Acanthodii or acanthodians is an extinct class of
gnathostomes (jawed
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% of ...
es), typically considered a
paraphyletic group. They are currently considered to represent a
grade
Grade most commonly refers to:
* Grade (education), a measurement of a student's performance
* Grade, the number of the year a student has reached in a given educational stage
* Grade (slope), the steepness of a slope
Grade or grading may also ref ...
of various fish lineages leading up to the
extant Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
, which includes living
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s,
rays
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
, and
chimaeras. Acanthodians possess a mosaic of features shared with both
osteichthyans (bony fish) and chondrichthyans (cartilaginous fish). In general body shape, they were similar to modern sharks, but their
epidermis was covered with tiny rhomboid platelets like the scales of
holostei
Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by a single living species, the bowfin ('' Amia calva''), as well as the Ginglymodi, the sole living representatives being the gars ...
ans (
gar
Gars are members of the family Lepisosteidae, which are the only surviving members of the Ginglymodi, an ancient holosteian group of ray-finned fish, which first appeared during the Triassic, over 240 million years ago. Gars comprise seven livin ...
s,
bowfins). A lower Silurian
species
In biology, a species is the basic unit of 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 the appropriate s ...
, ''
Fanjingshania renovata
''Fanjingshania'' is an extinct genus of acanthodian from the lower 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 beginnin ...
'', attributed to
Climatiiformes is the oldest chondrichthyan with known anatomical features.
The popular name "spiny sharks" is because they were superficially shark-shaped, with a streamlined body, paired
fins
A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...
, a strongly upturned tail, and stout, largely immovable bony spines supporting all the fins except the tail—hence, "spiny sharks". However, acanthodians are not true sharks; their close relation to modern cartilaginous fish can lead them to be considered "
stem-sharks". Acanthodians had a
cartilaginous
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...
skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. As a result, fossilized
spines and scales are often all that remains of these fishes in ancient
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s. The earliest acanthodians were marine, but during the
Devonian, freshwater species became predominant.
Acanthodians have been divided into four orders:
Acanthodiformes
Acanthodiformes is an order of acanthodian fishes which lived from the Early Devonian to Early Permian.
Subtaxa
* Family Acanthodidae
** Genus ''Acanthodes
''Acanthodes'' (from el, ἄκανθώδης , 'provided with spines') is an ex ...
,
Climatiiformes,
Diplacanthiformes
Diplacanthiformes (also known as Diplacanthida, Diplacanthoidei, or Diplacanthini) is an order of acanthodian fishes which lived during the Devonian Period
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60. ...
, and
Ischnacanthiformes.
"Climatiiformes" is a paraphyletic assemblage of early acanthodians such as
climatiids,
gyracanthids, and
diplacanthids; they had robust bony shoulder girdles and many small sharp spines ("intermediate" or "prepelvic" spines) between the pectoral and pelvic fins. The climatiiform subgroup Diplacanthida has subsequently been elevated to its own order, Diplacanthiformes. Ischnacanthiforms were predators with tooth plates fused to their jaws. Acanthodiforms were
filter feeders with a single dorsal fin, toothless jaws, and long
gill raker
Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...
s. They were the last and most specialized off the traditional acanthodians, as they survived up until the
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...
period.
Characteristics
The
scales of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.
Despite being called "spiny sharks," acanthodians predate sharks. Scales that have been tentatively identified as belonging to acanthodians, or "shark-like fishes" have been found in various Ordovician strata, though, they are ambiguous, and may actually belong to jawless fishes such as
thelodonts. The earliest unequivocal acanthodian fossils date from the beginning of the
Silurian Period, some 50 million years before the first sharks appeared. Later, the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the
Devonian and in the
coal swamps of
Carboniferous. By this time
bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...
times (approximately 250 million years ago).
Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes. Although their interior
skeletons were made of
cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower
shoulder girdle
The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of ...
. Others developed a bony flap over the gill openings analogous to the
operculum in later bony fishes. However, most of these characteristics are considered homologous characteristics derived from common
placoderm ancestors, and present also in basal
cartilaginous fish. Overall, the acanthodians'
jaws are presumed to have evolved from the first
gill arch
Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arc ...
of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage.
Taxonomy and phylogeny
In a study of early jawed vertebrate relationships, Davis ''et al.'' (2012) found acanthodians to be split among the two major clades
Osteichthyes (bony fish) and
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
(cartilaginous fish). The well-known acanthodian ''
Acanthodes
''Acanthodes'' (from el, ἄκανθώδης , 'provided with spines') is an extinct genus of spiny shark. Fossils have been found in Europe, North America, Asia, and Australia. ''Acanthodes'' was most common in the Carboniferous and Early Perm ...
'' was placed within Osteichthyes, despite the presence of many chondrichthyan characteristics in its braincase.
However, a newly described Silurian
placoderm, ''
Entelognathus
''Entelognathus primordialis'' (“primordial complete jaw”) is a maxillate placoderm from the late Silurian (Ludlow epoch) of Qujing, Yunnan, 419 million years ago.
A team led by Min Zhu of the Academy of Sciences' Institute of Vertebrate Pa ...
'', which has jaw anatomy shared with
bony fish and
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s, has led to revisions of this phylogeny: acanthodians were then considered to be a paraphyletic assemblage leading to cartilaginous fish, while bony fish evolved from placoderm ancestors.
Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to ''
Doliodus'' and ''
Tamiobatis''.
[ A 2017 study of ''Doliodus'' morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a ]transitional fossil
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...
and further reinforcing this idea.
References
Further reading
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External links
Acanthodii taxonomy
*
{{Authority control
Prehistoric fish classes
Teleostomi
Permian fish
Silurian fish
Silurian first appearances
Permian extinctions
Taxa named by Richard Owen
Paraphyletic groups