The Giant Trilobite Fossils of Arouca UNESCO Global Geopark Northern Portugal

Fossil Coast’s Geo Travel blog explores the giant Trilobite fossils of Canelas in the Arouca UNESCO Global Geopark of northern Portugal (40° 57' 55'' N 008° 13' 04'' W).

The Arouca Municipality is a mountainous area found on the western border of the Iberian Peninsula in northern Portugal.

It became a World Heritage Site and UNESCO Geopark in 2009 and is known as the “Motherland of the biggest trilobites in the world”.

This area forms part of an internationally important Middle Ordovician slate lagerstätte from the Darriwilian Age (467.3 – 458.4 million years ago) for several of the world’s largest and social species of trilobites.

Trilobites are among the earliest marine arthropods that first appeared in the fossil record during the Cambrian Period before becoming extinct by the end of the Permian Period. Trilobites have a recognisable body form characterised by their protective exoskeleton covering their back.

The Canelas Quarry of the Arouca UNESCO Global Geopark is among the “First 100 Geological Heritage Sites” designated by the International Union of Geological Sciences (IUGS) in 2022. The giant trilobites can be best viewed at the Geological Interpretation Centre of Canelas (CIGC) which opened in 2006.

The Arouca UNESCO Global Geopark has 41 geo sites in an area of 328km2 surrounded by the Freita (1100 meters), Montemuro (1222 meters - 8th highest mountain range in Portugal) and Serra da Arada (1,049 meters).

The most notable geo sites are the Castanheira Nodular Granite (Pedras Parideiras), the Vale do Paiva Ichnofossils, and the Canelas Giant Trilobites.

During the Darriwilian Age, the second of the two ages within the Mid Ordovician Epoch the region was under water and a marine environment in the Rheic Ocean. This ocean was created during the transition from the Cambrian Period to the Ordovician Period and the closure of the Iapetus Ocean.

The Rheic Ocean is recognised as the major Palaeozoic Ocean of southern Europe laying at the time between Laurentia and Gondwana from the Early Ordovician before it closed during the Ouachita-Alleghanian-Variscan orogeny or mountain building event that formed the supercontinent of Pangea.

The Arouca UNESCO Global Geopark is a major geological feature of the Portuguese landscape of the western half of the Iberian Peninsula known as the Iberian Massif or Hesperian Massif. This is the largest fragment of the Variscan basement that crops out in Europe.

The Variscan Orogeny generated a fold several kilometers long known as the Valongo Anticline and through metamorphism created tge Valongo Formation. This formation sits on the western side of Canelas near Porto.

The forces of this tectonic activity from the Variscan Orogeny compressed and heated the shale clays and minerals of the seafloor sediments to create a compact, finely-grained grey-blue to dark-grey slate called Valongo Slate.

The bedding planes of Valongo slate represent the only deposit of well-preserved soft-bodied fossils from the Middle Ordovician of northern Gondwana in Portugal.

These fossils are among the first examples of carbonaceous film preservation where the body forms of the fossils have been carbonised having been exposed to great pressure over long periods of time.

The slates of Canelas hold many invertebrate fossils in addition to hundred species of trilobites. Among the fossil record are graptolites, brachiopods, gastropods, cephalopods, bivalves, rostroshells, cystoids, crinoids, cephalopods, bryozoan, echinoderms, phytoliths, conularias, ostracodes, and ichnofossils.

The importance of the Canelas Quarry trilobites is down to their gigantism (up to 80cm) achieved by many species; the preservation of articulated exuvia or casting off by the process of ecdysis or moulting of their outer exoskeletons near the complete corpses; also, an indication that many trilobite species had similar life cycles for synchronous moulting, reproduction and a social behaviour where some species habitually lived or moved in large clusters together rather than being solitary or living alone.