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A relationship between the rate of molecular change and diversification has long been discussed, on both theoretical and empirical grounds. However, the effect on our understanding of evolutionary patterns is yet to be fully explored. Here, we develop a new model, the Covariant Evolutionary Tempo model, with the aim of integrating patterns of diversification and molecular evolution within a framework of a continuously changing "tempo" variable that acts as a master control for molecular, morphological, and diversification rates. Importantly, tempo itself is treated as being variable at a rate proportional to its own value. This model predicts that diversity is dominated by a small number of extremely large clades at any historical epoch including the present; that these large clades are expected to be characterised by explosive early radiations accompanied by elevated rates of molecular evolution; and that extant organisms are likely to have evolved from species with unusually fast evolutionary rates. Under such a model, the amount of molecular change along a particular lineage is essentially independent of its height, which weakens the molecular clock hypothesis. Finally, our model explains the existence of "living fossil" sister groups to large clades that are species poor and exhibit slow rates of morphological and molecular change. Our results demonstrate that the observed historical patterns of evolution can be modelled without invoking special evolutionary mechanisms or innovations that are unique to specific times or taxa, even when they are highly nonuniform.

Fossil material of echinoderms from the Cambrian of Morocco suggests a novel reconstruction of early echinoderm evolution and ties in with recent advances in molecular developmental biology.

Background

Jointed appendages represent one of the key innovations of arthropods, and thus understanding the development and evolution of these structures is important for the understanding of the evolutionary success of Arthropoda. In this paper, we analyze a cell cluster that was identified in a previous single-cell sequencing (SCS) experiment on embryos of the spider Parasteatoda tepidariorum. This cell cluster is characterized by marker genes that suggest a role in appendage patterning and joint development.

Results

We analyzed the expression profiles of these marker genes showing that they are expressed in the developing appendages and in a pattern that suggests a potential function during joint development. Several of the investigated genes represent new and unexpected factors such as dysfusion (dysf), spätzle3 (spz3), seven-up (svp). In order to study their evolutionary origin, we also investigated orthologs of the identified appendage-patterning genes in the harvestman Phalangium opilio, a distantly related chelicerate.

Conclusion

Our work highlights the usefulness of SCS experiments for the identification of potential new genetic factors that are involved in specific developmental processes. The current data provide potential new insights into the gene regulatory networks that underlie arthropod joint development.

Pockets of Ediacaran–Cambrian clastic sedimentary rocks are preserved across Fennoscandia, but the provenance, depositional setting and age of many such deposits remain uncertain. We report the first detrital zircon provenance study of the lowermost sediments deposited on sub-Cambrian bedrock in southern Sweden. We performed 285 ion microprobe U–Pb analyses on zircons from the Mickwitzia Sandstone (File Haidar Formation, Cambrian Series 2). Age peaks at c. 2100, 2000, 1800, 1550, 1500, 1225, 1150 and 950 Ma are consistent with Sveconorwegian and Fennoscandian source rocks, whereas a major peak at c. 550 Ma is attributed to the Timanian orogen to the NE. The youngest dates in the dataset, including multiple consistent dates from single grains, indicate a maximum depositional age of c. 550 Ma. This first documentation of Timanian-aged grains in southern Sweden connects reports of Timanian detritus as far afield as northern Norway, Estonia and even Poland. The Timanian detrital signature across thousands of square kilometres suggests that much of Baltica probably consisted of an extremely low-relief plain over which Timanian-sourced detritus spread extensively during the late Ediacaran and early Cambrian.

Background: Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior-posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy.

Results: In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes.

Conclusion: We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.

Background

The common house spider Parasteatoda tepidariorum represents an emerging new model organism of arthropod evolutionary and developmental (EvoDevo) studies. Recent technical advances have resulted in the first single-cell sequencing (SCS) data on this species allowing deeper insights to be gained into its early development, but mid-to-late stage embryos were not included in these pioneering studies.

Results

Therefore, we performed SCS on mid-to-late stage embryos of Parasteatoda and characterized resulting cell clusters by means of in-silico analysis (comparison of key markers of each cluster with previously published information on these genes). In-silico prediction of the nature of each cluster was then tested/verified by means of additional in-situ hybridization experiments with additional markers of each cluster.

