James Lamsdell | West Virginia University (original) (raw)
Papers by James Lamsdell
Palaeogeography Palaeoclimatology Palaeoecology, 2021
Much of the focus of palaeobiological studies in the last century can be summarized as seeking to... more Much of the focus of palaeobiological studies in the last century can be summarized as seeking to understand how evolutionary lineages occupy new regions of morphological, ecological, and geographic space, or are excluded from those spaces. A desire to understand the processes that lead to morphological change unites a variety of biological disciplines focusing on topics ranging from studies of organism genomics to broad scale macroevolutionary analyses. There is increasing recognition that a hierarchical approach, incorporating both intrinsic genealogical processes and external ecological factors, is necessary to understand the mechanisms behind the drivers of phenotypic change. One of the most important issues that remains to be resolved regards the generation and fixation of morphological changes within evolutionary lineages, including whether the evolution of novel morphologies facilitates expansion to previously unoccupied environments (a developmental push mechanism) or whether a shift in ecological occupation results in subsequent morphologic change (an ecological pull mechanism). The geological record affords a unique perspective on morphological change, preserving both evidence of environmental change through shifts in sedimentology and the changing morphology of evolutionary lineages; as such, palaeontology provides a long-term view of the relationship between ecological and morphological shifts. This review focuses on the ways that phylogenetic palaeoecology, which utilizes phylogenetic frameworks in concert with palaeoecological data, can be leveraged to explore these questions. It begins by reviewing the literature on novelty and innovationthe origination of new morphologies and their proliferation within ecosystemswithin a hierarchical framework and the role of heterochrony as the primary mechanism by which phenotypic change occurs before exploring evidence for developmental push and ecological pull as competing drivers of morphological shifts. Drivers of morphological shifts are examined through analysis of heterochronic trends in horseshoe crab evolution and comparison with case studies on angiosperm plants, giant ground sloths, and megatooth sharks.
Journal of Paleontology
One of the oldest fossil horseshoe crabs figured in the literature is Entomolithus lunatus Martin... more One of the oldest fossil horseshoe crabs figured in the literature is Entomolithus lunatus Martin, 1809, a Carboniferous species included in his Petrificata Derbiensia. While the species has generally been included within the genus Belinurus Bronn, 1839, it was recently used as the type species of the new genus Parabelinurus Lamsdell, 2020. However, recent investigation as to the appropriate authority for Belinurus (see Lamsdell and Clapham, 2021) revealed that all the names in Petrificata Derbiensia were suppressed in Opinion 231 of the International Commission on Zoological Nomenclature (1954) for being consistently nonbinomial under Article 11.4 of the International Code of Zoological Nomenclature (ICZN) (International Commission on Zoological Nomenclature, 1999). Despite the validation of several species names for anthozoans, brachiopods, and cephalopods described in Petrificata Derbiensia in subsequent rulings (International Commission on Zoological Nomenclature, 1956a, b), Bel...
Paleobiology
The burgeoning field of phylogenetic paleoecology (Lamsdell et al. 2017) represents a synthesis o... more The burgeoning field of phylogenetic paleoecology (Lamsdell et al. 2017) represents a synthesis of the related but differently focused fields of macroecology (Brown 1995) and macroevolution (Stanley 1975). Through a combination of the data and methods of both disciplines, phylogenetic paleoecology leverages phylogenetic theory and quantitative paleoecology to explain the temporal and spatial variation in species diversity, distribution, and disparity. Phylogenetic paleoecology is ideally situated to elucidate many fundamental issues in evolutionary biology, including the generation of new phenotypes and occupation of previously unexploited environments; the nature of relationships among character change, ecology, and evolutionary rates; determinants of the geographic distribution of species and clades; and the underlying phylogenetic signal of ecological selectivity in extinctions and radiations. This is because phylogenetic paleoecology explicitly recognizes and incorporates the qu...
Journal of Paleontology
In the first half of the nineteenth century, a marked shift occurred in our understanding and tre... more In the first half of the nineteenth century, a marked shift occurred in our understanding and treatment of the chelicerate fossil record, with the differentiation and recognition of entirely extinct genera for the first time. At the heart of this taxonomic revolution were the Eurypterida (sea scorpions) and Xiphosura (horseshoe crabs), although both groups were in fact considered crustaceans until Lankester's (1881) seminal comparative anatomical study of the extant xiphosuran Limulus Müller, 1785 and modern scorpions. The oldest available eurypterid genus is Eurypterus deKay, 1825; the oldest available fossil arachnid genus name is that of the scorpion Cyclophthalmus Corda, 1835. However, there has been considerable historical confusion over the oldest available fossil xiphosuran genus name, which has been recognized alternately as Belinurus König (with a publication date of either 1820 or 1851) or the synonymous Bellinurus Pictet, 1846. Most recent treatments (e.g., Selden and...
PeerJ
Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and... more Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed ‘living fossils’, recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura—set out in the Treatise of Invertebrate Paleontology in 1955—to be outdated and in need of revision, with several common gene...
Royal Society Open Science
Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, b... more Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, by at least the Silurian. The origin of myriapods and their land colonization have long remained puzzling until euthycarcinoids, an extinct group of aquatic arthropods considered amphibious, were shown to be stem-group myriapods, extending the lineage to the Cambrian and evidencing a marine-to-terrestrial transition. Although possible respiratory structures comparable to the air-breathing tracheal system of myriapods are visible in several euthycarcinoids, little is known about the mechanism by which they respired. Here, we describe a new euthycarcinoid from Upper Devonian alluvio-lagoonal deposits of Belgium. Synchrotron-based elemental X-ray analyses were used to extract all available information from the only known specimen. Sulfur X-ray fluorescence (XRF) mapping and spectroscopy unveil sulfate evaporation stains, spread over the entire slab, suggestive of a very shallow-water to the ...
