Giraffatitan

Giraffatitan, meaning "giant giraffe", is a genus of sauropod dinosaur that lived during the late Jurassic Period (Kimmeridgian-Tithonian stages). It was originally named as an African species of Brachiosaurus (B. brancai). Giraffatitan is one of the largest animals known to have walked the earth.

Description
Giraffatitan was a sauropod, one of a group of four-legged, plant-eating dinosaurs with long necks and tails and relatively small brains. It had a giraffe-like build, with long forelimbs and a very long neck. The skull had a tall arch anterior to the eyes, consisting of the bony nares, a number of other openings, and "spatulate" teeth (resembling chisels). The first toe on its front foot and the first three toes on its hind feet were clawed.

Traditionally, the distinctive high-crested skull has been seen as a characteristic of the genus Brachiosaurus to which Giraffatitan brancai was originally referred, but because within the traditional Brachiosaurus material it is known only from Tanzanian specimens now assigned to Giraffatitan, it is possible that Brachiosaurus altithorax did not show this feature.

Size
For many decades, Giraffatitan was claimed to be the largest dinosaur known, (ignoring the possibly larger but lost Amphicoelias) but in the later part of the 20th century several giant titanosaurians (Argentinosaurus, Puertasaurus and Futalognkosaurus) appear to have surpassed Giraffatitan in terms of sheer mass. However, Giraffatitan, along with Brachiosaurus are still the largest dinosaurs known from relatively complete material.[2]

All size estimates for Giraffatitan are based on the specimen HMN SII,[2] a subadult individual between 21.8–22.5 metres (72–74 ft) in length.[3][4] Mass estimates are more problematic and historically have varied from as little as 15 tonnes (17 short tons) to as much as 78 tonnes (86 short tons).[5][6] These extreme estimates are now considered unlikely due to flawed methodologies. More recent estimates based on models reconstructed from bone volume measurements, which take into account the extensive, weight-reducing air sac systems present in sauropods, and estimated muscle mass, are in the range of 23–37 tonnes (25–41 short tons).[2][7][8] However, HMN SII is not the largest specimen known (an assertion supported by its subadult status) but HMN XV2, represented by a tibia 13% larger than the corresponding material on HMN SII,[2] which might have attained 26 metres (85 ft) in length.

History and Classification
Giraffatitan brancai was first named and described by German paleontologist Werner Janensch in 1914 as Brachiosaurus brancai, based on several specimens recovered between 1909 and 1912 from the Tendaguru formation near Lindi, in what was then German East Africa, today Tanzania.[10] It is known from five partial skeletons, including three skulls and numerous fragmentary remains including skull material, some limb bones, vertebrae and teeth. It lived from 145 to 150 million years ago, during the Kimmeridgian to Tithonian ages of the Late Jurassic period.

In 1988, Gregory S. Paul noted that Brachiosaurus brancai (on which most popular depictions of Brachiosaurus were based) showed significant differences from the North American Brachiosaurus, especially in the proportions of its trunk vertebrae and in its more gracile build. Paul used these differences to create a subgenus he named Brachiosaurus (Giraffatitan) brancai. In 1991, George Olshevsky asserted that these differences were enough to place the African brachiosaurid in its own genus, simply Giraffatitan.[11]

Further differences between the African and North American forms came to light with the description in 1998 of a North American Brachiosaurus skull. This skull, which had been found nearly a century earlier (it is the skull Marsh used on his early reconstructions of Brontosaurus), is identified as "Brachiosaurus sp." and may well belong to B. altithorax. The skull is closer to Camarasaurus in some features such as the form of the front teeth and more elongated and less hollowed-out on top than the distinctive short-snouted and high-crested skull of Giraffatitan.[12]

The classification of Giraffatitan as a separate genus was not widely followed by other scientists at first, as it was not supported by a rigorous comparison of both species. However, a detailed comparison was published by Michael Taylor in 2009. Taylor showed that "Brachiosaurus" brancai differed from B. altithorax in almost every fossil bone that could be compared, in terms of both size, shape, and proportion, finding that the placement of Giraffatitan in a separate genus was valid.[2] Taylor found evidence of a sister relationship between Giraffatitan and Brachiosaurus, although his analysis omitted other Brachiosaurids.[2] A more recent study on Titanosauriform sauropods by D'Emic (2012)[13] places Giraffatitan as sister to a clade containing Brachiosaurus and a tritomy of Abydosaurus, Cedarosaurus, and Venenosaurus as shown below: