What Does the Fossil Record Show?

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November 11, 2011 Tags: History of Life

Today's entry was written by the BioLogos Editorial Team. You can read more about what we believe here.

What Does the Fossil Record Show?

Yesterday, in our BioLogos podcast, we looked at the question of transitional fossils. Today, to follow-up, we’d like to repost our recently revised FAQ on the fossil record – one of ten Questions we’ve updated. We’ve also edited how the Questions are organized, to help readers more easily find answers to the topics they care about, from explaining the BioLogos view to responding to arguments against God and Christianity. If you haven’t yet, we encourage you to take a look at the changes!

Evidence of Gradual Change

Organisms have changed significantly over time. In rocks more than 1 billion years old, only fossils of single-celled organisms are found. Moving to rocks that are about 550 million years old, fossils of simple, multicellular animals can be found. At 500 million years ago, ancient fish without jawbones surface; and at 400 million years ago, fish with jaws are found. Gradually, new animals appear: amphibians at 350 million years ago, reptiles at 300 million years ago, mammals at 230 million years ago, and birds at 150 million years ago.1 As the rocks become more and more recent, the fossils look increasingly like the animals we observe today.

The Transition to Land: Sea Creatures to Land Animals

Fossils of land animals, or tetrapods, first appear in rocks that are about 370 million years old. In older rocks, only sea creatures are found. But in 1998, scientists found a fossilized fin, 370 million years old, with eight digits similar to the five fingers humans have on their hands, as shown in Figure 1. However, the fin was undoubtedly that of a fish, which means this fossil is strong evidence of a transitional form.

Figure 1: An Illustration of the fossilized fin found in 1998. Its resemblance to a Tetrapod is an indication of gradual evolutionary change from sea creatures to land animals. Source: Image is used by permission from Darrel R. Falk, <em>Coming to Peace with Science: Bridging the Worlds between Faith and Biology</em> (Downers Grove, IL: InterVarsity Press, 2004), 113.Figure 1:An Illustration of the fossilized fin found in 1998. Its resemblance to a tetrapod is an indication of gradual evolutionary change from sea creatures to land animals. Source: Image is used by permission from Falk, Coming to Peace, 113.

One of the great success stories in the examination of the fossil record was the finding of a near-perfect fossilized transition between a vertebrate adapted for water and one adapted for land. Evolutionary biologist Neal Shubin set out to find a more complete transitional specimen than the 1998 fin. He determined the exact age of rock that he expected would yield a transitional land/water animal, and then he and his team spent four summers in the Arctic scouring rocks of that age to find one. The results (see Figure 2 below) were spectacular.2

From Reptiles to Mammals

Mammals first appeared in the fossil record about 230 million years ago, nearly 70 million years after reptiles first appeared. One group of reptiles, the cynodonts, first appeared about 260 million years ago and became increasingly mammal-like in more recent fossils—circa 245 million years ago. This change can be seen most clearly in the bone structure of the ear, as illustrated in Figure 3.

Figure 3: As shown in the image above, transitional fossils of cynodonts had two jaw hinges. These fossils date from a time when the dentary and squamosal bones were beginning to take over the role of jaw hinge (hinge #2). This allowed the articular and quadrate bones to evolve into the second and third bones of the mammalian ear, as shown on the right. Source: Image used by permission from Falk, Coming to Peace, 119. Originally from F. H. Pough, J. B. Heiser, and W. N. McFarland, Vertebrate Life, 4th ed. (Upper Saddle River, NJ: Prentice Hall, 1996), 607.

Scientists found a species of cynodonts, dating to just before the emergence of mammals, that had a double jaw hinge like that of a mammal. A pair of bones found in even earlier cynodont fossils seems to have transitioned slowly into the ear. No other fossils have been found that share a similar structure to the transitional cynodonts and date back before the time of mammals. Likewise, soon after mammals appeared, these cynodonts became extinct. This timing implies that the cynodont fossils record the transition from reptiles to mammals.3

Transitional Forms: Few and Far Between

Transitional forms occur just when one might expect to see a change from one body type to another. However, a common objection is that few transitional fossils have been discovered; thus many lineages cannot be traced smoothly.

There are several reason for these gaps in the fossil record. First, fossilization is a very rare event. Plus, transitional species tend to appear in small populations, where rapid changes in the environment can provide a stronger evolutionary drive. Finally, because fossilization itself is a rare event, smaller populations are sure to produce fewer fossils. The fact that transitional species have been found at all is remarkable, and it offers further support of gradual, evolutionary change.