Paleobiology
Eurypterids are generally considered to comprise a mixture of active nektonic to nektobenthic pre... more Eurypterids are generally considered to comprise a mixture of active nektonic to nektobenthic predators and benthic scavenger-predators exhibiting a mode of life similar to modern horseshoe crabs. However, two groups of benthic stylonurine eurypterids, the Stylonuroidea and Mycteropoidea, independently evolved modifications to the armature of their anterior appendages that have been considered adaptations toward a sweep-feeding life habit, and it has been suggested the evolution toward sweep-feeding may have permitted stylonurines to capture smaller prey species and may have been critical for the survival of mycteropoids during the Late Devonian mass extinction. There is a linear correlation between the average spacing of feeding structures and prey sizes among extant suspension feeders. Here, we extrapolate this relationship to sweep-feeding eurypterids in order to estimate the range of prey sizes that they could capture and examine prey size in a phylogenetic context to determine ...
Paleobiology
The occupation of new environments by evolutionary lineages is frequently associated with morphol... more The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic conditions. One primary mechanism by which phenotypic variants are known to arise is through changes in the timing or duration of organismal development resulting in alterations to adult morphology, a process known as heterochrony. While numerous studies have demonstrated heterochronic trends in association with environmental gradients, few have done so within a phylogenetic context. Understanding species interrelationships is necessary to determine whether morphological change is due to heterochronic processes; however, research is hampered by the lack of a quantitative metric with which to assess the degree of heterochronic traits expressed withi...
Journal of Vertebrate Paleontology
New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northea... more New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northeastern Colombia, including a new taxon of antiarch placoderm (Colombialepis villarroeli, gen. et sp. nov., previously reported as Asterolepis) and a new taxon of arthrodiran placoderm (Colombiaspis rinconensis, gen. et sp. nov.). We also report evidence of a stegotrachelid actinopterygian, a diplacanthid acanthodian (cf. Florestacanthus morenoi), a second antiarch placoderm (Bothriolepis sp.), a putative megalichthyid, and a putative tristichopterid. The absence of typical Euramerican markers, e.g., Asterolepis and Strepsodus, in this assemblage suggests that faunal interchange between Euramerica and Gondwana was less pronounced during the Frasnian–Famennian than previously thought (i.e., the Great Devonian Interchange hypothesis). Three arthropod taxa, including two families of spinicaudatan branchiopods and the eurypterid Pterygotus cf. bolivianus, are found to be associated with the reported vertebrate fauna. Pterygotus cf. bolivianus represents the first eurypterid described from Colombia and the youngest known pterygotid eurypterid, highlighting that pterygotids, which were competitors for large predatory fishes, did not go extinct during the major vertebrate radiation in the Early and Middle Devonian.
Paleobiology
Principal component analysis has been used to test for similarities in ecology and life habit bet... more Principal component analysis has been used to test for similarities in ecology and life habit between modern and fossil birds; however, the two main portions of the hind limb—the foot and the long bone elements—have not been examined separately. We examine the potential links between morphology, ecology, and phylogeny through a synthesis of phylogenetic paleoecological methods and morphospace analysis. Both hind limb morphologies and species’ ecologies exhibit extreme phylogenetic clumping, although these patterns are at least partially explainable by a Brownian motion style of evolution. Some morphologies are strongly correlated with particular ecologies, while some ecologies are occupied by a variety of morphologies. Within the morphospace analyses, the length of the hallux (toe I) is the most defining characteristic of the entire hind limb. The foot and hind limb are represented on different axes when all measurements are considered in an analysis, suggesting that these structure...
Cretaceous Research, 2020
Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are describ... more Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are described from the Late Cretaceous (CenomanianeTuronian) Gara Sbaa Lagerst€ atte of southeast Morocco. These most likely represent juveniles, as suggested by their small size and possession of a number of characteristics, such as short genal spines, that are characteristic of modern juvenile horseshoe crabs. Despite this, the development of the prosomal keel into a broader cardiac ridge and the scalloped lateral margins of the cardiac lobe clearly place these specimens within Mesolimulus. A further characteristic, the occurrence of only two tubercles on the thoracetron pleural ridges, marks Mesolimulus tafraoutensis sp. nov. as a distinct species. As Mesolimulus resolves phylogenetically as a total group limulid outside of the crown group, the new discovery indicates that stem-lineage limulids persisted into the Cretaceous and co-existed with crown-limulids as they underwent their major radiation.
Materials of the LXVI Session of the Paleontological Society of Russia, 2020
In the Belarusian Starobin potassium salt mines (Joint Stock Company «Belaruskali») the rare, but... more In the Belarusian Starobin potassium salt mines (Joint Stock Company «Belaruskali») the rare, but quite well-preserved remains of eurypterids have been found over several decades. Over the course of mining activities between 1976–2012, six eurypterid specimens were collected (Plax et al., 2009; Plax, Barbikov, 2012; Plax, Barbikov, 2013). All eurypterid remains came from the lower sylvinite bed of the third potassium horizon, which is part of the thirteenth salt rythmical bench of potassiferous substratum of the upper saliniferous stratum of the Starobin potassium salt deposits in the Pripyat Trough. According to the Stratigraphic Chart of the Devonian deposits of Belarus (Obukhovskaya et al., 2010), these deposits correspond to the Osovets Beds of the Streshin Regional Stage, the Middle Substage of Famennian, the Upper Devonian. In a recent systematic analysis of all eurypterid remains found up to that point, the authors (Plax et al., 2018) determined these all belong to the same new genus and species Soligorskopterus tchepeliensis Plax, Lamsdell, Vrazo et Barbikov. More recently, in 2019, two new well-preserved eurypterid specimens were found at the same general mining region and we assign them here to S. tchepeliensis.
