Trilobite

Trilobites
Fossil range: Atdabanian - Late Permian
Asaphus kowalewskii
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Trilobita
Walch, 1771[1]
Orders
  • Agnostida
  • Nectaspida
  • Redlichiida
  • Corynexochida
  • Lichida
  • Phacopida

Subclass: Librostoma

  • Proetida
  • Asaphida
  • Harpetida
  • Ptychopariida

Trilobites (pronounced traɪləˌbaɪt, meaning "three lobes") are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (526 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 appeared 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. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata),[2] although several alternative taxonomies are found in the literature.

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, with the possible exception for parasitism (where there is still scientific debate).[3] Some trilobites (particularly the family Olenida) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.[4]

Contents

Phylogeny

Despite their rich fossil record with thousands of genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties.[5] The systematic division of trilobites into nine distinct orders is represented by a widely held view that will inevitably change as new data emerges. Except possibly for the members of order Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Most scientists believe that order Redlichiida, and more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina. While many potential phylogenies are found in the literature, most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian, and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida is the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology,[6] what are now members of orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990[7] to encompass all of these orders, based on their shared ancestral character of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was erected in 2002.[8] The progenitor of order Phacopida is unclear.

Terminology

Fig 1. "Trilobite" (meaning "three-lobed") named for the three longitudinal lobes
Fig. 2 The trilobite body is divided into three major sections (tagmata)

As might be expected for a group of animals comprising c. 5,000 genera,[9] 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, see Fig 1); 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), see Fig 2. When describing differences between trilobite taxa, the presence, size, and shape of the cephalic features are often mentioned and shown in more detail in Figs 3 & 4.

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 millimetre (0.039 in) to 72 centimetres (28 in), with a typical size range of 3–10 cm (1.2–3.9 in). The world's largest trilobite, Isotelus rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.[10]

Exoskeleton

Kainops invius lateral and ventral view of exoskeleton showing hypostome, anterior & posterior doublure and many apodemes

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 moulting, 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 moulting. Similar to lobsters & crabs, trilobites would have physically "grown" between the moult stage and the hardening of the new exoskeleton.

Fig. 3 - Morphology of the cephalon
Fig. 4 - Detailed morphology of the cephalon

Cephalon

The cephalon of trilobites is highly variable with a lot of morphological complexity. The glabella (see Fig. 3) forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backwards at the rear edge of the hypostome. Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hyperstome have been described.[7] Size of the glabella and lateral fringe of the cephalon together with hypostome variation have been linked to different lifestyles, diets and specific ecological niches.[3] The lateral fringe of the cephalon is greatly exaggerated in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch.[11] Highly complex compound eyes are another obvious feature of the cephalon (see below). When trilobites moulted, the librigenae ("free cheeks") separated along the facial suture to assist moulting, leaving the cranidium (glabella + fixigenae) exposed (see Fig. 3).

Thorax

The thorax is a series of articulated segments that lie between the cephalon and pygidium. Number of segments varies between 2 and 61 with most species in the 2 to 16 range.[12]

Each segment consists of the central axial ring and the outer plurae which protected the limbs and gills. The plurae are sometimes abbreviated to save weight or extended to form long spines. Apodemes are bulbous projections on the ventral surface of the exoskeleton to which most leg muscles attached, although some leg muscles attached directly to the exoskeleton.[13] Distinguishing where the thorax ends and the pygidium begins can be problematic and many segment counts suffer from this problem.[12]

An enrolled Phacopid trilobite Phacops rana crassituberculata.

