Lecture 22: The Amniotes and Evolutionary Biology

Introduction to Evolutionary Biology

• Focus on amniotes and their evolution.

• Key traits of amniotes:

  • Amniotic eggs

  • Dry skin

  • Thoracic breathing

  • Improved kidneys

Amniotes Overview

• Defining Classification:

  • Monotreme

  • Placental

  • Marsupial

  • Aves (birds)

Adaptations for Terrestrial Life

• Challenges faced by early amphibians:

  • Heavy body movement

  • Gills function only in water

  • Body water loss

  • Egg desiccation

• Amniote Solutions:

  • New leg structures

  • Improved lung efficiency

  • Watertight skin and eggs

Amniote Reproductive Traits

• Internal fertilization is a key characteristic.

• The amniotic egg serves as a food source and ensures gas exchange and waste removal.

Types of Amniotes

• Synapsids:

  • One hole in skull (e.g., mammals)

• Diapsids:

  • Two holes in skull (e.g., reptiles, birds)

Thermoregulation

• Ectothermic vs. Endothermic:

  • Non-avian reptiles are ectotherms.

  • Birds and mammals are endotherms.

Avian Adaptations

• Birds evolved from theropod dinosaurs; features include feathers, flight skeletons, and endothermy.

• Unique Bird Traits:

  • Beak morphology reflects ecology and lifestyle.

Bird Classification

• Palaeognaths (flightless ratites):

  • More reptilian traits.

• Neognaths (modern birds):

  • Diverse forms and behaviors (includes most bird species).

Migration

• Significant percentage of bird species are migratory, employing various strategies for navigation.

Important Terminology

• Amniotes, ectothermy, endothermy, synapsids, diapsids, therapsids, archosaurs, etc.

• Examples and adaptations related to lizards, squamates, turtles, crocodilians, and birds.

🦎 Lecture 22: Reptiles and Birds (Amniotes)

🌍 Amniotes: The Perfect Transition to Terrestrial Life

• Amniotes include reptiles (including birds) and mammals, all descendants of vertebrates that fully adapted to land.

• Solved challenges amphibians only partially managed: support on land, breathing air, preventing desiccation, internal fertilization, and waste removal.

• Represent a major evolutionary leap from amphibians’ semi-aquatic life to true terrestrial dominance.

🧬 Defining Traits of Amniotes

• Amniotic egg: watertight, self-contained “life-support system.”

  • Contains yolk (food), shell (protection), and four membranes:

    • Amnion: cushions embryo

    • Chorion: gas exchange

    • Allantois: waste removal

    • Yolk sac: food supply

  • Benefit: embryo develops on land — no aquatic stage needed.

• Internal fertilization: required because shell prevents sperm entry post-laying.

• Watertight skin: prevents desiccation; covered in keratin.

• Thoracic breathing: expands ribcage to draw air (vs. amphibian glottal breathing using mouth cavity).

• Improved kidneys: excrete concentrated urine → conserve water.

🌡 Thermoregulation

• Ectothermy: rely on environment for body heat (most reptiles). Regulate via behavior (e.g., basking).

• Endothermy: internal metabolic regulation (birds, mammals).

• Evolved independently at least three times (mammals, plesiosaurs, archosaurs leading to birds).

💀 Amniote Skull & Jaws

• Stronger jaw muscles attached to temporal fenestrae (openings behind eyes).

  • → Powerful bite, better prey handling.

• Synapsids: 1 temporal opening → mammal lineage (pelycosaurs → therapsids → mammals).

• Diapsids: 2 openings → modern reptiles + birds (archosaurs + lepidosaurs).

🐊 Major Amniote Lineages

Synapsids

• Pelycosaurs: early dominant land vertebrates (~300–250 Mya); sail used for thermoregulation.

• Therapsids: had hair, were endothermic, led to mammals.

Diapsids

• Archosaurs: crocodilians, dinosaurs, birds.

• Dinosaur adaptations: upright limbs (legs under body) → faster, more efficient locomotion.

• Extinction: asteroid impact (~65 Mya) ended most non-avian dinosaurs.

🦎 Modern Reptiles

1. Tuatara

• Ancient lineage (only 2 species, New Zealand); once widespread in Mesozoic.

2. Lizards and Snakes (Squamates)

• Most diverse reptiles; usually carnivorous or insectivorous.

• Males: paired hemipenes.

• Cranial kinesis: flexible skull; snakes swallow prey larger than themselves.

• Venom evolved multiple times; most snakes are non-venomous.

3. Turtles and Tortoises

• Lost teeth; sharp beak.

• Shell fused to vertebrae + ribs → can’t leave shell.

• Turtles = mostly aquatic; tortoises = terrestrial.

• Lay eggs on land, even marine turtles.

4. Crocodilians (Crocodiles, Alligators, Caimans, Gharials)

• Aquatic ambush predators, eyes and nostrils on top of head.

• Strong jaws; twist prey to kill.

• Parental care: build nests, protect eggs, carry hatchlings in mouth — very bird-like behavior.

🦖 How Birds Evolved from Archosaurs

• Descend from small theropod dinosaurs (predatory group).

• Traits pre-adapted for flight: bipedal stance, hollow bones, swivel wrists, 3-fingered hands, backward-pointing pelvis.

• Archaeopteryx & Compsognathus-like theropods = transitional fossils showing evolution from dinosaurs to birds.

🐦 Birds (Aves)

Shared reptile traits

• Amniotic eggs, scaly legs/feet, thoracic breathing.

Unique derived traits

• Feathers: evolved first for insulation/display; later for flight.

  • Structure: barbs + barbules interlock (Velcro-like).

  • Asymmetrical feathers = flight, symmetrical = insulation.

• Lightweight skeleton: hollow bones, many fused for rigidity.

  • Keel (sternum) anchors pectoralis flight muscles.

• Endothermic: maintain ~40–42 °C (104–108 °F).

  • Requires high metabolic rate → highly efficient unidirectional lungs.

• No teeth, vestigial tail (pygostyle).

🧠 Bird Physiology & Behavior

• Bird lungs: constant airflow system (unlike mammals) for efficient O₂ uptake.

• Feathers: replaceable; regulate heat; key for thermoregulation.

• Beak and foot morphology: reflect diet/habitat (e.g., raptors = hooked beaks, webbed feet for waterfowl, etc.).

• Endothermy in birds: allows survival in nearly all climates.

🐧 Bird Diversity

• ~10,000 species — most diverse terrestrial vertebrates.

Paleognaths (Ratites = “old jaws”)

• Ostriches, Cassowaries, Rheas, Emus, Kiwis, Tinamous.

• Primitive jaw structure; mostly flightless.

• Evolved from flying ancestors (flightless by secondary loss).

Neognaths (“new jaws”)

• All other modern birds — enormous adaptive radiation.

  • Waterfowl (ducks, geese)

  • Owls, parrots, woodpeckers, flamingos, penguins, etc.

• Passeriformes (perching birds): most numerous (~6,000 species); small songbirds (e.g., purple martins, sparrows).

✈ Migration

• ~40% of species migrate annually.

• Arctic Tern: 55,000 mi/year (Arctic ↔ Antarctic).

• Bar-tailed Godwit: 8,425 mi nonstop (Alaska → Tasmania).

• Use multiple navigation cues:

  • Sun/stars, visual landmarks, olfaction, and magnetoreception (magnetite in beak/eyes).

  • Some learn routes socially (e.g., geese); others genetically programmed.

🧩 Quick Reference: Key Examples Mentioned