Old Earth Ministries Online Earth History Curriculum

Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)

This curriculum is presented free of charge for use by homeschooling families and schools.

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Chapter 3 - The Cambrian Period

Lesson 20: Species In-Depth: Trilobites


Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period (540 million years ago) and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago.

When trilobites first appearred in the fossil record they were already highly diverse and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left, with some 17,000 known species spanning Paleozoic time. Trilobites have provided important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics.

Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, except for parasitism. Some trilobites (particularly the family Olenida) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

 Chapter 3: The Cambrian Period


 Lesson 14: Cambrian Overview

  Lesson 15: Supercontinent Gondwana

  Lesson 16: Lagerstätte / Burgess Shale

  Lesson 17: The Cambrian Explosion Part 1

  Lesson 18: The Cambrian Explosion Part 2

  Lesson 19: The Cambrian Explosion Part 3

  Lesson 20: Species In-Depth: Trilobites




Olenoides erratus from the Mt. Stephen Trilobite Beds (Middle Cambrian) near Field, British Columbia, Canada.


Trilobite Morphology
"Trilobite" (meaning "three-lobed") named for the three longitudinal lobes (Picture Source)
Trilobite Morphology
The trilobite body is divided into three major sections (tegmata).

As might be expected for a group of animals comprising c.5,000 genera, the morphology and description of trilobites can be complex. However, despite morphological complexity and an unclear position within higher classifications, there are a number of characters that distinguish the trilobites from other arthropods: a generally sub-elliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes (from which the group gets its name); having a distinct, relatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium).

Physical Description


When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations (Lagerstätten) preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton.

Trilobites range in length from 1 mm to 72 cm (1/25 inch to 28 inches), with a typical size range of 3 to 10 cm (1 to 4 inches). The world's largest trilobite, Isotelus rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.




The exoskeleton is composed of calcite and calcium phosphate minerals in a protein lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the doublure. Three distinctive tagmata (sections) are present: cephalon (head); thorax (body) and pygidium (tail).
During molting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate molting. Similar to lobsters & crabs, trilobites would have physically "grown" between the molt stage and the hardening of the new exoskeleton.


Prosopon (surface sculpture)


Trilobite exoskeletons show a variety of small-scale structures collectively called prosopon. Prosopon does not include large scale extensions of the cuticle (e.g. hollow pleural spines) but to finer scale features, such as ribbing, domes, pustules, pitting, ridging and perforations. The exact purpose of the prosopon is not resolved but suggestions include structural strengthening, sensory pits or hairs, preventing predator attacks and maintaining aeration while enrolled. In one example, alimentary ridge networks (easily visible in Cambrian trilobites) might have been either digestive or respiratory tubes in the cephalon and other regions. Later, more advanced trilobites developed thicker cuticles (making alimentary prosopon harder to see) against predation by cephalopods.


Trilobite spines
Ceratarges sp., an example of a species with elaborate spines from the Devonian Hamar Laghdad Formation, Alnif, Morocco.


Some trilobites such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period. Examples of these specimens have been found in the Hamar Laghdad Formation of Alnif in Morocco. There is, however, a serious counterfeiting and fakery problem with much of the Moroccan material that is offered commercially. Spectacular spined trilobites have also been found in western Russia; Oklahoma, USA; and Ontario, Canada. These spiny forms could possibly have been a defensive response to the evolutionary appearance of fish.
Some trilobites had horns on their heads similar to those of modern beetles. Based on the size, location, and shape of the horns the most likely use of the horns was combat for mates, making the Asaphida family Raphiophoridae the earliest exemplars of this behavior. A conclusion likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents.
An exceptionally well preserved trilobite from the Burgess Shale. The antennæ and legs are preserved as reflective carbon films. An exceptionally well preserved trilobite from Beecher's Trilobite Bed. Segmented legs are clearly visible. At this Lagerstätte soft body parts are preserved by pyrite.


Digestive tract


The toothless mouth of trilobites was situated on the rear edge of the hypostome (facing backwards), in front of the legs attached to the cephalon. The mouth is linked by a small oesophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backwards from there to the pygidium. The "feeding limbs" attached to the cephalon are thought to have fed food into the mouth, possibly "slicing" the food on the hypostome and/or gnathobases first. Alternative lifestyles are suggested, with the cephalic legs used to disturb the sediment to make food available. A large glabella, (implying a large stomach), coupled with an impendent hypostome has been used as evidence of more complex food sources, i.e. possibly a carnivorous lifestyle.


Sensory Organs


Many trilobites had complex eyes; they also had a pair of antennae. Some trilobites were blind, probably living too deep in the sea for light to reach them. As such, they became secondarily blind in this branch of trilobite evolution. Other trilobites (e.g. Phacops rana and Erbenochile erbeni) had large eyes that were for use in more well lit, predator-filled waters.




The pair of antennae suspected in most trilobites (and preserved in a few examples) were highly flexible to allow them to be retracted when the trilobite was enrolled. The antennae are probably similar to those in extant arthropods and as such could have sensed touch, water motion, heat, vibration (sound), and especially olfaction (smell) or gustation (taste).


