''Forfarella'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

chasmataspidid Chasmataspidids, sometime referred to as chasmataspids, are a group of extinct chelicerate arthropods that form the order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida), with ...

, a group of extinct aquatic

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

Fossil 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 ...

s have been discovered in

deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, current accounts or any of several other types of accounts explained below. ...

of the Early

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. The single and

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...

, ''F. mitchelli'', is known from one only specimen found in

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...

, in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...

. Known as BMNH In 60023, it is poorly preserved and its type locality is uncertain, although it might be the Kelly Den stream section near the

village A village is a clustered human settlement or community, larger than a hamlet but smaller than a town (although the word is often used to describe both hamlets and smaller towns), with a population typically ranging from a few hundred to ...

Arbirlot Arbirlot (Gaelic: ''Obar Eilid'') is a village in a rural parish of the same name in Angus, Scotland. The current name is usually presumed to be a contraction of Aberelliot''Statistical Account of Scotland'', edited by Sir John Sinclair of Ulbster ...

. ''Forfarella'' was very small, only reaching of length. It had a nearly rectangular head, with its

eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and c ...

being unknown but possibly represented by a

tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projection ...

in the fossil. Its

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the tors ...

consisted of an almost trapezoidal preabdomen and a long, tapering postabdomen. The

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...

(the posteriormost division of the body) is not preserved, but it was probably short. The

appendages An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including antenn ...

, known only from a few fragments, probably had swimming paddles that ''Forfarella'' used to swim actively. The fossil of ''Forfarella'' was purchased in 1893 from a fossil collection. It was sent to the

Natural History Museum A natural history museum or museum of natural history is a scientific institution with natural history collections that include current and historical records of animals, plants, fungi, ecosystems, geology, paleontology, climatology, and more ...

London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...

. Decades later, in 1962, a

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...

studied it and determined that it was a chasmataspidid, naming it ''Forfarella mitchelli''. However, the paleontologist never formally published his findings. It would not be until 1999 when a group of three other paleontologists formally described ''Forfarella''. Forming part of the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Diploaspididae, it was similar to '' Diploaspis'' and other Devonian genera, although it is speculated that ''Forfarella'' may have existed during the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago (Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozoi ...

as well. It was a lacustrine

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

, meaning that it lived in

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much lar ...

Description

Like the other

s, ''F. mitchelli'' was a small

, with the size of the only known specimen estimated at only in length. Its

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the und ...

(

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

plate of the head) was wide and subrectangular (almost rectangular). It was broader than long; wide and long. The details on its surface are not preserved with the exception of a

in the left

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

corner of the carapace, only distinguished if it is placed under

plane polarized light Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of ...

. This tubercle most likely represents a

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

. The

(limbs) are only known from two fragments on both left and right sides, probably belonging to the sixth (and last) pair of them. The left one had a length of and a width of , while the right one was long and wide. Other

chasmataspidids had this pair of appendages modified into swimming "paddles", so it can be assumed that ''Forfarella'' had them as well. The

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...

(

), made up of 13 segments, had a total length of . The preabdomen (segments 1 to 4) was nearly trapezoidal (that is, subtrapezoidal) and narrowed

posteriorly Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

. Vague impressions distinguish the second, third and fourth

tergite A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...

s (dorsal half of the segments), but not the first one. The postabdomen (segments 5 to 13) also tapered posteriorly and was longer, with its 9 tergites distinguishable in the

fossil 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 ...

and with a length of approximately each. The

(the posteriormost division of the body) is unknown, but it was probably short as in other contemporary chasmataspidids. The specimen had two cracks all over the

prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''ce ...

(the head) and preabdomen, as well as dark patches probably corresponding to the

ornamentation An ornament is something used for decoration. Ornament may also refer to: Decoration *Ornament (art), any purely decorative element in architecture and the decorative arts * Biological ornament, a characteristic of animals that appear to serve o ...

History of research

''Forfarella'' is known from one single poorly preserved specimen, BMNH In 60023. It was found in the Dundee Formation (within the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also ext ...

) near

Arbroath Arbroath () or Aberbrothock ( gd, Obar Bhrothaig ) is a former royal burgh and the largest town in the council area of Angus, Scotland, with a population of 23,902. It lies on the North Sea coast some ENE of Dundee and SSW of Aberdeen. ...

, in the

. According to the label associated with the fossil, it was purchased in 1893 from the collection of a person surnamed Mitchell, who, along with other colleagues, was one of the fossil collectors in the area near

Forfar Forfar ( sco, Farfar, gd, Baile Fharfair) is the county town of Angus, Scotland and the administrative centre for Angus Council, with a new multi-million pound office complex located on the outskirts of the town. As of 2021, the town has a po ...

