Trilobite

From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the robot vacuum cleaner, see Electrolux Trilobite.
Trilobita
Trilobites
Temporal range: Atdabanian – Late Permian
Kainops invius
Scientific classification e
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: †Trilobitomorpha
Class: †Trilobita
Walch, 1771[1]
Orders

Trilobites (play /ˈtraɪləbaɪt/, /ˈtrɪləbaɪt/; 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 they 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. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.[2]

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),[3] 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 of parasitism (where there is still scientific debate).[4] Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.[5]

Contents

[edit] Phylogeny

Redlichida, such as this Paradoxides, may represent the ancestral trilobites.

Despite their rich fossil record with thousands of genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties.[6] The systematic division of trilobites into nine distinct orders is represented by a widely held view that will inevitably change as new data emerge. 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,[7] what are now members of orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990[8] 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.[9] The progenitor of order Phacopida is unclear.

[edit] Relationship to other taxa

Once soft-part anatomy had been recovered, the trilobites were originally allied to the Crustacea; however, this suggestion has since fallen out of favour. A relationship with the Chelicerata, in a clade termed Arachnomorpha (Arachnata), was in vogue for some time; a position in the Mandibulata (=Myriapoda + Crustacea + Hexapoda) stem-group may be a more parsimonious alternative.[10]

[edit] Physical description

Triarthrus eatoni - ventral side
Cast of Isotelus rex, the largest known trilobite.

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 (0.04 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.[11]

[edit] Exoskeleton

Trilobite external morphology

Trilobites are so named for the three longitudinal lobes: 1 – left pleural lobe; 2 – axial lobe; 3 – right pleural lobe; the antennae and legs are not shown in these diagrams
The trilobite body is divided into three major sections (tagmata): 1 – cephalon; 2 – thorax; 3 – pygidium

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).

[edit] Terminology

As might be expected for a group of animals comprising c. 5,000 genera,[12] 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). When describing differences between trilobite taxa, the presence, size, and shape of the cephalic features are often mentioned.

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 and crabs, trilobites would have physically "grown" between the moult stage and the hardening of the new exoskeleton. See Facial suture (trilobite anatomy).

[edit] Cephalon

Main article: Cephalon (arthropod anatomy)

[edit] Thorax

An enrolled phacopid trilobite Phacops rana crassituberculata

The thorax is a series of articulated segments that lie between the cephalon and pygidium. The number of segments varies between 2 and 61 with most species in the 2 to 16 range.[13]

Each segment consists of the central axial ring and the outer pleurae which protected the limbs and gills. The pleurae are sometimes abbreviated 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.[14] Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem.[13]

Trilobite fossils are often found "enrolled" (curled up) like modern pill-bugs for protection; evidence suggests enrollment helped protect against the inherent weakness of the arthropod cuticle that was exploited by anomalocarid predators.[15]

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.[13] In Phacops, the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure.[14] The doublure carries a Panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal.[14] 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.[16]

[edit] 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 pygidium being micropygous (pygidium smaller than cephalon), isopygous (pygidium equal in size to cephalon), or macropygous (pygidium larger than cephalon).

[edit] 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.[13] 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.[17]

[edit] Spines

Koneprusia brutoni, 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.[18] A conclusion likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents.[19]

An exceptionally well preserved trilobite from the Burgess Shale. The antennæ and legs are preserved as reflective carbon films.

[edit] Soft body parts

Only 21 or so species are described from which soft body parts are preserved,[14][20] so some features (e.g. the posterior antenniform cerci preserved only in Olenoides serratus)[21] remain difficult to assess in the wider picture.[22]

[edit] 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).[14][20] Each exopodite (walking leg) had 6 or 7 segments,[20] homologous to other early arthropods.[22] 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.[22] The base of the coxa, the gnathobase, sometimes have heavy, spiny adaptations which were used to tear at the tissues of prey.[23] The last exopodite segment usually had claws or spines.[14] Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.[22]

[edit] 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.[14] 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.[4]

[edit] 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.[22]

[edit] 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.[14]

[edit] 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.

