Onset of Late Cretaceous diversification in Europe's freshwater gastropod fauna links to global climatic and biotic events

Cranial anatomy of Acynodon adriaticus and extreme durophagous adaptations in Eusuchia (Reptilia: Crocodylomorpha)

Acynodon adriaticus, a small eusuchian from the Late Cretaceous of Italy, is known for its well-preserved cranial and postcranial material. Despite its excellent preservation, many details remain hidden due to the physical overlap between the elements and matrix obliteration. We used Micro-CT scans to reveal previously overlooked anatomical features and describe in detail the cranial and dental anatomy of this taxon, shedding new light on its palaeoecology. The holotypic specimen, SC 57248, represents a mature individual exhibiting signs of hyperossification, developed ornamentation, and various pathologies, including jaw arthritis and a possible dental anomaly. Acynodon adriaticus exhibits significant durophagous adaptations, including a robust, brevirostrine skull optimized for powerful biting and stress-load capacity. Its specialized dentition, lacking caniniform teeth, features anterior chisel-like teeth and hypertrophic posterior molariforms with thick enamel, indicative of a diet specializing in hard-shelled prey. The dentition pattern, accelerated molariform replacement rate, and reduced orbit size suggest adaptations for durophagous foraging in turbid, densely vegetated aquatic environments. The paleoecological context during the Late Cretaceous, characterized by increased freshwater habitats and high invertebrate diversity, likely facilitated the evolution of such specialized traits in A. adriaticus. This small crocodylomorph likely foraged slowly in shallow, benthic environments, using its powerful bite to process mollusks and large arthropods. The study of A. adriaticus, along with comparisons with other crocodylomorphs and ecomorphologically similar taxa like Iharkutosuchus makadii and Gnatusuchus pebasensis, provides a valuable morphofunctional model for understanding the evolutionary pathways of extinct crocodylians to durophagy.

Late Cretaceous ammonoids show that drivers of diversification are regionally heterogeneous

Palaeontologists have long sought to explain the diversification of individual clades to whole biotas at global scales. Advances in our understanding of the spatial distribution of the fossil record through geological time, however, has demonstrated that global trends in biodiversity were a mosaic of regionally heterogeneous diversification processes. Drivers of diversification must presumably have also displayed regional variation to produce the spatial disparities observed in past taxonomic richness. Here, we analyse the fossil record of ammonoids, pelagic shelled cephalopods, through the Late Cretaceous, characterised by some palaeontologists as an interval of biotic decline prior to their total extinction at the Cretaceous-Paleogene boundary. We regionally subdivide this record to eliminate the impacts of spatial sampling biases and infer regional origination and extinction rates corrected for temporal sampling biases using Bayesian methods. We then model these rates using biotic and abiotic drivers commonly inferred to influence diversification. Ammonoid diversification dynamics and responses to this common set of diversity drivers were regionally heterogeneous, do not support ecological decline, and demonstrate that their global diversification signal is influenced by spatial disparities in sampling effort. These results call into question the feasibility of seeking drivers of diversity at global scales in the fossil record.

Fossil marine vertebrates (Chondrichthyes, Actinopterygii, Reptilia) from the Upper Cretaceous of Akkermanovka (Orenburg Oblast, Southern Urals, Russia)

Upper Cretaceous coastal marine deposits are widespread in the Southern Urals with a number of marine vertebrates previously reported from this region. However, previous studies on the vertebrate faunas in this region often lack detailed taxonomic descriptions and illustrations, rendering comparisons to other faunal assemblages difficult. A new diverse vertebrate assemblage comprising cartilaginous and bony fishes, as well as marine reptiles, is described here from the Orenburg region near Akkermanovka (Southern Urals, Russia). Thirty five taxa are identified, including three holocephalans (Elasmodus sp., Ischyodus yanschini, Chimaeroid indet.), two hybodontiform sharks (Meristodonoides sp., cf. Polyacrodus sp.), 17 neoselachians (Paraorthacodus cf. andersoni, Paraorthacodus sp., Synechodus sp., Cederstroemia nilsi, Acrolamna acuminata, Archaeolamna ex gr. kopingensis, Cretalamna sarcoportheta, Cretoxyrhina mantelli, Eostriatolamia segedini, E. venusta, Hispidaspis horridus, H. cf. gigas, Pseudocorax laevis, Pseudoscapanorhynchus compressidens, Scapanorhynchus rhaphiodon, Squalicorax kaupi, Ptychodus rugosus), a holostean (Lepisosteidae indet.), nine teleosts (Protosphyraena sp., Saurodontidae indet., cf. Pachyrhizodus sp., Pachyrhizodontidae indet., Enchodus petrosus, E. ferox, E. cf. gladiolus, E. spp., Alepisauroidei indet.), two plesiosaurs (Polycotylidae indet., Plesiosauria indet.), and one mosasaurid (Tylosaurinae indet.). Based on the faunal assemblage, a Santonian-?early Campanian age is proposed. Lamniform sharks are the best represented group in terms of taxic diversity and relative abundance, probably reflecting the peak in diversity this group experienced following the Cenomanian radiation in the Late Cretaceous. The faunal assemblage of Akkermanovka exhibits significant taxonomic overlaps with assemblages reported from Asia and North America, but not from Southern Hemisphere continents, indicating east-west dispersal of several marine taxa during the Late Cretaceous.

