Animals With Backbones: Definition, Facts, And More

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Animals with backbones, also known as vertebrates, are a well-known group of animals, including mammals, birds, reptiles, amphibians, and fishes.

The main feature of animals belonging to this group is their backbone, an anatomical feature that first appeared in the fossil record about 500 million years ago during the Ordovician period.

Here are the different groups of vertebrates in the order of their evolution.

What are Animals With Backbones (vertebrates)?

The vertebrates (animals with backbones) are divided into 5 standard classes, including:

  • Fishes
  • Mammals
  • Birds
  • Amphibians
  • Reptiles

Currently, about 58,000 species of vertebrates have been identified.

Basics of vertebrate evolution

Let’s dig deeper into the different groups of vertebrates in the order of their evolution.

Jawless fish (Agnatha)

Agnatha

The first vertebrates were Agnatha. These fish-like animals have rigid bony plates covering their bodies.

As their name suggests, they have no backbones. In addition, these early fish did not have a pair of fins.

Agnatha is thought to have relied on filter-feeding for food and most likely sucked water and debris from the seafloor into their mouths, excreting water and waste through their gills.

Jawless fish that lived during the Ordovician period became extinct by the end of the Devonian period. Yet, some fish still do not have jaws, such as lamp fish and hagfish.

Today’s jawless species are not direct survivors of the Class Agnatha. Instead, they are distant cousins of cartilaginous fish.

Armored Fish (Placodermi)

Armored Fish

Placodermi evolved during the Silurian period. Like their predecessors, they also lack jaws but possess pairs of fins.

Placodermi diversified during the Devonian period but declined and fell into extinction by the late Permian.

Cartilaginous fish (Chondrichthyes)

Cartilaginous fish

Cartilaginous fish, including sharks, and rays, evolved during the Silurian. Cartilaginous fish have skeletons made of cartilage, not bone.

They also differ from other fish because they do not have swimming organs and lungs.

Bony fish (Osteichthyes)

Bony fish first appeared at the end of the Silurian period. Most modern fish belong to this group.

Amphibians

Amphibians 

Amphibians were the first vertebrates to venture on land. The early amphibians retained many fish-like characteristics but diversified during the Carboniferous period.

However, they retain a close relationship with water, require a humid environment to keep their skin moist and produce fish-like eggs that lack a hard protective layer.

In addition, amphibians go through a larval stage that is entirely aquatic. Therefore, only adult animals can survive in terrestrial habitats.

Reptilia

Reptiles appeared during the Carboniferous period and quickly took over the dominant position of terrestrial vertebrates.

Reptiles have freed themselves from aquatic habitats, which amphibians cannot.

Reptiles have developed hard-shelled eggs that they can lay on dry land. Also, they have dry skin consisting of scales that protect and help retain moisture.

Reptiles develop larger and stronger legs than amphibians.

In addition, the reptile’s legs below the body (instead of on the side like in amphibians) gives them better mobility.

Bird (Aves)

Around the beginning of the Jurassic period, two groups of reptiles acquired the ability to fly. One of these groups later gave rise to birds.

Birds have evolved various adaptations, such as feathers, hollow bones, and warm blood that allow them to fly.

Mammalia

Mammals, like birds, evolved from reptilian ancestors. Mammals develop a four-chambered, hair-covered heart, and most (except for single species such as platypuses and echidnas) do not lay eggs. 

The following table shows the evolution of vertebrates. The creatures listed at the top of the table will evolve earlier than those at the bottom.

Animal group Main feature
Jawless fish
  • No function
  • No pairing fins
  • Give rise to armored fish, cartilaginous fish, and bony fish
Placoderms
  • No jaws
  • Armored Fish
Cartilaginous fish
  • Cartilaginous skeleton
  • No bladder
  • No lungs
  • Internal fertilization
Bony fish (Osteichthyes)
  • Have gills
  • Have Lung
  • Have Bladder
  • Contribute to the creation of amphibians
Amphibians
  • The first vertebrate to venture on land
  • Still quite attached to the aquatic habitat
  • External fertilization
  • Eggs without a shell or cover
  • Moist skin
Reptilia
  • Body with scales
  • Hard-shell eggs
  • Stronger legs lie just below the body
Bird (Aves)
  • Hairy
  • Have hollow bones
Mammalia
  • Hair
  • Mammary glands
  • Hot blood

 

Structure and function of parts

Additional features of this group are:

  • The muscule system is composed mainly of fleshy masses forming pairs.
  • The central nervous system is partially located within the spine.

Other defining features of an animal of the vertebrate type are the backbone or spinal cord and the internal skeleton.

The internal skeleton that can identify vertebrates comprises cartilage or bone, or sometimes both.

The exoskeleton in the form of bony armor was the first bony substance that vertebrates evolved.

The basic function of this skeleton likely is to store phosphate, which is secreted as calcium phosphate and stored around the body and contributes to the body’s protection.

The skeleton provides support for other organs during growth.

The skeletons of most vertebrates, except for most primitive forms, the skeletons of most vertebrates consist of a skull, spine, and two pairs of limbs.

The skull facilitates cognitive development because it protects vital organs such as the brain, eyes, and ears.

Both the spine and the limbs as a whole support the vertebrate body. This support facilitates movement.

The body of a vertebrate is a hollow cavity containing internal organs. The heart and respiratory organs are protected inside the body. The heart is usually located between the lungs.

The central nervous system of vertebrates consists of the brain and spinal cord.

In lower vertebrates, the brain mainly controls the function of receptors.

In higher vertebrates, the ratio of brain to body size is greater, making the ability to exchange information between parts of the brain higher.

Invertebrates and Vertebrates: What’s the Difference?

Following are the critical differences between Invertebrates and Vertebrates:

Backbone And Skull

The most notable difference between invertebrates and vertebrates is the presence of a backbone, the skull (outer shell of the brain).

These organs are entirely absent in invertebrates and are well developed and functional in animals with backbones.

Other Features

Other features such as the nervous system, respiratory system, digestive system, gastrointestinal tract, and circulatory system are not well developed and organized in invertebrates.

Body Structure

  • The body structure of invertebrates is simple, with symmetry such as radial or bilateral.
  • Vertebrates have a complex and organized body structure with only bilateral body symmetry.

Popularity

Invertebrates have an overwhelming presence, with 98% of the total kingdom Animalia.

Meanwhile, vertebrates make up only 2% of the total kingdom of Animalia.

It means 98 out of 100 animals in the world today are invertebrates.

Habitat

Both types of animals group live in a variety of habitats.

However, vertebrates can fit and adapt to any habitat with ease.

The highly developed nervous system and skeleton of vertebrates allow them to adapt to land, sea, and air.

However, invertebrates are also found in various habitats, from forests and deserts to caves and seafloor mud.

The Bottom Line

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