The first specimen of S. tchepeliensis was found 600 m west of the southern outskirts of the agro-town of Tanezhitsy in Slutsk District, Minsk Region, at a depth of 578 m. It occurs in a clayey interbed with a thickness of about 2–3 cm that separates intercalations of halite (above) and sylvinite (below) of the sixth sylvinite bed, which is a part of the lower sylvinite bed of the third potassium horizon (the thirteenth lithological bench of clayey-halite substratum of the upper
saliniferous stratum). The specimen is near 6 cm long and includes fully articulated prosomal appendages (Fig. 1). Plant remains, including stalks of carbonized plants (from 0,5 to 1,5 cm long) and 2 cm long branch imprint of Adiantites sp. (Yurina A. L., pers. comm.) were also found together with the first specimen.
The second new eurypterid specimen was found at a nearby mining locality, 750 m to the west of the village of Velikiy Bykov (Slutsk District, Minsk Region), at a depth of 746 m. This specimen occurs in a clayey interbed with a thickness of about 4–6 cm that separates the third
sylvinite bed and an overlying halite bed. These beds are also part of the lower sylvinite stratum of the third potassium horizon of the thirteenth lithological bench of clayey-halite substratum of the upper saliniferous stratum. The second specimen is much larger (18 cm long) than the first
one but the appendages are not well preserved (Fig. 2). At a separate location in the same mine, 400 m south of the village of Pogost-2, a single branch of the plant Sphenopteris sp. (Yurina A. L., pers. comm.) was found at a depth of about 722 m in a clay unit interbedded within halite that are part of the lower sylvinite stratum of the third potassium horizon.
The new eurypterid material reveals additional information about the morphology of Soligorskopterus, specifically the broad metastoma with flattened posterior reminiscent to the metastoma of the Silurian stylonuroid Laurieipterus (Waterston, 1962) and the occurrence of a moveable spine posterodistally on podomere seven of appendages V and VI. All new eurypterid and plant specimens are concordant with the ages of the eurypterid specimens previously identified by authors
(Plax et al., 2018) as the Osovets Beds of the Streshin Regional Stage, the Middle Substage of the Famennian Stage, the Upper Devonian.
BMC Evolutionary Biology, 2019
Background: Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assi... more Background: Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assigned to ten genera, with a geologic range extending from the Ordovician to the Devonian. The Late Ordovician (Richmondian) fauna of the Big Hill Lagerstätte includes a new species of chasmataspidid represented by 55 specimens. This taxon is only the second chasmataspidid described from the Ordovician and preserves morphological details unknown from any of the previously described species. Results: The new chasmataspidid species is described as Hoplitaspis hiawathai gen. et sp. nov.. Comparison with all other known chasmataspidids indicates that Hoplitaspis occupies an intermediate morphological position between the Ordovician Chasmataspis and the Silurian-Devonian diploaspidids. While the modification of appendage VI into a broad swimming paddle allies Hoplitaspis to the Diploaspididae, the paddle lacks the anterior 'podomere 7a' found in other diploaspidids and shows evidence of having been derived from a Chasmataspis-like chelate appendage. Other details, such as the large body size and degree of expression of the first tergite, show clear affinities with Chasmataspis, providing strong support for chasmataspidid monophyly.
Journal of Vertebrate Paleontology, 2019
New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northea... more New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northeastern Colombia, including a new taxon of antiarch placoderm (Colombialepis villarroeli, gen. et sp. nov., previously reported as Asterolepis) and a new taxon of arthrodiran placoderm (Colombiaspis rinconensis, gen. et sp. nov.). We also report
evidence of a stegotrachelid actinopterygian, a diplacanthid acanthodian (cf. Florestacanthus morenoi), a second antiarch placoderm (Bothriolepis sp.), a putative megalichthyid, and a putative tristichopterid. The absence of typical Euramerican markers, e.g., Asterolepis and Strepsodus, in this assemblage suggests that faunal interchange between Euramerica and Gondwana was less pronounced during the Frasnian–Famennian than previously thought (i.e., the Great Devonian Interchange hypothesis). Three arthropod taxa, including two families of spinicaudatan branchiopods and the eurypterid
Pterygotus cf. bolivianus, are found to be associated with the reported vertebrate fauna. Pterygotus cf. bolivianus represents the first eurypterid described from Colombia and the youngest known pterygotid eurypterid, highlighting that pterygotids, which were competitors for large predatory fishes, did not go extinct during the major vertebrate radiation in the Early and Middle Devonian.
Geological Magazine, 2019
The Upper Famennian (Upper Devonian) Strud locality has yielded very abundant and diversified flo... more The Upper Famennian (Upper Devonian) Strud locality has yielded very abundant and diversified flora as well as vertebrate and arthropod faunas. The arthropod fauna, mostly recovered from fine shales deposited in a calm, confined floodplain habitat including temporary pools, has delivered a putative insect and various crustaceans including eumalacostracans and notostracan, spinicauda-tan and anostracan branchiopods. Here we present the Strud eurypterids, consisting of semi-articulated juvenile specimens assigned to Hardieopteridae recovered from the pool and floodplain deposits, as well as larger isolated fragments of potential adults recovered from strati-graphically lower, coarser dark sandy layers indicative of a higher-energy fluvial environment. The Strud fossils strongly suggest that, as proposed for some Carboniferous eurypterids, juvenile freshwater eurypterids inhabited sheltered nursery pools and migrated to higher-energy river systems as they matured.
Journal of Paleontology, 2018
We describe a new stylonurid eurypterid from the evaporitic potassium-salt deposits of the Upper ... more We describe a new stylonurid eurypterid from the evaporitic potassium-salt deposits of the Upper Devo-nian (Famennian) Soligorsk Formation in the Pripyat Trough of Belarus. All specimens are assigned to Soli-gorskopterus tchepeliensis new genus new species, which represents the first formally described eurypterid species from Belarus. The occurrence of well-preserved eurypterids in these unusual evaporite deposits is most likely due to transport from freshwater stream habitats into a hypersaline setting following death. Soligorskopterus tchepeliensis n. gen. n. sp. appears to be intermediate between the traditionally considered parastylonurids and stylonurids and thus extends our understanding of stylonurid evolution in the mid-Paleozoic. Soligorskopterus n. gen. extends the occurrence of Famennian eurypterids into eastern Laurussia and the Stylonuridae into the Upper Devonian, and this taxon could be part of a global eurypterid habitat shift that took place in the Late Devonian.
