Linkage mapping and QTL analysis of growth traits in Rhopilema esculentum.

R. esculentum is a popular seafood in Asian countries and an economic marine fishery resource in China. However, the genetic linkage map and growth-related molecular markers are still lacking, hindering marker assisted selection (MAS) for genetic improvement of R. esculentum. Therefore, we firstly used 2b-restriction site-associated DNA (2b-RAD) method to sequence 152 R. esculentum specimens and obtained 9100 single nucleotide polymorphism (SNP) markers. A 1456.34 cM linkage map was constructed using 2508 SNP markers with an average interval of 0.58 cM. Then, six quantitative trait loci (QTLs) for umbrella diameter and body weight were detected by QTL analysis based on the new linkage map. The six QTLs are located on four linkage groups (LGs), LG4, LG13, LG14 and LG15, explaining 9.4% to 13.4% of the phenotypic variation. Finally, 27 candidate genes in QTLs regions of LG 14 and 15 were found associated with growth and one gene named RE13670 (sushi, von Willebrand factor type A, EGF and pentraxin domain-containing protein 1-like) may play an important role in controlling the growth of R. esculentum. This study provides valuable information for investigating the growth mechanism and MAS breeding in R. esculentum.

The Diversity of the Endobiotic Bacterial Communities in the Four Jellyfish Species.

The associated microbiota plays an essential role in the life process of jellyfish. The endobiotic bacterial communities from four common jellyfish Phyllorhiza punctata, Cyanea capillata, Chrysaora melanaster, and Aurelia coerulea were comparatively analyzed by 16S rDNA sequencing in this study. Several 1049 OTUs were harvested from a total of 130 183 reads. Tenericutes (68.4%) and Firmicutes (82.1%) are the most abundant phyla in P. punctata and C. melanaster, whereas C. capillata and A. coerulea share the same top phylum Proteobacteria (76.9% vs. 78.3%). The classified OTUs and bacterial abundance greatly decrease from the phylum to genus level. The top 20 matched genera only account for 9.03% of the total community in P. punctata, 48.9% in C. capillata, 83.05% in C. melanaster, and 58.1% in A. coerulea, respectively. The heatmap of the top 50 genera shows that the relative abundances in A. coerulea and C. capillata are far richer than that in P. punctata and C. melanaster. Moreover, a total of 41 predictive functional categories at KEGG level 2 were identified. Our study indicates the independent diversity of the bacterial communities in the four common Scyphomedusae, which might involve in the metabolism and environmental information processing of the hosts.The associated microbiota plays an essential role in the life process of jellyfish. The endobiotic bacterial communities from four common jellyfish Phyllorhiza punctata, Cyanea capillata, Chrysaora melanaster, and Aurelia coerulea were comparatively analyzed by 16S rDNA sequencing in this study. Several 1049 OTUs were harvested from a total of 130 183 reads. Tenericutes (68.4%) and Firmicutes (82.1%) are the most abundant phyla in P. punctata and C. melanaster, whereas C. capillata and A. coerulea share the same top phylum Proteobacteria (76.9% vs. 78.3%). The classified OTUs and bacterial abundance greatly decrease from the phylum to genus level. The top 20 matched genera only account for 9.03% of the total community in P. punctata, 48.9% in C. capillata, 83.05% in C. melanaster, and 58.1% in A. coerulea, respectively. The heatmap of the top 50 genera shows that the relative abundances in A. coerulea and C. capillata are far richer than that in P. punctata and C. melanaster. Moreover, a total of 41 predictive functional categories at KEGG level 2 were identified. Our study indicates the independent diversity of the bacterial communities in the four common Scyphomedusae, which might involve in the metabolism and environmental information processing of the hosts.

A SLC6 transporter cloned from the lion's mane jellyfish (Cnidaria, Scyphozoa) is expressed in neurons.

