The Ediacaran Weng'an Biota of South China yields embryo-like microfossils preserved with sub-cellular fidelity, previously interpreted as the oldest evidence of animals. Megasphaera dominates the assemblage and preserves the earliest stages of binary reductive division. It is assumed to develop into Megaclonophycus, which is composed of hundreds to thousands of cells; however, this developmental link has not been tested. We used synchrotron and computed tomography to characterize hundreds of specimens of Megaclonophycus and determine cell division patterns. Specimen cell counts range from 14 to 10 201, with counts clustering around 2048 and 4096, consistent with binary reductive cell division. However, the specimens have asynchronous binary cell division with cell sizes within a specimen varying by two- to threefold. The volume of Megaclonophycus is variable during development, showing no evidence of gastrulation, characteristic of metazoan development. Megaclonophycus and Megasphaera exhibit a similar sequence of development, size and taphonomy indicating a developmental sequence of early to later stages of the same organism. These findings are incompatible with the crown group metazoan affinity suggested for this taxon, and so molecular clock estimates for the origin of animals must rely on fossil calibration from sources other than Megaclonophycus.

Crocodyliforms display a diverse range of skull morphologies through their evolutionary history. Extant crocodilians possess platyrostral (broad and flat) snouts, thought to be sub-optimal for resisting feeding loads due to the conflicting demands of feeding and hydrodynamic constraints. In contrast, numerous Mesozoic crocodyliforms possessed oreinirostral (dome-shaped) skulls, were terrestrial and hence free from hydrodynamic constraint. This study aims to examine the role of function in determining skull shape in crocodyliforms and assesses the difference in stress resistance between oreinirostral and platyrostral taxa. We hypothesize that in the absence of hydrodynamic constraints, oreinirostral taxa have skulls that are better suited for resisting feeding-induced loads. Using finite element analysis, we evaluated biomechanical performance in oreinirostral notosuchian taxa Baurusuchus salgadoensis, Montealtosuchus arrudacamposi and Caipirasuchus paulistanus, compared to the extant platyrostral Alligator mississippiensis, Crocodylus niloticus and Paleosuchus palpebrosus. Results show that oreinirostral morphologies are comparatively better suited for resisting forces generated during feeding, with increased muscular efficiency, supporting the hypothesis that hydrodynamic constraints influence crocodyliform skull evolution.

Students must develop higher order cognitive skills (HOCS) that allow them to think critically and use their learning in novel situations. However, little is known about how including HOCS in teaching and assessment affects students' perception of learning. We combine quantitative and qualitative data to determine whether the inclusion of HOCS and the presence of constructive alignment influences students' perceptions of learning within the first-year practical components of an undergraduate biology degree at a UK research intensive university. We applied the Blooming Biology Tool (BBT) and Bloom's Dichotomous Key (BDK) to quantify the proportion of HOCS present during practical sessions and their related assessments and found that a combination of tools can be used to reliably assess the requisite cognitive skills required to complete tasks. Students completed an online survey and provided free-text responses regarding which practical sessions they perceived had the most beneficial effects on their learning. Students valued both LOCS and HOCS for their learning but could only recognise and value HOCS in practical sessions featuring high proportions of HOCS. Our research provides methods for assessing and improving constructive alignment in the teaching of biology and furthers our understanding of when students will recognise and value HOCS.

The timing of origin of eukaryotes and the sequence of eukaryogenesis are poorly constrained because their fossil record is difficult to interpret. Claims of fossilized organelles have been discounted on the unsubstantiated perception that they decay too quickly for fossilization. We experimentally characterized the pattern and time scale of decay of nuclei, chloroplasts, and pyrenoids in red and green algae, demonstrating that they persist for many weeks postmortem as physical substrates available for preservation, a time scale consistent with known mechanisms of fossilization. Chloroplasts exhibit greater decay resistance than nuclei; pyrenoids are unlikely to be preserved, but their presence could be inferred from spaces within fossil chloroplasts. Our results are compatible with differential organelle preservation in seed plants. Claims of fossilized organelles in Proterozoic fossils can no longer be dismissed on grounds of plausibility, prompting reinterpretation of the early eukaryotic fossil record and the prospect of a fossil record of eukaryogenesis.

Chondrichthyan dentitions are conventionally interpreted to reflect the ancestral gnathostome condition but interpretations of osteichthyan dental evolution in this light have proved unsuccessful, perhaps because chondrichthyan dentitions are equally specialized, or else evolved independently. Ischnacanthid acanthodians are stem-Chondrichthyes; as phylogenetic intermediates of osteichthyans and crown-chondrichthyans, the nature of their enigmatic dentition may inform homology and the ancestral gnathostome condition. Here we show that ischnacanthid marginal dentitions were statodont, composed of multicuspidate teeth added in distally diverging rows and through proximal superpositional replacement, while their symphyseal tooth whorls are comparable to chondrichthyan and osteichthyan counterparts. Ancestral state estimation indicates the presence of oral tubercles on the jaws of the gnathostome crown-ancestor; tooth whorls or tooth rows evolved independently in placoderms, osteichthyans, ischnacanthids, other acanthodians and crown-chondrichthyans. Crown-chondrichthyan dentitions are derived relative to the gnathostome crown-ancestor, which possessed a simple dentition and lacked a permanent dental lamina, which evolved independently in Chondrichthyes and Osteichthyes.

Photogrammetry allows overlapping photographs of fossils to be taken and converted into photo-realistic three-dimensional (3-D) digital models. These models offer potential advantages in teaching palaeontology: they are cheap to produce, can be easily shared and allow the study of rare and delicate specimens. Here I describe approaches for using photogrammetric models in the teaching and learning of palaeontology. Little is known about how students perceive these models and whether they find them valuable in their learning. To address this, first-year university students taught using both fossil specimens and digital models were surveyed about their experience through an anonymous online survey. Most students found that the digital models were easy to use, helped them understand anatomy and were more useful than studying photographs. However, most did not see the models as a substitute for studying real fossils and felt they could learn more from physical models. Digital models are a useful addition to palaeontological education that can supplement real fossils and allow palaeontological education to take place in circumstances where handling of specimens is not possible.

The challenge of identifying fossilized organelles has long hampered attempts to interpret the fossil record of early eukaryote evolution. We explore this challenge through experimental taphonomy of nuclei in a living eukaryote and microscale physical and chemical characterization of putative nuclei in embryo-like fossils from the early Ediacaran Weng'an Biota. The fossil nuclei exhibit diverse preservational modes that differ in shape, presence or absence of an inner body and the chemistry of the associated mineralization. The nuclei are not directly fossilized; rather, they manifest as external moulds. Experimental taphonomy of epidermal cells from the common onion (Allium cepa) demonstrates that nuclei are more decay resistant than their host cells, generally maintaining their physical dimensions for weeks to months post-mortem, though under some experimental conditions they exhibit shrinkage and/or become shrouded in microbial biofilms. The fossil and experimental evidence may be rationalized in a single taphonomic pathway of selective mineralization of the cell cytoplasm, preserving an external mould of the nucleus that is itself resistant to both decay and mineral replication. Combined, our results provide both a secure identification of the Weng'an nuclei as well as the potential of a fossil record of organelles that might help arbitrate in long-standing debates over the relative and absolute timing of the evolutionary assembly of eukaryote-grade cells.

The Ediacaran Weng’an Biota (Doushantuo Formation, 609 Ma old) is a rich microfossil assemblage that preserves biological structure to a subcellular level of fidelity and encompasses a range of developmental stages [1]. However, the animal embryo interpretation of the main components of the biota has been the subject of controversy [2, 3]. Here, we describe the development of Caveasphaera, which varies in morphology from lensoid to a hollow spheroidal cage [4] to a solid spheroid [5] but has largely evaded description and interpretation. Caveasphaera is demonstrably cellular and develops within an envelope by cell division and migration, first defining the spheroidal perimeter via anastomosing cell masses that thicken and ingress as strands of cells that detach and subsequently aggregate in a polar region. Concomitantly, the overall diameter increases as does the volume of the cell mass, but after an initial phase of reductive palinotomy, the volume of individual cells remains the same through development. The process of cell ingression, detachment, and polar aggregation is analogous to gastrulation; together with evidence of functional cell adhesion and development within an envelope, this is suggestive of a holozoan affinity. Parental investment in the embryonic development of Caveasphaera and co-occurring Tianzhushania and Spiralicellula, as well as delayed onset of later development, may reflect an adaptation to the heterogeneous nature of the early Ediacaran nearshore marine environments in which early animals evolved.

