Trilobites on Land

Two overlapping trilobite fossils in rock.

These Olenellus romensis are from the Rome Formation. Similar examples may have crawled out of the sea and onto adjacent tidal flats. 

     Despite the long-standing, and well-established notion that trilobites were strictly-and-solely marine inhabitants, back in 2014, scientists investigating a series of Lower Cambrian deposits in the Appalachian Mountains of Tennessee discovered something they had not anticipated. Within outcrops of the 515 million-year-old Rome Formation, a team from the University of Saskatchewan found unmistakable fossil evidence which indicated that soon after beginning their lengthy crawl through Deep Time, some olenellid trilobites had already started journeying onto dry land – or at least onto sea-adjacent tidal flats.

    The early Paleozoic rocks emerging from that expedition revealed two distinct types of trace fossils:  cracks and tracks. The former were indicative of a shallow sea floor habitat that upon being exposed to direct sunlight, began to rapidly bake and crack. 

     The latter, however, were a decidedly different matter. These particular trilobite trackways, also known as Cruziana rugosa, presented telltale signs that more than half-a-billion years ago those highly adaptable animals were exploring the surrounding shoreline and taking advantage of the resource-filled tidal flats which bordered their aquatic home.

     Considering the evident ability of these creatures to adjust to both environmental conditions and ecological opportunities – something they did continually throughout their 270 million-year history -- it seems only logical to assume that even in the nascent stages of their quest for survival, some trilobite species had found shallow water marine ecosystems to their liking. From there, it would be just a matter of evolutionary expediency for these tri-lobed invertebrates to occasionally march their morphologically advanced body designs a few extra meters towards the surf’s edge…. and possibly beyond!

Apparently, as long as they prevented their gills from drying out, trilobites could have partaken in sundry seaside activities while functioning outside of their protective oceanic cocoon. After all, nature had already furnished them with hard, calcite shells, and thus equipped, they may have been perfectly prepared to be the first animals on Earth to 

temporarily venture out from under the planet’s blue waves.

  In retrospect, perhaps such a revelation shouldn’t come as a total surprise to those intimately familiar with the workings of the arthropod line. After all, horseshoe crabs are routinely found out-of-water during times of spawning. At full moons, especially in spring months, these “living fossils” emerge from the briny depths to deposit their eggs along marine perimeters. And since many trilobites apparently exhibited (and perhaps initiated) behavioral patterns akin to those displayed by modern arthropods, such Cambrian conduct – involving spawning, feeding, molting or mating -- is certainly not out of the realm of Paleozoic possibility.

     Additionally, the sum-total of this time-tested, trilobite-centric information serves to provide a major boost to the growing scientific speculation that terrestrial lifeforms may have initially evolved from marine animals. It had long been academically accepted that species of advanced organic life had first appeared in fresh water pools, or at least in brackish intertidal bays, prior to launching their primary land invasion… but now such attitudes are being called into serious question.

     In fact, despite the still-limited fossil evidence supporting any claim that they helped bridge this momentous sea-to-land transition, it seems that trilobites may have indeed played a pivotal role in pioneering a process that led to the eventual emergence of terrestrial creatures across the face of our planet.