Brontosaurus
Othniel Marsh, a Professor of Paleontology at Yale University who described and named an incomplete (and juvenile) skeleton of Apatosaurus ajax in 1877, two years later announced the discovery of a far larger and more complete specimen at Como Bluff Wyoming— which, because of discrepancies including the size difference, Marsh incorrectly identified as belonging to an entirely new genus and dubbed Brontosaurus excelsus. [Etymology: from Latin excelsus, "to exceed in number", suggesting the greater number of vertebrae in the sacrum than in any other genus of sauropod then known.]
To perfect the find — the largest dinosaur ever discovered at the time and nearly complete, lacking only a head, feet, and portions of the tail — for what was to be the first ever display of a sauropod skeleton, at Yale's Peabody Museum in 1905, some feet that were discovered at the same quarry were added, as well as a tail fashioned to appear as Marsh believed it should and what he apparently felt was the "correct" skull for the massive creature. This was not a delicate Diplodocus-style skull, matching what was actually a large Apatosaurus skeleton but, instead, a chimaera composed of "the biggest, thickest, strongest skull bones, lower jaws and tooth crowns from three different quarries", primarily those of Camarasaurus. (This "scientific sloppiness" is considered to be symptomatic of undue haste resulting from Marsh's notorious rivalry with Edward Drinker Cope, which would later become known as the "Bone Wars".) There is, however, some debate on whether Marsh was actually involved in this patchwork skeleton or whether it was other Yale professors, as he has been reported to have died in 1899, six years before the display aparantly took shape.
In 1903 Elmer Riggs published a paper in Geological Series of the Field Columbian Museum which identified B. excelsus as an Apatosaur (ie. A. excelsus):
...In view of these facts the two genera may be regarded as synonymous. As the term "Apatosaurus" has priority, "Brontosaurus" will be regarded as a synonym.
Nevertheless, the mistake was perpetuated by conservative paleontologists and museum curators (who would in fact reverse a number of corrective identifications of Apatosaurus skulls over the years) until it was officially laid to rest in 1975 with the publication of a paper by John S. (“Jack”) McIntosh and David S. Bermanbase, based on twenty years of research review.
Despite this some paleontologists — most notably Robert Bakker — argue that A. ajax and A. excelsus are in fact sufficiently distinct that the latter continues to merit a separate genus. Bakker recently re-classified A. yahnahpin as the type-species for the new genus Eobrontosaurus.
Tyrannosaurus Rex
Tyrannosaurus (from the Greek "τυραννόσαυρος", meaning 'tyrant lizard') is a genus of tyrannosaurid theropod dinosaur. The species Tyrannosaurus rex, commonly abbreviated to T. rex, is one of the dinosaurs most often featured in popular culture around the world. It hails from what is now western North America. Some scientists consider the slightly older Tarbosaurus bataar from Asia to represent a second species of Tyrannosaurus, while others maintain Tarbosaurus as a separate genus.
Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small and retained only two digits. Although other theropods rivaled or exceeded T. rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring over 12 metres (40 feet) in length and weighing as much as an elephant.
Fossils of some T. rex have been found in North American rock formations dating to the very end of the Cretaceous Period (late Maastrichtian stage, 65 million years ago); it was among the last dinosaurs to exist prior to the Cretaceous-Tertiary extinction event. More than 30 specimens of T. rex have now been identified, some nearly complete, which has allowed significant research into many aspects of its biology, including its life history and biomechanics. The feeding habits and potential speed of T. rex remain controversial.