Chordates are a group of animals that are defined by common anatomical features like having a notochord, a dorsal hollow nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. This includes well-known animals like vertebrates, tunicates, and lancelets.

If you’re short on time, here’s a quick answer to your question: Not all chordates have a vertebral column, jaws, paired appendages, and a cranium.

In this comprehensive guide, we will explore the key characteristics of chordates and go through what traits are not universally shared by all members of this phylum.

Overview of Chordates

Chordates are a remarkable group of animals that share key anatomical features that sets them apart from other animal groups. Here’s an overview of what defines chordates and the major chordate groups.

Chordates are deuterostomes with a notochord

The presence of a notochord, a flexible rod that runs the length of the embryo, is one of the defining features of chordates. The notochord provides stiffness and structure to the developing embryo. In some chordates like vertebrates, it eventually develops into the backbone.

Chordates are also deuterostomes, meaning the embryo’s anus forms before its mouth. This sets them apart from protostomes like arthropods where the mouth develops first. So chordates develop “ass-first” compared to “mouth-first” in protostomes!

Key defining features of chordates

There are four key anatomical features that define chordates at some point in their development:

  • A notochord
  • A dorsal, hollow nerve cord
  • Pharyngeal slits or clefts
  • A muscular post-anal tail

The nerve cord eventually develops into a spinal cord in vertebrates. The pharyngeal slits become gill structures in aquatic chordates, allowing for filter feeding. And the muscular tail provides locomotion.

Major chordate groups

There are three major living groups of chordates:

  1. Vertebrates – Animals with backbones like fish, amphibians, reptiles, birds and mammals. Vertebrates make up over 90% of all chordates.
  2. Tunicates – Also called sea squirts. Tunicates are filter feeding animals found in coastal waters. They have a tadpole-like larval stage with chordate features, but become sessile as adults.
  3. Cephalochordates – Small, fish-like marine animals known as lancelets. They retain chordate features like a notochord and gill slits throughout their lives.

There are over 65,000 living species of chordates on Earth today! They display a remarkable diversity of forms and lifestyles, from 30-foot long blue whales to microscopic tunicates. Yet their shared developmental and anatomical hallmarks testify to their close evolutionary relationships.

Vertebral Column

The vertebral column is unique to vertebrates

The vertebral column, also known as the backbone or spine, is a defining characteristic of vertebrates including mammals, birds, reptiles, amphibians, and fish. It provides structure and support for the body and protects the spinal cord.

Invertebrate chordates like lancelets, tunicates, and hagfish notably lack a vertebral column. So while all vertebrates have a vertebral column, not all chordates do.

Invertebrate chordates lack a vertebral column

There are three groups of invertebrate chordates that lack a vertebral column:

  • Lancelets (subphylum Cephalochordata) – small, fish-like marine animals that retain the notochord throughout their lives. They do not develop vertebrae or joints.
  • Tunicates (subphylum Tunicata/Urochordata) – sac-like marine filter feeders like sea squirts that only retain the notochord in their larval stage before becoming sessile adults.
  • Hagfish (class Myxini) – eel-shaped slime producing marine animals that represent the most primitive group of living vertebrates. They have a cranium and rudimentary vertebrae but lack a true vertebral column.

So while lancelets and tunicates never develop vertebrae, hagfish have primordial vertebral elements but lack development of a full vertebral column.

The notochord is replaced by the vertebral column in vertebrates

In vertebrate embryonic development, the earliest axial support structure is the notochord which guides development of the neural tube. In vertebrates, segmented vertebral bodies later form around the notochord from paraxial mesoderm to create the vertebral column bones.

Over time, the notochord largely disappears and is replaced structurally by the vertebral column and intervertebral discs in most vertebrates. For example:

Vertebrate Group Notochord Status
Fish Replaced by vertebral column and persists only as nucleus pulposus of intervertebral discs
Amphibians Same as fish
Reptiles and birds Same as fish
Mammals Same as fish

In contrast, invertebrate chordates like lancelets retain the notochord for body support since vertebrae never develop. So while vertebrates replace the embryonic notochord with the vertebral column, invertebrate chordates depend on the notochord throughout their lives in the absence of vertebrae.

Paired Appendages

Paired appendages are not universal in chordates

The key defining characteristic of chordates, which sets them apart from other animal groups, is the presence of a notochord and a dorsal hollow nerve cord at some point during their development. However, in terms of morphological characteristics, not all chordates share the same features like paired appendages.

Paired appendages like fins and limbs are found in vertebrates but are not universally present across all chordates.

Vertebrates have paired fins and limbs

The vertebrates are the group of chordates that are characterized by having a backbone or vertebral column. Vertebrates like fish, amphibians, reptiles, birds and mammals all share the presence of paired fins and limbs that help with movement and navigating the environment.

For example, fish have paired pectoral and pelvic fins. Terrestrial vertebrates like birds, mammals, reptiles have paired forelimbs and hindlimbs with digits to support movement and grasping.

Invertebrate chordates lack paired appendages

In contrast to vertebrates, invertebrate chordates like the lancelets (amphioxus) and tunicates do not possess paired appendages. They retain the defined notochord and dorsal hollow nerve cord of chordates but show significant degeneration and simplification of organ systems.