Palaeogeography Palaeoclimatology Palaeoecology, 2021
Much of the focus of palaeobiological studies in the last century can be summarized as seeking to... more Much of the focus of palaeobiological studies in the last century can be summarized as seeking to understand how evolutionary lineages occupy new regions of morphological, ecological, and geographic space, or are excluded from those spaces. A desire to understand the processes that lead to morphological change unites a variety of biological disciplines focusing on topics ranging from studies of organism genomics to broad scale macroevolutionary analyses. There is increasing recognition that a hierarchical approach, incorporating both intrinsic genealogical processes and external ecological factors, is necessary to understand the mechanisms behind the drivers of phenotypic change. One of the most important issues that remains to be resolved regards the generation and fixation of morphological changes within evolutionary lineages, including whether the evolution of novel morphologies facilitates expansion to previously unoccupied environments (a developmental push mechanism) or whether a shift in ecological occupation results in subsequent morphologic change (an ecological pull mechanism). The geological record affords a unique perspective on morphological change, preserving both evidence of environmental change through shifts in sedimentology and the changing morphology of evolutionary lineages; as such, palaeontology provides a long-term view of the relationship between ecological and morphological shifts. This review focuses on the ways that phylogenetic palaeoecology, which utilizes phylogenetic frameworks in concert with palaeoecological data, can be leveraged to explore these questions. It begins by reviewing the literature on novelty and innovationthe origination of new morphologies and their proliferation within ecosystemswithin a hierarchical framework and the role of heterochrony as the primary mechanism by which phenotypic change occurs before exploring evidence for developmental push and ecological pull as competing drivers of morphological shifts. Drivers of morphological shifts are examined through analysis of heterochronic trends in horseshoe crab evolution and comparison with case studies on angiosperm plants, giant ground sloths, and megatooth sharks.
Journal of Paleontology
One of the oldest fossil horseshoe crabs figured in the literature is Entomolithus lunatus Martin... more One of the oldest fossil horseshoe crabs figured in the literature is Entomolithus lunatus Martin, 1809, a Carboniferous species included in his Petrificata Derbiensia. While the species has generally been included within the genus Belinurus Bronn, 1839, it was recently used as the type species of the new genus Parabelinurus Lamsdell, 2020. However, recent investigation as to the appropriate authority for Belinurus (see Lamsdell and Clapham, 2021) revealed that all the names in Petrificata Derbiensia were suppressed in Opinion 231 of the International Commission on Zoological Nomenclature (1954) for being consistently nonbinomial under Article 11.4 of the International Code of Zoological Nomenclature (ICZN) (International Commission on Zoological Nomenclature, 1999). Despite the validation of several species names for anthozoans, brachiopods, and cephalopods described in Petrificata Derbiensia in subsequent rulings (International Commission on Zoological Nomenclature, 1956a, b), Bel...
Paleobiology
The burgeoning field of phylogenetic paleoecology (Lamsdell et al. 2017) represents a synthesis o... more The burgeoning field of phylogenetic paleoecology (Lamsdell et al. 2017) represents a synthesis of the related but differently focused fields of macroecology (Brown 1995) and macroevolution (Stanley 1975). Through a combination of the data and methods of both disciplines, phylogenetic paleoecology leverages phylogenetic theory and quantitative paleoecology to explain the temporal and spatial variation in species diversity, distribution, and disparity. Phylogenetic paleoecology is ideally situated to elucidate many fundamental issues in evolutionary biology, including the generation of new phenotypes and occupation of previously unexploited environments; the nature of relationships among character change, ecology, and evolutionary rates; determinants of the geographic distribution of species and clades; and the underlying phylogenetic signal of ecological selectivity in extinctions and radiations. This is because phylogenetic paleoecology explicitly recognizes and incorporates the qu...
Journal of Paleontology
In the first half of the nineteenth century, a marked shift occurred in our understanding and tre... more In the first half of the nineteenth century, a marked shift occurred in our understanding and treatment of the chelicerate fossil record, with the differentiation and recognition of entirely extinct genera for the first time. At the heart of this taxonomic revolution were the Eurypterida (sea scorpions) and Xiphosura (horseshoe crabs), although both groups were in fact considered crustaceans until Lankester's (1881) seminal comparative anatomical study of the extant xiphosuran Limulus Müller, 1785 and modern scorpions. The oldest available eurypterid genus is Eurypterus deKay, 1825; the oldest available fossil arachnid genus name is that of the scorpion Cyclophthalmus Corda, 1835. However, there has been considerable historical confusion over the oldest available fossil xiphosuran genus name, which has been recognized alternately as Belinurus König (with a publication date of either 1820 or 1851) or the synonymous Bellinurus Pictet, 1846. Most recent treatments (e.g., Selden and...
PeerJ
Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and... more Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed ‘living fossils’, recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura—set out in the Treatise of Invertebrate Paleontology in 1955—to be outdated and in need of revision, with several common gene...
Royal Society Open Science
Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, b... more Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, by at least the Silurian. The origin of myriapods and their land colonization have long remained puzzling until euthycarcinoids, an extinct group of aquatic arthropods considered amphibious, were shown to be stem-group myriapods, extending the lineage to the Cambrian and evidencing a marine-to-terrestrial transition. Although possible respiratory structures comparable to the air-breathing tracheal system of myriapods are visible in several euthycarcinoids, little is known about the mechanism by which they respired. Here, we describe a new euthycarcinoid from Upper Devonian alluvio-lagoonal deposits of Belgium. Synchrotron-based elemental X-ray analyses were used to extract all available information from the only known specimen. Sulfur X-ray fluorescence (XRF) mapping and spectroscopy unveil sulfate evaporation stains, spread over the entire slab, suggestive of a very shallow-water to the ...
