Early vertebrate evolution

Debate over the origin and evolution of vertebrates has occupied biologists and palaeontologists alike for centuries. This debate has been refined by molecular phylogenetics, which has resolved the place of vertebrates among their invertebrate chordate relatives, and that of chordates among their deuterostome relatives. The origin of vertebrates is characterized by wide‐ranging genomic, embryologic and phenotypic evolutionary change. Analyses based on living lineages suggest dramatic shifts in the tempo of evolutionary change at the origin of vertebrates and gnathostomes, coincident with whole‐genome duplication events. However, the enriched perspective provided by the fossil record demonstrates that these apparent bursts of anatomical evolution and taxic richness are an artefact of the extinction of phylogenetic intermediates whose fossil remains evidence the gradual assembly of crown gnathostome characters in particular. A more refined understanding of the timing, tempo and mode of early vertebrate evolution rests with: (1) better genome assemblies for living cyclostomes; (2) a better understanding of the anatomical characteristics of key fossil groups, especially the anaspids, thelodonts, galeaspids and pituriaspids; (3) tests of the monophyly of traditional groups; and (4) the application of divergence time methods that integrate not just molecular data from living species, but also morphological data and extinct species. The resulting framework will provide for rigorous tests of rates of character evolution and diversification, and of hypotheses of long‐term trends in ecological evolution that themselves suffer for lack of quantitative functional tests. The fossil record has been silent on the nature of the transition from jawless vertebrates to the jawed vertebrates that have dominated communities since the middle Palaeozoic. Elucidation of this most formative of episodes likely rests with the overhaul of early vertebrate systematics that we propose, but perhaps more fundamentally with fossil grades that await discovery.     

Evaluating the predicted extinction risk of living amphibian species with the fossil record

Bridging the gap between the fossil record and conservation biology has recently become of great interest. The enormous number of documented extinctions across different taxa can provide insights into the extinction risk of living species. However, few studies have explored this connection. We used generalised boosted modelling to analyse the impact of several traits that are assumed to influence extinction risk on the stratigraphic duration of amphibian species in the fossil record. We used this fossil‐calibrated model to predict the extinction risk for living species. We observed a high consensus between our predicted species durations and the current IUCN Red List status of living amphibian species. We also found that today's Data Deficient species are mainly predicted to experience short durations, hinting at their likely high threat status. Our study suggests that the fossil record can be a suitable tool for the evaluation of current taxa‐specific Red Listing status.     

The fossil record of extant elasmobranchs

Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various anthropogenic pressures. The abundant fossil record of fossil taxa has allowed the tracing of the evolutionary history of modern elasmobranchs to at least 250 MYA; nonetheless, exactly how far back the fossil record of living taxa goes has never been collectively surveyed. In this study, the authors assess the representation and extent of the fossil record of elasmobranchs currently living in our oceans by collecting their oldest records and quantifying first appearance dates at different taxonomic levels (i.e., orders, families, genera and species), ecological traits (e.g., body size, habitat and feeding mechanism) and extinction risks (i.e., threatened, not threatened and data deficient). The results of this study confirm the robust representation of higher taxonomic ranks, with all orders, most of the families and over half of the extant genera having a fossil record. Further, they reveal that 10% of the current global species diversity is represented in the geological past. Sharks are better represented and extend deeper in time than rays and skates. While the fossil record of extant genera (e.g., the six gill sharks, Hexanchus) goes as far back as c. 190 MYA, the fossil record of extant species (e.g., the sand shark, Carcharias taurus Rafinesque 1810) extends c. 66 MYA. Although no significant differences were found in the extent of the fossil record between ecological traits, it was found that the currently threatened species have a significantly older fossil record than the not threatened species. This study demonstrate that the fossil record of extant elasmobranchs extends deep into the geologic time, especially in the case of threatened sharks. As such, the elasmobranch geological history has great potential to advance the understanding of how species currently facing extinction have responded to different stressors in the past, thereby providing a deep-time perspective to conservation.     

The role of character displacement in the molarization of hominin mandibular premolars

Closely related species are likely to experience resource competition in areas where their ranges overlap. Fossil evidence suggests that hominins in East Africa c. 2–1.5 million years ago may have lived synchronically and sympatrically, and that competition may have contributed to the different tooth sizes observed in Homo and Paranthropus. To assess the likelihood that these taxa overlapped, we applied a character displacement model to the postcanine tooth size of fossil hominins and validated this model in populations of living primates. Mandibular fourth premolar (P₄) crown size was measured from fossil taxa and from living primate species where dietary overlap is established. Dimensions of the P₄ crown were fitted to a character matrix and described as the response variables of a generalized linear model that took taxon and location as input variables. The model recovered significant divergence in samples of closely related, living primates. When applied to fossil hominins the same model detected strong indications of character displacement between early Homo and Paranthropus (P = 0.002) on the basis of their P₄ crown size. Our study is an example of how ecologically informed morphologies measured in appropriate extant referents can provide a comparative context for assessing community and ecological evolution in the fossil record.     

The fossil record of Glypturus Stimpson, 1866 (Crustacea,Decapoda, Axiidea,Callianassidae) revisited,with notes on palaeoecology and palaeobiogeography

Abstract: The fossil record of the callianassid genus Glypturus (Decapoda, Axiidea) is re‐evaluated. Our systematic revision, both of extant and fossil taxa, is based on major cheliped morphology only, thus providing an important impetus for palaeontological studies. Both spination and tuberculation of chelipeds are herein considered of great taxonomic importance. Presence of spines on the upper margins of the merus and propodus and the lower margin of the carpus are significant for generic assignment, whereas the extent of tuberculation on lateral surfaces of the propodus is important for assignment at the species level. Altogether, four extant and six exclusively fossil species of Glypturus are recognized. Several extinct callianassid taxa are now transferred to the genus, namely Callianassa berryi, Callianassa fraasi, Callianassa munieri, Callianassa pugnax and Callianassaspinosa; Callianassa pseudofraasi is considered a junior synonym of C. fraasi. Based on a comparison of ecological preferences of extant representatives, the presence of Glypturus in the fossil record is considered to be linked with tropical to subtropical, nearshore carbonate environments of normal salinity. We argue that Glypturus is of Tethyan origin, with a stratigraphical range going as far back as the Eocene. Since then, the genus migrated both westwards and eastwards, establishing present‐day communities in the western Atlantic and Indo‐West Pacific which both comprise several distinct species. In the presumed area of origin, the genus does no longer occur today. The exlusively fossil (middle Eocene) genus Eoglypturus from Italy is considered closely related to Glypturus and is thus assigned to the subfamily Callichirinae as well.     

A palaeobiological examination of the geological evidence for recurring outbreaks of the crown-of-thorns starfish,Acanthaster planci (L.)

Geological investigations of the crown-of-thorns starfish (COTS) have concluded that outbreaks are not only recent but also have occurred in the past. The evidence lies in the abundance of COTS skeletal elements found both on the sea floor and within the underlying sedimentary record. These studies are flawed in three respects. First, the processes of fossil preservation from the living population to the fossil assemblage have been ignored. Second, it has not been demonstrated that the fossil skeletal elements representing alleged outbreak populations of starfish are of the same age. Third, the existence of a relationship between the number of COTS skeletal elements sampled from the sedimentary record and the relative abundance of COTS in the once living population has not been substantiated. The limitations introduced when studying the fossil record need to be established through taphonomic analyses of the COTS. Techniques which will allow greater temporal resolution of skeletal element age include amino-acid geochronology, analysis of sedimentation mode and rate, and correlation among sub-surface cores. In order to establish a relationship between the number of fossil COTS elements and the original population size, methods must be developed which will relate the number of fossil skeletal elements to the relative abundance of starfish in both the fossil and death assemblages and then to relate the latter to the relative size of the original population. When these approaches are used together it may be possible to make some estimate of relative COTS abundances based on data contained in the fossil record.     

Ancient DNA sheds new light on the Svalbard foraminiferal fossil record of the last millennium

Recent palaeogenetic studies have demonstrated the occurrence of preserved ancient DNA (aDNA) in various types of fossilised material. Environmental aDNA sequences assigned to modern species have been recovered from marine sediments dating to the Pleistocene. However, the match between the aDNA and the fossil record still needs to be evaluated for the environmental DNA approaches to be fully exploited. Here, we focus on foraminifera in sediments up to one thousand years old retrieved from the Hornsund fjord (Svalbard). We compared the diversity of foraminiferal microfossil assemblages with the diversity of aDNA sequenced from subsurface sediment samples using both cloning and high‐throughput sequencing (HTS). Our study shows that 57% of the species archived in the fossil record were also detected in the aDNA data. However, the relative abundance of aDNA sequence reads and fossil specimens differed considerably. We also found a limited match between the stratigraphic occurrence of some fossil species and their aDNA sequences, especially in the case of rare taxa. The aDNA data comprised a high proportion of non‐fossilised monothalamous species, which are known to dominate in modern foraminiferal communities of the Svalbard region. Our results confirm the relevance of HTS for studying past micro‐eukaryotic diversity and provide insight into its ability to reflect fossil assemblages. Palaeogenetic studies including aDNA analyses of non‐fossilised groups expand the range of palaeoceanographical proxies and therefore may increase the accuracy of palaeoenvironmental reconstructions.     

Evolving between land and water: key questions on the emergence and history of the Hippopotamidae (Hippopotamoidea,Cetancodonta, Cetartiodactyla)

The fossil record of the Hippopotamidae can shed light on three major issues in mammalian evolution. First, as the Hippopotamidae are the extant sister group of Cetacea, gaining a better understanding of the origin of the Hippopotamidae and of their Paleogene ancestors will be instrumental in clarifying phylogenetic relationships within Cetartiodactyla. Unfortunately, the data relevant to hippopotamid origins have generally been ignored in phylogenetic analyses of cetartiodactyls. In order to obtain better resolution, future analyses should consider hypotheses of hippopotamid Paleogene relationships. Notably, an emergence of the Hippopotamidae from within anthracotheriids has received growing support, leading to reconciliation between genetic and morphological evidence for the clade Cetancodonta (Hippopotamidae + Cetacea). Secondly, full account needs to be taken of the Hippopotamidae when studying the impact of environmental change on faunal evolution. This group of semi‐aquatic large herbivores has a clear and distinct ecological role and a diverse and abundant fossil record, particularly in the African Neogene. We examine three major phases of hippopotamid evolution, namely the sudden appearance of hippopotamines in the late Miocene (the “Hippopotamine Event”), the subsequent rampant endemism in African basins, and the Pleistocene expansion of Hippopotamus. Each may have been influenced by multiple factors, including: late Miocene grass expansion, African hydrographical network disruption, and a unique set of adaptations that allowed Hippopotamus to respond efficiently to early Pleistocene environmental change. Thirdly, the fossil record of the Hippopotamidae documents the independent emergence of adaptive character complexes in relation to semiaquatic habits and in response to insular isolation. The semiaquatic specializations of fossil hippopotamids are particularly useful in interpreting the functional morphology and ecology of other, extinct groups of large semiaquatic herbivores. Hippopotamids can also serve as models to elucidate the evolutionary dynamics of island mammals.     