In the course of recent comparative genomic studies conducted on nervous systems across the phylogeny, current thinking is leaning in favor of more heterogeneity among nervous systems than what was initially expected. The isolation and characterization of molecular components that constitute the cnidarian neuron is not only of interest to the physiologist but also, on a larger scale, to those who study the evolution of nervous systems. Understanding the function of those ancient neurons involves the identification of neurotransmitters and their precursors, the description of nutrients used by neurons for metabolic purposes and the identification of integral membrane proteins that bind to those compounds. Using a molecular cloning strategy targeting membrane proteins that are known to be present in all forms of life, we isolated a member of the solute carrier family 6 from the scyphozoan jellyfish Cyanea capillata. The phylogenetic analysis suggested that the new transporter sequence belongs to an ancestral group of the nutrient amino acid transporter subfamily and is part of a cluster of cnidarian sequences which may translocate the same substrate. We found that the jellyfish transporter is expressed in neurons of the motor nerve net of the animal. To this end, we established an in situ hybridization protocol for the tissues of C. capillata and developed a specific antibody to the jellyfish transporter. Finally, we showed that the gene that codes for the jellyfish transporter also expresses a long non-coding RNA. We hope that this research will contribute to studies that seek to understand what constitutes a neuron in species that belong to an ancient phylum.

Transcriptome analysis of scyphozoan jellyfish Rhopilema esculentum from polyp to medusa identifies potential genes regulating strobilation.

Strobilation is a unique asexual reproduction mode of scyphozoan jellyfish, through which benthic polyp develops into pelagic medusa. It is an orderly metamorphosis process triggered by environmental signals. However, the knowledges of molecular mechanisms under the drastic morphological and physiological changes are still limited. In this study, the transcriptomes from polyps to juvenile medusae at different stages were characterized by RNA-seq in scyphozoan jellyfish Rhopilema esculentum. Among 96,076 de novo assembled unigenes, 7090 differentially expressed genes (DEGs) were identified during the developmental stages. The co-expression pattern analysis of DEGs yielded 15 clusters with different expression patterns. Among them, a cluster with 388 unigenes was related to strobila. In this specific cluster, the GO terms related to "sequence-specific DNA binding transcription factor activity" and "sequence-specific DNA binding" were significantly enriched. Transcription factors, including segmentation protein even-skipped-like, segmentation polarity protein engrailed-like, homeobox proteins Otx-like, Twist-like and Cnox2-Pc-like, as well as genes such as RxR-like and Dmrtf-like, were identified to be potentially involved in strobilation. Their expression patterns and the other 11 TFs/genes involved in strobilation were confirmed with qRT-PCR methods. The present study pointed out the role of transcription factors in strobilation and produced a list of novel candidate genes for further studies. It could provide valuable information for understanding the molecular mechanisms of jellyfish strobilation.

Loss of metagenesis and evolution of a parasitic life style in a group of open-ocean jellyfish.

Loss or stark reduction of the free-swimming medusa or jellyfish stage is common in the cnidarian class Hydrozoa. In the hydrozoan clade Trachylina, however, many species do not possess a sessile polyp or hydroid stage. Trachylines inhabiting freshwater and coastal ecosystems (i.e., Limnomedusae) possess a metagenetic life cycle involving benthic, sessile polyp and free-swimming medusa. In contrast, the paradigm is that open ocean inhabiting, oceanic trachylines (in the orders Narcomedusae and Trachymedusae) develop from zygote to medusa via a free-swimming larva, forgoing the polyp stage. In some open-ocean trachylines, development includes a sessile stage that is an ecto- or endoparasite of other oceanic organisms. We expand the molecular-based phylogenetic hypothesis of trachylines significantly, increasing taxon and molecular marker sampling. Using this comprehensive phylogenetic hypothesis in conjunction with character state reconstructions we enhance understanding of the evolution of life cycles in trachyline hydrozoans. We find that the polyp stage was lost at least twice independently, concurrent with a transition to an oceanic life style. Further, a sessile, polypoid parasitic stage arose once, rather than twice as current classification would imply, in the open ocean inhabiting Narcomedusae. Our results also support the hypothesis that interstitial species of the order Actinulida are directly descended from direct developing, oceanic trachylines.

High occurrence of jellyfish predation by black-browed and Campbell albatross identified by DNA metabarcoding.

