Animals: Vertebrates

Learning Objectives

  1. Place chordates (and vertebrates!) on a phylogenetic tree
  2. Identify and describe key adaptations of chordates (notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail)
  3. Identify and use key features to differentiate between vertebrate groups, including, including Fishes (bony and cartilaginous), Amphibians, and Amniotes (reptiles, birds, and mammals)
  4. Organize the appearance and/or flourishing of major vertebrate groups in chronological order in geologic time

Animals: A recap

To recap, here again is the animal phylogeny, and we’ve marked now the branch ‘Chordata’ to orient ourselves:

Overarching phylogenetic tree of animals and their ancestor. Note that this is one hypothesis for the evolution of animals based on the criteria shown in blue (credit: Emily Weigel)

Now we’re going to take a special look at the chordates.

Chordates (Chordata)

Overarching phylogenetic tree of chordates and their ancestor. Note that this is one hypothesis for the evolution of animals based on the criteria shown in blue. Common group names referring to several phyla are shown in green (credit: Emily Weigel)

Viewing the tree, note that not all chordates are vertebrates! The most conspicuous and familiar members of Chordata are vertebrates, but not all have vertebrae. So what makes a chordate, a chordate?

Animals in the phylum Chordata share four key features that appear at some stage during their development:

  1. a notochord,
  2. a dorsal hollow nerve cord,
  3. pharyngeal slits, and
  4. a post-anal tail.

The chordates are named for the notochord, which is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates and in the adult stage of some chordate species. It is located between the digestive tube and the nerve cord, and provides support through the length of the body.

The dorsal hollow nerve cord derives from ectoderm that rolls into a hollow tube during development. In chordates, it is located dorsal to the notochord. In contrast, other animal phyla are characterized by solid nerve cords that are located either ventrally or laterally. The nerve cord found in most chordate embryos develops into the brain and spinal cord, which compose the central nervous system.

Pharyngeal slits are openings in the pharynx (the region just posterior to the mouth) that extend to the outside environment. In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food out of the water that enters the mouth. In vertebrate fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, into jaw supports. In tetrapods (amphibians, reptiles, birds, and mammals), the slits are modified into components of the ear and tonsils. Tetrapod literally means ‘four-footed,’ which refers to the phylogenetic history of various groups that evolved accordingly, even though some now possess fewer than two pairs of walking appendages.

The post-anal tail is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near. In humans, the post-anal tail is vestigial, that is, reduced in size and nonfunctional.

And here is the Crash Course take on Chordates (note, as awesome as Hank Green is, there is no such thing as a “living fossil”):

Non-vertebrate chordates: Cephalochordata (lancelets), Urochordata (Tunicates), and Myxini (hagfishes)

These groups are the chordates which do not possess vertebrae. Many are hermaphroditic, sessile or buried within the sand of aquatic environments, and hatch from eggs within the parent’s body. Some undergo metamorphosis into their adult form. Most are suspension-feeders, feeding on algae or small invertebrates.

In particular, the first fish, Myxini (hagfish) fall within this group. They possess a ‘skull’, that is, a bony, cartilaginous, or fibrous structure encasing the head. However, they lack a backbone, the key feature defining vertebrates.

Vertebrate chordates: an overview

Vertebrates display the four characteristic features of the chordates; however, members of this group also share derived characteristics that distinguish them from invertebrate chordates. Vertebrata is named for the vertebral column, composed of vertebrae, a series of separate bones joined together as a backbone. In adult vertebrates, the vertebral column replaces the notochord, which is only seen in the embryonic stage.

Vertebrates are the largest group of chordates, with more than 62,000 living species categorized based on anatomical and physiological traits. Here we will consider the traditional groups which constitute classes in the subphylum Vertebrata. Note that many modern authors classify birds within Reptilia, which correctly reflects their evolutionary heritage. We consider them separately only for convenience.

Vertebrate Groups

Fishes were the earliest vertebrates, with jawless species being the earliest and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders.

Agnathostomes (jawless fishes)

Jawless fishes, such as lampreys, represent the first true vertebrate lineage. They are craniates that represent an ancient vertebrate lineage that arose over one half-billion years ago. A defining feature is the lack of paired lateral appendages (fins), such that they appear tubular.

