Diplodocidae

Diplodocidae is a family of neosauropods characterized by their long, slender bodies. Unlike the vertically reaching brachiosaurids, diplodocids were built for low and horizontal browsing. Their anatomy included a long horizontal neck, which could reach 8 m or more, a roughly level back, and hindlimbs that were slightly longer than the forelimbs. Their exceptionally long tails tapered to a thin, whip-like end. The name "double-beam" refers to the diagnostic forked chevron bones on the underside of the tail vertebrae.

The skulls of these sauropods were square and low. They had peg-like teeth limited to the front of the jaw, an arrangement suited for stripping foliage off branches rather than chewing. Because the skull was small relative to the animal's overall size, it helped mitigate the biomechanical challenges of such a long neck.

The family includes several well-known genera such as Diplodocus, Apatosaurus, Barosaurus, Supersaurus, Galeamopus, and Kaatedocus. Supersaurus is potentially the longest sauropod in the fossil record, with estimated lengths exceeding 33 m.

In the Morrison Formation, diplodocids acted as versatile browsers. Their long necks allowed them to feed on conifer crowns or reach across large patches of ferns and horsetails without needing to move their massive bodies. Fossil trackways provide evidence that they lived in herds. The whip-like tip of the tail may have been used for display, defense, or as a deterrent, and some theories even suggest it could achieve supersonic speeds (Myhrvold & Currie 1997). Additionally, bifid neural spines on the neck vertebrae supported a strong dorsal ligament. This system functioned like a suspension bridge, allowing the dinosaur to hold its neck up with minimal muscular effort.

• Dippy (CM 84, Diplodocus carnegii) — Carnegie Museum of Natural History; multiple cast copies sent to museums worldwide by Andrew Carnegie 1905–1930s.
• Apatosaurus louisae holotype (CM 3018) — Carnegie Museum.
• Supersaurus vivianae (BYU 9024 + WDC DMJ-021) — Brigham Young University and Wyoming Dinosaur Center.
• Seismosaurus specimens — now considered a junior synonym of Diplodocus longus.

Tschopp et al. (2015) — a comprehensive specimen-level phylogenetic analysis — proposed substantial taxonomic revision: resurrecting Brontosaurus as distinct from Apatosaurus, recognising new species, and restructuring relationships. The "Brontosaurus is back" headline came from this paper; not all subsequent workers accept it. Whether tail-whip supersonic cracks were actually possible is debated. The validity of fragmentary giants like Maraapunisaurus (formerly Amphicoelias fragillimus) is permanently uncertain because the type material was lost.

Apatosaurus (or "Brontosaurus") and Diplodocus are the platonic long-necked dinosaur shape — The Land Before Time's Littlefoot is generically diplodocid, the Sinclair Oil dinosaur is Apatosaurus, and Dippy the cast diplodocid lived in the Natural History Museum of London for over a century.

• diplodocus
• apatosaurus
• sauropodomorpha
• brachiosauridae

• Tschopp, E., Mateus, O., & Benson, R. B. J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae. PeerJ, 3, e857.
• Foster, J. (2007). Jurassic West: The Dinosaurs of the Morrison Formation. Indiana University Press.
• Myhrvold, N. P., & Currie, P. J. (1997). Supersonic sauropods? Tail dynamics in the diplodocids. Paleobiology, 23, 393–409.
• Wedel, M. J., & Taylor, M. P. (2013). Caudal pneumaticity in sauropod dinosaurs. PLOS ONE, 8, e78213.