Chondrocranium and Basal Plate Formation

Chondrocranium

• Formed by the fusion of prechordal plates, which remain free and widely separated in lower vertebrates with broad heads (platytrabic condition)
• Sometimes joined by a plate of cartilage and fused at the anterior end to form a Y-shaped structure (tropitrabic condition)
• Composed of various elements distinct in the embryo, including the notochord, prechordal cartilage or trabeculae cranii, and parachordal cartilages

Basal Plate

• Formed by the merger of parachordal cartilages above, below, or around the notochord
• Anterior part of the plate encloses a large cavity with lateral edges pierced by foramina for the passage of cranial nerves
• Posterior part of the plate has two or more projections called occipital condyles, which articulate with the first vertebra of the spine

Neural Arches and Sense Capsule

• Neural arches arise from the posterior angles of the basal plate to encircle the spinal cord at the point where it enters the skull
• Sense capsule houses the nasal chambers and inner ear, and joins the anterior ends of the trabeculae cranii
• Otic (auditory) capsules join the margins of the occipital arch
• Eye capsules remain free to allow for independent movement of the eyes

Visceral Skeleton

• Skeleton of the pharyngeal pouches in vertebrates, modified to perform new functions on land
• In fishes, it is the skeleton of the jaws and gill arches
• In jawless vertebrates, each gill bar is supported by a single cartilaginous rod

Horns and Antlers

• Hair horns are entirely composed of keratinized hair-like epidermal fibers, located on a roughened area of the nasal bone
• Antlers are characteristic of the deer family, formed by branched dermal bones, and are shed and replaced annually

Skeletal System

• Divided into axial skeleton (notochord, vertebral column, ribs, sternum, skull, and visceral skeleton) and appendicular skeleton (pectoral and pelvic girdles, skeleton of paired fins and limbs, and skeleton of median fins of fishes)
• Components of the head skeleton include chondrocranium, visceral skeleton, and dermatocranium

Chondrocranium and its Derivatives

• Supports the brain and organs of special sense (olfactory organs, eye, internal ears)
• Composed of various elements, including the notochord, prechordal cartilage, and parachordal cartilages
• May remain cartilaginous or become bony, but is always present

Dermatocranium

• Completes the framework of the skull
• Always bony and usually present, but has been lost in several major vertebrate groups
• Components include roofing bones, dermal bones of the upper jaw, bones of the primary palate, and opercular bones

Dermatocranium Elements

• Roofing bones provide a protective shield over the brain and special sense organs
• Dermal bones of the upper jaw include nasals, frontals, parietals, and postparietals
• Bones of the primary palate include lacrimal, prefrontal, post-frontal, post-orbital, and infraorbital
• Opercular bones include intertemporal, supratemporal, tabular, squamosal, and quadratojugal bones