There is a story that has become part of trilobite folklore. It tells of how back in the early 1970s, a Berber tribesman living on the edge of Morocco's Sahara Desert was walking near the mountains that encircled his home. As he was strolling past one, he heard the distressing sound of falling rocks and noticed a particular fist-sized piece that was cascading in his direction down the mountainous slope.
Out of curiosity, he bent to investigate what the mountain had yielded, and much to his surprise, he found a perfectly enrolled Drotops trilobite lying at his feet. The unexpected discovery intrigued the Berber and empowered him to seek the spot from which this “strange rock” had originated. For weeks he searched, climbing and exploring, until finally he found the fossil-bearing layer upon the Magic Mountain that would soon ignite the entire Moroccan trilobite explosion.
This tale serves as a somewhat roundabout means for us to broach the subject of trilobite enrollment and ask some very basic questions… 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, 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. And while paleontologists will state that there were various types of trilobite enrollment -- including Sphaeroidal (when all the thoracic segments flex as part of the enrolling procedure) and Discoidal (when only the anterior segments of the thorax flex) -- the bottom line is that each served the same basic purpose… to protect the host trilobite from hostile situations.
The fossil record shows that a majority of the 25,000 recognized trilobite species could enroll, and that it was a common occurrence among virtually all orders of trilobites. But while we know that such a feature lasted until the demise of trilobites at the end of the Permian, when did this notable evolutionary development begin?
While a few Cambrian trilobites may have possessed the ability to enroll (though the rare fossilized remains of such specimens seem more “folded” than truly enrolled), by the Lower Ordovician, some 470 million years ago, it is clear that trilobite enrollment was already quite advanced. Many of the magnificent specimens emerging from Russia's famed Ordovician quarries are found in various states of enrollment, often presenting the appearance of nearly symmetrical fossil “balls.” By the Devonian, perhaps the most renowned of these trilobite “rollers” had emerged. These are the beautifully preserved Eldredgeops species from Ohio, whose 400 million year old circular conformity is broken only by the protruding stance of their compound eyes.
Indeed, judging by all the fossil evidence, it is apparent that the ability to enroll played a vital role in helping trilobites to not only survive, but to flourish, allowing them to dominate the planet's seas for more than 270 million years.
Here is a look at some of the most notable enrolled trilobites in the fossil record:
The discovery of an enrolled Drotops megalomanicus similar to this sparked the Moroccan trilobite explosion of the 1970s
The Devonian trilobite Dipleura dekayi enrolled for protection from predators.
Hundreds of enrolled Eldredgeops crassituburculata specimens have been found in the 400 million year old rocks of Ohio.
As proven by this large Paraomalonotus calvus, not only small trilobites enrolled.
This Ordovician llaenus from Russia displays an example of Sphaeroidal enrollment.
This 450 million year old Pterygometopus angulatus shows nearly perfect enrollment
The cephalon of this Moroccan Illaenid fits snugly into its pygidium.
Ptyocephalus yeserni from the Ordovician of Utah formed a nearly spherical “ball” upon enrolling.