Trilobite Evolution

Trilobites US

Trilobite Origins

The story of trilobite evolution must begin with that of the arthropods. Trilobites were a member of what is far and away the largest phylum of animals, Arthropoda, with well more than a million described species of out of a total of perhaps 5 to 10 million extant, comprising around 80% of all animal species (Ødegaard, 2000). Prior to modern sequencing, scientists can only conjecture as to the characteristics of the first common ancestor of arthropods, but it surely lived in the Precambrian, a period for which the fossil record is exceedingly sparse and enigmatic. The evolutionary origins of trilobites is unsettled science, with the distinct possibility it always will be. Consensus exists that their origins were in the Precambrian time among the diverse bilaterians, probably in the Ediacaran Period (Lieberman, 2002). Their ancestors (or a common ancestor are likely among from the early Ediacaran arthropods, that are often labeled arachnomorphs or trilobitamorphs in the early Cambrian. A place to begin is the ancestory and sequencing-based phylogeny of Ecdysozoa, the moulting animals, among which is trilobita.

Ecdysis unites trilobites with arthropods

From phylogenetic analyses constructed using 18S ribosomal RNA genes, Aguinaldo et al., 1997 defined Superphylum Ecdysozoa as a monophyletic clade uniting eight phyla, arthropoda, tardigrades and onychophorans that 18S ribosomal RNA Used for Ecdysozoa and 18S ribosomal RNA Phylogeneticshare segmentation and appendages as well as nematodes, nematomorphs, priapulids, kinorhynchs and loriciferans, that are worms with an anterior proboscis. Telford, et. al. (2008) reviews subsequent sequencing work replicating Aguinaldo's results, or approaches using Hox gene signature peptides. The whole area of arthropod (and therefore trilobite origins) continues unabated, as illustrated by recent wotk by Janssen, et. al (2014) supporting Onychophora as a sister group of Arthropoda based embryonic expression patterns og Hox genes. Body plans of numerous members of Arthropoda have evolved significantly differences since the Cambrian, lobopodians now represented by still extant onychophorans exhibit but minor change (Whittington, 1978). Many of the major morphological traits of the arthropods such as jointed Superphylum Ecdysozoa Phylogenylimbs and distinct adult body segmentation are absent in onychophorans, and, considering onychophorans as an arthropod stem-group, these absences are best interpreted as being primitive. Taken together, evisence grows that the early onychophorans, such as lobopodia, are ancestral to arthropods, and therefore trilobites.

Through modern sequencing, there is strong support combining the phyla Arthropoda, Tardigrada, and Onychophora into the unranked taxon Panarthropoda within superphylum Ecdysozoa. The phyla are united by common characteristics of ventral nervous system, a segmented body plan, and legs and claws.

Ediacaran animals Parvancorina and Spriggina,

García-Bellido, D. C.; Collins, D. H. (2004) Marrella from Burgess Shale earliest unequivocal evidence of moulting. [ (May 2004),

Moulting arthropod caught in the act", Nature 429 (6987): 4]

Superphylum: Ecdysozoa

Examples of Trilobite Evolution

Evolutionary Secondary Blindness Among Trilobites

The old saying goes, "use it or lose it". Charles Darwin spoke about seconadary blindness (progressive loss of vision through natural selection) in Origin of Species:

"The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind. One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse."

EocryphopsOften, the visual organs become vistigal, that is, while the organ/structure remains, the function that existed in ancestors is lost, in part or in whole. A function no longer needed in the fight for survival is no longer subject to positive natural selection, and function can be lost (e.g., ostrich wings). Further, if the function (or organ) in any way hinders survival, it might be selected against, and the organ further atrophy or disappear. Nature is replete with examples among extant animals (blind cave animals) of secondary blindness, and it occured among trilobites too. Secondary blindness is exhibited in evolutionary series among numerous species of trilobite. Examples are particularly notable among Order Agnostida, Superfamily Trinucleioidea of Asaphada, Order Proetida and Superfamily Phacopina of Asaphida (Clarkson, 1997). (Also see Trilobite Eyes and Secondarly Blindness)

Secondary Eye Loss in Trilobite Family Proetidae

The first three trilobites below are closely related and from Order Proetida, Family Proetidae. The left most has typical holochroal eyes, the next a Gerastos with reduced eye size, and the next is entirely eyeless Gerastos. The last trilobite, Eocryphops, is a phacopid trilobite with but a few facets in its schizochroal Eyes, way less than is typical for the Phacopids. Interestingly, like the reduced-eyes and blind Gerastos, it is also from Jorf, Morocco. Mysteriously, trilobites with reduced eyes are found in association with species with typical eyes. While the environment is believed to have been in deep water, it was still one in the photic zone, where good eye sight would seem to have been a survival benefit and selected for. Was there a particular niche in the environment where good sight was not selected for, or even selected against, leading to secondary eye loss? Was the Eocryphops trilobite we see below actually blind, and its reduced eyes just vistigal organs?
Proetus tuberculatus morocensis
Gerastos with Reduced Eyes
Gerastos Blind Trilobite
Order Proetida
Family Proetidae
Foum Ziguid, Morocco 
Order Proetida
Family Proetidae
Jorf, Morocco
Order Proetida
Family Proetidae
Jorf, Morocco
Order Phacopida
Family Phacopidae
Jorf, Morocco

