Ceratosaurs Of The Cretaceous

The earliest theropods—the Coelophysoidea—lived during the Late Triassic and Early Jurassic Epochs and represented the first radiation of carnivorous dinosaurs. Several other primitive lines of theropods persisted well into the Late Cretaceous, particularly in regions of the Southern Hemisphere (now Argentina, North Africa, India, and Madagascar). These were the ceratosaurs, many of which exhibited bizarre adaptations not seen in theropods of the Northern Hemisphere.

The body plan of most ceratosaurs included a short, stocky neck, long hind limbs, a stout tail, and strong but short forelimbs tipped with four-fingered hands. The top of the skull of ceratosaurs was often embellished with horns or lumps. Neoceratosaur skulls were also broad at the snout and tall, with a deep premaxilla positioned below the nasal openings.

Ceratosaurus (Late Jurassic, Colorado, Wyoming, and Utah), first described in 1884, was one of the first large carnivorous dinosaurs to be well understood. Ceratosaurus was a medium to large predator that measured up to 20 feet (6.6 m) long. Recent computed tomography (CT) studies of the braincase of a new specimen of Ceratosaurus suggests a posture for the head and neck that was more horizontal than upright, eyesight that was probably about average for a theropod, and a well-formed sense of smell comparable to that of birds.

The Cretaceous ceratosaurian relatives of Ceratosaurus are represented by several excellent specimens from the Southern Hemisphere. These ceratosaurian predators were the most abundant theropods of Gondwana during the Late Cretaceous and are known from partial and nearly complete specimens from Argentina, Brazil, North Africa, Madagascar, India, and southern Europe.

Partial remains of large Cretaceous theropods from India were some of the earliest remains of ceratosaurians discovered. Consisting of dissociated bones from the skull, vertebral column, girdles, and limbs, as well as incomplete skeletons, some of these taxa— including Laevisuchus, Indosuchus, and Indosaurus—still remain enigmatic today many years after their discovery in the 1920s and 1930s. A partial skull roof from Madagascar announced in 1979 was originally thought to be that of a dome-headed dinosaur, the likes of which had never been discovered in the Southern Hemisphere. Named Majungatholus ("Majunga dome") after a region in Madagascar, its affinities with ceratosaurian predatory dinosaurs would remain a mystery until 1998. This same dinosaur would be re-christened Majungasaurus. The discovery in Argentina in the early 1980s of such partial specimens as Noasaurus ("northwestern Argentina lizard") and Xenotarsosaurus ("strange ankle lizard,"

O Infobase Publishing A Ceratosaurus skull

after its unusually fused ankle bones) in the early 1980s encouraged famed Argentine paleontologist José Bonaparte (b. 1928) to expand his fieldwork in the Cretaceous fossil beds of his home country. It was Bonaparte whose breakthrough discoveries of Carnotaurus and Abelisaurus in the 1980s began to unlatch a window on the history of Gondwanan theropods that had yet to be opened more than a crack.

Carnotaurus (Late Cretaceous, Argentina) is known from a remarkably complete skeleton and skull. It was in some ways an extreme predator, with an exceptionally short and tall skull, a short, strong neck, and a broad muzzle. The name Carnotaurus means "meat-eating bull"; the animal was given its name because of the prominent bull-1 ike horns on the brow of each eye. This medium-sized theropod was about 25 feet (7.5 m) long and had a stout, strong neck, forelimbs that were shorter than those of Tyrannosaurus, and four claws each instead of the two seen in tyrannosaurs. With its strong legs and muscular jaws and neck, Carnotaurus possibly attacked nose first, thrusting its mouth into its prey and ripping flesh away with a turn of its head. On the other hand, the teeth of Carnotaurus were remarkably short, and its lower jaws were weak compared to those of most other theropods. Perhaps Carnotaurus attacked with its upper jaws in a chopping, axe-like fashion.

While further specimens of Carnotaurus have yet to turn up, a similar theropod named Aucasaurus (Late Cretaceous, Argentina) was named by Argentine paleontologists Rodolfo Coria and Luis Chiappe and American paleonologist Lowell Dingus in 2002 based on another nearly complete skeleton. Aucasaurus was 30 percent smaller than Carnotaurus, lacked the brow horn, had a somewhat longer, narrower skull, and also had forelimbs that were somewhat longer than those of its bigger cousin.

The true nature of Majungasaurus (previously named Majun-gatholus; Late Cretaceous, Madagascar), once thought to be a bone-headed pachycephalosaur, became evident in the late 1990s when a team of paleontologists led by American David Krause went to Madagascar and uncovered several new partial specimens, including a spectacularly preserved skull. The skull did indeed have a bony knob on top of its head, but it was actually a rounded horn on the forehead, not a skull cap as found in pachycephalosaurs. The skull was tall and blunt nosed, like its ceratosaur cousins from South America. Majungasaurus was a medium-sized carnivore measuring about 21 feet (6 m) long with strong hind legs and short, t hree-clawed forelimbs. A curious discovery among the fossils of Majungasaurus were bones with tooth marks that can be attributed to Majungasaurus itself; this provided evidence that this theropod may have either fed on its own kind or scavenged dead members of its clan. In either case, this evidence shows that some dinosaurs may have been cannibalistic.

The story of ceratosaurs from Gondwana also includes a the-ropod with one of the most unusual jaws ever seen in a predatory dinosaur—Masiakasaurus (Late Cretaceous, Madagascar). Named in 2001 by a team led by American paleontologist Scott Sampson, Masiakasaurus ("vicious lizard") is so named because its teeth protrude outward, especially along the front of the jaws. In some ways, its mouth more closely resembles those of extinct marine reptiles (see Chapter 8), which also suggests that Masiakasaurus was adapted to eat fishes, snakes, and other small, scampering animals. Sampson remarked that when they first found the lower jaw, they did not think it was from a dinosaur because they had never seen anything like it. The 6-foot-long (2 m) Masiakasaurus turns out to be just one of many bizarre evolutionary sidetracks that took place on the island of Madagascar. That island, along with the India landmass, had separated from the rest of the southern continents during the Cretaceous Period. Masiakasaurus is in good company with equally bizarre birdlike dinosaurs (or dinosaurlike birds); blunt-nosed, plant-eating crocodiles; and horned theropods such as Majungasaurus that are also found on the island. The complete scientific name of this dinosaur also gives us the rare opportunity to mix dinosaur trivia with rock and roll trivia: The complete name is Masiakasaurus knopfleri, in honor of rock guitarist Mark Knopfler, whose music the scientists listened to while digging up the fossils.

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