Wetland megabias: ecological and ecophysiological filtering dominates the fossil record of hot spring floras

Siliceous hot spring deposits form at Earth's surface above terrestrial hydrothermal systems, which create low‐sulphidation epithermal mineral deposits deeper in the crust. Eruption of hot spring waters and precipitation of opal‐A create sinter apron complexes and areas of geothermally influenced wetland. These provide habitat for higher plants that may be preserved in situ, by encrustation of their surfaces and permineralization of tissues, creating T⁰ plant assemblages. In this study, we review the fossil record of hot spring floras from subfossil examples forming in active hot spring areas, via fossil examples from the Cenozoic, Mesozoic and Palaeozoic to the oldest known hot spring flora, the Lower Devonian Rhynie chert. We demonstrate that the well‐known megabias towards wetland plant preservation extends to hot spring floras. We highlight that the record of hot spring floras is dominated by plants preserved in situ by permineralization on geothermally influenced wetlands. Angiosperms (members of the Cyperaceae and Restionaceae) dominate Cenozoic floras. Equisetum and gleicheniaceous ferns colonized Mesozoic (Jurassic) geothermal wetlands and sphenophytes and herbaceous lycophytes late Palaeozoic examples. Evidence of the partitioning of wetland hydrophytic and dryland mesophytic communities, a feature of active geothermal areas, is provided by well‐preserved and well‐exposed fossil sinter apron complexes, which record flooding of dryland environments by thermal waters and decline of local forest ecosystems. Such observations from the fossil record back‐up hypotheses based on active hot springs and vegetation that suggest removal of taphonomic filtering in hot spring environments is accompanied by an increase in ecological and ecophysiological filtering. To this end we also demonstrate that in the hot spring environment, the wetland bias extends beyond broad ecology. We show that ecosystems preserved from the Cenozoic to the Mesozoic provide clear evidence that the dominant plants preserved in situ by hot spring activity are also halophytic, tolerant of high pH and high concentrations of heavy metals. By extension, we hypothesize that this is also the case in Palaeozoic hot spring settings and extended to the early land plant flora of the Rhynie chert.     

Extinction of South American sparassodontans (Metatheria): environmental fluctuations or complex ecological processes?

Sparassodontans are a diverse but now extinct group of metatherians that were apex predators in South America during most of the Cenozoic. Studying their decline has been controversial mainly due to the scarcity of the fossil record, and different methodological approaches have led to contradictory hypotheses. In an effort to explore questions about their extinction, we developed a novel multi‐model statistical approach to analyse all of the currently available data at a continental scale. Using multiple regression analysis and new advances in beta diversity analysis, we used all currently available fossil data at a continental scale to test four competing hypotheses to account for the decline of sparassodontans: competition with placental carnivorans, competition with avian phorusrhacids, non‐competitive ecological interactions, and environmental fluctuations. Our results show that the sparassodontan extinction was a gradual process with species disappearing throughout the Cenozoic. Multiple regression analysis supported non‐competitive ecological interactions as the best extinction model. Native South American ungulates, African migrants (caviomorph rodents and platyrrhine primates) and didelphimorphians were the groups with the highest statistical significance. Sparassodontan beta diversity increased between South American Land Mammal Ages after the Paleocene–Eocene boundary. Our results demonstrate that ecological modelling techniques illuminate aspects of extinction processes whilst mitigating the limitations of the fossil record. Our study suggests that non‐competitive ecological interactions could have been the main driver for sparassodontan extinction rather than, as commonly assumed, a result of competition and/or abiotic fluctuations.     

Darwin and palaeontology: a re-evaluation of his interpretation of the fossil record

Charles Darwin's empirical research in palaeontology, especially on fossil invertebrates, has been relatively neglected as a source of insight into his thinking, other than to note that he viewed the fossil record as very incomplete. During the Beagle voyage, Darwin gained extensive experience with a wide diversity of fossil taxa, and he thought deeply about the nature of the fossil record. That record was, for him, a major source of evidence for large-scale transmutation, but much less so for natural selection or single lineages. Darwin's interpretation of the fossil record has been criticised for its focus on incompleteness, but the record as he knew it was extremely incomplete. He was compelled to address this in arguing for descent with modification, which was likely his primary goal. Darwin's gradualism has been both misrepresented and exaggerated, and has distracted us from the importance of the fossil record in his thinking, which should be viewed in the context of the multiple, sometimes competing demands of the multifaceted argument he presented in the Origin of Species.     

