Brachiosauridae

Brachiosaurids Temporal range: Late Jurassic – Early Cretaceous, 157–93 Ma PreЄ Є O S D C P T J K Pg N
Mounted Brachiosaurus skeleton cast, Field Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Order:	Saurischia
Suborder:	†Sauropodomorpha
Clade:	†Neosauropoda
Clade:	†Macronaria
Clade:	†Titanosauriformes
Family:	†Brachiosauridae Riggs, 1904
Genera
†Abydosaurus †Brachiosaurus †Cedarosaurus †Daanosaurus †Duriatitan[1] †Europasaurus †Giraffatitan †Lusotitan?[2] †Padillasaurus †Sonorasaurus †Venenosaurus

Brachiosauridae ("armed lizards", from Latin brachium = "arm" and Greek saurus = "lizard") is a family of herbivorous, quadrupedal sauropod dinosaurs.^[3] Brachiosaurids had long necks that enabled them to access the leaves of tall trees that other sauropods would have been unable to access.^[4] In addition, they possessed thick spoon-shaped teeth which helped them to consume tough plants more efficiently than other sauropods.^[4] They have also been characterized by a few unique unambiguous synapomorphies; dorsal vertebrae with 'rod-like' transverse processes and an ischium with an abbreviate pubic peduncle.^[4]

Brachiosaurus is one of the best-known members of the Brachiosauridae family, and was once thought to be the largest land animal to ever live.^[3] They thrived in the regions which are now North and South America, Africa, Europe, and Asia.^[5][6] They first appear in the fossil record in the Middle/Late Jurassic Period and disappear in the late Early Cretaceous Period.^[7]

The broad distribution of Brachiosauridae in both northern and southern continents suggests that the group originated prior to the breakup of Pangaea.^[5][6][8] In the Early Cretaceous the distribution of the family is dramatically reduced. It is still unclear whether this reduction is due to local extinctions or to the limited nature of the Early Cretaceous fossil record.^[5]

Brachiosauridae has been defined as all titanosauriforms that are more closely related to Brachiosaurus than to Saltasaurus.^[5][7] It is one of the three families that belongs to the Titanosauriformes clade of sauropod dinosaurs, which also includes Euhelopdidae and Titanosauria.^[5]

Description: anatomy and key features

Brachiosaurus humerus bone.

The Brachiosauridae family is composed of quadrupedal dinosaurs that are extremely large.^[8] The giant animals of this family can be distinguished from other taxon by their broad, thick and spoon-shaped teeth.^[8] The maxillary teeth were twisted apically, and the shape of these teeth were optimal for biting off resistant vegetation.^[4] While brachiosaurids didn't perform significant food processing in their mouths (similar to other members of the sauropod family), their teeth enabled them to slice through food instead of having to pull it off of tree branches.^[9] Evidence for this precision shearing consists of apical wear facets on the teeth and distinctive bone structure that suggests orthal jaw action.^[9]

In addition, the characteristic long necks of brachiosaurids are distinct from those of other long-necked dinosaur taxa.^[9] They possessed a narrow neck composed of 12–13 extremely long cervical bones that was laterally inflexible and dorsoventrally flexible.^[9] This meant that brachiosaurids could angle their necks up and lift their heads, enabling them to graze from treetops up to a height of about 14 meters.^[9] Other sauropods lacked this dorsoventral flexibility and thus their necks stretched outwards in front of them instead of upwards.^[9] Additionally, brachiosaurids have more often been found in the conifer-rich Tendangi beds dig site than in the Morrison depots, suggesting that their fitness was increased by the presence of taller conifer food sources.^[9]

However, the giant size and long necks of brachiosaurids meant that they required tremendous pressure to bring oxygenated blood to their brains.^[10] It has been proposed that they possessed a four-chambered double pump heart, with one pump for oxygenated and one pump for deoxygenated blood.^[10]

As in all macronarians, the forelimbs of brachiosaurids are long relative to the hindlimbs, but this trait is more pronounced in brachiosaurids.^[6] The forelimbs were very slender and the metacarpus bone of the forelimb was elongated.^[6][9] These adaptations overall increased the stride length of the forelimbs, likely resulting in an uneven gait.^[9] However, it was previously argued that they were hindlimb dominant like other sauropods, and thus had the ability to rear up on their hindlimbs.^[3] Based on their bone structure, it is likely that they had slow walking speeds (20–40 km/day), but were capable of moving faster when necessary (close to 20–30 km/hour).^[10]

In addition, brachiosaurids shared other unambiguous synapomorphies specific to the taxa. They possessed middle and posterior dorsal vertebrae with long, 'rod-like' transverse processes as well as an abbreviated pubic peduncle of the ischium.^[4] Their humerus had a large deltopectoral crest and they had wide supratemporal fenestrae.^[6] They had anteriorly placed neural arches, expanded distal blades, irregularly shaped coracoids, and ventral triangular projections on the anterior ramus of their quadratojugal bones.^[4]

History of findings

Changing classifications

Brachiosaurus mounted at Field Museum of Natural History.

