Age of the Giants Notes

After the Great Dying: Rise of the Diapsids

• The Permian mass extinction eliminated many basal lineages.
• This was followed by the diversification of multiple diapsid and synapsid groups.
• Synapsids became smaller, while diapsids became larger.
• The Mesozoic Era became the age of diapsids.
• There was a wide variety of diapsids, many of which were not dinosaurs.

Major Diapsid Lineages

• Lepidosauria ("Scaly lizards")
  • Plesiosauria - Marine.
  • Ichthyosauria - Marine.
  • Extant Lepidosauria – mostly terrestrial.
    • Squamata + Tuatara.
• Archosauria ("Ruling lizards")
  • Crocodylomorpha – terrestrial, freshwater, marine.
  • Dinosauria (including birds) – terrestrial, flying.
  • Pterosauria – flying.
• Turtles are likely a sister group of archosaurs.

Marine Lepidosauria – Ichthyosauria and Plesiosauria

• Marine lineages are apparently closest to extant Lepidosauria.
• They existed from the Triassic to the Late Cretaceous periods.
• All were strictly marine predators.

Plesiosaurs

• Existed from the Late Triassic to the end of the Cretaceous period.
• Fully marine animals.
• Limbs were paddle-like, extended by hyperphalangy (additional finger bones), similar to cetaceans.
  • Viviparous.
  • K-selected, producing few large offspring.

Ichthyosaurs

• Fully marine animals.
• Lived from the Triassic to the mid-Cretaceous period, facing an "unforced" extinction.
• Superficially dolphin-like but possessed a vertical, heterocercal tail.
• Both fore- and hind-limbs were retained as paddles.
• They were fast swimmers that ate cephalopods and fish.
• Viviparous.

Major Diapsid Lineages: Archosauria

• Pterosauria – “Winged Lizards.”
• Sister group of Dinosaurs + Birds.
• BUT NOT DINOSAURS!!!!
• Late Triassic – End Cretaceous.
• First vertebrates to evolve powered flight.

Pterosaur Adaptations for Flight

• Wings formed by a membrane stretched over the arm and extended by the 4th finger.
• The 1st-3rd fingers were retained as claws.
• Quadrupedal terrestrial locomotion.
• Endothermy.
• Body covered in hair-like pycnofibres for thermoregulation.
• Tooth loss in later forms.
• Air sacs.
• Unidirectional airflow through lungs.

Pterosaur Biology

• Oviparous.
• Diverse ecology, morphology, and diet.
• Largest ever flying animals.
  • Maximum wingspan ~ 11 meters (largest extant birds: 3.5 m; largest fossil birds: 6.4 m).
  • Maximum mass estimated ~ 200 kg.
  • Minimum wingspan ~ 25 cm.

The Dinosaur Renaissance

• An ongoing revolution in how we see dinosaurs.
• Moving from views of maladapted, lumbering failures to highly successful, complex, warm-blooded, adaptable animals.
• Initiated by Robert T. Bakker (US palaeontologist).
• Published the article "Dinosaur Renaissance" in the April 1975 issue of Scientific American.
• Authored the book “Dinosaur Heresies” in 1986.

Dinosaur Diversity

• A highly diverse group in terms of species and morphology.
• Primitively bipedal.
• Over 1000 known species.
• Size: from 1 kg to > 100 tons, 75 cm - ? 60 m.

Major Lineages - Traditional Interpretation

• Saurischia
  • Sauropodomorpha
  • Theropoda -> Aves.
• Ornithischia.

Major Lineages

• Ornithischia - "bird-hipped dinosaurs".
• Saurischia - "lizard-hipped dinosaurs".
  • Sauropoda
  • Theropoda

Alternative View: Baron et al. 2017

• Ornithischia as sister taxa to Theropoda and Saurischia.

Multiple alternative Views

• Relationships among major dinosaur groups remain poorly understood; more analyses and data are needed!

Ornithischia

• All herbivores.
• Very diverse body forms.
• Complex social behaviours (horns, frills, resonating chambers, sexual dimorphism, colonial nesting).

Saurischia

• Traditionally two major groups:
  • Sauropods (quadrupedal, herbivores).
  • Theropods (bipedal, carnivores) – including birds.
• However, see Baron et al., 2017 and Lovegrove et al., 2024.

Saurischia - Sauropods

• Large, adult length 5-?60 metres (c.f. blue whale: 30 metres).
• Weight: to over 100 tons (cf. African elephant max. approx. 10 tons, blue whale ~ 200 tons).
• Quadrupedal.
• Long necks and tails.
• Likely gregarious, herd-living, with communal nesting.

Why So Big?

