Tetrapods are vertebrates with four limbs. They belong to the superclass Tetrapoda, with over 30,000 species, including all extant and extinct anamniotes (amphibians) and anamniotes (reptiles and mammals).
Their name is obtained from the Ancient Greek words ‘tetra,’ meaning ‘four,’ and ‘pous,’ meaning ‘foot,’ referring to their ‘four legs.’ These vertebrates evolved during the Devonian Period from semi-aquatic tetrapodomorphs, a group of primitive bony fish of the clade Sarcopterygii.
These vertebrates have several anatomical and physiological features distinguishing them from their fish ancestors. They have a distinct head and neck, an appendicular skeleton, well-developed middle ears, more sensitive eyes, and more efficient hearts and lungs for gas and nutrient exchange throughout their bodies.
In early tetrapods, the brain occupied only half of the skull, while the other half was filled with fatty tissue or fluid. Unlike fish, tetrapods had elongated skulls at the front, which pushed their eye sockets farther back.
Early tetrapodomorph fishes had a neck region covered by opercular bones that protected their gills. Basal tetrapods, however, lost these bones while retaining the underlying gill arches. They also lost the gular bones covering the throat and other structures like the extrascapular and supracleithral bones. This loss allowed tetrapods to develop a neck, which helps in their independent head rotation. The neck joint is a pivot for the spine that pushes against the back of the skull.
The shoulder or pectoral girdle of tetrapods remains separate from the skull to facilitate efficient movement on land. In primitive forms, the clavicle and interclavicle bones join ventrally to form a broad chest plate. The upper part of this plate evolves into a shoulder bone or scapular plate, which includes the glenoid cavity, enabling the articulation of the forelimb bone, the humerus.
The pelvic girdle consists of the ilium, ischium, and pubis bones, meeting at the center to form the pelvic triangle or acetabulum. This structure provides the space for articulating the hindlimb bone, the femur. A series of sacral ribs connect the pelvic girdle to the backbone.
Tetrapods are characterized by having four limbs: a pair of forelimbs and a pair of hindlimbs. Each limb features a sturdy bone attached to the body at one end and two long bones at the other.
These two long bones connect to smaller carpal bones in the forelimbs, forming the hand or manus. In the hindlimbs, they connect to tarsal bones, forming the foot or pes. The number of digits on each limb was initially as high as seven or eight in early tetrapods, but this number decreased in more advanced groups through evolution.
Early tetrapods had wide, gaping jaws with moderately sized palatal, vomerine, and coronoid fangs, facilitating suction feeding and biting prey underwater.
In more advanced terrestrial tetrapods, jaws function through two distinct systems: the static and kinetic inertial systems. In the static system, jaw muscles exert maximum force on prey when nearly closed. In contrast, the kinetic system, also known as snapping, achieves maximum biting force when the jaws are wide open.
The tongue, attached to the hyoid bone of the skull, includes muscles that originally controlled gill openings in ancestral forms.
Early tetrapods were characterized by labyrinthodont or labyrinthodont plicidentine teeth, notable for their infolded outer enamel layer. These teeth were especially strong in juveniles, facilitating the consumption of soft prey. However, in adults, this dental structure became less advantageous.
Early tetrapods, like modern amphibians and reptiles, had a three-chambered heart in which oxygenated blood from the lungs and deoxygenated blood from the rest of the body mix up in a single ventricle. The oxygenated blood then moves to the aorta, a large blood vessel that distributes blood throughout the body.
With evolution, advanced tetrapods like birds and mammals developed four-chambered hearts with two atria and two ventricles. This adaptation improved the efficiency of oxygenation and blood circulation.
Tetrapods respire through gills, lungs, skin, and sometimes even through the lining of their digestive tract.
Some primitive tetrapods, such as Acanthostega and Archegosaurus, had functional gills through which gases were exchanged with the surrounding water. A few aquatic temnospondyls retained their internal gills till the early Jurassic Period.
Air enters a pair of lungs through buccal pumping, which helps contract thoracoabdominal muscles along the ribs. In mammals, the characteristic muscular diaphragm helps expand and contract the ribs during respiration.
Some modern-day tetrapods, like fish and amphibians, utilize their skin for respiration, a process known as cutaneous respiration. It is also hypothesized that some early tetrapods could have employed similar skin-based respiration methods.
One hypothesis proposes that early tetrapods may have used a passive exoskeletal recoil mechanism for breathing, akin to the primitive ray-finned fish Polypterus. In this mechanism, muscle contraction during exhalation indents the bony scales of the body. Upon relaxation of these muscles during inhalation, the scales spring back to their original position, creating a negative pressure that draws air into the body through openings known as spiracles.
Since early tetrapods had switched from an aquatic environment to land, they developed eyelids and tear ducts to keep their eyes moist. They also possess a set of rod and cone cells called opsins that help detect a range of light wavelengths.
In tetrapods, the otic notch is covered by a thin connective tissue membrane known as the tympanum or eardrum. The hyomandibula, typically found in fish, persists as the columella bone in tetrapods. This columella bone transmits vibrations from the environment to the interior of the head. It serves as an impedance-matching system by channeling sound waves to the receptors of the inner ear, which also contributes to balance.
Terrestrial tetrapods developed a tympanic middle ear around the early Triassic Period. The stapes, a distinctive bone in the mammalian middle ear, evolved as a reduced form of the columella bone.