Paleobiology
Eurypterids are generally considered to comprise a mixture of active nektonic to nektobenthic pre... more Eurypterids are generally considered to comprise a mixture of active nektonic to nektobenthic predators and benthic scavenger-predators exhibiting a mode of life similar to modern horseshoe crabs. However, two groups of benthic stylonurine eurypterids, the Stylonuroidea and Mycteropoidea, independently evolved modifications to the armature of their anterior appendages that have been considered adaptations toward a sweep-feeding life habit, and it has been suggested the evolution toward sweep-feeding may have permitted stylonurines to capture smaller prey species and may have been critical for the survival of mycteropoids during the Late Devonian mass extinction. There is a linear correlation between the average spacing of feeding structures and prey sizes among extant suspension feeders. Here, we extrapolate this relationship to sweep-feeding eurypterids in order to estimate the range of prey sizes that they could capture and examine prey size in a phylogenetic context to determine ...
Paleobiology
The occupation of new environments by evolutionary lineages is frequently associated with morphol... more The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic conditions. One primary mechanism by which phenotypic variants are known to arise is through changes in the timing or duration of organismal development resulting in alterations to adult morphology, a process known as heterochrony. While numerous studies have demonstrated heterochronic trends in association with environmental gradients, few have done so within a phylogenetic context. Understanding species interrelationships is necessary to determine whether morphological change is due to heterochronic processes; however, research is hampered by the lack of a quantitative metric with which to assess the degree of heterochronic traits expressed withi...
Journal of Vertebrate Paleontology
New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northea... more New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northeastern Colombia, including a new taxon of antiarch placoderm (Colombialepis villarroeli, gen. et sp. nov., previously reported as Asterolepis) and a new taxon of arthrodiran placoderm (Colombiaspis rinconensis, gen. et sp. nov.). We also report evidence of a stegotrachelid actinopterygian, a diplacanthid acanthodian (cf. Florestacanthus morenoi), a second antiarch placoderm (Bothriolepis sp.), a putative megalichthyid, and a putative tristichopterid. The absence of typical Euramerican markers, e.g., Asterolepis and Strepsodus, in this assemblage suggests that faunal interchange between Euramerica and Gondwana was less pronounced during the Frasnian–Famennian than previously thought (i.e., the Great Devonian Interchange hypothesis). Three arthropod taxa, including two families of spinicaudatan branchiopods and the eurypterid Pterygotus cf. bolivianus, are found to be associated with the reported vertebrate fauna. Pterygotus cf. bolivianus represents the first eurypterid described from Colombia and the youngest known pterygotid eurypterid, highlighting that pterygotids, which were competitors for large predatory fishes, did not go extinct during the major vertebrate radiation in the Early and Middle Devonian.
Paleobiology
Principal component analysis has been used to test for similarities in ecology and life habit bet... more Principal component analysis has been used to test for similarities in ecology and life habit between modern and fossil birds; however, the two main portions of the hind limb—the foot and the long bone elements—have not been examined separately. We examine the potential links between morphology, ecology, and phylogeny through a synthesis of phylogenetic paleoecological methods and morphospace analysis. Both hind limb morphologies and species’ ecologies exhibit extreme phylogenetic clumping, although these patterns are at least partially explainable by a Brownian motion style of evolution. Some morphologies are strongly correlated with particular ecologies, while some ecologies are occupied by a variety of morphologies. Within the morphospace analyses, the length of the hallux (toe I) is the most defining characteristic of the entire hind limb. The foot and hind limb are represented on different axes when all measurements are considered in an analysis, suggesting that these structure...
Cretaceous Research, 2020
Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are describ... more Two specimens of a new species of horseshoe crab, Mesolimulus tafraoutensis sp. nov., are described from the Late Cretaceous (CenomanianeTuronian) Gara Sbaa Lagerst€ atte of southeast Morocco. These most likely represent juveniles, as suggested by their small size and possession of a number of characteristics, such as short genal spines, that are characteristic of modern juvenile horseshoe crabs. Despite this, the development of the prosomal keel into a broader cardiac ridge and the scalloped lateral margins of the cardiac lobe clearly place these specimens within Mesolimulus. A further characteristic, the occurrence of only two tubercles on the thoracetron pleural ridges, marks Mesolimulus tafraoutensis sp. nov. as a distinct species. As Mesolimulus resolves phylogenetically as a total group limulid outside of the crown group, the new discovery indicates that stem-lineage limulids persisted into the Cretaceous and co-existed with crown-limulids as they underwent their major radiation.
Materials of the LXVI Session of the Paleontological Society of Russia, 2020
In the Belarusian Starobin potassium salt mines (Joint Stock Company «Belaruskali») the rare, but... more In the Belarusian Starobin potassium salt mines (Joint Stock Company «Belaruskali») the rare, but quite well-preserved remains of eurypterids have been found over several decades. Over the course of mining activities between 1976–2012, six eurypterid specimens were collected (Plax et al., 2009; Plax, Barbikov, 2012; Plax, Barbikov, 2013). All eurypterid remains came from the lower sylvinite bed of the third potassium horizon, which is part of the thirteenth salt rythmical bench of potassiferous substratum of the upper saliniferous stratum of the Starobin potassium salt deposits in the Pripyat Trough. According to the Stratigraphic Chart of the Devonian deposits of Belarus (Obukhovskaya et al., 2010), these deposits correspond to the Osovets Beds of the Streshin Regional Stage, the Middle Substage of Famennian, the Upper Devonian. In a recent systematic analysis of all eurypterid remains found up to that point, the authors (Plax et al., 2018) determined these all belong to the same new genus and species Soligorskopterus tchepeliensis Plax, Lamsdell, Vrazo et Barbikov. More recently, in 2019, two new well-preserved eurypterid specimens were found at the same general mining region and we assign them here to S. tchepeliensis.