Living phosphatic stromatolites in a low‐phosphorus environment: Implications for the use of phosphorus as a proxy for phosphate levels in paleo‐systems

In the geological record, fossil phosphatic stromatolites date back to the Great Oxidation Event in the Paleoproterozoic, but living phosphatic stromatolites have not been described previously. Here, we report on cyanobacterial stromatolites in a supratidal freshwater environment at Cape Recife, South African southern coast, precipitating Ca carbonate alternating with episodes of Ca phosphate deposition. In their structure and composition, the living stromatolites from Cape Recife closely resemble their fossilized analogues, showing phosphatic zonation, microbial casts, tunnel structures and phosphatic crusts of biogenic origin. The microbial communities appear to be also similar to those proposed to have formed fossil phosphatic stromatolites. Phosphatic domains in the material from Cape Recife are spatially and texturally associated with carbonate precipitates, but form distinct entities separated by sharp boundaries. Electron Probe Micro‐Analysis shows that Ca/P ratios and the overall chemical compositions of phosphatic precipitates are in the range of octacalcium phosphate, amorphous tricalcium phosphate and apatite. The coincidence in time of the emergence of phosphatic stromatolites in the fossil record with a major episode of atmospheric oxidation led to the assumption that at times of increased oxygen release the underlying increased biological production may have been linked to elevated phosphorus availability. The stromatolites at Cape Recife, however, form in an environment where ambient phosphorus concentrations do not exceed 0.28 μM, one to two orders of magnitude below the previously predicted minimum threshold of >5 μM for biogenic phosphate precipitation in paleo‐systems. Accordingly, we contest the previously proposed suitability of phosphatic stromatolites as a proxy for high ambient phosphate concentrations in supratidal to shallow ocean settings in earth history.     

Evaluation of the fossil fish‐specific diversity in a chadian continental assemblage: Exploration of morphological continuous variation in Synodontis (Ostariophysi,Siluriformes)

In the fossil record, the quantification of continuous morphological variation has become a central issue when dealing with species identification and speciation. In this context, fossil taxa with living representatives hold great promise, because of the potential to characterise patterns of intraspecific morphological variation in extant species prior to any interpretation in the fossil record. The vast majority of catfish families fulfil this prerequisite, as most of them are represented by extant genera. However, although they constitute a major fish group in terms of distribution, and ecological and taxonomic diversity, the quantitative study of their past morphological variation has been neglected, as fossil specimens are generally identified based on the scarcest remains, that is, complete neurocrania that bear discrete characters. Consequently, a part of freshwater catfish history is unprospected and unknown. In this study, we explored the morphological continuous variation of the humeral plate shape in Synodontis catfishes using Elliptic Fourier Analysis (EFA), and compared extant members and fossil counterparts. We analysed 153 extant specimens of 11 Synodontis species present in the Chad basin, in addition to 23 fossil specimens from the Chadian fossiliferous area of Toros Menalla which is dated around 7 Ma. This highly speciose genus, which is one of the most diversified in Africa, exhibits a rich fossil record with several hundred remains mostly identified as Synodontis sp. The analysis of the outline of the humeral plate reveals that some living morphological types were already represented in the Chad Basin 7 My ago, and allows for the discovery of extinct species. Beside illuminating the complex Neogene evolutionary history of Synodontis, these results underline the interest in the ability of isolated remains to reconstruct a past dynamic history and to validate the relevance of EFA as a tool to explore specific diversity through time. J. Morphol. 277:1486–1496, 2016. © 2016 Wiley Periodicals, Inc.     

