Palaeos Vertebrates 90.200 Teleostomi: Osteichthyes

*Palæos:*		Unit 90: Teleostomi
*The* *Vertebrates*		200: Osteichthyes

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Teleostomi: Osteichthyes

Cheirolepis and Cladistians

Taxa on This Page

Fish Bones

The Devonian is frequently called, with ample reason, the "Age of Fishes." The stage for this explosion of aquatic vertebrates was undoubtedly set in the Silurian, perhaps due to the duplication of ancestral hox genes. But, whatever the cause, the earliest Devonian saw the rapid development of chondrichthyans, placoderms, acanthodians, and the two most successful groups of vertebrates ever known: the actinopterygians and sarcopterygians -- collectively, the bony fish, the Osteichthyes. Much has been written on the unique adaptations of the osteichthyans. Some of these points are summarized in the main entry in these Notes. However, what everyone remembers is that osteichthyans were the first group with endochondral bone.

In the more general vertebrates there are various types of calcified tissues: dentine, enamel (or "enameloids" to be precise) and bone, plus variants, characterized by their ontogeny, chemistry, form and location. However, endochondral bone is said to be unique because it begins life as cartilage. In more basal vertebrates, cartilaginous structures can become superficially calcified in at least two different modes. Chondrichthyans develop microscopic bone platelets which cover the surface of the cartilage to create prismatic calcified cartilage. All gnathostomes and probably some more basal groups can also produce layers of bone over the surface of cartilage to create dense lamellar or perichondral bone. However, the bone-forming osteoblasts are eventually entombed in their own work product, cut off from all living connections, and drowned in their own metabolic wastes -- rather like post-modern philosophers.

In osteichthyans, the circulatory system actually invades the cartilaginous matrix. This permits the local osteoblasts to continue bone formation within the cartilage and also recruits additional, circulating osteoblasts (who have, presumably, been cruising around looking for some action). Other cells gradually eat away at the surrounding cartilage. The net result is that the cartilage is replaced from within by a somewhat irregular vascularized network of bone. Structurally, the effect is to create a relatively lightweight, flexible, "spongy" bone interior, surrounded by an outline of dense, lamellar periostial bone (since this bone now surrounds other bone, rather than cartilage, it is referred to as periostial rather than perichondral). Developmentally, in a sort of teleological, nineteenth century way, the process is viewed as "pre-forming" bone in cartilage, since the effective result is that a structure created in cartilage is replaced by the same structure composed of bone. This is the unique endochondral bone from which the osteichthyans derived their name, as well as countless structural advantages.

The problem here is that, like a lot of things, it ain't necessarily so. That is, endochondral bone is not really unique to the osteichthyans. Endochondral bone is found, for example, in the rhinocapsular ossifications of some arthrodire placoderms. Janvier (1996). In the Chondrichthyes there may be a long-term evolutionary trend to incorporate small, but increasing amounts of endochondral bone. Maisey (1988). There have even been reports of endochondral bone in acanthodians, although it seems that these are now discredited. Smith & Hall (1990). Smith & Hall, as well as Janvier, tend to dismiss these occurrences as not really comparable to the wholesale use of endochondral bone in osteichthyans, and certainly there is a significant quantitative difference. However, it would seem that Maisey has the better argument and that endochondral bone is plesiomorphic, a generalized gnathostome character.

Second, it is not clear that endochondral bone was much of an advantage. If one ignores the aberrant tetrapods, the indiscriminate internal calcification of cartilage may be viewed as a relatively short-lived experiment in evolving a fish strong enough to carry around massive armored scales while maintaining a relatively high degree of mobility. The actinopterygians quickly jettisoned their ganoid scales and thus dispensed with the need for most endochondral bone. Sarcopterygians retained their heavy cosmine scales, and endochondral bone, but -- except for the tetrapods -- became virtually extinct in the process.

Note added in disproof: An excellent textbook writer, who presumably would not wish to be quoted here by name, has kindly suggested that we go back to the drawing board on this essay. APW 040806.

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

Characters: Pattern of cranial bones, rooted teeth, medial insertion of mandibular muscle in lower jaw; head & pectoral girdles covered with large dermal bones; sclerotic ring has 4 plates; labyrinth contains large otoliths; nostrils completely separated; neurocranium frequently divided into separated anterior and posterior sections divided by fissure; lung or swim bladder, pattern of opercular and pectoral girdle dermal bones; no fin spines; lepidotrichia (bone fin rays); endochondral bone, i.e. bone ossified internally, by replacement of cartilage, as well as perichondrally, as "spongy bone."

Note: Zhu et al.'s (1999) description of Psarolepis from the Upper Silurian and Lower Devonian of China threatens to create chaos out the conventional, orderly treatment of this group. Psarolepis shares a number of characteristics previously believed to be unique to actinopterygians or sarcopterygians. In addition, it has several features, such as the fin spines and characters of the shoulder plate, which are associated with placoderms, chondrichthyans, or acanthodians. In overall appearance, Psarolepis most resembles a sarcopterygian, but the plates in isolation look strikingly like placoderm material. Depending on the data set used, PAUP suggests that it is either (a) the sister group of all (other) osteichthyans or (b) part of a paraphyletic Actinopterygii and sister of all (other) Sarcopterygii. The many strikingly primitive features and the pattern of cranial dermal bones probably favor (a), but these are admittedly a poor basis on which to make major phylogenetic decisions. We have elected to follow current fashion and place Psarolepis among the Sarcopterygii, as the sister of all other sarcopterygians.

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.

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.

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.

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).

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

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.

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.

Teleostomi: Osteichthyes

Cheirolepis and Cladistians

Abbreviated Cladogram

Contents

Taxa on This Page

Fish Bones

Polypteriformes

Descriptions