Trilobite eye

The Schizochroal eye of Erbenochile erbenii; the eye shade is unequivocal evidence that some trilobites were diurnal.


Even the earliest trilobites had complex, compound eyes with lenses made of calcite (a characteristic of all trilobite eyes), confirming that the eyes of arthropods and probably other animals could have developed before the Cambrian. Improving eyesight of both predator and prey in marine environments has been suggested as one of the evolutionary pressures furthering an apparent rapid development of new life forms during what is known as the Cambrian Explosion.
Trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In compound eyes, the lenses were typically arranged hexagonally. The fossil record of trilobite eyes is complete enough that their evolution can be studied through time, which compensates to some extent the lack of preservation of soft internal parts.


Sensory pits


There are several types of prosopon that have been suggested as sensory apparatus collecting chemical or vibrational signals. The connection between large pitted fringes on the cephalon of Harpetida and Trinucleoidea with corresponding small or absent eyes makes for an interesting possibility of the fringe as a "compound ear".


Fossil Record


The earliest trilobites known from the fossil record are "Fallotaspids" (order Redlichiida, suborder Olenellina, superfamily Fallotaspidoidea) and Bigotinids (order Ptychopariida, superfamily Ellipsocephaloidea) dated to some 520 to 540 million years ago. All trilobites are thought to have originated in present day Siberia, with subsequent distribution and radiation from this location.


Trilobite fossils are found worldwide, with many thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year.




Early trilobites show all of the features of the trilobite group as a whole; there do not seem to be any transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. early arthropods).  Evidence suggests significant diversification had already occurred prior to the preservation of trilobites in the fossil record, easily allowing for the "sudden" appearance of diverse trilobite groups with complex, derived characteristics (e.g. eyes).




Phylogenetic biogeographic analysis of Early Cambrian Olenellid and Redlichid trilobites suggests that a uniform trilobite fauna existed over Laurentia, Gondwana and Siberia before the tectonic breakup of the super-continent Pannotia between 600 to 550 Ma. Tectonic break up of Pannotia then allowed for the diversification and radiation of the species. Break up of Pannotia significantly pre-dates the first appearance of trilobites in the fossil record, supporting a long and cryptic development of trilobites extending perhaps as far back as 700 million years ago or possibly further.


Paleozoic Era


Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian - Redlichiida, Ptychopariida, Agnostida and Corynexochida. A Middle Cambrian and end Cambrian extinction event marked major changes in trilobite fauna. During the Ordovician trilobites were successful at exploiting new environments, notably reefs.  The end Ordovician extinction event again altered the trilobite fauna, with 74% of the dominant Late Ordovician trilobite fauna surviving into the Silurian.  Drastic Middle and Late Devonian extinctions almost wiped out the trilobites.  Three orders and all but five families were exterminated by the combination of sea level changes, and other contributing factors, including a possible meteorite impact.  Only a single order, the Proetida, survived into the Carboniferous.


The Proetida survived for millions of years, continued through the Carboniferous period and lasted until the end of the Permian (where the vast majority of species on Earth were wiped out). It is unknown why order Proetida alone survived the Devonian. The Proetida maintained relatively diverse faunas in deep water and shallow water, shelf environments throughout the Carboniferous. For many millions of years the Proetida existed untroubled in their ecological niche. An analogy would be today's crinoids which mostly exist as deep water species; in the Paleozoic era, vast 'forests' of crinoids lived in shallow near-shore environments.


Final Extinction


Exactly why the trilobites became extinct is not clear, although it may be no coincidence that trilobite numbers began to decrease with the appearance of the first sharks and other early gnathostomes in the Silurian and their subsequent rise in diversity during the Devonian period. With repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, it is clear that more than one, or a combination of causes is likely.


The closest extant relatives of trilobites may be the horseshoe crabs, or the cephalocarids.


Trace Fossils


There are three main forms of trace fossils associated with trilobites: Rusophycus; Cruziana & Diplichnites.  These trace fossils represent the preserved life activity of trilobites active upon the sea floor. Rusophycus, the resting trace, are trilobite excavations which involve little or no forward movement and ethological interpretations suggest resting, protection and hunting. Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding. Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface. However, care must be taken as similar trace fossils are recorded in freshwater and post Paleozoic deposits, representing non-trilobite origins.

Gallery - From Wikipedia

Pyritised Lloydolithus lloydi, lower Ordovician age, England.

Hypostome of Isotelus sp., Ordovician age, southern Ohio, USA.

Balizoma variolaris (Brongniart, 1822), Silurian age, Dudley, England.

Cyphaspis tafilalet - Proetid trilobites, Devonian age, Morocco.

Kolihapeltis sp., Devonian age, Morocco.

Crotalocephalus sp., Devonian age, Morocco.

Diplichnites sp. a trilobite trackway, Devonian age, northeastern Ohio, USA.

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Source: Trilobite