Holostei is a group 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 ...
. It is divided into two major clades, the
Halecomorphi, represented by the single living genus, ''
Amia'' with two species, the
bowfins (''
Amia calva
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 species, relict, being one of only two sur ...
'' and ''
Amia ocellicauda''), as well as the
Ginglymodi, the sole living representatives being the
gars (Lepisosteidae), represented by seven living species in two genera (''
Atractosteus'', ''
Lepisosteus'').
The earliest members of the clade, which are putative "
semionotiforms" such as ''
Acentrophorus'' and ''
Archaeolepidotus'', are known from the
Middle to
Late Permian and are among the earliest known
neopterygians.
Holostei was thought to be regarded as
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 ...
. However, a recent study provided evidence that the Holostei are the closest living relatives of the
Teleostei, both within the
Neopterygii. This was found from the morphology of the Holostei, for example presence of a paired
vomer. Holosteans are closer to teleosts than are the
chondrosteans, the other group intermediate between teleosts and cartilaginous fish, which are regarded as (at the nearest) a sister group to the Neopterygii.
The
spiracles of holosteans are reduced to vestigial remnants and the bones are lightly ossified. The thick
ganoid scales of the gars are more primitive than those of the bowfin.
Characteristics
Holosteans share with other non-teleost
ray-finned fish
Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...
a mixture of characteristics of
teleosts and
shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s. In comparison with the other group of non-teleost ray-finned fish, the
chondrosteans, the holosteans are closer to the teleosts and further from sharks: the pair of spiracles found in sharks and chondrosteans is reduced in holosteans to a remnant structure: in gars, the spiracles do not even open to the outside; the skeleton is lightly
ossified: a thin layer of bone covers a mostly
cartilaginous skeleton in the bowfins. In gars, the tail is still
heterocercal but less so than in the chondrosteans. Bowfins have many-rayed dorsal fins and can breathe air like the
bichirs.
In the holosteans a primary pulmonoid (respiratory)
swim bladder is still present, a trait that was independently lost in both chondrostei and teleostei, the only other two lineages of fish with a swim bladder (in some teleosts the swim bladder have since evolved to become secondarily respiratory again).
The gars have thick
ganoid scales typical of
sturgeons whereas the bowfin has thin bony scales like the teleosts. The gars are therefore in this regard considered more primitive than the bowfin.
The name ''Holostei'' derives from the
Greek words ''holos'', meaning whole, and ''osteon'', meaning bone: a reference to their bony skeletons.
Systematics of Neopterygii
The
evolutionary relationships of gars, bowfin and teleosts were a matter of debate. 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 ...
on the systematics of
neopterygians:
Halecostomi hypothesis
The
Halecostomi hypothesis proposes
Halecomorphi (
bowfin and its fossil relatives) 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 ...
of
Teleostei, the major group of living neopterygians, rendering the Holostei
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 ...
.
Holostei hypothesis
The Holostei hypothesis, where the gars and bowfin form 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 ...
Holostei 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 ...
to Teleostei, is better supported than the Halecostomi hypothesis, rendering the latter paraphyletic. It proposes Halecomorphi as the sister group of
Ginglymodi, the group which includes living
gars (
Lepisosteiformes) and their fossil relatives.
It is estimated that the
last common ancestor of gars and bowfin lived at least 250 million years ago.
Ginglymodi comprises three
orders:
Lepisosteiformes,
Semionotiformes and
Kyphosichthyiformes. Lepisosteiformes includes 1
family
Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
, 2
genera
Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
, and 7
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), ...
that are commonly referred to as gars. Semionotiformes and Kyphosichthyiformes are extinct orders.
Halecomorphi contains the orders
Parasemionotiformes,
Panxianichthyiformes,
Ionoscopiformes, and
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 ...
. In addition to many
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 ...
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), ...
, Amiiformes includes only 1
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 ...
species that is commonly referred to as the bowfin. Parasemionotiformes, Panxianichthyiformes, and Ionoscopiformes have no living members.
Gars and bowfins are found in North America and in
freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
ecosystems. The differences in each can be spotted very easily from just looking at the fishes. The gars have elongated jaws with fanlike teeth, only 3 branchiostegal rays, and a small dorsal fin. Meanwhile the bowfins have a terminal mouth, 10–13 flattened branchiostegal rays, and a long dorsal fin.
Phylogeny of bony fishes

The
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 ...
shows the relationships of holosteans to other living groups of
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 ...
(Osteichthyes), the great majority of which are
teleost
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 ...
s,
and to the terrestrial vertebrates (tetrapods) that evolved from a related group of
lobe-finned fish.
Approximate dates are from Near et al. (2012).
[
]
Notes
References
External links
Holostei on The University of Liverpool website
{{Taxonbar, from=Q846164
Neopterygii
Extant Jurassic first appearances