Lower Devonian trilobite diversity

The Devonian of the Ardennes, and especially the Eifelian, is very rich in trilobites. During this period, the Ardennes were situated north of the closing Rheic Ocean, along the southern margin of the Old Red Continent. Gondwana, to which present-day Morocco belongs, was on the southside. Lower Devonian strata in the Ardennes are characterised by near-shore clastic sediments unfavourable to many trilobites.

Traditionally, the Lower Devonian of the Ardennes has been characterised as comprising only “Rhenish” trilobites of the families Homalonotidae (Digonus, Burmeisterella, Wenndorfia, etc.) and Acastidae (Treveropyge, Acastava, etc.). The transition between the Lower Devonian and Middle Devonian is clearly defined by the introduction of trilobites of “Bohemian” (or sometimes called "Hercynian") type along with initiation of widespread carbonate deposition. These trilobites include Scutelluidae (Goldius, Scabriscutellum, Septimopeltis), Proetidae (Basidechenella, Gerastos, Tropidocoryphe, Astycoryphe, etc.), Lichidae (Ceratarges, etc.) and Odontopleuridae (Kettneraspis, Koneprusia, etc.). The early Eifelian fossil site "Mur des douaniers" near Vireux-Molhain serves as a good example of such a fauna which was until then believed to be exotic to the Ardennes. According to a recent study (van Viersen & Prescher, 2009) the situation in southeastern Belgium is different, where “Bohemian” trilobites already occur in the Pragian and thus in the middle part of the Lower Devonian - much earlier than had been expected. These trilobites encompass members of Lichidae, Phacopidae, Scutelluidae and Proetidae occur in the Longlier Formation (Pragian to possibly lowermost Emsian) in the area of Longlier and Martelange.

Left: an articulated specimen of Belenopyge sp. (formerly Lobopyge) and right: Scutelluinae gen. & sp. indet., both from the Longlier Formation (Pragian) in the Martelange area. Specimens are in the collection of the IRScNB.

 Lower Devonian  Middle Devonian  Upper Devonian

Trilobite diversity was recently subjected to a preliminary study (Crônier & van Viersen, 2007) which focused on the southern border of the Dinant Synclinorium.

Localisation of “Bohemian” trilobites in the Lower Devonian of the Ardenno-Rhenish Mountains (banded areas).

Digonus sp. (left) and Digonus ornatus disornatus Richter & Richter, 1932 (right) from the Pragian in the La Roche-en-Ardenne area.