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Early echolocating bat, clawed bat, hypothetical protobat

The prehistoric mammals of the air are bats, two early bats, Icaronycteris and Onychonycteris, and an even earlier protobat, which is hypothetical. Icaronycteris (“Icarus bat”) evidently hunted moths with echolocation about 50 Mya, which bats commonly do today. Onychonycteris (“clawed bat”) is the most primitive bat found so far. It had claws on all five of its wing fingers, unlike two for Icaronycteris and one for modern bats. These flying bats evolved from hypothetical protobats that could glide. They may have evolved from tree-climbing mammals that hung upside down to eat fruit or worms in the fruit. Today we think of bats as living in caves, but originally they evolved in trees, as did the first placental mammals.

Valenictus, Enliarctos, and Puijila

The prehistoric mammals of the water are pinnipeds (“fin foots”): the walrus-like Valenictus; the seal-like Enaliarctos; and the otter-like Puijila. Valenictus lived around 5 Mya, and it had no teeth other than its tusks. It evidently ate mollusks, like modern walruses do, rather than fish, as the earliest pinnipeds did. Enaliarctos (“Arctic sea bear”) lived around 20 Mya. It caught fish but took them back to land to eat them, rather than eating them in the water as modern seals do. Puijila (“young seal”) looked like an otter but is more closely related to seals. Charles Darwin speculated that a land-living, fish-hunting species could evolve into an aquatic species as they spent more and more time in the water, and the Pujila fossils provide strong evidence for just this process.

Glyptodont, ground sloth, and mammoth

The prehistoric mammals of the land are two xenarthrans (“strange joints”) and an afrothere (“Africa beast”). The xenarthrans are a glyptodont and a ground sloth, while the afrothere is a mammoth. Glyptodonts (“carved tooth”) were giant armadillos that arose about 20 Mya. Their armor protected them from giant flightless birds and other predators. Ground sloths evolved great size to protect themselves from most predators. They appeared about 35 Mya. Sloths are closely related to anteaters. Like all xenarthrans, they have long, clawed toes and low body temperatures. Xenarthrans evolved in South American after it split from Africa, and they spread to North America when the two land masses connected to each other 3 Mya. Afrotheres evolved in Africa when it was separate from the other land masses, leading to aardvarks, shrew-like tenrecs, manatees, and elephants. Elephants later spread into Asia, and mammoths reached North America. Glyptodonts, ground sloths, and mammoths died out with the end of the last ice age and the arrival of human hunters. Xenarthra and Afrotheria diverged from the other placental mammals long ago in the Mesozoic. Their ancestors either split off separately but at nearly the same time or they split off together and then diverged from each other soon afterwards.

Millipedes, pauropods, and centipedes

The prehistoric arthropods of the land are Myriapoda (“countless feet”): a gigantic millipede, Arthropleura; a millipede relative, a pauropod; and a predatory centipede. Myriapods were the first land animals. They breathed air in and out through spiracles, which are holes in their exoskeletons. Early myriapods ate rotting plants. Like pancrustaceans, the myriapods had mandibles that were good for chewing food. Millipedes (“thousand feet”) appeared over 400 million years ago, and their body segments were “doubled”, so they had two pairs of limbs per segment. Arthropleura (“jointed ribs”) lived 300 Mya, before the Permian extinction. The biggest Arthropleura were over 2 meters long, as big as the big eurypterids, and the biggest land arthropods ever. On the other hand, Pauropoda (“few feet”) are tiny, and scientists have found no fossil traces of them. They evidently split from the millipedes over 400 million years ago. Centipedes (“hundred feet”) split off even earlier. Unlike their immediate ancestors, centipedes were predators. Their two front legs no longer helped them walk but instead formed pincer-like venomous stingers called forcipules. It’s a sting, not a bite, because forcipules are limbs, not mouthparts. The centipede clade is called Chilopoda (“thousand feet”), and the millipede clade is called Diplopoda (“double feet”).

Two eurypterids and an early horseshoe crab

The extinct arthropods of the water are chelicerates: a eurypterid, Hughmilleria; a Stylonurid eurypterid; and an early horseshoe crab, Lunataspis. Eurypterids walked on legs and swam with paddles. They also had chelicerae (“claw horns”), which were claws by their mouths that helped them catch and eat prey. This adaptation helped them dominate the seas. Some, such as Hughmilleria, adapted to habitats with brackish or fresh water. Others, such as most of the Stylonurids, lost their swimming legs and only crawled. Like Eurypterids, horseshoe crabs are descended from the original chelicerate ancestors, along with scorpions, spiders, and other arachnids. These arthropods all arose during the Paleozoic Era, which ended in a great extinction event 252 Mya. Many sorts of arthropods survived, but not the eurypterids.

