Did the evolution of the phytoplankton fuel the diversification of the marine biosphere?

We discuss the possible links between the fossil record of marine biodiversity, nutrient availability and primary productivity. The parallelism of the fossil records of marine phytoplankton and faunal biodiversity implicates the quantity (primary productivity) and quality (stoichiometry) of phytoplankton as being critical to the diversification of the marine biosphere through the Phanerozoic. The relatively subdued marine biodiversity of the Palaeozoic corresponds to a time of relatively low macronutrient availability and poor food quality of the phytoplankton as opposed to the diversification of the Modern Fauna through the Mesozoic–Cenozoic. Increasing nutrient runoff to the oceans through the Phanerozoic resulted from orogeny, the emplacement of Large Igneous Provinces (LIPs), the evolution of deep-rooting forests and the appearance of more easily decomposable terrestrial organic matter that enhanced weathering. Positive feedback by bioturbation of an expanding benthos played a critical role in evolving biogeochemical cycles by linking the oxidation of dead organic matter and the recycling of nutrients back to the water column where they could be re-utilized. We assess our conclusions against a recently published biogeochemical model for geological time-scales. Major peaks of marine diversity often occur near rising or peak fluxes of silica, phosphorus and dissolved reactive oceanic phosphorus; either major or minor ⁸⁷Sr/⁸⁶Sr peaks; and frequently in the vicinity of major (Circum-Atlantic Magmatic Province) and minor volcanic events, some of which are associated with Oceanic Anoxic Events. These processes appear to be scale-dependent in that they lie on a continuum between biodiversification on macroevolutionary scales of geological time and mass extinction.     

A tale of two eras: Phytoplankton composition influenced by oceanic paleochemistry

We report the results of simple experiments which support the hypothesis that changes in ocean chemistry beginning in the Mesozoic Era resulted in an increase in the nutritional quality per mole of C and per cell of planktonic algal biomass compared to earlier phytoplankton. We cultured a cyanobacterium, a diatom, a dinoflagellate, and a green alga in media mimicking aspects of the chemistry of Palaeozoic and Mesozoic‐Cenozoic oceans. Substantial differences emerged in the quality of algal biomass between the Palaeozoic and Mesozoic‐Cenozoic growth regimes; these differences were strongly affected by interspecific interactions (i.e., the co‐existence of different species alters responses to the chemistry of the medium). The change was in the direction of a Mesozoic‐Cenozoic biomass enriched in protein per mole C, although cells contained less carbon overall. This would lead to a lower C:N ratio. On the assumption that Mesozoic‐Cenozoic grazers’ assimilation of total C was similar to that of their earlier counterparts, their diet would be stoichiometrically closer to their C:N requirement. This, along with an increase in mean cell size among continental shelf phytoplankton, could have helped to facilitate observed evolutionary changes in the Mesozoic marine fauna. In turn, increased grazing pressure would have operated as a selective force for the radiation of phytoplankton clades better equipped with antigrazing capabilities (sensu lato), as found widely in phytoplankton with biomineralization. Our results emphasize potential links between changing seawater chemistry, increased predation pressure and the rise to ecological dominance of chlorophyll a+c algae in Mesozoic oceans. The experiments also suggest a potential role for ocean chemistry in changes of marine trophic structure from the Palaeozoic to the later Mesozoic Era.     

The Late Palaeozoic phytoplankton blackout — Artefact or evidence of global change?

