Heterostracans are pivotal in examining the evolution of feeding and the head in jawed vertebrates as the stem-gnathostomes in which the oral apparatus is more commonly preserved. However, most analyses of their feeding, especially recent studies using modern 3D imaging methods, have focused on a specific subgroup, the Pteraspidiformes. Athenaegis chattertoni from the Wenlockian (Silurian) of Canada is the stratigraphically oldest heterostracan preserving an articulated feeding apparatus and is critical for reconstructing ancestral states in the lineage. Here, we use x-ray and synchrotron microtomography to re-examine the oral apparatus of Athenaegis, revealing a number of differences with the original reconstruction. Most importantly, the oral region of Athenaegis consists of numerous imbricated oral plates which functioned as a single integrated unit rather than a V-shaped arrangement separated by a medial seam. We show that Athenaegis, contrary to the original description, does not possess a medial postoral plate. The oral region was not capable of opening substantially as shown by the interdigitation of the oral plates, a close articulation with the surrounding plates, and an absence of muscle or ligament attachment points. We conclude a microphagous suspension feeding function of the oral apparatus for Athenaegis consistent with previous hypotheses of suspension feeding in heterostracans.
Abstract Fossil resins preserve not only body fossils but also a diverse record of trace fossils reflecting organism–substrate interactions during and after resin secretion. This study develops an ichnological framework for resin‐hosted traces, distinguishing between borings preserved on resin surfaces and behaviour‐related structures caught within the resin matrix, termed ichnoinclusions. The analysis is based on comprehensive sample of fossil resin material, including Baltic, Lublin, Ethiopian, Mexican (simojovelite) and Lebanese ambers. The documented ichnofossils represent multiple ichnofacies and record successive stages of resin taphonomy. Woodground‐related borings (e.g. Teredolites , Apectoichnus ) are associated with early stages of resin accumulation, whereas lithic‐type borings assigned to Gastrochaenolites isp. reflect later phases of redeposition, burial and substrate induration. These observations demonstrate that fossil resin does not represent a single ichnotaxobase, but passes through a succession of substrate conditions, recording a multi‐stage ichnological history. In addition, ichnoinclusions provide independent taphonomic information. Structures such as silk threads, spider webs, larval cases, and frass accumulations record resin viscosity, flow dynamics and microenvironmental conditions at the moment of entrapment. The results support a taphonomic model linking ichnofacies succession with resin redeposition and demonstrate that fossil resins should be treated as dynamic substrates recording both biological activity and sedimentary processes.
Abstract The number of described species underestimates true diversity in virtually any clade, yet robust species richness estimates are essential for inferring total diversity, enabling cross‐taxon comparisons, and informing conservation. Here, I evaluate the use of species discovery rates to estimate unknown diversity in fossil clades by projecting future descriptions from historical trends to the year 2300. Two predictive methods are employed: parametric cumulative distribution functions (CDFs) and Bayesian dynamic generalized additive models (DGAMs). Both are applied to three fossil mollusc groups with well‐documented records: non‐marine molluscs (freshwater bivalves and freshwater and terrestrial gastropods) globally; freshwater gastropods from the European fossil record; and Mesozoic–Cenozoic scallops (Pectinoidea). For the European gastropod record, comprehensive occurrence data additionally allow abundance‐based diversity estimation (iChao1) for comparison. To assess model robustness, I simulate data incompleteness through subsampling at varying ratios across selected time periods and by removing recent records. Results show that the best‐fitting distribution model is highly sensitive to the degree and timing of subsampling: depending on which historical period is most affected, the preferred model type may shift dramatically, yielding richness estimates that range from imminent saturation to gains exceeding 200% by 2300. DGAMs perform more reliably, producing more conservative and subsampling‐robust projections on average. Together, the results urge caution when predicting species richness from discovery rates and highlight the critical importance of complete, well‐curated fossil databases for meaningful diversity estimates.
Abstract Conodonts are known primarily from their microscopic tooth‐like skeletal feeding elements that occur commonly in jumbled assemblages in marine rocks from the late Cambrian to the Triassic–Jurassic boundary interval. Interpreting their evolution requires a scheme of positional homology of these elements within the feeding apparatus, information that is provided only by exceedingly rare articulated natural assemblages of elements. Here we describe fused clusters and bedding plane assemblages of the middle Triassic conodont Pseudofurnishius murcianus which are composed of 15 elements, consistent with the established model for Gondolelloidea, and more generally for Ozarkodinina, including symmetrical pairs of P 1 , P 2 , M, and S 1–4 elements and an asymmetrical unpaired S 0 element. We use these data to reconstruct the three‐dimensional architecture of the P. murcianus feeding apparatus which is closely comparable to that of Nicoraella , the only other gondolellid for which apparatus architecture is known, demonstrating evolutionary conservation within Gondolelloidea superfamily. Our results highlight evolutionary constraints on conodont dental architecture and offer new insights into the functional and developmental aspects of these early vertebrates.
Abstract Archosauriforms were the most successful reptiles of the Mesozoic, becoming widespread and diverse during the Triassic. Yet, little is known of their origin, although it seems clear that it was before the end‐Permian mass extinction. An additional source of information comes from the early archosauriform footprint record. Based on well‐preserved material including newly described specimens, we revise the earliest footprints attributed to archosauriforms from the late Permian Val Gardena Sandstone Formation of northern Italy. This material clearly belongs to the ichnofamily Chirotheriidae, but differs from all known ichnogenera belonging to this group. Accordingly, we introduce the new ichnotaxon Pachychirotherium gardenaensis . Biostratigraphically this ichnotaxon marks the first appearance of Chirotheriidae and belongs to the late Wuchiapingian–Induan Paradoxichnium footprint biochron. Through a synapomorphy‐based track–trackmaker correlation, we attribute the newly described ichnotaxon to Erythrosuchidae or non‐eucrocopod archosauriforms with a similar pes morphology. We analysed posture and locomotion of early archosauromorphs using six different late Permian to Early Triassic ichnogenera. Some key features of non‐eucrocopod archosauromorph footprints (e.g. Paradoxichnium , Pachychirotherium ) and skeletons are consistent with a sprawling locomotion during the late Permian (Lopingian), while some Early Triassic forms ( Synaptichnium , Protochirotherium ) were transitional. A fully erect posture in archosauriforms was acquired probably during the Middle Triassic ( Isochirotherium , Chirotherium ). Moreover, there is evidence of a stepwise acquisition of digitigrady (favoured by the appearance of a crurotarsal ankle) and mesaxony, as well as a progressive reduction of the manus. These factors were important for the later transition to an erect posture and bipedality.