• Increases the efficiency of digestion of poor-quality plant material.
• Likely through hindgut fermentation.
• Likely interspecific competition è selective advantages to reaching greater heights.

Saurischia - Theropoda

• Bipedal carnivores.
• Includes the largest ever land carnivores, including Tyrannosaurus rex, as well as Velociraptor.
• Some later lineages were endothermic and feathered.
• Birds descended from within Theropods.

Social Behaviour

• Dinosaurs displayed:
  • Extensive parental care.
  • Use of vocalisation and colour.
    • The distribution of colour is known in some species, suggesting strongly contrasting patterns!
  • Sexual dimorphism, suggesting sexual selection (i.e., complex courtship).
    • Sexual dimorphism in the nasal crest of Parasaurolophus.

Ecology

• Dinosaur communities: high levels of predation and counteradaptation (e.g., crypsis, countershading in Borealopelta) – similar to modern mammalian communities.

How Do We Know This Stuff?

• Direct evidence from fossils.
• Extant phylogenetic bracket: common derived traits of birds and crocodiles likely to be shared by dinosaurs.

Dinosaurs in a New Light

• Discoveries over the last five decades have changed and are still changing our view of dinosaurs:
  • A highly successful group existed over nearly 200 My.
  • Highly adaptable.
  • Complex social lives and interactions.
  • Many were endothermic.
  • Ecology similar to modern mammals.
  • And…

… And Anyway, They Are Not Extinct!

• You know those small flying dinosaurs you see all over the place?
• I think some people call them "birds."

So What Was Everyone Else Doing in the Mesozoic?

• Synapsids: first stem-mammals evolved in the late Triassic; first crown-group mammals ~167 Mya in the Jurassic.
• Two-thirds of mammalian history was lived in the shadow of the dinosaurs!
• They stayed small until the end of the Cretaceous.

The Other Archosaurs: Crocodylomorphs

• Hesperosuchus - Sphenosuchia = stem crocodilian.
  • Late Triassic, 220 Mya.
  • To 1.5 metres.
• Ornithosuchus (“bird crocodile”) – Sphenosuchia = stem crocodilian.
  • Late Triassic.
  • Partly bipedal predator, to 2.2 metres.
• Dakosaurus - large carnivorous fully aquatic form.
  • Late Jurassic - Early Cretaceous.
  • Dakosaurus: 4.5m.
• Simosuchus - late Cretaceous herbivorous crocodylomorph.
  • 0.75 metres.

So What Was Everyone Else Doing in the Mesozoic? Squamata

• Diversification of main lineages since the early Jurassic.
• Mostly small.

Mosasaurs: Squamate Marine Giants

• Giant marine lizards (likely relatives of varanid lizards).
• To 17 metres total length.
• Occupied niches of extinct ichthyosaurs and declining plesiosaurs in the late Cretaceous – apex marine predators.

So What Was Everyone Else Doing in the Mesozoic? Squamata

• Diversification of main lineages since the early Jurassic.
• Snakes diverged in the Jurassic, poorly known until the Cretaceous.
• Many large snakes – some documented to feed on dinosaurs.

Mid-Cretaceous: Nice Snakes, Shame About the Legs

• At first, thought to share some derived features with advanced snakes.
• Puzzle: convergent evolution of advanced features, or convergent re-evolution of hind limbs?
• Recent analyses suggest they were stem snakes, with retained primitive legs.

The End of the Giants - Chicxulub Crater

• Yucatán Peninsula.

It Was Good While It Lasted…

• Dinosaurs and other large diapsids were highly diverse.
• Dominant large vertebrates for > 160 My (cf mammals: 65 My).
• Reached sizes and diversities unmatched since (largest land animals, largest flying animals, some of the largest marine predators).
• Still evolving new lineages and evolutionary novelty until the end of the Cretaceous.
• Might well have continued to flourish but for the meteorite.

Take-Home Messages

• Diversity of extinct lineages, body plans, etc.
• Levels of diversity within many extinct groups.
• Many examples of convergent evolution across extinct and living lineages.
• The role of the 3 major mass extinctions in shaping tetrapod diversity:
  • Late Devonian: extinction of ichthyostegalians, followed by the rise of major anamniote groups and amniotes.
  • End Permian: extinction of most anamniotes, reduction of synapsids, followed by diversification of diapsids.
  • End Cretaceous: extinction of large diapsids, followed by diversification of birds and mammals as the dominant tetrapod groups.
• Palaeontology is a flourishing field!
• Numerous new discoveries due to:
  • Improved fossil record.
  • Constantly developing tools of examination and analysis.
• It’s a very exciting time to be a palaeontologist!