6. Osteichthyes – Comprehensive Study Notes

Lecture Learning Outcomes

• Section: Fishes
  • Explain why the Devonian is nick-named the “Age of Fishes”.
• Section: Sarcopterygians (lobe-fins)
  • Contrast fin architecture in actinopterygians vs. sarcopterygians.
  • State why the coelacanth is labelled a “living fossil”.
  • List the three surviving sarcopterygian lineages.
• Section: Actinopterygians (ray-fins)
  • Summarise the contribution of actinopterygians to overall vertebrate diversity.

Fundamental Osteichthyan (Bony-Fish) Synapomorphies

• Gas-filled lung initially ventral to, and derived from, embryonic gut; later modified into a swim bladder in many lineages.
• Dermal bone armour with a distinctive jaw/skull pattern that is retained in early tetrapods:
  • Upper jaw: maxilla + premaxilla.
  • Lower jaw: dentary.
• Extensive dermal contribution to the pectoral girdle; dermal bones form an operculum over gills.
• Branchiostegial rays line gill-chamber floor → expand buccal cavity, improve water flow.
• Hard exterior tissues:
  • “Ancestral enamel” on scales + dermal bones.
  • Two specialised scale enameloids recognized later: ganoine and cosmine.

Devonian – “Age of Fishes” (≈$419\text{–}359\,\text{Ma}$)

• Roughly $48\,\text{Myr}$ interval in which all major fish lineages co-existed:
  • Acanthodians, jawless ostracoderms, placoderms, early cartilaginous fishes, and BOTH bony-fish clades.
• First known bony-fish remains resemble acanthodians; diversify early–middle Devonian along two osteichthyan trunks:
  1. Sarcopterygii – rays extend from a central shaft; fin musculature external to body wall.
  2. Actinopterygii – multiple fin rays fan outward; musculature remains in body wall.

Overview of Osteichthyan Phylogeny (Fig. 7.2)

• Basal traits (plesiomorphies):
  • Monobasic paired fins, cosmine covering, endochondral bone, intracranial joint, two dorsal fins, lung ventral to gut.
• Stepwise innovations by clade (selected highlights):
  • Polypteriformes (bichirs) – finlets, everted forebrain hemispheres, ganoine.
  • Acipenseriformes (sturgeons) – reduced endochondral bone, abbreviated heterocercal tail.
  • Neopterygii – reduction of basals, mobile maxilla; sub-groups Holostei and Teleostei.
  • Teleostei – homocercal caudal fin, mobile premaxilla, elasmoid scales, lung modified into dorsal gas bladder.

Ray-Fins vs. Lobe-Fins (anatomical contrasts)

• Basal osteichthyan pectoral fin has propterygium, mesopterygium, metapterygium + distal radials supporting fin rays.
• Sarcopterygian pattern (e.g. Neoceratodus):
  • Monobasic: single proximal element (humerus) joins shoulder.
  • Radials arranged in pre- and post-axial series; muscles extend into the fin lobe.
  • Precursor of tetrapod one-to-two limb pattern ($\text{humerus} \to \text{radius+ulna}$).
• Actinopterygian (teleost) pattern:
  • Multiple parallel radials articulate with pectoral girdle inside body wall – creates light, fanned fin.

Dermal Skeletal & Scale Types (Fig. 7.4)

• Bowfin (Amia) dermatocranium retains classic osteichthyan jaw bones: premaxilla, maxilla, dentary + opercular suite.
• Longnose gar (Lepisosteus)
  • Ganoid scales (enamel-like ganoine over dentine) + abbreviate heterocercal tail.
• Devonian lungfish cosmoid scales – thick cosmine layer with pore canals.

Sarcopterygii – Core Diagnostic Triad

1. Cosmine on dermal bones/scales.
2. Monobasic paired fins with scaled, muscular lobes.
3. Intracranial joint between anterior & posterior braincase.

• Early/mid-Devonian lobe-fins: cylindrical, $20\,\text{cm}$ – >4\,\text{m}; thick scales; two dorsal fins; heterocercal tail.

Surviving Sarcopterygian Lineages

• Dipnoi – lungfishes (6 spp.).
• Actinistia – coelacanths (2 spp.).
• Tetrapodomorph fishes – lead to crown Tetrapoda (~$40{,}000$ terrestrial/secondarily aquatic spp.).

Geological Timeline Snapshot (Fig. 8-1)

• Divergence of extant lungfish families in Carboniferous (≈$359\text{–}299\,\text{Ma}$).
• Coelacanthiformes persist from Devonian through Quaternary.
• Tetrapodomorph sequence: $+Eusthenopteron \to +Panderichthys \to +Tiktaalik \to +Acanthostega$ etc.

Dipnoi (Lungfishes)

• Feeding: tooth-bearing dermal bones lost; palatoquadrate fused to cranium; marginal teeth fused into crushing ridges.
• Fins: dorsal + caudal + anal merge; caudal fin becomes symmetrical.
• Skull roof simplifies from many small plates to few large ones.

