High School Oceanography: Sea Life

Ocean Books

Facebook Twitter

Life in the Ocean
Topics Covered in this Lecture:
Slideshow: The Deep
Slideshow: Plankton
Video: Whales and Plankton
Video: Plankton


Once the single-celled organisms had the Earth's ocean and atmospheric chemistry under control, the stage was set for the evolution of a myriad of forms of sea life. Many of the organisms we have popularized as fossils and many that still live today appeared during the Paleozoic. And not to be outdone, the sea boasted a few of its own dinosaurs, complete with flippers and voracious appetites. Underlying this explosion of life, and its subsequent catastrophic extinction, are the threads of Gaia, stretching so tenderly around the Earth, orchestrating the balance of physical, geological, chemical, and biological forces.

An Explosion of Sea Life: The Paleozoic (540 to 250 MYA)

The Paleozoic marks the beginning of a literal "explosion" in the diversity of life on Earth. Once the Earth's atmosphere and oceans had "stabilized" (thanks to 2 billion years of hard labor by the bacteria), multicellular organisms were free to adapt and develop in a multitude of forms and patterns. Whereas early plants and animals spent much of their energy battling the changing chemistry of the Earth, the plants and animals of this period spent much of their energy battling each other. These organisms exploited every possible means to make a living, whether it was feeding at the great depths of the sea, feeding near the surface or shores, or eating other organisms. Nearly all the forms of eating we know were invented during this period -- herbivory, carnivory, omnivory, coprophagy, detritivory, etc. Within a few short million years, nearly all the known living animal phyla came into being.

At the other end of the spectrum -- at the end of the Paleozoic, nearly 90% of the marine animal species were wiped out during one of the largest mass extinctions in the Earth's history. Why this happened is not clear, but I suspect that the colonization of animals onto land had something to do with it.

During this period, we should also be aware that the land masses of the Earth (ever-moving, ever-changing) were divided into four to six continents. At about this time, California ran east-west along the equator (totally tropical, dude!) and Africa was at the South Pole. As these continents moved around, various shallow seas were formed, giving rise to local groups of marine organisms that are still preserved today. One of these marine fossil formations resides in the North Cascades in British Columbia (known as the Burgess Shale) and another is present in Guadalupe, Texas. We'll visit these sites shortly.

The Cambrian (540 - 500 MYA)

The first period of the Paleozoic is known as the Cambrian and the rapid proliferation of plants and animals during this period is known as the "Cambrian Explosion". Marine organisms ruled during this period. Life on land was probably limited to ponds of algae, various fungi, and rocks covered with lichens.

As mentioned above, one of the best preserved examples of this period is the Burgess Shale. Formed about 530 MYA, this formation represents a community of mud-loving organisms living next to an algal reef (probably similar to the stromatolites mentioned earlier). At its peak, this algal reef stretches several hundred feet high. It is believed that periodic mud slides along the reef led to the remarkable preservation of soft-bodied and other organisms found here.

Not surprisingly, algae and cyanobacteria have been found here. These thread-like organisms probably built mats attached to the sea floor.

A variety of sponges lived here, revealed by the remnants of spicules, tiny slivers of silica, that make up their bodies. Like modern sponges, these primitive sponges probably extracted nutrients from seawater and may have eaten suspended phytoplankton (plant plankton).

Arthropods (relatives of insects and lobsters) are one of the most abundant fossil types found here. A fossil that looks to me like a modern day ghost shrimp, Aysheaia pedunculata, is alleged to have eaten sponges and may have hid among them for protection (some gratitude!). Paleontologists believe that centipedes and millipedes may have evolved from this Aysheaia. Another arthropod found here is a bottom feeder called Canadaspis perfecta. This creature is notable for its teeth-like mandible which it used to grind large pieces of food, possibly dead carcasses. This guy looks to me like a modern-day relative of the sand hopper.

One of the more weird and science-fiction-like animals that came into being during this period is Dinomischus, an organism that bears no relation to any other known animal. Appearing like sculptured wine glasses, these organisms were only an inch long, but appeared in profusion across the muddy floor. In some ways, they resemble sea anemones, and I suspect that they feed on particles floating in the water.

Though not present at the Burgess Shale, a very important modern-day arthropod evolved during this time -- the horseshoe crab. A visitor (from outer space, no doubt) walking along a Cambrian beach could have witnessed a family of horseshoe crabs with babies latched onto their backs much as we do today. What our alien visitor might not have known is how important these creatures are to modern man. Currently, major investigations are underway to understand compounds in the blood of horseshoe crabs that have a variety of medical applications.