Gelatinous zooplankton are a large component of the animal biomass in all marine environments, but are considered to be uncommon in the diet of most marine top predators. However, the diets of key predator groups like seabirds have conventionally been assessed from stomach content analyses, which cannot detect most gelatinous prey. As marine top predators are used to identify changes in the overall species composition of marine ecosystems, such biases in dietary assessment may impact our detection of important ecosystem regime shifts. We investigated albatross diet using DNA metabarcoding of scats to assess the prevalence of gelatinous zooplankton consumption by two albatross species, one of which is used as an indicator species for ecosystem monitoring. Black-browed and Campbell albatross scats were collected from eight breeding colonies covering the circumpolar range of these birds over two consecutive breeding seasons. Fish was the main dietary item at most sites; however, cnidarian DNA, primarily from scyphozoan jellyfish, was present in 42% of samples overall and up to 80% of samples at some sites. Jellyfish was detected during all breeding stages and consumed by adults and chicks. Trawl fishery catches of jellyfish near the Falkland Islands indicate a similar frequency of jellyfish occurrence in albatross diets in years of high and low jellyfish availability, suggesting jellyfish consumption may be selective rather than opportunistic. Warmer oceans and overfishing of finfish are predicted to favour jellyfish population increases, and we demonstrate here that dietary DNA metabarcoding enables measurements of the contribution of gelatinous zooplankton to the diet of marine predators.

Respiratory response to temperature of three populations of Aurelia aurita polyps in northern Europe.

The benthic life stage (polyp or scyphistoma) of the bloom-forming jellyfish, Aurelia aurita (Linnaeus, 1759), also known as the moon jellyfish, contributes to the seasonal occurrence and abundance of medusa blooms via asexual reproduction. A. aurita is widely distributed in coastal areas in northern Europe, and one of the most studied jellyfish species. While the physiology of the visible medusa is largely understood, understanding of the physiology of the perennial benthic life-stage is scarce. To measure the physiological tolerance of A. aurita, the scyphistoma's temperature sensitivity across its distributional range was investigated. Respiration rates of polyps from three northern European locations exposed to 11 temperatures between 2 and 22°C were measured. There was a significant difference in respiration rate among the three polyp populations, which may reflect on differences in their thermal tolerance window. A critical temperature was reached at 14°C with the metabolic rate decreasing below and above that temperature. This pattern was less pronounced in the Norwegian population but polyps were able to survive, at least temporarily, those temperatures exceeding their natural range. While polyps collected from northern Norway, with a narrow environmental thermal window, displayed a low baseline metabolism with a Q10 value of 1.2, polyps from southern England and Scotland had Q10 values of 1.6 and 2.5, respectively. Differences in polyps' respiration rates across their distributional range suggest that populations have evolved adaptations to local environmental thermal conditions.

Three routes to crypsis: Stasis, convergence, and parallelism in the Mastigias species complex (Scyphozoa, Rhizostomeae).

Evolutionary inference can be complicated by morphological crypsis, particularly in open marine systems that may rapidly dissipate signals of evolutionary processes. These complications may be alleviated by studying systems with simpler histories and clearer boundaries, such as marine lakes-small bodies of seawater entirely surrounded by land. As an example, we consider the jellyfish Mastigias spp. which occurs in two ecotypes, one in marine lakes and one in coastal oceanic habitats, throughout the Indo-West Pacific (IWP). We tested three evolutionary hypotheses to explain the current distribution of the ecotypes: (H1) the ecotypes originated from an ancient divergence; (H2) the lake ecotype was derived recently from the ocean ecotype during a single divergence event; and (H3) the lake ecotype was derived from multiple, recent, independent, divergences. We collected specimens from 21 locations throughout the IWP, reconstructed multilocus phylogenetic and intraspecific relationships, and measured variation in up to 40 morphological characters. The species tree reveals three reciprocally monophyletic regional clades, two of which contain ocean and lake ecotypes, suggesting repeated, independent evolution of coastal ancestors into marine lake ecotypes, consistent with H3; hypothesis testing and an intraspecific haplotype network analysis of samples from Palau reaffirms this result. Phylogenetic character mapping strongly correlates morphology to environment rather than lineage (r=0.7512, p<0.00001). Considering also the deeper relationships among regional clades, morphological similarity in Mastigias spp. clearly results from three separate patterns of evolution: morphological stasis in ocean medusae, convergence of lake morphology across distinct species and parallelism between lake morphologies within species. That three evolutionary routes each result in crypsis illustrates the challenges of interpreting evolutionary processes from patterns of biogeography and diversity in the seas. Identifying cryptic species is only the first step in understanding these processes; an equally important second step is exploring and understanding the processes and patterns that create crypsis.

