Actinopterygii

Cheirolepis and Cladistians

Taxa on This Page

1. Actinopterygii
2. Cheirolepis X
3. Cladistia
4. Guildayichthyiformes X
5. Polypteriformes

Polypteriformes

The Polypteriformes contains the single family Polypteridae, with the single genus Polypterus.  It is known only from the Pleistocene and Recent, although this lineage obviously goes back much further in time.

Polypterus, also called "the bichir" is the most primitive living Actinopterygians. Eleven species of bichirs inhabit shallow floodwater areas in tropical Africa rivers, where they feed on worms, insect larvae, and small insects. This prehistoric looking fish resembles early actinopterygians from the Devonian in having a covering of thick rhombiodal scales, hardly overlapping but connected by fibres. The scales are covered with ganoin, a dense shiny substance like enamel, which gives the fish an archaic armoured appearance and makes it rather clumsy.

The paired lung-like swim bladders are connected to the esophagus and are used for respiration; the fish die if denied access to the surface to gulp air. Conversely though, the fish is able to survive for hours out of water. Compare with the lungfish, another example of living fossil. In the intestine there is a spiral valve, like that in the dogfish (a type of shark), which is not present in more advanced bony fishes. The skeleton of bichirs is mostly cartilage (again like the shark). APW 040806.

Descriptions

Actinopterygii:  ray-finned fish

Range: from the Late Silurian or Early Devonian

Phylogeny: Osteichthyes: Sarcopterygii + *: Cheirolepis + (Cladistia + Actinopteri).

Description: acrodine cap on teeth; premaxilla fused to other bones; maxilla projects posterodorsally; methyostylic jaw suspension (i.e. based on hyostylic, but palatoquadrate and Meckel's cartilage replaced by dermal bone, hyomandibular reduced to jaw element); single median postrostral; single nasal; 4 sclerotics; 1 pair of suprascapulars; clavicle elongate; pelvic girdle formed by metapterygium (not homologous with other vertebrates); $ fins supported by fin rays; $ skeleton with bone; scales with or approaching peg & socket articulation; $ scales rhombic, with overlying dentine denticles and cancellous bone; enamel "ganoine") and $ acrodine present.

Links: Fossil Actinopterygii; Introduction to the Actinopterygii; Actinopterygii; Animal Diversity Web: Class Actinopterygii; Actinopterygii; Peixes marinhos dos Açores; Class Osteichthyes; actinopterygii.htm Japanese version; see also English version); URMO: Actinopterygii; Bony fish showcase; Biology 356; IWR: Taxa: Actinopterygii; ISIS - taxon actinopterygii.

Note: some of the features in the "description" section are taken from Lund's (2000) synapomorphies of his "basal node" (#23) which is probably not quite Actinopterygii, but close, probably corresponding to the Late Devonian crown group of actinopterygians.

APW 040806.

Cheirolepis:

Description: Possibly earliest actinopterygian with "standard" dermal skull bones; large orbit; long (~50 cm) body; broad-based pelvic fin; fins other than pelvic fin long; strongly heterocercal; fringing scales small or absent; $ minute scales with two ganoine (probably same as enamel) layers; acrodine absent.

Links: Cheirolepis.

Image: Cheirolepis by 19th-century paleontologist and journalist Hugh Miller, reproduced courtesy of University of California Museum of Paleontology. APW 040806.

Cladistia:

Range: from the Early Carboniferous

Phylogeny: Actinopterygii:: Actinopteri + *: Guildayichthyiformes + Polypteriformes.

Description: $ rostral present as separate bone(s); $ postrostrals median & paired; $ maxilla narrow posteriorly; $ median gular absent; $ lateral gulars extended; $ postspiracular bones present; $ parasphenoid extends length of braincase; $ clavicles reduced or absent; $ pectoral fin based on extended lobe; $ caudal fin outline rounded.

Image: Discoserra pectinodon from the Lower Carboniferous of Montana, from Lund (2000).

Links: Untitled Document; Evolution der Fische German); Cladistia Cope, 1871 after Lund, 2000; Lund; Lecture 2; Untitled Document; New specimens of Serenoichthys kemkemensis; Cladistia.

References: Lund (2000). APW 040806.

Guildayichthyiformes: Discoserra, Guildayichthys. Deep-bodied marine fish with strong medial bones and unique cheek area.

