Vertebrates

The term ‘Vertebrate’ is derived from the Latin word’ vertebratus,’ which means ‘joint of the spine.’ Thus, vertebrates refer to animals with a characteristic bony or cartilaginous axial endoskeleton known as the vertebral column, spine, or backbone. Fish, amphibians, reptiles, birds, and mammals are all examples of vertebrates. 

They are members of the subphylum Vertebrata (also known as Craniata) within the phylum Chordata. Of more than 81,000 chordate species, over 69,963 are vertebrates, accounting for less than 5% of all animals living on Earth, the rest being invertebrates.

Scientific Classification

  • Kingdom Animalia
  • Superphylum Deuterostomia
  • Phylum Chordata
  • Clade Olfactores
  • Subphylum Vertebrata

Description

Size

They could be as tiny as the microhylid frog (Paedophryne amanuensis), the smallest vertebrate with a length of 7.7 mm (0.30 inches) to the colossal Blue Whale (the largest) that attains a length of almost 33 m (108 feet).

Body Plan

All vertebrates follow a general body plan containing the vertebral column (except hagfish, which retain the embryonic notochord till adulthood) with a hollow nerve cord situated dorsally. A series of elements replace the embryonic notochord in vertebrates collectively called the vertebrae, separated by the fibrocartilaginous discs. 

They also possess pharyngeal arches and the endostyle that develops into the thyroid gland in adults.  

Gills and Gill Arches

All aquatic vertebrates breathe through a series of gills, each supported by a bony or cartilaginous gill arch. While primitive jawless fish have seven pairs of gill arches, the numbers decreased through evolution, with cartilaginous fishes now having five to seven pairs and bony fish having just three. In some primitive bony fish and amphibians, the larvae possess external gills but are replaced either by internal gills covered by slits or lungs in adulthood.

Although the gills have been completely eliminated in advanced vertebrates, the fetal gill arches are retained in the form of jaws, thyroid gland, larynx, or ear bones.

Organ System

  • They have a closed circulatory system with body fluid circulating through blood vessels; however, the blood cells can move in and out of the body fluid.
  • Respiration involves specialized structures like lungs or gills, which exchange gases between the organism’s body and the environment and between the blood system and the body’s tissues.
  • They have a digestive system comprising an alimentary canal and ancillary organs for breaking down food and nutrient assimilation.
  • A unique feature of their excretory system is the unit nephron, which filters blood and reabsorbs wastes in a network of blood vessels called the glomeruli.
  • The central nervous system is characterized by a hollow nerve cord running dorsally along the notochord. The neural tube expands into three primary brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).

Taxonomy

Initially, on the basis of anatomical and physiological traits, vertebrates were grouped into seven classes (Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammals). With time, scientists made further modifications to this classification, focusing more on the phylogenetic relationships between early reptiles and amphibians. However, this traditional method of classifying vertebrates did not consider all the descendants of the common ancestors, and hence, most groups ended up as paraphyletic. 

Thus, scientists have presently resorted to phylogenetic relationships and revised the conventional scheme solely based on their known evolutionary history.

  • Agnatha (jawless fish)
    • Cyclostomi (hagfish and lampreys)
    • Conodonta (Extinct)
    • Ostracodermi (Extinct)
  • Gnathostomata (jawed vertebrates)
    • Placodermi (Extinct)
    • Acanthodii (Extinct)
    • Chondrichthyes (cartilaginous fish, like sharks, rays, and ratfish)
    • Osteichthyes (bony fish)
      • Actinopterygii (ray-finned fish) 
        • Cladistia (bichirs and relatives)
        • Chondrostei (sturgeons and paddlefish)
        • Holostei (bowfins and gars)
        • Teleostei
      • Sarcopterygii (lobe-finned fish)
        • Actinistia (coelacanths)
        • Dipnomorpha (lungfish)
        • Tetrapoda (limbed vertebrates)
          • Amphibians (lissamphibians and the extinct temnospondyls and lepospondyls)
          • Amniotes (true land vertebrates)
            • Sauropsids (reptiles and birds, & their extinct para reptiles, dinosaurs, pterosaurs, and Mesozoic marine reptiles)
            • Synapsids (mammals & their extinct relatives and pelycosaurid or therapsid ancestors)

Vertebrates (Vertebrata)

Evolution

  • Vertebrates originated during the Cambrian Explosion, with the earliest known fossils unearthed from the Maotianshan Shales in China (518 million years ago). Some of the earliest species were Haikouichthys, Myllokunmingia, and Haikouella, which had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.
  • The Late Ordovician Period (about 445 million years ago) marked the advent of the first jawed vertebrates that further branched into the two groups of bony fishes, the actinopterygii and sarcopterygii, during the Devonian Period.
  • By mid-Devonian, environmental circumstances led to the rise of amphibians, and by the early Carboniferous Period, the amniotes branched from amphibious tetrapods.
  • While diapsid amniotes were dominant during the late Paleozoic Era, diapsid amniotes became dominant during the Mesozoic Era. In the Jurrasic Period, cynodonts gave rise to endothermic mammals, and diapsids eventually gave rise to endothermic birds.
  • Finally, bony fishes, amphibians, reptiles, birds, and mammals diversified during the Cenozoic Era.

Lifespan

While the shortest-living vertebrate, Eviota sigillata, a tiny coral reef fish, completes its life cycle within just eight weeks, the longest-living vertebrate, the Greenland Shark, could live for as long as 400 years.

Reproduction

Most vertebrates undergo sexual reproduction, in which a smaller, motile gamete called spermatozoa (haploid) fuses with a larger, non-motile gamete known as ova (haploid) to produce a diploid zygote through fertilization. Two varieties of killifish, Kryptolebias marmoratus and Kryptolebias hermaphroditus, are the only known vertebrates to produce their own eggs and sperm by meiosis and self-fertilize.

However, some species, like the Colombian Rainbow boa, copperhead snake, and cotton mouth snake, undergo facultative parthenogenesis, in which haploid males develop from unfertilized eggs produced by the females.

References

  1. Vertebrate: Wikipedia.org 
  2. Vertebrate: Britannica.com 
  3. Vertebrate: Britannica.com
  4. Introduction to Vertebrates: Berkeley.edu