Development of a rapid assay to detect the jellyfish Cyanea nozakii using a loop-mediated isothermal amplification method.

Blooms of the harmful jellyfish Cyanea nozakii, which are a severe nuisance to fisheries and tourisms, frequently occur in the northern East China Sea, Yellow Sea, and Bohai Sea. To provide early warning of this species, a simple and effective molecular method for identifying C. nozakii was developed using the loop-mediated isothermal amplification method (LAMP). The LAMP assay is highly specific and uses a set of four primers that target six different regions on the mitochondrial cytochrome c oxidase subunit I (COI) gene of C. nozakii. The amplification conditions, including the dNTP and betaine concentrations, the inner primer to outer primer concentration ratio, reaction time and temperature, were optimized. The LAMP assay amplified DNA extracted from tissue samples of C. nozakii but did not amplify DNA from other common scyphozoans and hydrozoans collected in the same region. In addition, the LAMP assay was more sensitive than conventional PCR. Therefore, the established LAMP assay is a sensitive, specific, fast, and easily performed method for detection of C. nozakii at different stages in their life cycle.

Phylogenomic Analyses Support Traditional Relationships within Cnidaria.

Cnidaria, the sister group to Bilateria, is a highly diverse group of animals in terms of morphology, lifecycles, ecology, and development. How this diversity originated and evolved is not well understood because phylogenetic relationships among major cnidarian lineages are unclear, and recent studies present contrasting phylogenetic hypotheses. Here, we use transcriptome data from 15 newly-sequenced species in combination with 26 publicly available genomes and transcriptomes to assess phylogenetic relationships among major cnidarian lineages. Phylogenetic analyses using different partition schemes and models of molecular evolution, as well as topology tests for alternative phylogenetic relationships, support the monophyly of Medusozoa, Anthozoa, Octocorallia, Hydrozoa, and a clade consisting of Staurozoa, Cubozoa, and Scyphozoa. Support for the monophyly of Hexacorallia is weak due to the equivocal position of Ceriantharia. Taken together, these results further resolve deep cnidarian relationships, largely support traditional phylogenetic views on relationships, and provide a historical framework for studying the evolutionary processes involved in one of the most ancient animal radiations.

[Reason to split the genus Aurelia. Mesoglein from two populations differ].

The medusa, Aurelia aurita (Scyphozoa, Cnidaria), is considered to be a cosmopolitan species with a worldwide distribution in most seas from the poles to the tropics. Cnidarian is thought to possess two tissue layers: endoderm (gastroderm) and ectoderm, which are separated by huge mesoglea in medusa. The basic morphology of medusa is similar in different populations. Previously we have determined a new protein "mesoglein" as one of the main components of mesoglea. Deduced amino acid sequence of mesoglein contains Zona Pellucida (ZP) domain. In this paper, we have comparied of mesoglein and its gene in medusa from three habitats (White Sea (WsA), Black Sea (BsA), Japonic Sea (JsA)). The set of the mesoglea protein bands after SDS-PAGE is similar in all samples. Nevertheless, JsA mesogleins' M(r) is 53-55 kDa, while WsA and BsA mesogleins have M(r) of 47 kDa. Antibodies raised against WsA mesoglein recognize only mesogleins with M(r) of 47 kDa, but not 53-55 kDa, both on immunoblot and immunocytochemistry. Mesogleal cells and elastic fibrils are stained intensively in the mesoglea both from WsA and BsA but not from JsA. The possibility of gene divergency was checked by PCR with primers specific for WsA mesoglein gene. PCR products of expected length obtained on polyA-cDNA template from mesogleal cells of WsA and BsA medusa but not on cDNA of JsA medusa. Our results evidence that there are two different species in genus Aurelia: Aurelia aurita inhabits White and Black Seas while Aurelia sp. inhabits Japonic Sea. This is consistent with findings of other recept molecular biological studies.

Scyphomedusae of the Mediterranean: state of the art and future perspectives.

