7.2 Vertebrate Groups

Hagfishes are eel-like scavengers that live on the ocean floor and feed on dead or dying animals. They are known to enter the bodies of these carcasses to devour them from the inside. They are entirely marine and are found in oceans worldwide, except in polar regions. A unique feature of hagfishes is their ability to secrete large amounts of slime (mucus) from specialized glands to help them escape from the grip of predators. Their skeleton is made of cartilage and includes a flexible notochord and a skull, but they lack a vertebral column, so they are not considered true vertebrates

Lampreys resemble hagfishes in shape and size but differ in having a braincase and rudimentary vertebrae. They also lack paired fins and bones. Adult lampreys have a toothed, funnel-like mouth used by some species to parasitize other fish. Most lampreys, however, are free-living. They are found in coastal and freshwater environments across temperate regions. All lampreys spawn in freshwater, where their larvae live as suspension feeders for several years before maturing, reproducing, and dying shortly after.

Cartilaginous fishes, such as sharks and rays, have skeletons made of cartilage. Some parts of their skeletons are reinforced with calcium carbonate, but this is not true bone. Most cartilaginous fishes are marine but there are a few freshwater species. Sharks are typically carnivorous, using their sharp teeth to capture and tear prey. Sharks have evolved replenishing teeth – as older teeth are lost or worn down, new ones move forward from rows within the jaw to take their place, ensuring they always have functional teeth for feeding.

Sharks also possess highly developed sensory systems, including a keen sense of smell and electroreception (the ability to detect electrical fields produced by other organisms). Many shark species must swim continuously to maintain water flow over their gills and to keep

Rays are closely related to sharks but have flattened bodies, enlarged pectoral fins fused to the head, and gill slits on their underside. They are mostly bottom-dwellers in marine environments.

Bony fishes are the largest and most diverse group of vertebrates, with around 30,000 known species. They are characterized by a skeleton made primarily of bone which provides strong structural support and allows for a wide range of body shapes.Their skin is typically covered in overlapping scales, and mucus-secreting glands help reduce drag while swimming. Unlike many cartilaginous fishes, bony fishes often rely heavily on vision to locate prey. Additionally, their gills covered by a protective flap, called the operculum, which allows them to ventilate their gills without needing to swim. Another key adaptation in this group is the swim bladder, a gas-filled organ that helps regulate buoyancy.

Bony fishes are divided into two groups:

• Ray-finned fishes: This group includes most familiar fishes such as tuna, trout, bass, and salmon. Their fins are supported by long, flexible rays.
  

  

• Lobe-finned fishes: These fishes have fleshy, lobed fins supported by bones. Living examples include the less familiar fishes, such as lungfishes. These fishes are especially important in evolutionary history, as they gave rise to the first land vertebrates.

Salamanders typically have elongated bodies, tails, and four limbs. There are around 500 species, with some living in water, others on land, and some transitioning between the two.

Frogs are the most diverse amphibian group, with about 5000 species. They have a body plan specialized for jumping, with long, muscular hind limbs. Frogs use various strategies to avoid predators, including camouflage and poisonous skin secretions.

Crocodilians (“small lizard”) include alligators and crocodiles. They have long snouts and powerful tails. They live in tropical regions and are found in freshwater habitats, such as rivers and lakes. Some species are able to move on land but they spend most of their time in water. Unlike most reptiles, many crocodilian species do show parental care. Females guard their nests and protect their young from predators.

Lizards and Snakes are the most diverse group of reptiles. Lizards typically have four limbs, eyelids, and external ears, while snakes lack these features. Lizards range in size from chameleons and geckos that are a few centimeters in length to the Komodo dragon, which is about 3 meters in length. Snakes evolved from lizard-like ancestors and range in size from 10 centimeter-long thread snakes to 7.5 meter-long pythons and anacondas. They are entirely carnivorous, feeding on a variety of prey including small mammals, birds, and insects.

Turtles are characterized by a bony or cartilaginous shell (carapace) made from modified ribs. Turtles lay eggs on land, even if they live in aquatic environments. They range in size from a few centimeters to the massive leatherback sea turtles (over 2m). The term “turtle” is often used to describe those that live in the sea, where as “tortoise” refers to those that live on land.

Birds are a specialized group of reptiles that evolved from theropod dinosaurs. Unlike reptiles, birds are endothermic, generating their own body heat through metabolism. Their most distinctive feature is feathers, which are modified scales. Birds have several different types of feathers that are specialized for specific functions, like contour feathers that streamline the bird’s exterior and loosely structured down feathers that insulate.

Birds have evolved numerous adaptations for flight:

• Feathers that streamline the body and provide lift
• Hollow bones to reduce weight while maintaining strength
• A fused skeleton for stability during flight
• The absence of teeth and presence of only one ovary to reduce body mass

To help keep their feathers in top condition, birds engage in a maintenance behavior known as preening. This involves using their beak to clean, realign, and waterproof their feathers by spreading oil from a gland near the base of the tail. This oil helps maintain feather flexibility and water resistance, which is especially important for flight and insulation.

Migration is an important and widespread behavior among birds, allowing them to exploit seasonal resources and favorable climates. Many bird species travel thousands of kilometers between breeding and wintering grounds, often navigating with remarkable precision using environmental cues like the sun, stars, and Earth’s magnetic field. Migration helps birds avoid harsh conditions, access abundant food, and find suitable nesting sites.

Monotremes are the most primitive group of living mammals and include just three species: the platypus and two species of echidnas. Monotremes are unique among mammals, as they lay leathery eggs, similar to those of reptiles, rather than giving birth to live young. However, the eggs are retained within the mother’s reproductive tract until they are almost ready to hatch. Once the young hatch, the female begins to secrete milk from pores in a ridge of mammary tissue along the ventral side of her body.

Marsupials give birth to underdeveloped young, which then continue to grow and nurse inside a pouch on the mother’s body. While they do form a placenta, it is less complex than that of placental mammals. Well-known marsupials include kangaroos, koalas, bandicoots, and the Tasmanian devil. Marsupials are found primarily in Australia and nearby islands, with a few species in the Americas, such as opossums.

Eutherians, or placental mammals, are the most widespread and diverse group of mammals. They are distinguished by a complex placenta that supports the developing fetus by facilitating the exchange of nutrients, gases, and wastes between mother and offspring. This allows for a longer gestation period and more fully developed young at birth.Eutherians include a wide range of groups. Some examples include:

• Rodents – Have continuously growing incisors adapted for gnawing (e.g. mice, squirrels)
• Bats – Only mammals capable of true powered flight, with wings formed from elongated fingers and a thin membrane (e.g. big brown bats, vampire bats)
• Carnivorans – Have specialized teeth (canines) for capturing and slicing meat (e.g. dogs, cats, bears)
• Hoofed animals – Herbivorous mammals with hooves (e.g. horses, deer, pigs, whales)
• Primates – Have large brains, forward-facing eyes, and grasping hands adapted for complex behaviors (e.g. monkeys, apes, humans)