20 Important Firsts in the Animal Kingdom

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From the First, Everything Follows

Megazostrodon (London Natural History Museum).
As a rule, biologists and evolutionary scientists don't like the word "first"--evolution proceeds by slow increments, over millions of years, and it's technically impossible to pick out the exact moment when, say, the first true reptile evolved from its amphibian ancestors. Paleontologists take a different view: since they're constrained by the fossil evidence, they have an easier time picking out the "first" member of any given animal group, with the important proviso that they're talking about the first identified member of that animal group. That's why these "firsts" are constantly changing: all it will take is a new, spectacular fossil discovery to knock Archaeopteryx (the "first bird") off its comfortable perch. So without any further ado, here, to the best of our knowledge, are the first members of various different animal groups.
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The First Dinosaur - Eoraptor

Eoraptor, the first dinosaur. Wikimedia Commons

Some time during the middle Triassic period, about 230 million years ago, the very first dinosaurs evolved from their archosaur ancestors. Eoraptor, the "dawn raptor," wasn't a true raptor--that family of theropods only appeared toward the start of the Cretaceous period--but it's as good a candidate as any for the first true dinosaur. Befitting its early place on the dinosaur family tree, Eoraptor was only about two feet long from head to tail and weighed five pounds soaking wet, but it compensated for its puny size with sharp teeth and grasping, five-fingered hands.

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The First Dog - Hesperocyon

Hesperocyon, the first dog (Wikimedia Commons).

The genus to which all modern dogs belong, Canis, evolved in North America about six million years ago, but it was preceded by various dog-like "canid" mammals--and the mammalian genus that was immediately ancestral to the canids was the late Eocene Hesperocyon. About the size of a fox, Hesperocyon possessed an inner-ear structure similar to that of modern dogs, and also like its modern descendants it probably roamed in packs (though whether these communities lived high up in trees, burrowed underground, or trekked across the open plains is a matter of some dispute).

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The First Tetrapod - Tiktaalik

Tiktaalik, the first tetrapod (Alain Beneteau).

It's especially difficult to identify the first true tetrapod, given gaps in the fossil record and a blurring of the lines dividing lobe-finned fish from "fishapods" from true tetrapods. Tiktaalik lived during the late Devonian period (about 375 million years ago); its skeletal structure was more advanced than the lobe-finned fish that preceded it (such as Panderichthys), but less articulated than more advanced tetrapods like Acanthostega. It's as good a candidate as any for the first fish that crawled up out of the primordial ooze on four stubby legs!

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The First Horse - Hyracotherium

Hyracotherium, the first horse (Heinrich Harder).

If the name Hyracotherium sounds unfamiliar, that's because this ancestral horse was once known as Eohippus (you can thank the rules of paleontology for the change; it turns out that the more obscure name had priority in the historical record). As is often the case with a "first" mammal, the 50-million-year-old Hyracotherium was extremely small (about two feet long and 50 pounds) and it possessed many un-horse-like characteristics, such as a preference for low-lying leaves rather than grass (which had yet to spread widely across the North American continent).

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The First Turtle - Odontochelys

Odontochelys, the first turtle (Nobu Tamura).

Odontochelys ("toothed shell") is a case study in how slippery the title of "first" anything can be. When this late Triassic turtle was discovered in 2008, it immediately took precedence over the then-reigning turtle ancestor, Proganochelys, which lived 10 million years later. Odontochelys' toothed beak and semi-soft carapace point to its kinship with the obscure family of Permian reptiles--most likely the pareiasaurs--from which all modern turtles and tortoises evolved. And yes, in case you were wondering, it was pretty small: only about a foot long and one or two pounds.

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The First Bird - Archaeopteryx

Archaeopteryx, the first bird (Alain Beneteau).

Of all the "first" animals on this list, the standing of Archaeopteryx is the least secure. First, ss far as paleontologists can tell, birds evolved multiple times during the Mesozoic Era, and the odds are that all modern genera are descended not from the late Jurassic Archaeopteryx but the small, feathered dinosaurs of the ensuing Cretaceous period. And second, most experts will tell you that Archaeopteryx was closer to being a dinosaur than it was to being a bird--all of which hasn't prevented the public from bestowing it the title of "first bird."

