Spinosauroidea

The Spinosauroidea were a group of related theropods considered to be basal within the Tetanurae. The group is further broken down into two smaller groups. The Megalosauridae were medium- to large-sized theropods that mostly lived during the Middle and Late Jurassic Epochs. The Spinosauridae included several large-bodied forms that reigned as the largest carnivores in their ecosystems up until the early part of the Late Cretaceous, at which time they were replaced by more derived tetanurans.

Megalosauridae

The namesake of the Megalosauridae is Megalosaurus (Middle Jurassic, England), which has the distinction of being the first dinosaur ever described scientifically. The original specimen of Megalosaurus was a partial skeleton that included a lower jaw; it was studied by William Buckland in 1825. The group of dinosaurs that goes by the name of megalosaurids has long been something of a wastebasket for a variety of fragmentary Jurassic theropod remains. While recent work by paleontologists including Thomas Holtz, Ralph Molnar, Philip Currie, and Paul Sereno (b. 1957) has done much to establish a firmer basis for defining theropods of this group, even the best specimens of megalosaurs are only partial and often lack much of the skull and postcranial skeleton. Megalosaurs have been found in England, France, and the United States as well as in Niger and Argentina, thus extending the reach of this clade to the Northern and Southern Hemispheres of the dinosaur world.

One of the most complete megalosaurid specimens is that of Afrovenator ("African hunter"), discovered in Niger in 1994. The specimen represents a 30-foot (9 m) predatory dinosaur that lived between 125 million and 136 million years ago. The skull is nearly complete, except for the lower jaw, and the rest of the skeleton reveals a lightly built theropod with strong forelimbs with three robust claws, a trait characteristic of this animal's tetanuran relatives, such as Allosaurus. The evolutionary links between Afrovenator and megalosaurids from the Northern Hemisphere suggests that a land bridge still existed between the continents above and below the Equator as late as the Early Cretaceous Epoch.

Other noteworthy megalosaurids include Torvosaurus (Late Jurassic, Utah, Colorado, and Wyoming); Piatnitzkysaurus (Middle Jurassic, Argentina); Poekilopleuron (Middle Jurassic, France), Dubreuillosaurus (Middle Jurassic, France); and Eustreptospondylus (Middle Jurassic, England).

Spinosauridae

For many years, the Spinosauridae were represented only by an enigmatic large predator, Spinosaurus, discovered by German paleontologist Baron Ernst Stromer von Reichenbach (1870-1952) of the University of Munich. The story of the discovery of Spinosaurus is one of scientific adventure and hardships; it is also the tale of a dinosaur once lost but revealed again nearly six decades later.

The Spinosaurus story began when a young Ernst Stromer ventured into a desolate region of the Egyptian desert in 1910. Accompanied only by a guide, a cook, four camels, and two camel drivers, Stromer was hoping to find fossils of early mammals that had previously been discovered in other regions of the Egyptian desert. Instead, he stumbled upon a trove of dinosaur fossils dating from the first part of the Late Cretaceous Epoch, a time when Egypt was green and much wetter.

Stromer hired an experienced fossil hunter named Richard Markgraf to help him recover his finds. In three short years, the two men excavated more than 50 new kinds of animals and plants, including dinosaurs, fishes, turtles, snakes, marine reptiles, and crocodiles. Their rewards did not come easily. Fieldwork was best carried out in the early months of the year, before the onset of the devastating desert heat. Sandstorms were possible in the day or night, and the wind could whip up violently at any time, sending a fine, sandy dust through the air and into the eyes of the workers. Add to all that a veritable plague of camel fleas. It is a wonder that

Stromer and Markgraf lasted as long as they did, working that forbidding landscape for fossils.

The most spectacular of Stromer's dinosaur discoveries was Spi-nosaurus ("spine lizard"), an incredibly large predator that is estimated to have been about 50 to 57 feet (15 to 17 m) long. An unusual feature of Spinosaurus were long spines on its back that probably formed a large, sail-like structure. The spines alone were up to 6.6 feet (2 m) tall, adding height to a creature that must have stood about 16 to 20 feet (5 to 6 m) tall at the hips. Of the creature's skull, Stromer found only the lower jaws and a fragment of the upper, but these fossils, too, revealed an unusual trait. Whereas most thero-pods had bladelike teeth, the teeth of Spinosaurus were conical and unserrated, more like those of a crocodile.

Returning to Munich, Stromer mounted his partial skeleton of Spinosaurus on a wall in the Alte Akademie Museum, home to the Bavarian State Collection of Paleontology and Historical Geology. In 1915, he also published a detailed scientific description of Spinosaurus, illustrated with meticulous figures depicting the bones he had found.