[edit] 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.[21]

[edit] 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.[24] 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.[25]

Trilobite eyes were typically compound, with each lens being an elongated prism.[26] 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.[17] 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.[27]

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.[28] 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,[29] 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.[26][29] A living species with similar lenses is the brittle star Ophiocoma wendtii.[30]

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.[31]

The schizochroal eye of Erbenochile erbenii; the eye shade is unequivocal evidence that some trilobites were diurnal.[32]

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.[28]

Several other structures on trilobites have been explained as photo-receptors.[28] 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.[31]

[edit] 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".[28]

[edit] 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.[35]

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.[36]

Trilobite larvae are known from the Cambrian to the Carboniferous[37] and from all sub-orders.[36][38] 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.[36]

Despite the absence of supporting fossil evidence, their similarity to living arthropods has led to the belief that trilobites multiplied sexually and produced eggs.[39] [36] Some species may have kept eggs or larvae in a brood pouch forward of the glabella,[40] particularly when the ecological niche was challenging to larvae.[5] 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),[41] but so is calcification and hatching coinciding.[36]

The earliest post-embryonic trilobite growth stage known with certainty are the "protaspid" stages (anamorphic phase).[36] 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.[36][38]

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.[36]

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.[36] 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.[42] 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.[36] 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).[42] Pelagic larval life-style proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician.[11]

[edit] Fossil record

Walliserops trifurcatus, from Djebel Oufaten, Morocco

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 540 to 520 million years ago.[43][44] Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis sp. (western USA), Hupetina antiqua (Morocco)[45] and Serrania gordaensis (Spain).[46] All trilobites are thought to have originated in present day Siberia, with subsequent distribution and radiation from this location.[44]

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).[44] 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.[41] However, recent developments[47] suggest the picture is more complicated,[48] and likely to change as more information comes to light.[44]

[edit] 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).[17] 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.[41][49] 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.[50] 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).[24][44]

[edit] Divergence 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.[17]

[edit] Evolutionary trends

Principal evolutionary trends from primitive morphologies (e.g. eoredlichids)[51] 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.[17] Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.[51] Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures & hypostome specialization.[51] Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).[51]

Effacement is also a common evolutionary trend. It is the loss of surface detail in the cephalon, pygidium, or the thoracic furrows. The most notable examples of which happen in the orders Agnostida, Asaphida, and the suborder Illaenina of Corynexochida. It is believed that effacement is an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details (particularly of the glabella) can make the determination of phylogenetic relationships difficult.[52]

[edit] Pre-Cambrian

Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichidae 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.[44] 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).[44][53] 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.[53]

[edit] 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).[15] 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.[17] A continuing decrease in Laurentian continental shelf area[11] is recorded at the same time as the extinctions, suggesting major environmental upheaval.

[edit] 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.[17] Although intra-species trilobite diversity seems to have peaked during the Cambrian,[54] trilobites were still active participants in the Ordovician radiation event with a new fauna taking over from the old Cambrian one.[55] Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors.[17] 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.[41] 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.[55] 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.[17]

[edit] 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.[55]

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.[51] 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).[51] Only a single order, the Proetida, survived into the Carboniferous.[17]

[edit] 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).[17] 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.[51] For many millions of years the Proetida existed untroubled in their ecological niche.[17] 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.[17]

[edit] 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[56] 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.[51] With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobite is hardly surprising.[56]

The closest extant relatives of trilobites may be the horseshoe crabs,[43] or the cephalocarids.[57]

[edit] 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.[58] Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding.[59] Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface.[59] However, care must be taken as similar trace fossils are recorded in freshwater[60] and post Paleozoic deposits,[61] 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.[62]

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.

