A new acrioceratid ammonite, Dissimilites intermedius sp. nov., from the Barremian (Lower Cretaceous) of the Puez area
(Dolomites, northern Italy) is described. Dissimilites intermedius sp. nov. is an intermediate form between D. dissimilis
and D. trinodosum. The new species combines the ribbing style of D. dissimilis (bifurcating with intercalating single
ribs) with the tuberculation style of D. trinodosum (trituberculation on entire shell). The shallow-helical spire, entirely
comprising single ribs intercalated by trituberculated main ribs, is similar to the one of the assumed ancestor Acrioceras,
whereas the increasing curvation of the younger forms resembles similar patterns observed in the descendant Toxoceratoides.
These characters support the hypothesis of a direct evolutionary lineage from Acrioceras via Dissimilites to
Toxoceratoides. D. intermedius sp. nov. ranges from the upper Lower Barremian (Moutoniceras moutonianum Zone) to
the lower Upper Barremian (Toxancyloceras vandenheckii Zone). The new species allows to better understand the evolution
of the genus Dissimilites. The genus appears within the Nicklesia pulchella Zone represented by D. duboise, which
most likely evolved into D. dissimilis. In the Kotetishvilia compressissima Zone, two morphological forms developed:
smaller forms very similar to Acrioceras and forms with very long shaft and juvenile spire like in D. intermedius sp.
nov. The latter most likely gave rise to D. subalternatus and D. trinodosum in the M. moutonianum Zone, forms which
were probably ancestral to the genus Toxoceratoides.
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