Question

what are the two most important morphological or physiological innovations that evolved within the vertebrates?

Answer 1

Skull

Formation of the skull is one of the most dramatic evolutionary steps taken by the vertebrates (Craniata). The skulls of vertebrates serve to support and protect the brain, and both structures develop in relation to each other. There are different sources of bone in the skulls of vertebrates. The skulls of fishes and amphibians are composite structures formed from three distinct embryological sources: the splanchnocranium (neural crest origin), which first arose to support the pharyngeal slits; the chondrocranium (endochondral bone), which underlies and supports the brain; and the dermatocranium (dermal bone), which forms most of the outer casing of the skull.The skulls of the Amniota have additional innovations: a platform of endoskeletal bone supporting the brain and exoskeletal bones that encase the brain. There are differences in the skulls of different Amniota. The temporal region of their skulls varies with respect to the number of openings (temporal fenestrae) and the position of the temporal arches. In general, the braincase of the Amniota is enlarged in relation to the brain volume.

Bone and cartilage Cephalochordates and urochordates have a notochord, which is made up of a cartilage-like hard tissue. Most living vertebrates possess two types of hard tissue: cartilage and bone. Bone is one of the most distinctive tissues characterizing the vertebrates. Bone and cartilage are developmentally related in that they are formed from the same mesenchymal precursors, which may mature as either osteoblasts or chondroblasts. However, bone and cartilage differ greatly in their histological features, chemical composition, strength and weight. Cartilage in vertebrates is formed directly from connective tissue, whereas bone is formed in different ways. Bone can form superficially, developing directly from connective tissue of the dermis into dermal bone, for example, the external bones of the vertebrate skull. Other types of bones form from cartilage bone, for example the vertebrae. The sources for such bones include sclerotome-derived cells, which form most of the axial bones, neural crest cells, which form parts of the skull, and different types of somato-mesenchymal cells, which form the limbs Cephalochordates and urochordates have a notochord, which is made up of a cartilage-like hard tissue. Most living vertebrates possess two types of hard tissue: cartilage and bone. Bone is one of the most distinctive tissues characterizing the vertebrates. Bone and cartilage are developmentally related in that they are formed from the same mesenchymal precursors, which may mature as either osteoblasts or chondroblasts. However, bone and cartilage differ greatly in their histological features, chemical composition, strength and weight. Cartilage in vertebrates is formed directly from connective tissue, whereas bone is formed in different ways. Bone can form superficially, developing directly from connective tissue of the dermis into dermal bone, for example, the external bones of the vertebrate skull. Other types of bones form from cartilage bone, for example the vertebrae. The sources for such bones include sclerotome-derived cells, which form most of the axial bones, neural crest cells, which form parts of the skull, and different types of somato-mesenchymal cells, which form the limbs