Paleontology: A diverse biota in the shadow of a mass extinction?

Abstract Recent Euarthropoda phylogenies incorporating Cambrian taxa provide a robust framework for testing evolutionary scenarios. Despite considerable progress, the interrelationships of Artiopoda continue to be ambiguous. Here we reassess Acanthomeridion serratum from the Chengjiang Lagerstätte based on new material and updated phylogenetic analyses. Acanthomeridion is an unusual artiopodan characterized by lateral stalked eyes, a trilobite-like dorsal suture system and a postero-ventral plate resembling the postventral plate of aglaspidids, covering the anal region. Updated phylogenetic analyses consistently place Acanthomeridion within Vicissicaudata, closely related to the iconic Sidneyia from the Burgess Shale. Morphological evidence, including stalked lateral eyes, optic notch, gnathobasic cephalic appendages and a postero-ventral plate, also supports this relationship. Importantly, we refine the terminology of appendicular derivatives and propose two homologous categories derived from different trunk appendages: pre-terminal appendages and ventral sclerotized plates. In parallel, we identify potential evolutionary trends within Artiopoda, including the migration of eyes from lateral to dorsal positions and from mobile to sessile forms, as well as the emergence of complex suture systems. Collectively, these findings highlight Acanthomeridion as a key taxon for understanding the evolution of artiopodan vision, ecdysial systems and appendicular derivatives, and provide new evidence for resolving the internal phylogeny of Artiopoda.

Cambrian Burgess Shale-type (BST) fossil biotas document nearly complete snapshots of the oldest Phanerozoic marine ecosystems1-4. However, the rarity of deposits bearing high-diversity BST biotas5 has restricted our understanding of the evolutionary and ecological dynamics of theCambrian explosion. Here we report the Huayuan biota-a lower Cambrian (Stage 4, approximately 512 million years ago) BST Lagerstätte from an outer shelf, deep-water setting of the Yangtze Block in Hunan, South China. The Huayuan biota yields remarkable taxonomic richness, comprising 153 animal species of 16 phylum-level clades dominated by arthropods, poriferans and cnidarians, among which 59% of species are new. The biota is comprised overwhelmingly of soft-bodied forms that include preserved cellular tissues. The complex ecosystem contained diverse radiodonts and pelagic tunicates, filling a gap of high-diversity BST biotas from the Cambrian Stage 4. Critically, multivariate ordination based on a global dataset of Cambrian BST biotas places the Huayuan biota within a main transition of marine animal ecosystems between Cambrian Age 3 and Age 4. Network analysis reveals close faunal connections between the Huayuan and Burgess Shale biotas, indicating transoceanic dispersal. Dated shortly after the Sinsk event6-8, the Huayuan biota illuminates differences in the impacts of this extinction in shallow- versus deep-water settings during the first Phanerozoic mass extinction and offers critical insights into the transformation of global ecosystems in the early Cambrian.

The article explores the concept of “place” through Deep Time using fossil discoveries from the Burgess Shale, a 508-million-year-old fossil site which preserves some of the earliest lifeforms on Earth. I apply a performative analysis of semi-structured in-depth interviews and to ethnographic data collected on hikes to the shale; at museum exhibits, paleontological symposia, and conferences; and at two major North American paleontological collections. A performative analysis of place uncovers how places are always made and remade in actions upon and through materializing bodies and nonhuman entities. The present article addresses doings and makings of place through Deep Time, or vast passages of time that are unintelligibly long to humans. Place is not just a spatial and cultural concept, but also a temporal one, and the timeline is nonlinear. I argue that fossils contribute to the relativity, contested nature, and multi-synchronicity of place. With a focus on “place-making,” I argue that the shale is a process which never attains a fixity or finality to its meaning, but is rather negotiated and often, a site of struggle. As time-travellers from Deep Time, fossils are strictly traces of ancient critters and environments, so their meaning is contingent upon social, cultural, historical, and political conditions under which they are interpreted. Fossils are therefore sources of resistance on a changing and maybe-dying planet.