Early Cambrian Pseudooides prima has been described from embryonic and post-embryonic stages of development, exhibiting long germ-band development. There has been some debate about the pattern of segmentation, but this interpretation, as among the earliest records of ecdysozoans, has been generally accepted. Here, we show that the ‘germ band’ of P. prima embryos separates along its mid axis during development, with the transverse furrows between the ‘somites’ unfolding into the polar aperture of the hexa-radial theca of Hexaconularia sichuanensis, conventionally interpreted as a scyphozoan cnidarian; co-occurring post-embryonic remains of ecdysozoans are unrelated. We recognize H. sichuanensis as a junior synonym of P. prima as a consequence of identifying these two-form taxa as distinct developmental stages of the same organism. Direct development in P. prima parallels the co-occuring olivooids Olivooides, and Quadrapyrgites and Bayesian phylogenetic analysis of a novel phenotype dataset indicates that, despite differences in their tetra-, penta- and hexa-radial symmetry, these hexangulaconulariids comprise a clade of scyphozoan medusozoans, with Arthrochites and conulariids, that all exhibit direct development from embryo to thecate polyp. The affinity of hexangulaconulariids to extant scyphozoan medusozoans indicates that the prevalence of tetraradial symmetry and indirect development are a vestige of a broader spectrum of body-plan symmetries and developmental modes that was manifest in their early Phanerozoic counterparts.

: The Weng ’ an Biota is a fossil Konservat-Lagerstätte in South China that is c. 570 – 609 myr old and provides an unparalleled snapshot of marine life during the interval in which molecularclocks estimate that animal clades were diversifying. It yields fossils that are three-dimensionally preserved in calcium phosphate with cellular and sometimes subcellular fidelity. The biota includes candidates for the oldest animals in the fossil record, including embryonic, larval and adult forms. We argue that, although the Weng ’ an Biota includes forms that could be animals, none can currently be assigned to this group with confidence. Nonetheless, the biota offers a rare and valuable window on the evolution of multicellular and soft-bodied organisms in the prelude to the Cambrian radiation.

Early CambrianPseudooides primahas been described from embryonic and post-embryonic stages of development, exhibiting long germ-band development. There has been some debate about the pattern of segmentation, but this interpretation, as among the earliest records of ecdysozoans, has been generally accepted. Here, we show that the ‘germ band’ ofP. primaembryos separates along its mid axis during development, with the transverse furrows between the ‘somites’ unfolding into the polar aperture of the ten-sided theca ofHexaconularia sichuanensis, conventionally interpreted as a scyphozoan cnidarian; co-occurring post-embryonic remains of ecdysozoans are unrelated. We recognizeH. sichuanensisas a junior synonym ofP. primaas a consequence of identifying these two form-taxa as distinct developmental stages of the same organism. Direct development inP. primaparallels the co-occuring olivooidsOlivooides,andQuadrapyrgitesand Bayesian phylogenetic analysis of a novel phenotype dataset indicates that, despite differences in their tetra-, penta- and pseudo-hexa-radial symmetry, these hexangulaconulariids comprise a clade of scyphozoan medusozoans, withArthrochitesand conulariids, that all exhibit direct development from embryo to thecate polyp. The affinity of hexangulaconulariids and olivooids to extant scyphozoan medusozoans indicates that the prevalence of tetraradial symmetry and indirect development are a vestige of a broader spectrum of body-plan symmetries and developmental modes that was manifest in their early Phanerozoic counterparts.

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

The embryo-like microfossils from the Ediacaran Weng'an Biota (ca. 609 million years old) are among the oldest plausible claims of animals in the fossil record. Fossilization frequendy extends beyond the cellular, to preserve subcellular structures including contentious Large Intracellular Structures (LISs) that have been alternately interpreted as eukaryote nuclei or organelles, degraded remains, or abiological structures. Here we present new data on the structure, morphology, and development of the LISs in these embryo-like fossils, based on Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM) and quantitative computed tomographic analysis. All the lines of evidence, including consistency in the number, shape, position, and relative size (LlS-to-cytoplasm ratio) of the LISs, as well as their occurrence within preserved cytoplasm, support their interpretation as cell nuclei. Our results allow us to reject the view that nuclei cannot be preserved in early eukaryote fossils, offering new potential for interpreting the fossil record of early eukaryote evolution.