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The First Crocodile - Erpotesuchus

Erpetosuchus, the first crocodile (Wikimedia Commons).

Somewhat confusingly, the archosaurs ("ruling lizards") of the early Triassic period evolved into three distinct kinds of reptiles: dinosaurs, pterosaurs and crocodiles. That helps explain why Erpetosuchus, the "crawling crocodile," didn't look all that different from its near-contemporary Eoraptor, the first identified dinosaur. Just like Eoraptor, Erpetosuchus walked on two legs, and except for its elongated snout it looked more like a plain-vanilla reptile than a creature whose descendants would one day include the fearsome Sarcosuchus and Deinosuchus.

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The First Tyrannosaur - Guanlong

Guanlong, the first tyrannosaur (Andrey Atuchin).

Tyrannosaurs were the poster theropods of the late Cretaceous period, right before the K/T extinction that rendered the dinosaurs extinct. In the last decade or so, a series of spectacular fossil finds has pushed the origin of tyrannosaurs all the way back to the late Jurassic period, 160 million years ago. That's where we find the 10-foot-long, 200-pound Guanlong ("emperor dragon"), which had a very un-tyrannosaur-like crest on its head and a coat of shiny feathers (which implies that all tyrannosaurs, even T. Rex, may have sported feathers at some point in their life cycles).

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The First Fish - Pikaia

Pikaia, the first fish (Nobu Tamura).

When you delve back 500 million years into the history of life on earth, the honorific "first fish" loses some of its meaning. Thanks to the notochord (the primitive precursor of a true spinal column) that rand down the length of its back, Pikaia was not only the first fish, but the first vertebrate animal, and thus ancestral to mammals, dinosaurs, birds, and innumerable other types of creature. For the record, Pikaia was about two inches long, and so thin that it was probably translucent. It's named after Pika Peak in Canada, near where its fossils were discovered.

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The First Mammal - Megazostrodon

Megazostrodon, the first mammal (London Natural History Museum).

About the same time (the middle Triassic period) as the first dinosaurs were evolving from their archosaur predecessors, the earliest mammals were also evolving from the therapsids, or "mammal-like reptiles." A good candidate for the first true mammal was the mouse-sized Megazostrodon ("large girdled tooth"), a small, furry, insectivorous creature that had unusually well-developed sight and hearing, matched by a larger-than-average brain. Unlike modern mammals, Megazostrodon lacked a true placenta, but it may still have suckled its young.

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The First Whale - Pakicetus

Pakicetus, the first whale (Wikimedia Commons).

Of all the "firsts" on this list, Pakicetus may well be the most counterintuitive. This ultimate whale ancestor, which lived about 50 million years ago, looked like a cross between a dog and a weasel, and walked on four legs just like any other respectable terrestrial mammal. Ironically, the ears of Pakicetus weren't especially well adapted to hearing underwater, so this 50-pound furball probably spent more time on dry land than in lakes or rivers. Pakicetus is also notable as one of the few prehistoric animals ever to be discovered in Pakistan.

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The First Reptile - Hylonomus

Hylonomus, the first reptile (Nobu Tamura).

If you've gotten this far down the list, you may not be surprised to learn that the ultimate ancestor of dinosaurs, crocodiles and monitor lizards was the small, inoffensive Hylonomus ("forest dweller"), which lived in North America during the late Carboniferous period. The largest reptile of its time, by definition, Hylonomus weighed about a pound, and probably subsisted entirely on insects (which had only recently evolved themselves). By the way, some paleontologists claim that Westlothiana was the first reptile, but this creature was probably an amphibian instead.

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The First Sauropod - Vulcanodon

Vulcanodon, the first sauropod (Wikimedia Commons).

Paleontologists have had an especially hard time identifying the first sauropod (the family of plant-eating dinosaurs typified by Diplodocus and Brachiosaurus); the problem is that the smaller, two-legged prosauropods weren't directly ancestral to their more famous cousins. For now, the best candidate for the earliest true sauropod is Vulcanodon, which lived in southern Africa about 200 million years ago and "only" weighed about four or five tons. (Tantalizingly, early Jurassic Africa was also home to the famous prosauropod Massospondylus.)

