Faviel A. Lopez-Romero

We present a revised diagnosis of the extinct galeomorph shark † Bavariscyllium based on dental and skeletal material from the Upper Jurassic of Germany and test its purported carcharhiniform affinity through morphometric and phylogenetic analyses. Although † Bavariscyllium possesses a whisker-like throat barbel suggesting a closer relationship with orectolobiforms, our findings reveal insufficient evidence to confidently assign † Bavariscyllium to either Orectolobiformes or Carcharhiniformes. Additionally, we present quantitative evidence indicating that early galeomorphs, despite probably not being placed among extant orders, were exploring a variety of body forms, predating the divergence of most major body plans among modern representatives. † Bavariscyllium exhibits generalised clutching-type teeth with a hemiaulacorhize root characterised by strongly flared root lobes, closely resembling the supposed earliest carcharhiniforms from the Middle Jurassic. However, these features neither confirm nor refute a carcharhiniform affinity, questioning the reliability of these early galeomorphs as calibration fossils for dating the divergence of carcharhiniforms in phylogenomic analyses.

The understanding of the interaction between morphological diversity, phylogenetic history, and biodiversity at various temporal and spatial scales is crucial for conservation of communities and species facing extinction. Batoids (skates and rays) constitute a highly diverse group in morphology and habitat distribution, as evidenced by their long evolutionary history occupying essential ecological roles in their ecosystems. Currently over one-third of batoids are under threat of extinction. In this study, we examine the spatial patterns of morphological disparity, phylogenetic diversity, and beta diversity among batoids within Mexico’s economic exclusive zone (EEZ). Specifically, we evaluate the morphological disparity among species subjected to different extinction threats aiming to identify a possible trend toward homogenization of morphological variation. With this goal, we compiled geographic distribution data for all species reported within the EEZ, along with phylogenetic and morphological information. Our results suggest that species richness positively correlates with morphological disparity metrics (e.g., sum of variances and sum of ranges). This trend is also observed in regions with less species richness, particularly coral reefs. Notably, species in the highest extinction risk present the highest disparity. Overall, our results highlight the importance of assessing morphological traits to unveil hidden patterns of diversity and inform conservation strategies.

The fossil record of sharks and rays (Elasmobranchii) consists predominantly of isolated teeth, complicating their taxonomic and phylogenetic inferences. Consequently, the study of holomorphic specimens provides crucial information to reassess existing phylogenetic and taxonomic interpretations. The extinct Mesozoic shark genus †Sphenodus exemplifies these challenges. Initially classified based on dental traits, the genus has undergone multiple revisions, reflecting its taxonomic instability. However, the presence of skeletal remains assigned to †'Sphenodus' macer and †'S.' nitidus provides an opportunity to re-evaluate the phylogenetic and taxonomic relations of this genus. In this study, we critically reviewed the systematics and taxonomic relationships of †Sphenodus, through an exhaustive literature review and comparison with previous phylogenetic hypotheses. Our analysis of skeletal characters suggests the synonymy of †S. macer and †S. nitidus and supports previous Hexanchiformes classifications for the genus, placing it in a close relation to †Notidanoides forming a clade in sister relation to Hexanchidae. However, the teeth originally used to define †Sphenodus lack diagnostic traits for taxonomic identification, rendering†'Sphenodus' a nomen dubium. Consequently, we introduce a new name, †Archaeogracilidens, to include the species based on skeletal material and that present teeth with a well-preserved root and a crown, establishing a more stable systematic framework for this group.

The modern diversity of sharks, skates, and rays (Neoselachii) is the result of various diversification and extinction events during the Mesozoic (252–66 Ma). However, the key drivers of their diversity patterns remain poorly understood despite all the progress that has been accomplished in recent years. Here, we show that the interplay of climatic- and tectonic-linked trajectories, resulting in a high shallow marine habitat availability and lower atmospheric CO₂ concentration, were significant drivers and sustainers of Mesozoic neoselachian diversity. We show, for the first time, that higher atmospheric CO₂ content negatively affected neoselachian diversity in the past. The recognized gradual faunal changes throughout the Mesozoic and the two major diversification events during the Jurassic and Cretaceous, respectively, ultimately cumulated in an all-time diversity high in the Palaeogene despite the events during the end-Cretaceous extinction event, highlighting their remarkable resilience and adaptability despite severe environmental challenges. We thus provide novel perspectives on the processes underlying neoselachian diversification since the Mesozoic that contribute importantly to a better understanding of the selective forces that have shaped the long-term evolution and diversification of neoselachians. Given their vital role in modern ecosystems, our results provide information about possible future trends in the face of the current climate crisis.