Notes

  1. Darrel Falk, Coming to Peace with Science, 83-84.
  2. For a discussion of the science and the story of the discovery, see Darrel Falk, “In the Bones” (July 29, 2009), (accessed 10/21/2011) and Stephen Matheson, “New Limbs from Old Fins, Part 2” (Sept 16, 2011), (accessed 10/21/2011). Shubin’s 2008 book Your Inner Fish (Pantheon) is also very good.
  3. Falk, Coming to Peace, 115–120; F. H. Pough, J. B. Heiser, and W. N. McFarland, Vertebrate Life, 4th ed. (Upper Saddle River, NJ: Prentice Hall, 1996), 607; M. J. Benton, Vertebrate Palaeontology: Biology and Evolution (London: Unwin Hyman, 1990), 228–231; E. H. Colbert, M. Morales, and E. C. Minkoff, Colbert’s Evolution of the Vertebrates: A History of the Backboned Animals Through Time (New York: Wiley-Liss, 2001), 274–277; T. S. Kemp, The Origin and Evolution of Mammals (New York: Oxford University Press, 2005), 75–78.


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Tim - #66057

November 12th 2011

Dude, you didn’t…

“Evolutionary biologist Neal Shubin set out to find a more complete
transitional specimen…He determined the exact age of
rock that he expected would yield a transitional land/water animal, and
then…spent four summers in the Arctic scouring rocks of
that age to find one. The results…were spectacular.”

While Tiktaalik is truly an amazing morphologically transitional fossil, it is not stratigraphicaly transitional.  In research published in Nature in January 2010, we discover that full tetrapod tracks existed as far back as 395 million years ago.  These results are now widely accepted among paleontologists and other life scientists.  So, the rocks of the “exact age” of approximately 375 million years old are about 20 million years past the emergence of tetrapods.

Now, I’m fully convinced of Evolution/Common Descent based on the totality of evidence (not least of all paleontological and genetic).  However, you lose credibility when you publish stuff like this.  You’ve got to keep up with the science if you’re going to represent science to the masses, alright?


Ashe - #66063

November 14th 2011

“Rocks of the “exact age” of approximately 375 million years” ? It seems like a self-contradiction. Well-preserved tetrapod tracks from Velentia Island in Ireland were discovered as long ago as 1993, and are known to date to the latest Middle Devonian (around 385 million years ago) and are thus also older than Tiktaalik.


Darrel Falk - #66071

November 14th 2011

Tim,


We are aware of the footprints and indeed, I wrote a blog about it at that time.  See here http://biologos.org/blog/footprints-in-the-sand .

Everything we wrote above still holds.   Perhaps I should clarify, however.  You can never be sure that you have the transitional species in investigations of this sort.  Indeed, given the rarity of fossilization it is much more likely that for any “transitional species” all you have is cousin of the real species that was actually on the direct lineage.  Archaeoptrix is a transitional species in that same sense.  It is probably just a cousin. Wouldn’t it be great if we could go back in time to see what all was around at these transition times?   How may other cousin species were there?  Hundreds, perhaps?

I think we were all shocked to find that no sooner had Tiktaalik been discovered that it became apparent that the actual transition occurred even earlier than people had anticipated.  This illustrated clearly just how sparse the fossil record really is.   Given this, it is truly amazing just how much detail we can fill in.

HornSpiel - #66068

November 14th 2011

full tetrapod tracks existed as far back as 395 million years ago...So, the rocks of the “exact age” of approximately 375 million years old
are about 20 million years past the emergence of tetrapods.

“Rocks of the “exact age” of approximately 375 million years” ? I


I sympathize with your point of view that some of these statements seem self contradictory. However if you think about them with more nuance they are not.

Although some tetrapods may have existed prior to Tiktaalik rosea (shown above)the researchers were searching for a particular transitional species between certain lobed finned fish and certain fish-like tetrapods for which they knew that dates. what they found corresponded to that evolutionary transition. Whatever tetrapod made the 395 myr tracks, I would guess that it was not Tiktaalik rosea. If it were, then that would be a finding!

Second “exact age” of approximately. Perhaps you are aware from science class of error bars or significant figures. It means that there is always a limit to how accurate a measurement is. The above wording is shorthand for there being a a range of some tens of millions of years in which they would predict this creature existed.

The importance of science education is that it teaches us to think clearly, not just from our gut.


Jimpithecus - #66077

November 15th 2011

What is important to understand is that a species can have transitional characteristics without being in a direct ancestral line to the species that chronologically follow it.  For example, in the human evolutionary line, all australopithecine species had transitional characteristics relative to their ape ancestors and some of these characteristics appeared earlier than others, but only one of the australopithecines led to the genus Homo.  This post (http://biologos.org/blog/the-human-fossil-record-part-1-the-nature-of-transitional-fossils) ought to help.


HornSpiel - #66087

November 16th 2011

Jimp. and Darrel, thank you for pointing out the issue from a professional viewpoint. I apologize if my thoughts are more a stream of consciousness than a reasoned argument I do appreciate your well thought out
responses. My comment above about the dangers of thinking from the gut applies to me as well.

However your comments only weakly reaffirm the significant fact that the team that found Tiktaalik rosea using a prediction based on the evolutionary theory. Doesn’t that fact stand apart from the fact that later older tetrapod tracks were found? Or was it just a fluke that they found this fossil in the rocks that they did?

So why were professionals so “shocked to find that no sooner had Tiktaalik been
discovered that it became apparent that the actual transition occurred
even earlier….” Perhaps even professionals don’t understand all the implications and limits of the theory.


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