Tectonic trigger to the first major extinction of the Phanerozoic: The early Cambrian Sinsk event.

The Cambrian explosion, one of the most consequential biological revolutions in Earth history, occurred in two phases separated by the Sinsk event, the first major extinction of the Phanerozoic. Trilobite fossil data show that Series 2 strata in the Ross Orogen, Antarctica, and Delamerian Orogen, Australia, record nearly identical and synchronous tectono-sedimentary shifts marking the Sinsk event. These resulted from an abrupt pulse of contractional supracrustal deformation on both continents during the Pararaia janeae trilobite Zone. The Sinsk event extinction was triggered by initial Ross/Delamerian supracrustal contraction along the edge of Gondwana, which caused a cascading series of geodynamic, paleoenvironmental, and biotic changes, including (i) loss of shallow marine carbonate habitats along the Gondwanan margin; (ii) tectonic transformation to extensional tectonics within the Gondwanan interior; (iii) extrusion of the Kalkarindji large igneous province; (iv) release of large volumes of volcanic gasses; and (v) rapid climatic change, including incursions of marine anoxic waters and collapse of shallow marine ecosystems.

A giant stem-group chaetognath.

Chaetognaths, with their characteristic grasping spines, are the oldest known pelagic predators, found in the lowest Cambrian (Terreneuvian). Here, we describe a large stem chaetognath, Timorebestia koprii gen. et sp. nov., from the lower Cambrian Sirius Passet Lagerstätte, which exhibits lateral and caudal fins, a distinct head region with long antennae and a jaw apparatus similar to Amiskwia sagittiformis. Amiskwia has previously been interpreted as a total-group chaetognathiferan, as either a stem-chaetognath or gnathostomulid. We show that T. koprii shares a ventral ganglion with chaetognaths to the exclusion of other animal groups, firmly placing these fossils on the chaetognath stem. The large size (up to 30 cm) and gut contents in T. koprii suggest that early chaetognaths occupied a higher trophic position in pelagic food chains than today.

Homology of the head sensory structures between Heterotardigrada and Eutardigrada supported in a new species of water bear (Ramazzottiidae: Ramazzottius).

Phylum Tardigrada is represented by microscopic eight-legged panarthropods that inhabit terrestrial and marine environments. Although tardigrades are emerging model animals for areas of research including physiology, evolutionary biology, and astrobiology, knowledge of their external morphology remains insufficient. For instance, homologies between marine and terrestrial relatives largely remain unexplored. In the present study we provide detailed pictures of the head sensory organs in a new tardigrade, Ramazzottius groenlandensis sp. nov. Specimens were collected from a mixed moss and lichen sample on Ella Island, East Greenland. The new species differs from congeneric species in the presence of polygonal sculpturing on the dorsal cuticle, which is accentuated in the posterior region of the body, a lateral papilla on leg IV, and distinctive egg morphology. A Bayesian phylogenetic analysis (18S rRNA + 28S rRNA + COI) places the new species within the genus Ramazzottius with high confidence. Interestingly, the new species shows a full set of well-developed cephalic organs, which correspond to all sensory fields found in eutardigrades. Details on the full set of head organs were present only for heterotardigrades. The surface of these organs is covered with small pores, which presumably play a sensory role. This discovery suggests the homology of head sensory structures between heterotardigrades and eutardigrades, implying that the distinctive arrangement and positioning of sensory organs on the head is a plesiomorphic feature of tardigrades. Moreover, we find that the Ramazzottius oberhaeuseri morphotype forms a morphogroup, not a monophyletic species complex.

Trilobite hypostome as a fusion of anterior sclerite and labrum.

The trilobite hypostome is a biomineralized ventral plate that covers the mouth, but its evolutionary origin remains controversial. The labrum is a lobe-like structure that can take on variety of shapes in front of the mouth in arthropods, while the anterior sclerite refers to a cuticular plate articulated to the anterior margin of the head in some Cambrian arthropods. Here I present a perspective that views the trilobite hypostome as a fusion of the anterior sclerite and the labrum based on anatomical, topological, and developmental evidence. According to this perspective, the anterior lobe of the hypostome originated from the anterior sclerite, while the posterior lobe reflects a remnant of the sclerotized cover of the labrum. The convex anterior lobe housed the root of the eye stalks, represented by the palpebral ridges and the hypostomal wing, and the posterior lobe occasionally developed a pair of posterolateral extensions, as do the labra. The position of the antennal insertion was located in front of the posterior lobe, displaying a similar topology to the Cambrian arthropods with the labrum. The hypostome was present in many artiopodans except for the Conciliterga, in which the anterior sclerite was separate from the labrum.