Euarthropod appendages are specialized for diverse roles including feeding, walking, and mating, which require precise morphologies and ranges of motion. Cambrian fossils preserve exceptional details of extinct euarthropod appendages that can illuminate their anatomy and ecology. However, fossils are typically restricted by small sample sizes or incomplete preservation, and thus functional studies of the appendages usually rely on idealized reconstructions. The Burgess Shale Olenoides serratus is unique among trilobites owing to the availability of numerous specimens with soft tissue preservation that allow us to quantify its appendages' functional morphology. We measured the range of motion of the legs in Olenoides serratus and the extant horseshoe crab Limulus polyphemus. Despite repeated ecological comparisons between trilobites and xiphosurans, we find significant differences in the appendages' mobility between these taxa, with Limulus showing greater flexibility between the podomeres. O. serratus legs have a more restricted range of motion relative to L. polyphemus, particularly in their distal region. Flexure between the protopodite played a critical role in allowing the endopodite to create known trilobite trace fossils and bring food toward the ventral groove.

ABSTRACT The Guanshan Biota represents a Cambrian fossil Konservat-Lagerstätte stratigraphically positioned between the Chengjiang and Burgess Shale biotas, primarily discovered in the Kunming-Wuding area (west of the Xiaojiang Fault) in eastern Yunnan, South China. This study reports several new localities of the Guanshan Biota in the Malong-Yiliang area (east of the Xiaojiang Fault). Fossil assemblages predominantly occur in the lower Wulongqing Formation, and are dominated by arthropods and brachiopods, accompanied by a few cnidarians, vetulicolians, chancelloriids, hyoliths, eocrinoids and algae. Notably, Branchiocaris, Guangweicaris, Gangtoucunia, chancelloriids and algae are discovered for the first time from Malong. These new localities and their diverse fossil assemblages significantly alter previous perceptions of low taxonomic diversity in the stratotype area of the Wulongqing Formation. When integrated with previously documented fossils from Malong, the Guanshan Biota composition in this area is now very close to that of classical assemblages from Kunming. The progressive discovery of Guanshan Biota to the east of the Xiaojiang Fault suggests extensive distribution and considerable biodiversity of the biota in this area. Unlike the earlier Chengjiang Biota and Malong Fauna, whose distributions were constrained by the Xiaojiang Fault, the Guanshan Biota exhibits minimal tectonic influence on both its geographic distribution and faunal composition.

Lobopodians are an evolutionary grade of panarthropods characterized by their vermiform bodies and paired, unjointed lobopodous legs. A paraphyletic group, their study is of particular significance in understanding the evolution of extant panarthropods. Found exclusively in marine deposits from the Paleozoic, the great majority of species come from Cambrian Konservat-Lagerstätten, with only a few representatives known from the Ordovician, Silurian, and Carboniferous. Here we redescribe Palaeocampa anthrax from the Carboniferous Mazon Creek (USA) and Montceau-les-Mines (France) Lagerstätten as a lobopodian. First published in 1865, nearly fifty years before the discovery of the Burgess Shale, Palaeocampa is historically the first discovered lobopod, and its presence at the slightly younger Montceau-les-Mines (Gzhelian), makes this the youngest known fossil 'xenusiid' lobopodian species. We present the case that Palaeocampa most likely inhabited a freshwater environment, contesting the view that Paleozoic lobopodians were exclusively marine. Palaeocampa bears biomineralized dorso-lateral and lateral sclerite sets with a unique architecture unseen in other lobopodian sclerites, which may have been capable of secreting defensive chemicals at their tips. Palaeocampa anthrax represents a major evolutionary step in lobopodians, both in environmental adaptations and in defensive abilities.