Gnathostomes (jawed fishes)

Gnathostomes or ‘jaw-mouths’ are vertebrates that possess jaws. One of the most significant developments in early vertebrate evolution was the development of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately and become mobile predators. These two traits allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes. Most modern fishes are gnathostomes.

Dunkleosteous was an enormous placoderm from the Devonian period. It measured up to 10 meters in length and weighed up to 3.6 tons. (credit: Nobu Tamura)

Chondrichthyes (cartilaginous fishes)

The clade Chondrichthyes is diverse, consisting of sharks, rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras (ghost sharks). Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. They are thought to be descended from the placoderms, which had skeletons made of bone; thus, the cartilaginous skeleton of Chondrichthyes is a later development.

Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces, but some are suspension feeders that feed on plankton.

Hammerhead sharks tend to school during the day and hunt prey at night. (credit: Masashi Sugawara)

Osteichthyes (bony fishes)

The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today. The ‘bony fish’ group includes the Actinopterygii (ray-finned fishes), Actinistia (coelacanths), and Dipnoi (lungfish).

Nearly all bony fishes have an ossified skeleton. This characteristic has only reversed in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation.

Amphibians

The information below was adapted from OpenStax Biology 29.3

Amphibians are vertebrate tetrapods that include frogs, salamanders, and caecilians. The term amphibian loosely translates from the Greek as ‘dual life’ which is a reference to the metamorphosis that many frogs and salamanders undergo and their mixture of aquatic and terrestrial environments in their life cycle. Most species must remain forever associated with moist environment, at a minimum in early development as eggs.

As tetrapods, most amphibians are characterized by four well-developed limbs, although a few clades have since lost some or all limbs. An important characteristic of extant amphibians is a moist, permeable skin that is achieved via mucus glands that keep the skin moist; thus, exchange of oxygen and carbon dioxide with the environment can take place through it (cutaneous respiration). Importantly, as almost all extant adult amphibians are carnivorous predators, sensitive inner ear structures and sticky tongues are common in this clade.

Evolution of tetrapods from fishes represented a significant change in body plan from one suited to organisms that respired and swam in water, to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years during the Devonian period.

In 2006, researchers published news of their discovery of a fossil of a tetrapod-like fish, Tiktaalik roseae, which seems to be an intermediate form between fishes having fins and tetrapods having limbs. Tiktaalik likely lived in a shallow water environment about 375 million years ago. Watch the video below about the importance of the fossil to our understanding of tetrapod evolution:

Amniotes

The information below was adapted from OpenStax Biology 29.4

The amniotes (reptiles, birds, and mammals) are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs. Although the shells of various amniotic species vary significantly (from hard, to leathery, to using internal fertilization and development instead), they all allow retention of water.

The amniotic egg: a closer look

The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians.

However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois. Extra-embryonic membranes are membranes present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta.

Reptiles

Crocodilians, such as this Siamese crocodile (Crocodylus siamensis), provide parental care for their offspring. (credit: Keshav Mukund Kandhadai)

Reptiles are tetrapods, although some lineages have only vestigial structures since descending from four-limbed ancestors. Reptiles, even aquatic ones, lay eggs enclosed in shells on land. They usually reproduce sexually with internal fertilization, although some species show ovoviviparity (eggs remaining in the mother’s body until they are ready to hatch) or viviparity (live-born offspring).

One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin, containing the protein keratin and waxy lipids, which reduced water loss from the skin. This occlusive skin means that reptiles cannot use their skin for respiration, like amphibians, and thus all breathe with lungs.

Reptiles are ectotherms, animals whose main source of body heat comes from the environment. In addition to being ectothermic, reptiles are categorized as poikilotherms, or animals whose body temperatures vary rather than remain stable. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up and finding shady spots or going underground to cool down. The advantage of ectothermy is low metabolic energy needed to survive (e.g., about 10% of a similarly-sized endotherm).

Birds

In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves (Birds). Birds are descended, however, from dinosaurs, which are reptiles, so this classical scheme results in groups that are not true clades. We will consider birds as a group distinct from reptiles for the purpose of this discussion with the understanding that this does not completely reflect phylogenetic history and relationships.

