61. Sequence of evolutionary innovations in vertebrate classes

Sequence of evolutionary innovations in vertebrate classes

The earliest skeletal structure in chordates was the notochord, a flexible rod located beneath the nervous system and above the gut. It supported the body and helped guide embryonic development.
These organisms had a dorsal neural tube, formed from the dorsal ectoderm. The front end developed into a basic brain structure.
The skin was made up of a single layer of epidermal cells.
The feeding system involved cartilage-like structures, especially in species like Branchiostoma (Amphioxus). This cartilage was stiff but not mineralized and contained fibrillar collagen.
Cartilage likely evolved from a common ancestor of bilaterians and used specific transcription factors (SoxD and SoxE) for development.
Bone was unique to vertebrates and was composed of hydroxyapatite, a calcium phosphate-based compound.
Primitive chordates lived by filter-feeding, relying on water flow to trap food particles.

Agnathans were the earliest vertebrates, appearing about 500 million years ago. Modern examples include lampreys and hagfish.
Evolution of a distinct head compartment marked a shift from passive filter feeding to active food intake.
Their skulls were made of non-calcified cartilage, and they lacked jaws.
Neural crest cells appeared in these species. These cells contributed to the formation of cranial structures and were key to future evolutionary changes in vertebrates.
Head muscles were connected to internal cranial cartilage for the first time, enabling more efficient feeding and movement.
Although early vertebrates had bony exoskeletons, modern jawless fishes lost them over time.
A backbone began to develop, providing better support and allowing for increased body size.
Circulatory advances included the formation of a heart with an atrium and ventricle.
They also developed a primitive kidney system known as the pronephros.
Their brain was regionally differentiated, with the telencephalon acting primarily in olfactory processing.
The spinal cord was dorsoventrally flattened, with an unusual shape resembling spread bird wings in cross-section.
Only unpaired fins were present at this stage.

The invention of jaws was a major milestone. Jaws were formed from the first pharyngeal arch and made active predation possible.
Early jawed fishes, such as placoderms, had bony armor and jaws but initially no teeth.
The ability to produce bone evolved in this group. Though modern cartilaginous fishes use cartilage, early jawed fishes like placoderms contributed to bone evolution.
The development of articulated jaws provided a functional advantage, allowing vertebrates to become active hunters.