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The First Primate - Purgatorius

Purgatorius, the first primate (Nobu Tamura).

How ironic is it that the earliest identified primate ancestor, Purgatorius, hopped and skittered across the North American landscape at the same time as the dinosaurs went extinct? Purgatorius certainly didn't look like an ape, monkey or lemur; this small, mouse-sized mammal probably spent most of its time high up in trees, and it has been pegged as a simian precursor mainly because of the characteristic shape of its teeth. It was only after the K/T Extinction, 65 million years ago, that Purgatorius and pals were launched on their eons-long journey into Homo sapiens.

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The First Pterosaur - Eudimorphodon

Eudimorphodon, the first pterosaur (Wikimedia Commons).

Thanks to the vagaries of the fossil record, paleontologists know less about the early history of pterosaurs than they do about crocodiles and dinosaurs, which also evolved from archosaurs ("ruling lizards") during the middle Triassic period. For now, we'll have to content ourselves with Eudimorphodon, which (unlike some other animals on this list) was already fully recognizable as a pterosaur when it flew the skies of Europe 210 million years ago. Until an earlier transitional form is discovered, that's the best we can do!

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The First Cat - Proailurus

Proailurus, the first cat (Steve White).

The evolution of mammalian carnivores is a complicated affair, since dogs, cats, bears, hyenas and even weasels all share a common ancestor (and some other fearsome meat-eating mammals, like the creodonts, went extinct millions of years ago). For now, paleontologists believe that the earliest common ancestor of modern cats, including tabbies and tigers, was the late Oligocene Proailurus ("before the cats"). Somewhat oddly given the usual evolutionary trends, Proailurus was respectably sized, about two feet long from head to tail and weighing in the neighborhood of 20 pounds.

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The First Snake - Pachyrhachis

Pachyrhachis, the first snake (Karen Carr).

The ultimate origin of snakes, like the ultimate origin of turtles, is still a matter of ongoing debate. What we do know is that the early Cretaceous Pachyrhachis was one of the first identifiable members of its breed, a three-foot-long, two-pound, slithering reptile that possessed a pair of vestigial hind legs a few inches above its tail. Ironically, given the biblical connotations of snakes, Pachyrhachis and its hissing pals (Eupodophis and Haasiophis) were all discovered in the Middle East, either in or near the country of Israel.

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The First Shark - Cladoselache

Cladoselache, the first shark (Nobu Tamura).

The difficult-to-pronounce Cladoselache (its name means "branch-toothed shark") lived during the late Devonian period, about 370 million years ago, making it the earliest shark in the fossil record. If you'll forgive us for mixing our genera, Cladoselache was certainly an odd duck: it was almost completely devoid of scales, except for specific parts of its body, and it also lacked the "claspers" modern sharks use to mate with the opposite sex. Clearly Cladoselache figured this tricky business out, since it eventually went on to spawn Megalodon and the Great White Shark hundreds of millions of years later.

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The First Amphibian - Eucritta

Eucritta, the first amphibian (Dmitri Bogdanov).

If you're of a certain age, and still remember drive-in movies, you may appreciate the full name of this Carboniferous creature: Eucritta melanolimnetes, or "the creature from the black lagoon." As with the fish that preceded them and the tetrapods that succeeded them, it's difficult to identify the first true amphibians; Eucritta is as good a candidate as any, considering its small size, tadpole-like appearance, and strange mix of primitive characteristics. Even if Eucritta wasn't technically the first amphibian, its immediate descendant (which has yet to be discovered) almost certainly was!

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Strauss, Bob. "20 Important Firsts in the Animal Kingdom." ThoughtCo, Apr. 20, 2017, thoughtco.com/important-firsts-in-the-animal-kingdom-1093360. Strauss, Bob. (2017, April 20). 20 Important Firsts in the Animal Kingdom. Retrieved from https://www.thoughtco.com/important-firsts-in-the-animal-kingdom-1093360 Strauss, Bob. "20 Important Firsts in the Animal Kingdom." ThoughtCo. https://www.thoughtco.com/important-firsts-in-the-animal-kingdom-1093360 (accessed January 24, 2018).