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Psychopyge elegans

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.[1]

Exoskeleton

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The trilobite body is divided into three major sections (tagmata): 1 – cephalon; 2 – thorax; 3 – pygidium. Trilobites are so named for the three longitudinal lobes: 4 – right pleural lobe; 5 – axial lobe; 6 – left pleural lobe; the antennae and legs are not shown in these diagrams.

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).

Terminology

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As might be expected for a group of animals comprising c. 5,000 genera,[2] 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.

Cephalon

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Morphology of the trilobite cephalon
Labeled diagram of major parts of the cephalon
The major subdivisions of the cephalon.
The subdivisions can be further broken down into different areas used in describing trilobite cephalic morphology. 1 – preocular area; 2 – palpebral area; 3 – postocular area; 4 – posterolateral projection; 5 – occipital ring; 6 – glabella; 7 – posterior area; 8 – lateral border; 9 – librigenal area; 10 – preglabellar area

The cephalon of trilobites is highly variable with a lot of morphological complexity. The glabella 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 hypostome have been described.[3] The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.[4]

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.[5] Highly complex compound eyes are another obvious feature of the cephalon.

Facial sutures

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Facial or Cephalic sutures are the natural fracture lines in the cephalon of trilobites. Their function is to assist the trilobite in shedding its old exoskeleton during ecdysis (or molting).[6]

Primitive trilobites from the Early Cambrian (like Fallotaspis, Eofallotaspis, Schmidtiellus, and Profallotaspis) lacked facial sutures. They are all classified under the suborder Olenellina. They are believed to have never developed facial sutures, having pre-dated their evolution. Because of this (along with other primitive characteristics), they are thought to be the earliest ancestors of later trilobites.[7][8]

Some other later trilobites also lost facial sutures secondarily.[7] The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites.[9]

Dorsal sutures
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The dorsal surface of the trilobite cephalon (the frontmost tagma, or the 'head') can be divided into two regions - the cranidium and the fixigena ("fixed cheeks"). The cranidium can be further divided into the glabella (the central lobe in the cephalon) and the librigena ("free cheeks").[10] The facial sutures lie along the anterior edge, at the division between the cranidium and the librigena. They function as

Trilobite facial sutures on the dorsal side can be roughly divided into three main types according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge).[11]

In some trilobites the sutures may be difficult to see as they run along the margins of the cephalon.[10] This is considered a fourth type of suture known as Marginal (or Hypoparian sutures). In this type, the sutures run mostly or wholly along the margin of the cephalon. They are not considered to be a formal phylogenetic grouping like the previous types as they have independently arisen in several groups of trilobites. Hypoparian sutures usually develop from other types of sutures when trilobite groups acquire secondary blindnesss (a “devolution” of visual organs in some trilobites).[13] The marginal sutures exhibited by the harpetids and trinucleioids, for example, are derived from opisthoparian sutures.[14] Example genera from both groups are Harpes and Cryptolithus, respectively, both of which are blind.[9]

There are also two types of sutures in the dorsal surface connected to the compound eyes of trilobites.[13][9] They are:

  • Ocular suture - are sutures surrounding the edges of the compound eye. Trilobites with these sutures lose the entrie surface of the eyes when molting. It is common among Cambrian trilobites.
  • Palepebral suture - are sutures which form part of the dorsal facial suture running along the top edges of the compound eye.
Ventral sutures
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Dorsal facial sutures continue downward to the ventral side of the cephalon where they become the Connective sutures that divide the doublure. The following are the types of ventral sutures.[13]

  • Connective sutures - are the sutures that continue from the facial sutures past the front margin of the cephalon.
  • Rostral suture - is only present when the trilobite posseses a rostrum (or rostral plate). It connects the rostrum to the front part of the dorsal cranidium.
  • Hypostomal suture - separates the hypostome from the doublure when the hypostome is of the attached type. It is absent when the hypostome is free-floating (i.e. natant). it is also absent in some coterminant hypostomes where the hypostome is fused to the doublure.
  • Median suture - exhibited by asaphid trilobites, they are formed when instead of becoming connective sutures, the two dorsal sutures converge at a point in front of the cephalon then divide straight down the center of the doublure.

