Frasnian plants from the Dra Valley, southern Anti-Atlas, Morocco

Abstract

Anatomically preserved plant fragments are reported from Devonian marine deposits exposed in the Dra Valley of southern Anti-Atlas, Morocco. Associated conodont and tentaculite faunas indicate that the sediments yielding plants, which consist of black shales with intercalated calcareous concretions, are early Frasnian in age and most probably represent Zone 2 of the conodont zonation. This is the first record of Frasnian plants in North Africa. The specimens found all correspond to decorticated portions of axes. Six are referable to Callixylon, the organ genus corresponding to anatomically preserved axes of the progymnosperm tree Archaeopteris. Based on wood characters, especially ray structure, they are assigned to the species C. henkei, formerly described from the Famennian of Europe. One single specimen is compared to Xenocladia, a cladoxylopsid genus previously known from the Middle Devonian of Europe, USA and Kazakhstan. Interestingly, Archaeopteridales and Cladoxylopsida are two groups that dominate the younger plant assemblages of Famennian age recently described from the eastern Anti-Atlas. Callixylon henkei-type axes occur both in the Frasnian and in the Famennian deposits of the Anti-Atlas and they are all devoid of growth rings. These results are in accordance with a close position of Gondwana and Euramerica during Late Devonian times.