Batoids (skates and rays) are the most speciose group of cartilaginous fishes. Their body plan represents diverse ecologies and swimming modes. Early skeletal fossil remains, and recent phylogenetic analyses suggest that convergence has occurred within the batoids several times independently. The drivers for such disparity patterns and possible association with modularity and phenotypic integration among batoids are not fully understood. Here we used geometric morphometrics and phylogenetic comparative methods to characterize the evolutionary trends of the basal fin skeleton of batoids and sharks. Our analyses show that the morphological variation has a strong phylogenetic signal. Interestingly, the most speciose orders of batoids display low morphological disparity. Reef and freshwater species, show increased evolutionary rates. Meanwhile, the swimming mode shows different rates depending on the fin structure analyzed. A higher modularity and integration signal suggest that the pectoral fin of batoids has experienced mosaic evolution. The low morphological disparity might be associated with high integration. We find support for convergence between Jurassic, Cretaceous, and Extant guitarfishes, however, not completely between sharks and batoids. Our findings suggest that habitats and swimming mode have shaped the pectoral fin evolution among batoids, and at the same time batoids have constrained their basal fin skeleton.

The fossil record of chondrichthyans (chimaeras, sharks, rays and skates) consists largely of isolated teeth, with holomorphic specimens being extraordinary exceptions. However, numerous of these more or less completely preserved specimens are known from several Upper Jurassic deposits of Europe, enabling detailed analysis of their morphology. Batomorphs (rays and skates) resembling modern guitarfishes and wedgefishes (Rhinopristiformes) are among the most common Jurassic chondrichthyans found, but they have been only sporadically studied up to now, resulting in large knowledge gaps concerning their taxonomy and phylogeny. Here, we present the most detailed revision of Late Jurassic holomorphic batomorphs to date, quantitatively analysing body proportions of specimens from Germany (Solnhofen Archipelago), France (Cerin) and the UK (Kimmeridge), using both geometric and traditional morphometrics. Furthermore, we identify qualitative morphological characters for species discrimination, to clarify the taxonomic identity and diversity of Late Jurassic batomorphs based on holomorphic specimens. Our results support the validity of Belemnobatis sismondae, Kimmerobatis etchesi and Spathobatis bugesiacus, as well as that of the previously doubtful Asterodermus platypterus. Moreover, we describe Aellopobatis bavarica, a new taxon, which has hitherto been considered to be a large‐sized morphotype of Spathobatis bugesiacus. Our results highlight that the diversity of holomorphic batomorphs during the Late Jurassic was greater than previously thought, and suggest that this group was already well‐established and diverse by this time. This study thus provides vital information about the evolutionary history of Late Jurassic batomorphs and has direct implications for batomorph species that are based on isolated teeth only.

Environmental controls of species diversity represent a central research focus in evolutionary biology. In the marine realm, sharks are widely distributed, occupying mainly higher trophic levels and varied dietary preferences, mirrored by several morphological traits and behaviours. Recent comparative phylogenetic studies revealed that sharks present a fairly uneven diversification across habitats, from reefs to deep-water. We show preliminary evidence that morphological diversification (disparity) in the feeding system (mandibles) follows these patterns, and we tested hypotheses linking these patterns to morphological specialisation. We conducted a 3D geometric morphometric analysis and phylogenetic comparative methods on 145 specimens representing 90 extant shark species using computed tomography models. We explored how rates of morphological evolution in the jaw correlate with habitat, size, diet, trophic level, and taxonomic order. Our findings show a relationship between disparity and environment, with higher rates of morphological evolution in reef and deep-water habitats. Deep-water species display highly divergent morphologies compared to other sharks. Strikingly, evolutionary rates of jaw disparity are associated with diversification in deep water, but not in reefs. The environmental heterogeneity of the offshore water column exposes the importance of this parameter as a driver of diversification at least in the early part of clade history.