La explosión cámbrica es una radiación evolutiva que tuvo lugar en el Cámbrico temprano el cual arranca hace aproximadamente 538 millones de años. Es una de las más notables de la historia de la biota, ya que en 10 millones de años se desarrollaron la mayor parte de los planes corporales presentes en los grupos actuales de animales, aunque varias de sus formas originales no llegaron a día de hoy. Aparecieron los primeros esqueletos mineralizados, lo que facilitó el proceso de fosilización. Por otro lado, el aumento de oxígeno en la atmósfera fomentó el crecimiento de las estructuras corporales y el calentamiento global y la subida del nivel del mar crearon hábitats favorables. En los sedimentos cámbricos, como Burgess Shale, en Canadá, y la formación de Chenjiang, en China, se encontraron infinidad de fósiles de este periodo, algunos de ellos representados en esta ilustración: Los géneros Anomalocaris (1), Tamisiocaris (2), Opabinia (3), Hallucigenia (4), Marrella (5), Pikaia (6), la clase Trilobites (7), el género Vetulicola (8) y las especies Cotyledion tylodes (9) y Xianguangia sinica (10).

Much diversity in arthropod form is the result of variation in the number and differentiation of segments (tagmosis). Fossil evidence to date has suggested that the earliest-diverging arthropods, the radiodonts, exhibited comparatively limited variability in tagmosis. We present a new radiodont, Mosura fentoni n. gen. and n. sp., from the Cambrian (Wuliuan) Burgess Shale that departs from this pattern. Mosura exhibits up to 26 trunk segments, the highest number reported for any radiodont, despite being among the smallest known. The head is short, with a small, rounded preocular sclerite, three prominent eyes and appendages with curving endites tipped with paired spines, altogether suggesting a nektonic, macrophagous predatory ecology. The trunk is divided into a neck, mesotrunk with large swimming flaps and multisegmented posterotrunk with tightly spaced bands of gill lamellae and reduced flaps. Detailed preservation of expansive circulatory lacunae, closely associated with the gills, clarifies the nature of similar structures in other Cambrian arthropod fossils, including Opabinia. The morphology of the posterotrunk suggests specialization for respiration, unique among radiodonts, but broadly convergent with the xiphosuran opisthosoma, isopod pleon and hexapod abdomen. This reinforces the hypothesis that multiple arthropod lineages underwent parallel diversification in tagmosis, in tandem with their initial Cambrian radiation.

The trilobitomorphs are a megadiverse and ecologically versatile group of Paleozoic euarthropods that include the iconic trilobites, as well as non-biomineralized clades exclusively known from Konservat-Lagersttätten. The concilitergans, defined by the presence of a broad, variably effaced and flattened dorsal exoskeleton, have received comparatively little attention. This is particularly true for Helmetia expansa from the mid-Cambrian Burgess Shale (British Columbia) – the first chronologically reported species of Conciliterga, which was originally figured (based on a single specimen) by Walcott (1918). Here, we present the first comprehensive description of H. expansa based on all material available from the Burgess Shale, totalling 36 specimens housed at the Royal Ontario Museum and the Smithsonian Institution. The non-biomineralized dorsal exoskeleton is broad and flat, with a serrated margin throughout. The body consists of a cephalon with well-developed anterolateral spines, six thoracic tergites and a large pygidium bearing two pairs of lateral spines and a single terminal spine. The preserved appendages include short uniramous antennae followed by 15 pairs of homonomous and biramous limbs composed of a gracile endopodite and an exopodite with a broad lobe and thick lamellae. The digestive system consists of a straight gut tract, including a ‘J’-shaped foregut and five paired digestive glands on the anterior half of the body expressed as small and convex oblong structures with a submillimetric lamellar ultrastructure. Two specimens of H. expansa provide the first direct evidence of moulting in concilitergans. The lack of dorsal ecdysial sutures suggests a marginal moulting strategy similar to extant Xiphosura. A revised phylogeny of Trilobitomorpha supports Arthroaspis bergstroemi from the early Cambrian Sirius Passet of Greenland as the earliest branching concilitergan and prompts a new diagnosis for the clade. Our results inform the internal relationships within Conciliterga and formalize the families Helmetiidae (Helmetia, Rhombicalvaria, Haifengella and Kuamaia) and Tegopeltidae (Tegopelte, Skioldia, Saperion).

