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The ancestors of modern, squishy cephalopods like the octopus and the squid all had shells. In ancient times, their shell was their greatest asset but it eventually proved to be their biggest weakness.
Special thanks to Franz Anthony for the beautiful cephalopod reconstructions. You can see more of Franz's tremendous work at http://252mya.com
And thanks as always to Nobumichi Tamura for allowing us to use his wonderful paleoart: http://spinops.blogspot.com/
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References:
http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.860.9021&rep=rep1&type=pdf
http://onlinelibrary.wiley.com/doi/10.1002/bies.201100001/full
http://www.deepseanews.com/2017/10/how-the-squid-lost-its-shell/
https://www.researchgate.net/publication/269526655_An_endocochleate_experiment_in_the_Silurian_straight-shelled_cephalopod_Sphooceras
https://link.springer.com/article/10.1007/s13358-015-0100-3
http://onlinelibrary.wiley.com/store/10.1111/1475-4983.00155/asset/1475-4983.00155.pdf?v=1&t=jedjjjbl&s=df93664ab7ea8768c2832ea135e7dd76626b0ea3
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007262
http://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2008.00828.x/epdf
https://arstechnica.com/science/2017/11/if-you-liked-the-cambrian-explosion-youll-love-the-ordovician-radiation/
Squid Empire by Danna Staaf: http://www.cephalopodiatrist.com/p/squid-empire.html