Conclusions

Our data show that SCS data reliably group cells with similar genetic fingerprints into more or less distinct clusters, and thus allows identification of developing cell types on a broader level, such as the distinction of ectodermal, mesodermal and endodermal cell lineages, as well as the identification of distinct developing tissues such as subtypes of nervous tissue cells, the developing heart, or the ventral sulcus (VS). In comparison with recent other SCS studies on the same species, our data represent later developmental stages, and thus provide insights into different stages of developing cell types and tissues such as differentiating neurons and the VS that are only present at these later stages.

Background

Spiders evolved different types of eyes, a pair of primary eyes that are usually forward pointing, and three pairs of secondary eyes that are typically situated more posterior and lateral on the spider’s head. The best understanding of arthropod eye development comes from the vinegar fly Drosophila melanogaster, the main arthropod model organism, that also evolved different types of eyes, the larval eyes and the ocelli and compound eyes of the imago. The gene regulatory networks that underlie eye development in this species are well investigated revealing a conserved core network, but also show several differences between the different types of eyes. Recent candidate gene approaches identified a number of conserved genes in arthropod eye development, but also revealed crucial differences including the apparent lack of some key factors in some groups of arthropods, including spiders.

Results

Here, we re-analysed our published scRNA sequencing data and found potential key regulators of spider eye development that were previously overlooked. Unlike earlier research on this topic, our new data suggest that Hedgehog (Hh)-signalling is involved in eye development in the spider Parasteatoda tepidariorum. By investigating embryonic gene expression in representatives of all main groups of spiders, we demonstrate that this involvement is conserved in spiders. Additionally, we identified genes that are expressed in the developing eyes of spiders, but that have not been studied in this context before.

Conclusion

Our data show that single-cell sequencing represents a powerful method to gain deeper insight into gene regulatory networks that underlie the development of lineage-specific organs such as the derived set of eyes in spiders. Overall, we gained deeper insight into spider eye development, as well as the evolution of arthropod visual system formation.

The popularity of relaxed clock Bayesian inference of clade origin timings has generated several recent publications with focal results considerably older than the fossils of the clades in question. Here, we critically examine two such clades: the animals (with a focus on the bilaterians) and the mammals (with a focus on the placentals). Each example displays a set of characteristic pathologies which, although much commented on, are rarely corrected for. We conclude that in neither case does the molecular clock analysis provide any evidence for an origin of the clade deeper than what is suggested by the fossil record. In addition, both these clades have other features (including, in the case of the placental mammals, proximity to a large mass extinction) that allow us to generate precise expectations of the timings of their origins. Thus, in these instances, the fossil record can provide a powerful test of molecular clock methodology, and why it goes astray, and we have every reason to think these problems are general.

Strausfeld et al. (Report, 24 Nov 2022, p. 905) claim that Cambrian fossilized nervous tissue supports the interpretation that the ancestral panarthropod brain was tripartite and unsegmented. We argue that this conclusion is unsupported, and developmental data from living onychophorans contradict it.

Summary Morphology usually serves as an effective proxy for functional ecology,1,2,3,4,5 and evaluating morphological, anatomical, and ecological changes permits a deeper understanding of the nature of diversification and macroevolution.5,6,7,8,9,10,11,12 Lingulid (order Lingulida) brachiopods are both diverse and abundant during the early Palaeozoic but decrease in diversity over time, with only a few genera of linguloids and discinoids present in modern marine ecosystems, resulting in them frequently being referred to as “living fossils.”13,14,15 The dynamics that drove this decline remain uncertain, and it has not been determined if there is an associated decline in morphological and ecological diversity. Here, we apply geometric morphometrics to reconstruct global morphospace occupation for lingulid brachiopods through the Phanerozoic, with results showing that maximum morphospace occupation was reached by the Early Ordovician. At this time of peak diversity, linguloids with a sub-rectangular shell shape already possessed several evolutionary features, such as the rearrangement of mantle canals and reduction of the pseudointerarea, common to all modern infaunal forms. The end Ordovician mass extinction has a differential effect on linguloids, disproportionally wiping out those forms with a rounded shell shape, while forms with sub-rectangular shells survived both the end Ordovician and the Permian-Triassic mass extinctions, leaving a fauna predominantly composed of infaunal forms. For discinoids, both morphospace occupation and epibenthic life strategies remain consistent through the Phanerozoic. Morphospace occupation over time, when considered using anatomical and ecological analyses, suggests that the limited morphological and ecological diversity of modern lingulid brachiopods reflects evolutionary contingency rather than deterministic processes.