The first specimen of S. tchepeliensis was found 600 m west of the southern outskirts of the agro-town of Tanezhitsy in Slutsk District, Minsk Region, at a depth of 578 m. It occurs in a clayey interbed with a thickness of about 2–3 cm that separates intercalations of halite (above) and sylvinite (below) of the sixth sylvinite bed, which is a part of the lower sylvinite bed of the third potassium horizon (the thirteenth lithological bench of clayey-halite substratum of the upper
saliniferous stratum). The specimen is near 6 cm long and includes fully articulated prosomal appendages (Fig. 1). Plant remains, including stalks of carbonized plants (from 0,5 to 1,5 cm long) and 2 cm long branch imprint of Adiantites sp. (Yurina A. L., pers. comm.) were also found together with the first specimen.
The second new eurypterid specimen was found at a nearby mining locality, 750 m to the west of the village of Velikiy Bykov (Slutsk District, Minsk Region), at a depth of 746 m. This specimen occurs in a clayey interbed with a thickness of about 4–6 cm that separates the third
sylvinite bed and an overlying halite bed. These beds are also part of the lower sylvinite stratum of the third potassium horizon of the thirteenth lithological bench of clayey-halite substratum of the upper saliniferous stratum. The second specimen is much larger (18 cm long) than the first
one but the appendages are not well preserved (Fig. 2). At a separate location in the same mine, 400 m south of the village of Pogost-2, a single branch of the plant Sphenopteris sp. (Yurina A. L., pers. comm.) was found at a depth of about 722 m in a clay unit interbedded within halite that are part of the lower sylvinite stratum of the third potassium horizon.
The new eurypterid material reveals additional information about the morphology of Soligorskopterus, specifically the broad metastoma with flattened posterior reminiscent to the metastoma of the Silurian stylonuroid Laurieipterus (Waterston, 1962) and the occurrence of a moveable spine posterodistally on podomere seven of appendages V and VI. All new eurypterid and plant specimens are concordant with the ages of the eurypterid specimens previously identified by authors
(Plax et al., 2018) as the Osovets Beds of the Streshin Regional Stage, the Middle Substage of the Famennian Stage, the Upper Devonian.
BMC Evolutionary Biology, 2019
Background: Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assi... more Background: Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assigned to ten genera, with a geologic range extending from the Ordovician to the Devonian. The Late Ordovician (Richmondian) fauna of the Big Hill Lagerstätte includes a new species of chasmataspidid represented by 55 specimens. This taxon is only the second chasmataspidid described from the Ordovician and preserves morphological details unknown from any of the previously described species. Results: The new chasmataspidid species is described as Hoplitaspis hiawathai gen. et sp. nov.. Comparison with all other known chasmataspidids indicates that Hoplitaspis occupies an intermediate morphological position between the Ordovician Chasmataspis and the Silurian-Devonian diploaspidids. While the modification of appendage VI into a broad swimming paddle allies Hoplitaspis to the Diploaspididae, the paddle lacks the anterior 'podomere 7a' found in other diploaspidids and shows evidence of having been derived from a Chasmataspis-like chelate appendage. Other details, such as the large body size and degree of expression of the first tergite, show clear affinities with Chasmataspis, providing strong support for chasmataspidid monophyly.
Journal of Vertebrate Paleontology, 2019
New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northea... more New vertebrate remains are reported from the Late Devonian (?Frasnian) Cuche Formation of northeastern Colombia, including a new taxon of antiarch placoderm (Colombialepis villarroeli, gen. et sp. nov., previously reported as Asterolepis) and a new taxon of arthrodiran placoderm (Colombiaspis rinconensis, gen. et sp. nov.). We also report
evidence of a stegotrachelid actinopterygian, a diplacanthid acanthodian (cf. Florestacanthus morenoi), a second antiarch placoderm (Bothriolepis sp.), a putative megalichthyid, and a putative tristichopterid. The absence of typical Euramerican markers, e.g., Asterolepis and Strepsodus, in this assemblage suggests that faunal interchange between Euramerica and Gondwana was less pronounced during the Frasnian–Famennian than previously thought (i.e., the Great Devonian Interchange hypothesis). Three arthropod taxa, including two families of spinicaudatan branchiopods and the eurypterid
Pterygotus cf. bolivianus, are found to be associated with the reported vertebrate fauna. Pterygotus cf. bolivianus represents the first eurypterid described from Colombia and the youngest known pterygotid eurypterid, highlighting that pterygotids, which were competitors for large predatory fishes, did not go extinct during the major vertebrate radiation in the Early and Middle Devonian.
Geological Magazine, 2019
The Upper Famennian (Upper Devonian) Strud locality has yielded very abundant and diversified flo... more The Upper Famennian (Upper Devonian) Strud locality has yielded very abundant and diversified flora as well as vertebrate and arthropod faunas. The arthropod fauna, mostly recovered from fine shales deposited in a calm, confined floodplain habitat including temporary pools, has delivered a putative insect and various crustaceans including eumalacostracans and notostracan, spinicauda-tan and anostracan branchiopods. Here we present the Strud eurypterids, consisting of semi-articulated juvenile specimens assigned to Hardieopteridae recovered from the pool and floodplain deposits, as well as larger isolated fragments of potential adults recovered from strati-graphically lower, coarser dark sandy layers indicative of a higher-energy fluvial environment. The Strud fossils strongly suggest that, as proposed for some Carboniferous eurypterids, juvenile freshwater eurypterids inhabited sheltered nursery pools and migrated to higher-energy river systems as they matured.