Other notable marine organisms that developed during this time were the brachiopods, ancestors to modern day mollusks. Brachiopods survive to this day and you may have picked some up at local beaches in the form of jingle shells, often used to make wind chimes.

Finally, a recent major discovery of Cambrian fossils in Chengjiang, China, are shedding new and important clues on the evolution of life during this time period. Many Burgess-type fossils have been found here, including the alien-like animal Dinomischus. The ubiquity of many of these fossils throughout the world indicates the success of these animals in colonizing the world ocean.

The Ordovician (500 - 440 MYA)

Towards the end of the Cambrian and during the Ordovician (~500-440 MYA), one of the most fascinating oceanic creatures to walk this planet came into being -- the Trilobites. It has been said that next to the dinosaurs, trilobites are among the most famous fossils known to man. Even in prehistoric times, trilobites were valued for their intricate and striking beauty. A fossil trilobite found in France at a 15,000-year-old human settlement had a hole drilled in it, presumably so it could be worn as an amulet or good luck charm. Every time I think of trilobites I think of the Tribbles in Star Trek, and I have no idea if there is any connection, but their names and popularity appear similar.

Most trilobites walked along the bottom of shallow seas, much like their relatives the horseshoe crabs. They probably lived on scraps of algal material or other detritus, but their lack of major noshing parts suggests they weren't predatory (of course not, they were lovable little creatures, just like tribbles). Trilobites also had the ability to roll up into a ball -- either for protection or play -- and we can infer that they could bury themselves in the sediments for protection, too. Trilobites vanished from our planet approximately 245 MYA. I suspect they were abducted by aliens and turned into tribbles!

Primitive chordates also evolved at this time. The earliest known example is Pikaia gracilens, a small "fish-like" minnow that appeared to filter its food from the water as it swam along the surface.

Late in the Ordovician and throughout the Silurian, fossils have been found that resemble the scales of sharks. Because sharks are made of cartilage, no complete specimens are preserved, but it is apparent that distant relatives of sharks and manta rays began to evolve during this period.

Subsequent periods in the Paleozoic concern evolution of terrestrial organisms, such as land plants, trees, and reptiles. While important, we will restrict our discussion to creatures residing in the sea.

Question: Why do you think horseshoe crabs have been able to survive for hundreds of millions of years?

Dinosaurs in the Sea

Our coverage of the evolution of marine organisms leaps forward here with the introduction of the most beloved fossils on Earth, the dinosaurs. Most of the dinosaurs we know lived in the Mesozoic Period, which includes the Triassic, the Jurassic, and the Cretaceous (from 225 to 65 MYA). Before we examine a few of the inhabitants, we first need to quickly review the "state" of the world at this time.

Towards the end of the Paleozoic (about 280 MYA), the continents (we can't forget the continents) assembled into a kind of giant supercontinent known as Pangaea. As we will see in another lecture, the continents literally float in the molten mantle of the Earth. Since their formation 4.5 billion years ago, the continents have performed a kind of balletic movement, coming together, disbanding, and coming together again. While the early history is not well documented, we now have pretty good evidence of the movement of the continents since the Cambrian.

At about 245 MYA, the continents had begun to split up again, forming a new ocean called Tethys. The opening of the Tethys Ocean began a process which led to the formation of the Indian Ocean and the Atlantic Ocean, which features one of the largest mid-oceanic ridges on our planet. This ocean and the Panthallassa Ocean, the precursor of the Pacific Ocean, constituted the major oceans of the time.

It was in the Tethys Ocean that one of the more interesting oceanic dinosaurs has been found. Known as Cryptocleidus, this plesiosaur ruled the seas during the late Jurassic (190 - 136 MYA). Cryptocleidus was an impressive predator, boasting outwardly angled teeth that let it deftly grab its prey. Most likely, this dinosaur was an excellent swimmer, flying through the water with the greatest of ease, much like a sea turtle or sea lion.

Another denizen of the deep at this time was a type of marine crocodile. Looking very much like their modern counterparts, these beasts had flippers instead of claws. Their strong tails and flippered appendages allowed them to swim rapidly to feed on their favorite foods.

One of the favorite foods of many oceanic dinosaurs was the ammonites, which flourished in the Tethys Ocean during the Cretaceous (136 - 65 MYA). Relatives of the chambered nautilus, squids, and octopus, these creatures carried a calcified shell on their back filled with air pockets that allowed them to move through the water using a form of jet propulsion. Fossils of ammonites are found everywhere that ancient seas once existed.

