HomeVertebratesGnathostomes

Gnathostomes

Gnathostomata

Gnathostomes, also known as jawed vertebrates, belong to the infraphylum Gnathostomata, a name that originated from the Greek words ‘gnathos’ for ‘jaw’ and ‘stoma’ for ‘mouth.’ This group encompasses three extant subgroups: cartilaginous fish (Chondrichthyes), bony fish (Osteichthyes), and tetrapods (Tetrapoda). However, two extinct subgroups, placoderms (Placodermi) and acanthodians (Acanthodii), are also members of this broad group.

With approximately 60,000 species, gnathostomes represent 99% of all living vertebrates.

Characteristics

All gnathostomes possess a pair of bones forming the basic framework of the mouth. They also possess paired appendages, such as pectoral and pelvic fins, arms, legs, and wings. Unlike their jawless counterparts in the infraphylum Agnatha, gnathostomes have jaws hinged to their skulls, enabling them to bite and chew. Although lost in some species, the rest contain true teeth, aired limbs (such as fins, arms, legs, and wings), and a semicircular canal in their inner ear. 

Additionally, gnathostomes are distinguished by the presence of elastin, a protein that provides elasticity, myelin sheaths that insulate neurons, and a sophisticated immune system that includes lymphoid organs like the spleen and thymus and utilizes V(D)J recombination to generate diverse antigen recognition sites.

These vertebrates possess myelinated neurons (covered by a myelin sheath) and the protein elastin in the epidermis of skin, ligaments, cartilages, and other body parts that provide elasticity.

All extant and extinct subgroups of gnathostomes, along with their members, are listed below with their features.

Extant SubgroupMembersFeatures
Cartilaginous Fish 
(Over 1,000 species is divided into 2 subclasses)
Sharks, rays, skates, sawfish, and chimaeras.1. Their skeletons are composed of cartilage.
2. They have paired fins and nares, scales, and a heart with chambers arranged in series.
3. They possess specialized sense organs for olfaction and electroreception, such as the Ampulla of Lorenzini in sharks, which detects electromagnetic fields underwater.
4. They have a series of sensory organs known as the lateral line system, which detects vibrations and movement in water.
Bony Fish (Around 28,000 species are grouped into 45 orders and 435 families)Ray-finned and lobe-finned fish.1. They are characterized by an ossified (bony) skeleton.
2. Their bones contain bone cells (osteocytes) that regulate calcium balance in the body.
3. All bony fishes breathe through gills, which are protected by a gill cover (operculum), and utilize their lateral line system to detect vibrations.
Tetrapods (Over 30,000 species are divided into 3 classes and 11 subclasses)Amphibians, reptiles (including birds), and mammals.1. They are distinguished by four limbs (often modified into different structures, like flippers, based on the niches) and well-developed pelvic and pectoral girdles.
2. Several groups, like reptilian snakes and mammalian cetaceans, have lost these limbs as an adaptation for their respective lifestyles.
Extinct SubgroupMembersFeatures
PlacodermiPerished by the end of the Devonian Period (around 420 to 359 million years ago)Armored Fish1. They were among the first jawed fish with heads and thorax covered by armored plates.
2. Depending on the species, their bodies were scaled or naked.
3. Their jaws probably evolved from the first pair of gill arches.
AcanthodiiPerished by the end of the Permian Period (around 250 million years ago)Spiny Sharks1. They resembled sharks in appearance but possessed tiny rhomboid platelets like the scales of bowfins and gars.
2. These fish had a cartilaginous skeleton, but the bases of their fins were bony.
3. They possessed an upturned tail and stout, bony spines on all fins except the caudal (hence their name).

Taxonomy

Gnathostomata has been traditionally considered an infraphylum, which was classified into three broad groups: Chondrichthyes (cartilaginous fish), Placodermi (armored fish, now extinct), and Teleostomi (bony fish, birds, mammals, reptiles, and amphibians). Although Acanthodii (a class under Teleostomi) have gone extinct, the other groups, the bony fish (Osteichthyes) and tetrapods (amphibians, reptiles, birds, and mammals), are still extant and constantly evolving with time.

Around 60,000 species of gnathostomes are currently classified under the classes Chondrichthyes, Osteichthyes, and the superclass Tetrapoda.

A group of scientists also considers the class Ostreostraci (not included in the above list), comprising armored jawless fish, a sister group to Gnathostomata.

Evolution and Fossil Records

Jawless fish struggled to survive compared to their jawed counterparts, leading to the extinction of most jawless species during the Triassic Period. However, research on the surviving cyclostomes, which include jawless hagfishes and lampreys, has provided some insight into the significant changes that occurred in the vertebrate skull during the evolution of early jaws.

The ancestors of gnathostomes experienced two rounds of whole genome duplication. The first round occurred before the divergence of jawed and jawless fish, while the second round happened after their split, possibly due to the hybridization of the two lineages.

Recent research indicates that jawed fish evolved from a branch of placoderms. Entelognathus, a 419-million-year-old placoderm fossil, exhibits traits common to both bony and cartilaginous fish. Additionally, primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived simultaneously as Entelognathus, possess features like pelvic girdles, strengthening the ancestral connection between placoderms and jawed fish.

It is believed that the jaws developed from the supporting gill arches, helping these fish push water easily into the mouth. On constantly using the jaw, the gnathostomes also developed the ability to bite.

By the Devonian Period (around 420 to 359 million years ago), these vertebrates had diversified and spread globally, and is thus called the ‘Age of Fishes.’ Late Ordovician (485 to 443 million years ago) microfossils of acanthodian scales mark the first appearance of gnathostomes in the fossil records. Later, Qianodus and Fanjingshania fossils from the early Silurian Period (around 439 million years ago) provided the oldest undisputed evidence of the existence of jawed vertebrates.

References Article last updated on 10th July 2024
Comments

Leave a Reply

Your email address will not be published. Required fields are marked *