For example, lancelets have an extensively simplified body with no distinct paired fins, limbs or complex organ systems. So not all chordates share all morphological features like limbs, which are limited to the vertebrates.

Some key references:

Cranium

A cranium protects the brain in vertebrates

All vertebrates have a bony or cartilaginous cranium that encloses and protects the brain. The cranium forms a protective vault around the brain and sense organs, providing mechanical support and protection.

In humans, the cranium is made up of 8 bones which fuse together along suture lines as the skull matures. The bones of the cranium include the frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones. These bones articulate with one another at fixed joints called sutures.

The cranium has openings like the foramen magnum through which the spinal cord connects to the brain. There are also openings for blood vessels, cranial nerves and special senses like vision, smell, and hearing.

The cranium varies in shape and structure across vertebrate species based on ecological niche and dietary needs, but its key function is always to house and protect the brain.

Invertebrate chordates lack a cranium

Invertebrate chordates like lancelets and tunicates do not have a bony or cartilaginous cranium. As primitive chordates, they have a notochord but lack a true vertebral column. While lancelets have a simple tubular nerve cord running along the back, they do not have an enclosed brain or specialized sense organs found in vertebrates.

Therefore, a rigid cranium for protection is unnecessary. The nervous system in invertebrate chordates is diffuse and decentralized. In contrast, vertebrates have centralized nervous systems encased in bony cranial vaults.

The evolution of the cranium allowed development of larger, more complex brains in vertebrates compared to invertebrate ancestors. So while all chordates share common characteristics like the notochord, only vertebrates developed the critical innovation of the cranium and its protective capacity.

The cranium forms from cartilage or bone in vertebrates

In vertebrate embryos, the cranium originates from mesodermal tissue that forms cranial cartilage precursors. In humans, this cartilage model of the skull is replaced by bone in a process called endochondral ossification.

The cartilage acts as a template upon which bony cranial elements are laid down. In other vertebrates like sharks, the cranium remains composed of cartilage into adulthood. Whether bone or cartilage, the vertebrate cranium arises from cranial neural crest cells that migrate and coalesce to surround the developing brain.

Malformations of the cranium like craniosynostosis can occur if this complex developmental process is disrupted. While the material may vary, a hard cranium protects the brain across all vertebrates from jawed fishes to mammals.

This key innovation provided mechanical support and defense for larger, more complex brains to evolve.

Jaws

Jaws are restricted to vertebrates

While jaws may seem ubiquitous in the animal kingdom today, jawless fish were actually the first vertebrates. Primitive jawless fish like lampreys and hagfish do not have true jaws and instead use their round sucking mouths to latch onto prey and rasp away tissue.

It’s hard to imagine, but around 505 million years ago, all vertebrates were jawless!

Jaws evolved from the first gill arch and are unique to gnathostomes, which includes all vertebrates except jawless fish. Jaws allowed early gnathostomes to grasp prey and tear pieces using their teeth—a huge advantage.

Soon after jaws debuted, vertebrates diversified into sharks, bony fish, tetrapods, and more. Jaws were so effective that jawless vertebrates declined. Today, only around 120 species remain jawless.

Jaws are modified gill arches

Gill arches are bony curved bars that support the gills of fish and some amphibians. All vertebrate embryos develop pharyngeal arches that develop into structures like jaws and gills. While jawless fish retain the ancestral state offilter feeding gill arches, jawed vertebrates modified the first arch into the upper and lower jaw.

The jaw’s top section, the maxilla, and bottom section, the mandible, are hinged together by jointed bones and muscles for controlled biting and chewing. This novel ability changed early vertebrates’ survival strategies and diets.

Today, all jawed vertebrates retain this essential modification of the first embryonic gill arch that creates this useful bodypart.

Invertebrate chordates use a filtration feeding system

Chordates are defined by four key traits: a notochord, dorsal hollow nerve cord, pharyngeal slits or clefts, and a post-anal tail. The pharyngeal slits filter seawater over the gills to trap food particles in all chordate groups—from primitive invertebrate groups like lancelets and tunicates to vertebrates.

Unlike jawed vertebrates, invertebrate chordates like lancelets capture food using mucus nets or beat cilia to sweep water into their mouths passively. They lack true jaws for active hunting or biting.

Instead, they use their many gill slits lined with filtration meshes to sieve the water for microorganisms and detritus washed over their gills. This more primitive system limits them to small, passive prey instead of vertebrates’ advanced ability to grasp and tear large, active animals.

Conclusion

In summary, while all chordates share common anatomical features like a notochord, nerve cord and pharyngeal slits, traits like a vertebral column, jaws, paired appendages and cranium are specific to vertebrates.

The invertebrate chordates – tunicates and lancelets – lack these specializations present in vertebrates. So not all chordates have the same complex bodies as vertebrates like mammals, birds, reptiles, amphibians and fishes.

Understanding the diversity within chordates provides insight into how vertebrates evolved and adapted over time from simpler invertebrate ancestors.

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