The Valongo Formation, Portugal - Land of the Giants


Outdoor shot of a rocky area covered with small broken slabs of flat rocks and a large section of rock protruding from the earth. There is also sparse scrubby ground vegetation.

     Near the small Portuguese town of Arouca is situated one of the most fascinating Ordovician-age trilobite sites in the world. It's not that the species strikingly showcased within this quarry's smooth slate blocks are particularly unusual. In fact, virtually all of the fossils found here have been previously reported from other European Paleozoic locales throughout France, Spain and the Czech Republic. Nor is the preservation of the trilobites found within the Arouca shale especially impressive, with flat, often metamorphosed, ghostly white specimens contrasting dramatically against a jet black mudstone matrix. What makes these ancient arthropods drawn from the rich layers of what has become internationally recognized as the Valongo Formation of pronounced scientific significance are the dimensions that some of these trilobites once attained. 
     With species such as Ogyginus forteyi, Hungioides bohimicus and Uralichas hispanicus reaching lengths of a foot, or more, these Paleozoic treasures rank among the largest trilobites found anywhere on Planet Earth-- with one partially enrolled Ogyginus specimen estimated to have been 30 inches long… a world record!  And not only were the trilobites of the Valongo Formation often huge in size, they were also incredibly prolific. In the spring of 2009, the revelation that large clusters of super-sized trilobites were discovered in these hilly Portuguese outcrops generated a major scientific stir.
Debates discussing possible mating assemblages, protective burrowing behaviors and mass mortality scenarios dotted paleontological literature, and even the mainstream media seemed to briefly enjoy jumping aboard the trilobite train. In fact, this unexpected flood of press attention directed towards the fossil finds at Arouca did a great deal to promote the concept that these 450 million year old creatures were much more than mere impressions left upon some ancient rock surface. The reports seemed to bring these long-gone arthropods back to life, providing previously unknown-- and unimagined--insight into the lives, habits and deaths surrounding a variety of trilobite species. 
     The appearance of these intriguing ancient layers has helped provide paleontologists around the world with a unique window into what life may have been like back in the primordial oceans surrounding what was then the super-continent known as Pangea… the land mass that existed some 400 million years ago before the continents began to break apart and ever-so-slowly assume their more familiar shapes and geographical positions. The impressive trilobite assemblages of the Valongo Formation feature some matrix blocks with upwards of two dozen creatures lying literally one-upon-the-other. 
     Such mass mortality plates help lend additional credence to theories concerning everything from ancient planet-wide climatic shifts to fluctuating Ordovician sea levels. These insights appear particularly relevant today in light of growing concerns regarding the escalating impact of global warming upon our planet and its inhabitants. And with the entire region surrounding these significant sedimentary layers recently being designated as a UNESCO (United Nations Educational Scientific & Cultural Organization) sponsored Geopark-- created specifically for the preservation of the world's most notable geological resources-- the Valongo Formation has now earned world-wide recognition as being home to the largest trilobites in the world.
     “It is without question one of the most interesting paleontological discoveries made this decade,” said Francisco Alonso Couce, a resident of Madrid who specializes in collecting the giant trilobites found in the Valongo Formation. “Finding these huge trilobites was very unexpected… especially in the numbers that they've been found. Most of the layers that have been uncovered in this formation are barren of trilobites, as well as of all other fossils. That's why there was so much excitement when these highly fossiliferous layers began to be found. Then the sheer numbers of trilobites began to garner a lot of attention… and, of course, then there was the size.”

     Generally when one imagines trilobites, images of small, hard-shelled creatures scurrying across ancient sea floors spring instantly to mind. And for the most part, such a perception isn't far from the truth. The vast majority of the more than 25,000 scientifically recognized species that existed during the trilobites' 300 million year reign from the Lower Cambrian to the end of the Permian were diminutive-- an inch, or smaller in length. Indeed, among scientists and collectors, trilobites that fall into the 3 to 5 inch range are generally considered “monsters.” But by no means should such information be interpreted as indicating that trilobites didn't grow bigger than that… much bigger! 