The fossil record of the Palaeozoic documents one of the most dramatic changes in Phanerozoic marine primary production, although causes and effects of these changes have been the subject of rather controversial discussions. During the early Palaeozoic the marine phytoplankton experienced an enormous radiation and diversification of taxa especially among acritarchs, which was punctuated by a few extinction events possibly associated with climate changes. Phytoplankton diversity was drastically reduced at the Devonian/Carboniferous boundary, a phenomenon here designated as the “Phytoplankton Blackout”.After a lapse of about 130 million years phytoplankton diversity was gradually restored during the Late Triassic with the appearance of dinoflagellates and somewhat later of coccolithophorids and diatoms. Evidence from recent phytoplankton suggests that the dominant groups of phytoplankton differ from those of the Palaeozoic in their nutritional requirements and that fundamental changes in ocean chemistry have played an important role.The remarkable temporal coincidence of the Phytoplankton Blackout with plate tectonic processes during the assembly and breakup of Pangaea and the concomitant rise of land plants towards the end of the Devonian provide arguments that nutrient depletion in the ocean may have played a decisive role in controlling the phytoplankton blackout. Contrary arguments in favour of oceanic eutrophication, changes in life cycles of dominant phytoplankton groups or selective preservation are discussed and weighed against the scenario presented here. Metazoan evolution seems only loosely linked with the phytoplankton blackout. However, there is good agreement between phytoplankton and reef evolution throughout the Phanerozoic.     

The Ordovician Biodiversification: revolution in the oceanic trophic chain

The Early Palaeozoic phytoplankton (acritarch) radiation paralleled a long-term increase in sea level between the Early Cambrian and the Late Ordovician. In the Late Cambrian, after the SPICE δ¹³C_carb excursion, acritarchs underwent a major change in morphological disparity and their taxonomical diversity increased to reach highest values during the Middle Ordovician (Darriwilian). This highest phytoplankton diversity of the Palaeozoic was possibly the result of palaeogeography (greatest continental dispersal) and major orogenic and volcanic activity, which provided maximum ecospace and large amounts of nutrients. With its warm climate and high atmospheric CO₂ levels, the Ordovician was similar to the Cretaceous: a period when phytoplankton diversity was at its maximum during the Mesozoic. With increased phytoplankton availability in the Late Cambrian and Ordovician a radiation of zooplanktonic organisms took place at the same time as a major diversification of suspension feeders. In addition, planktotrophy originated in invertebrate larvae during the Late Cambrian–Early Ordovician. These important changes in the trophic chain can be considered as a major palaeoecological revolution (part of the rise of the Palaeozoic Evolutionary Fauna of Sepkoski). There is now sufficient evidence that this trophic chain revolution was related to the diversification of the phytoplankton, of which the organic-walled fraction is partly preserved.     

The influence of the biological pump on ocean chemistry: implications for long‐term trends in marine redox chemistry,the global carbon cycle,and marine animal ecosystems

The net export of organic matter from the surface ocean and its respiration at depth create vertical gradients in nutrient and oxygen availability that play a primary role in structuring marine ecosystems. Changes in the properties of this ‘biological pump’ have been hypothesized to account for important shifts in marine ecosystem structure, including the Cambrian explosion. However, the influence of variation in the behavior of the biological pump on ocean biogeochemistry remains poorly quantified, preventing any detailed exploration of how changes in the biological pump over geological time may have shaped long‐term shifts in ocean chemistry, biogeochemical cycling, and ecosystem structure. Here, we use a 3‐dimensional Earth system model of intermediate complexity to quantitatively explore the effects of the biological pump on marine chemistry. We find that when respiration of sinking organic matter is efficient, due to slower sinking or higher respiration rates, anoxia tends to be more prevalent and to occur in shallower waters. Consequently, the Phanerozoic trend toward less bottom‐water anoxia in continental shelf settings can potentially be explained by a change in the spatial dynamics of nutrient cycling rather than by any change in the ocean phosphate inventory. The model results further suggest that the Phanerozoic decline in the prevalence ocean anoxia is, in part, a consequence of the evolution of larger phytoplankton, many of which produce mineralized tests. We hypothesize that the Phanerozoic trend toward greater animal abundance and metabolic demand was driven more by increased oxygen concentrations in shelf environments than by greater food (nutrient) availability. In fact, a lower‐than‐modern ocean phosphate inventory in our closed system model is unable to account for the Paleozoic prevalence of bottom‐water anoxia. Overall, these model simulations suggest that the changing spatial distribution of photosynthesis and respiration in the oceans has exerted a first‐order control on Earth system evolution across Phanerozoic time.     

