Plate tectonics in the Palaeozoic
Moving continents and fossils alike
Discoveries of very similar, if not identical fossils at nowadays opposite ends of the world were, for a considerable period of time, some of the great mysteries in palaeontology. As for trilobites, a good example can be found in the Lower Ordovician epipelagic species Carolinites genacinaca ROSS, 1951, appearing in sedimentary rocks not only of northern Australia and the western United States but in the Arctic, eastern Siberia and southeast China as well.
If you look up these places on a map of our time, you will realize that geographically they are located not only as distant as can be but neither do they share the same or a at least similar geographic latitude. And there are other trilobites, coming from places as far apart as the Moroccan Anti-Atlas range and the quarries of Oklahoma in the United States, astonishingly similar in appearance as are, e.g., Odontochile and Huntonia. How do we explain those conformities in morphology? And what is the solution to the riddle of Carolinites genacinaca?
The first step in solving this puzzle was taken by the German scientist Alfred Wegener (1880-1930). And although he was not the first one to realize the very similar shapes of the western African coast and the eastern coast of South America, he can be attributed with having been the first to substantiate a theory of moving continental plates by geological survey. The pattern of distribution of trilobite fossils on today's continents may be surprising at first glance, but it becomes quite understandable when plate tectonics and continental drift are taken into consideration.
Certainly, Wegener in his time did not know anything about convection currents in the hot inside of the planet and so he thought the movement of the Earth's continental plates to be driven by centrifugal and tidal forces – forces we now know to be far too insignificant to have such effects. This may have been one reason why the bottom line of his theory had been contended throughout his time and was entirely forgotten soon after he departed from this life. It was not until after the end of World War II that further and deeper investigations into the matter vindicated his conclusions and since the 1970's plate tectonics have become a scientific fact.
On this page, we would like to tell a little about the discovery of the continental drift and how planet Earth continued to change its face during the 300 million years of the trilobite reign, using palaeogeographic maps of C. R. Scotese's PALEOMAP Project and Dr. Sam Gon's trilobite website. These maps are based on results of research into palaeomagnetism, linear magnetic anomalies, palaeobiogeography and other fields of science, giving convincing evidence of the multiple arrangements of land masses during this incredible time span.
In his book „The Formation of Continents and Oceans“, first published in 1915, Alfred Wegener inferred from the astounding similarity in shape of the coast lines of the South American and African continents that in a remote past both land masses may have been part of a far bigger super-continent which broke apart at some point during the planet's history. The fitting accuracy becomes even more obvious when looking at the continental shelves themselves, submerged as they are, rather than at the easily visible coastlines raising above sea level. It was down there that plate tectonics turned from theory to reality.
Wegener, convinced of the importance of his observations, undertook to reconstruct a primeval super-continent which he named Pangäa („whole Earth“) and which included not only the southern continents but all present land masses. He theorized about specifically light continents, predominantly made up of of silicium and aluminium, both elements characteristic of granite, „floating“ on top of more dense and heavier masses, where silicium and magnesium were the predominant elements to form a basaltic composition. Just like an iceberg floating in water, the lighter continent would float on top of the heavier material below.
As to the powers at work in breaking up continents and making them drift apart, Wegener sought for explanations in various astronomical forces like the deceleration of the planet's rotation by the tidal friction of the moon or forces resulting from changes in the earth's rotational axis.
He called the latter „Polflucht“ (flight from the poles), a pull induced by the planet's rotation, and looked at it as a potential driving force in bringing the continents ever closer to the equator. Wegener himself, however, soon realized that this force was in no way substantial enough to explain the drifting of entire continents and this doubtful idea played a considerable role in his entire drift theory being vigorously challenged by most contemporary geologists throughout his lifetime.
It was not until the early 1960's that upcoming deep sea exploration lead to a fundamental increase in knowledge of the geology of the oceans' sea-floors. Scientists realized that areas like the mid-ocean ridges were volcanically active, erupting huge amounts of basaltic lava, mostly pillow lava, through wide-stretched fissures in regular intervals. Palaeomagnetic studies on these basaltic rocks lead to the discovery of repeated reversals of the earth's magnetic field during the planet's history - events which left their traces as mirrored stripe patterns on both sides of e.g. the Mid-Atlantic Ridge, a result of the alignment of magnetized particles along the globe's magnetic field lines inside the rock during its blistering birth. It was furthermore established that sedimentary rocks covering the deep sea floors became the older and more massive the more distant they were located from the mid-ocean ridges.
