Background: Many pathologies that modify the shell geometry and ornamentation of ammonoids are known fromthe fossil record. Since they may reflect the developmental response of the organism to a perturbation (usually asublethal injury), their study is essential for exploring the developmental mechanisms of these extinct animals.Ammonoid pathologies are also useful to assess the value of some morphological characters used in taxonomy, aswell as to improve phylogenetic reconstructions and evolutionary scenarios.Results: We report on the discovery of an enigmatic pathological middle Toarcian (Lower Jurassic) ammonoidspecimen from southern France, characterized by a pronounced left-right asymmetry in both ornamentation andsuture lines. For each side independently, the taxonomic interpretations of ornamentation and suture lines arecongruent, suggesting a Hildoceras semipolitum species assignment for the left side and a Brodieia primaria speciesassignment for the right side. The former exhibits a lateral groove whereas the second displays sinuous ribs. Thisspecimen, together with the few analogous cases reported in the literature, lead us to erect a new forma-typepathology herein called “forma janusa” for specimens displaying a left-right asymmetry in the absence of any clearevidence of injury or parasitism, whereby the two sides match with the regular morphology of two distinct, knownspecies.Conclusions: Since “forma janusa” specimens reflect the underlying developmental plasticity of the ammonoidtaxa, we hypothesize that such specimens may also indicate unsuspected phylogenetic closeness between the twodisplayed taxa and may even reveal a direct ancestor-descendant relationship. This hypothesis is not, as yet,contradicted by the stratigraphical data at hand: in all studied cases the two distinct taxa correspond tocontemporaneous or sub-contemporaneous taxa. More generally, the newly described specimen suggests that ahitherto unidentified developmental link may exist between sinuous ribs and lateral grooves. Overall, werecommend an integrative approach for revisiting aberrant individuals that illustrate the intricate links among shellmorphogenesis, developmental plasticity and phylogeny.