Only six lungfish species guard a 410-million-year line
Lungfish are the closest living relatives of tetrapods—the clade that includes amphibians, reptiles, birds, and mammals. Just six freshwater species survive in Africa, South America, and Australia, yet fossils reach the Early Devonian more than 410 million years ago, when the first members still swam in the sea.
Dipnoans carry lobed fins with robust internal skeletons and tooth plates for crushing hard prey. All show an unbroken cartilaginous notochord; modern forms reduce skull-roof bones and lose the mineralised cosmine armour of Devonian ancestors. Their lungs connect to larynx and pharynx without a trachea and are subdivided into many air sacs—unlike the simple vascularised swim bladders some other fishes use. Most living species have two lungs; the Australian lungfish has one. Circulatory shunts reconfigure when the animal switches between gill and air breathing. The name dipnoi itself points to that dual breathing kit.
African and South American lungfish can burrow into mud and estivate through dry seasons, slowing metabolism to as little as one-sixtieth of normal and converting ammonia waste to urea. Burrowing appears already in fossil gnathorhizids. The Queensland lungfish (Neoceratodus forsteri) cannot survive total water loss but can last several days emersed if kept moist; fossils of Neoceratodus in New South Wales imply at least 100 million years in Australia. "Granddad" at Chicago's Shedd Aquarium lived from 1933 to 2017 and was euthanised at about 109; "Methuselah," still living as of 2022 reporting, is estimated over ninety.
South America's Lepidosiren paradoxa, an obligate air-breather in Amazon and Paraná basins, may reach 125 centimetres with nearly useless adult gills. Africa's marbled lungfish (Protopterus aethiopicus) can hit 200 centimetres and holds the largest known vertebrate genome—133 billion base pairs—while the gilled lungfish P. amphibius, about 44 centimetres, is the smallest living species. Roughly 420 million years ago the lungfish and tetrapod lines parted; today's living lungfish last shared a common ancestor somewhere in the long stretch from Late Carboniferous time through the Jurassic.
Source: Lungfish
