10 Missing Links in Vertebrate Evolution

01
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Missing Links? You'll Find Them Right Here

archaeopteryx
A specimen of Archaeopteryx (Wikimedia Commons).

As useful as it is, the phrase "missing link" is misleading in at least two ways. First, most of the transitional forms in vertebrate evolution aren't missing, but in fact have been conclusively identified in the fossil record. Second, it's impossible to pick out a single, definitive "missing link" from the broad continuum of evolution; for example, first there were theropod dinosaurs, then a large array of bird-like theropods, and only then what we consider true birds. With that said, here are 10 so-called missing links that help fill in the story of vertebrate evolution.

02
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The Vertebrate Missing Link - Pikaia

Pikaia
Pikaia (Nobu Tamura).

One of the most important events in the history of life was when vertebrates--animals with protected nerve cords running down the lengths of their backs--evolved from their invertebrate ancestors. The tiny, translucent, 500-million-year-old Pikaia possessed some crucial vertebrate characteristics: not only that essential spinal cord, but also bilateral symmetry, V-shaped muscles, and a head distinct from its tail, complete with forward-facing eyes. (Two other proto-fish of the Cambrian period, Haikouichthys and Myllokunmingia, also deserve "missing link" status, but Pikaia is the best-known representative of this group.)

03
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The Tetrapod Missing Link - Tiktaalik

tiktaalik
Tiktaalik (Alain Beneteau).

The 375-million-year-old Tiktaalik is what some paleontologists call a "fishapod," a transitional form perched midway between the prehistoric fish that preceded it and the first true tetrapods of the late Devonian period. Tiktaalik spent most, if not all, of its life in the water, but it boasted a wrist-like structure under its front fins, a flexible neck and primitive lungs, which may have allowed it to climb occasionally onto semi-dry land. Essentially, Tiktaalik blazed the prehistoric trail for its better-known tetrapod descendant of 10 million years later, Acanthostega.

04
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The Amphibian Missing Link - Eucritta

eucritta
Eucritta (Dmitry Bogdanov).

Not one of the better-known transitional forms in the fossil record, the full name of this "missing link"--Eucritta melanolimnetes--underlines its special status; it's Greek for "creature from the black lagoon." Eucritta, which lived about 350 million years ago, possessed a weird blend of tetrapod-like, amphibian-like and reptile-like characteristics, especially with regard to its head, eyes and palate. No one has yet identified what the direct successor of Eucritta was, though whatever the identity of this genuine missing link, it probably counted as one of the first true amphibians.

05
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The Reptile Missing Link - Hylonomus

reptile
Did all modern reptiles evolve from Hylonomus? (Wikimedia Commons).

About 320 million years ago, give or take a few million years, a population of prehistoric amphibians evolved into the first true reptiles--which, of course, themselves went on to spawn a mighty race of dinosaurs, crocodiles, pterosaurs and sleek, marine predators. To date, the North American Hylonomus is the best candidate for the first true reptile on earth, a tiny (about one foot long and one pound), skittering, insect-eating critter that laid its eggs on dry land rather than in the water. (The relative harmlessness of Hylonomus is best summed up by its name, Greek for "forest mouse.").  

06
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The Dinosaur Missing Link - Eoraptor

eoraptor
Eoraptor (Wikimedia Commons).

The first true dinosaurs evolved from their archosaur predecessors about 230 million years ago, during the middle Triassic period. In missing link terms, there's no particular reason to single out Eoraptor from other, contemporary South American theropods like Herrerasaurus and Staurikosaurus, other than the fact that this plain-vanilla, two-legged meat-eater lacked any specialized features and thus may have served as the template for later dinosaur evolution. (For example, Eoraptor and its pals seem to have predated the historical split between saurischian and ornithischian dinosaurs.)

07
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The Pterosaur Missing Link - Darwinopterus

darwinopterus
Darwinopterus (Nobu Tamura).

Pterosaurs, the flying reptiles of the Mesozoic Era, are divided into two main groups: the small, long-tailed "rhamphorhynchoid" pterosaurs of the late Jurassic period and the larger, short-tailed "pterodactyloid" pterosaurs of the ensuing Cretaceous. With its large head, long tail and relatively impressive wingspan, the appropriately named Darwinopterus appears to have been a classic transitional form between these two pterosaur families; as one of its discoverers has been quoted in the media, it's "a really cool creature, because it links the two major phases of pterosaur evolution."

08
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The Plesiosaur Missing Link - Nothosaurus

nothosaurus
Nothosaurus (Wikimedia Commons).

Various kinds of marine reptiles swam the earth's oceans, lakes and rivers during the Mesozoic Era, but the plesiosaurs and pliosaurs were the most impressive, some genera (like Liopleurodon) achieving whale-like sizes. Dating to the Triassic period, slightly before the golden age of plesiosaurs and pliosaurs, the slender, long-necked Nothosaurus may well have been the genus that spawned these marine predators. As is often the case with the smallish ancestors of large aquatic animals, Nothosaurus spent a fair amount of its time on dry land, and may even have behaved like a modern seal.

09
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The Therapsid Missing Link - Lystrosaurus

lystrosaurus
Lystrosaurus (Wikimedia Commons).

No less an authority than evolutionary biologist Richard Dawkins has described Lystrosaurus as the "Noah" of the Permian-Triassic Extinction 250 million years ago, which killed almost three-quarters of land-dwelling species on earth. This therapsid, or "mammal-like reptile," wasn't any more of a missing link than others of its kind (such as Cynognathus or Thrinaxodon), but its worldwide distribution at the start of the Triassic period makes it an important transitional form in its own right, paving the way for the evolution of Mesozoic mammals from therapsids millions of years later.

10
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The Mammal Missing Link - Megazostrodon

megazostrodon
Megazostrodon (Wikimedia Commons).

More so than with other such evolutionary transitions, it's difficult to pinpoint the exact moment when the most advanced therapsids, or "mammal-like reptiles," spawned the first true mammals--since the mouse-sized furballs of the late Triassic period are represented mainly by fossilized teeth! Even still, the African Megazostrodon is as good a candidate as any for a missing link: this tiny creature didn't possess a true mammalian placenta, but it still seems to have suckled its young after they hatched, a level of parental care that put it well toward the mammalian end of the evolutionary spectrum.  

11
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The Bird Missing Link - Archaeopteryx

archaeopteryx
Archaeopteryx (Emily Willoughby).

Not only does Archaeopteryx count as "a" missing link, but for many years in the 19th century it was "the" missing link, since its spectacularly preserved fossils were discovered only two years after Charles Darwin published On the Origin of Species. Even today, paleontologists disagree about whether Archaeopteryx was mostly dinosaur or mostly bird, or whether it represented a "dead end" in evolution (it's possible that prehistoric birds evolved more than once during the Mesozoic Era, and that modern birds descend from the small, feathered dinosaurs of the late Cretaceous period rather than the Jurassic Archaeopteryx).