''Agaricocrinus americanus'', also known as the mushroom crinoid or American crinoid, is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

. Its

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

can be found in the U.S. states of

Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...

Tennessee Tennessee (, ), officially the State of Tennessee, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Kentucky to the north, Virginia to the northeast, North Carolina t ...

and

Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

. They date back to the Lower Mississippian, about 345 million years ago.

Fossil beds

One of the locations from which ''Agaricocrinus americanus'' is known is the Edwardsville Formation, in the vicinity of Crawfordsville,

Montgomery County, Indiana Montgomery County is a county in the U.S. state of Indiana. As of the 2020 United States census, it had a population of 37,936. Its county seat is Crawfordsville. The county is divided into eleven townships which provide local services. Mont ...

, in the United States. In this fossil-rich bed have been found the fossils of sixty species of crinoid, distributed among more than forty genera. It is thought that the various species had different length stalks so that they could capture

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

drifting past at various heights above the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

. The fossil beds were formed at a time when the seabed was much higher than it is today. It is believed that the crinoids were buried in sediment from nearby

deltas A river delta is a landform, wikt:archetype#Noun, archetypically triangular, created by the deposition (geology), deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or ...

during storms. The resulting

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

deposits are soft enough for the fossils to be extracted in three-dimensional relief. Other fossils of this species have been found in similar sedimentary rocks in Whites Creek in

Davidson County, Tennessee Davidson County is a county in the U.S. state of Tennessee. It is located in the heart of Middle Tennessee. As of the 2020 census, the population was 715,884, making it the 2nd most populous county in Tennessee. Its county seat is Nashville, ...

, from Osagean rocks in Kentucky, and more recently from the Fort Payne Formation near the boundary between Tennessee and Kentucky. Certain morphological differences have been found between various specimens from these different locations and new finds have often been claimed as new species. David Meyer and William Ausich studied the genus ''Agaricocrinus'' in 1997 and proposed that ''A. arcula'', ''A. dissimilis'', ''A. elegans'', ''A. podagricus'', ''A. ponderosus'', ''A. profundus'' and ''A. tugurium'' should be brought into

synonymy A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

with ''Agaricocrinus americanus''. A closely related but less common species is '' Agaricocrinus crassus''.

Description

extant Extant or Least-concern species, least concern 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 * Exta ...

crinoids, ''Agaricocrinus americanus'' was anchored to a hard surface by a

holdfast Holdfast most often refers to: *Holdfast (biology), a root-like structure that anchors aquatic sessile organisms to their substrate *Holdfast (tool), a tool used to secure a workpiece to a workbench or anvil Holdfast or hold fast may also refer t ...

out of which grew an articulated stalk. On top of this was a

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with a number of feather-like arms. Each arm bore short branches known as pinnules and from these cirri were extended which sifted plankton from the water flowing past.

References

{{Taxonbar, from=Q4691331 History of Missouri Prehistoric crinoids