animal diversification

Animal Evolution

  • Course Information: BIOL:1412

  • Instructor: Dr. Waltz

  • Image Credits: Ray Troll, Nicolle R. Fuller, Sayo-Art Fuller

Objectives

  • Review general phylogenetic relationships of major animal clades.

  • Describe the timeline of animal evolution/diversification.

  • Explain factors that promoted diversification in different animal lineages.

  • Describe major extinction events.

Review: What is an Animal?

  • Characteristics of animals:

    • Multicellular.

    • Heterotrophic (obtain food from other organisms).

    • Internal digestion.

    • Possess movement and nervous systems.

    • Animals are monophyletic (share a common ancestor).

Animal Origins

  • Closest Living Relative: Choanoflagellates.

  • Common ancestor: approximately 900 million years ago (mya).

  • Initial diversification was slow; this was followed by the Cambrian "Explosion".

  • Resulted in a significant increase in diversity, evidenced in the fossil record.

Cambrian Explosion (~540 mya)

  • Rapid Diversification: Established many basic body forms still present today.

  • Dominated marine environments

    • Key organisms: sponges, crustaceans, sea stars, mollusks, worms, chordates.

  • Many modern phyla appeared and evolved during this period.

  • Concept of punctuated equilibrium: Evolution features long periods of stability interrupted by rapid changes.

  • Land was not yet colonized during this time.

Why the Cambrian Explosion?

  • Factors contributing to this event:

    • Increase in global oxygen levels.

    • Rise in ocean calcium ion concentration.

    • Expansive continental shelf and shallow lagoons created new habitats.

    • Evolution of Hox genes, leading to changes in body patterns.

Post-Cambrian Evolution

  • Increased orders and families within phyla, particularly during the Ordovician Period (485 – 420 mya).

  • Modern spinal column formed, leading to the appearance of first fishes.

  • Plants began to colonize land (~470 mya), followed by animals (~440 mya).

Fish Evolution

  • Early Ordovician: Appearance of the first fishes.

  • Development of armored plates; by the late Ordovician, bony jaws evolved, creating large predators.

  • Formation of two major lineages:

    1. Acanthodians - ancestors of sharks.

    2. Placoderms - ancestors of bony fishes.

  • Devonian Period (~420 – 360 mya): Marked diversification of sharks and bony fishes.

  • Emergence of first lobe-finned fishes and the initial vertebrate colonization of land.

Fish Evolution Continued

  • End of Triassic Period (~200 mya): Bony fishes diversified but faced an extinction event.

  • Jurassic Period (~200 – 145 mya): Increased body sizes; niches opened up post-extinction allowing larger reptiles to thrive.

  • Cretaceous Period (~145 – 66 mya): Modern fish ancestors evolved, alongside larger predatory species.

Significant Extinction Event: K-Pg Boundary**

  • Occurred ~65 mya with the Cretaceous-Paleogene extinction, likely due to an asteroid impact.

  • This event allowed radiation of bony fishes and involved the extinction of ammonites.

Lobe-finned Fishes

  • Originated and diversified during the Devonian period.

  • Key modifications included:

    • Skeletal adaptations (girdles, vertebral column, skull).

    • Muscle adaptations led to the evolution of tetrapods.

  • Included the line leading to amphibians, mammals, birds, crocodilians, and squamates (lizards and snakes).

Evolution of Amphibians

  • Transition: Early species were fully aquatic, later formed a terrestrial lifestyle (~80 mya).

  • Derived from a Sarcopterygian ancestor with key adaptations:

    1. Lungs for breathing air.

    2. Appendages with internal support.

  • Challenges faced on land:

    • Gravity.

    • Desiccation (drying out).

    • Breathing medium differences (air vs. water).

Reptiles, Birds & Crocodilians

  • First ancestors appeared during the Pennsylvanian period as part of the Sauropsid lineage.

  • This lineage led to:

    • Dinosaurs.

    • Modern birds.

    • Crocodilians.

    • Other reptiles like snakes and lizards.

  • K-Pg extinction event eradicated non-avian dinosaurs.

Evolution of Mammals

  • First ancestors also from the Pennsylvanian period; classified as Synapsids.

  • During the Jurassic Period, the lineage leading to modern mammals developed.

  • K-Pg extinction opened ecological niches, leading to diversification in mammal forms and functions.

Amniote Evolution

  • Diversification led to various reptiles, tuataras, lepidosaurs, squamates, turtles, crocodilians, pterosaurs (extinct), and many dinosaurs.

  • Lineage includes:

    • Theropods (including birds).

    • Mammals.

Trends in Animal Evolution

  • Complexity: Focused on evolution leading to more complex structures and systems.

  • Centralization: Development of centralized nervous systems.

  • Cephalization: The concentration of sensory organs and nerve cells at the front end (head) of the organism.

Major Extinction Events

  1. Late Ordovician (~445 mya): Extinction of 85% of all species; causes included volcanism and anoxia.

  2. Late Devonian (~360 mya): Extinction of 70% of species, affecting reef organisms due to ocean anoxia and cooling fluctuations.

  3. Permian-Triassic (~250 mya): Largest extinction event; 81% marine and 70% terrestrial species lost; associated with global warming and ocean acidification. Resulted in diversification of archosaurs (ancestors of birds and crocodilians).

  4. Triassic-Jurassic (~200 mya): 70-75% species lost due to volcanic activity and sea changes, paving the way for terrestrial dinosaurs.

  5. Cretaceous-Paleogene (~65 mya): Approximately 75% of species lost, notably non-avian dinosaurs due to asteroid impact, which allowed mammal diversification.