[edit] 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.[63][64][65]

Identification of the 'Atlantic' and 'Pacific' trilobite faunas in North America and Europe[66] implied the closure of the Iapetus Ocean (producing the Iapetus suture),[67] thus providing important supporting evidence for the theory of continental drift.[68][69]

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.[70][71]

Trilobites are excellent stratigraphic markers of the Cambrian period: researchers who find trilobites with alimentary prosopon, and a micropygium, have found Early Cambrian strata.[72] Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils.[73][74][75]

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.[76]

[edit] See also

Paleontology portal
Arthropods portal

[edit] References

  1. ^ Robert Kihm; James St. John (2007), Walch's trilobite research – A translation of his 1771 trilobite chapter, in Donald G. Mikulic, Ed Landing and Joanne Kluessendorf, "Fabulous fossils – 300 years of worldwide research on trilobites" (PDF), New York State Museum Bulletin (University of the State of New York) 507: 115–140, http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Kihm-and-St.John-2007.pdf. 
  2. ^ http://firstlifeseries.com/learn-more/
  3. ^ 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 
  4. ^ a b Fortey, Richard (2004), "The Lifestyles of the Trilobites" (PDF), American Scientist 92: 446–453, http://www.cornellcollege.edu/geology/courses/Greenstein/paleo/trilobites.pdf 
  5. ^ a b 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, PMC 18664, PMID 10841557, http://www.pnas.org/cgi/pmidlookup?view=long&pmid=10841557 
  6. ^ Fortey, R. A. (2001), "Trilobite systematics: The last 75 years", Journal of Paleontology 75: 1141–1151, doi:10.1666/0022-3360(2001)075<1141:TSTLY>2.0.CO;2, http://findarticles.com/p/articles/mi_qa3790/is_200111/ai_n8958763/ 
  7. ^ 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 
  8. ^ Fortey, R. A. (1990), "Ontogeny, Hypostome attachment and Trilobite classification" (PDF), Palaeontology 33 (3): 529–576, http://palaeontology.palass-pubs.org/pdf/Vol%2033/Pages%20529-576.pdf, retrieved June 22, 2009 
  9. ^ Ebach, M. C.; McNamara, K. J. (2002), "A systematic revision of the family Harpetidae (Trilobita)", Records of the Western Australian Museum 21: 135–167 
  10. ^ Gerhard Scholtz & Gregory D. Edgecombe (2005). "Heads, Hox and the phylogenetic position of trilobites". In Stefan Koenemann & Ronald A. Jenner (PDF). Crustacea and Arthropod Relationships. Volume 16 of Crustacean issues. CRC Press. pp. 139–165. ISBN 0-8493-3498-5. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.130.2585&rep=rep1&type=pdf. 
  11. ^ a b c Rudkin, D.A.; Young, G. A.; Elias, R. J.; Dobrzanske, E. P. (2003), "The world's biggest trilobite: Isotelus rex new species from the Upper Ordovician of northern Manitoba, Canada", Palaeontology 70 (1): 99–112, doi:10.1666/0022-3360(2003)077<0099:TWBTIR>2.0.CO;2, http://www.bioone.org/perlserv/?request=get-document&doi=10.1666%2F0022-3360(2003)077%3C0099%3ATWBTIR%3E2.0.CO%3B2&ct=1 
  12. ^ Jell, P. A.; Adrain, J. M. (2003), "Available generic names for trilobites", Memoirs of the Queensland Museum 48 (2): 331–553 
  13. ^ a b c d Whittington, H. B. (1997), "Morphology of the Exoskeleton", in Kaesler, R. L., Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida, Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 1–85, ISBN 0-8137-3115-1 
