Astraspis ('star shield') is an extinct genus of primitive jawless fish from the Ordovician of Central North America and Bolivia (Gagnier, 1993) . It is related to other Ordovician fishes, such as the South American Sacabambaspis, and the Australian Arandaspis. Basic Anatomy Astraspids are hypothesized to have been about 200mm in length. They are supposed to have had a mobile tail covered with small protective plates (<1mm) and a head region covered with much larger plates (>>2mm). The specimen from North America (described by Sansom et al., 1997) is to have had relatively large, lateral eyes and a series of eight gill openings on each side. The specimen was generally oval in cross-section. The protective bony plates covering the animal were composed of aspidin (chemically similar to modern shark's teeth), covered by tubercles composed of dentine. It is from these tubercles (which are generally star-shaped) that the name 'Astraspis' (literally "star-shield") is derived. In popular culture It has a small cameo in the 2003 BBC series Sea Monsters: A Walking with Dinosaurs Trilogy, a spin-off to the famed Walking with Dinosaurs, where Nigel Marven uses a dead Astraspis as bait to attract a sea scorpion. The creature is identified in the series' novelisation. It is also said to be an armour-plated fish in the documentary, whereas is more closely related to the modern lamprey. Astraspis was also featured in Animal Armageddon. The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds. While tails are primarily a feature of vertebrates, some invertebrates including scorpions and springtails, as well as snails and slugs, have tail-like appendages t at are sometimes referred to as tails. Tailed objects are sometimes referred to as "caudate" and the part of the body associated with or proximal to the tail are given the adjective "caudal". A tooth (plural teeth) is a small, calcified, whitish structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness. The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position. The teeth of mammals have deep roots, and this pattern is also found in some fish, and in crocodilians. In most teleost fish, however, the teeth are attached to the outer surface of the bone, while in lizards they are attached to the inner surface of the jaw by one side. In cartilaginous fish, such as sharks, the teeth are attached by tough ligaments to the hoops of cartilage that form the jaw. Teeth are among the most distinctive (and long-lasting) features of mammal species. Paleontologists use teeth to identify fossil species and determine their relationships. The shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding. Carnivores, on the other hand, need canines to kill prey and to tear meat. Mammals are diphyodont, meaning that they develop two sets of teeth. In humans, the first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful.