Lagerstatten

ABSTRACT The Guzhangian Weeks Formation (House Range, Utah, USA) contains a virtually unstudied but diverse assemblage of ‘soft-bodied’ organisms. This fauna includes several enigmatic appendages of arthropods that are described in this contribution. Six appendages (two isolated and four paired appendages) are interpreted as frontal appendages of a probably new species of Anomalocaris. They are characterized by a slender morphology, 14 podomeres, ventral spines alternating in size, up to three auxiliary spines per ventral spine, and only two dorsal spines. Another isolated appendage is also tentatively assigned to Anomalocaris, but it exhibits a more robust morphology, a stronger distal tapering, and apparently simple ventral spines, suggesting that it may represent a distinct taxon. These frontal appendages represent the youngest occurrence of anomalocaridids in Laurentia and demonstrate the persistence of older, Burgess Shale-type taxa in the Weeks Formation. An assemblage of four antenniform and six robust and heavily-armed appendages is also described. These are interpreted as the serially arranged, anterior appendages of a single individual of an undetermined arthropod species. This association of three pairs of robust, spiny appendages with two pairs of antenniform structures in a Cambrian arthropod is unique.

The Cretaceous greenhouse 'waterworld' was dominated by shallow epicontinental seas. Modern-day Colombia, northern South America, had a Cretaceous palaeolatitude of 5-7 degrees north of the equator, and the current Alto Ricaurte region (Eastern Cordillera) preserves the Lower Cretaceous (Hauterivian-Aptian) deposits of the Paja Formation. The Paja Formation is dominated by black to variegated mudrocks, with lenses of gypsum and calcite, abundant pyrite, and an exceptional fossil fauna. Fossils include virtually complete and articulated marine reptiles, including both long-and short-necked plesiosaurs, numerous ichthyosaurs, and marine turtles (some with eggs preserved inside the body cavity), diverse fish, numerous pelagic ammonites, and pseudo-planktonic wood-boring bivalves. However, the Paja Formation benthic fauna is depauperate, mainly thin-shelled bivalves with abundant evidence for bottom-living microbial mats. The exquisite preservation of the pelagic fauna permits the Paja Formation of the Alto Ricaurte to be recognised as a rare, Lower Cre-taceous marine vertebrae Lagerst´'atte. The fossil fauna strongly suggests well-oxygenated surface waters, but dysoxic-anoxic bottom waters, separated by a temperature and/or density defined pycnocline. This interpretation argues against the previously proposed intertidal evaporitic (sabkha) environment for Paja Formation seas, and sedimentological, palaeobio-logical, taphonomic and diagenetic data is presented to argue for deeper water conditions for the Paja Formation Lagerst´'atte.

The Lower Cretaceous (Hauterivian-Aptian) Paja Formation Lagerstatte of the modern-day Alto Ricaurte region, Eastern Cordillera of Colombia (northern South America) resulted from flooding of an extensional back-arc basin close to the palaeo-equator. The Paja Formation fossil fauna is dominated by pelagic organisms (marine reptiles, fish, ammonites), with abundant evidence of continental influence, including a dinosaur and abundant terrestrial plant remains. Within the Arcillolitas Abigarradas (middle) Member of the Paja Formation, some horizons preserve abundant wood, which is frequently bored by pseudoplanktonic pho-ladoid bivalves, commonly referred to as 'shipworms' or 'piddocks'. The presence of wood boring bivalves in Paja Formation seas indicates the continued presence of xylic substrates, and long residence time of floating wood. The abundance of wood was a result of nearby landmasses with a well-lit, equatorial location 5-7 degrees north of the palaeoequator. These surrounding landmasses were under the ascending limb of the Hadley atmospheric cell, with high rainfall and high terrestrial nutrient production due to subaerial weathering, which taken with elevated Cretaceous partial pressures of atmospheric CO2, was conducive to high ('tropical rainforest') terrestrial plant primary productivity. Plentiful rainfall led to abundant runoff, and proximity to the northern 'hurricane belt' meant the area was frequently affected by tropical storms, leading to a constant supply of wood to the Paja Formation basin. However, not all Paja Formation fossil wood is bored, indicating some woods were not suitable for Cretaceous wood boring bivalve infestation. This may have been due to excessive wood hardness, rapid water logging and sinking, or chemical or other factors that prevented pholadoid larval attachment. However, that some wood floated for long periods is indicated by the presence of different size classes of wood boring bivalves in a single fossil log. Paja Formation bivalves are preserved in wood replaced by dark-coloured (organic-rich) sparry (coarsely crystalline) calcite, whereas rarer silicified wood does not preserve pholadoid borings. This suggests different taphonomic pathways for the two modes of Paja Formation wood preservation.