The most obvious characteristic that sets birds apart from other modern vertebrates is the presence of feathers, which are modified scales. Birds are endothermic, and because they fly, they require large amounts of energy, necessitating a high metabolic rate. Like mammals, which are also endothermic, birds have an insulating covering that keeps heat in the body: feathers. Specialized feathers called down feathers are especially insulating, trapping air in spaces between each feather to decrease the rate of heat loss. Certain parts of a bird’s body are covered in down feathers, and the base of other feathers have a downy portion, whereas newly hatched birds are covered in down.

Feathers not only act as insulation but also allow for flight, enabling the lift and thrust necessary to become airborne. The feathers on a wing are flexible, so the collective feathers move and separate as air moves through them, reducing the drag on the wing.

Like the question of how flight evolved, the question of how endothermy evolved in birds still is unanswered. Feathers provide insulation, but this is only beneficial if body heat is being produced internally. Similarly, internal heat production is only viable if insulation is present to retain that heat. It has been suggested that one or the other– feathers or endothermy– evolved in response to some other selective pressure.

Birds as modern dinosaurs

The evolutionary history of birds is still somewhat unclear. Due to the fragility of bird bones, they do not fossilize as well as other vertebrates. Birds belong to a group called the archosaurs, which also includes crocodiles and dinosaurs.

Dinosaurs (including birds) are subdivided into two groups, the Saurischia (‘lizard like‘) and the Ornithischia (‘bird like‘). Despite the names of these groups, most evidence suggests it was not the bird-like dinosaurs that gave rise to modern birds (but there is dissent and an alternative hypothesis has been proposed). Rather, the lizard-like dinosaurs gave rise to bipedal predators called theropods, which includes birds.

One important fossil of an animal intermediate to dinosaurs and birds is Archaeopteryx, which is from the Jurassic period. Archaeopteryx is important in establishing the relationship between birds and dinosaurs, because it is an intermediate fossil, meaning it has characteristics of both dinosaurs and birds. Some scientists propose classifying it as a bird, but others prefer to classify it as a dinosaur. The fossilized skeleton of Archaeopteryxlooks like that of a dinosaur, and it had teeth whereas birds do not, but it also had feathers modified for flight, a trait associated only with birds among modern animals. Fossils of older feathered dinosaurs exist, but the feathers do not have the characteristics of flight feathers.

Mammals

The information below was adapted from OpenStax Biology 29.6

Mammals are vertebrates that possess several defining characteristics, including certain features of the hair, the jaw and skeletal system, integument (skin), and internal anatomy.

The presence of hair is one of the most obvious signs of a mammal. Although it is not very extensive on certain species, such as whales, hair has many important functions for mammals. Mammals are endothermic, and hair provides insulation to retain heat generated by metabolic work. Hair traps a layer of air close to the body, retaining heat. Along with insulation, hair can serve as a sensory mechanism via specialized hairs called vibrissae, better known as whiskers. These attach to nerves that transmit information about sensation, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling.

The skeletal system of mammals possesses many unique features. Unlike other vertebrates, the lower jaw of mammals consists of just one bone, the dentary. The additional jaw bones found in other vertebrates have been modified to function in hearing and form bones in the middle ear (the malleus, incus, and stapes) and is one way of distinguishing fossil mammals from fossils of other synapsids.

Bones of the mammalian inner ear are modified from bones of the jaw and skull. (credit: NCI)

The adductor muscle that closes the jaw is composed of two muscles in mammals the temporalis and the masseter. These allow side-to-side movement of the jaw, making chewing possible, which is unique to mammals. Most mammals have heterodont teeth, meaning that they have different types and shapes of teeth rather than just one type and shape of tooth.

In addition to defining mammary glands produce milk that is used to feed newborns, other specific glands appear to mark mammals uniquely:

  • Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin for water resistance and lubrication.
  • Eccrine glands which produce sweat, may be limited to certain parts of the body or absent in some species, but when present, aid with thermoregulation.
  • Apocrine glands, or scent glands, secrete substances that are used for chemical communication, such as in skunks.

Modern mammals belong to just three clades: eutherians (or placental mammals, like us), marsupials (eg., kangaroo, opossum), and monotremes (platypus and echidna).

Key Events in Vertebrate Evolution