''Triarthrus'' is a genus of

Upper Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

found in

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Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virginia ...

, and

Indiana Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...

, eastern and northern

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, China and

Scandinavia Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and S ...

. It is the last of the Olenid trilobites, a group which flourished in the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

period. The specimens of ''T. eatoni'' that are found in the

Beecher's Trilobite Bed Beecher's Trilobite Bed is a Konservat-Lagerstätte of Late Ordovician (Caradoc) age located within the Frankfort Shale in Cleveland's Glen, Oneida County, New York, USA.Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus ( legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...

area are exquisitely preserved showing soft body parts in

iron pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue gi ...

. Pyrite preservation has given scientists a rare opportunity to examine the

gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

, walking legs, antennae, digestive systems, and eggs of trilobites, which are rarely preserved. ''Triarthrus'' is therefore commonly used in science texts to illustrate trilobite anatomy and physiology.

Distribution

* ''T. beckii'' Upper Caradoc and Ashgill, Snake Hill Formation, Cohoes, New York State; and Kentucky. * ''T. billingsi'' Ashgill?, Quebec * ''T. canadensis'' is known from the Upper Ordovician of Canada (Katian, lower Member of the Whitby Formation, Craigleith vicinity, Georgian Bay area; middle Member of the Whitby Formation, Whitby, Rogue River and Pickering, all Lake Simcoe area, Ontario) ''cited in'' * ''T. eatoni'' Upper Caradoc-Ashgill, N.Y., is known from the Upper Ordovician of Canada (Ashgill, lower Member of the Whitby Formation, Craigleith vicinity, Georgian Bay area, Lake Simcoe area, Ontario; and Quebec) and the United States (New York) * ''T. glaber'' Ashgill, Quebec * ''T. huguesensis'' Ashgill, Quebec * ''T. latissimus'' Sweden * ''T. linnarssoni'' Norway and Sweden * ''T. rougensis'' Ashgill, Ontario * ''T. sichuansis'' China * ''T. spinosus'' is known from the Upper Ordovician of Canada (Katian, middle Member of the Whitby Formation, Rogue River and Pickering, both Lake Simcoe area, Ontario and Quebec), and the United States (New York).

Taxonomy

''T. beckii'' and ''T. eatoni'' have long been considered closely related and possibly synonymous. Recent comparative analysis showed that there is no sharp distinction between the two, but that they seem to represent opposing morphotypes. ''T. beckii'' dominates earlier in the distribution and in shallow water, while ''T. eatoni'' does so later and in deeper waters. It is probable that the transgression of the sea level aided the eventual disappearance of the ''T. beckii'' morph. The occurrence of the supposed pluriform species continued for more than two million years.

Species previously assigned to ''Triarthrus''

* ''T. angelini'' = '' Bienvillia angelini'' * ''T. belli'' = '' Parabolinella'' sp. * ''T. caenigenus'' = '' Porterfieldia caenigena'' * ''T. convergens'' = ''Porterfieldia convergens'' * ''T. fisheri'' = ''Porterfieldia fisheri'' * ''T. goldwyerensis'' = ''Porterfieldia goldwyerensis'' * ''T. humilis'' = ''Porterfieldia humilis'' * ''T. jachalensis'' = ''Porterfieldia jachalensis'' * ''T. pacificus'' = ''Porterfieldia pacifica'' * ''T. papilosus'' = ''Porterfieldia papilosa'' * ''T. parapunctatus'' = ''Porterfieldia parapunctata'' * ''T. parchaensis'' = ''Bienvillia parchaensis'' * ''T. punctatus'' = ''Porterfieldia punctata'' * ''T. rectifrons'' = ''Bienvillia rectifrons'' * ''T. reedi'' = ''Porterfieldia caenigena'' * ''T. shinetonensis'' = ''Bienvillia shinetonensis'' * ''T. sinensis'' = ''Porterfieldia sinensis'' * ''T. tetragonalis'' = ''Bienvillia tetragonalis'' * ''T. thor'' = ''Porterfieldia thor'' * ''T. variscorum'' = '' Parabolina frequens''

Description

Exoskeleton

''Triarthrus'' is an average size trilobite (up to about ) and its moderately convex body is about twice as long as wide (excluding spines). Like in all Olenidae, the headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

) of ''Triarthrus'' has opisthoparian sutures, and the right and left free cheeks that they define are yoked. The cephalon in ''Triarthrus'' is semicircular. The central raised area (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior t ...