Members of some groups, like lepidosaurs, possess a specialized vomeronasal organ for detecting pheromones and other air-borne biological compounds on land. This organ is vestigial in some lineages, like archosaurs and catarrhines.
Different scientists have different views on the definition of tetrapods. While some resort to cladistic classification based on apomorphies, others consider crown groups for classifying tetrapods.
Based on Apomorphy
Most paleontologists defined tetrapods based on the presence of four limbs and their digits. This definition is scientifically supported, as limbs and digits represent apomorphies, or newly evolved traits that define these animals.
However, relying solely on apomorphies could be misleading as different lineages could have evolved these traits independently through convergent evolution. Thus, scientists have made a cladistic analysis of tetrapods, revealing them as a monophyletic clade (all members share a recent common ancestor).
Based on the Crown Group
A group of scientists led by the French paleontologist Michel Laurin considered tetrapods to be a crown group representing the most recent common ancestor of all living members. In this node-based cladistic definition, tetrapods are considered a crown group to all living amphibians (anamniotes) and amniotes, such as reptiles, birds, and mammals.
However, this approach leaves out many extinct four-limbed vertebrates, like Ichthyostega and Acanthostega, which would otherwise be considered tetrapods. These vertebrates would then have to be placed under the stem group or stem tetrapods, representing members related to, but not within, the crown group.
Crown tetrapods have been further classified into two groups: Batrachomorpha and Reptiliomorpha. While batrachomorphs share a common ancestry with living anamniotes or amphibians, reptiliomorphs are more closely related to living amniotes.
Although tetrapods have historically been among the best-understood animals, there has been a perpetual debate on their classification. By the 18th century, Carl Linnaeus had already included tetrapods in his six broad classes of animals, and by the early 19th century, the French zoologist Pierre André Latreille had classified tetrapods into four classes: amphibians, reptiles, birds, and mammals. Since birds are considered reptiles according to the cladistic approach, there are currently three broad classes of tetrapods.
The following classification scheme is based on the works of Hildebrand and Goslow (2001).
This classification scheme, however, was contradictory from a cladistic point of view. Although the earliest tetrapods have been grouped under the class Amphibia, cladistics reveal that most tetrapods are more closely related to amniotes than modern-day amphibians. It also places birds under Reptilia since they are thought to be descendants of theropod dinosaurs.
Such considerations often compel scientists to focus on phylogeny rather than anatomical or physiological traits.
Tetrapods are believed to have evolved from early bony fishes during the Emsian stage of the Early Devonian Period. Their ancestors, the tetrapodomorphs of shallow waters, evolved from lobe-finned sarcopterygians.
The earliest tetrapods were similar to Acanthostega and were primarily aquatic. Since fossil records have been unearthed from continents once part of Laurasia, Gondwana, and even parts of North China, it is believed that tetrapod ancestors could swim and disperse to distant places through the seas of shallow continental shelves.
Several groups of tetrapodomorph fishes are considered ancestors of tetrapods. These include the rhizodonts, the osteolepidids, the tristichopterids, the elpistostegalians, and the genus Tiktaalik.
Tetrapods have evolved through major historical phases over millions of years to give rise to modern-day forms.
The earliest stem tetrapods appeared in the Early Devonian Period. They were similar to Ichthyostega, having legs, lungs, and gills but could not venture on land.
Two major Late Devonian extinction events washed off these fish-like stem tetrapods. However, around 10 million years after the extinction events, they reappeared during the early Carboniferous Period with features adapted for a terrestrial lifestyle.
The first crown tetrapods evolved by the Visean age of the Early Carboniferous, with Westlothiana being the oldest undisputed member. Around this time, the number of digits on these tetrapods’ limbs reduced to a maximum of five.
By the Mid-Carboniferous, these ancestors had already diversified into two lineages: the anamniotes or amphibians (from either the temnospondyls or the lepospondyls) and the amniotes (from the anthracosaurs). Basal amniotes, such as batrachomorphs and reptiliomorphs, were small and emerged in the Late Carboniferous.
Both amniotes and anamniotes were affected by the Carboniferous rainforest collapse (CRC), a major extinction event that took place around 300 million years ago.
The shift in ecological conditions that followed the collapse helped the amniotes occupy new niches and feed on plants and animals, including other tetrapods.
During the Permian Period, the amniotes further diversified into two major groups: sauropsids and synapsids, the latter of which was more successful. The Permian ended with the Permian-Triassic Extinction Event (around 252 million years ago), eliminating many large and diverse tetrapod groups.
During the Triassic Period, a subgroup of sauropsids known as diapsids emerged. This group of tetrapods included turtles, crocodiles, dinosaurs, and lepidosaurs.
Some lepidosaurs from the Jurassic Period evolved into lizards, which later gave rise to snakes during the Cretaceous Period. By the late Mesozoic Era, several once-prominent groups, such as temnospondyls, anomodonts, and therocephalians, had become extinct. However, a specific group of synapsids called cynodonts persisted, eventually leading to mammals.
The Cretaceous Period concluded abruptly with the Cretaceous-Paleogene extinction event, leading to the extinction of dinosaurs and many other large groups. Despite this mass extinction, birds survived and continued to diversify during the Cenozoic Era.
The early stem tetrapods were primarily aquatic but later adapted to a semi-aquatic or amphibious lifestyle. Modern-day amphibians are aquatic in their larval or tadpole stage and are partly terrestrial during adulthood. Similarly, most amniotes, like reptiles, birds, and mammals, mostly occupy terrestrial habitats.
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