Mollusc evolution

Early molluscan evolution

Molluscs form a diverse phylum that diversified in the early Cambrian. They make for a very interesting research subject that’s definitely worth hiring a paper writing service or spending time looking up for info about them yourself. The fossil record of molluscs is relatively good compared to many other phyla. Yet, there are still many questions lingering and several fundamental issues of their interrelationships needs to be resolved. For a long time the aplacophorans have been considered the most primitive molluscs and thus been considered of key importance to help us polarize the evolution of molluscs. Modern aplacophorans are vermiform in the sense that they all have a rounded outline, no shells and a mantle with miniscule imbricating sclerites. Given their aberrant appearance compared to the other molluscs and their simple, wormy shape they have thus been considered to be primitive.  However, the fossil record  of aplacophorans are limited and the only fossil until recently that has been ascribed to the aplacophorans are the bisarre Acaenoplax from the Silurian of the UK. This fossil exhibits a curious mix of aplacophoran and chiton like characters.

A growing idea has been that aplacophorans might be derived and that they are sister groups to chitons. Fossils, like Acaenoplax, suggest that their ancestor might even have had shells. This is echoed in extant aplacophoran embryology, which show that both major groups of aplacophorans exhibits a 7 fold iteration on the dorsal surface in late stages of trochophore larvae, which is lost during metamorphosis.

The idea that aplacophorans should have evolved from a chiton like ancestor necessitates that they should have diverged from chitons after the appearance of chiton-like fossils, which are know down to the late Cambrian. I tested this hypothesis with molecular phylogenetics and relaxed molecular divergence estimates (Vinther et al. 2012a, Proceedings B). This study shows that aplacophorans and chitons diversified most likely in the Early Ordovician, which is after the oldest known chiton like forms, which are thus stem groups to both of these groups.


Fossil colors

One of the exciting projects I have been fortunate to be a part of was the discovery of fossil melanin and developing methods to put colors on fossils, such as for example feathered dinosaurs. The study has made a significant impact with respect to how we can perceive fossil feathered dinosaurs – we can now reconstruct them with their original color patterns!

My collaborators on these projects are Derek Briggs, Matthew Shawkey, Julia Clarke, Richard Prum, Liliana D`Alba, Li Quanguo, Gao Keqin, Gerald Mayr.

A dedicated essay editor was called in to polish the end result.

National Geographic and NSF has funded these projects


Squid Fossils History

It all started in the fall of 2006. I was in my first year of graduate studies when I did a project on exceptionally preserved squids and their ink sacs. My question was mainly to figure out why the ink sac is preserved in three dimensions while the rest is completely squashed, even the shell. I began this task as one should approach it as a student of Derek Briggs, by decaying a bunch of squids.

However, After a few days (picture right), everything had dissolved into a sludge. the stench is not even possible to describe without loosing the ability to eat calamari for a year. Not much came out of these experiments, the squids rotted away, and the ink sac dissappeared too. So I could only conclude that it is truly exceptional when you find a fossil softbodied squid.

While the decay experiments failed miserably, my studies of the fossil ink sacs lend insights to a great idea. The fossil ink sacs were preserved as solid organic blobs. When looking at these organic blobs, one could see that they were composed of the melanin from the ink preserved at microscopic level.

As the picture shows above, the fossil ink (lower right) is identical to modern squid ink (lower left). The fact that the ink was preserved has been known for some years, but this made me think that there must be fossil melanin elsewhere. I could make that assertion because melanin is chemically the same in all animals and if it preserved so well here, it must be preserved in other organisms and if so, we might be able to put colors on a fossil feather on perhaps a dinosaur.

My first attempts to test this hypothesis were done back home in Denmark on a Christmas trip in 2007. We have a fossil scull of a bird with a halo of feathers in it and I managed to get the preparator to cut down the block to a size so that I could get it in the SEM at the museum. When I zoomed in I was met by an ocean of little sausages. Exactly how the melanin is contained in bird feathers as well as in mammal hair, within the melanosomes.

I showed these to Derek, who noted that these structures has been identified as bacteria and he found an additional good candidate to prove the concept that these structures are melanosomes and not bacteria with a Cretaceous (~100 ma) fossil feather from Brazil (Crato FM) preserved with color patterns.

The fossil clearly showed the presence of melanosomes in the dark bands and not in the light bands as would be expected if the structures were color-imparting melanosomes.

Subsequently we continued our studies to the well preserved feathers from Messel in Germany, Middle Eocene (47 ma). There we found and described a feather with structural coloration.

More recently we have developed methods to empirically predict colors by measuring the shape and distribution of melanosomes and analysing them statistically with modern feathers. This has culminated with the description of plumage color patterns of one of the oldest know feathered dinosaurs, Anchiornis huxleyi, and a giant Eocene penguin, Incayacu paracaensis.

Liliana and Matt holding the Microraptor specimen that recently was studied (Li et al. 2012), photo credit: Li Quanguo.