Family Homalonotidae
Subfamilies: Homalonotinae
Genera: Arduennella, Burmeisterella, Digonus, Scabrella, Parahomalonotus, Wenndorfia.
Homalonotinae are classically known to occur in the Lower Devonian of the Ardennes. They frequent the old literature where they are usually listed under the name Homalonotus but all of these species were reassigned to different genera through time. Digonus is probably the most widely known of these genera. It is already present in the lowermost Lochkovian in the Mondrepuis area (Digonus roemeri). These rocks are part of the Mondrepuis Formation and were deposited in a young sea that flooded the lands of the present-day Ardennes from the south towards the north during much of the Lower Devonian. Members of Digonus persist until the Upper Emsian where they can locally become abundant.
List of species
Lochkovian
– Digonus roemeri (DE KONINCK, 1876)
Pragian
- Digonus ornatus disornatus RICHTER & RICHTER, 1932
- Scabrella asselberghsi (MAILLIEUX, 1936)
- Parahomalonotus sp.
- Digonus sp.
Emsian
- Digonus gigas gigas (ROEMER, 1843)
- Burmeisterella armata (BURMEISTER, 1843)
- Burmeisterella quadrispinosa WENNDORF, 1990
- Wenndorfia mutabilis (KOCH, 1880)
- Wenndorfia sp.
- Arduennella maillieuxi (ASSELBERGHS, 1923)
Interesting about certain homalonotines such as Digonus and Burmeisterella is the acuminate rostral plate (not to be confused with the hypostome [=mouth plate]) which may be visible in homalonotines both ventrally and dorsally. Wenndorf (1990) described how the sharp end of this plate fitted under the pygidial doublure, acting like a lock. When the trilobite enrolled, this locking system ensured that its carapace became an impenetrable ball. The powerful spines of Burmeisterella armata will only have added to the defensive effect.
Digonus gigas gigas, a characteristic homalonotid in the Upper Emsian that is locally abundant. After Wenndorf.