Evolutionary arms races are often postulated as the cause of rapid evolutionary escalations, however, they are not always easy to demonstrate. In an excellent study of both living examples and fossils, researchers from the University of Michigan have demonstrated that crinoids evolved their defensive strategies in response to being preyed upon by sea urchins (Baumiller led the research, who also led the research in one of the papers we cited in our poster).
Modern crinoids were quite recently found to be able to crawl away from stressful situations and reattach to substrate in a new location. Palaeozoic crinoids were unable to do this and so invested in stronger "armour" in order to provide protection. The researchers placed sea urchins in a tank with both crinoid fragments and living crinoids. They observed the sea urchins feeding on the fragments and the living crinoids. This confirmed that sea urchins do feed on crinoids and also provided the necessary clues for scrutinising the fossil record. The undigested parts bore characteristic teeth marks from the sea urchins.
The palaeontologists then analysed 2,500 stalked crinoids from the Triassic period, looking for the same bite marks and scratches. Over 500 had such markings, suggesting that predation by sea urchins was a strong ecological presence for early Mesozoic crinoids and the likely driver behind the evolution of motility.
The timing of the occurrence also has significance, as most examples of evolutionary arms races comes from the Mesozoic Marine Revolution, which occurred around 75 Ma later. For crinoids it appears that the arms race had long been raging due to the evolution of stronger, more active feeding apparatus in echinoids.
The journal article can be found here.
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