Vertebrates are animals with backbones. The subphylum Vertebrata is contained in the phylum Chordata, which also includes lancelets (small, jawless, fish-like creatures) and tunicates (small ocean-dwelling creatures that are sessile in their adult forms). Because these are ocean-dwelling animals that are rarely seen and largely unknown by the public, we typically divide animals into vertebrates and non-vertebrates, but that leaves out these two groups. But we should always keep in mind that the basic distinction we should make is between chordates and non-chordates.
Vertebrates can be separated into two broad groups: agnathans, or jawless vertebrates such as hagfish and lampreys, and gnathostomes, or the jawed vertebrates. Gnathostomes can be classified into three extant groups: Chondrichthyes, Ostichthyes, and Tetrapoda.
Chondrichthyes are the cartilaginous fishes, which include sharks, rays, skates, and chimaeras. They are jawed fish with paired fins, paired nares, scales, a heart with chambers in a series, and skeletons made out of cartilage, rather than bone.
Osteichthyes are the bony fishes, which are similar to the Chondrichthyes in many ways, but have skeletons made of bone, rather than cartilage. They are an extremely diverse and abundant group around the world.
Tetrapoda are the group of all four-limbed vertebrates, including amphibians, reptiles, birds, and mammals. The tetrapods evolved from the lobe-finned fishes during the Devonian period, approximately 395 million years ago. Some members, such as snakes and some lizards, of the group have lost one or both pairs of limbs, while in the birds, the front pair of limbs have evolved into wings.
Tetrapods have evolved into four major groups: amphibians (class Amphibia), reptiles (class Reptilia), birds (class Aves), and mammals (class mammalia).
\paragraph{Virginia Opossum (\textit{Didelphis virginiana})} Also known as the North American opossum, the Virginia opossum is the only marsupial living north of Mexico. Typically nocturnal, with a naked, scaled tail, they can vary greatly in size ranging in size from 33 to 55 centimeters from the tip of the snout to the base of the tail, with the tail adding another 25–54 cm (9.8–21.3 in). Males are slightly larger, with an average body length of 40.8 cm (16.1 in) with an average tail length of 29.4 cm (11.6 in), while females are 40.6 cm (16.0 in) long with a 28.1 cm (11.1 in) tail. Weight for males ranges from 2.1–2.8 kg (4.6–6.2 lb) and for females from 1.9–2.1 kg (4.2–4.6 lb).
\caption{Virginia opossum carrying her young on her back. \\{\scriptsize (Photo by Specialjake - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20584629)}}
\paragraph{Snowshoe Hare (\textit{Lepus americanus})} So called because of its large hind feet, which prevent it from sinking into the snow when it walks and hops. Its fur is rusty brown in summer, turning white in winter, which helps to disguise it in a snowy landscape. It is mainly active at night, sometimes feeding in small groups. It does not hibernate. It is found across the northern portion of the lower peninsula and throughout the upper peninsula.
\paragraph{Eastern Cottontail (\textit{Sylvilagus floridanus})} Common throughout the lower peninsula, it is the most common rabbit species in North America.