Rostrum

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The rostrum (or the rostral plate) is a distinct part of the doublure located at the front of the cephalon. It is separated from the rest of the doublure by the rostral suture.

During molting in trilobites like Paradoxides, the rostrum is used to anchor the front part of the trilobite as the cranidium separates from the librigena. The opening created by the arching of the body provides an exit for the molting trilobite.

It is absent in some trilobites like Lachnostoma.

Hypostome

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The hypostome is the hard mouthpart of the trilobite found on the ventral side of the cephalon typically below the glabella. The hypostome can classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella.

  • Natant - Hypostome not attached to doublure. Aligned with front edge of glabella.
  • Conterminant - Hypostome attached to rostral plate of doublure. Aligned with front edge of glabella.
  • Impendent - Hypostome attached to rostral plate but not aligned to glabella.

Below is an illustration of the three types. The doublure is shown in light brown, the inside surface of the cephalon in gray, and the hypostome in light blue. The glabella is outlined in red broken lines.

Thorax

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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.[15]

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.[16] Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem.[15]

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.[17]

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.[15] In Phacops, the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure.[16] The doublure carries a Panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal.[16] 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.[18]

Pygidium

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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)

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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.[15] 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.[19]

Spines

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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.[20] A conclusion likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents.[21]

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

Soft body parts

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Only 21 or so species are described from which soft body parts are preserved,[16][22] so some features (e.g. the posterior antenniform cerci preserved only in Olenoides serratus)[23] remain difficult to assess in the wider picture.[24]

Appendages

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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).[16][22] Each exopodite (walking leg) had 6 or 7 segments,[22] homologous to other early arthropods.[24] 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.[24] The base of the coxa, the gnathobase, sometimes have heavy, spiny adaptations which were used to tear at the tissues of prey.[25] The last exopodite segment usually had claws or spines.[16] Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.[24]

Digestive tract

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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.[16] 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.[26]

Internal organs

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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.[24]

Musculature

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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.[16]

Sensory organs

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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

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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.[23]

Eyes

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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.[27] 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.[28]

Trilobite eyes were typically compound, with each lens being an elongated prism.[29] 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.[19] 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.[30]

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.[31] 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,[32] 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.[29][32] A living species with similar lenses is the brittle star Ophiocoma wendtii.[33]

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.[34]

  • Holochroal eyes had a great number (sometimes over 15,000) of small (30–100 μm, rarely larger)[30] lenses. Lenses were hexagonally close packed, touching each other, with a single corneal membrane covering all lenses.[31] 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.[30] Little is known of the early history of holochroal eyes; Lower and Middle Cambrian trilobites rarely preserve the visual surface.[30]
The schizochroal eye of Erbenochile erbenii; the eye shade is unequivocal evidence that some trilobites were diurnal.[35]
  • 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.[31] Schizochroal eyes appear quite suddenly in the early Ordovician, and were presumably derived from a holochroal ancestor.[30] 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.[30] Modern eyes which are functionally equivalent to the schizochroal eye were not thought to exist,[31] but are found in the modern insect species Xenos peckii.[36]
  • Abathochroal eyes are found only in Cambrian Eodiscina, had around 70 small separate lenses that had individual cornea.[37] The sclera was separate from the cornea, and did not run as deep as the sclera in schizochroal eyes.[31] 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.[31] Environmental conditions seem to have resulted in the later loss of visual organs in many Eodiscina.[31]

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.[31]

Several other structures on trilobites have been explained as photo-receptors.[31] 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.[34]

Sensory pits

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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".[31]

Development

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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.[38]

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.[39]

Trilobite larvae are known from the Cambrian to the Carboniferous[40] and from all sub-orders.[39][41] 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.[39]

Despite the absence of supporting fossil evidence, their similarity to living arthropods has led to the belief that trilobites multiplied sexually and produced eggs.[42] [39] Some species may have kept eggs or larvae in a brood pouch forward of the glabella,[43] particularly when the ecological niche was challenging to larvae.[44] 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),[7] but so is calcification and hatching coinciding.[39]

The earliest post-embryonic trilobite growth stage known with certainty are the "protaspid" stages (anamorphic phase).[39] 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.[39][41]

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.[39]

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.[39] 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.[45] 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.[39] 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).[45] Pelagic larval life-style proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician.[1]

References

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