Plesiosauria

Plesiosauria (from the Ancient Greek plesios, "near to," and sauros, "lizard," coined by William D. Conybeare and Henry De la Beche in 1821 after Mary Anning discovered the first complete skeleton of Plesiosaurus dolichodeirus at Lyme Regis, England in 1823) is an extinct order of secondarily marine diapsid reptiles (Sauropterygia, Sauropsida) that thrived in global seas and estuaries from the latest Triassic (Rhaetian, ~203 million years ago) until the Cretaceous–Paleogene (K–Pg) mass extinction (66.0 Ma). Descended from Triassic coastal amphibious nothosaurs† (Nothosaurus) and semi-rigid pistosauroids† (Pistosaurus, Augustasaurus) within the superorder Sauropterygia (which phylogenetic analyses often place inside Archelosauria or as sister to Testudines), plesiosaurs possessed a body plan completely unlike that of any living animal: a broad, rigid, turtle-like teardrop trunk braced ventrally by massive flat coracoid and pubis/ischium plates and tightly interlocking belly ribs (gastralia), a short tapering tail, and two pairs of nearly identical, wing-like hydrofoil flippers!

Whereas ichthyosaurs, sharks, and whales propel themselves with their tail and use their foreflippers only for steering, and sea turtles, penguins, and sea lions fly underwater using only their front flippers, plesiosaurs are the only known vertebrates in Earth's history to fly underwater using all four limbs as large, symmetrical hydrofoil wings!

• For over a century, biomechanists debated whether plesiosaurs rowed their four flippers backward like oars on a rowboat, flapped all four flippers up and down in unison, or alternated front and back flippers.
• In 2015 and 2017, Luke E. Muscutt, Gabriel D. Weymouth, Colin Palmer, and colleagues built a robotic wind/water-tunnel tandem-flipper physical model and ran 3D fluid-structure simulations based on fossil joint geometry (Araújo et al., 2015; Liu et al., 2015):
  • Both the foreflippers and hindflippers executed a figure-eight underwater flying stroke (dominated by dorso-ventral lift generation rather than drag-based rowing);
  • Crucially, when the hindflippers beat at a specific phase lag behind the foreflippers, the hindflippers weave directly into the swirling wake vortices (Kármán vortex street) shed off the trailing edges of the front flippers! This tandem wake-harvesting interaction boosted hindflipper thrust by up to 60% and overall propulsive efficiency by 40%, explaining why plesiosaurs retained four equally giant hydrofoils for 140 million years!

Across the Jurassic and Cretaceous, plesiosaurs repeatedly evolved back and forth between two contrasting ecological body plans (morphotypes; O'Keefe, 2001, 2002):