Scyphomedusae (Phylum Cnidaria, Class Scyphozoa) are perceived as a nuisance due to their sudden outbreaks that negatively affect human activities (particularly tourism and fisheries) mainly because of their stings. A brief review of the history of scyphozoan blooms in the Mediterranean and updated information available after 2010 point to an increase in scyphozoan outbreaks. Whilst the negative effects on public health, aquaculture, coastal industrial activities and fisheries operations are undeniable, the effects on the ecosystem are not well defined. We focus on the trophic interactions between scyphomedusae and fish, highlighting that the negative effects of scyphomedusae on fish stocks exerted through direct predation on early life stages of fish and competition for plankton are at present speculative. In favor of a positive effect of scyphomedusae on fish populations, the reports of predation upon scyphozoans are increasing, which suggests that predators may benefit from the availability of scyphozoans by shifting their diet toward jelly prey. Additionally, scyphomedusae may provide nursery habitats to early life stages of ecologically and economically important forage fishes and other organisms which shelter underneath their bells. Together with these ecosystem services, compounds extracted from scyphozoan tissues and venoms are having a variety of biomedical applications and are likely to contribute to treat a growing number of diseases, including cancer. Our analysis highlights that a re-evaluation of the balance between "positive" and "negative" effects of scyphomedusae on the ecosystem and human activities is needed and provides indications on potential directions for future studies.

Pentaplex PCR as screening assay for jellyfish species identification in food products.

Salted jellyfish, a traditional food in Asian Countries, is nowadays spreading on the Western markets. In this work, we developed a Pentaplex PCR for the identification of five edible species (Nemopilema nomurai, Rhopilema esculentum, Rhizostoma pulmo, Pelagia noctiluca, and Cotylorhiza tuberculata), which cannot be identified by a mere visual inspection in jellyfish products sold as food. A common degenerated forward primer and five specie-specific reverse primers were designed to amplify COI gene regions of different lengths. Another primer pair targeted the 28SrRNA gene and was intended as common positive reaction control. Considering the high level of degradation in the DNA extracted from acidified and salted products, the maximum length of the amplicons was set at 200 bp. The PCR was developed using 66 reference DNA samples. It gave successful amplifications in 85.4% of 48 ready to eat products (REs) and in 60% of 30 classical salted products (CPs) collected on the market.

Pelagia benovici sp. nov. (Cnidaria, Scyphozoa): a new jellyfish in the Mediterranean Sea.

A bloom of an unknown semaestome jellyfish species was recorded in the North Adriatic Sea from September 2013 to early 2014. Morphological analysis of several specimens showed distinct differences from other known semaestome species in the Mediterranean Sea and unquestionably identified them as belonging to a new pelagiid species within genus Pelagia. The new species is morphologically distinct from P. noctiluca, currently the only recognized valid species in the genus, and from other doubtful Pelagia species recorded from other areas of the world. Molecular analyses of mitochondrial cytochrome c oxidase subunit I (COI) and nuclear 28S ribosomal DNA genes corroborate its specific distinction from P. noctiluca and other pelagiid taxa, supporting the monophyly of Pelagiidae. Thus, we describe Pelagia benovici sp. nov. Piraino, Aglieri, Scorrano & Boero.

Internal anatomy of Haliclystus antarcticus (Cnidaria, Staurozoa) with a discussion on histological features used in Staurozoan taxonomy.

Stauromedusae have relatively few macromorphological characters, making both their taxonomy and identification difficult. For this reason, histological characters are also employed in the taxonomy of the group. This study presents a detailed description of the histomorphology of Haliclystus antarcticus Pfeffer, 1889 (Cnidaria, Staurozoa). We make new observations for the species and for the class, and address functional, taxonomical, and evolutionary aspects of staurozoan histo-anatomy. A complete reconstruction of H. antarcticus body plan is used to guide a more detailed observation, based on light microscopy, of structures rarely cited in the literature, such as the intertentacular lobules, the ostia between adjacent perradial pockets, and the male and female gonadal vesicles. Two possible regions of nematocyst formation are hypothesized and discussed. We also provide a review of the current use of histological characters in the taxonomy of the group. Understanding the body plan of stauromedusae is a challenge, because each single individual presents characters found in medusae and in polyps of other medusozoans. Comprehensive histological descriptions are important to establish relations of homology within Staurozoa and Cnidaria, providing crucial data on their evolution.

Diversification of animal venom peptides-were jellyfish amongst the first combinatorial chemists?

An ocean of data: Huge numbers of protein toxins are found in animal venoms. This diversity is widely believed to have arisen by gene duplication events. However, recent data now challenges this tradition view. Here we highlight how jellyfish could hold the key to unravelling toxin diversification, with a view towards future combinatorial biosynthesis of toxin libraries.