Journal of Paleontology, 2018
We describe a new stylonurid eurypterid from the evaporitic potassium-salt deposits of the Upper ... more We describe a new stylonurid eurypterid from the evaporitic potassium-salt deposits of the Upper Devo-nian (Famennian) Soligorsk Formation in the Pripyat Trough of Belarus. All specimens are assigned to Soli-gorskopterus tchepeliensis new genus new species, which represents the first formally described eurypterid species from Belarus. The occurrence of well-preserved eurypterids in these unusual evaporite deposits is most likely due to transport from freshwater stream habitats into a hypersaline setting following death. Soligorskopterus tchepeliensis n. gen. n. sp. appears to be intermediate between the traditionally considered parastylonurids and stylonurids and thus extends our understanding of stylonurid evolution in the mid-Paleozoic. Soligorskopterus n. gen. extends the occurrence of Famennian eurypterids into eastern Laurussia and the Stylonuridae into the Upper Devonian, and this taxon could be part of a global eurypterid habitat shift that took place in the Late Devonian.
"The original Arachnomorpha hypothesis placed trilobites and a number of predominantly early Pala... more "The original Arachnomorpha hypothesis placed trilobites and a number of predominantly early Palaeozoic arthropod taxa, includ-ing megacheirans, in the chelicerate stem lineage. As an alternative hypothesis, Lamellipedia (Arachnomorpha excluding mega-cheirans) was originally interpreted as a paraphyletic grade of derivatives of the chelicerate stem lineage but,are increasingly being discussed as possible stem-lineage mandibulates under a modified lamellipedian hypothesis.
Here, we present a broad analysis aimed at testing the Arachnomorpha hypotheses versus the modified lamellipedian hy-pothesis. Most of the over 80 included taxa are extinct, and the more than 170 characters coded focus on external morphology recognizable in the fossils. Euchelicerates are represented by three extant and 14 extinct species spanning synziphosurines, xiphosurids, chasmataspids, eurypterids, and scorpions. Three extant myriapods are included in acknowledgement of the Paradoxopoda versus Mandibulata controversy. Crustacea is represented by five extant and seven extinct taxa, including cephalocarids, branchiopods, mystacocarids, remipedes, and malacostracans, as well as four supposed derivatives of the crustacean stem lineage. Lamellipedians are represented by marrellomorphs, 16 artiopod species, three of which are trilo-bites, as well as Cheloniellon calmani, Sidneyia inexpectans, Squamacula clypeata, and Aglaspis spinifer. Five megachei-rans were included. Also, several controversial or problematic taxa were included, such as Canadaspis perfecta, Agnostus pisiformis, Sanctacaris uncata, Kiisortoqia soperi, and Sarotrocercus oblita. The stem-lineage euarthropod Shankouia zhengei was selected as outgroup. The matrix was run in TNT.
Results strongly support the position of all ingroup taxa except Fuxianhuia protensa within a well-supported Euarthropoda. Arthropods with a great appendage and Kiisortoqia are well supported as derivatives of the stem lineage of a monophyletic Chelicerata (Pycnogonida + Euchelicerata). Canadaspis is retrieved as an early derivative of the mandibulate stem line-age, and artiopods are rendered a paraphyletic grade giving rise to Crustacea. These results falsify the Arachnomorpha hypothesis, but are more in line with the modified lamellipedian hypothesis. "
The description of new eurypterids from the Lower Devonian Cottonwood Canyon formation of Wyoming... more The description of new eurypterids from the Lower Devonian Cottonwood Canyon formation of Wyoming reveals that at least some species underwent relatively drastic changes in morphology from juvenile to adult stages. Ontogenetic data can be a great source of information for reconstructing the relationships of organisms, with modern evolutionary theory recognising the importance of heterochronic changes in producing novel morphologies and that juveniles can sometimes exhibit plesiomorphic characteristics absent in adults. Furthermore, the juveniles of some arthropod groups exhibit potentially clade-defining characteristics that are not expressed in adults. The impact that ontogenetic data can have on phylogenetic reconstruction of relationships has received relatively little attention among palaeontologists, for whom such data is usually sparse or lacking altogether. Consequently, it is unclear how coding juvenile instars into a phylogeny of adult exemplars would affect the relationships and character polarity of ingroup taxa. Using the example of the Cottonwood Canyon eurypterids, the impact of coding juvenile individuals into established parsimony-based eurypterid phylogenies is explored and a method for dealing with phylogenetic data in morphological phylogenies is suggested. The potential impact of paedomorphosis and peramorphosis on tree topology is discussed and the importance of differentiating between adult and juvenile arthropod morphologies is highlighted.
Trilobites are the iconic invertebrate fossils of the Palaeozoic yet consensus on their relations... more Trilobites are the iconic invertebrate fossils of the Palaeozoic yet consensus on their relationships with other arthropods remains elusive. The Arachnomorpha hypothesis, formulated in the first half of the last century, placed trilobites with a range of predominantly early Palaeozoic arthropod taxa in the chelicerate stem lineage. The alternative placement of trilobites in the crustacean or mandibulate stem lineage has found its advocates but, during the last 60 years, most pundits have favoured Arachnomorpha. Among the non-trilobite taxa affiliated with the arachnomorphs, the megacheirans have garnered particular attention. Early proponents of the Arachnomorpha hypothesis identified these taxa with a large, preoral, ostensibly raptorial appendage as particularly close to the chelicerate crown group. This great appendage was considered to be homologous to the chelicera, a hypothesis that has recently been revived by some researchers. Subsequent workers have stressed the similarity of the postoral appendages in many arachnomorph taxa to the exclusion of megacheirans. The new Lamellipedia was interpreted as a paraphyletic grade of derivatives of the chelicerate stem lineage. Homology of the great appendage with the chelicera was discounted and the megacheirans considered derivatives of the euarthropod stem.
In the last decade, neuroanatomical and developmental data brought renewed momentum to the great appendage–chelicera hypothesis. This also led to a reevaluation of the relationships of lamellipedians, which are increasingly being discussed as possible stem-lineage mandibulates under a modified lamellipedian hypothesis. Most cladistic studies of fossil arthropods published since have focussed on the interrelationships of arachnomorph or lamellipedian taxa. Taxon sampling is usually limited to the supposed ingroups or only part thereof, with the outgroup frequently being a supposed basal lamellipedian or arachnomorph.