(SCFs) have disclosed a record of organically preserved faunas from Cambrian epeiric seas. Their phylogenetically and functionally derived components, including probable crown-group crustaceans and molluscs, are absent from the 'exceptional' palaeoenvironmental settings captured by Burgess Shale-type (BST) macrofossil biotas. This apparent segregation of SCF and BST-macrofossil deposits has led to contrasting hypotheses on whether their faunal differences reflect genuine ecological patterns or overriding taphonomic controls. We report a new, exceptionally diverse SCF biota from the Cambrian Hess River Formation of the Northwest Territories (Canada), which occupied an offshore slope setting. The Hess River biota, hosted by a single shale sample, rivals the Burgess Shale in its disparity of bilaterian body plans, providing a microfossil counterpoint to the regional record of BST-macrofossil faunas from similar deeper-water palaeoenvironments. The Hess River SCFs comprise exceptionally preserved ecdysozoan and spiralian sclerites, arthropod mouthparts, semi-articulated wiwaxiids, problematica and pterobranchs, but no recognizable crown molluscs or crustaceans. The similarities between the Hess River fauna and classic deeper-water BST-macrofossil biotas suggest significant palaeoecological overlap, robust to their distinct taphonomic expressions. This upholds the existence of comparatively modern communities in Cambrian epeiric settings, distinct from the faunas populating both BST-macrofossil biotas and SCF assemblages sampling similar palaeoenvironments.

Abstract Over the last 50 years, paleobiology has made great strides in illuminating organisms and ecosystems in deep time through study of the often-curious nature of the fossil record itself. Among fossil deposits, none are as enigmatic or as important to our understanding of the history of life as Konservat-Lagerstätten, deposits that preserve soft-bodied fossils and thereby retain disproportionately large amounts of paleobiological information. While Konservat-Lagerstätten are often viewed as curiosities of the fossil record, decades of study have led to a better understanding of the environments and circumstances of exceptional fossilization.Whereas most types of exceptional preservation require very specific sets of conditions, which are rare but can occur at any time, Seilacher noted the problem of “anactualistic” modes of exceptional preservation, defined as modes of fossilization that are restricted in time and that no longer occur. Here, we focus on anactualistic preservation and the widely recognized overrepresentation of Konservat-Lagerstätten in the Ediacaran and early Paleozoic. While exceptional fossil deposits of Ediacaran, Cambrian, and Early Ordovician age encompass a number of modes of fossilization, the signal of exceptional preservation is driven by only two modes, Ediacara-type and Burgess Shale–type preservation. Both are “extinct” modes of fossilization that are no longer present in marine environments. We consider the controls that promoted widespread anactualistic preservation in the Ediacaran and early Paleozoic and their implications for the environmental conditions in which complex life first proliferated in the oceans.

The diversity of cephalic morphologies in mandibulates (myriapods and pancrustaceans) was key to their evolutionary success. A group of Cambrian bivalved arthropods called hymenocarines exhibit diagnostic mandibulate traits that illustrate this diversity, but many forms are still poorly known. These include the odaraiids, typified by Odaraia alata from the Burgess Shale (Wuliuan), characterized by its unique tubular carapace and rudder-like tail fan, and one of the largest Cambrian euarthropods at nearly 20 cm in length. Unfortunately, odaraiid cephalic anatomy has been largely unknown, limiting evolutionary scenarios and putting their mandibulate affinities into question. Here, we reinvestigate Odaraia based on new specimens from the Burgess Shale and describe exquisitely preserved mandibles with teeth and adjacent structures: a hypostome, maxillae and potential paragnaths. These structures can be homologized with those of Cambrian fuxianhuiids and extant mandibulates, and suggest that the ancestral mandibulate head could have had a limbless segment but retained its plasticity, allowing for limb re-expression within Pancrustacea. Furthermore, we show the presence of limbs with spinose endites which created a suspension-feeding structure. This discovery provides morphological evidence for suspension feeding among large Cambrian euarthropods and evinces the increasing exploitation of planktonic resources in Cambrian pelagic food webs.

