Learning from beautiful monsters: phylogenetic and morphogenetic implications of left-right asymmetry in ammonoid shells

Abstract

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.

When less is more and more is less: the impact of sampling effort on species delineation

“…The observation of transitional morphologies is not new in conodont studies, nor in other clades (see for instance Jattiot et al . 2019), and the use of biometric measurements and statistics to delineate species is common in palaeontology (Malmgren & Kennett 1981; Ezard et al . 2010; De Meulemeester et al .…”

On the Rarity and Peculiarity of the Early Toarcian (Lower Jurassic) Leukadiellinae Ammonites—Systematic Review and Insights on the Interplay of Environmental Stress, Evolution and Biodiversity

“…Whatever the mechanism by which the archaic genetic information is maintained and reactivated, the frequently observed re-occurrence of archaic morphologies, also in pathological or injured ammonites [92], is an indication that genetic information relative to ancestral structures is not (completely) lost during phylogenesis [93]. In addition, experimental evidence indicates that environmental stress (thermal or chemical) during the early developmental stages of organisms may alter the ontogenetic expression of phenotypes [88,94,95]; these developmental effects may be key to the understanding of how archaic morphologies are preserved and resumed.…”

“…In addition, pathological specimens documented in the literature show, apart from cripple-like anomalies affecting ornamental features [92], also abnormal morphology of the kind herein referred to as coherent phenotypic anomaly. A typical case is that in which the two sides of an ammonite are different, one reproducing the exact pattern of a related species [93]. However, the limits between "cripple" and "coherent" morphological anomalies are not always obvious.…”