     Perhaps the most famous of these large trilobite species have been the Ordovician-age Isotelus maximus from the Midwestern United States, a type that grew up to 14 inches in length and became recognized as the State Fossil of Ohio back in 1985. Impressively-sized Silurian-age trilobites have also been found in Western New York State, with species such as Trimerus delphinocephalus attaining lengths up to 8 inches. 
     A variety of species have been found in the prolific Ordovician quarries near St. Petersburg, Russia,that on rare occasion exceed 10 inches from head to tail. And while over the decades, partial specimens (usually disarticulated cephalons) have given ample evidence of the great size that some trilobites might have attained-- such as with the legendary, spinose Devonian lichid, Terataspis grandis, which may have grown to two feet in length-- complete examples of such extraordinary discoveries had been few and far between… at least before the fossil “revolution” that has occurred world-wide over the last 25 years.
     Prior to the late 1980s, when such fossil hotbeds as Morocco, China and Russia first opened their doors to the prying eyes of eager scientists, museum curators and serious collectors, most trilobites had been found in Eastern Canada, the Western United States, Bolivia, and Europe. These were generally small specimens rarely exceeding four inches in length. Few within the scientific community had any reason to expect that a veritable treasure trove of giant-sized Paleozoic relics were out there, just waiting to be unleashed into their fossil-loving hands. But then, strange things started to happen. 
     First, the now-legendary trilobite beds of Northwest Africa started to produce Cambrian-age Paradoxides trilobites of prodigious size… and in equally prodigious numbers. Thousands of specimens, some reaching 16 inches in length, began to emerge on the world stage, highlighting museum displays, flooding fossil trade shows and even appearing in local rock shops. These massive specimens almost single-handedly changed public perceptions about trilobites. No longer were they fingernail-sized fossils designed for well-manicured cabinets of curiosities or carefully displayed museum shelves. Now they were often larger-than-life monstrosities designed to be seen from across the room… even if that room happened to be a grand gallery filled with 100 invited guests.
     Even before the paleontological community had begun to fully grasp the scope of this Invasion of Cambrian Giants, Morocco started to unleash its next oversized trilobite barrage… Ordovician-age asaphids that often grew to more than 20 inches in length. Indeed, one particular layer produced over 30 of these oceanic monsters, many in such close proximity that they apparently indicated some sort of real-life interaction. It rapidly became clear that many parts of the Moroccan lagerstatte had served as a macrofauna, giving rise to some of the most astounding trilobites ever seen. But while the appearance of these North African behemoths made many within the scientific community begin to reconsider their views on trilobite dimensions and lifestyles, they also made these scholars ponder why other sedimentary layers around the globe weren't producing similarly over-sized fossil specimens. 
     Some of their questions were soon to be answered. At roughly the same time that the giant Moroccan trilobites were hitting the world market, ten inch examples of the Ordovician Dikelocephalid trilobite Asaphopsoides yongshunensis were emerging from central China by the dozens. And in 1998, along the shores of Hudson Bay in Manitoba, Canada, a 28 inch-long specimen of Isotelus rex (roughly “The King of Trilobites”) was being collected by members of the Royal Ontario Museum, making it the largest complete trilobite yet found. Soon after, teams of commercial diggers in Portugal, who had been supplying the flat plates of black Valongo Formation slate as tiles for housing since the late 1950s, began to notice that they had hit upon an interesting layer-- one that was not only producing large trilobites, but producing them in significant quantities as well. 
     In light of the amazing fossil finds made around the globe over the last two decades, an obvious question began to emerge: why did trilobites in certain locations grow to astounding dimensions, while in other outcrops representatives of the same species seemingly remained within more conventional size boundaries? It is a query that still has scientific tongues wagging from Tokyo to Toronto. One of the current hypotheses speculates that gigantism can be brought on as an adaption to a colder climate. And since back in the mid-Ordovician, the undersea location of present-day Portugal was located much closer to the South Pole, it is possible that the Arouca trilobites grew larger in order to meet new pressures placed on them by their rapidly changing environment. However, during that same Paleozoic timeframe, Hudson Bay, where Isotelus rex was found, was believed to be a sub-tropical tidal estuary. Thus, this debate rages on!