Unveiling an ignored taphonomic window in the early Cambrian Chengjiang Biota

The evolution of articulated sclerites via soft membranes, termed arthrodization, is arguably one of the most critical innovations in animals. Defining the megaphylum Arthropoda, the arthrodization of appendages, or arthropodization, likely predated that of the body, the combination of both being diagnostic of true arthropods (Euarthropoda) – all of these innovations occurring during the Cambrian explosion. Here, thanks to dozens of exceptionally preserved fossils from the Cambrian Wuliuan Stage Burgess Shale (Tulip Beds locality on Mount Stephen, British Columbia, Canada), we show that a distinct but comparable system of imbricated sclerotic elements evolved in the paraphyletic sister group of arthropods, the lobopodians. Entothyreos synnaustrus gen. et sp. nov. has characteristic body plan features of the Collinsovermidae (order Luolishaniida), including anterior limbs for suspension-feeding and stout anchoring posterior limbs. Uniquely, however, E. synnaustrus also displays segmental sclerotic sheets along the trunk, covered in a thin layer of integument, as well as overlapping sclerotized annuli on posterior-most limbs. While the latter elements likely served a protective function, the dorsolateral trunk sheets, which also carry spines, may have facilitated body erection and suspension-feeding. Other luolishaniids possess separate ring-like structures connecting the base of metameric spines which are covered by the apical layer of the lobopodian integument. E. synnaustrus and related taxa illustrate, therefore, an arguably parallel evolution of arthropod-like morphoanatomical features early during the rise of panarthropods. This finding broadens our perspectives on the uniqueness of major synapomorphies and the importance of including canalization in macroevolutionary narratives. http://zoobank.org/urn:lsid:zoobank.org.pub:D4A01784-E587-481A-AB04-A812B4AAE422

Fault-controlled, hydrothermal dolomitization often occurs at margins between shallow-water carbonate platforms and deep-water sedimentary basins. In western Canada, for example, the platform margin between the Cathedral Formation and the Burgess Shale Formation has been dolomitized at temperatures up to ~200 °C, with local magnesite, talc, and clinochlore mineralization. At the same time, the Burgess Shale Formation includes exceptional fossils that provide key evidence of the radiation of the animal phyla during the Cambrian Period (541 to 485.4 Ma). This lagerstätte and Mg-rich minerals within the adjacent and underlying strata, however, have been critically understudied. Here we show, using carbonate U-Pb geochronology, that western Canada was tectonically active and subject to hydrothermal dolomitization during the Middle Cambrian (Miaolingian Epoch) to Middle Ordovician (488.1 ± 18.8 Ma). These results extend the latest stages of rifting along the western margin of Laurentia into the Paleozoic, while also suggesting that the dolomitization of the Cathedral Formation occurred at the same time as the deposition of the Burgess Shale lagerstätte.

BackgroundThe early Cambrian arthropod clade Megacheira, also referred to as great appendage arthropods, comprised a group of diminutive and elongated predators during the early Palaeozoic era, around 518 million years ago. In addition to those identified in the mid-Cambrian Burgess Shale biota, numerous species are documented in the renowned 518-million-year-old Chengjiang biota of South China. Notably, one species, Tanglangia longicaudata, has remained inadequately understood due to limited available material and technological constraints. In this study, we, for the first time, examined eight fossil specimens (six individuals) utilizing state-of-the-art μCT and computer-based 3D rendering techniques to unveil the hitherto hidden ventral and appendicular morphology of this species.ResultsWe have identified a set of slender endopodites gradually narrowing distally, along with a leaf-shaped exopodite adorned with fringed setae along its margins, and a small putative exite attached to the basipodite. Our techniques have further revealed the presence of four pairs of biramous appendages in the head, aligning with the recently reported six-segmented head in other early euarthropods. Additionally, we have discerned two peduncle elements for the great appendage. These findings underscore that, despite the morphological diversity observed in early euarthropods, there exists similarity in appendicular morphology across various groups. In addition, we critically examine the existing literature on this taxon, disentangling previous mislabelings, mentions, descriptions, and, most importantly, illustrations.ConclusionsThe μCT-based investigation of fossil material of Tanglangia longicaudata, a distinctive early Cambrian euarthropod from the renowned Chengjiang biota, enhances our comprehensive understanding of the evolutionary morphology of the Megacheira. Its overall morphological features, including large cup-shaped eyes, raptorial great appendages, and a remarkably elongated telson, suggest its potential ecological role as a crepuscular predator and adept swimmer in turbid waters.

