Dinosaurs made their first appearance in the fossil record in the earliest part of the Late Triassic Epoch. While a few of these early dinosaurs are known from excellent fossils, many are only understood on the basis of fragmentary remains. The complete story of early dinosaurs is woefully incomplete in most cases. Even the partial record of early dinosaurs shows that they existed at about the same time in several widely separated locations around the world, however; these locations include what are now Brazil, the United States (New Mexico and Texas), and India. Having become widely distributed by the Late Triassic suggests that the dinosaurs currently recognized as being the first were the descendants of even older dinosaurs yet to be discovered.
If a paleontologist were to bet on the most likely location in which to find additional clues about the origins of dinosaurs, a good place to start would be South America because that is where the earliest remains currently known are found. Staurikosaurus ("lizard of the Southern Cross"), from Brazil, dates from the earliest part of the Late Triassic, about 228 million years ago. First described in 1970 by Edwin Colbert, Staurikosaurus is known only from a partial skeleton and lower jaw, but the features exhibited by its backbone, pelvis, and jaw clearly identify it as a true dinosaur. Staurikosaurus measured about 6 feet (2 m) long and probably weighed about as much as a medium-sized dog. It had a sliding joint in its lower jaw—the intramandibular j oint—that allowed the part of the jaw that contained its teeth to move forward and backward, making it more difficult for prey to wriggle free.
Two additional small to medium-sized carnivorous dinosaurs from the Early Triassic have been found in Argentina, Eoraptor and Herrerasaurus. Both are known from excellent skeletons. Eoraptor ("dawn thief") was the smaller of the two; it measured a mere 3 feet (1 m) long and had a lightweight body. At up to 15 feet (5 m) long, Herrerasaurus was the larger of the two. It was named after Victorino Herrera, the Argentine goatherd who found the first specimen in 1963. The evolutionary position of Herrerasaurus was not well understood until the 1990s, when additional specimens were studied.
All three of these early carnivores exemplified the body plan from which all other dinosaur body plans appear to have evolved for more than 160 million years. They had long hind limbs to provide mobility; a center of gravity near the hips, so that the animal could
easily lean over to maintain a balanced gait while running; flexible arms for grasping prey; a short body; a flexible, S-curved neck; and a long tail for additional balance.
Staurikosaurus, Eoraptor, and Herrerasaurus were all "lizard-hipped" saurischians, one of the two main types of dinosaurs based on hip design. As a group, these three dinosaurs are classed as basal saurischians, a primitive group of saurischians that preceded the radiation of more advanced and closely related carnivorous dinosaurs.
The "lizard-hipped" dinosaurs were not limited to carnivores, and the first evidence of plant-eating saurischians also dates from the early part of the Late Triassic. These herbivores were the "pro-sauropods," an early group of long-necked, plant-eating dinosaurs that appear in the fossil record only slightly later than the Staurikosaurus, Eoraptor, and Herrerasaurus. One of the earliest of these is
Saturnalia (Brazil), which is known from three partial skeletons and measured about 6 feet (1.75 m) long, a medium-sized animal within its domain. Saturnalia lived at roughly the same time as the earliest known carnivores of Argentina.
The roots of "prosauropods" are somewhat obscure. In 1999, two incomplete specimens of a saurischian from Brazil was discovered that might provide a common link between the "prosauropods" and theropods. Guaibasaurus dates from approximately the same time as Herrerasaurus but appears to have been more primitive in such skeletal features as its dorsal vertebrae, pelvis, upper leg bone (femur), and foot, giving it some close similarities to "prosauro-pods." Guaibasaurus may in fact represent a common ancestor to both theropod and "prosauropods."
"Prosauropods" were plentiful by the end of the Triassic. They grew large and bulky, sometimes exceeding 33 feet (10 m) in length—a clear sign of the gigantism to come in succeeding lines of dinosaurian herbivores. "Prosauropod" remains are plentiful and found in all corners of the world—i ncluding Germany, Argentina, South Africa, Lesotho, Zimbabwe, China, England, and Antarctica—but, strangely, not in the American Southwest. They
are among the best known of the Triassic dinosaurs and were certainly abundant.
Dinosaurs of the Late Triassic are not limited to those with saurischian hips. One basal member of the Ornithischia, the plant-eater Pisanosaurus, was discovered in the same rock formation that yielded fossils of Herrerasaurus and Eoraptor. Known only from a fragmentary skull and skeleton, Pisanosaurus includes traits that connect it with later ornithischians, but its lineage remains somewhat clouded by a lack of diagnostic evidence. Even so, this small dinosaur, measuring perhaps 3.3 feet (1 m) long, clearly lived alongside the first known predatory dinosaurs. This suggests that larger carnivores such as Herrerasaurus may have fed on this herbivore. Only a few small ornithischian dinosaurs are known from the Late Triassic. Yet from this tiny handful of taxa, they would later eventually develop into many diverse lines of plant eaters, including armored and plated dinosaurs, iguanodonts, "duck-bills," and the horned dinosaurs.