Trilobite Size

Size matters. Size matters a lot in evolution. But, it’s the species that can best adapt to its environment, particularly a changing environment that survives. In survival, the benefits accrued by larger size do not always endow the animal with a survival advantage – size matters in both providing advantages and disadvantages – there are tradeoffs. Famed American paleontologist Edward Drinker Cope (Hone, et al., 2005) postulates (in what some call Cope’s rule) that population members tend to increase in body size over evolutionary time scales. While larger body size provides higher fitness for many reasons, there are also many disadvantages for both individual organisms and entire populations. Moreover, the fossil record suggests that clades of larger animals are more susceptible to extinction as generational time period is longer and fecundity is reduced, clearly the worst outcome for the bullies and brutes. Here are some pros and cons to size to consider:

Evolutionary fitness advantages:

  • Better defense from big bullies, unless hiding is harder
  • Better able to be the bully, and overcome prey
  • More types of food can be overcome and eaten
  • Can beat out competition for mating
  • Increased longevity and opportunities to mate
  • Bigger brain to outsmart predators and prey
  • Reduced heat loss per unit mass

Evolutionary fitness disadvantages:

  • Longer development time, and juvenile vulnerability
  • Need more food
  • Inter-generational time lengthened, with associated reduction in ability to adapt
  • Lowered fecundity since parents must invest more in offspring.

It’s difficult to know which of the above affected trilobite size changes, and which were more important and when, but trilobites clearly exhibited changes in size over geologic time. Trilobites have a more than 700 fold adult size range, enormous by any standard. The minimum is on the order of one mm, and the maximum more than 700 mm. The largest trilobite found in the fossil record is Isotelus rex, an Ordovician Asaphid trilobite discovered in 1998 (Rudkin, 2003) near the Hudson Bay in Canada. Bell, et al (2012) studied trilobite size variation over geologic time (click for larger image):

That is, while trilobites exhibit a fast size increase from the lower Cambrian to middle Cambrian, as well as into the Ordovician, size decreases thereafter. There is a gradual size decline through the Silurian. An initial size increase in the early Devonian is attributed to evolution of spiny exoskeletons in some species (Chatterton, 2006), noting also that the Frasnian/Famennian mass extinction removed all predatory species among orders Phacopida and Lichida, leaving generally more diminutive trilobites through the Carboniferous and into the Permian, when trilobites all went extinct.

Trilobite Size Examples
World's Largest Agnostid      
Middle Cambrian
Order Agnostida
Family Agnostidae
Christmas Hills, Tasmania


Russian Ordovician Asaphidae Trilobite Eye Stalk Evolutionary Adaptation

Lower Ordovician Asaphus broggeri, with no eye stalks from which other asaphids descendedThe trilobite fossil record is among the best of all the animal groups. It is not surprising then that it sometimes reveals sequences of evolutionary transition, recording the adaptation, or descent with modification, to new selective pressures as a matter of survival, or perhaps record failure to adapt, leading to extinction. One great example is the radiation of Baltic asaphid trilobites of family Asaphidae from the region that is now Saint Petersburg, Russia from late in the lower Ordovician and into the middle Ordovician. A hundred years of collecting of these strata has provided tremendous data set for Russian Ordovician Trilobite Asaphus kowalewskii with Evolved High Eye Stalkscladistics analysis. The region that has long yielded a tremendous diversity of trilobites is believed to have been an inland sea during the earlier Ordovician that was cut off from the ocean to the west. At some point toward the late Ordovician the inland sea again became connected to the ocean. The resulting flow of sediment (turbidity) clouded the water and settled on the seafloor. Through some 50 foot of limestone encompassing some two million years, some Asaphidae lines of descent developed stalked eyes of ever increasing height, ostensibly to enable them to better spot either prey or predators, or both. The figure shows selected species and lines of descent. Not all lines produced elevated eyes. The figure below depicts the two best known: 1) Asaphus lepidurus to Asaphus expansus, which branches into Asaphus cornutus with moderate stalked eyes, and then to Asaphus kowalewskii, with very high eye stalks that almost seem unnatural. The other branch from Asaphus expanses leads to three species with successively higher eyes, Asaphus kotlukovi, Asaphus punctatus and Asaphus intermedius. It also warrants mention that the period beginning in the early Ordovician is at times called "The Great Ordovician Radiation", or even the "Ordovician Explosion", denoting an enormous diversification of marine animals, that had been decimated in the Cambrian-Ordovician Extinction Event.
Russian Asaphus Trilobite Evolution
Figure above: Turbidity flows are selective pressure for trilobite in family Asaphidae to evolves eyes on ever higher stalks. Not all species in evolutionary lines are shown - click image to open new page with larger image. (Reference, Adamek, 2014)