Course Information: BIOL:1412
Instructor: Dr. Waltz
Image Credits: Ray Troll, Nicolle R. Fuller, Sayo-Art Fuller
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.
Characteristics of animals:
Multicellular.
Heterotrophic (obtain food from other organisms).
Internal digestion.
Possess movement and nervous systems.
Animals are monophyletic (share a common ancestor).
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.
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.
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.
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).
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:
Acanthodians - ancestors of sharks.
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.
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.
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.
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).
Transition: Early species were fully aquatic, later formed a terrestrial lifestyle (~80 mya).
Derived from a Sarcopterygian ancestor with key adaptations:
Lungs for breathing air.
Appendages with internal support.
Challenges faced on land:
Gravity.
Desiccation (drying out).
Breathing medium differences (air vs. water).
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.
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.
Diversification led to various reptiles, tuataras, lepidosaurs, squamates, turtles, crocodilians, pterosaurs (extinct), and many dinosaurs.
Lineage includes:
Theropods (including birds).
Mammals.
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.
Late Ordovician (~445 mya): Extinction of 85% of all species; causes included volcanism and anoxia.
Late Devonian (~360 mya): Extinction of 70% of species, affecting reef organisms due to ocean anoxia and cooling fluctuations.
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).
Triassic-Jurassic (~200 mya): 70-75% species lost due to volcanic activity and sea changes, paving the way for terrestrial dinosaurs.
Cretaceous-Paleogene (~65 mya): Approximately 75% of species lost, notably non-avian dinosaurs due to asteroid impact, which allowed mammal diversification.