  14. ^ a b c d e f g h Bruton, D. L.; Haas, W. (2003), "Making Phacops come alive", in Lane, P. D., D. J. Siveter & R. A. Fortey, Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001, Special Papers in Palaeontology 70, Blackwell Publishing & Palaeontological Association, pp. 331–348, ISBN 9780901702814, http://books.google.co.uk/books?id=2E2fDXCkUEkC 
  15. ^ a b Nedin, C. (1999), "Anomalocaris predation on nonmineralized and mineralized trilobites", Geology 27 (11): 987–990, doi:10.1130/0091-7613(1999)027<0987:APONAM>2.3.CO;2, http://geology.geoscienceworld.org/cgi/content/abstract/27/11/987 
  16. ^ Bruton, D. L.; Nakrem, H. A. (2005), "Enrollment in a Middle Ordovician agnostoid trilobite" (PDF), Acta Palaeontologica Polonica 50: 441–448, http://app.pan.pl/archive/published/app50/app50-441.pdf, retrieved June 22, 2009 
  17. ^ a b c d e f g h i j k l m Clarkson, E. N. K. (1998), Invertebrate Paleontology and Evolution (4th ed.), Oxford: Wiley/Blackwell Science, p. 452, ISBN 0-632-05238-4, http://www.google.co.uk/books?id=g1P2VaPQWfUC&pg=PP1&lpg=PP1 
  18. ^ Knell, R. J.; Fortey, R. A. (2005), "Trilobite spines and beetle horns: sexual selection in the Palaeozoic?", Biology Letters 1 (2): 196–199, doi:10.1098/rsbl.2005.0304, PMC 1626209, PMID 17148165, http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1626209 
  19. ^ New Scientist magazine (2005), Earliest combatants in sexual contests revealed (published May 28, 2005), http://www.newscientist.com/channel/life/mg18625015.100/ 
  20. ^ a b c Hughes, Nigel (2003), "Trilobite tagmosis and body patterning from morphological and developmental perspectives.", Integrative and Comparative Biology 43: 185–205, doi:10.1093/icb/43.1.185, http://icb.oxfordjournals.org/cgi/content/full/43/1/185 
  21. ^ a b Whittington, H. B. (1980), "Exoskeleton, moult stage, appendage morphology, and habits of the Middle Cambrian trilobite Olenoides serratus", Palaeontology 23: 171–204 
  22. ^ a b c d e Whittington, H. B. (1997), "The Trilobite Body.", in Kaesler, R. L., Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida., Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 137–169, ISBN 0-8137-3115-1 
  23. ^ Ramskold, L.; Edgecombe, G. D. (1996), "Trilobite appendage structure – Eoredlichia reconsidered", Alcheringa 20: 269–276, doi:10.1080/03115519608619471 
  24. ^ a b McCall, G. J. H. (2006), "The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion", Earth-Science Reviews 77 (1-3): 1–229, doi:10.1016/j.earscirev.2005.08.004 
  25. ^ Parker, Andrew (2003), In the Blink of an Eye, Cambridge, MA: Perseus Books, ISBN 0738206075, OCLC 52074044 
  26. ^ a b Levi-Setti, Riccardo (1993), Trilobites (2 ed.), Chicago, IL: University of Chicago Press, p. 342, ISBN 0-226-47451-8 
  27. ^ a b c d e f Clarkson, E. N. K. (1979), "The Visual System of Trilobites", Palaeontology 22: 1–22, doi:10.1007/3-540-31078-9_67 
  28. ^ a b c d e f g h i j Clarkson, E. N. (1997), "The Eye, Morphology, Function and Evolution", in Kaesler, R. L., Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida., Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 114–132, ISBN 0-8137-3115-1 
  29. ^ a b Clarkson, E. N. K.; Levi-Setti, R. L. (1975), "Trilobite eyes and the optics of Descartes and Huygens", Nature 254 (5502): 663–7, doi:10.1038/254663a0, PMID 1091864 
