Glossary

Mosasaurs (family Mosasauridae) are an extinct group of highly adapted aquatic squamate reptiles that inhabited the world's oceans during the Late Cretaceous, from approximately 94 to 66 million years ago. Belonging to Squamata—the same order as modern lizards and snakes—mosasaurs evolved from semi-aquatic aigialosaur ancestors, developing limbs modified into paddle-like flippers, streamlined bodies, and hypocercal crescent-shaped tail fins convergent with those of sharks. They rose to ecological dominance as apex marine predators following the decline and extinction of ichthyosaurs and the recession of pliosaurs, diversifying into a wide range of ecological niches from small coastal shell-crushers roughly 3 metres long to giant open-ocean predators reaching an estimated 17 metres. Over 40 valid genera and dozens of species have been described, exhibiting varied dental morphologies reflecting diets spanning fish, cephalopods, ammonites, sea turtles, plesiosaurs, and even other mosasaurs. Mosasaurs were viviparous, giving birth to live young in the open ocean, thus completing an entirely marine life cycle. They went extinct during the Cretaceous–Paleogene (K–Pg) mass extinction event approximately 66 million years ago, alongside non-avian dinosaurs and ammonites.

**Plesiosauria** is an order (or clade) of extinct, secondarily aquatic marine reptiles within the superorder Sauropterygia that ranged from the latest Triassic (Rhaetian, approximately 203 million years ago) to the end of the Cretaceous Period (66 million years ago), spanning over 140 million years. They are not dinosaurs; instead, they are a phylogenetically distinct lineage of diapsid reptiles that returned to the ocean from land-dwelling ancestors. The most distinctive feature of plesiosaurs is their unique four-flipper propulsive system: they possessed four nearly identical, wing-shaped flippers and swam via dorso-ventral 'underwater flight.' Controlled water-tank experiments by Muscutt et al. (2017) demonstrated that, when properly phased with the fore flippers' vortex wake, the hind flippers generated up to 60% more thrust and 40% higher efficiency than when operating alone—an arrangement unparalleled among any other living or extinct vertebrate. Their body plan featured a broad, rigid trunk, expanded ventral girdle plates, a short tail, well-developed gastralia (belly ribs), and hyperphalangy in the digits. Plesiosauria is traditionally divided into two superfamilies: the long-necked, small-headed **Plesiosauroidea** and the short-necked, large-headed **Pliosauroidea**. However, phylogenetic analyses since O'Keefe (2001) have revealed that the 'pliosauromorph' body plan evolved independently at least three times and the short-necked Polycotylidae actually nest within Plesiosauroidea, rendering neck-length-based classification unreliable. Multiple lines of evidence—oxygen isotope paleothermometry, bone histomorphometry, and molecular metabolic markers—indicate that plesiosaurs were endothermic, and a preserved gravid female of *Polycotylus* (O'Keefe & Chiappe 2011) confirms viviparity with a K-selected reproductive strategy.

**Pterosauria** is an extinct order of flying reptiles that lived throughout the Mesozoic Era, from the Late Triassic to the end of the Cretaceous (approximately 228 to 66 million years ago). Pterosaurs are not dinosaurs but belong to the same broader group, Archosauria ("ruling reptiles"), and are classified within the bird-line archosaur clade Avemetatarsalia, specifically within Ornithodira alongside Dinosauria. They were the first vertebrates to evolve true powered flight, achieved through a unique wing structure in which a membrane of skin, muscle, and connective tissue stretched from an enormously elongated fourth finger to the hindlimbs. Their hollow, thin-walled bones represent an adaptation for reducing body mass during flight, and their bodies were covered in hair-like integumentary filaments called pycnofibers. Pterosaurs ranged enormously in size, from species with wingspans of approximately 50 cm to the giant azhdarchid Quetzalcoatlus northropi, which had a wingspan estimated at 10–11 meters and stood roughly 5 meters tall. Ecologically diverse, pterosaurs filled niches as piscivores, insectivores, filter feeders, and terrestrial predators. They went extinct at the Cretaceous–Paleogene (K–Pg) boundary along with non-avian dinosaurs, likely as a result of the Chicxulub bolide impact and its cascading environmental consequences.