Editing Trilobite (section)

===Eyes===
[[File:Phacops eye.jpg|thumb|left|Unknown ''Phacops'' sp. eye]]

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.<ref name="McCall2006">{{Citation | author=McCall, G. J. H. | year=2006 | doi=10.1016/j.earscirev.2005.08.004 | title=The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion | journal=[[Earth-Science Reviews]] | volume=77 | issue=1–3 | pages=1–229 |bibcode = 2006ESRv...77....1M }}</ref> Improving eyesight of both predator and prey in marine environments has been suggested as one of the [[evolutionary pressure]]s furthering an apparent rapid development of new life forms during what is known as the [[Cambrian explosion]].<ref>{{Citation | author=Parker, Andrew | title=In the Blink of an Eye | publisher=Perseus Books | location=Cambridge, MA | year=2003 | isbn=978-0-7382-0607-3 | oclc=52074044 | url=https://archive.org/details/inblinkofeye00park }}</ref>

Trilobite eyes were typically [[compound eye|compound]], with each lens being an elongated prism.<ref name='levi-setti93' /> 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.<ref name=Clarkson98 /> The fossil record of trilobite eyes is complete enough that their evolution can be studied through time, which compensates to some extent for the lack of preservation of soft internal parts.<ref name=Clarkson79 />

[[Lens (anatomy)|Lens]]es of trilobites' [[Arthropod eye|eyes]] were made of [[calcite]] ([[calcium carbonate]], CaCO<sub>3</sub>). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals to form each lens of each eye.<ref name=Clarkson97>{{Citation | last=Clarkson | first=E. N. | author-link=Euan Clarkson | contribution=The Eye, Morphology, Function and Evolution | editor-last=Kaesler | editor-first=R. L. | title=Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida | pages=[https://archive.org/details/treatiseoninvert0002unse/page/114 114–132] | publisher=The Geological Society of America, Inc. & The University of Kansas | place=Boulder, CO & Lawrence, KA | year=1997 | isbn=978-0-8137-3115-5 | url=https://archive.org/details/treatiseoninvert0002unse/page/114 }}</ref> Rigid calcite lenses would have been unable to [[accommodation (eye)|accommodate]] to a change of focus like the soft lens in a human eye would; in some trilobites, the calcite formed an internal [[doublet (lens)|doublet]] structure,<ref name='Clarkson75'>{{Citation | last1=Clarkson | first1=E. N. K. | last2=Levi-Setti | first2=R. L. |year=1975 | title=Trilobite eyes and the optics of Descartes and Huygens |journal=[[Nature (journal)|Nature]] |volume=254 |pages=663–7 | doi=10.1038/254663a0 | pmid=1091864 | issue=5502|bibcode = 1975Natur.254..663C | s2cid=4174107 }}</ref> giving superb [[depth of field]] and minimal [[spherical aberration]], according to optical principles discovered by French scientist [[René Descartes]] and Dutch physicist [[Christiaan Huygens]] in the 17th century.<ref name='levi-setti93'>{{citation|first=Riccardo|last=Levi-Setti|title=Trilobites|publisher=University of Chicago Press|location=Chicago, IL|year=1993|page=[https://archive.org/details/trilobites00levi/page/342 342]|isbn=978-0-226-47451-9|edition=2|url=https://archive.org/details/trilobites00levi/page/342}}</ref><ref name='Clarkson75' /> A living species with similar lenses is the [[brittle star]] ''[[Ophiocoma wendtii]]''.<ref name='Aizenberg-etal01'>{{Citation |author1=Joanna Aizenberg |author2=Alexei Tkachenko |author3=Steve Weiner |author4=Lia Addadi |author-link4=Lia Addadi|author5=Gordon Hendler |year=2001 |title=Calcitic microlenses as part of the photoreceptor system in brittlestars |journal=[[Nature (journal)|Nature]] |volume=412 |pages=819–822 |doi=10.1038/35090573 |pmid=11518966 |issue=6849|bibcode = 2001Natur.412..819A |s2cid=4327277 }}</ref>

In other trilobites, with a Huygens interface apparently missing, a [[Gradient-index optics|gradient-index lens]] is invoked with the [[refractive index]] of the lens changing toward the center.<ref name=Bruton03b>{{Citation | last1=Bruton | first1=D. L. | last2=Haas | first2=W. | contribution=The Puzzling Eye of ''Phacops'' | editor-last=Lane, P. D. |editor2=Siveter, D. J. |editor3=Fortey R. A. |series=Special Papers in Palaeontology |volume= 70 |title=Trilobites and Their Relatives: Contributions from the Third International Conference, Oxford 2001 |pages=349–362|publisher=Blackwell Publishing & Palaeontological Association |year=2003b |contribution-url=https://books.google.com/books?id=2E2fDXCkUEkC}}</ref>

Sublensar sensory structures have been found in the eyes of some [[Phacopida|phacopid]] trilobites.<ref name="Schoenemann-2013">{{Cite journal|last1=Schoenemann|first1=Brigitte|last2=Clarkson|first2=Euan|year=2013|title=Discovery of some 400{{nbsp}}million year-old sensory structures in the compound eyes of trilobites|journal=Scientific Reports|volume=3|page=1429|doi=10.1038/srep01429|pmid=23492459|pmc=3596982|bibcode=2013NatSR...3.1429S}}</ref> The structures consist of what appear to be several sensory cells surrounding a rhadomeric structure, resembling closely the sublensar structures found in the eyes of many modern arthropod [[apposition eye]]s, especially ''[[Limulus]]'', a genus of horseshoe crabs.<ref name="Schoenemann-2013" />

