A case of mistaken identity.
In 2002 the small slab containing Acadoparadoxides remains in the image above was purchased from local excavators at Tarhoucht. Later, the label which had been attached to it fell off, so that when the time came for the juvenile, or meraspid, carapace on the right to be included in the first major article to be published on the early paradoxidid trilobites from Morocco in 2015, and memory being faulty, the specimen was thought to have originated from the large excavation pits at Tarhia, to the north of Tarhoucht, and the remains were assigned to Acadoparadoxides levisettii in Fig. 17 D of Geyer & Vincent (2015).
In 2002 the small slab containing Acadoparadoxides remains in the image above was purchased from local excavators at Tarhoucht. Later, the label which had been attached to it fell off, so that when the time came for the juvenile, or meraspid, carapace on the right to be included in the first major article to be published on the early paradoxidid trilobites from Morocco in 2015, and memory being faulty, the specimen was thought to have originated from the large excavation pits at Tarhia, to the north of Tarhoucht, and the remains were assigned to Acadoparadoxides levisettii in Fig. 17 D of Geyer & Vincent (2015).
However, all available Acadoparadoxides cranidia from the Tarhoucht area, including Tarhia, were subsequently subjected to geometrical analysis (see this website), unnecessary in the case of this meraspid trilobite as its image can be easily measured and found to be exactly equal in width across both anterior border and eye-lobes. This strongly suggests that it is not, in fact, of A. levisettii, since in all known cranidia of that species the cranidial width across the anterior border always exceeds the width across the eye-lobes, so the specimen must represent one of the other early species instead. Other early species known from the pits near Tarhia include Acadoparadoxides cf. mureroensis and Acadoparadoxides ovatopyge, but in these species small cranidia have a short anterior border, so that in a meraspid the cranidial width across the anterior border should be considerably shorter than that across the eye-lobes. The specimen therefore remained puzzling until documentary evidence came to light in the form of an old photocopy of an information card prepared for friends in 2002 or soon after, which clearly states that the specimen was found at Bou Tiouit mountain, near Tarhoucht (see below). This means that the meraspid is almost certainly of A. pampalius, which underlies A. levisettii at Bou Tiouit, but is not exposed in the pits at Tarhia. In A. pampalius the cranidial width across the anterior border is always exactly equal to that across the eye-lobes, so this fits with the evidence provided by the meraspid cranidium. As can be seen, the meraspid is preserved on the slab just a few centimetres from the impression of one of the smallest complete Moroccan Acadoparadoxides adults known, and the shape of the pygidium in this impression is comparable with known pygidia of A. pampalius.
However, all available Acadoparadoxides cranidia from the Tarhoucht area, including Tarhia, were subsequently subjected to geometrical analysis (see this website), unnecessary in the case of this meraspid trilobite as its image can be easily measured and found to be exactly equal in width across both anterior border and eye-lobes. This strongly suggests that it is not, in fact, of A. levisettii, since in all known cranidia of that species the cranidial width across the anterior border always exceeds the width across the eye-lobes, so the specimen must represent one of the other early species instead. Other early species known from the pits near Tarhia include Acadoparadoxides cf. mureroensis and Acadoparadoxides ovatopyge, but in these species small cranidia have a short anterior border, so that in a meraspid the cranidial width across the anterior border should be considerably shorter than that across the eye-lobes. The specimen therefore remained puzzling until documentary evidence came to light in the form of an old photocopy of an information card prepared for friends in 2002 or soon after, which clearly states that the specimen was found at Bou Tiouit mountain, near Tarhoucht (see below). This means that the meraspid is almost certainly of A. pampalius, which underlies A. levisettii at Bou Tiouit, but is not exposed in the pits at Tarhia. In A. pampalius the cranidial width across the anterior border is always exactly equal to that across the eye-lobes, so this fits with the evidence provided by the meraspid cranidium. As can be seen, the meraspid is preserved on the slab just a few centimetres from the impression of one of the smallest complete Moroccan Acadoparadoxides adults known, and the shape of the pygidium in this impression is comparable with known pygidia of A. pampalius.
Disarticulated, moulted meraspid carapace of A. pampalius figured in Geyer & Vincent (2015), Fig. 17 D as A. levisettii. Note the diminutive detached hypostome and the long pleural spines on the second segment of the thorax. The combined length is about 1.6 cm to thoracic segment 14, and the free cheeks are missing.
Disarticulated, moulted meraspid carapace of A. pampalius figured in Geyer & Vincent (2015), Fig. 17 D as A. levisettii. Note the diminutive detached hypostome and the long pleural spines on the second segment of the thorax. The combined length is about 1.6 cm to thoracic segment 14, and the free cheeks are missing.
Impression of a very small adult carapace of A. pampalius on the same slab as the meraspid. The thorax has a full adult complement of 18 segments, as in other known adults of this species. The full length is about 2.1 cm, but 1.7 cm to thoracic segment 14. Note the impression of a genal spine on the right-hand side.
Impression of a very small adult carapace of A. pampalius on the same slab as the meraspid. The thorax has a full adult complement of 18 segments, as in other known adults of this species. The full length is about 2.1 cm, but 1.7 cm to thoracic segment 14. Note the impression of a genal spine on the right-hand side.
Interestingly, while the combined length of the cranidium and thorax of the meraspid remains to thoracic segment 14 is about 1.6 cm, the length of the impression of the tiny adult carapace to segment 14 is 1.7 cm, its full length to the posterior margin of the pygidium being about 2.1 cm. Could it be, therefore, that both carapaces are of the same individual animal? The meraspid remains have clearly been moulted, and the adult impression a few centimetres from it is only fractionally larger, and is inverted relative to the meraspid remains. Although there is no absolute proof, it may be conjectured that immediately after moulting and crawling a short distance, the unfortunate creature suffered an accident of unknown nature which resulted in it ending its life on its back, the position in which it became buried by sediment along with evidence of its previous existence as a juvenile, or meraspid, trilobite!
Interestingly, while the combined length of the cranidium and thorax of the meraspid remains to thoracic segment 14 is about 1.6 cm, the length of the impression of the tiny adult carapace to segment 14 is 1.7 cm, its full length to the posterior margin of the pygidium being about 2.1 cm. Could it be, therefore, that both carapaces are of the same individual animal? The meraspid remains have clearly been moulted, and the adult impression a few centimetres from it is only fractionally larger, and is inverted relative to the meraspid remains. Although there is no absolute proof, it may be conjectured that immediately after moulting and crawling a short distance, the unfortunate creature suffered an accident of unknown nature which resulted in it ending its life on its back, the position in which it became buried by sediment along with evidence of its previous existence as a juvenile, or meraspid, trilobite!
[Thanks are due to Prof. Dr. Gerd Geyer of Würzburg, Germany, for the loan of the photographs.]
[Thanks are due to Prof. Dr. Gerd Geyer of Würzburg, Germany, for the loan of the photographs.]
This is the old photocopy of the information card from 2002/2003, which confirms the origin of the slab as Bou Tiouit ~
This is the old photocopy of the information card from 2002/2003, which confirms the origin of the slab as Bou Tiouit ~
Reference:- GEYER, G. & VINCENT, T., 2015. The Paradoxides puzzle resolved: the appearance of the oldest paradoxidines and its bearing on the Cambrian Series 3 lower boundary. Paläontologische Zeitschrift 89(3), 335 - 98.
Reference:- GEYER, G. & VINCENT, T., 2015. The Paradoxides puzzle resolved: the appearance of the oldest paradoxidines and its bearing on the Cambrian Series 3 lower boundary. Paläontologische Zeitschrift 89(3), 335 - 98.
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