. As they did not usually specify the exact type locality of their material, its tracking was problematic. After studying the fossil's label and comparing it with other fossils from Mitchell's collection that had more details about their origins, it was concluded in a subsequent study that the Kelly Den stream section, south of the

, could be the original site. However, this cannot be confirmed. Following the purchase, the fossil of ''Forfarella'' was not instantly registered in the

like other fossils from Mitchell's collection. In 1962, the British

Charles D. Waterston borrowed the still unregistered specimen, as well as four other fossils of juvenile

eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is ...

s, to describe them. He noted the chasmataspidid nature of what would become the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

of ''Forfarella'', registered it as In 60023 and coined the name ''Forfarella mitchelli''. However, Waterston would never publish his description, returning the fossil to the

museum A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical History (derived ) is the systematic study and th ...

three years later, where it would be labeled with the same name given by the paleontologist. Thus, ''Forfarella mitchelli'' remained as a '' nomen manuscriptum'' (a

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. A ...

name that appears in an informally published document). Despite the turbulent history of the fossil, the paleontologists Jason A. Dunlop, Lyall I. Anderson and Simon J. Braddy formally proposed and described ''Forfarella mitchelli'' for the first time in 1999 as a new

of chasmataspidid. The name suggested by Waterston was kept to avoid future confusion. Anderson also studied a troublesome specimen allegedly at the

National Museum of Scotland The National Museum of Scotland in Edinburgh, Scotland, was formed in 2006 with the merger of the new Museum of Scotland, with collections relating to Scottish antiquities, culture and history, and the adjacent Royal Scottish Museum (opened in ...

(later discovering that it was not there) which could represent another specimen of ''Forfarella''. However, upon examination, it was discovered that it was not a chasmataspidid, but a poorly preserved undetermined arthropod fragment.

Classification

''Forfarella'' is classified as part of the

Diploaspididae, one of the two families in the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Chasmataspidida. It includes one single

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

, ''F. mitchelli'', from the Early Devonian of Scotland. At the time ''Forfarella'' was described, members of Diploaspididae were defined as small chasmataspidids with a subrectangular or semicircular carapace, a tapering postabdomen and a short telson. Excluding the last one, which is uncertain, ''Forfarella'' possessed these features. Regarding other chasmataspidids, ''Forfarella'' was considerably similar to '' Diploaspis casteri'', although it had a longer postabdomen, perhaps because of a

taphonomic Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efrem ...

distortion (that is, a defect product of the fossilization of the

organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...

) of the specimen. The genus also resembled other Devonian chasmataspidids, but differed from the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

Chasmataspis ''Chasmataspis'' is a genus of chasmataspidid, a group of extinct aquatic chelicerate arthropods. It was found in the Early Ordovician deposits of Tennessee, USA. Morphology Just like other chasmataspidids, the body of ''Chasmataspis'' comp ...

'', which was much larger and had genal spines (spines protruding from the posterolateral corners of the carapace). What differentiated ''Forfarella'' from the rest of the chasmataspidids was the dimensions of its body, its size, the shape of its carapace and the distinctive subtrapezoidal preabdomen. Even if ''Forfarella''s affiliation with Diploaspididae is certain, the genus has not been included to date in any

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

analysis or

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

Paleoecology

The specimen of ''Forfarella'' was discovered in Early Devonian

of Scotland. It was found at the Old Red Sandstone, in the Dundee Formation. The fossil is preserved in a gray

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.Blatt ''et al.'' 1980, ...

. Its

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Li ...

(the physical characteristics of the

rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...

of the fossil) is similar to that of the

laminate Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materi ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...

-bearing

beds A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...

Tealing Tealing (Scottish Gaelic: Tèalainn) is a village in Angus in eastern Scotland, nestled at the foot of the Sidlaw Hills. It is just north of the city of Dundee and south of Forfar. With a population of just over 500, scattered across of ferti ...

, a Scottish village near from the suspected type locality. However, the exact location from which it was recollected is still unknown. It is thought that ''Forfarella'', just as other Devonian chasmataspidids, had its sixth pair of appendages modified into paddles. These would have made it an

active Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * Active Records, a record label Ships * ''Active'' (ship), several commercial ships by that name * HMS ''Active'', the name of various ships of the British Royal ...

swimmer. Furthermore, the deposits in which ''Forfarella'' was found were

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

, that is, they formed on the bottom of an ancient

. This type of chasmataspidids subsequently invaded

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does in ...

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

s, maybe due to the

Late Devonian extinction The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch (geology), Epoch, which collectively represent one of the five largest extinction event, mass extinction events in the history of life on Earth. The t ...

event. In 2017, a new species of ''Diploaspis'', ''D. praecursor'', was described. Unlike the other two, this species was

(which goes before the Devonian), extending the fossil range of the genus. This also suggests that there could be specimens from earlier times of other diploaspidids similar to ''Diploaspis'', such as '' Achanarraspis'' or ''Forfarella'', that have yet to be discovered (i.e. a "ghost" range).

References

Citations

Bibliography

* * * {{Taxonbar, from=Q19842642 Diploaspididae Early Devonian first appearances Devonian chasmataspidids Chasmataspidids of Europe Fossils of Scotland Fossil taxa described in 1999