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The Paradoxides Paradox: Plate Tectonics and Continental Drift

     

Pangea Map image

     

     Trilobites are more than mere remnants of an incredibly distant time. The fact is that their fossils can reveal fascinating insights about our planet's frequently violent geologic history. Examining these ancient arthropods often provides us with a unique window into a mysterious undersea world that existed half a billion years ago, a world that in many ways is as alien to us as the moons of Jupiter.   
     Back in the early Paleozoic, trilobites filled the oceans. It was during this time that they emerged as the first dominant life form on earth to feature a hard exoskeleton. Due to that evolutionary advance, their remains are pervasive in the fossil record, providing important stratigraphic markers to key geologic horizons worldwide. Indeed, when trilobites of a similar age are viewed together, and then compared and contrasted with other existing specimens, their fossils often provide an unrivalled perspective on the primordial forces that helped shape Planet Earth. 
     Perhaps nowhere is this phenomenon more evident then when dealing with the Middle Cambrian trilobite Paradoxides. If you closely view the specimens presented within this site's Gallery of Trilobites, you will note something interesting. You will see that fossils of closely related Paradoxides species have been found in such now-disparate locations as Eastern Canada, Sweden, Wales, the Czech Republic and Morocco. 
     An obvious question then emerges: How can fossils of the same trilobite genus exist in rocks that are now thousands of miles apart? After all, it is believed that trilobites were relatively territorial animals, with their lives controlled by such factors as the depth and temperature of their aquatic environment. Few, if any, trilobite species were equipped to traverse the open ocean, even on a limited basis. So with that in mind, how did the fossilized remains of similar trilobite species find themselves in such diverse corners of the globe? It is the Paradoxides paradox.
     As it happens, the answer to this question is quite simple… it all has to do with Plate Tectonics and Continental Drift. These concepts explain how over an extended period of time, continents move across the earth's outer crust, often propelled by volcanic forces at play along the key fault lines that lie directly under the planet's surface. And correspondingly, as these continents continue to shift their global position ever-so-slowly during the passage of countless millennia, so do the trilobite fossils contained within their sedimentary layers.
    “Continental Drift explains why the eastern 'bulge' of South America seems to fit perfectly into the western recess of Africa,” said Sam Stubbs, an associate with the Houston Museum of Natural Science. “The reason is because at one time in the distant past, it did!”
     Some 510 million years ago, the earth was a far different place than it is today. The now instantly familiar outlines of the continents would then have been unrecognizable to our modern eyes as they were joined together in haphazard shapes to form supercontinents.  At that time, these immense bodies of land, which carried exotic names such as Gondwana and Avalonia, were aligned in radically different geographical positions across the face of the planet, often separated by huge bodies of water such as the Panthalassic Sea and Iapetus Ocean. 
      What would one day become the heartland of North America lay along the Cambrian equator, a part of the supercontinent known as Laurentia. In proximity to the south was the supercontinent Baltica, which consisted of what would eventually emerge as western Europe. The areas that would later become Wales and Newfoundland were co-joined off the coast of Gondwana. It was in the cool tidal estuaries located along this coast that Paradoxides trilobites swam by the thousands.
     Due to the incredibly turbulent geologic forces that lay behind the formation of these continents, the world's oceans were forced to continually expand and contract, in the process often flooding much of the continental landmasses that surrounded them. Indeed, during the Middle Cambrian, sea levels may have been higher than at any other time in earth history. Over the course of the next five-to-ten million years, this oceanic activity may have provided trilobite genus' such as Paradoxides with a unique opportunity to expand their range, while evolving into many closely-related species within a fairly limited geographical territory.
    The fossil record tells us that many of these species thrived within their chosen oceanic environment. In certain places, such as the Jince fossil beds in the Czech Republic and the Manuels River outcrops of Newfoundland, disarticulated Paradoxides carapaces are found scattered throughout the sedimentary layers. In the fossil stronghold of Morocco, thousands of complete Paradoxides fossils have been commercially mined over the last two decades. 
     Indeed, there may well be a reason why Paradoxides were so pervasive; they were the largest of Middle Cambrian trilobites, often exceeding 12 inches in length. Their size, along with their hydrodynamically streamlined body shape, allowed them to dominate the offshore continental shelf environments that they preferred to inhabit. It is believed by many scientists that Paradoxides were predatory animals, with other smaller trilobites potentially on their menu. Their size, as well as their top-predator status, apparently allowed a disproportionate number of Paradoxides remains to eventually become fossilized.
      “At that time in earth history, there's little doubt that larger trilobites had an evolutionary advantage over smaller ones,” said Jason Cooper, a leading trilobite authority. “That's why large Olenoides trilobites dominate the Middle Cambrian fossil record of the western US and Canada, while Paradoxides are so prevalent in rocks rimming the Atlantic Ocean -- whether in Wales, Newfoundland or Morocco.”
        As members of these various Paradoxides species perished -- either through natural attrition or a sudden shift in environmental pressures -- their remains were subsequently buried under eons of mudslides and sand deposits, which would then gradually be transformed into mudstone or sandstone. For hundreds of millions of years, the slowly fossilizing shells of these trilobites remained trapped within their final resting places, first as the oceans receded… and then as the continents broke apart and drifted towards their current locations atop the global lithosphere.
     Thus, it is relatively easy to understand why a Pardoxides davidis specimen uncovered amid the rocky shores of Wales possesses more than a passing resemblance to a Paradoxides davidis specimen found 3,000 miles away along the coast of Newfoundland. At one time, more than 500 million years ago, these trilobites shared an identical and overlapping ecological niche off the coast of Gondwana. Much the same can be said for the closely related Paradoxides gracilis in the Czech Republic, Eccaparadoxides mediterraneus in Spain and Acadopardoxides Levi-setti in Morocco; at key moments in our planet's distant past, all lived within the same thriving trilobite community. Only time, along with the rampant powers of nature, have served to subsequently drive their fossilized remains to distant corners of the globe.
     “The similarities between the Paradoxides davidis specimens found in Wales and the type found in Newfoundland are amazing,” said professor Riccardo Levi-setti. “The rock itself may appear slightly different, but the trilobites themselves are virtually identical. We know that these species must have lived in close proximity back in the Middle Cambrian. The dynamic pressures that caused their separation bespeak of the internal forces that still dominate our planet.”

 

Worldwide distribution of Paradoxides species

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