The most plausible and comprehensible explanation seemed to rest with those never-ceasing basaltic magma flows. Once released a smoldering mass and solidifying on both sides of those wide-stretched fissures that mark the mid-ocean ridges they seemed to be spreading the sea-floor in opposite directions.
There were and is no evidence, though, that this sea-floor spreading leads to any change in the planet's radius as could be expected and was actually described in the old 19th century expansion theory. Those masses of magma, that appear to have been released for eons, did not add a single inch to the globe's diameter, which ultimately leads to the conclusion that any newly formed oceanic crust will sooner or later be met with destruction at a place very distant from its origin. And we have never come across any sea-floor the age of which in excess of 200 million years.
As a matter of fact a large percentage of ocean floors explored by man turned out to be no older than 65 million years. These results finally refuted the early idea of unalterability which regarded oceans to be basins which were formed by collapsing crust soon after its first formation during the planet's early days..
So where is the geologic scrapyard that recycles oceanic crust formed millions of years ago? In the 1970's scientists identified the dark abysses of the deep sea trenches, most notably those of the Pacific Ocean, to be the most promising candidates due to their strong seismic and volcanic activity - the „Pacific Ring of Fire“. Geophysical studies revealed sloping areas of seismic friction where the heavier oceanic crust appears to subduct beneath a lighter continental (or other oceanic) crust until it melts in the high temperatures of the Earth's mantle and is thereby recycled.
Sea quakes, at times of enormous magnitudes, with epicentres usually at depths between 320 and 720 kilometres, are typical of such subduction zones. It was such an undersea megathrust earthquake with an epicentre off the west coast of Sumatra, Indonesia, that caused the disastrous tsunami on Dec. 26, 2004 claiming the lives of more than 200,000 victims when it hit coastlines along the Indian Ocean.
The continental crusts move on top of what is called the asthenosphere, a highly viscous mechanically weak ductily-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~ 62 and 124 miles) below the surface, but perhaps extending as deep as 400 km (~ 249 miles).
Today, with state-of-the-art satellites at our disposal to survey planet Earth from high above, taking images of outstanding quality and resolution and capable of measuring distances with an unprecedented accuracy, we are able to provide unquestionable evidence of our continents drifting. On average, sea-floor spreading occurs at a rate of a few centimetres per annum, albeit variable from ocean to ocean. Drift rates between the large tectonic plates have been established to range between 2 and 20 centimetres per year. The continents are moving as we speak and will be moving as long as the planet's core remains the blistering hell that it is.
In summary, it can be said that as early as 1915 Alfred Wegener postulated that both continental and oceanic crusts were drifting, for whatever reason, on the more dense and viscous asthenosphere, either apart or toward each other. Upon collision, the lighter crust (usually continental) would stay on top while the heavier would sink beneath it. But it was not until the 1970's that the combined results from different branches of geo science and the model of plate tectonics were finally ready to replace the older theories of mountain formation and why the planet's surface looked the way it did.
And it also brought the solution to the riddle of the Lower Ordovician epipelagic trilobite Carolinites genacinaca and the astonishing similarity between many Moroccan and American species. Looking back at the distribution of land masses during the Ordovician will reveal that all the places we mentioned to yield fossils of this small trilobite were once located in tropical regions, in the same latitude close to the equator. It took 400 million years to move these fossilized remains of Carolinites in different directions to the quarries they are found in today, a journey that took place partly submerged, partly as dry land. And because these fossils are so incredibly old, they can tell of the boundless forces that rule the inside of our globe.
The importance of the trilobites' role in palaeobiogeography must not be underestimated. Like the remains of other life forms, the fossils of these early arthropods and their geographic distribution in the fossil record of today represent another strong thread in the scientific steel rope on which any theory of evolution is suspended, biological or geological - „Panta rhei!“
Line drawings of Carolinites and tectonic maps © Dr. Sam Gon III (www.trilobites.info)
Plate tectonic maps and Continental drift animations © by C. R. Scotese,
PALEOMAP Project (www.scotese.com)