Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.3 Gnathostomes Are Vertebrates That Have Jaws
Gnathostomes, or “jawed vertebrates,” include sharks, ray-finned fishes, lobe-finned fishes, amphibians, reptiles (including birds), and mammals. The evolution of jaws and paired fins enabled these animals to become effective predators and highly successful in aquatic and terrestrial environments.
Derived Traits of Gnathostomes
- Jaws:
- Evolved from modified skeletal rods supporting anterior pharyngeal slits
- Enabled strong grasping and cutting of prey
- Other Traits:
- Duplication of Hox genes → allowed greater complexity
- Enlarged forebrain → enhanced vision and smell
- Lateral line system in aquatic forms → detects vibrations in water
- Paired fins and tail → improved swimming and control
Fossil Evidence
- First appeared ~440 MYA
- Early jawed vertebrates include:
- Placoderms: armored, jawed vertebrates
- Acanthodians: spiny fishlike vertebrates
- Innovations like fin rays and efficient gill ventilation supported active predation
Chondrichthyans (Cartilaginous Fishes)
- Includes sharks, rays, skates, and ratfishes
- Skeleton mainly of cartilage; sometimes lightly mineralized
- Adaptations:
- Streamlined bodies, strong tails, oil-rich liver for buoyancy
- Continual swimming for respiration
- Spiral valve in intestine → increases nutrient absorption
- Acute senses: smell, electroreception, lateral line system
- Internal fertilization using claspers; reproductive modes:
- Oviparous: eggs hatch outside body
- Ovoviviparous: eggs hatch inside mother
- Viviparous: nourishment via placenta-like connection
- Rays: Bottom-dwellers; large pectoral fins for swimming, often with venomous tails
- Many face sharp declines due to overfishing
Osteichthyans (Bony Fishes and Tetrapods)
- Have ossified skeletons made of calcium phosphate
- Most use gills protected by operculum for breathing
- Swim bladder maintains buoyancy (derived from lungs)
- Skin has bony scales and secretes mucus
- Most reproduce through external fertilization
Ray-Finned Fishes (Actinopterygii)
- Include common fishes like trout, tuna, eels, seahorses
- Fins supported by bony rays
- Diverse in form and ecological role
- Human impact: overfishing and habitat destruction (e.g., cod collapse)
Lobe-Finned Fishes (Sarcopterygii)
- Fins have rod-shaped bones surrounded by muscle
- Surviving groups:
- Coelacanths (rediscovered in 1938)
- Lungfishes: use lungs to breathe air; live in stagnant waters
- Tetrapods: evolved from lobe-finned ancestors; adapted to land
In a Nutshell
Gnathostomes revolutionized vertebrate evolution with the origin of jaws and paired fins. Sharks and rays dominate the oceans as cartilaginous predators, while bony fishes (ray-finned and lobe-finned) have diversified into countless aquatic niches. Lobe-fins ultimately gave rise to tetrapods, setting the stage for life on land.