Invasion of a naticid predator and associated changes in death assemblages of bivalve prey in northern Japan: implications for palaeoecological studies

The recent invasion of a naticid predator (Laguncula pulchella) and associated changes in the death assemblages of bivalve prey (Ruditapes philippinarum) provide a baseline for interpreting predator–prey interactions in the fossil record. This article presents quantitative data on size‐frequency distributions (SFDs) of living and death assemblages, prey size selectivity and drillhole site selectivity from the Tona Coast, northern Japan. Before the appearance of the predator, the SFD of the death assemblage exhibited a right‐skewed platykurtic distribution, and there were very few predatory drillholes. Once the predator appeared, frequencies of predatory drillholes increased, particularly in the smallest size class (2–10 mm shell length). Furthermore, juvenile peaks in the SFDs of death assemblages sharpened, and thus, SFDs exhibited strongly right‐skewed leptokurtic distributions. These changes suggest that intense naticid predation precluded juvenile clams from growing to adulthood, and thus, many dead shells of juvenile clams were introduced into the sediment. The changes in SFDs may also indicate intensification of predation pressure in the fossil record. No temporal shifts in prey size selectivity and drillhole site selectivity were noted, despite substantial changes in the demographics of Ruditapes philippinarum. This suggests that lack of specific size classes of preferred prey species is unlikely to be a primary factor accounting for size mismatches between predator and prey, because, in such situations, naticid predators probably attack other prey species. Therefore, such a factor is unlikely to primarily explain the less stereotypical predatory behaviour (i.e. low prey size selectivity and low drillhole site selectivity), which has been frequently recognized in fossil assemblages. Such less stereotypical predatory behaviour in fossil assemblages is likely to be explained by other factors, such as the existence of multiple predator taxa and lack of specific size classes of all available prey.     

Recognizing pulses of extinction from clusters of last occurrences

The distribution of last occurrences of fossil taxa in a stratigraphic column are used to infer the pattern, timing and tempo of extinction from the fossil record. Clusters of last occurrences are commonly interpreted as an abrupt pulse of extinction. However, stratigraphic architecture alone can produce clusters of last occurrences that can be misinterpreted as an extinction pulse. These clusters will typically occur in strata that immediately underlie facies changes and sequence‐stratigraphic surfaces. It has been proposed that a basin‐wide analysis of the fossil record within a sequence‐stratigraphic framework can be used to distinguish between clusters of last occurrences caused solely by extinction pulses from those generated by sequence‐stratigraphic architecture. A basin‐wide approach makes it possible to observe lateral facies shifts in response to sea‐level change, mitigating the effects of stratigraphic architecture. Using computer simulations of plausible Late Ordovician mass‐extinction scenarios tuned to an inferred Late Ordovician sea‐level curve, we show that stratigraphically‐generated clusters of last occurrences are observed even in basin‐wide analyses of the simulated fossil records due to the basin‐wide loss of preferred facies for many taxa. Nonetheless, we demonstrate that by coarsening the stratigraphic resolution to the systems‐tract level and identifying facies preferences of simulated taxa, we can filter out taxa whose last occurrences coincide with the basin‐wide loss of their preferred facies. This enables consistent identification of the underlying extinction pattern for a wide variety of extinction scenarios. Applying this approach to empirical field data can help to constrain underlying extinction patterns from the fossil record.     

Brief communication: Paleobiological inferences on the locomotor repertoire of extinct hominoids based on femoral neck cortical thickness: The fossil great ape hispanopithecus laietanus as a test-case study

The relationship between femoral neck superior and inferior cortical thickness in primates is related to locomotor behavior. This relationship has been employed to infer bipedalism in fossil hominins, although bipeds share the same pattern of generalized quadrupeds, where the superior cortex is thinner than the inferior one. In contrast, knuckle‐walkers and specialized suspensory taxa display a more homogeneous distribution of cortical bone. These different patterns, probably related to the range of movement at the hip joint and concomitant differences in the load stresses at the femoral neck, are very promising for making locomotor inferences in extinct primates. To evaluate the utility of this feature in the fossil record, we relied on computed tomography applied to the femur of the Late Miocene hominoid Hispanopithecus laietanus as a test‐case study. Both an orthograde body plan and orang‐like suspensory adaptations had been previously documented for this taxon on different anatomical grounds, leading to the hypothesis that this fossil ape should display a modern ape‐like distribution of femoral neck cortical thickness. This is confirmed by the results of this study, leading to the conclusion that Hispanopithecus represents the oldest evidence of a homogeneous cortical bone distribution in the hominoid fossil record. Our results therefore strengthen the utility of femoral neck cortical thickness for making paleobiological inferences on the locomotor repertoire of fossil primates. This feature would be particularly useful for assessing the degree of orthograde arboreal locomotor behaviors vs. terrestrial bipedalism in putative early hominins. Am J PhyAnthropol 2012. © Wiley Periodicals, Inc.     

