What are Craniates, derived characteristics, and origin?

Introduction

The craniates colonize practically every environment on the earth.  They are the most adaptable animals in marine and freshwater habitats and they also occur on the surface of the ground, in the trees, under the ground, in the air, etc.  They also range in size from animals of just a few centimeters and grams to giant animals like whales, dinosaurs, elephants, etc.  Certainly, the weight of a female blue whale is truly surprising that it is the largest vertebrate that has ever lived till now.  

As it is clear from the name Craniata which indicates that these animals have a well-developed brain, anterior to their bodies that are enclosed in a bony, tough, and hard cranium which is made up of cartilage or bone.  The most primitive members of this phylum are the hagfishes, which have the standard chordate features and characteristics additional to a cranium.

Almost all the members of this phylum have clear and distinct skeletal elements which are the vertebral columns, that may develop and replace the notochord. Those organisms with a vertebral column are called Vertebrata, and without the vertebral column are called invertebrates. The internal skeleton which is developed by the vertebrates is highly prosperous and allows them the development of frames that help them to swim, walk, run, fly, hoop, etc.  They are likely to have appeared in the early Paleozoic era and became successful and dominant aquatic organisms in the Devonian era, sometimes called the era or age of fishes.   Animals having four legs, Tetrapods also appeared also in the early  Devonian era.

Scientific Classification:

Kingdom:     Animalia

Phylum:      Chordata

Subphylum: Olfactores

Clade:        Craniata

Derived Characteristics of Craniates:

• Head 

All the craniates have skulls having brains and well-developed heads.

• Hox Genes

All the craniates have 2 clusters or pairs of “Hox genes” but the early chordates like tunicates and lancelets have only one pair of Hox Genes. Other than the “Hox genes”,  all other genes which generate signaling molecules, stimulus, and transcription factors are also duplicated in craniates. This genetic variation and complexity made it possible for the craniates to develop more complex and developed body structures and morphologies as compared to tunicates and lancelets. 

• Neural Crest

 The Craniates also possess a unique neural crest. Which is formed by the accumulation of cells close to the dorsal sides of the neural tube where it pinches inside to form the ectoderm.  All these cells spread throughout the embryo and give rise to different types of body structures like teeth, the skull's bones and cartilages, the dermis which is the inner layer of skin of the facial region, different types of neurons, and various sensory capsules in which the eyes and other sense organs area develop. 

• Pharyngeal slits

In the aquatic craniates, the pharyngeal slits are evolved into gill slits. On the other side the lancelets used their pharyngeal slits typically for suspension feeding,  The gill slits of craniates are connected with nerves and muscles and help to pump out the water through the slits. This pumping also helps in the suction of food and helps in gas exchange. 

•  Craniates are more active

 The Craniates are more active as compared to lancelets and tunicates. They possess a well-developed metabolism and extensive muscular system. The lining of muscles in the craniates in their digestive system helps in digestion by moving food through the tract.

• Skeleton

Craniates also possess an endoskeleton, which is basically cartilaginously based but also becomes mineralized in many ways like bone and calcified cartilage in vertebrates. Only some craniates, like the gnathostome, and some fossil jawless vertebrates, possess a mineralized exoskeleton.

• Circulatory  System

Craniates have a circulatory system comprising the arteries, capillaries, and veins, and a heart which is enclosed in a chamber. They have red blood cells having hemoglobin along with kidneys that help to remove waste from the blood. The circulatory system is totally closed, and the heart has at least 2 large chambers named, the atrium and the ventricle.

• Digestive System

Craniates also have a complete and well-developed digestive system. The digestive tract comprises the mouth and oral cavity, pharynx, esophagus, intestine, rectum, and anus. All craniates also have a pancreas that produces and secretes digestive enzymes and hormones, like insulin and glucagon, that regulates the blood sugar level and aid in digestion.

• Hepatic Organ 

All craniates have a liver or hepatic organ that performs various functions, including storage of food and the production of bile.

Evolution:

The very first known craniates were marine organisms that evolved almost, about 480 million years ago. These primitives craniates are considered to have diverged from the lancelets.

At the stage of embryos, craniates have diversified tissues that are called the neural crest. The neural crest helps in the development of different types of structures in the adult animal like ganglia, nerve cells, endocrine glands, tissues, and connective tissue of the skull. Craniates, like all chordates, develop a notochord that is present in hagfishes and lampreys it disappears in most vertebrates and is mostly replaced by the vertebral column.

 

Among all the craniates, the most primitive organisms are the hagfishes. Hagfishes do not possess a bony skull. But their skull is made up of cartilage which is a strong but flexible substance consisting of the protein called keratin.  Among all the craniates, Hagfishes are the only living craniates that have a skull but lack a vertebral column or backbone.

Classes of Craniates:

• Agnatha -Jawless Fishes

• Chondrichthyes -Sharks, skates, rays, and chimeras

• Osteichthyes -Bony Fishes

• Amphibia -Amphibians

• Reptilia -Reptiles

• Mammalia -Mammals

• Aves -Birds