Trilobite fossils are often found enrolled (curled up) like modern woodlice for protection; evidence suggests enrollment helped protect against inherent weakness of arthropod cuticle that was exploited by Anomalocarid predator attacks.[14]

Some trilobites achieved a fully closed capsule (e.g. Phacops), while others with long pleural spines (e.g. Selenopeltis) left a gap at the sides or those with a small pygidium (e.g. Paradoxides) left a gap between the cephalon and pygidium.[12] In Phacops the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure.[13] The doublure carries a panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal.[13] Even in an Agnostid, with only 2 articulating thoracic segments, the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition.[15]

Pygidium

The pygidium is formed from a number of segments and the telson fused together. Segments in the pygidium are similar to the thoracic segments (bearing biramous limbs) but, are not articulated. Trilobites can be described based on the pydigium being micropygous (pydigium smaller than cephalon), isopygous (pydigium equal in size to cephalon), or macropygous (pydigium larger than cephalon).

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.[12] 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.[16]

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.

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.[17] A conclusion likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents.[18]

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.

Soft body parts

Only 21 or so species are described from which soft body parts are preserved,[13][19] so some features (e.g. the posterior antenniform cerci preserved only in Olenoides serratus)[20] remain difficult to assess in the wider picture.[21]

Appendages

Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs (2, 3 or 4 cephalic pairs, followed by a variable number of thorax + pygidium pairs).[13][19] Each exopodite (walking leg) had 6 or 7 segments,[19] homologous to other early arthropods.[21] Expodites are attached to the coxa which also bore a feather-like epipodite, or gill branch, which was used for respiration and, in some species, swimming.[21] The base of the coxa, the gnathobase, sometimes have heavy, spiny adaptations which were used to tear at the tissues of prey.[22] The last expodite segment usually had claws or spines.[13] Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.[21]

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 esophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backwards from there to the pygidium.[13] 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.[3]

Internal organs

While there is direct and implied evidence for the presence and location of the mouth, stomach and digestive tract (see above) the presence of heart, brain and liver are only implied (although "present" in many reconstructions) with little direct geological evidence.[21]

Musculature

Although rarely preserved, long lateral muscles extended from the cephalon to mid way down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.[13]

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.

Antennae

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. Also, one species (Olenoides serratus) preserves antennae-like cerci that project from the rear of the trilobite.[20]

Eyes

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.[23] 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.[24]

Trilobite eyes were typically compound, with each lens being an elongated prism.[25] 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.[16] 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.[26]

Lenses of trilobites' eyes were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals to form each lens of each of their eyes.[27] Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would; however, in some trilobites the calcite formed an internal doublet structure,[28] giving superb depth of field and minimal spherical aberration, as discovered by French scientist René Descartes and Dutch physicist Christiaan Huygens in the 17th century.[25][28] A living species with similar lenses is the brittle star Ophiocoma wendtii.[29]

In other trilobites, with a Huygens interface apparently missing, a gradient index lens is invoked with the refractive index of the lens changing towards the center.[30]

Holochroal eyes had a great number (sometimes over 15,000) of small (30-100μm, rarely larger)[26] lenses. Lenses were hexagonally close packed, touching each other, with a single corneal membrane covering all lenses.[27] Holochroal eyes had no sclera, the white layer covering the eyes of most modern arthropods. Holochroal eyes are the ancestral eye of trilobites, and are by far the most common, found in all orders and through the entirety of the Trilobites' existence.[26] Little is known of the early history of holochroal eyes; Lower and Middle Cambrian trilobites rarely preserve the visual surface.[26]

The Schizochroal eye of Erbenochile erbenii; the eye shade is unequivocal evidence that some trilobites were diurnal.[31]

Schizochroal eyes typically had fewer (to around 700), larger lenses than holochroal eyes and are found only in Phacopida. Lenses were separate, with each lens having an individual cornea which extended into a rather large sclera.[27] Schizochroal eyes appear quite suddenly in the early Ordovician, and were presumably derived from a holochroal ancestor.[26] Field of view (all around vision), eye placement and coincidental development of more efficient enrollment mechanisms point to the eye as a more defensive "early warning" system than directly aiding in the hunt for food.[26] Modern eyes which are functionally equivalent to the schizochroal eye were not thought to exist,[27] but are found in the modern insect species Xenos peckii.[32]