Oceanic oxygenation events in the anoxic Ediacaran ocean

The ocean‐atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635 Ma), middle (ca. 580 Ma), or late (ca. 560 Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi‐proxy paleoredox study of a relatively continuous, deep‐water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran–early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox‐static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well‐oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran–early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis.     

Metal limitation of cyanobacterial N2 fixation and implications for the Precambrian nitrogen cycle

Nitrogen fixation is a critical part of the global nitrogen cycle, replacing biologically available reduced nitrogen lost by denitrification. The redox‐sensitive trace metals Fe and Mo are key components of the primary nitrogenase enzyme used by cyanobacteria (and other prokaryotes) to fix atmospheric N₂ into bioessential compounds. Progressive oxygenation of the Earth's atmosphere has forced changes in the redox state of the oceans through geologic time, from anoxic Fe‐enriched waters in the Archean to partially sulfidic deep waters by the mid‐Proterozoic. This development of ocean redox chemistry during the Precambrian led to fluctuations in Fe and Mo availability that could have significantly impacted the ability of prokaryotes to fix nitrogen. It has been suggested that metal limitation of nitrogen fixation and nitrate assimilation, along with increased rates of denitrification, could have resulted in globally reduced rates of primary production and nitrogen‐starved oceans through much of the Proterozoic. To test the first part of this hypothesis, we grew N₂‐fixing cyanobacteria in cultures with metal concentrations reflecting an anoxic Archean ocean (high Fe, low Mo), a sulfidic Proterozoic ocean (low Fe, moderate Mo), and an oxic Phanerozoic ocean (low Fe, high Mo). We measured low rates of cellular N₂ fixation under [Fe] and [Mo] estimated for the Archean ocean. With decreased [Fe] and higher [Mo] representing sulfidic Proterozoic conditions, N₂ fixation, growth, and biomass C:N were similar to those observed with metal concentrations of the fully oxygenated oceans that likely developed in the Phanerozoic. Our results raise the possibility that an initial rise in atmospheric oxygen could actually have enhanced nitrogen fixation rates to near modern marine levels, providing that phosphate was available and rising O₂ levels did not markedly inhibit nitrogenase activity.     

The role of phytoplankton photosynthesis in global biogeochemical cycles

Phytoplankton biomass in the world's oceans amounts to only 1–2% of the total global plant carbon, yet these organisms fix between 30 and 50 billion metric tons of carbon annually, which is about 40% of the total. On geological time scales there is profound evidence of the importance of phytoplankton photosynthesis in biogeochemical cycles. It is generally assumed that present phytoplankton productivity is in a quasi steady-state (on the time scale of decades). However, in a global context, the stability of oceanic photosynthetic processes is dependent on the physical circulation of the upper ocean and is therefore strongly influenced by the atmosphere. The net flux of atmospheric radiation is critical to determining the depth of the upper mixed layer and the vertical fluxes of nutrients. These latter two parameters are keys to determining the intensity, and spatial and temporal distributions of phytoplankton blooms. Atmospheric radiation budgets are not in steady-state. Driven largely by anthropogenic activities in the 20th century, increased levels of IR- absorbing gases such as CO₂, CH₄ and CFC's and NO_x will potentially increase atmospheric temperatures on a global scale. The atmospheric radiation budget can affect phytoplankton photosynthesis directly and indirectly. Increased temperature differences between the continents and oceans have been implicated in higher wind stresses at the ocean margins. Increased wind speeds can lead to higher nutrient fluxes. Throughout most of the central oceans, nitrate concentrations are sub-micromolar and there is strong evidence that the quantum efficiency of Photosystem II is impaired by nutrient stress. Higher nutrient fluxes would lead to both an increase in phytoplankton biomass and higher biomass-specific rates of carbon fixation. However, in the center of the ocean gyres, increased radiative heating could reduce the vertical flux of nutrients to the euphotic zone, and hence lead to a reduction in phytoplankton carbon fixation. Increased desertification in terrestrial ecosystems can lead to increased aeolean loadings of essential micronutrients, such as iron. An increased flux of aeolean micronutrients could fertilize nutrient-replete areas of the open ocean with limiting trace elements, thereby stimulating photosynthetic rates. The factors which limit phytoplankton biomass and photosynthesis are discussed and examined with regard to potential changes in the Earth climate system which can lead the oceans away from steady-state. While it is difficult to confidently deduce changes in either phytoplankton biomass or photosynthetic rates on decadal time scales, time-series analysis of ocean transparency data suggest long-term trends have occurred in the North Pacific Ocean in the 20th century. However, calculations of net carbon uptake by the oceans resulting from phytoplankton photosynthesis suggest that without a supply of nutrients external to the ocean, carbon fixation in the open ocean is not presently a significant sink for excess atmospheric CO₂.The submitted paper has been authored under Contract No. DE-AC02-76H00016 with the US Department of Energy. Accordingly, the US Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes.     

