Short answers, pulled from the story.
Chondrichthyes is the class of jawed cartilaginous fishes that represents a biological lineage surviving for over 400 million years. These creatures rely on a flexible framework of cartilage instead of rigid internal skeletons found in bony vertebrates. This anatomical choice allows them to dominate the seas as apex predators ranging from the tiny finless sleeper ray to the massive whale shark.
The earliest unequivocal fossils of acanthodian-grade cartilaginous fishes such as Qianodus and Fanjingshania date back to the early Silurian period around 439 million years ago. Isolated scales made of dentine and bone appeared during the Middle and Late Ordovician Period over 450 million years ago. By the start of the Early Devonian 419 million years ago jawed fishes had already divided into three distinct groups including the clade that includes spiny sharks and early cartilaginous fish.
Chondrichthyes possess a sensory system that allows them to detect the faintest electrical fields generated by the muscle contractions of their prey. This capability is mediated by the ampullae of Lorenzini a network of jelly-filled pores that act as electroreceptors distributed across the head and snout. The lateral line system allows these animals to sense motion vibration and pressure changes in the water providing them with a three-dimensional map of their surroundings.
Fertilization always occurs internally and development ranges from egg-laying to live birth in Chondrichthyes. Most species are ovoviviparous meaning that the eggs hatch inside the mother's body and the young are born live. A disturbing phenomenon known as capture-induced parturition occurs frequently in sharks and rays when they are fished where the stress of capture causes premature birth or abortion affecting 88 species to date.
The subclass Holocephali which includes chimaeras rabbit-fishes and elephant-fishes represents a group of cartilaginous fishes that have preserved some features of elasmobranch life from the Paleozoic era. These animals lack a stomach with the gut being simplified and the stomach merged with the intestine. Unlike their elasmobranch relatives chimaeras lack both the Leydig's and epigonal organs suggesting a divergent evolutionary path that has preserved ancient traits while developing unique adaptations for deep-sea life.