The Weeks Formation in Utah is the youngest (c. 499 Ma) and least studied Cambrian Lagerstätte of the western USA. It preserves a diverse, exceptionally preserved fauna that inhabited a relatively deep water environment at the offshore margin of a carbonate platform, resembling the setting of the underlying Wheeler and Marjum formations. However, the Weeks fauna differs significantly in composition from the other remarkable biotas of the Cambrian Series 3 of Utah, suggesting a significant Guzhangian faunal restructuring. This bioevent is regarded as the onset of a transitional episode in the history of life, separating the two primary diversifications of the Early Paleozoic. The Weeks fossils have been strongly affected by late diagenetic processes, but some specimens still preserve exquisite anatomical details. Supplementary material: a supplementary text (material and methods), four supplementary tables (compositions of the lower and upper Weeks faunas, structure of the upper Weeks fauna, and results of the similarity analyses), and a supplementary data file (generic presence/absence matrix) are available at https://doi.

– A new solutan echinoderm, Pahvanticystis utahensis gen. et sp. nov. is described from the upper part of the Weeks Formation (Guzhangian). The Cambrian (Series 3) succession of the central House Range in western Utah documents the early diversification of the class Soluta, which is characterized by a major ecological transition from sessile, 'pelmatozoan' primitive taxa (Colei-carpus, Wheeler Formation), to more and more vagile, temporarily attached (Castericystis, Marjum Formation), to mostly unattached, 'homalozoan' derived forms (Pahvanticystis, Weeks Formation). The morphology of Pahvanticystis is remarkably intermediate between those of Castericystis and Minervaecystis. Its twisted, flattened dististele possibly represents an adaptation for a more efficient crawling atop soft substrates. This morphological feature also questions the phylogenetic relationships between syringocrinid and dendrocystitid solutans, and the possible evolution of the latter from basal, Pahvanticystis-or Minervaecystis-like syringocrinids by paedomorphosis.

The Drumian Wheeler Formation preserves one of the most diverse exceptionally preserved faunas of the Cambrian period. Here we describe Messorocaris magna gen. et sp. nov., a new non-biomineralizing arthropod from this formation tentatively assigned to the family Sanctacarididae. The new taxon exhibits a vaulted axial region, and wide pleural regions forming sickle-shaped lateral extensions in the trunk, a character particularly distinctive within the Sanctacarididae. This description provides an opportunity to stress the fact that the 'Wheeler fauna' encompasses two distinct assemblages, as confirmed by similarity analysis. These contemporaneous faunas lived at different bathymetries, and should be treated as separate entities.

ABSTRACT The Guzhangian Weeks Formation (House Range, Utah, USA) contains a virtually unstudied but diverse assemblage of ‘soft-bodied’ organisms. This fauna includes several enigmatic appendages of arthropods that are described in this contribution. Six appendages (two isolated and four paired appendages) are interpreted as frontal appendages of a probably new species of Anomalocaris. They are characterized by a slender morphology, 14 podomeres, ventral spines alternating in size, up to three auxiliary spines per ventral spine, and only two dorsal spines. Another isolated appendage is also tentatively assigned to Anomalocaris, but it exhibits a more robust morphology, a stronger distal tapering, and apparently simple ventral spines, suggesting that it may represent a distinct taxon. These frontal appendages represent the youngest occurrence of anomalocaridids in Laurentia and demonstrate the persistence of older, Burgess Shale-type taxa in the Weeks Formation. An assemblage of four antenniform and six robust and heavily-armed appendages is also described. These are interpreted as the serially arranged, anterior appendages of a single individual of an undetermined arthropod species. This association of three pairs of robust, spiny appendages with two pairs of antenniform structures in a Cambrian arthropod is unique.