Heavily loaded flight and limits to the maximum size of dragonflies (Anisoptera) and griffenflies (Meganisoptera)

An original hypothesis is presented that the maximum mass and size of living anisopteran dragonflies are constrained by a physiological performance limit: the wing muscle power required to permit reproductively successful males to carry heavier females in the so‐called ‘wheel position’ in flight. It is proposed that the same limit cannot have applied to all fossil Odonatoptera. As the physiology of the giant Carboniferous griffenfly Namurotypus sippeli precludes flight in the wheel position, it did not need to carry any substantial load aside from exogenous aerial prey. Based on its thorax dimensions, it is argued that Namurotypus flew with a relatively low maximum specific muscle power output in comparison with living Anisoptera. The extinction of some families of large Mesozoic Odonatoptera may have been exacerbated by competition with smaller (stem‐) Anisoptera that evolved higher specific power outputs and superior flight performance similar to living Anisoptera. To investigate the credibility of this flight‐performance size‐limit hypothesis and its consequences, an analysis of the scaling of the required flight power and available muscle power is presented using allometric relations. It is found that for living Anisoptera and fossil Odonatoptera, there are different limiting sizes, above which the required specific flight power would exceed the available muscle specific power. These limits are directly related to maximum load‐carrying capacity and the atmospheric air density at the habitual altitude. It is suggested that the largest living species of Petaluridae, Petalura ingentissima, is close to the proposed Anisoptera size limit at current near‐sea‐level air density conditions.     

Major issues in the origins of ray‐finned fish (Actinopterygii) biodiversity

Ray‐finned fishes (Actinopterygii) dominate modern aquatic ecosystems and are represented by over 32000 extant species. The vast majority of living actinopterygians are teleosts; their success is often attributed to a genome duplication event or morphological novelties. The remainder are ‘living fossils’ belonging to a few depauperate lineages with long‐retained ecomorphologies: Polypteriformes (bichirs), Holostei (bowfin and gar) and Chondrostei (paddlefish and sturgeon). Despite over a century of systematic work, the circumstances surrounding the origins of these clades, as well as their basic interrelationships and diagnoses, have been largely mired in uncertainty. Here, I review the systematics and characteristics of these major ray‐finned fish clades, and the early fossil record of Actinopterygii, in order to gauge the sources of doubt. Recent relaxed molecular clock studies have pushed the origins of actinopterygian crown clades to the mid‐late Palaeozoic [Silurian–Carboniferous; 420 to 298 million years ago (Ma)], despite a diagnostic body fossil record extending only to the later Mesozoic (251 to 66 Ma). This disjunct, recently termed the ‘Teleost Gap’ (although it affects all crown lineages), is based partly on calibrations from potential Palaeozoic stem‐taxa and thus has been attributed to poor fossil sampling. Actinopterygian fossils of appropriate ages are usually abundant and well preserved, yet long‐term neglect of this record in both taxonomic and systematic studies has exacerbated the gaps and obscured potential synapomorphies. At the moment, it is possible that later Palaeozoic‐age teleost, holostean, chondrostean and/or polypteriform crown taxa sit unrecognized in museum drawers. However, it is equally likely that the ‘Teleost Gap’ is an artifact of incorrect attributions to extant lineages, overwriting both a post‐Palaeozoic crown actinopterygian radiation and the ecomorphological diversity of stem‐taxa.