Most trilobites were able to roll themselves up into a protective ball – a behavior known as enrollment or volvation – exposing just their heavily armored backs to attackers. They’re often found fossilized curled up like this, and rare preservation of soft tissues shows that they had a complex system of muscles to help them quickly achieve this pose while simultaneously tucking their antennae and all their limbs safely inside their enrolled shells.
Some species also developed sharp defensive spines and spikes that jutted out when they enrolled, making themselves even more daunting to potential predators in one of the earliest known examples of an evolutionary “arms race”.
The biggest stars of the Cambrian euarthropods, and most of the Paleozoic Era, were of course the trilobites. Known from literally tens of thousands of species spanning over 270 million years, they’re some of the most recognizable and popular fossils.
Trilobites’ exact evolutionary origins and transitional forms are unknown, but they’re thought to have originated in Siberia in the very early Cambrian and their leg anatomy indicates they were a part of the artiopodan lineage. They made a sudden and dramatic entrance to the fossil record about 521 million years ago, appearing fully-formed and rapidly diversifying and spreading all around the world within just a couple of million years.
Their hard calcified exoskeletons made them much more likely to fossilize than soft-bodied animals, with a distinctive three-part body plan consisting of a head shield, three-lobed thorax segments, and a tail shield. Each individual regularly molted their carapace throughout their life, meaning that most trilobite remains are actually empty discarded shells rather than actual carcasses.
Trilobites were one of the most successful groups of early animals, existing for over 300 million years – and during that time they developed a huge diversity of weird heads, with various arrangements of spines, horns, eyestalks, and even long snouts and tridents.
But perhaps one of the oddest was the genus Odontocephalus, known mainly from the early-to-mid Devonian and represented here by Odontocephalus aegeria.
Living about 390 million years ago in northeast North America, this trilobite grew up to around 9cm long (3.5″). And although it wasn’t overall very elaborately ornamented, the front margin of its head had a row of extensions that flared out to meet at their tips, forming something resembling the cowcatchers used on trains.
The actual function of this structure is unknown. It might have been purely used for visual display since trilobites had excellent vision – but Odontocephalus was also a fast-moving bottom-dweller, and its “cowcatcher” may have served the same sort of purpose as its modern equivalent, deflecting small obstacles in its path as it trundled along the seabed.