Among the busy coastal Devonian reefs crawling with predators of every shape and size, it would have been easy to miss a shy refugee further inland. She was a strange looking fish, two-feet long, most of which was tail. And as we open she's swimming for her life up a fresh water stream with a larger predator in hot pursuit. Under normal conditions this little lady--we'll call her Mother -- and the eggs she carries wouldn't have a chance. Her stalker outweighs her by a factor of ten. But to our great benefit, she has a whole suite of secret weapons.
To understand what those secrets were and how they came about, we need to first go back almost two-hundred millions years before Mother's time, when ancestors of the first fish appear in the fossil record during the Cambrian Period. They're known today as conodonts and they're not worms, they're chordates, but they probably evolved from something very much like a worm. Early conodonts are a candidate precursor to all vertebrates, those familiar animals with a spine we know and love in our own era. And because of their relatively large brains, conodonts were the geniuses of their day. They had to be! For many millions of years these modest creatures were mercilessly hunted, surviving only by their wits, caught up in war between the rival rulers of the primeval sea: Arthropods and Mollusks AKA bugs and snails.
Conodonts first appear in the fossil record about 500 million years ago. Note: There is some talk of a new basal chordate recently found and soon to be released that pushes back the origin of proto-vertebrates 'significantly'
By the time the conodonts were hiding in the warm mud of the Cambrian sea, the arthropods already dominated the world. They came in an amazing array of formidable super bugs. Anomalocaris was a free-swimming segmented predator over a meter in length, with a mouth that shredded prey like a garbage disposal cutting through hamburger. Another of the arthropods, Pterygotus, looked like a cross between a lobster and a scorpion; except they were over six-feet long!
Above, Ordovician Dawn, courtesy of artist and writer R. Gary Raham, depicts the giant nautiloid, Endoceras, capturing an armored fish. Three trilobites lie in the mud in the left foreground near a starfish. Rugose corals wave their tentacles from cup-like bodies attached to the ocean bottom. Other corals species appear like rounded domes. Flower-like starfish relatives, called crinoids, capture tasty morsels that float nearby. Gary has also written and illustrated a really neat series of science books for kids of all ages
But by 450 million years ago, during the Ordovician Period, the mega-bugs were being outclassed by a new and even more frightening menace. The reign of the super-snails had dawned: Or Cephalopods to be more precise, a taxon that includes modern squid and octopi. The Ordovician and later Silurian versions of Cephalopods include giant nautiloids and ammonoids which easily rank among the most nightmarish monsters to ever inhabit the earth! Some of these gals were enormous squid like creatures as long as a school bus which carried around a thick shell. Each of the many tentacles were equipped with hundreds of suckers lined with tiny, razor sharp teeth, all radiating around a parrot like beak located in the center of the creature that could lop off big chunks of flesh and bite through solid bone. It's not terribly surprising that during this same time, some primitive fish developed thick bony armor! But the saving grace for fish was just around the corner ...
In the Agnatha above, a type of early Ordovician fish, you might notice it lacks something most modern fish have: A jaw. Without a jaw, early fish were limited to slurping down small prey, or gooey algal slime and decomposing flesh. The jaw might seem like such a useful item to have that natural selection would craft it again and again. Yet as best we can tell, unlike wings or eyes, the jaw only evolved once in the entire history of life, and only in vertebrates. But once it did, the balance of power turned decidedly in favor of the fish.
With the capacity to chomp down on prey using the new hinged jaws--full of sharp pointy teeth-- that's exactly what the fish began doing. The newcomers went on a tear, radiating into all kinds of new econiches. And that's what marks the late Devonian: It was the golden age of fish, particularly sharks. There were sharks that ate shellfish, sharks that lived in the deep sea, and sharks that lived in sandy shallows. There were speed demon sharks, sharks that sat in stealthy ambush, and even sharks that could leap out of the water and glide for short distances!
Fish grew to the size of whales and the tables were turned on the super bugs and the giant tentacular monsters. Arthropods and nautiloids suddenly found themselves on the receiving end of what they'd been dishing out for eons. It wasn't long before toothy fish with gaping maws were chowing down on the meanest nautiloids and the nastiest arthropods, and eventually, even each other.
Life was hard enough for a little fish in a big ocean before the jawed giants came to be and now, because of them, by 375 million years ago it was more dangerous than ever. So in the interests of self preservation some of those smaller fish sought safety upstream in less crowded rivers or freshwater estuaries. But there were a couple of problems with that environment for a creature that evolved in the open ocean. For one, as any aquarium aficionado can attest, put most salt water fish in a fresh water aquarium and pretty soon they're floating belly up. So the early refugees from the sea had to adapt to varying levels of salinity before they could safely journey between lake and ocean. On top of that, the oxygen content in shallow tidal basins and fresh water ponds and streams declines greatly as a function of heat; and it was pretty warm back then.
Another huge problem was the unpredictable level of essential minerals. The ocean was plenty loaded down with calcium, sodium, potassium. and other substances essential to fishy physiology. The stuff they needed was everywhere, dissolved in the water, free for the drinking. But ponds and streams were not nearly so well endowed. Important trace minerals and salts would come and go depending on what kind of rock and soil happen to be exposed or washed in by local run off from rain at any given time.
Evolution addressed all these problems by modifying existing anatomical structures. Openings formed behind the gills that could absorb a little oxygen from the air when the water turned anaerobic. Kidney like organs developed to pump salt water out. And the fresh water explorers began storing calcium and other minerals in their rubbery cartilage during times of plenty so they could call on those reserves when they became scarce. This was the birth of the harder, bony skeletons found in most fish today.
