Abstract.  During the Arnsbergian, before the Mid-Carboniferous boundary (MCB), the majority
 of ammonoids displayed an advanced involute conch form.  The number of genera and species
 decreased stepwise toward the MCB.  After the MCB the dominating ammonoid groups, except
 for the Glaphyritidae, became extinct, and the Homoceratidae with a more evolute conch
 appeared and became widespread.  The increase was stepwise and ended in the following
 Kinderscoutian Stage giving rise to the complicatedly ornamented reticuloceratids.  The
 fluctuation of ammonoid faunas in the critical interval around the MCB showed a relatively
 slow stepwise decline in diversity, a short low point with the origination of new groups, and a
 rather rapid evolutionary diversification.  Possible causes of this pattern are discussed.  The
 stratigraphical range and the boundaries of the late Lower Namurian (late Namurian A) after
 the MCB are reviewed, and the ammonoid occurrences indicating this interval are documented.