Evolutionary morphology of oblique ribs of bivalves

Fossil bivalves bearing oblique ribs first appeared in the Mid Ordovician but their diversity remained low during the Palaeozoic. The diversity soon increased after the Early Triassic, peaking in the Early Cretaceous. The Palaeozoic–Mesozoic record is dominated by burrowing bivalves (mainly pholadomyoids and trigonioids), which developed oblique ribs with symmetric profiles, probably adapted for shell reinforcement, although there are indications that the ribs of trigonioids also enhanced burrowing efficiency. After the Paleocene, the main groups of burrowing bivalves were veneroids (primarily tellinoideans and lucinoideans) and nuculoids, which generated oblique ribs of the shingled type, adapted to increase burrowing efficiency. The inferred change in function at the Mesozoic/Cenozoic boundary can be correlated with an increase in mean mobility of the bivalve faunas bearing oblique ribs through time. This implies a major ecological cause for the observed temporal patterns, which forced bivalve faunas to burrow more rapidly and efficiently. In particular, either the Phanerozoic increase in the diversity of durophagous predators or the accelerating rate of sediment reworking (both being a consequence of the Mesozoic Marine Revolution), or both, could have provided the necessary evolutionary force.     

Life in the arena: infaunal gastropods and the late Phanerozoic expansion of marine ecosystems into sand

Marine ecosystems have expanded into the infaunal realm below the surface of soft sediments throughout the Phanerozoic eon. During the Palaeozoic era, this expansion largely involved sedentary animals living in permanent resting places. Active sand‐burrowing animals colonized the infaunal environment later, but when this happened and when specialization for infaunal life evolved remain open questions. Here, phylogenetic evidence, fossil occurrences and previously established criteria for recognizing the sand‐burrowing habit in marine gastropods are used to determine how many gastropod clades became infaunal and when the transitions from surface‐dwelling to infaunal life occurred. At least 20, and as many as 35, clades (all but one of post‐Palaeozoic age) contain actively infaunal species. The overwhelming majority (15 of 20 clades) became infaunal during the Cenozoic, and clades with hundreds of infaunal species in the living fauna diversified beginning in the Early Miocene. The repeated evolution of, and specialization to, the sand‐burrowing habit by gastropods and other animals was enabled by increased habitat availability and higher marine productivity, and was necessitated by intensifying predation. As a result, the infaunal realm was transformed from a marine refuge to an integrated part of the marine biosphere in which high performance in locomotion and defence has become the norm.     

