History of Trilobites
The trilobites first appeared in the fossil record during the Early Cambrian, about 540 million years ago, as a result of the Cambrian explosion, although there is evidence that the trilobites had forms in the Precambrian, about 600 – 550 million years ago (Benton & Harper, 2009) (Lieberman, 2002). The Cambrian explosion refers to the wide diversification of body plans and speciation, resulting in a wide number of species of trilobites (Benton & Harper, 2009). Then, they continued on into the Ordovician, where they experienced another radiation event during the Ordovician radiation, resulting in a large biodiversity increase at the family, genus, and species level (Benton & Harper, 2009). This is where trilobite diversity was at its highest in their entire history (Benton & Harper, 2009).
Picture showing the diversity in physical morphology of Trilobites Anomalocaris is shown eating a trilobite (Morris, 2002)
Graph showing the correlation between carbon dioxide levels and sea surface temperature showing a drop in carbon dioxide leading to global cooling resulting in an ice age and mass extinction event in the Ordovician (Young et al., 2009)
However, during the late Ordovician, a mass extinction occurred (Benton & Harper, 2009). This extinction was caused by an ice age, resulting in global cooling and increased glaciation (Benton & Harper, 2009). The ice age was caused by an increase in volcanic activity, releasing a large amount of silicate rocks, which absorbed CO2 out of the atmosphere (Young et al., 2009). This caused global temperatures to drop drastically, causing the well-known Hirnantian glaciation period (Young et al., 2010). This resulted in changes to the trilobite’s ecosystem, such as large fluctuations in ocean water temperature and salinity and substantial losses in phytoplankton populations. These combined effects severely affected pelagic (free swimming) trilobite species as well as trilobites with long planktotrophic (plankton eating) larval stages (Chatterton & Speyer, 1989). Benthic species, or species that crawled on the seafloor, were spared this fate, however they experienced losses as well (Chatterton & Speyer, 1989).
The dramatically changing environment and their inability to migrate far distances resulted in great losses of many benthic trilobite species (Chatterton & Speyer, 1989). Despite this, the benthic trilobites were able to maintain a substantial number of taxa to continue on to be the ancestors of trilobite genera that appeared in the Silurian period (Chatterton & Speyer, 1989). Another theory of the cause of the Ordovician extinction was that a gamma ray burst hit the Earth, causing the extinction of many lifeforms, although there is little to no evidence to support this theory (Chatterton & Speyer, 1989).
After the Ordovician extinction, trilobite taxa numbers were much lower than during the Cambrian and early Ordovician periods, with few survivors from the late Ordovician accounting for all post-Ordovician trilobite groups, with the exception of the order Harpetida (Adrain et al., 1998). These few genera survived and slightly diversified in small adaptive radiation bursts through the Silurian and the most of the Devonian period (Gon, 2013). However, during the Late Devonian, another mass extinction occurred, causing about 19% of families and 50% of genera to go extinct (Algeo et al., 1998). During the late Devonian, a great global cooling began, resulting in lower sea levels and anoxia of ocean water (Benton & Harper, 2009). Anoxia severely impacted the benthic trilobites, resulting in the loss of all trilobite orders except the Proetida (Gon, 2013) (Benton & Harper, 2009). There are multiple theories of how the anoxia occurred, such as global cooling, the evolution of plants, and extraterrestrial impacts (Benton & Harper, 2009) (Algeo et al., 1998). The current leading theory is that global cooling caused the oxygen levels to decrease, however there is building evidence for the evolution of plants to be the culprit. The evolution of plants during the Devonian made them grow larger, resulting in deeper root penetration and rhizoturbation (root erosion) (Algeo et al., 1998). This resulted in a higher amount of soil formation, which allowed nutrients in soils and upper bedrock to be washed away into rivers (Algeo et al., 1998). This caused a nutrient flux in rivers, and eventually, the oceans, resulting in eutrophic conditions in seaways around the world (Algeo et al., 1998). These eutrophic conditions allowed for algal blooms to form, which resulted in widespread anoxia in oceans (Algeo et al., 1998). This final trilobite order survived for millions of years through the Carboniferous and most of the Permian (Gon, 2013).
Picture of modern algal blooms creating an anoxic environment similar to algal blooms in the Devonian Sea. (Baranuik, 2015)
The Permian extinction was the largest and the most devastating extinction of all time, resulting in up to 95% of marine life and 57% of all families going extinct (Gon, 2013). The main culprit behind the Permian extinction was the Siberian traps, which released 2 million cubic kilometers of lava out onto the surface of the Earth, along with toxic levels of carbon dioxide and sulfate gases, which resulted in global warming and acid rain (Benton & Harper, 2009). The acid rain caused acidification of the oceans, making it extremely difficult for trilobites to form their shells (Clarkson et al., 2015). The released carbon dioxide caused the global temperature to increase by about 5-6°C (Benton & Harper, 2009). This increase in global temperature caused the circulation of ocean water and dissolved oxygen to decrease, resulting in the anoxic conditions of the water (Benton & Harper, 2009). Another contributor to the increase in temperature could have been the methane gas hydrates in the deep ocean (Benton & Harper, 2009). These were frozen blocks of methane that were locked under the sea via carbon sinking (Benton & Harper, 2009). An increase in temperature from the global warming caused by the carbon dioxide released from the Siberian traps may have caused these gas hydrates to turn into methane gas, bubbling up from the sea in huge volumes, causing further global warming (Benton & Harper, 2009). This could have resulted in a runaway greenhouse effect via melting of methane hydrates causing the temperature to rise which causes even more melting of methane gas hydrate (Benton & Harper, 2009). This volcanic induced anoxia resulted in the extinction of most sea-life, including the last trilobites, permanently ending the trilobites (Benton & Harper, 2009) (Song et al., 2013). Sadly, today there are no living descendants of the trilobites (Fortey, 2010). However, the closest relative of the trilobites are horseshoe crabs, and they share the common features that made trilobites so successful (Fortey, 2010).
Picture of a modern horseshoe crab, one of the closest relatives to trilobites (Horseshoe crabs info, 2016).
Figure of the possible chain of events following the eruption of the Siberian Traps, 251 Mya. Volcanism pumps carbon dioxide into the atmosphere and this causes global warming. Global warming leads to reduced circulation and reduced upwelling in the oceans, which produces anoxia, productivity decline and extinction in the sea. Gas hydrates may have released methane which produced further global warming in a "runaway greenhouse" scenario (Benton and Harper, 2009).