The problematic systematic position of Ceratostratiotes Gregor (Hydrocharitaceae ?) - morphological, anatomical and biochemical comparison with Stratiotes L.

The enigmatic fossil taxon Ceratostratiotes sinjanus (Kerner) Buzek has been considered as either a Hydrocharitaceae seed (monocot) or a Ceratophyllaceae fruit (dicot). The co-occurrence of seeds of Stratiotes kaltennordheimensis (Zenker) Keilhack (Hydrocharitaceae) and Ceratostratiotes in Early Miocene sediments in Langau (Lower Austria) enabled comparisons of morphological-anatomical features and lignin compositions, independent of diagenetic biases. The biochemistry of the Ceratostratiotes seed coat wall is not monocotyledon-like and is dissimilar to that of the co-occurring Stratiotes testae. In contrast, the Ceratostratiotes seed coat anatomy resembles that of Hydrocharitaceae genera with the micropyle and raphe being very similar to those in seeds of Stratiotes. Although the horizontal arrangement of the Ceratostratiotes spines is different from the longitudinal arrangement of the surface ridges in Stratiotes kaltennordheimensis and the spines in Blyxa (Hydrocharitaceae), the spine structures are very similar. A mixture of similarities and differences characterizes the morphological-anatomical features. Thus the systematic affinity of Ceratostratiotes still remains enigmatic.