BIOSCI 109: Lecture 20 - Fossils & Major Transitions

Fossils

Traces of past living organisms including their bones, shells, fur, feathers, and so on. Typically, organic material is replaced with minerals, leaving a cast that preserves the space and shape.

Ichnofossils are fossils of paths and tracks. Subfossils are the actual biological organic material, containing some (deteriorated) DNA.

Fossils Importance & Relevance: Aging

Aging of lineages with the use of relative (geologic period) and absolute (radiative) dating. It tells us how long lineages of modern species may have existed.

Fossils Importance & Relevance: Morphologies

Fossils show morphologies and character combinations that aren’t observed presently. It allows us to observe how different morphologies may have interacted and worked together.

Fossils Importance & Relevance: Biogeography

Discovering fossils help us to understand where fossil groups existed, when, and where they went when they were alive. Then, we can also link them to where their lineages may presently be, whether it’s still in the same place or somewhere else.

Fossils Importance & Relevance: Biodiversity

By recording the numbers of fossils at different time periods, it tells us how biodiverse those time periods are. This also helped us discover when major extinction events occured.

Fossils Importance & Relevance: Paleoecology

Fossils show us windows into past ecosystems and how they might’ve function, their structure, and so on. The surrounding fossils in the area allow us to infer how ecosystems may have looked and worked.

Fossils Importance & Relevance: Life History

Discovering fossils of the same species at different ages helps us to understand how they might’ve developed, what happened at specific stages, and so on.

A good example is how we were able to infer that T-Rexes got pregnant relatively early. Similarly to modern day birds, they’d store calcium in their bones near pregnancy to support the growth of their eggs.

Fossils Importance & Relevance: Behaviours

Fossils can be preserved in many different states and actions. They can be preserved as they’re resting, incubating, eating, and so on. Depending on how an individual was preserved, we can infer on what behaviours they might’ve adopted and practised.

Transitional Fossils

Fossils that help to reveal information and allows us to better understand “major transitions”. These transitions include changes in form, function, or habitats.

These transitional fossils have a collection of morphological characters that include some, but not all, of the characters defining some taxon and transition.

With the use of cladograms, we can easily understand the steps of transition from one point to another.

Transitional Fossils: Misconceptions

• They don’t have to be exact intermediates between two living species, there can be variations

• It’s not supposed to be a “morph” or hybrid between two species

• Any organisms can be “transitional” to something, even present day ones could be transitional to something in the future

• There is no one missing link that explains everything, but instead many transitional fossils that showcase the gradual changes in characters

It is important to rememebr that transitional fossils are not progressional linear ladder. There was not one species who just kept evolving and evolving to get where we are now. But instead, many different species that evolved, had certain characters, persisted, went exinct, and branched out.

Archaeopteryx & Transition to Flying/Birds

Arcaeopteryx are widely considered to be the “missing link” between ancient reptiles and modern day birds.

It had some characters modern birds have such as having feathers, being able to fly, and so on. But it also had characters birds didn’t have like teeth. Making it a good transitional fossil.

It was also one of the first fossils discovered that seemed to be a flying dinosaur that’s related to birds.

On cladograms, it is hypothesised to be one of the earliest to evolve flight feathers.

• Before that, there were fossils with different types of feather observed. The feather types before were transitional to flight feathers.

Origins of Tetrapods

Mainly asked questions about how life transitioned from an aquatic mode of life (fish) to a terrestrial mode of life (ambphibian initially).

There were many characters that were focused on and how they could’ve transitioned from ancient species to modern day species.

Some of them incldue how fins transitioned to limbs/feet, how soft skulls transitioned to hard/fused skulls, and different bone structures (hips, back, etc).