Morgane Brosse

Ammonoid recovery after the Permian–Triassic mass extinction: a re-exploration of morphological and phylogenetic diversity patterns

The explosive ammonoid rediversification after the Permian–Triassic mass extinction is now well understood in terms of taxonomic richness and biogeography. Using an updated dataset of Early Triassic ammonoids, we compare morphological disparity and taxonomic richness patterns at the regional and global scales. Disparity evolved similarly at both scales, suggesting a global influence of abiotic factors. Morphological diversification occurred early in the Smithian and a marked contraction of the morphospace took place during the end-Smithian extinction. We confirm that trends in disparity and richness were decoupled during the Griesbachian and Dienerian. Three macroevolutionary processes may be involved: (1) a nonselective extinction at the Permian–Triassic boundary; (2) a Dienerian constrained radiation with several homeomorphic genera; (3) a potential deterministic extinction during the end-Smithian crisis. We also demonstrate a superfamily imprint upon disparity for the Spathian when most superfamilies occupied a restricted part of the morphospace. Sphaerocones were the most affected by the Dienerian and end-Smithian extinction, but explanations remain elusive. On the one hand, this may be linked to widespread harsh conditions at those times. On the other hand, as the sphaerocones occurred episodically during the Early Triassic, this might be explained by a relaxing of ecological constraints or simply by convergent evolution. Supplementary materials: The database, including measurements of specimens illustrated in previously published plates and of unpublished specimens from Utah, South China and Spiti, as well as the number of genera present in each studied substage of the Early Triassic, are available at www.geolsoc.org.uk/SUP18571.

Timing of global regression and microbial bloom linked with the Permian-Triassic boundary mass extinction: implications for driving mechanisms

New high-resolution U-Pb dates indicate a duration of 89 ± 38 kyr for the Permian hiatus and of 14 ± 57 kyr for the overlying Triassic microbial limestone in shallow water settings of the Nanpanjiang Basin, South China. The age and duration of the hiatus coincides with the Permian-Triassic boundary (PTB) and the extinction interval in the Meishan Global Stratotype Section and Point, and strongly supports a glacio-eustatic regression, which best explains the genesis of the worldwide hiatus straddling the PTB in shallow water records. In adjacent deep marine troughs, rates of sediment accumulation display a six-fold decrease across the PTB compatible with a dryer and cooler climate as indicated by terrestrial plants. Our model of the Permian-Triassic boundary mass extinction (PTBME) hinges on the synchronicity of the hiatus with the onset of the Siberian Traps volcanism. This early eruptive phase released sulfur-rich volatiles into the stratosphere, thus simultaneously eliciting a short-lived ice age responsible for the global regression and a brief but intense acidification. Abrupt cooling, shrunk habitats on shelves and acidification may all have synergistically triggered the PTBME. Subsequently, the build-up of volcanic CO2 induced a transient cool climate whose early phase saw the deposition of the microbial limestone.

The oldest record of gnathostome fossils from Greece: Chondrichthyes from the Lopingian of Hydra Island

The Permian deposits of Hydra Island, Greece, have been known for over a century and host some of the best-studied and most diverse invertebrate assemblages of the ancient Paleotethys Ocean. However, until now, no Paleozoic fossils of jawed vertebrates had been reported from Greece. Recent fieldwork on Hydra Island brought to light rare cartilaginous fish remains, including a tooth belonging to an unknown hybodontiform shark, as well as an unidentifiable dermal denticle of an euselachian shark. Despite similarities with iconic Paleozoic and Mesozoic durophagous euselachians, the Hydriot tooth likely corresponds to a new species, but is provisionally left in open nomenclature until more material becomes available. The new chondrichthyan fossils from Hydra Island correspond to one of the few Lopingian (late Permian) occurrences known from the Paleotethys. Moreover, they constitute the oldest record of jawed-vertebrate fossils from Greece, predating younger occurrences by more than 50 million years.

Evolutionary trends of Triassic ammonoids.

The Triassic represents a key interval in the evolutionary history of ammonoids. Characterized by the dominance of the Ceratitida with their typical suture line indented on the lobes only, the Triassic quasi-monophyletic clade shows a remarkable biostratigraphic and geographic record. However, very few studies have thoroughly investigated their evolutionary trends, except for taxonomic richness. Although Triassic ammonoids show a very large range of morphologies, suture complexity and adult size, little is currently known about their trends, except for peculiar time intervals or taxonomic groups. Nevertheless, it seems that taxonomic diversity and morphological disparity of Triassic ammonoids are uncoupled, at least during part of the Early Triassic recovery. Finally, Triassic ammonoids still have many properties to contribute to evolutionary biology, but going further now requires the construction of quantitative databases of the various morphological characters and reconstruction of Triassic ammonoid phylogeny.