Giant trilobites and biotite nodules in Portugal

The generally accepted reason for the fame of Arouca is Princess Mafalda, born 1195, who was responsible for the convent becoming Cistercian. Here is an interesting story – she was beatified in 1793. However, I won’t go into that now, but it is well worth investigating. For this article, there are other reasons for its fame, at least among geologists.

Fig 1
Fig. 1. Map of Arouca Geopark.

Arouca is 38km to the south east of Oporto, in northern Portugal, and gives its name to one of two geoparks in Portugal. In Arouca Geopark (Fig. 1), which has an area of 330km2 (just a little smaller than the Isle of Wight), there are two quite remarkable geological features, one palaeontological and the other concerning igneous petrology.
A geopark is an area of significant size that has a particular geological heritage, with a certain number of sites of special importance – scientific quality, rarity, aesthetic appeal and educational value. It must also have a sustainable strategy for development to be accepted as a member of the worldwide network of geoparks. There are 42 in Europe, in 16 countries. The other Portuguese Geopark is Naturtejo, through which the River Tagus flows.

There are nine geoparks in the British Isles, for example, NW Highlands (Scotland), Copper Coast (Ireland), Fforest Fawr (Wales) and the English Riviera.
The website,, gives website addresses for all.
The geology of Portugal is very complex. There are no strata younger than Triassic, except for Holocene deposits in the valleys. The pre-Permian sediments have been metamorphosed and intruded by igneous rocks and there is also a considerable amount of mineralisation.

During the Lower Palaeozoic, there was an ocean environment with fine sediments accumulating on the floor. In this ocean lived a range of invertebrates. The animals that are the concern of this article are the trilobites, but there were also orthocone nautiloids, gastropods, bivalves, brachiopods, echinoderms and graptolites. There are also outstanding trace fossils.

The trilobites predominate and are what make the locality at Canelas exceptional. These fossils are found in the slates of the Valongo Formation – a fossiliferous succession of shales and siltstones, including slate beds giving material of roofing quality. This correlates with the Dapingian and Darriwilian of the Middle Ordovician, 471.8 to 460.9mya.

Fig. 2. Ogyginus forteyi – named after Richard Fortey, formerly of the Natural History Museum, London.

Complete specimens of some species reach up to 70cm in length and there seems to be a pattern to their occurrence – either isolated specimens or in clusters of up to thousands of specimens. While it is reasonable to suppose that these trilobites had a similar life style to the Horseshoe Crab (Limulus polyphemus), which number many thousands when they spawn on the beaches of Delaware Bay at full moon in Spring and Summer, the fact that the fossils are complete indicates that they died as a result of a catastrophic event such as a volcanic eruption. (In a natural death, the carapace would soon break apart along sutures, as it does when it moults as part of growing larger.) The asaphid, Ogyginus forteyi (Fig. 2), is abundant, but the most common trilobites are the calymenoids, Neseuretus and Bathycheilus, and the illaenid, Ectillaenus giganteus. These all indicate shallow water conditions.

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