Locality reports
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Locality code: F-032
Locality name: Mur des douaniers Vicinity: Vireux-Molhain, northern France Geology: Vieux Moulin Member of the Jemelle Formation (lower Eifelian) The Mur des douaniers is a famous rock outcrop along the route D47 connecting Treignes and Vireux-Molhain. The earliest reports of abundant phacopid trilobites in this area date from the late 19th century (van Viersen & Daumeries, 2012), as a result of exploration of shales originating from construction works of the nearby Najauge railway tunnel. For many decades the Mur des douaniers was frequented by collectors for its rich trilobite fauna although the specimens are decalcified and severely tectonically distorted, almost without exception. The site was declared a natural reserve by French authorities in 1991 and collecting is now prohibited. Ever since its closure the only way to acquire trilobites from this famous site is to purchase them. Material does sometimes appear on fossil markets or online, usually comprising frequently occurring taxa such as Geesops sparsinodosus gallicus (now called just Geesops gallicus). Among the rarer trilobites from Vireux-Molhain are lichids and odontopleurids; both are extremely hard to come by. |
The Mur des douaniers in 2004.
Map with the Mur des douaniers indicated by a cross.
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Misconceptions
Despite their popularity there has long been scant literature available about the trilobites of the Mur des douaniers. Most sources concerned booklets, websites, or brief sections in professional papers. Two errors about the Mur des douaniers consistently occur:
Misconception 1. Traditionally, the rocks that are exposed at the road cut were assigned to the "Assise de Couvin" [note that Co2a is type horizon of Septimopeltis magnispina (Maillieux, 1938)] or sometimes the underlying "Assise de Bure". The age is said to be Eifelian. According to a panel that is placed at the site, the outcropping rocks belong to the St. Joseph Formation. This formation is the basal unit of the Couvinian as perceived in the old literature by Maillieux and others, but is actually of Early Devonian (Emsian) age. The cause of the misconception lies in the fact that the Emsian-Eifelian boundary does not coincide with the Emsian-Couvinian boundary but is situated in the overlying Eau Noire Formation. Assignment to the St. Joseph Formation conflicts with lithology at the site and with the occurrences of the trilobites Geesops and Septimopeltis which strongly suggest an Eifelian age. The discovery of the goniatite Pinacites jugleri at the Mur des douaniers provides further biostratigraphic evidence for an early Eifelian age. Van Viersen (2006) positioned the outcrops in the lower part of the Jemelle Formation. In 2008, Dumoulin & Blockmans recognised a new member of the Jemelle Formation, the Vieux Moulin Member, to which the outcrops at the Mur des douaniers were also assigned.
Misconception 2. A large part of the various (literature and internet) sources provide wrong names that were adopted from other (similar) sources. The panel that is placed at the site mentions 10 names of trilobites, but only a few of these appear to be correct. A mistake made in many sources, is the attempt to compare the Vireux trilobite biota to those from other regions and of different age. Species such as Phacops latifrons, Cyphaspis ceratophthalmus (often incorrectly referred to as Otarion ceratophtalmum (sic!)), Asteropyge punctata, Gerastos cuvieri and Ceratarges armatus, which are claimed to have been discovered in the lower Eifelian of Vireux-Molhain and Treignes (Belgium), are typically known from the middle Eifelian of the German Eifel region. None of these are known from the Ardennes.
Naming of trilobite taxa
Maillieux (1938) was the first to formally establish a new trilobite species in describing Septimopeltis magnispina. Additionally, Geesops gallicus Struve, 1982, Gerastos catervus (van Viersen, 2006) and Ceratarges cognatus van Viersen, 2006 have been described. Van Viersen (2008) recorded several rare Koneprusia specimens but treated these in open nomenclature. Van Viersen et al. (2019) described additional new species: Asteropyge eonia, Kettneraspis eftychia, Pedinopariops ceuthonymus, Cyphaspis cf. iuxta and Diademaproetus cf. pertinax. Other species that remain to be properly documented are Septimopeltis cf. akatastasia and Scabriscutellum cf. archinalae.
