Acanthodiformes (alternatively spelled Acanthodida) is an order of "Acanthodii" which lived from the Early Devonian to the Early Permian. Members of the order have been found worldwide in rocks preserving both Freshwater fish and Saltwater fish environments, and are distinguished from other acanthodians by the presence of only a single dorsal fin and dorsal fin spine, and in most members a lack of Tooth and well-developed Gill raker. Some acanthodiforms are presumed to have fed by Filter feeder, and had large mouths and Branchial arch. While they have been suggested to be close relatives of modern Osteichthyes due to similarities in their skulls, recent research indicates that, like other acanthodians, they are more likely to be stem-group Chondrichthyes.
Classification
The order was first established by
Soviet Union Ichthyology Lev Berg in 1940, and contained only the family
Acanthodidae.
Later authors considered the order to also include the families
Mesacanthidae and
Cheiracanthidae, and sometimes opted for the alternate spelling "Acanthodida".
In 1979, Robert Denison considered the Mesacanthidae and Cheiracanthidae to be evolutionary grades, and considered them part of the family Acanthodidae. This classification scheme again limited the order to a single family, but Denison suggested further families could be established if the evolution of the group were better understood.
In a 1995 paper, paleontologist Jaroslav Zajic suggested the family
Howittacanthidae should also be included within the order because of similarities between the genus
Howittacanthus and members of Acanthodidae.
As defined by the Fifth Edition of Fishes of the World (2016), the order includes the distinct families Acanthodidae, Mesacanthidae and Cheiracanthidae. The family Howittacanthidae is not recognized here, and
Howittacanthus is considered a member of Acanthodidae under this classification scheme.
Below is a comprehensive list of acanthodiform genera, compiled from the Paleobiology Database, Handbook of Paleoichthyology volume 5 (1979), and organized in accordance with Fishes of the World (2016).
Description
As defined by Leo Berg, the family and order encompass the acanthodians with a single dorsal fin, no additional fins or spines on the underbelly between the
Pectoral fin and
Pelvic fin (termed intermediate fins),
Thorax that lack
Dermis bones, a single operculum that covers the
Gill, and that lacked teeth.
Members of the family Mesacanthidae, which are now also included in the order, possessed a single pair of intermediate fins and multiple gill covers and are assumed to be primitive members of the group.
The genus
Acanthodopsis uniquely possessed tooth-like structures in its jaws, which seem to have developed independently of typical acanthodian teeth.
In members of Acanthodidae, scales are restricted to the
lateral line and head in some taxa, while the rest of the body is scaleless. The cheiracanthids retained a scaly covering over their entire bodies.
The skull is entirely cartilaginous in mesacanthids,
while in cheiracanthids and acanthodids the skull is reinforced by a coating of bone (termed perichondral bone).
In later, more specialized acanthodids, the gill rakers have become heavily modified,
and their
Otolith were well-developed.
Some members have been characterized as very slender and elongated, with proportionally long bodies relative to the size of the head.
Research history
has a significantly more complete fossil record than any other group of acanthodians, and a much greater percentage of acanthodiform fossils preserve complete skeletons or soft tissue compared to other acanthodian orders.
In particular, species of the genus
Acanthodes historically represented the only acanthodian for which uncrushed, well-preserved skeletal anatomy was known,
and formed the primarily basis for the study of acanthodian
Endoskeleton.
Due to similarities between the skull of
Acanthodes bronni and early members of
Osteichthyes (bony fish) it was often concluded that the acanthodians as a group were related to the osteichthyans within the clade
Teleostomi.
However, recent research indicates that the class Acanthodii is instead a stem-group that was ancestral to
Chondrichthyes (cartilaginous fish), and that similarities between the skulls of
Acanthodes and bony fish resulted from convergent evolution and had skewed the perception of acanthodians as a group.
Paleobiology and extinction
Members of the Acanthodiformes show adaptations for filter-feeding, such as large, wide mouths and many gill rakers modified for straining small prey from the water.
Members of the order are known from both freshwater and marine habitats,
although the Late Carboniferous and
Cisuralian freshwater species are the best represented in the fossil record.
The diet of most acanthodiforms likely consisted of small
Crustacean and other
zooplankton, but
Acanthodopsis may have been a
Predation based on its large tooth-like structures. The slender bodies and flexible pectoral regions suggest that members of the group were fast, agile swimmers, and their well-developed otoliths were most likely adaptations towards a good sense of balance.
The Acanthodiformes first appeared during the Early Devonian, and members of the family Acanthodidae survived into the Early Permian of Europe and North America.
