Microanatomy of the dermal skeleton of Balticaspis latvica (Osteostraci, Middle Devonian)

Middle Devonian osteostracans are rare, and the microanatomy of their derma l skeleton has not been described in detail. Traquair (1894: pi. 7, figs. 2 , 3) figured the surface of tesserae of the large Scottish species Cephalas pis magnifica. Obruchev (1961), Afanassieva and Janvier (1985:fig. 5C(1.2)) , and Afanassieva (1991:pl. 23) described and figured Ilemoraspis kirkinska yae from the Khakass autonomous region from strata that probably date from the Middle Devonian. Another Middle Devonian osteostracan, Balticaspis latv ica, known only from the holotype (Nr. 58-5, Latvias Dabas Muzjeis, Riga), has been erected by Lyarskaya (1981) and redescribed in a monograph by Afan assieva (1991) dealing. with the histology of the dermal skeleton of osteos tracans. However, Afanassieva (1991) did not discuss the microanatomy or th e histology of Balticaspis, but only its macroanatomy. The new microanatomi cal data on Balticaspis presented below show that it is taxonomically disti nct from the tesserae recently described (Otto and Laurin, 1999) from the T ori member of the Early Middle Devonian Pernu Formation in southern Estonia , contrary to our initial suspicion. We also suggest that the reduction in thickness of the dermal skeleton of Middle and Upper Devonian osteostracans is not a synapomorphy of all late osteostracans, but that it probably occu rred at least three times independently (although in the case of Balticaspi s, this conclusion is supported only by indirect evidence).