A spinose appendage fragment of a problematic arthropod from the Early Ordovician of Morocco PETER VAN ROY and O. ERIK TETLIE Van Roy, P. and Tetlie, O.E. 2006. A spinose appendage fragment of a problematic arthropod from the Early Ordovician of Morocco. Acta Palaeontologica Polonica 51 (2): 239–246. A highly spinose fragment of a possibly raptorial appendage from the Arenig (Early Ordovician) of the Upper Fezouata Formation north of Zagora, southeastern Morocco is described as the arthropod Pseudoangustidontus duplospineus gen. et sp. nov. The single fragmentary specimen displays a unique morphology, carrying at least 39 pairs of spines (i.e., 78 spines) of very regularly alternating lengths. Pseudoangustidontus gen. nov. shows some similarities to a number of spinose arthropod appendages and appendage parts, most notably to the spine-bearing podomeres of the third prosomal appendage of megalograptid eurypterids and the problematic and incompletely known genus Angustidontus. However, megalograptids and Angustidontus both have a lower spine count, while the latter also carries only a single row of spines. Because no known arthropod displays a morphology closely comparable to that of Pseudoangustidontus gen. nov., the af- finities of the new fossil within Arthropoda remain uncertain. Key words: Arthropoda, raptorial appendage, Arenig, Ordovician, Morocco. Peter Van Roy [peter.vanroy@ugent.be], Department of Geology & Soil Science, Ghent University, Krijgslaan 281 / S8, B-9000, Ghent, Belgium; O. Erik Tetlie [erik.tetlie@yale.edu], Department of Geology and Geophysics, Yale University, PO Box 208109, New Ha- ven, CT 06520-8109, USA. Introduction Spinose raptorial appendages are known from a wide range of arthropods (Fig. 1). The spines on the appendages are generally used to impale prey, as for example in the terres- trial amblypygid, thelyphonid and schizomid arachnids, the marine spearing stomatopod crustaceans and the terrestrial mantid insects. Apart from directly wounding prey, spines can also function to subdue and immobilize it by simply en- tangling it, as observed in amblypygids (Ladle and Velan- der 2003; Weygoldt 2000). The purpose of this paper is to describe an enigmatic, extremely spinose raptorial arthro- pod appendage fragment from the Arenig (Early Ordovi- cian) of the Upper Fezouata Formation north of Zagora, southeastern Morocco (Fig. 2). Geology Traditionally, the classical British nomenclature for the Or- dovician (Fortey et al. 1995, 2000) has also been applied to Morocco. The first two stages of this regional British sys- tem, the Tremadoc and the Arenig, can be correlated to the international global stages as follows: (1) the British Tre- madoc corresponds to the global Tremadocian stage; (2) the British Arenig encompasses the entire global unnamed sec- ond stage of the Ordovician, and the base of the global un- named third stage (Gradstein et al. 2005; Ogg 2004; Webby et al. 2004). For the global unnamed second stage, the name “Floian” was recently proposed (Bergström et al. 2006) and this name will be used further in the text. For the purpose of this discussion, stage names will be given as: British Stage/ Global stage. The Upper Fezouata Formation is a geograph- ically extensive, mainly transgressive unit composed of yel- low-green mudstones and siltstones, becoming increasingly sandy towards the top. In the area around Zagora, southeast- ern Morocco (Fig. 2A), where this unit reaches its maxi- mum thickness of up to 700 m, it conformably overlies the siltstones of the Lower Fezouata Formation of Tremadoc/ Tremadocian age (Early Ordovician) and locally grades into the sandstones of the middle Arenig/ late Floian (Early Ordovician) Zini Formation (Destombes et al. 1985). The age of the Upper Fezouata Formation is constrained be- tween the early to middle Arenig/early to late Floian (Early Ordovician). Because the present specimen was found by local Mo- roccan collectors and reached the authors via a rather circu- itous route, it has been impossible to identify the exact lo- cality where it was found with absolute certainty. At least six sites to the north of Zagora are known to preserve labile parts (Fig. 2B). In addition to these sites, local Moroccan collectors have also started working at a number of other outcrops in the area. One of the largest and most intensely worked excavations is situated approximately 25 km north http://app.pan.pl/acta51/app51-239.pdf Acta Palaeontol. Pol. 51 (2): 239–246, 2006