The fossil record of freshwater Gastropoda - a global review

Gastropoda are an exceptionally successful group with a rich and diverse fossil record. They have conquered land and freshwater habitats multiple times independently and have dispersed across the entire globe. Since they are important constituents of fossil assemblages, they are often used for palaeoecological reconstruction, biostratigraphic correlations, and as model groups to study morphological and taxonomic evolution. While marine faunas and their evolution have been a common subject of study, the freshwater component of the fossil record has attracted much less attention, and a global overview is lacking. Here, I review the fossil record of freshwater gastropods on a global scale, ranging from their origins in the late Palaeozoic to the Pleistocene. As compiled here, the global fossil record of freshwater Gastropoda includes 5182 species in 490 genera, 44 families, and 12 superfamilies over a total of ~340 million years. Following a slow and poorly known start in the late Palaeozoic, diversity slowly increased during the Mesozoic. Diversity culminated in an all-time high in the Neogene, relating to diversification in numerous long-lived (ancient) lakes in Europe. I summarise well-documented and hypothesised freshwater colonisation events and compare the patterns found in freshwater gastropods to those in land snails. Furthermore, I discuss potential preservation and sampling biases, as well as the main drivers underlying species diversification in fresh water on a larger scale. In that context, I particularly highlight the importance of long-lived lakes as islands and archives of evolution and expand a well-known concept in ecology and evolution to a broader spectrum: scale-independent ecological opportunity.

Fifth mass extinction event triggered the diversification of the largest family of freshwater gastropods (Caenogastropoda: Truncatelloidea: Hydrobiidae)

The fifth mass extinction event (MEE) at the Cretaceous-Palaeogene (K-Pg) boundary 66 million years ago (Ma) led to massive species loss but also triggered the diversification of higher taxa. Five models have been proposed depending on whether this diversification occurred before, during or after the K-Pg boundary and the rate of species accumulation. While the effects of the K-Pg MEE on vertebrate evolution are relatively well understood, the impact on invertebrates, particularly in freshwater ecosystems, remains controversial. One example is the hyperdiverse Hydrobiidae-the most species-rich family of freshwater gastropods. Whereas some studies place its origin in the Jurassic or even Carboniferous, most fossil records postdate the K-Pg event. We therefore used robustly time-calibrated multi-locus phylogenies of >400 species representing >100 hydrobiid genera to unravel its evolutionary history and patterns of diversification. We found that the family started diversifying shortly after the K-Pg boundary (∼60 Ma; 95% highest posterior density 52-69 Ma). Lineage richness gradually increased to the present and phylogenetic diversity until ∼25 Ma. These findings suggest that diversification was not initially driven by ecological opportunity. Combining the two criteria of timing and rate of diversification, a soft-explosive diversification model of aquatic vertebrates best fits the patterns observed. We also show that most higher hydrobiid taxa (i.e. subfamilies) diversified from the Middle Oligocene to Middle Miocene (i.e. 12-28 Ma). Two of the 15 major clades delimited are described here as new subfamilies (i.e. Bullaregiinae n. subfam. and Pontobelgrandiellinae n. subfam.), whose members are restricted to subterranean waters. Our results are an important contribution to understanding how the fifth MEE has shaped evolution and patterns of biodiversity in continental aquatic systems. Given the high extinction risks faced by many hydrobiids today, they also emphasise the need to study the biodiversity of vulnerable ecosystems.

The extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) in the Upper Cretaceous of central-western Russia-The road to easternmost peri-Tethyan seas

Isolated teeth belonging to the genus Ptychodus Agassiz, 1834 (Chondrichthyes; Elasmobranchii) from the Upper Cretaceous of the Ryazan and Moscow Oblast regions (European Russia) are described and discussed in detail herein. The taxonomic composition of the Ptychodus assemblage from the Ryazan region is very diverse including the first records of the cuspidate species P. altior and P. anonymus, which thus is largely consistent with those from other contemporaneous European localities. Ptychodus ubiquitously inhabited epicontinental seas of Europe during most of the Cretaceous with the most diverse assemblages coming from southern England, northern Italy, Belgium, and European Russia. Additionally, the material documented here from the Cenomanian of Varavinsky ravine area (Moscow Oblast) represents the northernmost occurrence of Ptychodus hitherto reported from Europe. It is evident that the Late Cretaceous shallow seas of the Russian platform represented a crucial pathway for the dispersal of Ptychodus from the European peri-Tethys to the eastern margins of the Neo-Tethyan Ocean. The Albian-Campanian records of Ptychodus from Europe indicate that its dominance in the peri-Tethys persisted for most of its evolutionary history. A local temperature drop across most of the European shallow seas probably contributed to the narrowing of its geographic range in the peri-Tethyan seas towards the end of the Mesozoic Era. The fossil remains of Ptychodus documented herein are accordingly of utmost importance for better understanding the taxonomic composition of Russian fossil ichthyofaunas and also inform about the dispersal of Ptychodus towards western and eastern peri-Tethyan seas during the Late Cretaceous.

Biotic Interactions and the Future of Fishes on Coral Reefs: The Importance of Trait-Based Approaches

Biotic interactions govern the structure and function of coral reef ecosystems. As environmental conditions change, reef-associated fish populations can persist by tracking their preferred niche or adapting to new conditions. Biotic interactions will affect how these responses proceed and whether they are successful. Yet, our understanding of these effects is currently limited. Ecological and evolutionary theories make explicit predictions about the effects of biotic interactions, but many remain untested. Here, we argue that large-scale functional trait datasets enable us to investigate how biotic interactions have shaped the assembly of contemporary reef fish communities and the evolution of species within them, thus improving our ability to predict future changes. Importantly, the effects of biotic interactions on these processes have occurred simultaneously within dynamic environments. Functional traits provide a means to integrate the effects of both ecological and evolutionary processes, as well as a way to overcome some of the challenges of studying biotic interactions. Moreover, functional trait data can enhance predictive modeling of future reef fish distributions and evolvability. We hope that our vision for an integrative approach, focused on quantifying functionally relevant traits and how they mediate biotic interactions in different environmental contexts, will catalyze new research on the future of reef fishes in a changing environment.