Respiration & Circulation

• Lungs vs. Swim Bladders:
  • Both form as gut outpocketings in embryonic pharynx.
  • Lung – primarily gas exchange; walls subdivide (alveolar) or add lobes.
  • Swim bladder – primarily buoyancy.
• Primitive osteichthyan cardio-pulmonary circuit (Fig. 7.6A):
  • Heart pumps mixed blood; pulmonary artery sends deoxygenated blood to lung when gills inactive.
• General teleost condition (Fig. 7.6B): lungs → gas bladder, pulmonary circuit lost.
• General sarcopterygian condition (Fig. 7.6C): partial double circuit; spiral valve guides oxygenated blood to head.

Modern Lungfish Examples

1. Australian (Queensland) lungfish – Neoceratodus forsteri
   • Freshwater SE QLD; single lung used only under stress; nostrils open near upper lip to irrigate olfactory epithelium.
   • Famous captive “Granddad” lived $\approx109$ yrs (1933–2017).
2. African lungfish (Protopterus & relatives)
   • Weak gills ⇒ must breathe air; drown if denied surface access.
   • Aestivation in mucous-lined burrow during drought (norm <$6$ months; record $\approx4$ yrs).

Actinistia – Coelacanths (Living Fossils)

• Appear middle Devonian (~$408\,\text{Ma}$); show limited change to Cretaceous.
• Diagnostic traits: muscular lobes support fins; diphycercal (three-lobed) tail with vertebral axis to tip; absent maxilla.
• Presumed extinct after late Cretaceous until 1938 discovery off East London, South Africa:
  • Caught by Captain H. Goosen; recognised by museum curator Marjorie Courtney-Latimer; described by Prof. J. L. B. Smith.
  • Internal organs discarded, but external morphology confirmed fossil identity – hence “living fossil”.
• Subsequent finds:
  • 1952: Comoros Islands (secured after French embargo).
  • >$200$ specimens from western Indian Ocean; record female $178\,\text{cm},\;98\,\text{kg}$ (1991).
  • 1997: second species Latimeria menadoensis discovered NE Indonesia, >10{,}000\,\text{km} from L. chalumnae range.

Actinopterygii (Ray-Finned Fishes)

• Early (stem) actinopterygians: heterocercal tail, single dorsal fin, ganoine-covered ganoid scales, tightly packed radials.
• Largest radiation of vertebrates: ≈$30{,}000$ extant spp. (>$99\%$ of ray-fins are teleosts).

Key Evolutionary Trends (late Palaeozoic ↦ Mesozoic)

• Caudal fin becomes nearly symmetrical (homocercal) in teleosts.
• Fin rays reduced → lighter, more flexible appendages.
• Dermal armour diminished; swim bladder aids buoyancy, freeing fins for manoeuvring.
• Jaw suspension shifts: hyomandibula supports lower jaw; adductor mandibulae snaps jaws shut.
• Neopterygian innovation: posterior end of maxilla detached from cheek → protrusible jaws, stronger suction.

Non-Teleost Ray-Fin Groups

1. Polypteriformes (bichirs & reedfish)
   • Africa; $11$ spp.; many dorsal finlets; thick ganoid scales; ventral paired lungs (primitive).
2. Acipenseriformes (sturgeons & paddlefish)
   • $24$ sturgeon spp., NH temperate; up to $6\,\text{m}$; bottom-feeders; prized roe (caviar).
   • Greatly reduced dermal skeleton; mostly cartilaginous endoskeleton; dorsal scutes along caudal region.
   • Special spiracle-like dorsal opercular opening enables respiration while mouth busy feeding (Kardong Box 11.1).
3. Holostei (gars & bowfin)
   • Both with heavy ganoid/elasmoid scales, vascularised dorsal gas bladder for buoyancy + air breathing.

Teleostei (crown ray-fins)

• Defining characters: homocercal tail, mobile premaxilla, elasmoid scales, dorsal swim bladder.
• Teleost superradiation (Fig. 33):
  • Basal: Osteoglossomorpha (bony-tongues), Elopomorpha (tarpons & eels), Otocephala (herrings + Ostariophysi).
  • Ostariophysi subdivides into Otophysi (Cypriniformes, Characiformes, Siluriformes, Gymnotiformes) with Weberian apparatus.
  • Euteleostei → Acanthopterygii (spiny-rayed), culminating in hyper-diverse Percomorpha (cichlids, wrasses, gobies, tunas, etc.).

Functional Innovations in Teleosts

• Protrusible jaws (Kardong 7.25): premaxilla & maxilla form a sliding four-bar linkage; epaxial & hypaxial muscles elevate neurocranium and expand buccal cavity → rapid suction.
• Weberian apparatus (Fig. 7-17): modified anterior vertebrae & ossicles connect swim bladder to inner ear – bladder acts as pressure-wave amplifier, enhancing hearing in Otophysi.

Ecological & Practical Notes

• Many ray-fins have converted lung → swim bladder (buoyancy) or retained lung for air breathing.
• Ethical/economic issue: over-harvest of sturgeon roe for caviar threatens wild populations.
• Coelacanth discovery highlights importance of museum networks and local fishers in biodiversity science.

Examples of World’s Largest Freshwater Fish

• Giant freshwater stingray – total length $\le 5\,\text{m}$, disc $\le2.4\,\text{m}$, $\approx600\,\text{kg}$.
• Mekong giant catfish – length $\le3\,\text{m}$, $\approx300\,\text{kg}$.
• Chinese paddlefish – length reported $\le7\,\text{m}$, mass >300\,\text{kg} (possibly extinct).