     Other scholars have speculated that much like some modern-day arthropods, trilobites might have continued growing throughout their lives, molting their hard exoskeletons on a regular basis to reflect their ever-increasing dimensions. If one can surmise that the areas in and around Arouca were relatively free of predators (and no apparent trilobite-eaters have yet to be revealed in the Valongo Formation layers), then trilobites could have lived for relatively long periods of time, thus producing large specimens in life… and impressive fossils in death. Some scientists have also logically hypothesized that increased size may have helped ward off predation, providing certain trilobite species, which had inherited the “gigantic” gene, a better chance to survive and reproduce.
     Obviously, many key questions concerning trilobite growth remain to be answered; there is still much work to be done and knowledge to be garnered from the study of the giant trilobites of Arouca. For well over a century, Ordovician trilobites have been recognized throughout Portugal thanks to various well-known outcrops of the Valongo Formation. Prior to the discoveries at the Arouca site, however, scientific parties specifically searching for Paleozoic relics traditionally found most of these specimens in a rough limestone matrix. Species such as Nesuretus avus, Zeliszkella toledana, Illaenus giganteus and Bathycheilus castiliani became familiar to both academics and trilobite hobbyists around the globe-- but these were all relatively small examples, usually not exceeding 3 inches in length. So it wasn't until the commercial tile quarry at Arouca started yielding its amazing discoveries that a greater understanding of the possible dimensions and quantities that these trilobites attained became better understood.
     “As in a typical working quarry, the rock at Arouca is removed in layers,” Couce said. “After all, this has always been a commercial operation, not one specifically designed to reveal fossils. The trilobite finds that have occurred at Arouca have been a side-benefit of the operation, not the primary mission of the workers. But everyone involved quickly realized what they had uncovered; the right authorities were contacted, and special care was taken to extract these specimens in the best possible manner.”
     The surprising fact is that a vast abundance of the Arouca sedimentary layers have proven to be virtually barren of fossils, and for many years it was assumed that few, if any, significant paleontological finds would be made in the quarry. It wasn't until the first years of the 21st Century that the layers featuring huge trilobites in astounding numbers were uncovered… and quickly exhausted. Almost as soon as that “special” layer had been opened, workers ran into hillside overburden that ostensibly stopped their digging cold. Indeed, it is now estimated that if traditional tile work is allowed to continue at the quarry, and layers are removed at the same steady rate that they have been for over the last half century, it may take another 20 years-- or more-- before the trilobite-bearing layers are once again brought into play by quarry workers. 
     However, it now appears as if work of all sorts-- be it for roofing tile or the search for fossil treasures-- may soon come to a forced end at Arouca. With the area now falling under UNESCO Geopark jurisdiction, the quarrying of any materials drawn from the Valongo Formation layers may soon find itself greatly restricted, with all discoveries becoming instantly labeled as “national treasures.” To further solidify its new-found notoriety, as well as draw additional tourist interest towards Arouca, the quarry has now even created an impressive on-site museum. This facility is designed to display some of the most outstanding trilobite specimens found in the area's layers over the last few years, including large blocks featuring masses of possibly mating, possibly molting giant trilobites. 
     So somewhat ironically, just as interest in these amazing Portuguese “monster” trilobites has begun to reach a fever pitch, they may soon become something of an “endangered species” with all future digging at Arouca possibly being brought to a grinding halt. But despite such impending issues, there are already enough of these unique trilobites unearthed to keep academics busy for many years to come. Clearly, these amazing Paleozoic relics now rank among the most intriguing invertebrate fossils on earth, worthy of study and display in any museum, anywhere. These magnificent relics, drawn from the rich hillsides of Portugal, now enjoy the well-deserved recognition as being the true giants of the trilobite world.

Gallery of Valongo Trilobites

Layers of tilted slate or shale exposed at the surface, surrounded by broken-off rock pieces on the ground.