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Why Are There No Half-Fish Half-Mollusks in the Fossil Record?

Updated on January 20, 2012
Cowrie, Cypraea chinensis, with partially extended mantle.
Cowrie, Cypraea chinensis, with partially extended mantle. | Source

Debates with creationists and other assorted science deniers are usually fruitless endeavors. Some have quite appropriately likened it to playing chess with a pigeon. However, there are times when these debates can be quite useful. Many times, I have found myself challenged by their questions (or at least, the ones they copy-pasted from Answers in Genesis) to research further into a topic I don't know much about.

This hub is the product of one such discussion on an earlier hub. A commenter wondered why there are no half-fish half-mollusks in the fossil record, implying that this demonstrates evolution to be false. Knowing little about prehistoric fish and even less about prehistoric gastropods, I decided to look into this further. The answer, as you might expect, is rather complicated.

Nielsen's (2001) view of bilaterian relationships with mollusks and chordates highlated. Click to embiggen.
Nielsen's (2001) view of bilaterian relationships with mollusks and chordates highlated. Click to embiggen. | Source
Diagram showing differences in protostome  and deutorostome embryonic development.
Diagram showing differences in protostome and deutorostome embryonic development. | Source

One Tree, Multiple Branches

Fish and mollusks belong to separate sub-branches of the bilaterian branch of the tree of life. The split between these branches dates back to the dawn of the animal kingdom more than 500 million years ago.

The bilaterian group to which fish and mollusks both belong refers to the fact that these creatures have bilateral symmetry - the left side of their body is a mirror of their right side. Most animals fall into this category. This group is split into two major subgroups, or superphyla in scientific parlance:

  • deuterostomes - this group contains animals from birds to humans to sea urchins to dolphins, and includes the multiple subgroups of chordata that we classify in lay terms as fish.
  • protostomes - this group contains invertebrate animals from leeches to crabs to flatworms, and includes the mollusca subgroup.

Though there are a number of physical differences that separate these groups, the primary difference is in their embryonic development, illustrated at right. While protostomes develop their digestive tube mouth-first, we deuterostomes develop it anus-first. Seven-year-old boys and immature adults are permitted to giggle at this fact as needed, but this is nevertheless an important taxonomic distinction.

Current models of evolution put the split between these two lines somewhere between 560 to 550 million years ago.The exact lineage of creatures so early in Earths history - prior to the evolution of bony skeletons or shells - is extremely difficult to reconstruct as soft tissue doesn't preserve well and most creatures of the time did not have the courtesy to die in places where they would fossilize. Despite these obstacles, paleontologists studying ancient fossils have put together a rough picture of how these groups may have split.

Reconstruction of Haikouichthys ercaicunensis.
Reconstruction of Haikouichthys ercaicunensis. | Source
Fossil of Kimberella quadrata
Fossil of Kimberella quadrata | Source

The First Fish

The leading candidate for the earliest fish in the fossil record is a jawless creature known as Haikouichthys ercaicunensis that would likely have resembled a modern-day hagfish or lamprey. It was discovered in Yunnan Province, southern China in 1999. Haikouichthys fossils have been dated to around 530 million years ago. Although there is some disagreement among paleoicthyologists about how exactly to classify it, haikouichthys had a defined skull and was probably one of the first vertebrates on the planet.

The First Mollusk

A fossil known as Kimberella quadrata, dating from 558 to 555 million years ago, is a leading candidate for the first mollusk, but this classification is not without controversy. Originally classified as a species of jellyfish, later research has determined that it was most likely an early bilaterian. It is a subject of heated debate among paleontologists whether or not to also classify it as a mollusk.

Key to this debate is whether or not Kimberella had a radula - the chitinous "tooth-tongue" used by mollusks for feeding. To date, no Kimberella fossils have been found with an intact radula. However, scratch marks found in the rocks near Kimberella fossils strongly resemble radula markings of modern-day mollusks as they feed off microbial mats on the sea floor, prompting those in the pro-mollusk camp to classify Kimberella as a mollusk. Others argue that this evidence is circumstantial and consider the mollusk classification highly dubious.

So Where's The Follusk (Mollish?)

The proto-mollusk and proto-fish candidates above are both several branches of classification away from each other, indicating that their common ancestor is much farther back in evolutionary history. This creature, dubbed the urbilaterian, is the ancestor of both protostomes and deuterostomes.

Unfortunately, it has yet to be found. Though there are some controversial fossil candidates, none have been definitively classified as a true bilaterian.

This still-hypothetical creature would likely have been a type of flatworm with a central nervous system and segmented body, and would have lived somewhere around 570 million years ago. As it would not have had bony structures, fossils of this will be difficult to find, making it likely that the urbilaterian will remain a mystery for many years to come.