Range: Early Carboniferous of North America

Phylogeny: Cladistia: Polypteriformes + *.

Description: laterally compressed, discoidal form; mouth small; premaxillae paired and not sutured in midline and $ loosely attached; maxilla does not extend posteriorly past mid-orbit; 8-10 sclerotic bones; 2-3 rows of paired bones over orbit, including supraorbitals; suborbitals ($) in >1 row, numerous, variable within species; infraorbital series of 6-10 bones lacking a specialized postorbital; opercular & gular bones unique; $ narrow dorsal and ventral preoperculars; interopercular bones or rays between preopercular and branchiostegal rays; single large opercular; $ subopercular absent; few branchiostegal rays, not extending forward under mandible; median skull roof bones strongly developed, often unpaired (the rostral, posterior postrostral, and supraoccipital are unpaired in Guildayichthys); supraoccipital ($) prominent on skull roof, dividing parietals and posterior of frontals; parietals similar size to frontals; 3 bones in otic canal series; braincase ossified as several separate bones; parasphenoid extends entire length of braincase; spine with enlarged thoracic supraneurals articulating with $ stiff, overlapping median dorsal scales also providing very strong base for median musculature); caudal fin somewhat heterocercal; $ clavicle absent; fin rays well spaced; $ dorsal fin long & low; $ dorsal fin base scaled & lobate (as are paired fin bases); tall, rhombic, ganoid scales with peg & socket joints.

Note: Lund (2000) believes that the guildayichthyiforms were specialized to have strong medial bones concentrating the force of the bite in the small antorbital jaw and permitting high maneuverability in "geometrically complex environments." The sister group relationship with the polypteriforms is very strong and stable.

Image: Guildayichthys carnegiei modified from Lund 2000). The labels on the image reflect my surprise at the identification of the parietal. However, Lund makes a very convincing case that the large, unpaired supraoccipital is homologous to the 'B' bone of Dipnoi see the diagram at that entry -- the 'J' and 'I' bones are the parietals and postparietals, respectively). See also image at Cladistia.

Reference: Lund (2000). APW 040806.

Polypteriformes: Polypterus (bichirs), Erpetoichthys (reedfish).

Phylogeny: Cladistia: Guildayichthyiformes + *.

Description: Moderate-sized fresh water African fish with unique fins; maximum length about 90 cm, most species less than 30 cm; maxilla firmly united to skull; spiracular opening large but canal lost; pineal plate absent; a pair of gular plates, no branchiostegals; acrodine cap on teeth; body elongated, almost eel-like; well-ossified skeleton; vertebrae with ossified centra and neural canal; dorsal fin consisting of 5-18 finlets, each with a single spine to which is attached one or more soft rays; modified heterocercal tail, superficially symmetrical; postcleithrum differentiated from body scales; pectoral fin rays supported by numerous ossified radials which attach to a cartilaginous plate and two rods, thence to the scapula and coracoid; covered by thick, interlocking rhomboid ganoid scales; pectoral fins fleshy (apparently convergent with sarcopterygians) larval forms may have external gills; adults have two ventral lungs and are obligate air-breathers; air recoil breathing; intestine with spiral valve; inhabit edges of streams and flood plains, concealed by day, forage for worms, insect larvae, small fishes by night.

Note: The polypteriforms anchor the crown group Actinopterygii at what is believed to be a very early stage in actinopterygian evolution, and a wide phylogenetic gap lies between the polypteriforms and the next living group (the Chondrostei). Unfortunately, the polypteriforms have little fossil record and are clearly highly specialized. Under these circumstances it is not clear that the proposed synapomorphies for the crown group Actinopterygii have a great deal of credibility. The recent discovery of the Guildayichthyiforms adds substantial clarity to the picture and may change our idea of the Actinopterygii considerably.

Links: Erpetoichthys calabaricus (Rope Fish): Narrative; Bichir; Orders Summary; (Polypteriformes) - MavicaNET useful entry point to some good Russian sites, otherwise hard to find); Let's Talk About Fishes with Mr.Kaow (Thai); Flösselhechte German); Which came first, the lung or the breath- interesting & controversial abstract); Serwis akwarystyczny Polish: species level data from the point of view of an aquariist). APW 040806.