Short answers, pulled from the story.
Acanthodii are an extinct class of jawed fishes that lived from the Silurian to the Permian period. They are called "spiny sharks" because they had a streamlined, shark-like body with a strongly upturned tail, but all fins except the tail were supported by stout bony dentine spines. They are not true sharks; they represent a paraphyletic grade of fish lineages basal to modern cartilaginous fish.
The earliest unequivocal acanthodian fossils date from the beginning of the Silurian Period, roughly 50 million years before the first sharks appeared. The oldest remains attributed to acanthodian-grade chondrichthyans are Fanjingshania and Qianodus from the Early Silurian of China, dating to around 439 million years ago.
Acanthodians died out in Permian times, approximately 250 million years ago. The youngest known fossil evidence consists of isolated scales and fin spines from Middle-Late Permian strata in the Paraná Basin of Brazil.
Acanthodians are divided into four orders: Acanthodiformes, Climatiiformes, Diplacanthiformes, and Ischnacanthiformes. Ischnacanthiforms were predators with tooth plates fused to their jaws, while Acanthodiformes were filter feeders with toothless jaws that survived until the Permian.
Yes. Current research considers acanthodians to represent a paraphyletic grade of fish lineages basal to Chondrichthyes, which includes living sharks, rays, and chimaeras. A 2016 study by Burrow and colleagues found chondrichthyans nested within Acanthodii, most closely related to the genera Doliodus and Tamiobatis.
Acanthodian scales have distinctive ornamentation unique to each order, which allows palaeontologists to use them to determine the relative age of sedimentary rock layers. The scales have a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown, and their small size means fossilised scales and fin spines are often the only remains preserved in ancient rock.