Despite their popularity there has long been scant literature available about the trilobites of the Mur des douaniers. Most sources concerned booklets, websites, or brief sections in professional papers. Two errors about the Mur des douaniers consistently occur:
Misconception 1. Traditionally, the rocks that are exposed at the road cut were assigned to the "Assise de Couvin" [note that Co2a is type horizon of Septimopeltis magnispina (Maillieux, 1938)] or sometimes the underlying "Assise de Bure". The age is said to be Eifelian. According to a panel that is placed at the site, the outcropping rocks belong to the St. Joseph Formation. This formation is the basal unit of the Couvinian as perceived in the old literature by Maillieux and others, but is actually of Early Devonian (Emsian) age. The cause of the misconception lies in the fact that the Emsian-Eifelian boundary does not coincide with the Emsian-Couvinian boundary but is situated in the overlying Eau Noire Formation. Assignment to the St. Joseph Formation conflicts with lithology at the site and with the occurrences of the trilobites Geesops and Septimopeltis which strongly suggest an Eifelian age. The discovery of the goniatite Pinacites jugleri at the Mur des douaniers provides further biostratigraphic evidence for an early Eifelian age. Van Viersen (2006) positioned the outcrops in the lower part of the Jemelle Formation. In 2008, Dumoulin & Blockmans recognised a new member of the Jemelle Formation, the Vieux Moulin Member, to which the outcrops at the Mur des douaniers were also assigned.
Misconception 2. A large part of the various (literature and internet) sources provide wrong names that were adopted from other (similar) sources. The panel that is placed at the site mentions 10 names of trilobites, but only a few of these appear to be correct. A mistake made in many sources, is the attempt to compare the Vireux trilobite biota to those from other regions and of different age. Species such as Phacops latifrons, Cyphaspis ceratophthalmus (often incorrectly referred to as Otarion ceratophtalmum (sic!)), Asteropyge punctata, Gerastos cuvieri and Ceratarges armatus, which are claimed to have been discovered in the lower Eifelian of Vireux-Molhain and Treignes (Belgium), are typically known from the middle Eifelian of the German Eifel region. None of these are known from the Ardennes.
Naming of trilobite taxa
Maillieux (1938) was the first to formally establish a new trilobite species in describing Septimopeltis magnispina. Additionally, Geesops gallicus Struve, 1982, Gerastos catervus (van Viersen, 2006) and Ceratarges cognatus van Viersen, 2006 have been described. Van Viersen (2008) recorded several rare Koneprusia specimens but treated these in open nomenclature. Van Viersen et al. (2019) described additional new species: Asteropyge eonia, Kettneraspis eftychia, Pedinopariops ceuthonymus, Cyphaspis cf. iuxta and Diademaproetus cf. pertinax. Other species that remain to be properly documented are Septimopeltis cf. akatastasia and Scabriscutellum cf. archinalae.
A. Asteropyge eonia | B. Cyphaspis cf. iuxta | C. Gerastos catervus | D. Kettneraspis eftychia (digitally inverted photo of external mould) | E. Pedinopariops ceuthonymus | F. Ceratarges cognatus (internal mould) | G. Koneprusia sp. (silicone cast of external mould).
Note the exceptional quality of these rare specimens most of which have much of the mineralised cuticle preserved. Only the Ceratarges specimen is an internal mould.
Layer packed with Gerastos catervus specimens.