ABSTRACT he fossil record represents an important test to molecular divergence estimates, with known occurrences representing minimum divergence times for sister taxa. As such, accurately placing fossils in phylogenies is integral to understanding the patterns and processes that shape the tree of life. The chelicerate order Xiphosura comprises classic archetypes of morphological stasis, with the earliest known Ordovician representatives exhibiting all key morphological characteristics of the group. Molecular studies on the four extant species consistently retrieve a basal split between Limulinae and Tachypleinae, but conflict regarding the relationships of the three Asian species. Molecular divergence estimates using either no or a single fossil calibration point infer a Cretaceous or Palaeogene origin for Limulidae and a Palaeogene or Neogene origin for Tachypleinae and Tachypleus. Here, we present male and female specimens of Tachypleus syriacus (=‘Mesolimulus’ syriacus) from the Cretaceous of Lebanon, revealing an anterior scalloped carapace margin in males—a derived condition of sexual dimorphism shared with Tachypleus tridentatus. Morphological phylogenetic analysis of total group Limulidae retrieves a monophyletic Tachypleus with a minimum divergence time during the Cretaceous, while crown-group Tachypleinae and Limulidae are both present during the Triassic, showing that molecular clock analyses have significantly underestimated the divergence times for these taxa.

The Late Ordovician (Hirnantian, approximately 445 million years ago) extinction event was among the largest known, with 85% species loss [ 1 ]. Post-extinction survival faunas are invariably low diversity, especially benthic communities [ 2 ], but ecological structure was restored relatively rapidly [ 1 ]. This pattern, however, reflects organisms with robust skeletons, as only one exceptionally preserved Hirnantian fossil biota was previously known [ 3, 4 ]; in particular, almost no Hirnantian sponges have been recorded. Our study reveals an extraordinarily diverse, sponge-dominated community thriving immediately after the Hirnantian extinction in Zhejiang, South China. Several contemporaneous sites preserve a total diversity of over 75 sponge species, many with preserved soft tissues, in pronounced contrast to normal survival and early recovery faunas. This diversity is unprecedented for any Hirnantian fossil group, and the fauna provides a unique window into a post-extinction ecosystem. The sponges are often large and structurally complex and represent numerous different lineages that survived the extinction. Layers with abundant sponge remains were deposited after other mass extinctions [ 5, 6 ], suggesting a general pattern of sponge abundance during collapse of Phanerozoic marine ecosystems. It is possible that the conditions of ecological collapse increase the particulate food sources for sponges, while they themselves are relatively unaffected by the crises. Furthermore, the abundance of sponges in the Hirnantian sequence of South China may have aided post-extinction ecosystem recovery by stabilizing the sediment surface, allowing sessile suspension feeders such as brachiopods, corals, and bryozoans to recover rapidly.

An unusual fork-tailed coelacanth from the Lower Triassic Sulphur Mountain Formation of British Columbia, Canada, marks the first considerable departure in actinistian body form since the Mississippian Period. Rebellatrix divaricerca, gen. et sp. nov., is unique among coelacanths in its possession of a bifurcated caudal fin, reduced segmentation of fin rays, and fusion of caudal fin elements. Parsimony analysis of a character-taxon matrix of 109 characters and 29 taxa recovered R. divaricerca as sister taxon to Latimerioidei. The family Rebellatricidae is erected to include only R. divaricerca. This novel body shape of the new coelacanth raises questions about the idea that coelacanths were morphologically static following the Mississippian. Rebellatrix divaricerca also emphasizes the morphological diversity of coelacanths during the Early Triassic, a time when species diversity of coelacanths was also highest. The slender, fork-tailed body form suggests fast swimming and an active lifestyle, unique among known fossil and extant coelacanths.