Sauropodomorpha

Sauropodomorpha (from the Ancient Greek sauros, "lizard," pous, "foot," and morphe, "form," coined by Friedrich von Huene in 1932) is the suborder and clade of long-necked, herbivorous saurischian dinosaurs (Dinosauria) that sits as the sister clade to Theropoda (predatory dinosaurs and birds). Originating in the Late Triassic (Carnian, ~231 million years ago) as slender, 1.5-meter bipedal omnivores (Saturnalia, Buriolestes, Panphagia) and 6-to-10-meter bipedal/facultatively quadrupedal "prosauropods" (Plateosaurus, Massospondylus, Macrocollum), the clade gave rise by the Early Jurassic to the obligate quadrupedal Sauropoda (Vulcanodon, Cetiosaurus, Diplodocus, Apatosaurus, Brachiosaurus, and the Cretaceous Titanosauria)—which remained the dominant megaherbivores on Earth for >135 million years until the 66 Ma K–Pg boundary extinction.

At their zenith, giant titanosaurian and macronarian sauropods—including Argentinosaurus huinculensis, Patagotitan mayorum, Puertasaurus, Dreadnoughtus schrani, and Supersaurus—reached total body lengths of 30 to 39 meters and body masses of 50 to >70 metric tons! They surpassed the largest known land mammal in history (Paraceratherium, ~15–20 tons) by 3- to 4-fold and the largest living land animal (the African bush elephant, ~6 tons) by 10-fold, approaching the biomechanical ceiling of terrestrial animal life.

Why did no terrestrial mammal (or ornithischian dinosaur) ever exceed ~15–20 metric tons, whereas more than a dozen sauropod lineages independently exceeded 40 to 70 metric tons? An international research consortium led by P. Martin Sander and Marcus Clauss (2008, 2011) resolved this puzzle via the Sauropod Gigantism Evolutionary Cascade Model, showing that sauropods combined five non-mammalian traits that bypassed the physiological bottlenecks limiting mammalian body size:

1. No Mastication (Zero Chewing) → Small, Lightweight Head → 10-to-15-Meter Neck:
   • Because a herbivorous mammal (such as an elephant or rhino) must thoroughly chew (masticate) every mouthful of tough vegetation before swallowing, its head must house massive, heavy molars and giant jaw muscles. A 200-kg elephant-like head cannot be cantilevered at the end of a 12-meter neck without breaking the cervical spine!
   • In contrast, sauropods completely bypassed oral chewing: their tiny skulls (<0.2% of body mass—a 30-meter Diplodocus had a skull barely larger than a horse's head!) functioned strictly as lightweight cropping rakes (lined with spoon-shaped spatulate teeth in Camarasaurus/Brachiosaurus or pencil-shaped peg teeth in Diplodocus/Nigersaurus that were replaced as fast as every 14 to 35 days; D'Emic et al., 2013). Stripped conifer needles, horsetails, and ferns were swallowed whole without a single chew and fermented for days inside a multi-ton hindgut/foregut microbial fermentation vat, allowing the head to remain light enough to be carried at the end of a 10-to-15-meter neck (10 to 19 elongated cervical vertebrae).
   • Operating like a stationary crane boom, a 12-meter neck gave a sauropod a feeding envelope of >500–1,000 cubic meters of foliage from low ground ferns to 15-meter conifer canopies without taking a single step—saving immense locomotor energy!
2. Avian-Style Pneumatized Skeleton and Unidirectional Flow-Through Lungs: If a 12-meter sauropod neck had been built of solid bone and ventilated by a dead-end mammalian tidal lung, the neck would have been too heavy to lift and the animal would have suffocated inside its own tracheal dead space (re-inhaling 100 liters of stale exhaled air inside its 10-meter windpipe on every breath!). As demonstrated using CT scans by Mathew J. Wedel (2003, 2009), sauropods possessed avian-style unidirectional air-sac lungs:
   • Because air flows in a continuous one-way loop from posterior air sacs through the lung into anterior air sacs, tracheal dead space is less limiting;
   • Furthermore, cervical air-sac diverticula hollowed out the cervical and dorsal vertebrae into honeycomb lattices (camerate and camellate/somphospondylous bone) that were 60% to 75% empty air by volume—giving a 12-meter Sauroposeidon or Brachiosaurus neck the bulk density of cork (~0.3 g/cm³) while maintaining high bending strength via external bony struts (laminae)!
3. Columnar Graviportal Limbs with Sauropod Metacarpal Horseshoe Colonnades: Sauropod limbs were straight, vertical pillars (graviportal) lacking flexion at the knee and elbow. In derived Eusauropoda / Neosauropoda, the five metacarpal bones of the front foot (manus) were arranged vertically in a tight, U-shaped tubular horseshoe colonnade (looking like the base of a Greek column!) that transmitted tons of weight directly into the ground with zero toe splaying (with all finger phalanges and claws lost except a single large inner thumb claw in basal forms, and even the thumb claw lost in derived Titanosauria!), while the broad hindfoot (pes) rested on a gigantic elastic heel cushion pad behind three lateral sickle-shaped claws (digits I–III).
4. Oviparity (Egg-Laying) Bypassing the Mammalian Gestation Bottleneck: As a placental mammal grows larger, its gestation period stretches to 22 months in an elephant (producing a single calf every 4–5 years), making 20-ton mammals acutely vulnerable to extinction during environmental perturbations because population recovery rates collapse (Janis & Carrano, 1992). In contrast, even a 60-ton sauropod remained an r-selected egg-layer: nesting grounds such as Auca Mahuevo in Patagonia, Argentina (Chiappe et al., 1998) show that giant titanosaurs dug trenches and laid clutches of 15 to 40 spherical, grapefruit-sized eggs (only 13–22 cm in diameter, limited in size by eggshell O₂/CO₂ gas-conductance pores!) several times a year, hatching 3-kilogram babies that grew >10,000-fold to adulthood via rapid fibrolamellar bone deposition within 20–30 years!