*[[Holochroal eye]]s had a great number (sometimes over 15,000) of small (30–100&nbsp;μm, rarely larger)<ref name=Clarkson79 >{{Citation |last1=Clarkson |first1=E. N. K. |author-link=Euan Clarkson |year=1979 |title=The Visual System of Trilobites |journal=[[Palaeontology (journal)|Palaeontology]] |volume=22 |pages=1–22 |doi=10.1007/3-540-31078-9_67 |series=Encyclopedia of Earth Science |isbn=978-0-87933-185-6}}</ref> lenses. Lenses were [[Close-packing of equal spheres|hexagonally close packed]], touching each other, with a single [[cornea|corneal membrane]] covering all lenses.<ref name=Clarkson97/> Each lens was in direct contact with adjacent lenses. Holochroal eyes are the ancestral eye of trilobites, and are by far the most common, found in all orders except the Agnostida, and through the entirety of the Trilobites' existence.<ref name=Clarkson79 /> Little is known of the early history of holochroal eyes; Lower and Middle Cambrian trilobites rarely preserve the visual surface.<ref name=Clarkson79 /> The spatial resolving power of grated eyes (such as holochroal eyes) is dependent on [[irradiance|light intensity]], [[circular motion]], receptor density, registered light angle, and the extent to which the signal of individual [[Ommatidium|rhabdoms]] are neurally combined. This implies that lenses need to be larger under low light conditions (such as for ''[[Pricyclopyge]]'', when comparing it to ''[[Carolinites]]''), and for fast moving predators and prey. As the circular velocity caused by the forward speed of an animal itself is much higher for the [[Ommatidium|ommatidia]] directed perpendicular to the movement, fast-moving trilobites (such as ''Carolinites'') have eyes flattened from the side and more curved were ommatia are directed to the front or back. Thus eye morphology can be used to make assumptions about the ecosystem of trilobites.<ref name='McCormick&Fortey'>{{cite journal|last1= McCormick |first1= T.|last2= Fortey|first2= R.A.|year= 1998|title= Independent testing of a paleobiological hypothesis: the optical design of two Ordovician pelagic trilobites reveals their relative paleobathymetry|journal= Paleobiology|volume= 24|issue= 2|pages= 235–253|doi= 10.1666/0094-8373(1998)024[0235:ITOAPH]2.3.CO;2|jstor= 2401241|s2cid= 132509541}}</ref>[[File:Erbenochile eye.JPG|thumb|right|The schizochroal eye of ''[[Erbenochile|Erbenochile erbenii]]''; the eye shade is unequivocal evidence that some trilobites were [[diurnality|diurnal]].<ref name="Fortey&Chatterton2003">{{Citation |last1=Fortey |first1=R. |last2=Chatterton |first2=B.| year=2003 |title=A Devonian Trilobite with an Eyeshade |journal=[[Science (journal)|Science]] |volume=301 |page=1689 |doi=10.1126/science.1088713 |pmid=14500973 |issue=5640|s2cid=45993674 }}</ref>]]
*[[Schizochroal eye]]s typically had fewer (around 700), larger lenses than holochroal eyes and are found only in [[Phacopina]]. Each lens had a cornea, and adjacent lenses were separated by thick interlensar cuticle, known as sclera. Schizochroal eyes appear quite suddenly in the early Ordovician, and were presumably derived from a holochroal ancestor.<ref name=Clarkson79 /> 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.<ref name=Clarkson79/> Modern eyes that are functionally equivalent to the schizochroal eye were not thought to exist,<ref name=Clarkson97 /> but are found in the modern insect species ''[[Xenos (insect)|Xenos]] peckii''.<ref name=Buschbeck1999>{{citation |last1=Buschbeck | first1=Elke |last2=Ehmer | first2=Birgit |last3=Hoy | first3=Ron |year=1999 |title=Chunk Versus Point Sampling: Visual Imaging in a Small Insect |journal=[[Science (journal)|Science]] |volume=286 |issue=5442 |doi=10.1126/science.286.5442.1178 |pmid=10550059 |pages=1178–80}}</ref>
*Abathochroal eyes are found only in [[agnostida|Cambrian Eodiscina]], and have around 70 small separate lenses that had individual cornea.<ref name="Jell75">{{citation |last1=Jell | first1=P. A. |year=1975 |title=The abathochroal eye of ''Pagetia'', a new type of trilobite eye |journal=Fossils and Strata |volume=4 |pages=33–43| doi=10.18261/8200049639-1975-02 | isbn=8200049639 }}</ref> The sclera was separate from the cornea, and was not as thick as the sclera in schizochroal eyes.<ref name=Clarkson97 /> 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.<ref name=Clarkson97 /> Environmental conditions seem to have resulted in the later loss of visual organs in many Eodiscina.<ref name=Clarkson97 />

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.<ref name=Clarkson97 />

Several other structures on trilobites have been explained as photo-receptors.<ref name=Clarkson97 /> 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.<ref name=Bruton03b />