''Sinotubulites'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small, tube-shaped shelly fossils from near the terminal boundary of the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

period in formations within

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

and

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. It is often found in association with '' Cloudina'', and much like ''Cloudina'', is important to studies done on the Cambrian Explosion.

Discovery and naming

The first fossil material for ''Sinotubulites'' was found in the

Dengying Formation The Dengying Formation is an upper Ediacaran (551-541 Ma) fossiliferous geologic Formation (geology), formation found in South China. It was deposited on a shallow marine carbonate platform. Members Listed by ascending age: * The Hamajing Member ...

's Baimatuo Member, Yangtze Gorge

South China South China ( zh, s=, p=Huá'nán, j=jyut6 naam4) is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is ...

, and formally described in 1977 as ''?Cloudina sp.'', but was redescribed and named in 1981. The generic name ''Sinotubulites'' is derived from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

words ''sino'', to mean "China"; and ''tubulus'', to mean "tube"; and the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

suffix ''-ites''.

Description

''Sinotubulites'' is a tubular organism, which is noted to have a "tube-in-tube" structure composed of four to seven thin layers, with most specimens being straight or slightly curved, with openings at either end of all specimens. It has also been split up into "Small", "Medium", and "Large" types depending on width, but excluding length, with the longest known specimens coming in at in length. "Large" types have a width of , and consist of irregular polygonal cross-sections, with regular spaced annulations and troughs along the tube surface. "Medium" types have a width of , and range from having finely spaced annulations to irregular annulations along the tube surface. "Small" types have a width of , consisting of a more circular cross-section, and a smooth outer surface.

Preservation

Due to the wide distribution of ''Sinotubulites'', it also has a wide range of preservation. Tubes found in the Yangtze Gorges region of South China generally preserve

silicified In geology, silicification is a process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and abundant com ...

or dolomitized, leading to the interpretation that the tubes may have been

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...

in life, which was replaced by

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

during fossilisation. Tubes found in southern Shaanxi region preserve as

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

, silica and phosphate, or silica tubes within a dolomite matrix. Fully phosphatic specimens show much more weaker layered structures, whilst silica preserved tubes show more clear multi-layered walls. Specimens composed of both silica and phosphate show coating of the latter mineral on the surface of silica layers, replacing wall layers and infilling the spaces between the silica layers. Tubes found within North America are found preserved as calcareous tubes, with some specimens noted to have irregular and blotchy silica overgrowths, most likely due to the infilling of voids caused by diagenetic dissolution from the surrounding matrix, which consists of carbonate minerals. A recent study has further supported that tubes being made of a calcareous mineral, and going further to suggest

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

as a likely tube mineral, and also noted that close analysis on sections of tube have shown that the tubes first underwent silicification, and then phosphatization, both of which greatly preserve surviving cells and tissues in high detail.

Ecology

''Sinotubulites'' and ''Cloudina'' (discovered in 1972) are currently the two earliest known fossils of organisms that mineralized shells when alive, and are often found in the same fossil beds. It has been noted that ''Cloudina'' specimens often have tiny holes bored in them, which are attributed to predators, while no such borings have been found in ''Sinotubulites''. This suggests that ''Sinotubulites'' had evolved features that made it a much less attractive target than ''Cloudina''. As a result, both organisms are important in analyses of the Cambrian explosion, as predation and the appearance of mineralised components are often cited as possible causes of the "explosion". As for the mode of living for ''Sinotubulites'', it is noted that while specimens in Shaanxi are transported, hampering the ability to obtain ecological information from them, whilst specimens within the Dengying Formation are more likely to be preserved in situ. It is noted that the Dengying tubes lie parallel to the bedding plane in which they are preserved on, and their general morphology may give ecological clues as well. All specimens are open at either end of the tube, along with no attachment structures being found to date, indicating that neither end was attached to the substrate. The general structure of the tube itself is also more suited for laying on the substrate rather than standing, preventing currents from rolling the tube, suggesting a procumbent epifaunal lifestyle.

Taxonomy

First assigned to ''Cloudina'' in 1977 under ''?Cloudina sp.'' due to similarities between the two genera, and redescribed under a new genus in 1981, ''Sinotubulites'' has had 7 species assigned to it, although currently only 5 are considered valid :

References