Cambrian lobopodians shed light on the origin of the tardigrade body plan.

Phylum Tardigrada (water bears), well known for their cryptobiosis, includes small invertebrates with four paired limbs and is divided into two classes: Eutardigrada and Heterotardigrada. The evolutionary origin of Tardigrada is known to lie within the lobopodians, which are extinct soft-bodied worms with lobopodous limbs mostly discovered at sites of exceptionally well-preserved fossils. Contrary to their closest relatives, onychophorans and euarthropods, the origin of morphological characters of tardigrades remains unclear, and detailed comparison with the lobopodians has not been well explored. Here, we present detailed morphological comparison between tardigrades and Cambrian lobopodians, with a phylogenetic analysis encompassing most of the lobopodians and three panarthropod phyla. The results indicate that the ancestral tardigrades likely had a Cambrian lobopodian-like morphology and shared most recent ancestry with the luolishaniids. Internal relationships within Tardigrada indicate that the ancestral tardigrade had a vermiform body shape without segmental plates, but possessed cuticular structures surrounding the mouth opening, and lobopodous legs terminating with claws, but without digits. This finding is in contrast to the long-standing stygarctid-like ancestor hypothesis. The highly compact and miniaturized body plan of tardigrades evolved after the tardigrade lineage diverged from an ancient shared ancestor with the luolishaniids.

Interpreting fossilized nervous tissues.

Paleoneuranatomy is an emerging subfield of paleontological research with great potential for the study of evolution. However, the interpretation of fossilized nervous tissues is a difficult task and presently lacks a rigorous methodology. We critically review here cases of neural tissue preservation reported in Cambrian arthropods, following a set of fundamental paleontological criteria for their recognition. These criteria are based on a variety of taphonomic parameters and account for morphoanatomical complexity. Application of these criteria shows that firm evidence for fossilized nervous tissues is less abundant and detailed than previously reported, and we synthesize here evidence that has stronger support. We argue that the vascular system, and in particular its lacunae, may be central to the understanding of many of the fossilized peri-intestinal features known across Cambrian arthropods. In conclusion, our results suggest the need for caution in the interpretation of evidence for fossilized neural tissue, which will increase the accuracy of evolutionary scenarios. Also see the video abstract here: https://youtu.be/2_JlQepRTb0.

Fecal microbiota and diets of muskox female adults and calves.

In mammals, the gut microbiome is vertically transmitted during maternal lactation at birth. In this study, we investigated the gut microbiome and diets of muskox, a large herbivore inhabiting in the high Arctic. We compared the microbiota composition using bacterial 16S rRNA gene sequencing and diets using stable isotope analysis of muskox feces of six female adults and four calves on Ella Island, East Greenland. Firmicutes were the most abundant bacterial phylum in both the adults and calves, comprising 94.36% and 94.03%, respectively. Significant differences were observed in the relative abundance of the two Firmicutes families. The adults were primarily dominated by Ruminococcaceae (73.90%), and the calves were dominated by both Ruminococcaceae (56.25%) and Lachnospiraceae (24.00%). Stable isotope analysis of the feces in the study area revealed that both adults and calves had similar ranges of 13C and 15N, likely derived from the dominant diet plants. Despite their similar diets, the different gut microbiome compositions in muskox adults and calves indicate that the gut microbiome of the calves may not be fully colonized to the extent of that of the adults.

The complete mitochondrial genome of the Arctic fairy shrimp Branchinectapaludosa (Müller, 1788) (Anostraca, Branchinectidae) from Sirius Passet, North Greenland.