1. Viviparity with a Single Giant Fetus (Polycotylus latipennis): For two centuries—unlike ichthyosaurs, where hundreds of gravid mothers with embryos were found—no pregnant plesiosaur fossil had ever been discovered, leading paleontologists to speculate that plesiosaurs might have hauled themselves onto sandy beaches like giant sea turtles to lay clutches of eggs. In 2011, F. Robin O'Keefe and Luis M. Chiappe described a 4.7-meter Late Cretaceous female Polycotylus latipennis (LACM 129639, from the Pierre Shale of Kansas, ~78 Ma) preserved with a single, huge articulated fetus inside her body cavity! Whereas ichthyosaurs gave birth to litters of multiple small young (~15–20% of maternal length), the single Polycotylus fetus already measured >1.5 meters (~35–40% of its mother's body length)—matching the extreme K-selected reproductive strategy of modern toothed whales and dolphins (which invest heavily in a single large, precocial calf and provide prolonged maternal care and social protection)!
2. Endothermy (35–39 °C) and Fibrolamellar Bone Growth: Oxygen-isotope (δ¹⁸O_PO4) paleothermometry of plesiosaur tooth enamel (Bernard et al., 2010) and quantitative osteohistology of blood-vessel-rich fibrolamellar bone (Fleischle, Wintrich & Sander, 2018) demonstrate that plesiosaurs maintained elevated, constant core body temperatures of ~35–39 °C and resting metabolic rates in the avian–mammalian endothermic range, enabling them to colonize near-freezing high-latitude polar seas in the Arctic and Antarctica (Kear et al., 2006).
3. Why Did Long-Necked Elasmosaurs Swallow Hundreds of Smooth River Pebbles (Gastroliths)? Skeletons of elasmosaurid plesiosaurs are frequently discovered with a compact pile of dozens to >300 smooth, polished chert, granite, and quartzite cobbles (gastroliths) weighing up to 5–13 kg sitting directly inside the ribcage where the muscular stomach (gizzard) was located—sometimes carried hundreds of kilometers out into the open Western Interior Seaway far from any rocky shoreline! While early researchers suggested gastroliths functioned purely as heavy ballast weights to control buoyancy or roll stability (Taylor, 1993), hydrodynamic calculations (Henderson, 2006) and deep gouge marks on the stones alongside crushed fish bones and irrational cephalopod beaks (Cicimurri & Everhart, 2001; Wings, 2007) indicate that—because long-necked plesiosaurs had needle-like trapping teeth incapable of chewing food—gastroliths functioned primarily as a muscular gastric mill (gizzard) to grind whole swallowed fishes and shelled invertebrates (while providing secondary buoyancy trim in shallow lung-inflated diving!).

Why did Edward Drinker Cope originally put the skull of Elasmosaurus platyurus on the end of its tail in 1868 (triggering the "Bone Wars")?

When the pioneering American paleontologist Edward Drinker Cope (1868) reconstructed the type skeleton of the 10.3-meter Kansas plesiosaur Elasmosaurus platyurus at the Academy of Natural Sciences in Philadelphia, no scientist had ever seen a reptile with a 72-vertebra, 5.5-meter neck and a short 1.5-meter tail (since almost all lizards and dinosaurs have short necks and very long tails!). Misinterpreting the vertebral articular orientations, Cope placed the skull on the short end (the tail!) and reconstructed Elasmosaurus with a short neck and a 5.5-meter whip tail. In 1870, Joseph Leidy and Cope's rival Othniel Charles Marsh publicly pointed out that the atlas–axis skull joint sat at the tip of the 72-vertebra column—humiliating Cope, who frantically tried to buy back every printed copy of his monograph and ignited the bitter 30-year "Bone Wars" (Great Dinosaur Rush) between Cope and Marsh that discovered 136 new species of North American dinosaurs and marine reptiles!

1. O'Keefe, F. R. (2001). Ecomorphology of plesiosaur flipper geometry. Journal of Evolutionary Biology, 14(6), 987–991. doi:10.1046/j.1420-9101.2001.00347.x
2. Bernard, A., Lécuyer, C., Vincent, P., Amiot, R., Bardet, N., Buffetaut, E., ... & Prieur, A. (2010). Regulation of body temperature by some Mesozoic marine reptiles. Science, 328(5984), 1379–1382. doi:10.1126/science.1187443
3. O'Keefe, F. R., & Chiappe, L. M. (2011). Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science, 333(6044), 870–873. doi:10.1126/science.1205689
4. Kubo, T., Mitchell, M. T., & Henderson, D. M. (2012). Albertonectes vanderveldei, a new elasmosaur (Reptilia, Sauropterygia) from the Upper Cretaceous of Alberta. Journal of Vertebrate Paleontology, 32(3), 557–572. doi:10.1080/02724634.2012.658124
5. Muscutt, L. E., Dyke, G., Weymouth, G. D., Naish, D., Palmer, C., & Ganapathisubramani, B. (2017). The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion. Proceedings of the Royal Society B, 284(1861), 20170951. doi:10.1098/rspb.2017.0951
6. Fleischle, C. V., Wintrich, T., & Sander, P. M. (2018). Quantitative histological models suggest endothermy in plesiosaurs. PeerJ, 6, e4955. doi:10.7717/peerj.4955