Mosasauria

Mosasauria (commonly known as mosasaurs, named after the Meuse River [Mosa in Latin] near Maastricht, the Netherlands, where the first giant skull of Mosasaurus hoffmannii was quarried in 1764/1780 and studied by Georges Cuvier in 1808 to establish the scientific reality of species extinction) is an extinct clade of large, fully aquatic carnivorous marine lizards within the order Squamata (lizards and snakes). Emerging from small (~1-meter), semi-aquatic coastal varanoid/pythonomorph lizards (aigialosaurs, such as Dallasaurus and Haasiasaurus) at the beginning of the Late Cretaceous (Cenomanian, ~98–94 million years ago), mosasaurs underwent a dramatic adaptive radiation promptly following the Cenomanian–Turonian extinction of the last ichthyosaurs and giant pliosaurs (Plesiosauria).

For the final 28 million years of the Mesozoic Era (94 to 66.0 Ma), mosasaurs reigned as the undisputed apex predators of every ocean on Earth, reaching body lengths of 12 to 17 meters and masses exceeding 10 to 15 metric tons (Mosasaurus hoffmannii, Tylosaurus proriger, Hainosaurus, Prognathodon) before being abruptly extinguished at the pinnacle of their diversity by the end-Cretaceous (K–Pg) Chicxulub asteroid impact (66.0 Ma).

Unlike ichthyosaurs and plesiosaurs (which were diapsid archosauromorph/ichthyosauromorph reptiles with no close living lizard relatives), mosasaurs are true crown-group squamate lizards nested inside Toxicofera, placed by morphological and combined phylogenomics either inside Anguimorpha (sister to monitor lizards [Varanus] and beaded lizards [Heloderma]) or as the sister clade to Serpentes (snakes) inside Pythonomorpha:

For more than a century, museum mounts and paleoart depicted mosasaurs as giant sea-going monitor lizards with long, straight, ribbon-like tails that swam sluggishly by whole-body anguilliform (eel-like) undulation and ambushed prey from coastal kelp beds.

A series of spectacular soft-tissue fossil discoveries from Kansas, California, and Jordan (Platecarpus tympaniticus [Lindgren et al., 2010], Prognathodon overtoni [Lindgren et al., 2013], and Plotosaurus bennisoni) fully overturned that reconstruction:

1. Downward-Bent (Hypocercal) Tail Fluke: In derived mosasaurs (Platecarpus, Plotosaurus, Prognathodon, Mosasaurus), a series of wedge-shaped caudal vertebrae halfway down the tail bends the vertebral column sharply downward at 30°–45°, supporting the lower lobe of a crescentic, two-lobed vertical tail fluke whose upper, boneless fleshy lobe was stiffened by a dense cross-woven mesh of collagen fibers—making the mosasaur tail a mirror-image (hypocercal) counterpart to the upward-bent (heterocercal) tail of carcharhinid sharks and an precise convergence with the tail fluke of Jurassic ichthyosaurs!
2. Axial Stiffening and Carangiform/Thunniform Propulsion: In the most derived genus Plotosaurus (from the Maastrichtian of California), the ribcage deepened into a rigid, tear-drop-shaped anterior hull covered in tiny, keeled hydrodynamic scales (identical in drag-reducing riblet spacing to fast-swimming pelagic shark denticles!), while the fore- and hind-paddles narrowed into high-aspect-ratio hydrofoils with up to 10–12 phalanges per digit (hyperphalangy) used specifically for steering rather than rowing.
3. Endothermy (~ 35–39\,^°C) Proven by Tooth Enamel Oxygen Isotopes: Clumped-isotope (Δ₄₇) and phosphate oxygen-isotope (δ¹⁸O_PO4) paleothermometry of Tylosaurus, Platecarpus, and Clidastes tooth enamel compared against co-occurring ectothermic fish and endothermic sea birds (Hesperornis) (Bernard et al., 2010; Harrell et al., 2016) demonstrates that mosasaurs maintained elevated, endothermic core body temperatures of 35 °C to 39 °C across water temperatures ranging from 18 °C to 30 °C—powering sustained pelagic pursuit from the warm Western Interior Seaway to 65° S paleolatitude off Antarctica!