Here we report the complete mitochondrial genome of the Arctic fairy shrimp, Branchinectapaludosa (Müller, 1788) (Anostraca, Branchinectidae), which was collected in the High Arctic of North Greenland. A complete 16,059 bp mitochondrion of B.paludosa was sequenced and assembled with the Illumina next generation sequencing platform. The B.paludosa mitogenome contains 13 PCGs, 22 tRNAs and 2 rRNA genes that are commonly observed in most metazoans and shows the conserved gene arrangement pattern of Anostraca. Our results of the phylogenomic analysis are consistent with the previous phylogenetic relationship, based on nuclear 18S ribosomal DNA. The B.paludosa mitogenome will be useful for understanding the geographical distribution and phylogenetic relationship of anostracans.

Enduring evolutionary embellishment of cloudinids in the Cambrian.

The Ediacaran-Cambrian transition and the following Cambrian Explosion are among the most fundamental events in the evolutionary history of animals. Understanding these events is enhanced when phylogenetic linkages can be established among animal fossils across this interval and their trait evolution monitored. Doing this is challenging because the fossil record of animal lineages that span this transition is sparse, preserved morphologies generally simple and lifestyles in the Ediacaran and Cambrian commonly quite different. Here, we identify derived characters linking some members of an enigmatic animal group, the cloudinids, which first appeared in the Late Ediacaran, to animals with cnidarian affinity from the Cambrian Series 2 and the Miaolingian. Accordingly, we present the first case of an animal lineage represented in the Ediacaran that endured and diversified successfully throughout the Cambrian Explosion by embellishing its overall robustness and structural complexity. Among other features, dichotomous branching, present in some early cloudinids, compares closely with a cnidarian asexual reproduction mode. Tracking this morphological change from Late Ediacaran to the Miaolingian provides a unique glimpse into how a primeval animal group responded during the Cambrian Explosion.

Characterization of the complete mitochondrial genome of the scale worm, Eunoe nodosa (Phyllodocida; Polynoidae) from the Beaufort Sea.

To increase the mitogenome data available for robust phylogeny, we sequenced the complete mitochondrial DNA of the scale worm Eunoe nodosa (Sars, 1861) in the family Polynoidae of the order Phyllodocida. The complete mitogenome has 15,366 bp and has 28.9% A, 13.2% C, 19.0% G, and 38.8% T. Using MITOS and tRNAscan-SE, we identified the 13 typical protein-coding genes (PCGs), 2 ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes, and a non-coding region. Phylogenomic analysis based on 27 in-group taxa belonging to five families of the subclass Errantia show congruence with the published phylogenetic relationship within the Polynoidae, in which E. nodosa lies in the clade of shallow water species.

Complete mitochondrial genome of the marine polychaete, Nereis zonata (Phyllodocida, Nereididae) isolated from the Beaufort Sea.

Here, we present the first whole mitogenome sequence of the marine polychaete, Nereis zonata, isolated from the Beaufort Sea. The mitochondrial genome of N. zonata is 15,757 bp in length and consists of 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and a non-coding region that is typical of polychaetes. GC content of the N. zonata mitogenome is 37.2%. A maximum-likelihood gene tree based on the N. zonata mitogenome combined with previously published annelid mitogenome data revealed that N. zonata is clustered with Cheilonereis cyclurus, which form a sister group to Nereis sp.

The first complete mitochondrial genome from the family Solasteridae, Crossaster papposus (Echinodermata, Asteroidea).

The common sunstar, Crossaster papposus, belongs to the family Solasteridae whose ordinal classification has been unstable. Here, for the first time, we assembled and annotated the complete mitochondrial genome of the common sunstar, C. papposus Linnaeus, 1767. The circular genome of C. papposus is 16,335 bp in length and contains 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, a control region, and large and small ribosomal subunits. The overall genomic structure and gene arrangement were identical to the reported mitochondrial genomes of sea star species, and a phylogenetic analysis of 13 PCGs recovers a closest relationship with the derived cluster of the paraphyletic order Valvatida.

Integrative description of a new Dactylobiotus (Eutardigrada: Parachela) from Antarctica that reveals an intraspecific variation in tardigrade egg morphology.

Tardigrades constitute one of the most important group in the challenging Antarctic terrestrial ecosystem. Living in various habitats, tardigrades play major roles as consumers and decomposers in the trophic networks of Antarctic terrestrial and freshwater environments; yet we still know little about their biodiversity. The Eutardigrada is a species rich class, for which the eggshell morphology is one of the key morphological characters. Tardigrade egg morphology shows a diverse appearance, and it is known that, despite rare, intraspecific variation is caused by seasonality, epigenetics, and external environmental conditions. Here we report Dactylobiotus ovimutans sp. nov. from King George Island, Antarctica. Interestingly, we observed a range of eggshell morphologies from the new species, although the population was cultured under controlled laboratory condition. Thus, seasonality, environmental conditions, and food source are eliminated, leaving an epigenetic factor as a main cause for variability in this case.