Profuse complete specimens
A noteworthy aspect of the F-032 locality is the wealth of complete trilobite specimens that it has yielded. We have examined several thousands of trilobites in private and museum collections, the vast majority of which are fully articulated. These numbers are exceptional for any locality in the Ardenno-Rhenish Mountains. Of note is that nearby coeval strata just across the Belgian border have yielded the same trilobite species but the specimens are scarce, poorly preserved isolated sclerites (van Viersen & Daumeries, 2012). The abundance of complete trilobites at the Mur des douaniers remains largely unexplored since new excavations for scientific studies there are not permitted (Crônier & van Viersen, 2008; van Viersen, 2008). Some clues are nonetheless provided by the trilobites. From a taphonomic point of view the large quantities of complete trilobites, which were prone to postmortal disarticulation, are suggestive of deposition below fair weather wave base. Van Viersen (2006), who examined sizable rocks in private collections, reported mass occurrences of Gerastos catervus, articulated and in various orientations, in several cm thick banks. A geological context is lacking for such samples but based on current knowledge, we speculate that the most parsimonious explanation would be to regard these as obrution deposits.
A noteworthy aspect of the F-032 locality is the wealth of complete trilobite specimens that it has yielded. We have examined several thousands of trilobites in private and museum collections, the vast majority of which are fully articulated. These numbers are exceptional for any locality in the Ardenno-Rhenish Mountains. Of note is that nearby coeval strata just across the Belgian border have yielded the same trilobite species but the specimens are scarce, poorly preserved isolated sclerites (van Viersen & Daumeries, 2012). The abundance of complete trilobites at the Mur des douaniers remains largely unexplored since new excavations for scientific studies there are not permitted (Crônier & van Viersen, 2008; van Viersen, 2008). Some clues are nonetheless provided by the trilobites. From a taphonomic point of view the large quantities of complete trilobites, which were prone to postmortal disarticulation, are suggestive of deposition below fair weather wave base. Van Viersen (2006), who examined sizable rocks in private collections, reported mass occurrences of Gerastos catervus, articulated and in various orientations, in several cm thick banks. A geological context is lacking for such samples but based on current knowledge, we speculate that the most parsimonious explanation would be to regard these as obrution deposits.
Internal mould of Geesops gallicus.
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Trilobites from an old collection. All the specimens shown here are articulated (complete) internal moulds.
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A rare external mould of Gerastos catervus.
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Papers cited
Crônier, C. & Viersen, A.P. van, 2008. The 'Mur des douaniers', an exceptionally well-preserved early Eifelian fossil site. Bulletin de la Societe Geologique de France, 179: 89-95. [ download full-text article ] Maillieux, E., 1938. Le Couvinien de l’Ardenne et ses faunes. Mémoires du Musée royal d’Histoire naturelle de Belgique, 83: 1-57. Struve, W., 1982. Beiträge zur Kenntnis der Phacopina (Trilobita), 10: Neue Untersuchungen über Geesops (Phacopinae; Unter- und Mittel-Devon). Senckenbergiana lethaea, 63: 473-495. Viersen, A.P. van, 2006. New Middle Devonian trilobites from Vireux-Molhain (Ardennes, northern France). Senckenbergiana lethaea, 86: 63-75. [ download full-text article ] Viersen, A.P. van, 2008. First record of the odontopleurid trilobite Koneprusia from the lower Eifelian of Vireux-Molhain (northern France), with remarks on the associated trilobite fauna. Geologica Belgica, 11: 83-91. [ download full-text article ] Viersen, A.P. van & Daumeries, G., 2012. Les trilobites du Dévonien ardennais du “Mur des douaniers” à Vireux-Molhain. Fossiles, Revue française de Paléontologie, 8: 5-15. [ download full-text article ] Viersen, A.P. van, Taghon, P., Magrean, B. 2019. Early Middle Devonian trilobites and events in the Nismes - Vireux-Molhain area, southern border of the Dinant Synclinorium (Belgium, northern France). Geologica Belgica, 22: 7-33. [ download full-text article ] |
A rare, exceptionally well-preserved Geesops specimen from Vireux-Molhain. According to van Viersen et al. (2019) there appear to be several Geesops species that occur at this site. It is not certain which specimens are conspecific with Struve's holotype of Geesops gallicus because the latter is a poorly preserved internal mould.
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