Embryos, polyps and medusae of the Early Cambrian scyphozoan Olivooides.

The Early Cambrian organism Olivooides is known from both embryonic and post-embryonic stages and, consequently, it has the potential to yield vital insights into developmental evolution at the time that animal body plans were established. However, this potential can only be realized if the phylogenetic relationships of Olivooides can be constrained. The affinities of Olivooides have proved controversial because of the lack of knowledge of the internal anatomy and the limited range of developmental stages known. Here, we describe rare embryonic specimens in which internal anatomical features are preserved. We also present a fuller sequence of fossilized developmental stages of Olivooides, including associated specimens that we interpret as budding ephyrae (juvenile medusae), all of which display a clear pentaradial symmetry. Within the framework of a cnidarian interpretation, the new data serve to pinpoint the phylogenetic position of Olivooides to the scyphozoan stem group. Hypotheses about scalidophoran or echinoderm affinities of Olivooides can be rejected.

Mitochondrial genome of the freshwater jellyfish Craspedacusta sowerbyi and phylogenetics of Medusozoa.

The 17,922 base pairs (bp) nucleotide sequence of the linear mitochondrial DNA (mtDNA) molecule of the freshwater jellyfish Craspedacusta sowerbyi (Hydrozoa, Trachylina, Limnomedusae) has been determined. This sequence exhibits surprisingly low A+T content (57.1%), containing genes for 13 energy pathway proteins, a small and a large subunit rRNAs, and methionine and tryptophan tRNAs. Mitochondrial ancestral medusozoan gene order (AMGO) was found in the C. sowerbyi, as those found in Cubaia aphrodite (Hydrozoa, Trachylina, Limnomedusae), discomedusan Scyphozoa and Staurozoa. The genes of C. sowerbyi mtDNA are arranged in two clusters with opposite transcriptional polarities, whereby transcription proceeds toward the ends of the DNA molecule. Identical inverted terminal repeats (ITRs) flank the ends of the mitochondrial DNA molecule, a characteristic typical of medusozoans. In addition, two open reading frames (ORFs) of 354 and 1611 bp in length were found downstream of the large subunit rRNA gene, similar to the two ORFs of ORF314 and polB discovered in the linear mtDNA of C. aphrodite, discomedusan Scyphozoa and Staurozoa. Phylogenetic analyses of C. sowerbyi and other cnidarians were carried out based on both nucleotide and inferred amino acid sequences of the 13 mitochondrial energy pathway genes. Our working hypothesis supports the monophyletic Medusozoa being a sister group to Octocorallia (Cnidaria, Anthozoa). Within Medusozoa, the phylogenetic analysis suggests that Staurozoa may be the earliest diverging class and the sister group of all other medusozoans. Cubozoa and coronate Scyphozoa form a clade that is the sister group of Hydrozoa plus discomedusan Scyphozoa. Hydrozoa is the sister group of discomedusan Scyphozoa. Semaeostomeae is a paraphyletic clade with Rhizostomeae, while Limnomedusae (Trachylina) is the sister group of hydroidolinans and may be the earliest diverging lineage among Hydrozoa.

Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record.

The phylum Cnidaria is comprised of remarkably diverse and ecologically significant taxa, such as the reef-forming corals, and occupies a basal position in metazoan evolution. The origin of this phylum and the most recent common ancestors (MRCAs) of its modern classes remain mostly unknown, although scattered fossil evidence provides some insights on this topic. Here, we investigate the molecular divergence times of the major taxonomic groups of Cnidaria (27 Hexacorallia, 16 Octocorallia, and 5 Medusozoa) on the basis of mitochondrial DNA sequences of 13 protein-coding genes. For this analysis, the complete mitochondrial genomes of seven octocoral and two scyphozoan species were newly sequenced and combined with all available mitogenomic data from GenBank. Five reliable fossil dates were used to calibrate the Bayesian estimates of divergence times. The molecular evidence suggests that cnidarians originated 741 million years ago (Ma) (95% credible region of 686-819), and the major taxa diversified prior to the Cambrian (543 Ma). The Octocorallia and Scleractinia may have originated from radiations of survivors of the Permian-Triassic mass extinction, which matches their fossil record well.