In 1903 Riggs examined a brachiosaurid specimen and classified them as a new and unusual family of sauropods.^[3] He published a complete description of the phenotype after examining the humerus, femur, coracoid, and sacrum of a Brachiosaurus specimen that had been prepared at the Field Colombian Museum.^[3] Since then, the classification of these sauropods has been through many changes.^[11] Marsh's multifamily theory of sauropod classification prevailed until 1929, when Janesh proposed a two-family theory based on differences in sauropod teeth.^[11] Macronarians with broad, spatulate teeth, were placed in the Brachiosauridae family, while sauropods with more slender and peg-shaped teeth were considered titanosaurids.^[11] This put diplodocids and titanosaurids together in one group based on their similar teeth, despite the many other differences between the taxa.^[11] Today, most agree that there are 4–5 well established families within Macronaria.^[11]

In 1997, Salgado, Coria and Calvo studied the traits used to classify Brachiosauridae and determined that they were plesiomorphic for basal titanosaurs.^[3] They proposed that some characteristics that were used to differentiate Brachiosaurus were synapomorphies for Titanosauriformes.^[3] They determined that the family Brachiosauridae was actually a group of primitive titanosauriforms, and not a stable separate clade.^[3] They based this conclusion on similar humerus:femur length ratios known for titanosauriforms, basal titanosaurs, and other primitive sauropods.^[3] However, in 1998 Sereno & Wilson criticized conclusions in Salgado et al's article, and proposed instead that Brachiosauridae remain a stand-alone clade in Titanosauriformes.^[3]

Important findings

In 1943, Lapparent discovered the "French Bothriospondylus" from the Oxfordian of France which dates to the Late Jurassic, which was recently identified in 2013 by Mannion as a brachiosaurid.^[3] This specimen represents the oldest undisputed record of the brachiosaurid group.^[5]

Paleobiogeographic distribution

Definitive brachiosaurid remains have been found from the Late Jurassic Period to the Early Cretaceous, from about 157 to 93 million years ago.^[9] In addition, Macronaria in general first appear in the Late Jurassic. However, the almost simultaneous appearance of Camarosaurus, Brachiosaurus, and a possible titanosaur suggest that they originated earlier, closer to the Mid-Jurassic.^[9]

Trackway evidence also supports a Middle Jurassic origin for titanosaurs, which implies the same for all neosauropods.^[9] Brachiosaurids in particular have a broad distribution dating to the Late Jurassic.^[5] Late Jurassic specimens have been discovered in the northern and southern Hemispheres, including North America, Africa, Europe, and South America.^[5] This suggests that brachiosaurids originated in the Middle Jurassic, prior to the breakup of Pangaea, followed by diversification and dispersal that resulted in the global spread present in the Late Jurassic.^[5]

This conclusion is further supported by paleogeographic data.^[5] While many Late Jurassic dinosaur remains have been found in China, no brachiosaurid remains have been uncovered in East Asia.^[6] This would support the Middle Jurassic origin theory since East Asia was separated from the rest of Pangaea by water from the late Middle Jurassic to the Early Cretaceous.^[6]

While brachiosaurids were widely dispersed in the Late Jurassic, their geographic distribution narrowed in the Early Cretaceous.^[5] So far, brachiosaurid specimens have only been found in the Aptian-Albian region of North America.^[5] This reduction in distribution occurs immediately following the Jurassic-Cretaceous boundary.^[5] The brachiosaurid distribution in the Early Cretaceous has been interpreted as a result of regional extinctions in Europe, Africa, and South America.^[5] Overall, the Early Cretaceous seems to be a time of reduced sauropod diversity worldwide. It has been argued that this change may be due to an extinction event at the Jurassic-Cretaceous boundary.^[5] A second hypothesis is that the apparent lack of geographical diversity is due to sampling bias in the generally poor Early Cretaceous fossil record.^[5] Recently discovered evidence supports the conclusion that brachiosaurids existed outside of North America in lower latitudes of Gondwana in the Early Cretaceous.^[5] In 2013, Mannion et al. reported on the discovery of two isolated teeth found in Lebanon from the Early Cretaceous that possess posteriorly twisted crowns, which are characteristic of the brachiosaurids Giraffititan and Abydosaurus.^[5] In addition, a brachiosaurid originally named Padillasaurus leivaensis was discovered in Colombia from the Early Cretaceous and placed in the Brachiosauridae taxon, which suggests that Brachiosauridae survived in northwestern Gondwana after the Jurassic/Cretaceous boundary.^[5] In the Early Cretaceous, Colombia was located close to the equator in northwestern Gondwana while Lebanon was in the northeast of Gondwana.^[5] This suggests that brachiosaurids were in fact present outside of North America in the Early Cretaceous, and supports the theory that the apparent lack of specimens is due to an incomplete record.^[5] However, the rarity of these dinosaur specimens may also reflect a decrease in abundance of brachiosaurids acting in combination with the poor fossil record.^[5]

Classification

Brachiosauridae is one of the two major clades of Titanosauriformes, a diverse group of sauropods that existed in the Late Jurassic and Cretaceous in Laurasia and Gondwana. Europasaurus is considered the most basal brachiosaurid.^[11]

Titanosauriformes was a globally distributed, long-lived clade of dinosaurs that contained both the largest and smallest known sauropods.^[11] This clade was composed of three distinct groups: Brachiosauridae, a mix of Late Jurassic and Early Cretaceous sauropods, Euhelopodidae, a group of mid-Cretaceous East Asian sauropods, and Titanosauria, a large Cretaceous clade located mostly in Gondwana.^[11]

Traditionally, Brachiosauridae included Brachiosaurus and some other suggestively assigned genera, following the generic separation of Brachiosaurus species into B. altithorax and Giraffatitan brancai this have been the only members supported by cladistic analysis.

Cladogram of Brachiosauridae after D'Emic, Foreman, & Jud (2016).^[12]

References

External links

• Artist's rendering of fourteen brachiosaurs
• Brachiosauridae (German)