Extinct marine invertebrate
Fossil arthropod, one of the dominant forms of life during the Cambrian and Ordovician periods. Still abundant and diverse through the end of the Devonian period. Attained spectacular morphologic and taxonomic diversity over 300 million years of evolutionary history.
Sedimentology and Stratigraphy
The oldest trilobites are found in Lower Cambrian rocks roughly 525-530 million years old. The first records may be in Baltica (present day Scandinavia and the eastern European platform); however, shortly on the heels of these occurrence records, trilobites also appear in Lower Cambrian rocks from Siberia and China and then Antarctica, North America, and Australia (in no particular order). One interesting aspect of the early history of trilobites is that straightaway trilobites show a prominent pattern of biogeographic differentiation that implied a potentially long, hidden history of trilobites that might indicate the group’s origins extend well back into the Proterozoic. Lieberman (2003) and Meert and Lieberman (2004) used phylogenetic analysis, phylogenetic biogeography, and information from tectonics to constrain the earliest origins of trilobites to Siberia, which was once a separate continental bloc. Further, results suggested that the origins of Trilobita could be constrained somewhere within the interval 550-600Ma and occurred during the breakup of the supercontinent Pannotia: that is to say, anywhere from 20-70 million years before the group first appeared in the fossil record. This pattern of an early, hidden history with subsequent proliferation may be a more general phenomenon that perhaps has relevance for our understanding of how large scale evolution typically transpires.
Regarding the initial diversification of trilobites, it is likely that the breakup of the supercontinent Pannotia at the end of the Proterozoic may have provided fuel for the evolution of trilobites, and other taxa, during the Cambrian radiation by amplifying opportunities for allopatric speciation.
Trilobites may well represent the exemplar animal of the Cambrian, and continued to radiate throughout the Ordovician, but they were particularly hard hit during the end Ordovician mass extinction. This was one of five great debacles in the history of life when a large percentage of animal life was eliminated in a geologically short period of time. Although their diversity rebounded somewhat, again they were hard hit during the next so-called mass extinction, the Late Devonian biodiversity crisis. They stayed at relatively low diversity levels after that, succumbing at the end of the Permian during the largest mass extinction in the history of life.
One especially poignant pattern in trilobite evolution is that they basically appear to thrive, except during times of mass extinction. The precise reasons for the trilobite’s ultimate demise is one of those age old questions whose answer will likely never be known, but it appears tied up, paradoxically, with their proclivity to evolve rapidly. The reasons this matters is that groups that evolve rapidly also tend to have high rates of extinction. Trilobites are in this respect a highly volatile animal group, as were the ammonoids. The typically high rates of extinction in trilobites conspired with the inordinately high rates of extinction during a mass extinction to push the group to, and over, the proverbial edge.
Evolution and Systematics
Two of the major topics in trilobite phylogenetics have been the question of trilobite monophyly and addressing their place within the broader arthropod clade. Lauterbach (1980) was the first to challenge the age old assumption of trilobite monophyly. However, both Fortey and Whittington (1989) and Ramsköld and Edgecombe (1991) reconsidered Lauterbach’s proposal in greater detail and concluded that the trilobites were indeed monophyletic and several synapomorphies could be used to define the group. The principle divergence between Ramsköld and Edgecombe’s (1991) analysis and Fortey and Whittington’s (1989) concerns the position of the agnostoids, an appropriately named ‘trilobite-like’ group consisting of diminutive, blind, presumed pelagic forms that lack dorsal sutures (used for molting) on the exoskeleton and also lack a trilobite style hypostome. Ramsköld and Edgecombe (1991) treated these as outside of the trilobite ingroup whereas Fortey and Whittington (1989) viewed them as ingroup trilobites (see cladogram above).
Part and parcel with the issue of trilobite monophyly, phylogenetic approaches made it possible to consider the issue of trilobite relationships with other arthropods in greater detail. Until relatively recently, the consensus view was that among the major extant clades of arthropods the chelicerates were the group trilobites shared closest affinity with. For instance, Edgecombe and Ramsköld (1999) viewed trilobites as arachnates, nested within several clades of arachnomorph arthropods from such Cambrian soft-bodied faunas as the Burgess Shale and the Chengjiang biota. These in turn shared a common ancestor with the chelicerates. Hendricks and Lieberman (2008) reiterated their conclusions. Wills et al. (1998) incorporated a broad range of Cambrian soft-bodied arthropods into a phylogenetic analysis that also included trilobites and several extant representatives of chelicerates, crustaceans, and uniramians, as well as onychophorans. They recovered Schizoramia, with the crustacea sister to the arachnomorphs (see cladogram).
Dunlop (2005) and Scholtz and Edgecombe (2005, 2006), however, have presented an interesting discussion of the potential problems with the Schizoramia grouping, and the association of trilobites with some arachnomorphs and with the chelicerates. In particular, the studies endorsing trilobite affinities with chelicerates were not able to consider the relationships of relevant taxa like the arachnids and pycnogonids (Dunlop, 2005). Including arachnids and especially pycnogonids in such studies would result in many reversals and losses (Scholtz and Edgecombe, 2005).
Studies that concentrate on extant taxa and combine molecular and morphological data tend to retrieve the group Mandibulata (crustaceans and uniramians) which is in turn sister to the Chelicerata (Dunlop, 2005). Scholtz and Edgecombe (2005) argued against the notion that there even was an “arachnomorph” clade, and of greatest relevance here, suggested that trilobites did not group with the chelicerates. Instead, they are part of the stem-lineage of Mandibulata, sharing among other traits a sensorial antenna. In a sense, this was a return to what was the more traditional, pre-cladistic view of trilobites, that treated them as most closely related to crustaceans on account of their common biramous appendages (in addition to the aforementioned antennae), although Arachnomorpha including trilobites is also an old concept (Scholtz and Edgecombe, 2005).
Clearly the results of Wills et al. (1998) are distinctly at odds with the discussion of Scholtz and Edgecombe (2005). Notably the latter authors did consider in great detail the homology of various character complexes, and this culminated in a new analysis by Scholtz and Edgecombe (2006) that retrieved the cladogram shown. Thus, sadly at this time consensus about the higher-level position of trilobites within Arthropoda is lacking. Interestingly, the monophyly of trilobites was originally considered within the context of chelicerate affinity, although even among those workers endorsing mandibulate affinity monophyly for the group still is favored (e.g., Dunlop, 2005; Scholtz and Edgecombe, 2005, 2006).