In 1989, Stephen Jay Gould published his Wonderful Life: “High in the Canadian Rockies is a small limestone quarry formed 530 million years ago called the Burgess Shale. It holds the remains of an ancient sea where dozens of strange creatures lived a forgotten corner of evolution preserved in awesome detail. In this book Stephen Jay Gould explores what the Burgess Shale tells us about evolution and the nature of history”. Gould, based upon a reinterpretation of the famous fossils from the Burgess Shale, proposed a conclusion that was revolutionary at the time, because it was centred on the notion of contingency in the course of biological evolution. The very last paragraph displayed in particular the following sentences: “This response does not cite a single law of nature; it embodies no statement about predictable evolutionary pathways, no calculation of probabilities based on general rules of anatomy or ecology. I do not think that any ‘higher’ answer can be given, and I cannot imagine that any resolution could be more fascinating.” In this essay, I intend to criticise Gould’s conclusions and to provide a new reinterpretation of the history of life, based not on contingency but on the inevitability of evolution.

Abstract Burgess Shale–type (BST) Lagerstätten record an exceptional variety of Cambrian soft-bodied fauna, yet these deposits are typically restricted to outboard depositional settings >1000 km from the paleocoastline. For shallow, well-oxygenated shelf environments, our knowledge of non-mineralized animals (the majority of diversity) is severely limited, giving rise to substantial bias in our perception of Cambrian biotas. An alternate means of detecting soft-bodied Cambrian fauna, independent of paleobathymetry, is to use acid maceration to extract microscopic organic remains of non-mineralized animals, known as “small carbonaceous fossils” (SCFs). Here, a hitherto unknown diversity of Cambrian arthropod and mollusk remains are reported from shallow-marine sediments (Cambrian Stage 3 Mickwitzia Sandstone, Sweden). These microfossils comprise a variety of arthropod cuticles preserving sub-micron-scale anatomy alongside abundant radular mouthparts from mollusks—among the oldest known arthropod and molluscan SCFs on record. Significantly, at least three distinct types of fossil radula are identifiable (uniseriate, distichous, and polystichous forms), revealing that substantial diversification of the basic molluscan radula had already taken place by the early Cambrian. These cryptic elements of the biota—otherwise undetectable in such deposits—offer novel insights into Cambrian primary consumers as well as aspects of the fauna that are absent from deeper-water BST deposits.

It is often thought that the primitive is simpler, and that the complex is generated from the simple by some process of self-assembly or self-organization, which ultimately consists of the spontaneous and fortuitous collision of elementary units. This idea is included in the Darwinian theory of evolution, to which is added the competitive mechanism of natural selection. To test this view, we studied the early evolution of arthropods. Twelve groups of arthropods belonging to the Burgess Shale, Orsten Lagerstätte, and extant primitive groups were selected, their external morphology abstracted and codified in the language of network theory. The analysis of these networks through different network measures (network parameters, topological descriptors, complexity measures) was used to carry out a Principal Component Analysis (PCA) and a Hierarchical Cluster Analysis (HCA), which allowed us to obtain an evolutionary tree with distinctive/novel features. The analysis of centrality measures revealed that these measures decreased throughout the evolutionary process, and led to the creation of the concept of evolutionary developmental potential. This potential, which measures the capacity of a morphological unit to generate changes in its surroundings, is concomitantly reduced throughout the evolutionary process, and demonstrates that the primitive is not simple but has a potential that unfolds during this process. This means for us the first empirical evolutionary evidence of our theory of evolution as a process of unfolding.