Gnathostomes (jawed vertebrates) have not always existed in the fossil record, nor have modern gnathostomes existed since the first appearance of fossil gnathostomes. The first jawed vertebrates were preceded by a great diversification of jawless vertebrates. Modern gnathostomes form a clade which share modifications of the skeletal system (jaws, ossified vertebral regions, pectoral girdles, pelvic girdles, a hyoid arch, an otic capsule, an occipital region, true teeth, and new components of the skull), nervous system (reorganization of spinal tissue, new tracts of the spinal cord, an enlarged brain, more complex eyes and ears), endocrine system (androgen receptors, a portal system between the hypothalamus and pituitary, endocrine islets in the pancreas), muscular system (jaw muscles, fin muscles), cardiovascular system (duplications of globin genes, blood vessels to service the heart, an advanced intrinsic conduction system of the heart, and more blood vessels servicing skeletal muscle), lymphatic system (a true thymus and spleen, true antibodies, different antibody classes, additional components of the complement cascade, MHC proteins), digestive system (layers of the muscularis, a large liver divided into lobes, a stomach divided into regions), urinary system (fenestrations, a bladder in females), and reproductive systems (paired gonads, new male ducts, oviducts in females).