The Weng'an Biota is a fossil Konservat-Lagerstatte in South China that is c. 570 - 609 myr old and provides an unparalleled snapshot of marine life during the interval in which molecular clocks estimate that animal clades were diversifying. It yields fossils that are three-dimensionally preserved in calcium phosphate with cellular and sometimes subcellular fidelity. The biota includes candidates for the oldest animals in the fossil record, including embryonic, larval and adult forms. We argue that, although the Weng'an Biota includes forms that could be animals, none can currently be assigned to this group with confidence. Nonetheless, the biota offers a rare and valuable window on the evolution of multicellular and soft-bodied organisms in the prelude to the Cambrian radiation.

Fossilized embryos afford direct insight into the pattern of development in extinct organisms, providing unique tests of hypotheses of developmental evolution based in comparative embryology. However, these fossils can only be effective in this role if their embryology and phylogenetic affinities are well constrained. We elucidate and interpret the development of Olivooides from embryonic and adult stages and use these data to discriminate among competing interpretations of their anatomy and affinity. The embryology of Olivooides is principally characterized by the development of an ornamented periderm that initially forms externally and is subsequently formed internally, released at the aperture, facilitating the direct development of the embryo into an adult theca. Internal anatomy is known only from embryonic stages, revealing two internal tissue layers, the innermost of which is developed into three transversally arranged walls that partly divide the lumen into an abapertural region, interpreted as the gut of a polyp, and an adapertural region that includes structures that resemble the peridermal teeth of coronate scyphozoans. The anatomy and pattern of development exhibited by Olivooides appears common to the other known genus of olivooid, Quadrapyrgites, which differs in its tetraradial, as opposed to pentaradial symmetry. We reject previous interpretations of the olivooids as cycloneuralians, principally on the grounds that they lack a through gut and introvert, in embryo and adult. Instead we consider the affinities of the olivooids among medusozoan cnidarians; our phylogenetic analysis supports their classification as total‐group Coronata, within crown‐Scyphozoa. Olivooides and Quadrapyrgites evidence a broader range of life history strategies and bodyplan symmetry than is otherwise commonly represented in extant Scyphozoa specifically, and Cnidaria more generally.

The evolutionary emergence of animals is one of the most significant episodes in the history of life, but its timing remains poorly constrained. Molecular clocks estimate that animals originated and began diversifying over 100 million years before the first definitive metazoan fossil evidence in the Cambrian. However, closer inspection reveals that clock estimates and the fossil record are less divergent than is often claimed. Modern clock analyses do not predict the presence of the crown‐representatives of most animal phyla in the Neoproterozoic. Furthermore, despite challenges provided by incomplete preservation, a paucity of phylogenetically informative characters, and uncertain expectations of the anatomy of early animals, a number of Neoproterozoic fossils can reasonably be interpreted as metazoans. A considerable discrepancy remains, but much of this can be explained by the limited preservation potential of early metazoans and the difficulties associated with their identification in the fossil record. Critical assessment of both records may permit better resolution of the tempo and mode of early animal evolution.

Three-dimensional analyses of the early Ediacaran microfossils from the Weng'an biota (Doushantuo Formation) have focused predominantly on multicellular forms that have been interpreted as embryos, and yet they have defied phylogenetic interpretation principally because of absence of evidence from other stages in their life cycle. It is therefore unfortunate that the affinities of the various other Doushantuo microfossils have been neglected. A new conical fossil that is preserved at a cellular level is described here. The fossil contains distinct cell clusters that are characterized and analysed in three dimensions. These clusters are often exposed at the specimen surface, and the fossil preserves many hemispherical craters that are interpreted as positions where clusters have left the organism. The cell clusters may be either reproductive propagules or infesting organisms. Similar clusters are found in a variety of Doushantuo organisms including putative animal embryos and algae.