Lecture 8: Ornithischian Dinosaurs, Pterosaurs, and the K-Pg Extinction

Overview of Ornithischian Dinosaurs

• Ornithischians: "Bird-hipped" dinosaurs
• First fossils: Late Triassic
• Major Groups:
  • Ornithopods and Ceratopsians
  • Stegosaurs and Ankylosaurs

Ornithopod Features

• Time Period: Jurassic to Late Cretaceous
• Locomotion: Bipedal with fusion of pubis and ischium
• Adaptations: Efficient chewing mechanism and likely developed cheeks

Hadrosaurs (Duck-billed Dinosaurs)

• Description: Abundant in Late Cretaceous
• Bill Formation: Modification of maxilla and premaxilla to create a bill
• Feeding Mechanism: Tooth "batteries" for processing tough vegetation (e.g., conifer needles)
• Posture: Bipedal running posture

Hadrosaur Biology

• Nesting Behavior: Communal nesting with evidence of parental care
• Physical Features: Head crests – potentially for signaling; exhibited sexual dimorphism
• Skin Texture: Reptile-like skin, preserved skin casts found

Ceratopsians (Horned Dinosaurs)

• Morphology: Initially bipedal, later quadrupedal; large, modified skulls with beaks and frills
• Time Period: Lived predominantly during Late Cretaceous
• Neural Spines: Adapted for large muscle attachments to support heavy heads
  • Example genera: Psittacosaurus (Mid Cretaceous, Mongolia), Triceratops (Late Cretaceous, US)

Ceratopsian Biology

• Nesting: Evidence of nests and eggs suggests parental care
• Function of Frills and Horns: Debated; may relate to defense, mating displays, or species recognition
  • Example specimens: Kosmoceratops, Utahceratops, Torosaurus

Thyreophora: Armored Ornithischians

• Time Period: Mid Jurassic to Late Cretaceous
• Groups:
  • Stegosaurs:
  • Characteristics: Bony dermal plates, small tubular skulls, and minimal brain size (similar to a walnut)
  • Key Example: Stegosaurus
  • Ankylosaurs:
  • Characteristics: Extensive dermal bone, some with tail clubs

Pterosaurs – The Flying Reptiles

• Classification: Diapsids with uncertain origins within Triassic archosaurs; sister group to dinosaurs
• First Fossils: Late Triassic
• Two main groups:
  • Rhamphorhynchoids: Long tails, teeth for catching fish
  • Pterodactyloids: Short tails, some toothless, larger body sizes

Pterosaur Morphology

• Body Structure: Short body with large sternum; pneumatic bones; adapted for flight with elongated fingers for wing membranes
• Flight Mechanics: Bird-like muscle arrangement, with a combination of muscles that enhanced flight capability

Pterosaur Biology

• Adaptations: Coating of hair suggests a possible endothermic metabolism
• Reproductive Behavior: Laid eggs, evidence found in specimens like Darwinopterus

K-Pg (Cretaceous-Paleogene) Extinction Event

• Extinctions:
  • On Land:
  • Non-avian dinosaurs
  • Pterosaurs
  • Some bird families
  • Some marsupial mammals
  • In Sea:
  • Plesiosaurs
  • Mosasaurs
  • Various shark and teleost families
  • Numerous invertebrates (e.g., ammonites)
• Survivors:
  • Generally unaffected: Crocodiles, turtles, lizards, snakes, amphibians, placental mammals, most fish, and many plants
• Extinction Selectivity: No strong evidence for selective extinction; implications of chance events

Key Takeaways

• Ornithischian dinosaurs showcase a variety of adaptations and ecological strategies.
• The K-Pg extinction was a transformative event leading to the loss of numerous species and the survival of others.