Molecular palaeontology illuminates the evolution of ecdysozoan vision.

Colour vision is known to have arisen only twice-once in Vertebrata and once within the Ecdysozoa, in Arthropoda. However, the evolutionary history of ecdysozoan vision is unclear. At the molecular level, visual pigments, composed of a chromophore and a protein belonging to the opsin family, have different spectral sensitivities and these mediate colour vision. At the morphological level, ecdysozoan vision is conveyed by eyes of variable levels of complexity; from the simple ocelli observed in the velvet worms (phylum Onychophora) to the marvellously complex eyes of insects, spiders, and crustaceans. Here, we explore the evolution of ecdysozoan vision at both the molecular and morphological level; combining analysis of a large-scale opsin dataset that includes previously unknown ecdysozoan opsins with morphological analyses of key Cambrian fossils with preserved eye structures. We found that while several non-arthropod ecdysozoan lineages have multiple opsins, arthropod multi-opsin vision evolved through a series of gene duplications that were fixed in a period of 35-71 million years (Ma) along the stem arthropod lineage. Our integrative study of the fossil and molecular record of vision indicates that fossils with more complex eyes were likely to have possessed a larger complement of opsin genes.

Brain and eyes of Kerygmachela reveal protocerebral ancestry of the panarthropod head.

Recent discoveries of fossil nervous tissue in Cambrian fossils have allowed researchers to trace the origin and evolution of the complex arthropod head and brain based on stem groups close to the origin of the clade, rather than on extant, highly derived members. Here we show that Kerygmachela from Sirius Passet, North Greenland, a primitive stem-group euarthropod, exhibits a diminutive (protocerebral) brain that innervates both the eyes and frontal appendages. It has been surmised, based on developmental evidence, that the ancestor of vertebrates and arthropods had a tripartite brain, which is refuted by the fossil evidence presented here. Furthermore, based on the discovery of eyes in Kerygmachela, we suggest that the complex compound eyes in arthropods evolved from simple ocelli, present in onychophorans and tardigrades, rather than through the incorporation of a set of modified limbs.

Post-embryonic development of the Early Ordovician (ca. 480 Ma) trilobite Apatokephalus latilimbatus Peng, 1990 and the evolution of metamorphosis.

In many marine invertebrates metamorphosis entails a shift from a free-swimming larva to a benthic juvenile or adult. However, how the metamorphosis-entailing "indirect development" in arthropods arose from direct-developing ancestor is poorly understood. Trilobites left a rich fossil record, and some trilobite lineages had a metamorphosis-undergoing early developmental stage, termed the "asaphoid protaspis"-stage, providing a good opportunity to elucidate the rise of indirect development. Among others, the Ordovician representatives of Remopleuridioidea are known to possess a highly bulbous "asaphoid protaspis," while the Furongian (Late Cambrian) remopleuridioidean genus Haniwa did not possess it. Here we show the post-embryonic development of the remopleuridioidean trilobite, Apatokephalus latilimbatus, from the Tremadocian (485.4 Ma-477.7 Ma) Dongjeom Formation, Korea. The post-embryonic development of A. latilimbatus contains a free-swimming "commutavi protaspis" (a term replacing "asaphoid protaspis"). Interestingly, the earlier protaspid stage shows more similar morphology and size to the meraspis than the commutavi protaspid stage does. This indicates that the commutavi protaspid stage was intercalated into the ancestral direct development as a specialized stage for a better dispersal, and thus the "commutavi protaspis" of A. latilimbatus represents the initial phase of the evolution of indirect development. The duration of the free-swimming phase became longer in more derived remoplueridioidean trilobites, implying that the intercalated free-swimming strategy became emphasized during subsequent evolution. The morphological gap between the commutavi protaspis and the subsequent earliest meraspis provides a convincing case for the "selective independence" of developmental stages, explaining the various morphologies of commutavi protaspides in many trilobite lineages.