Learning from beautiful monsters: phylogenetic and morphogenetic implications of left-right asymmetry in ammonoid shells.
Résumé
Background: Many pathologies that modify the shell geometry and ornamentation of ammonoids are known from
the fossil record. Since they may reflect the developmental response of the organism to a perturbation (usually a
sublethal 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, as
well as to improve phylogenetic reconstructions and evolutionary scenarios.
Results: We report on the discovery of an enigmatic pathological middle Toarcian (Lower Jurassic) ammonoid
specimen from southern France, characterized by a pronounced left-right asymmetry in both ornamentation and
suture lines. For each side independently, the taxonomic interpretations of ornamentation and suture lines are
congruent, suggesting a Hildoceras semipolitum species assignment for the left side and a Brodieia primaria species
assignment for the right side. The former exhibits a lateral groove whereas the second displays sinuous ribs. This
specimen, together with the few analogous cases reported in the literature, lead us to erect a new forma-type
pathology herein called “forma janusa” for specimens displaying a left-right asymmetry in the absence of any clear
evidence of injury or parasitism, whereby the two sides match with the regular morphology of two distinct, known
species.
Conclusions: Since “forma janusa” specimens reflect the underlying developmental plasticity of the ammonoid
taxa, we hypothesize that such specimens may also indicate unsuspected phylogenetic closeness between the two
displayed 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 to
contemporaneous or sub-contemporaneous taxa. More generally, the newly described specimen suggests that a
hitherto unidentified developmental link may exist between sinuous ribs and lateral grooves. Overall, we
recommend an integrative approach for revisiting aberrant individuals that illustrate the intricate links among shell
morphogenesis, developmental plasticity and phylogeny.
Domaines
PaléontologieOrigine | Fichiers éditeurs autorisés sur une archive ouverte |
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