At the end of the Cretaceous, another catastrophe of epic proportions hit our planet, wiping out nearly 70% of the Earth's species at this time. The demise of the dinosaurs has been one of the great riddles of our time, and many scientific and popular theories have been proposed for their disappearance. However, there is now strong evidence that the extinction of the dinosaurs was caused by the impact of a major meteorite in the Gulf of Mexico. Known as the Chicxulub structure, named after the small Mayan village in its proximity, this crater has been dubbed the "smoking gun" of the Cretaceous-Tertiary boundary when the mass extinction occurred.

While Mexican geologists had long suspected that the circular geophysical anomalies in the area were of extraterrestrial (i.e. meteoric) origin, it wasn't until the bold proposition of Luis Alvarez and his geologist son, Walter, that the idea began to gain favor. Their proposition stemmed from their discovery in 1980 of a centimeter-thick layer of clay in limestone deposits dated at about the time of the mass extinction. By a fortuitous sequence of events, Walter learned of the Mexican scientists' samples and began a more intensive investigation of the area. By 1990, teams of scientists from the United States and Mexico had assembled an impressive body of evidence that the Chicxulub structure was indeed an impact crater.

Announced in the journal Nature in 1992, the theory has gained many converts. Subsequent measurements of the crater from the space shuttle and from marine seismic-reflection instruments have established that the crater diameter ranges from 110 to 180 miles across. This dimension makes it the largest of all impacts known to have occurred on our planet. More work remains to establish the complete dimensions of the impact, which lies beneath 1,000 to 3,000 feet of limestone. Still, this exciting "discovery" in our lifetime provides a sobering message for the power of the Universe in altering the course of life on our planet.

The Cenozoic (65 - 2 MYA)

As we move from the Paleozoic to the Cenozoic, we begin to see many of the species of organisms that are familiar to us today. By this time, the three major oceans as we know them -- the Pacific, the Atlantic, and the Indian -- were well established. In addition, the formation of the Mediterranean Sea and the separation of Australia from Antarctica was completed during this period. Of the animals that came into being during this period, I will restrict our discussion to two of our favorites -- great white sharks and whales.

The oldest known fossils from the ancient relative of the great white are teeth from a shark known as Carcharodon megalodon. An inhabitant of the Miocene (27 - 7 MYA), this monster of the sea reached lengths of 40 - 100 feet. Its teeth -- up to 7 inches in length -- were sharp and triangular with serrated edges that cut like a Ginsu knife. Without much trouble, this massive shark could stretch its jaws to accommodate a full grown man (or woman...I don't imagine it was particular). Fortunately, our present-day great whites, Carcharadon carcharias, have a maximum known length of 21 feet, as evidenced by a 7,300-pound specimen caught in Cuba in 1945. Their teeth have shrunk, too, down to a mere 2.5 inches. Still, their bite has been measured at 21 tons per inch, something I don't care to experience.

One of the most curious twists of evolution in the Earth's history has to be the return of mammals to the sea in the form of whales and dolphins. Land mammals evolved sometime around the time of the dinosaurs in the mid-Jurassic. Yet, sometime during the early Tertiary, about 50 MYA, a breed of wolf-like mammals living at the edge of the Tethys Sea decided to abandon terrestrial life and return to the sea. Fossils from India and Pakistan reveal the transformation of four-legged seaside scavengers into amphibious seal-like creatures that eventually became full-blown aquatic predators with small hind legs. Eventually, the hind legs disappeared as their bodies became streamlined with flippers and tail flukes. Nostrils, originally in the front of their heads, migrated to the top of their heads to become the familiar blowhole that is present in whales and dolphins today.

Why this happened is anybody's guess, but we know today that whales and dolphins are aquatic mammals with incredible grace and sophistication. In all ways, they are truly the gentle giants of the sea, both in the size of their bodies, and in the intelligence with which they conduct themselves. We will spend an entire lecture on whales and dolphins later in the semester.

Question: Who are the modern day dinosaurs of the sea?

Is Gaia Here?

The startling and pervasive expansion of life on Earth begs several questions concerning the nature of evolution and the processes which feed it. In a strict Darwinian sense, new life arose as competitive pressures made other life unsuccessful. Species at a disadvantage came to an end. On the other end of the spectrum, the cooperation between cells at a very early stage -- the process of symbiogenesis -- casts a different view of the development of life on the planet.