  30. ^ Joanna Aizenberg; Alexei Tkachenko; Steve Weiner; Lia Addadi; Gordon Hendler (2001), "Calcitic microlenses as part of the photoreceptor system in brittlestars", Nature 412 (6849): 819–822, doi:10.1038/35090573, PMID 11518966 
  31. ^ a b Bruton, D. L.; Haas, W. (2003b), "The Puzzling Eye of Phacops", in Lane, P. D., Siveter, D. J. & Fortey R. A., Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001, Special Papers in Palaeontology 70, Blackwell Publishing & Palaeontological Association, pp. 349–362, http://books.google.co.uk/books?id=2E2fDXCkUEkC 
  32. ^ Fortey, R.; Chatterton, B. (2003), "A Devonian Trilobite with an Eyeshade", Science 301 (5640): 1689, doi:10.1126/science.1088713, PMID 14500973 
  33. ^ Buschbeck, Elke; Ehmer, Birgit; Hoy, Ron (1999), "Chunk Versus Point Sampling: Visual Imaging in a Small Insect", Science 286 (5442): 1178, doi:10.1126/science.286.5442.1178, PMID 10550059 
  34. ^ Jell, P. A. (1975), "The abathochroal eye of Pagetia, a new type of trilobite eye", Fossils and Strata 4: 33–43 
  35. ^ Sam Gon III. "Trilobite Development". http://www.trilobites.info/ontogeny.htm. 
  36. ^ a b c d e f g h i j Chatterton, B. D. E.; Speyer, S. E. (1997), "Ontogeny", in Kaesler, R. L., Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida., Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 173–247, ISBN 0-8137-3115-1 
  37. ^ Lerosey-Aubril, R.; Feist, R. (2005), "First Carboniferous protaspid larvae (Trilobita)", Journal of Paleontology 79: 702–718, doi:10.1666/0022-3360(2005)079[0702:FCPLT]2.0.CO;2 
  38. ^ a b Rudy Lerosey-Aubril. "The Ontogeny of Trilobites". Archived from the original on October 27, 2009. http://web.archive.org/web/20091027001052/http://geocities.com/barracudaaa/index. Retrieved November 8, 2010. 
  39. ^ Zhang, X.; Pratt, B. (1994), "Middle Cambrian arthropod embryos with blastomeres", Science 266 (5185): 637–9, doi:10.1126/science.266.5185.637, PMID 17793458 
  40. ^ Fortey, R. A.; Hughs, N. C. (1998), "Brood pouches in trilobites", Journal of Paleontology 72: 639–649, http://findarticles.com/p/articles/mi_qa3790/is_199807/ai_n8785182. 
  41. ^ a b c d Clowes, Chris, Trilobite Origins, http://www.peripatus.gen.nz/Taxa/Arthropoda/Trilobita/TriOri.html, retrieved April 12, 2009 
  42. ^ a b Chatterton, B. D. E.; Speyer, S. E. (1989), "Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites", Paleobiology 15: 118–132 
  43. ^ a b Fortey, Richard (2000), Trilobite!, London: HarperCollins, ISBN 0-00-257012-2 
  44. ^ a b c d e f g B. S., Lieberman (2002), "Phylogenetic analysis of some basal early Cambrian trilobites, the biogeographic origins of the eutrilobita, and the timing of the Cambrian radiation", Journal of Paleontology 76: 692–708, doi:10.1666/0022-3360(2002)076<0692:PAOSBE>2.0.CO;2 
  45. ^ Hollingsworth, J. S. (2008), The first trilobites in Laurentia and elsewhere, in I. Rábano, R. Gozalo and D. García-Bellido, "Advances in trilobite research", Cuadernos del Museo Geominero, nº 9 (Madrid, Spain: Instituto Geológico y Minero de España) 
  46. ^ Linan, Eladio; Gozalo, Rodolfo; Dies Alvarez, María Eugenia (2008), "Nuevos trilobites del Ovetiense inferior (Cámbrico Inferior bajo) de Sierra Morena (España)", Ameghiniana 45 (1): 123–138, http://www.scielo.org.ar/scielo.php?script=sci_abstract&pid=S0002-70142008000100008&lng=es&nrm=iso&tlng=en 
