The American Museum of Natural History has a great trilobite website with information and pictures that do a thorough job describing trilobites.  Today, we are going to highlight the Trilobite Spines page.   From the very beginning of their history up to the end, armor was a big component of trilobite anatomy.  First, they had a calcium carbonate exoskeleton, which probably explains why they are preserved in the uncounted billions.  At some point, pretty early, some species evolved spines, most likely for defense.  It’s easy to imagine they looked less appetizing to the big predators that existed from the Cambrian to the Permian.

It seems safe to surmise that the life of a trilobite was filled with daunting challenges. From their first days in the Lower Cambrian to their last stand during the Permian, some 270 million years later, there were constant, ever-more-advanced threats facing these primitive arthropods. Their initial evolutionary response to such peril was perhaps the most important… growing a hard, calcite shell, which throughout their long history provided a degree of protection from attacks by sharp-jawed Cambrian sea monsters, five-foot long Silurian aquatic scorpions and razor-toothed Devonian fish.

However, it is what happened next to the trilobite body design that proved to be more revolutionary than evolutionary — the appearance of a complex, and apparently highly effective, system of spines which eventually emanated from virtually every part of some trilobites’ anatomy. Exactly when trilobites started to generate defensive spines is a question still open to scientific speculation and debate. It appears that a few early members of the Olenellid and Redlichid lines, such as Esmeraldina rowei, did possess a rudimentary form of this adaptation. But by the Middle Cambrian, such species as Kootenia randolphi and Olenoides superbus featured an imposing series of spines projecting along their axial lobe. 

 A number of scientists believe that these initial spines may have been somewhat flexible, and perhaps were used by trilobites as swimming rudders as much as for protection. Some have speculated that spines may have served as nerve-filled sensory organs, increasing a trilobite’s ability to “feel” disturbances in the water around them. Others postulate, however, that these primitive projections represented nothing more than the start of an undersea  “arms race”, which saw creatures around the globe gearing up for daily battles for survival.