Evolution of Marine Invertebrates


Trilobite photo from www.trilobite.com

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.

Appearance of Multicellular Life (1 BYA)

The appearance of multicellular organisms represents the true expression of organismal teamwork. From this point on, life's "problems" would be solved by mutual cooperation of multicellular assemblages of cells. These "confederacies" of cells gave rise to the development of specialized structures, such as holdfasts (for hanging on to a substrate), trichocysts (stinging cells, for protection), tentacles (for gathering food), and sex organs (for reproduction).

The earliest known multicellular organisms belong to a class of plant-like animals known as volvocines (from the Latin word volvere, which means "to roll"). These simple, spherical shaped organisms literally roll through the water. They are composed of a colony of cells resembling single-celled algae, but they have two flagella and an eyespot. The simplest volvocine is a colony called Gonium socilae, made up of four cells stuck together in a jelly-like matrix. Interestingly enough (and just to "prove" that they arose from individuals), any cell in this colony-of-four is capable of swimming off and starting a colony of its own.

A more complicated volvocine is Volvox, which is made up of about half a million cells. This organism actually produces specialized cells which can break off and form their own colonies. These offspring cells represent one of the first examples of asexual reproduction. Be that as it may, some species of Volvox have evolved sexual reproduction, producing and releasing eggs and sperm to create new progeny.

Unfortunately, we don't have time to consider all the interesting evolutionary innovations that occurred in the primitive seas. However, we should note that from these first simple colonies of "algal" cells, more elaborate forms of algae evolved. One familiar example is the kelp Macrocystis, which occurs off the California coast and is one of the fastest growing plants in the world. On a good day, a kelp may grow as much as three feet. Imagine if that happened to you and me!

On the animal side, soft-bodied sponges and sea anemones are implicated as the first inhabitants of the sea. Many of the early forms of animal life were soft-bodied and left no clues as to their existence. However, a few sandstone imprints of a soft-bodied animal known as Ediacarans have been found in Australia in sediments dating back to 700 million years ago (MYA). These organisms resemble sea worms, which suggests that annelids (of which earthworms are a member) had evolved by this time. Their ancestors, what Margulis and Sagan call "little societies of cells", were far simpler than the Ediacarans and resembled Trichoplax, one of the most primitive organisms in the world discovered in the 1960s climbing the wall of an aquarium! This non-descript, multicellular version of slime is believed to be one of the earliest forms of multicellular organisms.

This marks the end of the Precambrian Geological Time Period.

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?