''Gluteus minimus'' is an extinct species of

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

from the

Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding ...

(

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during ...

Famennian The Famennian is the later of two faunal stages in the Late Devonian epoch. The most recent estimate for its duration is that it lasted from around 371.1 to 359.3 million years ago. An earlier 2012 estimate, still used by the International Commis ...

) of

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

that is the only species within the genus ''Gluteus''. It was first collected in 1902, but only described in 1975 and the holotype is SUI 34978-91. Each animal appears as a bi-lobed lens up to , and almost all specimens share the same slight asymmetry. The species is of uncertain affinities, having been variously thought of as either fish teeth, some sort of

fish scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

s, or

otolith An otolith (, ' ear + , ', a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle (ear), utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule ...

Collection history

Specimens of ''G. minimus'' were first collected by

Stuart Weller Stuart Weller (December 26, 1870 – August 5, 1927) was an American paleontologist and geologist. Biography Stuart Weller was born in Maine, New York on December 26, 1870. Weller studied geology and paleontology at Cornell University with bache ...

in 1902, from the Maple Mill Shale near Maple Mill on the English River, Washington County,

. The Upper Devonian Maple Mill Shale appears as a series of

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

lenses A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

which are rich in fossils, including

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

remains, possible sporocarps,

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

, and

scolecodont A scolecodont is the jaw of a polychaete annelid, a common type of fossil-producing segmented worm useful in invertebrate paleontology. Scolecodonts are common and diverse microfossils, which range from the Cambrian period (around half a billion ...

s. At a similar time to Weller's collections, Charles Rochester Eastman collected specimens from the Kinderhook Beds at Burlington,

Des Moines County, Iowa Des Moines County is located in the U.S. state of Iowa. As of the 2020 census, the population was 38,910. The county seat and largest city is Burlington. It is one of Iowa's two original counties along with Dubuque County; both were organize ...

. C. H. Belanski collected specimens 30 years later from the Lime Creek Formation in

Floyd County, Iowa Floyd County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 15,627. The county seat is Charles City. History Floyd County was established in 1854 and was named for Sergeant Charles Floyd of the Lewi ...

. Thousands of specimens have since been collected from the Maple Mill Shale and nearby sites, and two have been found in a well at Columbus City, Iowa.

Description

''Gluteus minimus'' was

formally described A species description is a formal scientific description of a newly encountered species, typically articulated through a scientific publication. Its purpose is to provide a clear description of a new species of organism and explain how it differ ...

in a paper in the journal ''

Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

'' in 1975 by Richard Arnold Davis and Holmes A. Semken Jr., of the

University of Cincinnati The University of Cincinnati (UC or Cincinnati, informally Cincy) is a public university, public research university in Cincinnati, Ohio, United States. It was founded in 1819 and had an enrollment of over 53,000 students in 2024, making it the ...

and the

University of Iowa The University of Iowa (U of I, UIowa, or Iowa) is a public university, public research university in Iowa City, Iowa, United States. Founded in 1847, it is the oldest and largest university in the state. The University of Iowa is organized int ...

, respectively. They studied more than 3,200 specimens, many of which were deposited at the University of Iowa's Department of Geology and the University of Cincinnati Geology Museum. They referred to the fossils as "

horse collar A horse collar is a part of a horse harness that is used to distribute the load around a horse's neck and shoulders when pulling a wagon or plough. The collar often supports and pads a pair of curved metal or wooden pieces, called hames, to wh ...

s", a name which had been used colloquially among university staff. ''Gluteus minimus'' appears as lenticular (lens-shaped), bilobed fossils, with a size up to . In a sample of 400 specimens, the

median The median of a set of numbers is the value separating the higher half from the lower half of a Sample (statistics), data sample, a statistical population, population, or a probability distribution. For a data set, it may be thought of as the “ ...

diameter was , with a range of . Each fossil has two dissimilar surfaces – one smooth with one or rarely two furrows, and one with conspicuous "growth lines", parallel with the outer edge of the fossil. The overall shape is broadly symmetrical, but the furrow is almost always on the same side of the " re-entrant" (the cleft between the two halves). Opposite the re-entrant is a " salient", which separates two indentations in the margin. The specimens of ''G. minimus'' are made of

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

, comprising 56%

calcium oxide Calcium oxide (formula: Ca O), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term '' lime'' connotes calcium-containing ...

, 39%

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 ...

, 3.5%

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...

, and 1.5%

organic carbon Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...

. Davis and Semken could not rule out the possibility that the apatite was introduced after deposition, since the shell of an animal similar to ''

Bellerophon Bellerophon or Bellerophontes (; ; lit. "slayer of Belleros") or Hipponous (; lit. "horse-knower"), was a divine Corinthian hero of Greek mythology, the son of Poseidon and Eurynome, and the foster son of Glaukos. He was "the greatest her ...

'' in the same sediments had been replaced with apatite. No

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s could be recovered.

Classification

The identity of ''Gluteus minimus'' has been enigmatic since its original discovery. Weller's specimens collected in 1902 are labelled "Fish remains?" in his sister's handwriting, Burlington's specimens are labelled "Cone scales?", and Belanski described them as "apparently dermal ossifications of some fish". While ''Gluteus minimus'' has been interpreted as either a

tooth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tea ...

or a

, it cannot be confidently placed in any

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

, and more recent 3D analyses suggest it may be an

or statolith from a Devonian free-swimming animal rather than an entire organism or the shell of an organism. In 2011, specimens from the same fossil strata in Iowa, and nearly identical to the illustrations of ''Gluteus'' were identified as teeth of the cochliodont "shark", '' Synthetodus trisulcatus''.Fossil Forum: ''Synthetodus''
/ref>

References

External links

* {{Taxonbar, from=Q5572491 Fossil taxa described in 1975 Enigmatic prehistoric animal genera Devonian animals Late Devonian animals Devonian animals of North America