Lotagnostus
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''Lotagnostus'' is a genus of very small
trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...
s in the order
Agnostida Agnostida are an order of extinct arthropods which have classically been seen as a group of highly modified trilobites, though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopod ...
, which lived on the outer
continental shelves A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
worldwide, during the late Upper
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
. It was described by Whitehouse in 1936, and the type species is ''Lotagnostus trisectus'', which was originally described as a species of ''
Agnostus ''Agnostus'' is a genus of agnostid trilobites, belonging to the family Agnostidae, that lived during the late Middle Cambrian – Upper Cambrian (about 506 to 485 million years ago). It is the type genus of the family (biology), family Agnostida ...
'' by Salter in 1864.


Taxonomy

The
Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...
divides ''Lotagnostus'' into three subgenera that differ in the degree of effacement: ''L. (Lotagnostus)'' Whitehouse, 1936 (neither cephalon nor pygidium strongly effaced), ''L. (Distagnostus)'' Shergold, 1972 (strongly effaced on the outer/dorsal surface, but with clear furrows on the inner/ventral surface and ''L. (Eolotagnostus)'' Zhou ''in'' Zhiqiang Zhou, li & Qu, 1982 (with even the inner/ventral surface strongly effaced). Later authors however consider it likely that creating these subgenera would render the nominate subgenus
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
.


Biostratographic significance

The
first appearance datum First appearance datum (FAD) is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest fossil discovered, to date, of a partic ...
(FAD) of ''Lotagnostus americanus'' has been proposed to define the lower limit of
Cambrian Stage 10 Stage 10 of the Cambrian is the still unnamed third and final stage of the Furongian series. It follows the Jiangshanian and precedes the Ordovician Tremadocian Stage. The proposed lower boundary is the first appearance of the trilobite '' Lotagno ...
. This proposition is based on the interpretation that ''L. americanus'' is a polymorph species with a global distribution, a concept that included ''L. obscurus'', and ''Agnostus trisectus'', the
type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...
of ''Lotagnostus''. Recently however, the North American species of ''Lotagnostus'' have been revised and it was concluded that ''L. americanus'' in fact shows little variation and has a limited spatial distribution, and the variants from other locations belong to several other ''Lotagnostus'' species. This renders ''L. americanus'' unfit as
index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...
for the lower limit of Cambrian Stage 10. As alternatives the FADs of the conodonts '' Cordylodus andresi'' and '' Eoconodontus notchpeakensis'' have been suggested.


Distribution

* ''L. trisectus'' is known from the late Upper Cambrian of Canada (MacNeil Brook, Middle Chesley Drive Group, Nova Scotia), England ( White Leaved Oak Shale, Malvern, Gloucestershire), Wales ( Cwmhesgen Formation) and Sweden ( Andrarum). * ''L. americanus'' occurs at the late Upper Cambrian of Canada (in boulders in conglomerates of the Léwis Formation, North Ridge, near Léwis, Quebec). * ''L. asiaticus'' has been found in the late Upper Cambrian of China (Kuruktagh, Xinjiang) * ''L. germanicus'' is known from the late Upper Cambrian of Canada (North shore of East Bay, near Eskasoni, Cape Breton Island, Middle Chesley Drive Group, Nova Scotia) * ''L. hedeni'' occurs at the late Upper Cambrian of China (Kuruktagh, Xinjiang; Siyangshan Formation, western Zhejiang), Kazakhstan (''Euloma limitaris-Taoyuania'' zone) and Canada (''Phylacterus saylesi'' fauna, Shallow Bay Formation, western Newfoundland) * ''L. obscurus'' is present in the late Upper Cambrian of the United States ( Pogonip Limestone, Windfall Formation, near Hamburg Shaft, Eureka District, Nevada)) * ''L. peladensis'' is known from the late Upper Cambrian of Argentina (Precordillera) * ''L. punctuatus'' has been found in the late Upper Cambrian of China (Zejiang; elsewhere in Southeastern China) and Kazakhstan. ''Lotagnostus'' is also present in Siberia ( Ogon'or Formation along the Khos-Negele River), but its assignment to a particular species is difficult.


Description

Like all Agnostida, ''Lotagnostus'' is diminutive, with the headshield (or
cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...
) and tailshield (or
pygidium The pygidium (: 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 compos ...
) of approximately the same size (or isopygous) and outline. Like all Agnostina, ''Lotagnostus'' has only two thorax segments. The species are characterized by variable stages of effacement, more so on the outer/dorsal surface compared to the inner/ventral surface. This may complicate distinction from effaced species in other genera. The frontal lobe the central raised area of the cephalon (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 ...
) is long at about or more of the length of the glabella. The basal lobes at the rear of the glabella are narrow triangular and equal in length to the frontal lobe. The central part of the glabella has two side lobes. Opposite the tip of the side lobes there is a node on the midline. There is a furrow at midline connecting the glabella with the border furrow. The axis of the pygidium (or rhachis) has three sections. The frontal section is split into three parts. Both lateral parts are defined by furrows on all sides: those with the central section directed backward and very slightly outward, those with the middle section outward and slightly backward, and those with the pleural zone backward and slightly inward. The middle part is merged with the central section. The middle part carries a node. The furrow between the central and rear parts is transverse, and those with the pleural zone backward and slightly inward. The rear part is about twice as long as the frontal and central parts individually. The surrounding axial furrow is directed outward, then strongly bends backward and then curves regularly ever more inward. Over the posterior of the rear part a strip about as wide as the node on the central part may be discernible, which ends in a node that at the very rear of the rhachis. The distance between the rhachis and the border furrow (or post axial length) is about equal to of the rhachis. The pygidial border is about of the rhachis. Opposite to the rear of the rhachis the border may carry a small spine at each side.


References

{{Taxonbar, from=Q10787061 Agnostidae Cambrian trilobites Fossils of Great Britain Fossils of China Fossils of Sweden Fossils of Russia Fossils of Argentina Fossils of Kazakhstan Fossils of the United States Paleontology in Nova Scotia Paleontology in Quebec Paleozoic life of Newfoundland and Labrador Paleozoic life of Yukon Agnostida genera Index fossils