Complex sentence: Polyxenida is the only living order of the subclass Penicillata, the basal subclass of millipedes, although some authors place the extinct orders Arthropleurida and Eoarthropleurida (each represented by a single genus) within the Penicillata as a sister group to Polyxenida.
Split into simpler sentences: Polyxenida is the only living order of the subclass Penicillata, the basal subclass of millipedes. Some authors place the extinct orders Arthropleurida and Eoarthropleurida (each represented by a single genus) within the Penicillata as a sister group to Polyxenida.