Halecomorphi is a

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

of ray-finned

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

in the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Neopterygii Neopterygii (from Greek νέος ''neos'' 'new' and πτέρυξ ''pteryx'' 'fin') is a subclass of ray-finned fish (Actinopterygii). Neopterygii includes the Holostei and the Teleostei, of which the latter comprise the vast majority of extant ...

. The only extant Halecomorph species are the

bowfin The ruddy bowfin (''Amia calva'') is a ray-finned fish native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being one of only two surviving species ...

(''Amia calva'') and eyespot bowfin ('' Amia ocellicauda''), but the group contains many extinct

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), ...

in several

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

(including

Amiidae The Amiidae are a family of basal ray-finned fishes. The bowfin and the eyespot bowfin ('' Amia ocellicauda'') are the only two species to survive today, although additional species in all four subfamilies of Amiidae are known from Jurassic, Cre ...

Caturidae Caturidae is an extinct Family (biology), family of predatory Amiiformes, amiiform Actinopterygii, ray-finned fish, being the sister-group to the extant family Amiidae. Though their body form is very different than the modern bowfin, a number of f ...

, Liodesmidae, Sinamiidae) in the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Amiiformes The Amiiformes order (biology), order of fish has only two extant taxa, extant species, the bowfins: ''Amia calva'' and ''Amia ocellicauda'', the latter recognized as a separate species in 2022. These Amiiformes are found in the freshwater syste ...

, as well as the

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 ...

orders Ionoscopiformes, Panxianichthyiformes, and Parasemionotiformes. The fossil record of halecomorphs goes back at least to the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 251.9 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

The Halecomorphi exhibit a combination of

ancestral An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

features, such as most heavily mineralized

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

, but also by more derived or "modern" features, particularly in the structure of the skull (e.g. position and shape of preopercles). Unique derived traits (

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...

) of the Halecomorphi include: *Unique jaw articulation in which the quadrate and symplectic participate in the joint. *Lengthened dorsal fins (in some species) *Two biconcave

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

per segment in the posterior body region (a condition known as diplospondyly) *Fan like arrangement of small bones ( hypurals) in the tail.

Systematics and phylogeny

On the systematic position of the Halecomorphi, there are two competing

hypotheses A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific method, scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educ ...

: *The Halecostomi hypothesis proposes Halecomorphi as the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

, the major group of living neopterygians, rendering the

Holostei Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by the single living genus, '' Amia'' with two species, the bowfins (''Amia calva'' and '' Amia ocellicauda''), as well as the Gin ...

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. *The Holostei hypothesis proposes Halecomorphi as the sister group of Ginglymodi, the group which includes gars (

Lepisosteidae Gars are an ancient group of ray-finned fish in the Family (biology), family Lepisosteidae. They comprise seven living species of fish in two genera that inhabit Fresh water, fresh, Brackish water, brackish, and occasionally marine waters of eas ...

) and their fossil relatives, rendering the Halecostomi paraphyletic. The latter hypothesis is more widely accepted. The following

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

summarizes the evolutionary relationships of

(indicated with a

dagger A dagger is a fighting knife with a very sharp point and usually one or two sharp edges, typically designed or capable of being used as a cutting or stabbing, thrusting weapon.State v. Martin, 633 S.W.2d 80 (Mo. 1982): This is the dictionary or ...

, †) and

living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** Living species, one that is not extinct *Personal life, the course of an individual human's life * ...

orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

of Halecomorphi.

References

* Brian J. Gardiner, John G. Maisey, D. Tim J. Littlewood:
Interrelationships of Basal Neopterygians.
' S. 117-146 in: Melanie L. J. Stiassny, Lynne R. Parenti, G. David Johnson (Hrsg.): ''Interrelationships of Fishes.'' Academic Press, 1996, {{Taxonbar, from=Q4040348 Vertebrate unranked clades Taxa named by Edward Drinker Cope