From roly-polies to trilobites, animals have been curling up the same way for millions of years.
Simple behaviours sometimes evolve into entire lifestyles. From bugs to armadillos, thousands of animals today curl up; they are able to shape their bodies into a ball, becoming the ultimate defence strategy.
Among all animals able to “roll”, arthropods (insects, crustaceans…) have taken this behaviour a step further. Rolling has become their entire survival strategy, and their anatomy has evolved accordingly. These animals have a rigid exoskeleton made out of plates on their dorsal side that can imbricate, and a softer underside. Thanks to this exoskeleton, as well as their segmented body, they are able to fully become a “ball”, a behaviour often known as “enrolment” (or conglobation).
Trilobites developed this mechanism more than 500 million years ago. Since then, enrolment has become quite common among arthropods. This is a major case of convergent evolution that involves animals as different as the extinct trilobites, the roly-polies or pill bugs (terrestrial isopods) the pill millipedes (Oniscidea), the horseshoe crabs (Xiphosura), the cuckoo wasps (Chrysididae) and even some mantis shrimp larvae (Malacostraca).
Is evolving into a “ball” perhaps even more ubiquitous than evolving into an ant?
To understand the origins of this behaviour, we first need to examine its first inventors: trilobites.
Trilobites are extinct arthropods that lived between the Cambrian to the end of the Permian, going extinct 250 million years ago, just before the age of the dinosaurs.
Trilobites are characterized by their body*, which is subdivided into three distinct parts: the head, the thorax and the pygidium. Trilobites are also one of the few arthropods able to integrate minerals into their exoskeleton. It is thanks to these minerals that trilobites are easily preserved and have an extraordinary fossil record.
*The word “trilobite”, though derives from the three lobes of the thorax.
Trilobites have been found across all continents including Antarctica, and is not uncommon to find them in their curled-up “enrolled” positions.
Enrolling is a great defensive strategy for predation or to protect your most vulnerable parts from environmental danger
says Sarah Losso from Harvard University
Enrolment was probably key to the group’s long-term survival and large diversity and is often a topic of research interest for invertebrate palaeontologists. The dorsal plates of the trilobites (tergites) have provided a lot of information regarding how this behaviour evolved, but the role of the ventral plates (sternites) has remained largely unknown.
“Unlike the calcite exoskeleton on the dorsal side, the ventral structures are rigid, but not mineralized, and are decay-prone”, explains Sarah Losso, a palaeontologist at Harvard University.
Together with other palaeontologists, Losso has been examining exceptionally preserved trilobites from the state of New York to understand the ventral mechanisms that enabled rolling.
“These fossils weren’t worked on for over 100 years. They are very difficult fossils to work with”, mentions Losso.
Today, though, there are new technologies available. Using computed tomography (a CT scan), the team is able to look inside enrolled trilobites, observing the anatomy of the legs and sternites.
“The great thing is that we have the sternites and legs preserved, but in 3D, showing how they were held in life”, adds Losso.
The images show that when the body curls up, the ventral plates can imbricate with each other, facilitating the contraction of the entire body.
“Enrolment in isopods and millipedes is also very similar, they make a perfect sphere. Regardless of their type of enrolment, they have to move the sternites the same way”- explains Losso.
The mechanisms enabling enrolment, then, have remained almost unchanged for 500 million years.
But trilobites were probably not the only arthropods able to roll 500 million years ago.
The last segments of the trilobite body are often fused together into a large plate termed a pygidium. A pygidium is not strictly necessary for enrolment, but several other arthropods of that period have one, such as the the pac-man crab Pakucaris (know more about him at Behind the carapace– photo left) or Mollisonia (photo right) an ancestor of the chelicerates (spiders, horseshoe crabs and others). Were these animals also rolling?
But Losso and colleagues have discovered something else beyond the sternites: they have uncovered the shape of a trilobite’s leg base.
In trilobites, the base of the limbs is right at the centre of the body making it extremely difficult to observe. “A lot of previous studies tried not to show the cross-section of the limb’s base because we don’t know it very well, and when they did, they would draw it as an oval shape, a conservative guess. We now know it’s wedge-shaped, it kind of looks like a slice of pizza, which is great for enrolling, as the limbs can come together and fit snuggly”-explains Losso
But these wedge-shaped limbs are not shared across other rolling arthropods, like the pill millipedes and the roly-polies.
A figure illustrating many of the “rolling” arthropods still alive. Clockwise: a pill millipede (by Zac Herr), a giant larvae of a stomatopod, a horseshoe crab upside-down (image from here), terrestrial isopod (aka roly-poly, by Bennie Kästle).
Instead, when observing the cross-section of the legs of these animals, Losso and colleagues noticed that their shape was round. There is only one exception to this rule: horseshoe crabs.
The key here is another force driving convergent evolution: feeding. Both trilobites and horseshoe crabs use their limb bases to grind food particles, while other rolling arthropods have specialized mouthparts to do the same function, instead. The wedge shape probably evolved as a way to both enable rolling and food grinding.
But arthropods are not the only ones evolving into a “rolling” anatomy. Discover what else is evolving into a “ball” nowadays at our partner’s channel Darwin’s Chronicles:
References
Izquierdo‐López, A., & Caron, J. B. (2021). A Burgess Shale mandibulate arthropod with a pygidium: a case of convergent evolution. Papers in Palaeontology, 7(4), 1877-1894.
Losso, S. R., Affatato, P., Nanglu, K., & Ortega-Hernández, J. (2023). Convergent evolution of ventral adaptations for enrolment in trilobites and extant euarthropods. Proceedings of the Royal Society B, 290(2013), 20232212.
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