Copepod nauplii use phosphorus from bacteria,creating a short circuit in the microbial loop

In subtropical oceans phytoplankton carbon: phosphorus (C : P) ratios are high, and these ratios are predicted to increase further with rising ocean temperatures and stratification. Prey stoichiometry may pose a problem for copepod zooplankton nauplii, which have high phosphorus demands due to rapid growth. We hypothesised that nauplii meet this demand by consuming bacteria. Naupliar bacterial and phytoplankton carbon and phosphorus ingestion, assimilation and incorporation were traced using ³³P and ¹⁴C radioisotopes. Bacterial carbon was incorporated four times less efficiently into biomass than phytoplankton carbon. In contrast, bacterial and phytoplankton phosphorus were incorporated at similar efficiencies, and bacteria could meet a substantial amount of naupliar phosphorus requirements. As parts of the ocean become more oligotrophic, bacteria could help sustain naupliar growth and survival under suboptimal stoichiometric conditions. Thus, nauplii may be a shortcut for phosphorus from the microbial loop to the classical food web.     

遗迹化石Chondrites的指相意义和阶层分布

介绍和评述Chondrites的基本特征和流行的成因解释 ,记述最近发现于美国犹他州中西部北美下 中奥陶统之交层型剖面上的Chondrites遗迹组构的不寻常特征和属性 :细小的Chondrites被所有与之共生的遗迹化石交切 ,形成于水与沉积物界面附近缺氧的早期软底质中 ,Chondrites是形成最早和最浅的遗迹阶层。流行的遗迹相和遗迹阶层模式的不足是 :前者在操作上 ,简单地将遗迹化石及其组合处理为同沉积或准同沉积的物理沉积构造 ,忽略了造迹活动的多阶段和多阶层的特点 ;后者的资料基础仅依据中生代以来的地层记录 ,对重大生物 环境事件和造迹生物生态习性演化对遗迹阶层形成和分布的影响注重不够。指出Chondrites的阶层分布和对古氧相的示踪在古生代与中生代有重要差别     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

Limits to randomness in paleobiologic models: the case of Phanerozoic species diversity

The question of how random, or unconstrained, paleobiologic models should be is examined with a case study: Signor's (1982, 1985) inverse calculation of levels of marine species diversity through the Phanerozoic. His calculation involved an ingenious model that estimated species numbers and species abundances in the world oceans of the past by correcting known numbers of fossil species for variations in sedimentary rocks available for sampling and in effort paleontologists might devote to sampling. The model proves robust to changes in possible shapes of species-abundance distributions, but it is sensitive to alterations in the assumption that paleontologists collect fossils at random. If it is assumed that ease of collecting varies with age of sediment (with the Cenozoic offering easy sampling) or that paleontologists tend to seek out rarer fossils, results of the inverse calculation change. In particular, the magnitude of the calculated Cenozoic diversity increase always declines from the factor of about seven as originally reported to something considerably smaller. This leaves open the problem of the magnitude of Cenozoic increase in marine species diversity, awaiting better empirical data and, perhaps, more exacting models, random or otherwise.     

Omega-3: a link between global climate change and human health

In recent years, global climate change has been shown to detrimentally affect many biological and environmental factors, including those of marine ecosystems. In particular, global climate change has been linked to an increase in atmospheric carbon dioxide, UV irradiation, and ocean temperatures, resulting in decreased marine phytoplankton growth and reduced synthesis of omega-3 polyunsaturated fatty acids (PUFAs). Marine phytoplankton are the primary producers of omega-3 PUFAs, which are essential nutrients for normal human growth and development and have many beneficial effects on human health. Thus, these detrimental effects of climate change on the oceans may reduce the availability of omega-3 PUFAs in our diets, exacerbating the modern deficiency of omega-3 PUFAs and imbalance of the tissue omega-6/omega-3 PUFA ratio, which have been associated with an increased risk for cardiovascular disease, cancer, diabetes, and neurodegenerative disease. This article provides new insight into the relationship between global climate change and human health by identifying omega-3 PUFA availability as a potentially important link, and proposes a biotechnological strategy for addressing the potential shortage of omega-3 PUFAs in human diets resulting from global climate change.     