) is approximately quadrate, and considerably wider than the posterior margin of the fixed cheeks (or fixigenae). The front of the glabella is close to the anterior border (or the preglabellar field is short), but the border furrow is absent in front of the glabella. The facial suture crosses in front of the glabella on the top of an inflated rim. The posterior end of the glabella consists of the occipital ring that is defined by a furrow crossing over the midline. The occipital ring carries a small node at its centre point, and may be adorned with a backward directed spine as long as the glabella. In front of the occipital furrow two pairs of lateral furrows emerge from the axial furrow almost perpendicularly and curve backwards as they cut toward the midline, but without reaching it. Anteriorly, one or two further furrows are isolated shallow slits or depressions. The fixigenae are very narrow (less than ¼ of the width of the glabella at the back of the eye, and less than ½ at the occipital ring). The librigenae are narrow, and with or without spines. The border is prominent elsewhere, and the eyes of small to medium size, sitting at the end of small palpebral lobes next to the frontal half of the glabella. The thorax consists of 13 to 16 segments, with the axis wider than pleural regions, obliquely truncated or rounded pleural tips, and the fulcrum placed very close to the axis. The tailpiece (or

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) is small, with 3 to 5 axial rings, distinctly segmented pleural fields, and an entire, evenly rounded margin.

Uncalcified parts

The foremost pair of extremities in trilobites are the antennas. In ''Triarthrus eatoni'' the antennas are probably flexible. From their attachment adjacent to the

hypostome In zoology, the hypostome can refer to structures in distinct animal groups: * Hypostome (trilobite), the ventral mouthpart plate in trilobites * Hypostome (tick), the barbed attachment structure associated with the mouthparts of parasitic arachni ...

they stretch inward and forward, almost touching each other at the point where, from a

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

viewpoint, they appear from under the cephalic margin, then bending outward and then forward again in a slight S-curve. They are about twice as long as the headshield (or

), each consisting of 40 to 50 segments that are shorter than wide. As the preservation of soft tissue in ''T. eatoni'' is excellent, it is probable that it lacked cerci, the most backward pair of extremities known from '' Olenoides serratus''.

Life cycle

Due to the existence of an excellently preserved deposit in the Frankfurt Shale near Rome, New York State, that suggests an entire population of ''Triarthrus eatoni'' was killed and quickly buried, it has been possible to make a plausible reconstruction of its life history. After hatching from roughly spherical eggs averaging ~0.2mm in diameter, the small protaspid and early meraspid stages supposedly lived between the

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

in the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. This is derived from the fact that only

exuvia In biology, exuviae are the remains of an exoskeleton and related structures that are left after ecdysozoans (including insects, crustaceans and arachnids) have moulted. The exuviae of an animal can be important to biologists as they can often b ...

e of the early stages were found, making clear that they haven't died in the disaster that killed their elders. In this phase the larvae probably got dispersed over large areas. After a few months, at about 2mm long ''T. eatoni'' started living at the

sea floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

. Here, it must have fed by filtering suspended food particles, probably comparable to the trunk limb filter-feeding Cephalocarida and

Branchiopoda Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian ''Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus. Description ...

. The exoskeletons fall into five size categories, suggesting that ''T. eatoni'' had a distinct breeding season, likely annually, as is the case in almost all

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group c ...

ns (although some

amphipod Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descri ...

s have two breeding periods per year). It could live through at least four breeding seasons, and at that age reach approximately 4 cm in length.

References

Sources

''T. eatoni'' and ''T. rougensis'' photos.