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Why Are There No Half-Fish Half-Mollusks in the Fossil Record?
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.
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.
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.
Sources and More Information
- Bilateria - Wikipedia, the free encyclopedia
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside.
- DOI: 10.1144/SP286.12 (pdf)
New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.
- DOI: 10.1111/j.1420-9101.2004.00741.x
Zhang, X.-G. and Hou, X.-G. (2004), Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis. Journal of Evolutionary Biology, 17: 1162–1166.