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The evolutionary history of amphibians is a fascinating journey that dates back over 300 million years. Amphibians are believed to have evolved from lobe-finned fishes, specifically from a subgroup known as the tetrapodomorphs during the Devonian period. These early amphibian ancestors, like Tiktaalik, began to develop limbs and lungs to adapt to life on land, although they still depended on water for reproduction. The first true amphibians emerged in the Carboniferous period, diversifying into a variety of forms, some of which were quite large compared to today's species. Over time, amphibians branched into three main groups: Anura (frogs and toads), Caudata (salamanders), and Gymnophiona (caecilians), each adapting to different ecological niches.
Throughout their evolutionary history, amphibians have faced numerous challenges, including mass extinctions and climate changes. Despite these hurdles, they have persisted and currently represent over 7,000 known species worldwide, as reported by AmphibiaWeb (2023). However, they are now facing a new set of threats, including habitat loss, climate change, pollution, and disease, such as the chytrid fungus that has devastated populations globally. Amphibians are often considered environmental indicators due to their sensitive skin and life cycle, which ties closely to both aquatic and terrestrial ecosystems. Their evolutionary history not only tells the story of life transitioning from water to land but also highlights the delicate balance of ecosystems that modern conservation efforts strive to protect.
Amphibians are a class of animal that includes modern-day frogs, toads, salamanders, newts, and caecilians. They first evolved from lobe-finned fish and primitive tetrapods about 340 million years ago. Sometimes this date is incorrectly given as 400 or 380 million years ago, but fossils have not been unearthed from these periods.
About 380 million years ago, during the Devonian period, some fish began to evolve legs and digits. These early "tetrapodomorphs" lacked the characteristics that define amphibians, so they are classified as basal tetrapods. Decades ago, they were classified as amphibians, though taxonomists have changed their view on the matter. This is why the origin of this class is sometimes incorrectly cited as 380 million years ago.
Some of the earliest tetrapods include Tiktaalik, among the earliest with a weight-bearing wrist structure, and Acanthostega, which had eight digits on each foot. These early species would have been mostly aquatic, and used their limbs to navigate through swamps rather than taking extensive journeys over the land.
Between 380 and 360 million years ago is a period called "Romer's gap," in which barely any tetrapod fossils have been found, casting a cloud of mystery on the evolution of the first amphibians from the early basal tetrapods. Prior to the gap, no fossils are found, and the first known amphibian fossil appears shortly after the gap. After the gap, the world was in the Carboniferous period, where sea levels were high and the coasts were covered with flooded forests and swamps.
The first amphibians were temnospondyls, long-headed animals with a sprawling gait and distinctive look. These were the first truly terrestrial tetrapods, and would have eaten themselves silly by consuming insects that lacked specialized adaptations for defending against large vertebrate predators. The early temnospondyls were the size of large fish, ranging from about 1.6 to 5 feet (0.5 to 1.5 meters) in length. The earliest ones had stubby feet, and probably couldn't move very fast.
Throughout the Carboniferous period, temnospondyls grew in size and diversity until they occupied many of the predatory and herbivorous niches that terrestrial animals exploit today. By the late Permian, some even grew to 30 feet (9 m) in length, and resembled crocodiles. This animal, Prionosuchus is the largest amphibian known. In the Carboniferous, temnospondyls were joined by the diverse but less numerous lepospondyls. Lissamphibians, the group that includes all modern amphibians and their common ancestors, emerged about 300 million years ago.