The size of Stromer's Spinosaurus eclipsed that of the more famous Tyrannosaurus, first described in 1905. Spinosaurus might have become one of the most familiar of all dinosaurs were it not for World War II. As fate would have it, the events of that war were to thrust Stromer's work into relative obscurity for many years. The fossils that represented his life's work—including Spinosaurus—were stored in the Alte Akademie Museum in Munich. On April 24, 1944, Britain and Germany were at war. The British Royal Air Force, while dropping bombs on a nearby military target, accidentally set fire to the museum housing Stromer's fossils. After the fire, all that remained of the spectacular Spinosaurus were Stromer's stories, field notes, and published descriptions.

For many years, Stromer's lost dinosaur was nearly forgotten. Fortunately, several discoveries since 1986—including those of Bary-onyx (Early Cretaceous, England); Suchomimus (Early Cretaceous, Niger); and Irritator (Early Cretaceous, Brazil)—have revealed much more about the clade of dinosaurs to which Spinosaurus belonged. The spinosaurs not only had conical teeth similar to those of crocodiles, but also had skulls that were long and narrow and that featured a crocodilelike snout and a cluster of teeth at the front of the jaw. In the taxon Suchomimus, the bottom jaw has a rounded, chinlike "rosette" at its tip in which the largest teeth are housed. Baryonyx and Suchomimus also had powerful forelimbs and large claws on their hands; the longest of these claws was about 12 inches (30 cm) long. A combination of evidence—including their teeth, jaws, claws, and the sediments in which their remains are found—strongly suggests that the spinosaurs were fish eaters and used their powerful arms and claws to scoop fish from the water. The large sail-back found in Spinosaurus was not nearly as prominent in Baryonyx and Suchomimus.

Although Stromer's magnificent specimen of Spinosaurus was destroyed during World War II, the search for another specimen has been revived by a new generation of paleontologists. In 1999, armed only with Stromer's sketchy field notes, paleontologist Josh Smith and archaeologist Jennifer Smith visited Egypt in search of the famed fossil site. What they found during their quick foray were bone fragments strewn over the ground and enough evidence to compel them to return the following year. The subsequent expedition included teams from the University of Pennsylvania and Egyptian Geological Survey and Mining Authority headed by Josh Smith, Peter Dodson, Matthew Lamanna, and Yousry Attia.

The hardships of working in the desert had changed little since Stromer's time, but Smith and his t eam—unlike Stromer before them—were able to take up residence in a pleasant motel that served as their home for the duration of the dig. The team did not find another specimen of Spinosaurus apart from teeth, but it did make a major discovery of a sauropod from the time of Spinosaurus. Paralititan—or "tidal giant," after the coastal tidal environment in which the dinosaur once lived—i s among the largest known sauropod dinosaurs, after only Argentinosaurus, Puertasaurus, and possibly Sauroposeidon and Amphicoelias. Paralititan was a heavy and stocky beast, about 80 to 100 feet (24 to 30 m) long and weighing as much as 50 tons (45 metric tons). Paralititan adds significantly to the legacy of Egyptian dinosaurs and the ecosystem once documented by Stromer.

Although the fossil-hunting team led by Josh Smith was unable to find a significant new specimen of Spinosaurus, its efforts have helped reinstate the gigantic theropod as a legitimate contender for the title of largest known predatory dinosaur. Nonetheless, additional hard evidence in the form of new fossil remains of Spinosau-rus has been difficult to come by. In 1996, Canadian paleontologist Dale Russell (b. 1947) described fragmentary bones of Spinosaurus from Morocco including neck vertebrae, part of the spine, and pieces of the lower jaw. Other fragments have been reported from Tunisia and Algeria, but none of these specimens has significantly advanced knowledge of this enigmatic theropod. Even the rediscovery—in 2006, by Josh Smith and Matthew Lamanna—of two unpublished photographs of Stromer's Spinosaurus museum exhibit was an event, as the photographs provided long-unseen views of the original Spinosaurus specimen that was destroyed.

The most striking new fossil evidence for Spinosaurus was announced in 2005 by a multinational team of paleontologists led by Italian Cristiano Dal Sasso. This team had "rediscovered" a long-forgotten fossil fragment discovered in Morocco in 1975. The specimen is that of a remarkably preserved partial skull and upper jaw of Spinosaurus. The snout measures 3.25 feet (1 m) long, and the entire skull is estimated to have been 5.7 feet (1.7 m) long, the largest of any known spinosaur. The specimen is remarkably preserved in three dimensions—not squashed flat as can sometimes happen to bones during fossilization. The jaw includes many teeth that had not yet erupted to the surface.

Dal Sasso and his colleagues estimate that their specimen of Spinosaurus is about 20 percent larger than Stromer's. Using the body plan of Suchomimus as a guide to fill in the missing parts

Baryonyx and Neovenator, two large predatory dinosaurs from the Early Cretaceous of Great Britain

of the animal's body, this would make the Dal Sasso specimen of Spinosaurus 53 to 60 feet (16 to 18 m) long. This is 20 percent to 30 percent larger than Baryonynx and Suchomimus and even longer than Tyrannosaurus and Giganotosaurus, the two theropods most commonly assumed to be the largest. The table on the next page provides a comparison of the largest known theropods.

Was this article helpful?

0 0

Post a comment