  47. ^ Jell, P. (2003), "Phylogeny of Early Cambrian trilobites", in Lane, P. D., Siveter, D. J. & Fortey R. A., Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001, Special Papers in Palaeontology 70, Blackwell Publishing & Palaeontological Association, pp. 45–57 
  48. ^ Sam Gon III. "First Trilobites". http://www.trilobites.info/firsttrilos.htm. 
  49. ^ Sam Gon III. "Origins of Trilobites". http://www.trilobites.info/origins.htm. 
  50. ^ Sam Gon III. "Trilobite Classification". http://www.trilobites.info/triloclass.htm#trilobites. 
  51. ^ a b c d e f g h Fortey, R. A.; Owens, R. M. (1997), "Evolutionary History", in Kaesler, R. L., Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida., Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 249–287, ISBN 0-8137-3115-1 
  52. ^ Samuel M. Gon III (July 20, 2008). "Evolutionary Trends in Trilobites". A Guide to the Orders of Trilobites. http://www.trilobites.info/trends.htm. Retrieved April 14, 2011. 
  53. ^ a b Fortey, R. A.; Briggs, D. E. G.; Wills, M. A. (1996), "The Cambrian evolutionary "explosion": decoupling cladogenesis from morphological disparity", Biological Journal of the Linnean Society 57: 13–33, doi:10.1111/j.1095-8312.1996.tb01693.x 
  54. ^ Webster, Mark (2007), "A Cambrian peak in morphological variation within trilobite species", Science 317 (5837): 499–502, doi:10.1126/science.1142964, PMID 17656721 
  55. ^ a b c Adrain, Jonathan M.; Fortey, Richard A.; Westrop, Stephen R. (1998), "Post-Cambrian trilobite diversity and evolutionary faunas", Science 280 (5371): 1809, doi:10.1126/science.280.5371.1922, PMID 9632387 
  56. ^ a b Owens, R. M. (2003), "The stratigraphical distribution and extinctions of Permian trilobites.", in Lane, P. D., Siveter, D. J. & Fortey R. A., Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001, Special Papers in Palaeontology 70, Blackwell Publishing & Palaeontological Association, pp. 377–397 
  57. ^ Lambert, David (1985), The Field Guide to Prehistoric Life, Facts on File Publications, New York: the Diagram Group, ISBN 0-8160-1125-7 
  58. ^ Baldwin, C. T. (1977), "Rusophycus morgati: an asaphid produced trace fossil from the Cambro-Ordovician of Brittany and Northwest Spain", Journal of Paleontology 51 (2): 411–425, http://www.jstor.org/stable/1303619 
  59. ^ a b Garlock, T. L.; Isaacson, P. E. (1977), "An Occurrence of a Cruziana Population in the Moyer Ridge Member of the Bloomsberg Formation (Late Silurian)-Snyder County, Pennsylvania", Journal of Paleontology 51 (2): 282–287, http://www.jstor.org/pss/1303607 
  60. ^ Woolfe, K. J. (1990), "Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica", Palaeogeography, Palaeoclimatology, Palaeoecology 80: 301–310, doi:10.1016/0031-0182(90)90139-X 
  61. ^ John-Paul Zonneveld; S. George Pemberton; Thomas D. A. Saunders; Ronald K. Pickerill (2002), "Large, robust Cruziana from the Middle Triassic of northeastern British Columbia: ethologic, biostratigraphic, and paleobiologic significance", Palaios 17 (5): 435–448, doi:10.1669/0883-1351(2002)017<0435:LRCFTM>2.0.CO;2 
  62. ^ Robert R. Gaines; Mary L. Droser (2003), "Paleoecology of the familiar trilobite Elrathia kingii: an early exaerobic zone inhabitant" (PDF), Geology 31: 941–4, doi:10.1130/G19926.1, http://earthsciences.ucr.edu/docs/Gaines&Droser_2003.pdf 
