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Comment to the reader.
The previous entry ended in the late Carboniferous, in the swamp forests of what is now Nova Scotia, with a small lizard-like animal sheltering inside a hollow tree. That animal, Hylonomus, was an early amniote — a member of the lineage whose innovation, the amniotic egg, had freed vertebrate reproduction from the need for standing water. With that freedom, the dry interior of the continents was open to vertebrates for the first time.
This entry follows what happened next. Almost immediately after the amniotes appeared, their lineage split in two. One branch led to mammals, including the species writing these words. The other led to what readers today would loosely call reptiles — including the dinosaurs and their surviving descendants the birds. The history of the next two and a half hundred million years is, in large part, the history of those two branches: their divergence, their separate rises and falls, two near-extinctions, and the eventual displacement of one by the other.
I write in the spring of 2026 of the Common Era. Several of the dates and benchmarks given here were last revised within the past few years and may move again. Where a working benchmark has shifted recently or remains contested, I have flagged it in the prose.
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How did the descendants of a small Carboniferous amniote come to include both an albatross and a human being?
The previous entry traced life from the deep freeze of the Cryogenian to the appearance of vertebrates that could lay their eggs on dry ground. From here, the arc narrows. Two lineages, both descended from those early amniotes, will spend the next two hundred and fifty million years dividing the world's land vertebrates between them. One will rise spectacularly, dominate the planet for more than a hundred and thirty million years, suffer the worst ecological catastrophe since multicellular life began, and re-establish itself in radically different form. The other will live in its shadow for nearly the whole of that time, then inherit the world. A small subset of the second lineage will, much later, climb into trees and look out from them with forward-facing eyes — and from that vantage, eventually, write these words.
This is the story of that division and the events that decided it.
The split appears to have occurred in the late Carboniferous, around 318 to 315 million years ago, in the equatorial coal swamps of what was then a single supercontinent assembling toward Pangaea. The two daughter lineages are now distinguished by a pair of holes in the skull behind the eye socket. The arrangement, position, and number of these temporal fenestrae are a key diagnostic feature of the early amniotes that have left fossils, and they remain the marker by which the two halves of the modern tetrapod world are most often distinguished.
The lineage with one pair of openings, low on each side of the skull, became the synapsids. The lineage with two pairs, or with the openings positioned higher up, became the sauropsids. Sauropsids are what readers today would loosely call reptiles, plus the birds — turtles, lizards, snakes, crocodiles, dinosaurs, and the surviving descendants of the dinosaurs that still nest in hedgerows. Synapsids are the mammals, plus an enormous extinct radiation that filled out most of the Permian.
The earliest known animals firmly on the synapsid side of the split are Archaeothyris florensis and the more poorly preserved Echinerpeton intermedium, both described from late-Carboniferous rocks at Florence, in Cape Breton, Nova Scotia, not far from the locality where Hylonomus itself was recovered. They lived around 310 million years ago, in the late part of the Moscovian stage, on current stratigraphic calibration of the host rocks [1]. They are not impressive creatures by modern standards. They are unmistakably the kind of animal from which the rest of this entry's cast descends.
For the first stretch of their history, both branches were sidelined. Through the late Carboniferous, the dominant tetrapods of the equatorial swamps remained the large amphibian-grade vertebrates of the previous entry: Eryops-like predators, hippo-sized waterside hunters, and a variety of forms with no modern analogue. The early amniotes were small and ecologically marginal. What changed that, in the Permian, was the climate.
The Carboniferous coal forests collapsed at around 305 million years ago in an episode now known as the Carboniferous Rainforest Collapse, driven by a shift to drier and more seasonal climates as Pangaea continued to assemble. The amphibian-grade vertebrates, dependent on water for reproduction, suffered disproportionately. The amniotes, which did not need standing water to breed, did not. The early Permian — running from roughly 299 to 252 million years ago — opens with the synapsids expanding into ecological roles that the disappearing amphibian-grade animals had occupied.
The first wave of synapsids is the group informally called pelycosaurs — a paraphyletic collection of basal synapsid lineages that includes the most famous early-Permian animal of all, Dimetrodon. Pelycosaurs were not yet mammals in any meaningful sense. They were sprawling, lizard-shaped quadrupeds with limbs splayed sideways from the body, and their reproduction, metabolism, and skin were all closer to those of a modern reptile than to those of any modern mammal. What they brought to the world was teeth. For the first time among amniotes, the teeth in a single jaw were differentiated — small incisor-like teeth at the front for nipping, larger canine-like teeth for puncturing, and smaller cheek teeth behind for processing. Dimetrodon itself, with its enormous dorsal sail supported by elongated vertebral spines and its powerful canines, was the apex predator of its early-Permian ecosystem and probably the first vertebrate to occupy that role on dry continental ground.
The pelycosaurs did not last. Sometime in the late early Permian, around 280 million years ago, a more derived group of synapsids — the therapsids — appeared, evolved from one of the pelycosaur lineages, and progressively replaced their ancestors over the following several million years [2]. Therapsids were better adapted to active life on land. Their limbs were drawn in beneath the body rather than splayed out, their gait was more upright, their teeth more elaborately differentiated, and the bones of the lower jaw began the long simplification that would, much later, leave the mammalian jaw as a single bone on each side. By the late Permian, therapsids had radiated into a great variety of forms: tusked herbivores, sabre-toothed carnivores, dog-sized burrowers, and the small, slender, mammal-like cynodonts from which the mammals themselves would eventually derive. They were not mammals — fur, milk, and warm-bloodedness in the modern sense almost certainly came later, in scattered features, across multiple cynodont lineages — but they had the unmistakable look of animals on the way to becoming something else.
For roughly forty million years, the synapsids were the dominant land vertebrates on Earth. The branch that would eventually produce the dinosaurs — the sauropsids — remained relatively quiet through this stretch, diversifying in the background but not yet challenging the synapsid hold on terrestrial ecosystems. Then, at the very end of the Permian, the world ended.
The boundary between the Permian and the Triassic is the most catastrophic moment recorded anywhere in the fossil history of complex life. The formal boundary is now placed at 251.902 ± 0.024 million years ago, with the principal pulse of the extinction itself constrained, by high-precision uranium-lead dating of volcanic ash beds in the Chinese sections at Meishan that bracket the horizon, to an interval of about sixty thousand years between roughly 251.941 and 251.880 million years ago [3]. Around eighty to ninety per cent of marine species and the majority of terrestrial vertebrate species disappear from the rock record across this interval. The event is now informally referred to as the Great Dying. Geologically, the pulse was a flash; biologically, it was an epoch.
The principal cause is now broadly settled. Coincident with the extinction, a vast outpouring of basaltic lava and underlying intrusive sills was emplaced across what is now western Siberia. The Siberian Traps large igneous province covered millions of square kilometres at its peak and represents one of the largest volcanic events in the planet's history. Recent work has shown that the most lethal phase of the eruption was not the surface lava flows but the intrusion of magma sheets into the surrounding sedimentary rocks, including thick deposits of coal and evaporites, which were cooked by contact metamorphism and released vast quantities of carbon dioxide, methane, and halogenated compounds into the atmosphere [4]. The atmospheric loading drove rapid global warming, ocean acidification, and the spread of oxygen-poor and sulphide-rich conditions through much of the marine water column. Coral reefs collapsed. Marine invertebrate diversity reached its lowest point in the entire Phanerozoic. On land, the diverse late-Permian therapsid faunas that had ruled for forty million years were almost entirely wiped out.