Transitional fossils: What are they?

[Loudmouth]

So.... transitional fossils. Anyone want to talk about those?

I think the source of the confusion is that we are trying to use the same term to describe two different (but related) things. We are trying to describe how populations evolve on one hand with the statement "all fossils are transitional". This is true in the sense that each generation is transitional between the previous generation and the next generation.

We are also trying to describe how one tests the theory of evolution using the fossil record. We do this by looking at the patterns of homology and modification (i.e. synapomorphies and derived characteristics). A fossil with a mixture of avian and mammalian characteristics would be a transitional fossil between birds and mammals. At the same time this would falsify the theory of evolution.

 

[Loudmouth]

So, from when i last posted, two fossils from the same species may not be both transitional if one lacks the features labelled as transitional.

Are you talking about partial preservation or are you saying that the two fossils differ in morphology?

From my understanding the hierarchy will work something like this:
A large number of fossils are the so called 'normal' fossils, of land animals staying land animals, of reptiles staying reptiles, and so on. Just as evolution predicts.

If we look at the hominid transitional fossils they start out as land animals and end up as land animals but they are still transitional.

But then there are the 'transitions' where relatively large branching offs occur. Like land animals to sea animals (ambelocetus), reptiles to birds (archaopteryx, microraptor), fish/amphibian (tiktaalik) to name some of the more well known ones.

Every split in the tree is the same, but the time since these splits occurred is different. For example, if you travelled back in time to the point at which the cetaceans first split off you would not consider the differences between the lineages to be that great. It might be similar to the difference between wolves and coyotes, or the difference between llamas and camels that you mentioned in the previous post. The large differences we see NOW are due to the accumulation of changes over time in each lineage.

 

[sbvera13]

I think the source of the confusion is that we are trying to use the same term to describe two different (but related) things. We are trying to describe how populations evolve on one hand with the statement "all fossils are transitional"
...
We are also trying to describe how one tests the theory of evolution using the fossil record.

I think the former definition of transitional fossil is the scientifically established one. However, creationists use the "bird-man" hybrid definition; we use the population version to try and show them how far off the mark they are in asking for that.

 

[metherion]

Are you talking about partial preservation or are you saying that the two fossils differ in morphology

partial preservation.

If we look at the hominid transitional fossils they start out as land animals and end up as land animals but they are still transitional.

Well, yes. I would LOVE to say, well, you know what i mean, but the entire point of this thread is to try and define it better.

Every split in the tree is the same, but the time since these splits occurred is different. For example, if you travelled back in time to the point at which the cetaceans first split off you would not consider the differences between the lineages to be that great. It might be similar to the difference between wolves and coyotes, or the difference between llamas and camels that you mentioned in the previous post. The large differences we see NOW are due to the accumulation of changes over time in each lineage.

This too I am aware of. So would it be better to say that fossils indicating a branch we now see the distinct differences of are transitional, everything is transitional, what? That is, after all, the point of this thread.

Metherion

 

[Loudmouth]

So would it be better to say that fossils indicating a branch we now see the distinct differences of are transitional, everything is transitional, what? That is, after all, the point of this thread.

Metherion

You mentioned Ambulocetus earlier. This is good example. Ambulocetus means "walking whale". It is a tetrapod which harkens back to the common ancestor with other terrestrial mammals. Ambulocetus also has cranial features that are found in modern whales. Ambulocetus has a mixture of characteristics from two divergent taxa.

You might think of it in this way. Earlier generations in each branch will have more features in common because there has been less time to modify the structures that they share through common ancestry. As each branch moves further away from the common ancestor in time there is more opportunity to modify those structures. Thus, earlier generations in the whale lineage had legs that were modified over time to the point that they disappeared.

 

[Alunyel]

You mentioned Ambulocetus earlier. This is good example. Ambulocetus means "walking whale". It is a tetrapod which harkens back to the common ancestor with other terrestrial mammals. Ambulocetus also has cranial features that are found in modern whales. Ambulocetus has a mixture of characteristics from two divergent taxa.

You might think of it in this way. Earlier generations in each branch will have more features in common because there has been less time to modify the structures that they share through common ancestry. As each branch moves further away from the common ancestor in time there is more opportunity to modify those structures. Thus, earlier generations in the whale lineage had legs that were modified over time to the point that they disappeared.

Rodhocetus, Kutchicetus and Dorudon are also good examples. And the only three I can remember at the moment.