Middle Devonian trilobite diversity
The tranquil, warm waters of the Eifelian in the
Ardennes favoured the settlement of additional trilobite groups. This is shown by
a rapid diversity increase from the lower to the middle Eifelian. The lower Eifelian
‘Mur des douaniers’ locality near Vireux-Molhain (northern France) comprises at least
12 trilobite species that belong to Phacopinae, Proetinae, Asteropyginae, Cornuproetinae,
Otarioninae, Scutelluinae, Odontopleurinae, Koneprusiinae and Trochurinae. The middle
Eifelian of the Rochefort area has already yielded over 20 species comprising Tropidocoryphinae
and Harpetidae, in addition to those that are part of the Vireux-Molhain biota. These
localities illustrate well how rich trilobite associations can be in the Ardennes.
Trilobites from Rochefort (locality Loc004 of van Viersen & Prescher, 2008) include species that are remarkably similar to those recorded from the coeval ‘Trilobitenfelder’ of Gees in the Eifel. This can be explained by the fact that the Ardennes and Eifel were part of the same coastal line during the Middle Devonian. Not surprisingly, trilobite specimens from the Ardennes were assigned in the literature to species typically known from the Eifel, often regardless of age and morphology. However, it now appears that there are distinct differences at the species or subspecies level that require closer examination. For example, Kettneraspis bayarti from the Ardennes is similar to Kettneraspis elliptica from the Eifel and Sauerland, and Cyphaspis unguloides belgica from the Ardennes stands close to Cyphaspis unguloides unguloides from the Eifel, yet they are markedly different.
Holotype of Gerastos arcus from the Jemelle Formation (Eifelian) in Couvin in the Maillieux collection (IRSNB).
Paratype cranidium of Kettneraspis bayarti from the Jemelle Formation (Eifelian) in Jemelle.
Whereas trilobites are a major component of the benthic fauna of the Eifelian, in the Lower Givetian (Trois-Fontaines Formation), widespread development of reef complexes is observed in Belgium followed by lagoonal sediments unfavourable to trilobites. Higher in the (Lower) Givetian, trilobites reappear but diversity is much lower than during the uppermost Eifelian and assemblages appear to be dominated by Dechenella.