How could the semi-arid floodplains of the Late Jurassic Morrison Formation (~155–148 Ma across Wyoming, Colorado, and Utah) simultaneously support 8 to 10 coexisting genera of multi-ton sauropods (Diplodocus, Apatosaurus, Barosaurus, Supersaurus, Camarasaurus, Brachiosaurus, Haplocanthosaurus) alongside Stegosaurus and Allosaurus?

• Through craniodental, neck-posture, and isotopic (δ¹³C) niche partitioning (Fiorillo, 1998; Whitlock, 2011; Button et al., 2014):
  • High-shouldered Brachiosaurus browsed high conifer crowns (10–14 meters up) out of reach of all other herbivores;
  • Deep-jawed Camarasaurus had high mechanical-advantage spatulate teeth with heavy scratch microwear for crushing woody mid-height cycads, ginkgos, and seed ferns;
  • Narrow-snouted Diplodocus and Apatosaurus used unilateral branch-stripping peg teeth with fine pit/scratch microwear to strip low ground ferns and Equisetum horsetails at 0–4 meters height!

Why is Brontosaurus excelsus considered a valid dinosaur genus again after being synonymized with Apatosaurus for 112 years?

In 1877, Othniel Charles Marsh named a juvenile sauropod skeleton Apatosaurus ajax, and two years later (1879) named a second, more complete skeleton Brontosaurus excelsus ("thunder lizard"). In 1903, Elmer S. Riggs argued that the two skeletons were too similar to belong to separate genera, making Brontosaurus a junior synonym of Apatosaurus for over a century. In 2015, a landmark 300-page specimen-level phylogenetic analysis of 81 diplodocid skeletons across 477 anatomical characters by Emanuel Tschopp, Octávio Mateus, and Roger B. J. Benson demonstrated that Apatosaurus and Brontosaurus differ across dozens of consistent osteological characters (with Apatosaurus having much thicker, wider cervical vertebrae and a stouter scapulocoracoid than Brontosaurus)—officially resurrecting Brontosaurus (B. excelsus, B. parvus, and B. yahnahpin) as a valid, distinct genus alongside Apatosaurus!

1. Wedel, M. J. (2003). Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology, 29(2), 243–255. doi:10.1666/0094-8373(2003)029<0243:VPASAT>2.0.CO;2029%3C0243:VPASAT%3E2.0.CO;2)
2. Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., ... & Witzel, U. (2011). Biology of the sauropod dinosaurs: The evolution of gigantism. Biological Reviews, 86(1), 117–155. doi:10.1111/j.1469-185X.2010.00137.x
3. D'Emic, M. D., Whitlock, J. A., Smith, K. M., Fisher, D. C., & Wilson, J. A. (2013). Evolution of high tooth replacement rates in sauropod dinosaurs. PLOS ONE, 8(7), e69235. doi:10.1371/journal.pone.0069235
4. Lacovara, K. J., Lamanna, M. C., Ibiricu, L. M., Poole, J. C., Schroeter, E. R., Ullmann, P. V., ... & Novas, F. E. (2014). A gigantic, exceptionally complete titanosaurian sauropod dinosaur from nerve Patagonia, Argentina. Scientific Reports, 4, 6196. doi:10.1038/srep06196
5. Tschopp, E., Mateus, O., & Benson, R. B. J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ, 3, e857. doi:10.7717/peerj.857
6. Carballido, J. L., Pol, D., Otero, A., Cerda, I. A., Salgado, L., Garrido, A. C., ... & Krause, J. M. (2017). A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs. Proceedings of the Royal Society B, 284(1860), 20171219. doi:10.1098/rspb.2017.1219