アメリカ自然史博物館のインスタグラム(amnh) - 10月12日 00時22分

The subject of this week's #TrilobiteTuesday is trilobite enrollment; more specifically why did these primitive arthropods enroll, and when did they begin to do so? Of course, many modern creatures from isopods to armadillos enroll for safety, and it can be logically surmised that trilobites assumed such a defensive position for much the same reason. With their thick calcite shells, an enrolled trilobite could present quite the daunting challenge for any predator seeking an easy meal. In addition, the compact shape provided by enrolling may have also aided them in battling unfavorable conditions during the heights of oceanic storms. During enrollment, as shown on this Dipleura Dikayi from the Devonian of Pennsylvania, the flexibility provided by their thoracic segments allowed a trilobite's rigid cephalon and pygidium to interlock in order to encase and protect the creature's delicate limbs, antennae and soft ventral parts. It was a key evolutionary development in allowing the trilobite line to survive for more than a quarter of a billion years.

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