That's Not A Very Good Answer!

Reconstructing the fossil record is a bit like trying to put together a puzzle for which you've lost the box and are missing an untold number of pieces. Although you don't exactly know what the finished picture will look like, you can make some educated guesses about where pieces go and occasionally find some that fit together. Other pieces don't seem to fit at all, forcing you to re-think what you believe the final picture will look like.

Science, particularly a squishy science such as evolutionary biology, is not always about definitive answers. Sometimes the goal is just to find the right questions.


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    • Trish_M profile image

      Tricia Mason 6 years ago from The English Midlands

      Hi :)

      Very good.

      I can't understand why creationists assume that the lack of, for example, a creature that was half zebra and half toad, is evidence against evolution.

      We are learning all the time, and, all the time, the evidence for evolution becomes ever stronger.

    • Teresa Coppens profile image

      Teresa Coppens 6 years ago from Ontario, Canada

      Very well written hub Scott. Great job. I love the analogy of scientific investigation to putting together a puzzle. As it should be evolutionary theory is modified as new puzzle pieces are found. We will probably never know all of the answers but as curious humans we will continue to try.

    • profile image

      Onthepath 6 years ago

      Hey @somethgblue , Interesting opinion. Do you mind if I ask you what exactly it is you DO believe? Or at least what conclusions you have made for yourself (other than the obvious disbelief in both evolution and creationism).

      P.S. @scottcgruber, great hub! Keep up the good work!

    • Apostacy profile image

      Apostacy 6 years ago from Near Toronto

      What Theists fail to realize is, the answer "We don't know" becomes the default position, and "God did it" Remains a claim lacking sufficient evidence. Very informative Hub my friend, I enjoyed it.

    • somethgblue profile image

      somethgblue 6 years ago from Shelbyville, Tennessee

      You don't have to be convinced, I would never claim to be 100% sure of anything, not even death, although so far taxes seem to be a sure bet.

      However even that being said I read somewhere that when you file your first tax return that it is in effect a contract made with the IRS, which you then are liable for the rest of your life.

      Anyway I used to believe whole heartedly in evolution, until I began to research it and then a crack in my arguments turned into a hole and now I've gone to the other side.

      I'm not a creationist either and see science and spiritualism as two sides of the same coin. My one pet peeve is contempt prior to investigation, as it is too easy to believe others and would much rather decide for myself.

    • scottcgruber profile image

      scottcgruber 6 years ago from USA

      If there's a copy of that in the library, maybe I'll check it out sometime. Though if those semantic arguments are her best shot at proving evolution a "fraud," I don't see myself being convinced.

    • somethgblue profile image

      somethgblue 6 years ago from Shelbyville, Tennessee

      I could but what would be the point, as others have already done it far better than I could.

      Perhaps I should've said it doesn't work for me, however this is implied and anyone with half a brain would have picked up on that. I suggest you simple read a book, there are many out there for public consumption, off the top of my head I would suggest Forbidden Archeology.

      The Evolution Conspiracy by Lisa Shiel is actually really good and exposes the fraud in a very easy to understand way.

      • Evolution fails to meet standard criteria for a fact or a theory.

      • No one can satisfactorily define what a species is.

      • The usual definition of evolution is so vague as to be meaningless.

      • No one knows how life originated, or where the original living cell came from.

      • DNA is not infallible (despite what CSI would lead you to believe).

      You seem to want everything handed to you on a silver platter, if for instance you don't like reading just go to YT, there are dozens of videos about it.

      Quoting science isn't proof of anything, everything ever written is fiction or to put it simple . . . someone's version of the truth!

    • scottcgruber profile image

      scottcgruber 6 years ago from USA

      Do explain how the science of evolutionary biology "doesn't work" when 150 years of research into comparative anatomy, embryonic development, paleontology, genome analysis, and observed speciation clearly show that it does.

    • somethgblue profile image

      somethgblue 6 years ago from Shelbyville, Tennessee

      Very good, you are definitely the master of copy and paste.

      Being humble enough to admit that the answer hasn't been found yet, is a sign of intelligence, that far surpasses a mollusk.

      Of course all of this is based on a science that doesn't work, but it is well thought out and shows how disinformation can be spread . . . kind of like fertilizer!

    • dadibobs profile image

      dadibobs 6 years ago from Manchester, England

      Very interesting piece, i like the human element, placed in the commentary. this is a very educational hub. :)

      voted up.

    • davenmidtown profile image

      David Stillwell 6 years ago from Sacramento, California

      An interesting hub. I enjoyed the methodology and the layout as well as the gentle message that we know so very little. Well done!


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