Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.2 Vertebrates Are Chordates That Have a Backbone
Vertebrates are distinguished from other chordates by the presence of a backbone and a more complex skeletal and nervous system. This innovation allowed for more efficient locomotion, feeding, and sensory processing, enabling vertebrates to become dominant animals in many ecosystems.
Key Derived Traits of Vertebrates
- Backbone (vertebrae):
- Replaces the notochord in most vertebrates
- Provides structural support and protection for the spinal cord
- Rudimentary in hagfishes and lampreys
- Skull and Brain:
- Well-defined head with a brain encased in a skull
- Enhanced sensory organs: eyes, ears, olfactory structures
- Duplicated Hox genes:
- Two or more sets allow complex body development
- Supports nervous system and skeletal innovations
- Neural crest:
- Embryonic cells unique to vertebrates
- Form teeth, skull bones, neurons, and sensory organs
Jawless Vertebrates: Hagfishes and Lampreys
These two lineages are the only living jawless vertebrates and form a clade called cyclostomes.
Hagfishes (Myxini):
- Have reduced vertebrae, cartilaginous skull, and a notochord
- Scavengers that produce defensive slime to deter predators
Lampreys (Petromyzontida):
- Have a notochord and rudimentary vertebrae
- Many are parasites that use keratinized teeth to suck blood from host fish
- Larvae are suspension feeders resembling lancelets
- Skeleton is made of non-collagenous cartilage
Recent evidence supports that both groups are true vertebrates.
Fossil Evidence of Early Vertebrates
- Haikouella (~530 MYA): Had brain, eyes, muscles, but no skull or ears
- Myllokunmingia: First known chordate with a head and skull elements; lacked vertebrae
- Conodonts: Early jawless vertebrates with mineralized dental elements; widely fossilized
- Armored jawless vertebrates (Ordovician–Devonian):
- Had paired fins and mineralized exoskeletons
- Extinct by late Devonian
- Skeleton Evolution:
- First vertebrate skeletons made of unmineralized cartilage
- Mineralization began externally and moved inward to bone
In a Nutshell
Vertebrates are distinguished by their backbone, skull, and neural crest, features that support complex movement, feeding, and sensory functions. Jawless vertebrates like hagfishes and lampreys provide a living window into early vertebrate evolution, while fossils such as Haikouella and Myllokunmingia help trace the transition from simple chordates to more advanced vertebrate forms.