Pilina unguis
Encyclopedia
Pilina unguis is an extinct species of Paleozoic Silurian
monoplacophora
n. It was first named as Tryblidium unguis and described by Gustaf Lindström
in Latin from the Silurian deposits of Gotland
in Sweden, in 1880.
The shell
has an obovate outline, anteriorly acuminate, posteriorly expanded, with the greatest width somewhat behind the median, transverse axis of the shell. It is regularly and moderately convex, being the most elevated near the median transverse axis.
The apex
is close to the anterior margin and only very little prominent. The surface of the shell is covered with thread-fine ornamental lines parallel to the regular concentric lines of growth, somewhat interrupted by deeper sulci. It is generally even, excepting some wavy, irregular, shallow, longitudinal furrows.
The aperture
is oval with comparatively thin margins, somewhat reflexed outwards. When seen from the side it forms a moderately elevated arch, being highest near the median line of the shell. The central space of the interior surface is surrounded by an oval ring of six pairs of muscular
scars, open anteriorly. The posterior ones are narrow and elongate, the middle ones are transversally broad and much enlarged towards the margin of the shell. The surface of these pairs is smooth or only partially scrobiculated. The uppermost pair is more complicated and consists, as in the kindred species, of a more shallow, elongated interior portion, and of a larger exterior one, from the inner corner of which there projects a narrow sinuous groove, directed obliquely upwards and leaving a small, smooth space between itself and the opposite similar one. The lower, interior part of this upper, muscular scar is pearshaped, wide below, with a narrow, stalklike neck upwards and its surface is finely reticulated by shallow pits and intervening ridges. Some dark, narrow streaks are directed from the enclosed central space of the shell towards the interstices between the muscular scars. The whole central part of the shell inside the scars and to their outer edges is of dark color, while the outer border
is lighter.
Near the apex there is on the inside a little oval depression or pit, which quite resembles a scar, filled up, as if there had been a foramen. Lindström considered this as the mark of the outlines of the initial shell, where now the apex is seen on the outside.
The shell is very thin, scarcely exceeding 0.5 mm in thickness and composed of thin, glossy lamellas, which are not perforated by any parasite.
. It has
been found in several specimens in Fårö
at Lutterhorn, Lansa
and Norsholm
, in Svarfvare huk, the canal near Westöös in Hall, Westkinde, Häftingsklint, Lummelund, Stor Wede in Follingbo, the limestone cliffs near Visby
and Kyrkberget in Visby. It has only been found in the strata b and c or the upper and lower limestone
and never in the shale of a. This is rather unexpected as the same species or at least a nearly allied variety, as mentioned above, has been found in the Lower Silurian strata of Estonia
at Borkholm and also in the Upper Silurian at Koik, in the »Jördensche Schicht». These are only a little more elongate and not so enlarged as the specimens from Gotland.
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
monoplacophora
Monoplacophora
Monoplacophora, meaning "bearing one plate", is a polyphyletic class of mollusks with a cap-like shell, living on the bottom of deep sea. Extant representatives were unknown until 1952; previously they were known only from the fossil record.- Definition :...
n. It was first named as Tryblidium unguis and described by Gustaf Lindström
Gustaf Lindström
Gustaf Lindström was a Swedish paleontologist.He was born in Visby on Gotland. In 1848 he entered Uppsala University, and in 1854 he took his doctor's degree...
in Latin from the Silurian deposits of Gotland
Gotland
Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...
in Sweden, in 1880.
Shell description
The length of the shell is 59–67 mm, width 47–51 mm and the height of the shell is 15–16 mm. With is shell size up to 67 mm was at least twice larger than any recent known living monoplacophoran species.The shell
Mollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
has an obovate outline, anteriorly acuminate, posteriorly expanded, with the greatest width somewhat behind the median, transverse axis of the shell. It is regularly and moderately convex, being the most elevated near the median transverse axis.
The apex
Apex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
is close to the anterior margin and only very little prominent. The surface of the shell is covered with thread-fine ornamental lines parallel to the regular concentric lines of growth, somewhat interrupted by deeper sulci. It is generally even, excepting some wavy, irregular, shallow, longitudinal furrows.
The aperture
Aperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
is oval with comparatively thin margins, somewhat reflexed outwards. When seen from the side it forms a moderately elevated arch, being highest near the median line of the shell. The central space of the interior surface is surrounded by an oval ring of six pairs of muscular
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
scars, open anteriorly. The posterior ones are narrow and elongate, the middle ones are transversally broad and much enlarged towards the margin of the shell. The surface of these pairs is smooth or only partially scrobiculated. The uppermost pair is more complicated and consists, as in the kindred species, of a more shallow, elongated interior portion, and of a larger exterior one, from the inner corner of which there projects a narrow sinuous groove, directed obliquely upwards and leaving a small, smooth space between itself and the opposite similar one. The lower, interior part of this upper, muscular scar is pearshaped, wide below, with a narrow, stalklike neck upwards and its surface is finely reticulated by shallow pits and intervening ridges. Some dark, narrow streaks are directed from the enclosed central space of the shell towards the interstices between the muscular scars. The whole central part of the shell inside the scars and to their outer edges is of dark color, while the outer border
is lighter.
Near the apex there is on the inside a little oval depression or pit, which quite resembles a scar, filled up, as if there had been a foramen. Lindström considered this as the mark of the outlines of the initial shell, where now the apex is seen on the outside.
The shell is very thin, scarcely exceeding 0.5 mm in thickness and composed of thin, glossy lamellas, which are not perforated by any parasite.
Distribution
This species has nearly the same geographical distribution as Tryblidium reticulatumTryblidium reticulatum
Tryblidium reticulatum is an exctinct species of a paleozoic Silurian monoplacophoran.This species have been discovered and originally described by Gustaf Lindström from Silurian of Gotland in Sweden in 1880...
. It has
been found in several specimens in Fårö
Fårö
Fårö is a small Baltic Sea island north of the island of Gotland, off Sweden's southeastern coast. It is the second-largest island in the province. It has a population of fewer than 600 and has become a popular summer resort. The island has no banks, post offices, medical services or police...
at Lutterhorn, Lansa
LANSA
LANSA is a development environment for generating applications on multiple computer systems.LANSA consists of an entire family of integrated software development tools....
and Norsholm
Norsholm
Norsholm is a locality situated in Norrköping Municipality, Östergötland County, Sweden with 573 inhabitants in 2005. It lies around 15 kilometres southwest of Norrköping....
, in Svarfvare huk, the canal near Westöös in Hall, Westkinde, Häftingsklint, Lummelund, Stor Wede in Follingbo, the limestone cliffs near Visby
Visby
-See also:* Battle of Visby* Gotland University College* List of governors of Gotland County-External links:* - Visby*...
and Kyrkberget in Visby. It has only been found in the strata b and c or the upper and lower limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
and never in the shale of a. This is rather unexpected as the same species or at least a nearly allied variety, as mentioned above, has been found in the Lower Silurian strata of Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
at Borkholm and also in the Upper Silurian at Koik, in the »Jördensche Schicht». These are only a little more elongate and not so enlarged as the specimens from Gotland.