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1. Scaphopoda is the sister taxon to Bivalvia: Evidence of ancient incomplete lineage sorting

   https://doi.org/10.1073/pnas.2302361120  

   Song, Hao; Wang, Yunan; Shao, Haojing; Li, Zhuoqing; Hu, Pinli; Yap-Chiongco, Meghan K; Shi, Pu; Zhang, Tao; Li, Cui; Wang, Yiguan; et al (October 2023, Proceedings of the National Academy of Sciences)

   The almost simultaneous emergence of major animal phyla during the early Cambrian shaped modern animal biodiversity. Reconstructing evolutionary relationships among such closely spaced branches in the animal tree of life has proven to be a major challenge, hindering understanding of early animal evolution and the fossil record. This is particularly true in the species-rich and highly varied Mollusca where dramatic inconsistency among paleontological, morphological, and molecular evidence has led to a long-standing debate about the group’s phylogeny and the nature of dozens of enigmatic fossil taxa. A critical step needed to overcome this issue is to supplement available genomic data, which is plentiful for well-studied lineages, with genomes from rare but key lineages, such as Scaphopoda. Here, by presenting chromosome-level genomes from both extant scaphopod orders and leveraging complete genomes spanning Mollusca, we provide strong support for Scaphopoda as the sister taxon of Bivalvia, revitalizing the morphology-based Diasoma hypothesis originally proposed 50 years ago. Our molecular clock analysis confidently dates the split between Bivalvia and Scaphopoda at ~520 Ma, prompting a reinterpretation of controversial laterally compressed Early Cambrian fossils, includingAnabarella,Watsonella,andMellopegma,as stem diasomes. Moreover, we show that incongruence in the phylogenetic placement of Scaphopoda in previous phylogenomic studies was due to ancient incomplete lineage sorting (ILS) that occurred during the rapid radiation of Conchifera. Our findings highlight the need to consider ILS as a potential source of error in deep phylogeny reconstruction, especially in the context of the unique nature of the Cambrian Explosion. 

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2. The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an “Explosion”

   https://doi.org/https://doi.org/10.1093/icb/icy088  

   Sperling, E.A.; Stockey, R.G. (January 2018, Integrative and comparative biology)

   Animals originated and evolved during a unique time in Earth history—the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560–540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity—or at least lower quantities of organic carbon reaching the seafloor—compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the “fire triangle” metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event. 

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3. Cambrian origin but no early burst in functional disparity for Class Bivalvia

   https://doi.org/10.1098/rsbl.2023.0157  

   Zhou, Sharon; Edie, Stewart M.; Collins, Katie S.; Crouch, Nicholas M.; Jablonski, David (May 2023, Biology Letters)

   Both the Cambrian explosion, more than half a billion years ago, and its Ordovician aftermath some 35 Myr later, are often framed as episodes of widespread ecological opportunity, but not all clades originating during this interval showed prolific rises in morphological or functional disparity. In a direct analysis of functional disparity, instead of the more commonly used proxy of morphological disparity, we find that ecological functions of Class Bivalvia arose concordantly with and even lagged behind taxonomic diversification, rather than the early-burst pattern expected for clades originating in supposedly open ecological landscapes. Unlike several other clades originating in the Cambrian explosion, the bivalves' belated acquisition of key anatomical novelties imposed a macroevolutionary lag, and even when those novelties evolved in the Early Ordovician, functional disparity never surpassed taxonomic diversity. Beyond this early period of animal evolution, the founding and subsequent diversification of new major clades and their functions might be expected to follow the pattern of the early bivalves—one where interactions between highly dynamic environmental and biotic landscapes and evolutionary contingencies need not promote prolific functional innovation. 

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4. Breathless through Time: Oxygen and Animals across Earth’s History

   https://doi.org/10.1086/721754  

   Sperling, Erik A.; Boag, Thomas H.; Duncan, Murray I.; Endriga, Cecilia R.; Marquez, J. Andres; Mills, Daniel B.; Monarrez, Pedro M.; Sclafani, Judith A.; Stockey, Richard G.; Payne, Jonathan L. (October 2022, The Biological Bulletin)

   Oxygen levels in the atmosphere and ocean have changed dramatically over Earth history, with major impacts on marine life. Because the early part of Earth’s history lacked both atmospheric oxygen and animals, a persistent co-evolutionary narrative has developed linking oxygen change with changes in animal diversity. Although it was long believed that oxygen rose to essentially modern levels around the Cambrian period, a more muted increase is now believed likely. Thus, if oxygen increase facilitated the Cambrian explosion, it did so by crossing critical ecological thresholds at low O2. Atmospheric oxygen likely remained at low or moderate levels through the early Paleozoic era, and this likely contributed to high metazoan extinction rates until oxygen finally rose to modern levels in the later Paleozoic. After this point, ocean deoxygenation (and marine mass extinctions) is increasingly linked to large igneous province eruptions—massive volcanic carbon inputs to the Earth system that caused global warming, ocean acidification, and oxygen loss. Although the timescales of these ancient events limit their utility as exact analogs for modern anthropogenic global change, the clear message from the geologic record is that large and rapid CO2 injections into the Earth system consistently cause the same deadly trio of stressors that are observed today. The next frontier in understanding the impact of oxygen changes (or, more broadly, temperature-dependent hypoxia) in deep time requires approaches from ecophysiology that will help conservation biologists better calibrate the response of the biosphere at large taxonomic, spatial, and temporal scales. 

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5. Ecological novelty at the start of the Cambrian and Ordovician radiations of echinoderms

   https://doi.org/10.1111/pala.12688  

   Novack‐Gottshall, Philip M.; Purcell, Jack; Sultan, Ali; Ranjha, Isa; Deline, Bradley; Sumrall, Colin D. (January 2024, Palaeontology)

   Abstract The Cambrian and Ordovician radiations marked the origins of all major echinoderm clades and established their Phanerozoic ecological blueprint. Recent claims of modest innovation of early echinoderms and other animals suggest constraints on novelty during the origins of phyla. Here, we document the life‐habit richness, body size, tiering, habitat usage, mobility, diet and foraging habits of 366 Cambrian–Ordovician echinoderm genera across a time‐scaled phylogeny to identify the timing and impact of novelty. Most early echinoderms were sedentary, filter‐feeding microbivores, and their body sizes, diets and modes of locomotion were largely unchanged through time, despite major volatility in taxonomic composition. Cambrian echinoderms lived close to the seafloor, with stylophorans first evolving semi‐infaunality. Many Ordovician echinoderms lived farther from the seafloor, with more complex filter‐feeding organs and tiering structure, often using other organisms as substrates. Mobile carnivores and mass‐feeders emerge then, too, across diverse asterozoans and echinozoans. Most of these novelties coincide with the origins of individual clades during the early and late Cambrian (Terreneuvian/Series 2 and Furongian), and 10–20 million years pass before most become ecologically important. Life‐habit evolution within most taxa involved new variations in these traits, with crinoids and asterozoans better able to shift strategies. The Ordovician radiation records more new variability in pre‐existing Cambrian life habits rather than radical re‐invention, with important novelties (like mobile carnivory and mass‐feeding, increased height off the seafloor, and biotic substrates) becoming ecologically more impactful. Taxa with these strategies were those best able to capitalize on the newly heterogeneous Ordovician world. 

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