The Late Jurassic-Early Cretaceous (164–100 Ma) represents one of the main transitional periods in life history. Recent studies unveiled a complex scenario in which abiotic and biotic factors and drivers on regional and global scales due to the fragmentation of Pangaea resulted in dramatic faunal and ecological turnovers in terrestrial and marine environments. However, chondrichthyan faunas from this interval have received surprisingly little recognition. The presence of numerous entire skeletons of chondrichthyans preserved in several localities in southern Germany, often referred to as Konservat-Lagerstätten (e.g., Nusplingen and the Solnhofen Archipelago), provides a unique opportunity of to study the taxonomic composition of these assemblages, their ecological distributions and adaptations, and evolutionary histories in detail. However, even after 160 years of study, the current knowledge of southern Germany’s Late Jurassic chondrichthyan diversity remains incomplete. Over the last 20 years, the systematic study and bulk sampling of southern Germany’s Late Jurassic deposits significantly increased the number of known fossil chondrichthyan genera from the region (32 in the present study). In the present work, the fossil record, and the taxonomic composition of Late Jurassic chondrichthyans from southern Germany are reviewed and compared with several contemporaneous assemblages from other sites in Europe. Our results suggest, inter alia, that the Late Jurassic chondrichthyans displayed extended distributions within Europe. However, it nevertheless also is evident that the taxonomy of Late Jurassic chondrichthyans is in urgent need of revision.

The lifelong tooth replacement in elasmobranch fishes (sharks, rays and skates) has led to the assemblage of a great number of teeth from fossil and extant species, rendering tooth morphology an important character for taxonomic descriptions, analysing phylogenetic interrelationships and deciphering their evolutionary history (e.g. origination, divergence, extinction). Heterodonty (exhibition of different tooth morphologies) occurs in most elasmobranch species and has proven to be one of the main challenges for these analyses. Although numerous shark species are discovered and described every year, detailed descriptions of tooth morphologies and heterodonty patterns are lacking or are only insufficiently known for most species. Here, we use landmark‐based 2D geometric morphometrics on teeth of the tiger shark Galeocerdo cuvier to analyse and describe dental heterodonties among four different ontogenetic stages ranging from embryo to adult. Our results reveal rather gradual and subtle ontogenetic shape changes, mostly characterized by increasing size and complexity of the teeth. We furthermore provide the first comprehensive description of embryonic dental morphologies in tiger sharks. Also, tooth shapes of tiger sharks in different ontogenetic stages are re‐assessed and depicted in detail. Finally, multiple cases of tooth file reversal are described. This study, therefore, contributes to our knowledge of dental traits across ontogeny in the extant tiger shark G. cuvier and provides a baseline for further morphological and genetic studies on the dental variation in sharks. Therefore, it has the potential to assist elucidating the underlying developmental and evolutionary processes behind the vast dental diversity observed in elasmobranch fishes today and in deep time.

Background: The onset of morphological differences between related groups can be tracked at early stages during embryological development. This is expressed in functional traits that start with minor variations, but eventually diverge to defined specific morphologies. Several processes during this period, like proliferation, remodelling, and apoptosis for instance, can account for the variability observed between related groups. Morphological divergence through development is often associated with the hourglass model, in which early stages display higher variability and reach a conserved point with reduced variability from which divergence occurs again to the final phenotype.Results: Here we explored the patterns of developmental shape changes in the lower jaw of two shark species, the bamboo shark (Chiloscyllium punctatum) and the catshark (Scyliorhinus canicula). These two species present marked differences in their foraging behaviour, which is reflected in their adult jaw morphology. By tracing the developmental sequence of the cartilage condensation, we identified the onset of cartilage for both species at around stage 31. Other structures that developed later without a noticeable anlage were the labial cartilages, which appear at around stage 33. We observed that the lower jaw displays striking differences in shape from the earliest moments, without any overlap in shape through the compared stages.Conclusions: The differences observed are also reflected in the functional variation in feeding mechanism between both species. Likewise, the trajectory analysis shows that the main differences are in the magnitude of the shape change through time. Both species follow a unique trajectory, which is explained by the timing between stages.