Patterns of Phanerozoic carbonate platform sedimentation

Carbonate platforms changed substantially in spatial extent, geometry, composition and palaeogeographical distribution through the Phanerozoic. Although reef construction and carbonate platform development are intimately linked today, this was not the case for most of the Phanerozoic. Carbonate production by non-enzymatic precipitation and non-reefal organisms is mostly responsible for this decoupling. Non-reefal carbonate production was especially prolific during times of depressed reef growth, balancing losses in reef carbonate production. Palaeogeographical distribution and spatial extent of Phanerozoic carbonate platforms exhibit trends related to continental drift, evolutionary patterns within carbonate platform biotas, climatic change and, possibly, variations in ocean chemistry. Continental drift moved large Palaeozoic tropical shelf areas into higher latitudes, thereby reducing the potential size of tropical platforms. However, the combined global size of carbonate platforms shows no significant decline through the Phanerozoic, suggesting that availability of tropical shelf areas was not a major control of platform area. This is explained by the limited platform coverage of low-latitude shelves (42% maximum) and occasional high-latitude excursions of platform carbonates. We speculate that reduced tropical shelf area in the icehouse tropics forced the migration of the many carbonate-secreting organisms into higher latitudes and, where terrigenous input was sufficiently low, extensive carbonate platform could develop.     

Bacteria in the cold deep-sea benthic boundary layer and sediment-water interface of the NE Atlantic

This is a short review of the current understanding of the role of microorganisms in the biogeochemistry in the deep-sea benthic boundary layer (BBL) and sediment-water interface (SWI) of the NE Atlantic, the gaps in our knowledge and some suggestions of future directions. The BBL is the layer of water, often tens of meters thick, adjacent to the sea bed and with homogenous properties of temperature and salinity, which sometimes contains resuspended detrital particles. The SWI is the bioreactive interface between the water column and the upper 1 cm of sediment and can include a large layer of detrital material composed of aggregates that have sedimented from the upper mixed layer of the ocean. This material is biologically transformed, over a wide range of time scales, eventually forming the sedimentary record. To understand the microbial ecology of deep-sea bacteria, we need to appreciate the food supply in the upper ocean, its packaging, passage and transformation during the delivery to the sea bed, the seasonality of variability of the supply and the environmental conditions under which the deep-sea bacteria grow. We also need to put into a microbial context recent geochemical findings of vast reservoirs of intrinsically labile organic material sorped onto sediments. These may well become desorped, and once again available to microorganisms, during resuspension events caused by deep ocean currents. As biotechnologists apply their tools in the deep oceans in search of unique bacteria, an increasing knowledge and understanding of the natural processes undertaken and environmental conditions experienced by deep-sea bacteria will facilitate this exploitation.     

Horseshoe crab phylogeny and independent colonizations of fresh water: ecological invasion as a driver for morphological innovation

Xiphosurids are an archaic group of aquatic chelicerate arthropods, generally known by the colloquial misnomer of ‘horseshoe crabs’. Known from marine environments as far back as the early Ordovician, horseshoe crabs are generally considered ‘living fossils’ – descendants of a bradytelic lineage exhibiting little morphological or ecological variation throughout geological time. However, xiphosurids are known from freshwater sediments in the Palaeozoic and Mesozoic; furthermore, the contention that xiphosurids show little morphological variation has never been tested empirically. Attempts to test this are hampered by the lack of a modern phylogenetic framework with which to explore different evolutionary scenarios. Here, I present a phylogenetic analysis of Xiphosurida and explore patterns of morphospace and environmental occupation of the group throughout the Phanerozoic. Xiphosurids are shown to have invaded non‐marine environments independently at least five times throughout their evolutionary history, twice resulting in the radiation of major clades – bellinurines and austrolimulids – that occupied novel regions of morphospace. These clades show a convergent ecological pattern of differentiation, speciation and subsequent extinction. Horseshoe crabs are shown to have a more dynamic and complex evolutionary history than previously supposed, with the extant species representing only a fraction of the group's past ecological and morphological diversity.