Abathochroal eyes are found only in Cambrian Eodiscina, had around 70 small separate lenses that had individual cornea.[33] The sclera was separate from the cornea, and did not run as deep as the sclera in schizochroal eyes.[27] Although well preserved examples are sparse in the early fossil record, abathochroal eyes have been recorded in the lower Cambrian, making them among the oldest known.[27] Environmental conditions seem to have resulted in the later loss of visual organs in many Eodiscina.[27]

Secondary blindness is not uncommon, particularly in long lived groups such as the Agnostida and Trinucleioidea. In Proetida and Phacopina from western Europe and particularly Tropidocoryphinae from France (where there is good stratigraphic control), there are well studied trends showing progressive eye reduction between closely related species that eventually leads to blindness.[27]

Several other structures on trilobites have been explained as photo-receptors.[27] Of particular interest are macula, the small areas of thinned cuticle on the underside of the hypostome. In some trilobites macula are suggested to function as simple ventral eyes that could have detected night and day or allowed a trilobite to navigate while swimming (or turned) upside down.[30]

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".[27]

Development

Trilobites grew through successive moult stages called instars, in which existing segments increased in size and new trunk segments appeared at a sub-terminal generative zone during the anamorphic phase of development. This was followed by the epimorphic developmental phase, in which the animal continued to grow and moult, but no new trunk segments were expressed in the exoskeleton. The combination of anamorphic and epimorphic growth constitutes the hemianamorphic developmental mode that is common among many living arthropods.[34]

Trilobite development was unusual in the way in which articulations developed between segments, and changes in the development of articulation gave rise to the conventionally recognized developmental phases of the trilobite life cycle (divided into 3 stages), which are not readily compared with those of other arthropods. Actual growth and change in external form of the trilobite would have occurred when the trilobite was soft shelled, following moulting and before the next exoskeleton hardened.[35]

Trilobite larvae are known from the Cambrian to the Carboniferous[36] and from all sub-orders.[35][37] As instars from closely related taxa are more similar than instars from distantly related taxa, trilobite larvae provide morphological information important in evaluating high-level phylogenetic relationships among trilobites.[35]

By comparison with living arthropods, trilobites are thought to have reproduced sexually, producing eggs,[38] albeit without undoubted examples in the fossil record.[35] Some species may have kept eggs or larvae in a brood pouch forward of the glabella,[11] particularly when the ecological niche was challenging to larvae.[4] Size and morphology of the first calcified stage are highly variable between (but not within) trilobite taxa, suggesting some trilobites passed through more growth within the egg than others. Early developmental stages prior to calcification of the exoskeleton are a possibility (suggested for fallotaspids),[39] but so is calcification and hatching coinciding.[35]

The earliest post-embryonic trilobite growth stage known with certainty are the protaspid stages (anamorphic phase).[35] Starting with an indistinguishable proto-cephalon and proto-pygidium (anaprotaspid) a number of changes occur ending with a transverse furrow separating the proto-cephalon and proto-pygidium (metaprotaspid) that can continue to add segments. Segments are added at the posterior part of the pygidium but, all segments remain fused together.[35][37]

The meraspid stages (anamorphic phase) are marked by the appearance of an articulation between the head and the fused trunk. Prior to the onset of the first meraspid stage the animal had a two-part structure — the head and the plate of fused trunk segments, the pygidium. During the meraspid stages, new segments appeared near the rear of the pygidium as well as additional articulations developing at the front of the pygidium, releasing freely articulating segments into the thorax. Segments are generally added one per moult (although two per moult and one every alternate moult are also recorded), with number of stages equal to the number of thoracic segments. A substantial amount of growth, from less than 25% up to 30%–40%, probably took place in the meraspid stages.[35]

The holaspid stages (epimorphic phase) commence when a stable, mature number of segments has been released into the thorax. Moulting continued during the holaspid stages, with no changes in thoracic segment number.[35] Some trilobites are suggested to have continued moulting and growing throughout the life of the individual, albeit at a slower rate on reaching maturity.