Whereas marine crocodiles, ichthyosaurs, and whales have rigid, fused skulls, early and mid-grade mosasaurs (Tylosaurus, Platecarpus, Clidastes) inherited and amplified the streptostylic and kinetic skull of their squamate lizard ancestors to swallow huge struggling prey whole (including 2-meter plesiosaurs, sharks, diving birds Hesperornis, and other mosasaurs preserved as stomach contents inside Tylosaurus and Prognathodon!):

1. The Intramandibular Hinge Joint: Halfway along the length of each lower jaw ramus—between the anterior tooth-bearing dentary/splenial unit and the posterior angular/surangular/articular unit—sat a mobile, ball-and-socket synovial hinge (intramandibular joint) bridged by flexible ligaments. When biting down on a large fish or marine reptile, the lower jaws bowed outward and flexed at the mid-mandibular hinge to absorb struggling forces and widen the pharyngeal gullet.
2. Backward-Hooked Pterygoid Palatal Teeth ("The Snake Ratchet"): Unique among marine reptiles (and shared with macrostomatan snakes!), mosasaurs possessed four tooth rows in the upper jaw: two standard marginal rows on the premaxillae and maxillae, plus two inner rows of large, backward-curved teeth mounted on the mobile pterygoid bones of the roof of the mouth!
   • Powered by protracting and retracting pterygoid muscles via the movable quadrate bone (streptostyly), the left and right pterygoid tooth rows ratcheted forward and backward alternately to walk large, slippery prey down the throat without needing to tear it into small pieces on the seafloor.
   • *(In the latest Maastrichtian apex giants—Mosasaurus hoffmannii, Prognathodon, and Thalassotitan atrox—cranial kinesis was secondarily locked down into a rigid, akinetic box skull armed with faceted, serrated cutting blades or conical bone-crushers capable of dismembering prey larger than their own heads, precisely like modern killer whales [Orcinus orca]!)*

• Pelagic Live Birth (Viviparity) and Oceanic Nurseries: Already in 95-million-year-old Carsosaurus marchesetti (preserving four curled fetuses inside the mother's torso oriented for tail-first birth so the neonate would not drown before clearing the birth canal!) and in open-ocean neonate fossils of Clidastes and Platecarpus discovered by Daniel Field and colleagues (2015) in pelagic chalk hundreds of kilometers from shore, mosasaurs gave birth to live, precocial young directly in the open ocean—and evolved a forked tongue and paired vomeronasal (Jacobson's) organs on the palate to track chemical scent plumes underwater!
• Violent Intraspecific Cranial Combat: Skulls of Tylosaurus, Mosasaurus, and Hainosaurus frequently display partially healed, spacing-matched puncture wounds and gouges across the snout and braincase (as well as broken and fused caudal spines caused by ramming blows from the solid bony rostrum of rival Tylosaurus!).
• Abrupt Extinction at the Chicxulub Boundary (66.0 Ma): Unlike ichthyosaurs (which declined 28 million years before the end of the Cretaceous), mosasaurs were at their highest morphological disparity and trophic breadth in the final million years of the Maastrichtian—spanning freshwater river species (Pannoniasaurus), clam-crushing durophages (Globidens, Carinodens), fish-specialist Plotosaurus, and 15-meter apex Mosasaurus hoffmannii. When the Chicxulub impact triggered global darkness and the collapse of calcareous phytoplankton (Haptophyta) and ammonite/teleost food webs, the high metabolic demands of endothermic apex mosasaurs led to their total extinction at the 66.0 Ma boundary.

1. Russell, D. A. (1967). Systematics and Morphology of American Mosasaurs (Bulletin 23). Peabody Museum of Natural History, Yale University. Google Scholar
2. Lindgren, J., Caldwell, M. W., Konishi, T., & Chiappe, L. M. (2010). Convergent evolution in aquatic tetrapods: insights from an exceptional fossil mosasaur. PLOS ONE, 5(8), e11998. Google Scholar
3. Lindgren, J., Kaddumi, H. F., & Polcyn, M. J. (2013). Soft tissue preservation in a fossil marine lizard with a bilobed tail fin. Nature Communications, 4, 2423. Google Scholar
4. Harrell, T. L., Pérez-Huerta, A., & Suarez, C. A. (2016). Endothermic mosasaurs? Possible thermoregulation of Late Cretaceous mosasaurs (Reptilia, Squamata) indicated by stable oxygen isotopes in fossil tooth enamel. Palaeontology, 59(3), 351–363. Google Scholar

• Squamata
• Ichthyosauria
• Plesiosauria
• Mass Extinction
• Haptophyta