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THINK ABOUT IT
Several generations of the world's most prominent paleontologists have been engaged in the search for dinosaur beginnings. Discoveries made over many decades have gradually improved our knowledge of the first dinosaurs.
Hermann von Meyer (1801-1869) was a German naturalist and fossil hunter during the formative years of dinosaur science. His most celebrated fossil was that of the bird-reptile Archaeopteryx that he described in 1861. Long before then, however, he had been working in the fossil beds of southern Germany, where a rich fauna of terrestrial life was recorded. In 1837, several years before the word "dinosaur" was coined by Richard Owen, von Meyer had come across some large, stocky bones and given them the name Plateosaurus, or "flat lizard," to describe what was clearly a large, bulky animal. The original specimen was not complete and consisted primarily of limbs, vertebrae, some digits, and a fragmentary skull. Without a complete pelvic girdle or teeth, the true affinities of the "gigantic saurian" were not clear. A complete understanding of Plateosaurus did not come for another 84 years, until another German paleontologist, Friedrich von Huene (1875-1969), completed his own landmark study. Von Huene had recovered thousands of bones, making up several complete skeletons of Plateosaurus, from a wooded hillside near Trossingen, Germany. What was revealed in these bones was a large, bulky, plant-eating dinosaur that measured almost 30 feet (9 m) long, with a small head affixed to a moderately long neck, a stout body, and a long, heavy tail. Von Huene's famous paper describing Plateosaurus made him something of an international star among fossil hunters. The energetic German continued to work well into his eighties and named more new dinosaurs than any other paleontologist before or during his time.
The discovery and understanding of the first carnivorous dinosaurs in Argentina is another story that involved decades of detective work. In 1959, Argentine paleontologist Osvaldo Reig (1929-1992) explored the badlands of the Ischigualasto region of northwest Argentina with his guide, a local artisan and goatherd named Don Victorino Herrera. The region contained rocks dating from the earliest part of the Late Triassic Epoch, and from these he described Herrerasaurus, a sizable predatory dinosaur, based on some fragmentary limbs, pelvic elements, and vertebrae. From these rocks, Reig described Her-rerasaurus, a sizable predatory dinosaur. He based his description on some fragmentary limb, pelvic, and vertebrae elements. The discovery was significant because it pushed back the origin of dinosaurs even further than Plateosaurus, but the fossil material was incomplete and left many unanswered questions about the evolutionary position of Herrerasaurus.
The fossil location that yielded Herrerasaurus was visited by some of the most famous paleontologists of the twentieth century, including von Huene in 1930 and Alfred Romer in 1958. But it was 30 years later before significant new fossils of Herrerasaurus were unearthed. In 1988, American paleontologist Paul Sereno mounted an eight-week expedition to the Ischigualasto region to search for more fossils of early dinosaurs. This was Sereno's first expedition as a leader. He was joined by Argentine
colleagues, including the legendary paleontologist José Bonaparte (b. 1928), whose numerous discoveries span the entire duration of the dinosaur age, and Fernando Novas. Since that time, Sereno and Novas have become two of the superstars of dinosaur science; in 1988, however, Sereno in particular was still trying to make his mark in the field.
Three weeks into the expedition, Sereno made the discovery of a lifetime, an articulated skeleton of Herrerasaurus eroding out of the dry badland sandstone. Sereno immediately understood the significance of the find. "Unlike the early finds reported 25 years before," explais Sereno, "this skeleton had a skull and forelimbs, which allowed the first accurate reconstruction of this very primitive dinosaur." The specimen included the first skull found for Herrerasaurus; in subsequent expeditions, Sereno's team found several other partial skeletons. Sereno and Novas went on to describe the spectacular fossil and also had another surprise up their sleeves. In 1991, during one of their follow-up expeditions to the same territory, expedition member Ricardo Martinez of Argentina discovered the remains of a contemporary of Herrerasaurus, the 3-foot (1-m) long Eoraptor.
Here is one footnote to the Herrerasaurus story. Recall that the dinosaur was originally named by Reig in 1963 after Don Victorino Herrera, the local man who led Reig to the site. Sereno had the chance to meet Herrera and his wife, still living in their humble village home at the foothills of the Andes mountains, just after Serano's discovery of the new specimen of Herrerasaurus. Herrera died in 1990.
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Unambiguous evidence of the first dinosaurs from the Late Triassic paints a picture of a world already populated with a variety of these creatures. These early dinosaurs were outnumbered by other kinds of reptiles and mammal-l ike reptiles; the presence of dinosaurs in the fossil record increases dramatically toward the end of the Triassic. This clearly shows that their fortunes had turned, and the world was falling under the control of the ruling reptiles. How this transition occurred is the subject of the discussion below.
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