Some trilobites showed a marked transition in morphology at one particular instar, which has been called trilobite metamorphosis. Radical change in morphology is linked to the loss or gain of distinctive features that mark a change in mode of life.[40] A change in lifestyle during development has significance in terms of evolutionary pressure, as the trilobite could pass through several ecological niches on the way to adult development and changes would strongly affect survivor-ship and dispersal of trilobite taxa.[35] It is worth noting that trilobites with all protaspid stages solely planktonic and later meraspid stages benthic (e.g. asaphids) failed to last through the Ordovician extinctions, while trilobites that were planktonic for only the first protaspid stage before metamorphosing into benthic forms survived (e.g. lichids, phacopids).[40] Pelagic larval life-style proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician.[10]

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.[41][42] Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis sp. (western USA), Hupetina antiqua (Morocco)[43] and Serrania gordaensis (Spain).[44] All trilobites are thought to have originated in present day Siberia, with subsequent distribution and radiation from this location.[42]

Fallotaspids lack facial sutures, that is to say fallotaspids are thought to pre-date facial sutures (as opposed to a group that secondarily lost facial sutures).[42] Fallotaspids are strongly suggested to be the ancestral trilobite stock: absence of facial sutures; apparently un-calcified protaspid stages and fallotaspids underlying (pre-dating) or co-existing with all other trilobite occurrences.[39] However, recent developments[45] suggest the picture is more complicated,[46] and likely to change as more information comes to light.[42]

Origins

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).[16] Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina, Parvancorina, and other trilobitomorphs of the Ediacaran period of the Precambrian are ambiguous enough to make detailed analysis of their ancestry far from compelling.[47][39] Morphological similarities between early trilobites and other Cambrian arthropods (e.g. the Burgess Shale fauna and the Maotianshan shales fauna) make analysis of ancestral relationships difficult.[48] However, it is still reasonable to assume that the trilobites share a common ancestor with other arthropods prior to the Ediacaran-Cambrian boundary. 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).[23][42]

Radiation and extinction

For such a long lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where unsuccessful groups perished while surviving groups diversified to fill ecological niches with more successful adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn out decline in the Devonian culminating in final extinction of the last few survivors at the end of the Permian period.[16]

Evolutionary trends

Principal evolutionary trends from primitive morphologies (e.g. eoredlichids)[49] include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy) and development of extreme spinosity in certain groups.[16] Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.[49] Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures & hypostome specialization.[49] Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).[49]

Pre-Cambrian

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 million years ago and 550 million years ago.[42] Tectonic break up of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive Olenellid province (Laurentia, Siberia and Baltica) and the separate Redlichid province (Australia, Antarctica and China).[42][50] 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.[50]

Cambrian

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

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. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian, surviving orders developed isopygus or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax above).[14] The end Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks went extinct.[16] A continuing decrease in Laurentian continental shelf area[10] is recorded at the same time as the extinctions, suggesting major environmental upheaval.

Ordovician

Cheirurus sp., middle Ordovician age, Volkhov River, Russia

The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptoloids with many groups appearing in the fossil record for the first time.[16] Although intra-species trilobite diversity seems to have peaked during the Cambrian,[51] trilobites were still active participants in the Ordovician radiation event with a new fauna taking over from the old Cambrian one.[52] Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors.[16] The Phacopida and other "new" clades almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.[39] Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician allowing many families to continue into the Silurian with little disturbance.[52] Ordovician trilobites were successful at exploiting new environments, notably reefs. However, the end Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Trinucleioidea and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites, very few entirely new patterns of organisation arose post-Ordovician; later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction vigorous trilobite radiation has stopped and gradual decline beckons.[16]

Silurian and Devonian

Phacopid trilobite, Devonian age, Ohio, United States

Most Early Silurian families constitute a subgroup of the Late Ordovocian fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida.[52]

Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida (including the well-known Calymenina). However, a number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of drastic Middle and Late Devonian extinctions.[49] Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium (a meteorite impact has also been suggested as a cause).[49] Only a single order, the Proetida, survived into the Carboniferous.[16]

Carboniferous and Permian

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).[16] 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.[49] For many millions of years the Proetida existed untroubled in their ecological niche.[16] 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.[16]

Final extinction

Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity[53] of genera limited to shallow water, shelf habitats coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end Permian mass extinction event.[49] With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobite design is hardly surprising.[53]

The closest extant relatives of trilobites may be the horseshoe crabs,[41] or the cephalocarids.[54]

Fossil distribution

Cruziana, fossil trilobite furrowing trace
A trilobite fragment (T) in a thin-section of an Ordovician limestone; E = echinoderm; scale bar is 2 mm

Trilobites appear to have been exclusively marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected. The remnants of trilobites can range from the preserved body to pieces of the exoskeleton, which it sheds in the process known as ecdysis. In addition, the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils.

There are three main forms of trace fossils associated with trilobites: Rusophycus; Cruziana & Diplichnites – such 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.[55] Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding.[56] Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface.[56] However, care must be taken as similar trace fossils are recorded in freshwater[57] and post Paleozoic deposits,[58] representing non-trilobite origins.

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.

Rusophycus, a "resting trace" of a trilobite; Ordovician of southern Ohio. Scale bar is 10 mm.

A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Group. This trilobite is featured on the town's coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known Elrathia kingi trilobite is found in abundance in the Cambrian age Wheeler Shale of Utah.[59]

Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York State, U.S.A. (Ordovician Walcott-Rust quarry, near Russia, and Beecher's Trilobite Bed, near Rome); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador.


Importance

The study of Paleozoic trilobites in the Welsh-English borders by Niles Eldredge was fundamental in formulating and testing Punctuated Equilibrium as a mechanism of evolution.[60][61][62]

Identification of the 'Atlantic' and 'Pacific' trilobite faunas in North America and Europe[63] implied the closure of the Iapetus Ocean (producing the Iapetus suture),[64] thus providing important supporting evidence for the theory of continental drift.[65][66]

Trilobites have been important in estimating the rate of speciation during the period known as the Cambrian Explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian.[67][68]

Trilobites are excellent stratigraphic markers of the Cambrian period: researchers who find trilobites with alimentary prosopon, and a micropygium, have found Early Cambrian strata.[69] Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils.[70][71][72]

Trilobites are the state fossils of Ohio (Isotelus), Wisconsin (Calymene celebra) and Pennsylvania (Phacops rana).

Until the early 1900s, the Ute Indians of Utah wore trilobites, which they called Pachavee (little water bug), as amulets. A hole was bored in the head and the fossil was worn on a string.[73]

See also

References

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  2. ↑ Cotton, T. J.; Braddy, S. J. (2004), "The phylogeny of arachnomorph arthropods and the origins of the Chelicerata", Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169–193, doi:10.1017/S0263593303000105 
  3. ↑ 3.0 3.1 3.2 Fortey, Richard (2004), "The Lifestyles of the Trilobites" (PDF), American Scientist 92: 446–453, http://www.cornellcollege.edu/geology/courses/Greenstein/paleo/trilobites.pdf 
  4. ↑ 4.0 4.1 Fortey, Richard (June 2000), "Olenid trilobites: the oldest known chemoautotrophic symbionts?", Proceedings of the National Academy of Sciences 97 (12): 6574–6548, doi:10.1073/pnas.97.12.6574, PMID 10841557, PMC 18664, http://www.pnas.org/cgi/pmidlookup?view=long&pmid=10841557 
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  6. ↑ Moore, R. C., ed. (1959), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, Boulder, CO & Lawrence, KA: The Geological Society of America & The University of Kansas Press, pp. xix + 560 pp., 415 figs., ISBN 0-8137-3015-5 
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