like limbs and backbones of higher than 5 feet, the Pata-gonian titanosaur Argentinosaurus could have reached lengths of 120 feet.

fragmentary remains, duckbills and their primitive relatives are much better represented. Both primitive duckbills and their primitive forerunners are known from late Cretaceous rocks of northern Patagonia. Anabisetia and Gasparinisaura are two small, distant cousins of the duckbills known from Neuquen and Rio Negro provinces, respectively. True duckbills, known from the late Cretaceous of northern Patagonia, include Secernosaurus, Kritosaurus, and another unnamed crested form. In contrast to their North American relatives, duckbills appear to have been less successful in South America, where late Cretaceous ecosystems were dominated by huge, herbivorous sauropods.

The earliest known Patagonian theropod is from the early middle Jurassic of Chubut, but it is very fragmentary. Much more complete is Piatnitzkysaurus from the middle Jurassic of Chubut. Piat-nitzkysaurus was a moderately sized, meat-eating, primitive tetanu-ran. Many more Cretaceous theropods have come from Patagonia, the most abundant of which are the abelisaurs. An early member of this group is the early Cretaceous Ligabueino, an animal the size of a pigeon. Much larger abelisaurs lived in the late Cretaceous of Patagonia, including the horned Carnotaurus, one of the largest carnivores of its time.

A diverse assemblage of sauropodomorphs has also been found in Patagonia. In the Triassic, prosauropods roamed the land and nested there. Several specimens of these, including adults, juveniles, hatch-lings, and eggs, were collected by Jose Bonaparte in the late Triassic rocks of Santa Cruz province, in the southern end of Patagonia. The spectacularly preserved hatchlings, known by the name of Mussaurus, which means "mouselike lizard," were little more than six inches long. The earliest known sauropod from Patagonia is Amygalodon, from the early part of the middle Jurassic, which is known from isolated teeth and a vertebra that were collected in the province of Chubut. Bonaparte and his collaborators found better sauropod remains in the middle Jurassic of Chubut in the late 1970s. An incomplete skeleton constitutes the only known specimen of the dinosaur Volkheimeria. Another middle Jurassic sauropod from Patagonia is Patagosaurus, which Bonaparte discovered in the same rock layers that produced Volkheimeria and the theropod Piatnitzkysaurus. The primitive Patagosaurus is known from several specimens of both adults and juveniles.

Abundant remains of sauropods are also known from the Cretaceous of Patagonia. A peculiar one from the early Cretaceous is the dicraeosaur Amargasaurus, a close relative of the late Jurassic Dicraeosaurus from Africa, unearthed from the same strata as the tiny theropod Ligabueino. One nearly complete skeleton was found in which most of the animal's bones were still articulated. These dinosaurs have a double row of long spines that project upward from their backbones, as well as peglike teeth like those found in their cousins the diplodocids. Another relative of the whip-tailed diplodocids is Rebachisaurus, a middle Cretaceous sauropod that is also known from Africa.

Titanosaurs are the most common sauropods from the late Cretaceous of Patagonia. As we've seen, these were the first dinosaurs to be found on the South American continent. Numerous remains of these dinosaurs have been found in the northern half of the region, particularly in rocks within the province of Neuquen, where we were searching. In addition to Argentinosaurus, other Patagonian titanosaurids include Andesaurus, Epachthosaurus, Aeolosaurus, Neuquensaurus, and several other species.

The list of candidates that could have laid the eggs at Auca Mahuevo was extensive. So we began by paring it down to those that came from late-Cretaceous rocks of Patagonia, since we knew from previous studies that the rocks we were collecting were from this age. The embryos from Auca Mahuevo could have belonged to dinosaur groups that lived millions of years earlier or on other continents, but to start, our list of the most likely candidates included only the dinosaurs known to have lived in the region at that time. Possible candidates among ornithischians included duckbills and their forerunners; among theropods, the abelisaurs were candidates; and among sauropods, titanosaurids constituted likely victims.

We were now ready to begin to search for more definitive clues to help us solve the mystery. The bony evidence we needed to make a positive identification of the victims at Auca Mahuevo was hidden inside the eggs.

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