Whether life arose and took command of planetary processes for its own good, as proposed by Lovelock in his Gaia theory; whether life arose as the sum of populations of individual genomes with an expressed fitness or ability to survive under a given set of environmental conditions, as popularly expressed by evolutionists and molecular biologists; or whether some laws of non-equilibrium thermodynamics pulsate at the quantum level of life such that symbiotic organisms, communities of organisms, and entire ecosystems had to arise to satisfy these pulses, as suggested by some physicists and myself, are questions that will continue to drive our science. The importance of looking backwards into the fossil record, and indeed, into space, cannot be overstated. The evolution of life and biogeochemical processes on this planet will be most easily understood by gaining some sense of what has happened in the past, be it on our planet or in the far reaches of interstellar space.

In this quest, we must also look ahead. We must gauge the effects of our own mechanization and evolution on the globe. We must scrutinize the smallest impacts and synthesize their individual fluxes into a global model that gives us the best picture of what might happen to our planet. We have to give it our best shot. As evidenced by the rapid demise of many marine organisms 245 million years ago and the dinosaurs 65 million years ago, the Universe waits for no one.

What does this have to do with Gaia? Only that it is prudent to keep our minds open to all possibilities. Life's extraordinary "gift" of oxygen 2 billion years ago, and its maintenance of that "gift" for another 2 billion, exemplify the power that life -- microbial life -- holds over the planet. We would be wise not to dismiss the possibility that microorganisms hold the purse strings of other equally powerful planetary processes. If the foundations of Gaia lie in the microbial world, and a good case could be made for this premise, then man's reign here is not because of his own advanced evolutionary state; rather, man is here by permission.

Considering that every cell in our body contains a mitochondrion, which originated as a bacterial cell, and considering that our own bodies contain as many as ten thousand billion bacterial cells -- nearly ten percent of our body weight -- we might be wise to reflect on who is ruling whom!

Be that as it may, we will discuss at a later date the possible mechanisms and evidence for them that would support the idea of an interconnected, Gaian planet. In the meantime, I will leave you with these few paragraphs from Margulis and Sagan, which sums up much of which we have just learned. 

It is an illuminating peculiarity of the microcosm that explosive geological events in the past have never led to the total destruction of the biosphere. Indeed, like an artist whose misery catalyzes beautiful works of art, extensive catastrophe seems to have immediately preceded major evolutionary innovations.

Life on Earth answers threats, injuries, and losses with innovations, growth, and reproduction. The disastrous loss of needed hydrogen from the gravitational field of the earth led to one of the greatest evolutionary innovations of all time: the use of water (H2O) in photosynthesis. But it has also led to a tremendous pollution crisis, the accumulation of oxygen gas, which was originally toxic to the vast majority of organisms living on the planet. Nonetheless, the oxygen crisis 1,000 million years ago promoted the evolution of respiring bacteria which used oxygen to derive biochemical energy more efficiently than ever before. These bacteria were symbiotic and merged with other bacteria to form eukaryotic cells -- which, becoming multicellular, evolved into fungi, plants, and animals. The most severe mass extinctions the world has ever known, at the Permo-Triassic boundary 245 million years ago, were rapidly followed by the rise of mammals, with their sharp eyes and large receptive brains. The Cretaceous catastrophe, including the disappearance of dinosaurs 66 million years ago, cleared the way for the development of the first primates, whose intricate eye-hand coordination led to technology. World War II ushered in radar, nuclear weapons, and the electronic age. And the holocaust of Hiroshima and Nagasaki over forty years ago decimated Japanese industry and culture, unwittingly clearing the way for a new beginning in the form of a rising red sun of the Japanese information empire.

With each crisis the biosphere seems to take one step backwards and two steps forward -- the two steps forward being an evolutionary solution that surmounts the boundaries of the original problem. Not only meeting but going beyond challenges confirms that the biosphere is extremely resilient, that it recovers from tremors with renewed vigor. Nuclear conflagration in the northern hemisphere would kill hundreds of millions of human beings. But it would not be the end of life on Earth, and, as heartless as it sounds, a human Armageddon might prepare the biosphere for less self-centered forms of life. As different from us as we are from dinosaurs, such future beings may have evolved through matter, life, and consciousness to a new superordinate stage of organization, and in doing so, consider human beings as impressive as we do iguanas.

While boldly stated and even startling, these paragraphs in their book at least help us to step back from our human-centric view of life, and help us to consider the enormity of life forces of which we are a part. Much as ancient men had to reconcile the fact that the Earth was not the center of the Universe, so too might modern man have to realize that he is not the dominant player in the circle of Life on this planet.

Question: If Gaia did begin to operate during the evolution of the Earth, when might it have started?

from The Remarkable Ocean World