Pliosaurus, Plesiosaurus, and Odontochelys

The sauropsids of the water are a Plesiosaurus, a Pliosaurus, and an Odontochelys, an early turtle relative. Early in the Mesozoic Era, several sorts of lizard-like sauropsids swam and hunted in the water. While some lines of sauropsids led to land-loving lizards, pterosaurs, and dinosaurs, other lines evolved into ichthyosaurs, plesiosaurs, turtles, and crocodiles. Plesiosaurus and its close relatives hunted by sight, but scientists aren’t sure what their long, stiff necks were for. Pliosaurus is also part of the clade of plesiosaurs, and they were top predators, guided by sight to ambush or pursue prey. No one is sure where the turtle line originated, but one popular idea is that they arose from the same clade that led to plesiosaurs, called Pantestudines. Ondontochelys had a shell only on its belly. The turtle line later diverged into some lines that live exclusively in the water, some only on land, and many that both swim in the water and crawl on land.

Cockroach (300 million years ago), butterfly (35 Mya), and giant dragonfly relative (300 Mya)

The arthropods of the air are flying insects, a cockroach, Aphthoroblattina; a butterfly, Prodryas; and a dragonfly relative, Meganeura. Over 300 million years ago, insects appeared that could lay their eggs on land they could fold their wings behind them, allowing them to crawl easily. These lines evolved into cockroaches, such as Aphthoroblattina, and later into termites, grasshoppers, walking sticks, and their relatives. The butterfly Prodryas represents the clade of insects that develop first as larvas, then as pupas, and finally as adults. These insects, including ants, bees, and flies, have been even more successful than the roaches and their relatives. Among these insects, butterflies are relatively recent. You can think of butterflies as daytime moths, and they evolved from nighttime moths about 50 million years ago. Scientists know about Prodryas from a single, 35 million year old fossil, but it is one of the best butterfly fossils ever found. Meganeura was a predator living 300 million years ago. With a wingspan of 60 centimeters (2 feet), it was one of the biggest flying insects ever. Their clade died out, but the closely related dragonflies have survived for over 300 million years.

Theropod, sauropod, ornithischian

In Clades: Prehistoric, the sauropsids of the land are dinosaurs, specifically a feathered Utahraptor (a theropod), an Apatosaurus (a sauropod), and a Stegosaurus (an ornithischian). The earliest dinosaur precursors were two-legged relatives of crocodiles, as were the first theropods, sauropods, and ornithischians. Walking efficiently on two legs gave them an advantage over crocodiles, lizards, and early mammal relatives. Dinosaurs first appeared about 230 million years ago (Mya), after the Permian–Triassic extinction event of 250 Mya. The plant-eating ”bird-hipped” dinosaurs (Ornithischia) evolved greater size, and several lines evolved a four-legged gait, especially the frilled and armored dinosaurs. The “lizard-hipped” dinosaurs (Saurischia) developed into two major clades. The sauropods evolved long necks to eat tree foliage and large size to protect themselves against theropod predators. Their large size necessitated a four-legged gait. Theropods ate meat, and they, too, became larger over time. Theropods evolved feathers, and some developed flight. Some of these flying dinosaurs were the only dinosaurs to survive the Cretaceous-Paleogene extinction event of 66 million years ago. We call them birds (Aves).

It’s easy to get the names sauropsid and sauropod mixed up. A sauropod (“lizard foot”) is one type of sauropsid (“lizard face”).

Robin, eagle, and ostrich

In Clades, sauropsids of the air are represented by birds. Today’s birds all descend from a species of flying theropod dinosaur that lived before the Cretaceous-Tertiary extinction event of 70 million years ago. Unlike today’s common birds, these early birds did not perch in trees. Four bird lineages survived the extinction event that wiped out the other dinosaurs. One surviving lineage led to today’s ostriches and their close relatives (Paleognatha, or “old jaws”). The other lineage, the Neognatha or “new jaws”, survived in three separate lines, one leading to ducks and fowl, another to sea gulls and their relatives, and the third to “earth birds” (Telluraves). Telluraves predominate today, and they mostly perch and nest in trees. Telluraves split into Afroaves (or “African birds”) and Australaves (or “southern birds”), represented in Clades by a bald eagle and an American robin, respectively. The common ancestor of all eagles and robins was evidently a carnivorous bird that perched on tree branches. “Old-jaw” birds and “new-jaw” birds together form the clade Aves, or birds. If one includes the many birdlike dinosaurs that didn’t survive the last extinction event, this larger clade is called Avialae (“bird wings”). Archaeopteryx are in the clade Avialae, but they are a separate branch from Aves.