  63. ^ Eldredge, Niles & Gould, Stephen Jay (1972), "Punctuated equilibria: an alternative to phyletic gradualism", in Schopf, Thomas J. M., Models in Paleobiology, San Francisco, CA: Freeman, Cooper, pp. 82–115, ISBN 0-87735-325-5, http://www.blackwellpublishing.com/ridley/classictexts/eldredge.asp  Reprinted in Eldredge, Niles (1985), Time frames: the rethinking of Darwinian evolution and the theory of punctuated equilibria, New York, NY: Simon and Schuster, ISBN 0-671-49555-0 
  64. ^ Mayr, Ernst (1992), "Speciational Evolution or Punctuated Equilibria?", in Peterson, Steven A. & Somit, Albert, The Dynamics of evolution: the punctuated equilibrium debate in the natural and social sciences, Ithaca, NY: Cornell University Press, pp. 25–26, ISBN 0-8014-9763-9, http://www.stephenjaygould.org/library/mayr_punctuated.html 
  65. ^ Shermer, Michael (2001), The borderlands of science: where sense meets nonsense, Oxford, UK: Oxford University Press, ISBN 0-19-514326-4 
  66. ^ Windley, B. F. (1996), The Evolving Continents (3 ed.), John Wiley & Sons, pp. xvi, 526, ISBN 0471917397 
  67. ^ Harland, W. B.; Gayer, R. A. (1972), "The Arctic Caledonides and earlier oceans", Geological Magazine 109: 289–314, doi:10.1017/S0016756800037717 
  68. ^ Hughes Patrick, "Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle", On the shoulders of giants (Earth Observatory, NASA), archived from the original on August 8, 2007, http://web.archive.org/web/20070808235956/http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_2.html, retrieved December 26, 2007, "... on January 6, 1912, Wegener ... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography." 
  69. ^ Alfred Wegener (1966), The origin of continents and oceans, Biram John, Courier Dover, p. 246, ISBN 0486617084 
  70. ^ Lieberman, BS (1999), "Testing the Darwinian Legacy of the Cambrian Radiation Using Trilobite Phylogeny and Biogeography", Journal of Paleontology 73 (2): 176–181, http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176 
  71. ^ Lieberman, B. S. (2003), "Taking the pulse of the Cambrian radiation", Integrative and Comparative Biology 43: 229–237, doi:10.1093/icb/43.1.229 
  72. ^ Schnirel, B.L. (2001), Trilobite Evolution and Extinction, Dania, Florida: Graves Museum of Natural History 
  73. ^ Geyer, Gerd (1998), "Intercontinental, trilobite-based correlation of the Moroccan early Middle Cambrian", Canadian Journal of Earth Science 35 (4): 374–401, doi:10.1139/cjes-35-4-374, http://rparticle.web-p.cisti.nrc.ca/rparticle/AbstractTemplateServlet?calyLang=eng&journal=cjes&volume=35&year=1998&issue=4&msno=e97-127 
  74. ^ Babcock, L. E.; Peng, S.; Geyer, G.; Shergold, J. H. (2005), "Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian System", Geoscience Journal 9 (2): 101–106, doi:10.1007/BF02910572 
  75. ^ "International Sub-commission on Cambrian Stratigraphy". http://www.palaeontologie.uni-wuerzburg.de/. 
  76. ^ Joleen Robinson (October 1970), "Tracking the Trilobites", Desert magazine 

[edit] Further reading


[edit] External links

v · d · eTrilobites
Psychopyge elegans Taxonomy
Geochronology
Notable fossils
Notable trilobitologists
Fossil sites
Morphology
Category:Trilobites

Retrieved from "http://en.wikipedia.org/wiki/Trilobite"
Personal tools
Namespaces
Variants
Actions
Navigation
Interaction
Toolbox
Print/export
Languages