All these adaptations were a smashing success. The new freshwater adapted fish proliferated and invaded rivers, creeks, and lakes. Of course, sooner or later, a big freshwater bully had to evolve that liked to eat the smaller guys and the little fish were right back to square one!
Back to our Devonian drama and Mother, the fleeing fish hoping to avoid being eaten. Early on in the Devonian several species of fish developed a bony extension supporting the front and back fins called lobes, which helped them hold steady while resting on the bottom in the face of strong currents.
Eusthenoptern, an early 'lobe-finned' fish. This is the actual drawing done for Carl Zimmer's book "At The Water's Edge" by paleo-wildlife artist Carl Buell. It lived about 380 million years ago and those fins exhibit fossil homologies to our own limbs. Although they're not easy to make out, the forerunners of the familiar arm and leg bones are there. See also PZ's post on Panderichthys for the skinny on another transitional a little further along
Mother was one of these kind of critters and her secret weapons included the new bony skeleton, and the latest deluxe model of air bladder which could extract a little oxygen from the air in a pinch. But her biggest innovation was her lobed fins. Instead of small lobes with long spines, she had evolved longer, bony paddles and reduced the spines to webbed hooks on the ends. So Mother -- or Acanthostega as she is formally known -- could scramble upstream through fast moving water just a few inches deep better than any of her contemporaries, by pumping that big tail and scrambling along the bottom with her leggy paddles. No large predator could follow her into that environment. And that's how she got away.
In these shallow water retreat[s], Mother and her children were much safer. Over time they would forge further and further inland, learning to catch and feast upon unwary arthropods, tiny worms, and the small snails that patrolled the shallow water's edge. Eventually her descendants would develop even more robust bony paddles with articulated pads, all fastened to a stronger skeleton, that allowed them to take short walks out of the water altogether. And thus, swimmers became walkers; fish became amphibians.
The new walking fish would go on to diversify into other amphibians, reptiles, dinosaurs and birds, and a small group would eventually become the mammals. And in a textbook display of evolutionary biology, all of these clades would produce subsequent members that returned to the water, a couple of which are pictured below.
Enlarge
Enlarge
These critters may look closely related and they're both descendants of land dwelling animals. But they're separated by hundreds of millions of years of divergent evolution. The one on the left is a Tylosaurus, one of the last of the great Mosasaurs. On the right is Ambulocetus, an early whale from 50 million years ago and as much a mammal as you or I. Illustrations courtesy of Carl Buell
The sobering thing to contemplate is that if Mother or just one of her critical descendants or forebears hadn't made it, most of the familiar animals we know today wouldn't exist. No cats or dogs, no turtles or crocodiles, not a single bird or dinosaur would have ever lived. In their stead would have been completely different kinds of vertebrates -- or no land vertebrates at all. The bugs, worms, and snails were doing fine on land. They had beaten fish out of the sea by a hundred-million years. Had the terrestrial bugs and snails been allowed unfettered evolution with no brainy walking fish to worry about, today we could have giant browsing arthropods preyed upon by lightning fast land octopi, instead of antelope and tigers.
Enlarge
The star of our story: Mother Acanthostega, the one that got away. She may look like a land dweller, but careful analysis of her bones reveals that without the buoyancy provided by at least a few inches of water, she was about as helpless as a catfish on land. Her skeleton would have to be reinforced, the developing legs better braced, and ankles evolved, before she could leave the water completely. Fossil deposits containing her remains have been found in strata over 350 million years old
Which brings me back to my evening meal and the lady who was willfully missing out on one of the great wonders of evolution, even though it was literally staring at her in the face from both sides of the aquarium. Not all the fish that inhabited fresh water in the Devonian stayed there or went on to become tetrapods. Some of those guys went back to salt water eventually and the new morphologies they had acquired in the lakes and streams granted them several competitive advantages over their cartilaginous cousins that remained behind. The primitive lung turned into an air bladder, a precise buoyancy control mechanism. And the hard mineral matrix in their bones allowed more powerful muscles to anchor and operate stiffer tails and ray-fins. Both these adaptations significantly improved their agility and speed.
Almost every fish on the menu last night was one of these bony salt water denizens that made the evolutionary voyage to freshwater and back again. That includes Atlantic Cod, Yellowfin tuna, red snapper, and flounder. Salmon, one of my favorites, still to this day retrace their fresh water roots, returning the sanctuary of the lake and pond to spawn. Even the brightly colored triggerfish looking warily out of the tank at my skeptical lady friend and I made the journey to fresh water and back to the salty sea.
And of course, Acanthostega (Or something close to her) leads to another lineage: Primates. One of which would learn to walk on two legs rather than four. About 70,000 years ago one of those bipedal creatures would walk out of Africa into the Middle East. By 40,000 years ago they would walk into Europe and 12,000 years ago they would stroll into the New World. In a geological instant they're walking on the moon and eating dinner at Red Lobster.
The story my fellow diner was missing out on connected both of us to the fish we came to consume as well as the ones on display. It's an epic that joins mankind by way of shallow water, bony minerals, and lobed fins, to every whale, hippo, bison, lizard, squirrel, snake, grizzly bear, frog, eagle, dino, and pterosaur, just to name a handful. It's a wondrous tale of predators avoided and conquests won, any of which could have gone the other way. And if even one of those Devonian links in the long chain of creatures leading to present day forms were broken, you and I, and the world we know, wouldn't be here.
The third rock from the sun might instead be a planet of intelligent squid or technological insects, peering with inhuman eyes at the ancient fossils of long forgotten, curious fish-like creatures with paddling fins that mysteriously died out. And who knows, had it not been for the one that got away, perhaps they too would have wondered what might have been, and dreamed of a strange world that never was, ruled by modified walking fish and their bipedal descendants.
