Taxonomic revision of genus Carinolithus (Early - Middle Jurassic) based on morphometric analyses and diagenesis observations: implications for biostratigraphy and evolutionary trends

We applied morphometric analyses performed with a polarizing light microscope on a total of 100 specimens of Carinolithus superbus and C. magharensis to clarify the taxonomy based on measurements of total height (TH), stem height (SH), stem width (SW), proximal shield width (PS), distal shield width (DS) and thickness of the distal shield (TDS). Statistical analyses showed that three of these parameters, namely the DS, TDS and SW, are diagnostic for taxa discrimination. New taxa include C. superbus subsp. crassus subsp. nov. and C. magharensis subsp. minor subsp. nov. Specimens of C. cantaluppii were qualitatively investigated to assess the potential role of secondary modifications and our results revealed that, indeed, C. cantaluppii is a diagenetic artefact due to different degrees of overgrowth on specimens of C. poulnabronei, C. superbus subsp. crassus, C. superbus subsp. superbus, C. magharensis subsp. minor and C. magharensis subsp. magharensis. This implies that C. cantaluppii is not a true taxon, and the overgrowth phase of Carinolithus specimens is an effective proxy to assess the diagenetic degree. The morphometry-based revised taxonomy of the genus Carinolithus was applied to improve the biostratigraphic resolution of the Toarcian-Aalenian interval, with further implications for the reconstruction of evolutionary trends.