Here, we present a broader analysis, aimed at testing the Arachnomorpha hypotheses versus the modified lamellipedian hypothesis. Most of the over 80 included taxa are extinct, and the more than 170 characters coded focus on external morphology recognizable in the fossils. Euchelicerates are represented by three extant and 14 extinct species spanning synziphosurines, xiphosurids, chasmataspids, eurypterids, and scorpions. Three extant myriapods are included in acknowledgement of the Paradoxopoda versus Mandibulata controversy. Crustacea is represented by five extant and seven extinct taxa, including cephalocarids, branchiopods, mystacocarids, remipedes, and malacostracans, as well as four supposed derivatives of the crustacean stem lineage. Lamellipedians are represented by marrellomorphs, 16 artiopod species, three of which are trilobites, as well as Cheloniellon calmani, Sidneyia inexpectans, Squamacula clypeata, and Aglaspis spinifer. Among arthropods with a great appendage, five megacheirans and the bivalved Isoxys acutangulus were included. Also, several controversial or problematic taxa were included, such as Canadaspis perfecta, Agnostus pisiformis, Sanctacaris uncata, Kiisortoqia soperi, and Sarotrocercus oblita. The stem-lineage euarthropod Shankouia zhengei was selected as outgroup. The matrix was run in TNT.
Results strongly support the position of all ingroup taxa except Fuxianhuia protensa within a well-supported Euarthropoda, contradicting earlier hypotheses that arthropods with a great appendage, Canadaspis, or Sarotrocercus represent stem-lineage euarthropods. Arthropods with a great appendage and Kiisortoqia are well supported as derivatives of the stem lineage of a monophyletic Chelicerata (Pycnogonida + Euchelicerata). Canadaspis is retrieved as an early derivative of the mandibulate stem lineage, and Sarotrocercus falls among the artiopod taxa which are rendered a paraphyletic grade giving rise to Crustacea. These results falsify the Arachnomorpha hypothesis, but are more in line with the modified lamellipedian hypothesis. Trilobites are likely representatives of the crustacean or pancrustacean/tetraconatan stem lineage.
Recent restudy of a number of fossil arthropods has revealed inconsistencies in the treatment of ... more Recent restudy of a number of fossil arthropods has revealed inconsistencies in the treatment of the articulation devices between their trunk segments: for example the definition of an ‘articulating half-ring’ is rather liberally applied. A number of different articulation morphologies are identified; an anterior axial recess with attachment to
the preceding tergite via arthrodial membrane, an anterior articulating ridge or shelf constraining overlap with the preceding tergite, a posterior articulating ridge or shelf constraining overlap with the succeeding tergite, a transverse articulating boss, or an articulating half-ring and furrow, as well as telescope-like joining without obvious articulating devices other than arthrodial membrane and possibly pivot joints. The anterior articulating ridge morphology appears particularly widespread among arthropods, appearing to be part of the ground pattern for Artiopoda, Megacheira and Chelicerata. However, some synziphosurines (currently considered the paraphyletic stem-lineageto Xiphosurida) have been described as possessing articulating half-rings, and the ramifications of this are considered in light of a number of other characters. A new interpretation of basal chelicerate relationships is considered where Xiphosura is para- or even polyphyletic, with synziphosurines representing a basal grade to a clade consisting of Xiphosurida, Chasmataspidida, Eurypterida and Arachnida; further characters supporting this topology are reviewed.
Ever since Henry Woodward proposed to unite horseshoe crabs, eurypterids and trilobites as Crusta... more Ever since Henry Woodward proposed to unite horseshoe crabs, eurypterids and trilobites as Crustacea within the group Merostomata in 1866 there has been much debate surrounding the affinities of the group, the relationship of its constituent taxa and its validity as a monophylum. It is now accepted that ‘merostomes’ (comprising xiphosurans, eurypterids and chasmataspidids) are a paraphyletic grouping of basal chelicerates that form the stem lineage to Arachnida, but the phylogenetic topology of this part of the chelicerate tree is still unclear and several groups as currently defined (including Xiphosurida and Eurypterida) lack any convincing synapomorphies. Recent restudy of a number of fossil arthropods has revealed inconsistencies in the treatment of the articulation devices between their abdominal segments. For example the definition of an ‘articulating half-ring’ is rather liberally applied. When the morphology of these articulation devices is compared across arthropod groups several trends become apparent, one being that for several groups (such as Artiopoda, Megacheira and Chelicerata) the ground pattern for articulations appears to involve a narrow transverse ridge on the tergite preceded by a smooth articulation facet. However, some synziphosurines (currently considered the paraphyletic stem lineage to Xiphosurida) have been described as possessing articulating half-rings and the ramifications of this are considered in light of a number of other characters. A new interpretation of ‘merostome’ relationships is considered where Xiphosura is para- or even polyphyletic, with synziphosurines representing a basal grade to a clade consisting of Xiphosurida, Chasmataspidida, Eurypterida and Arachnida. Eurypterids are considered monophyletic, and the potential autapomorphy of having the opercular plates of somites VIII and IX fused into a genital operculum is identified, while xiphosurids are united by the possession of a fused thoracetron. Diploaspid chasmataspidids are considered to represent a monophyletic clade, however the monophyly of the order is still unclear due to their extreme morphological differences from the Ordovician Chasmataspis. Finally, what ramifications this revision might have on the arachnid ground pattern will be discussed in brief.
Restudy of the Silurian Pentland Hills eurypterid fauna helps to resolve the poorly understood ea... more Restudy of the Silurian Pentland Hills eurypterid fauna helps to resolve the poorly understood early phase of eurypterid evolution. ‘Drepanopterus’ bembycoides and ‘D’. lobatus are not stylonurines, but identified as synonyms of ‘Nanahughmilleria’ conica, representing a distinct new genus with an intriguing set of characters; an epistoma, appendages II-IV with poorly-expressed paired spines, and a pediform appendage VI with a modified podomere 7a (as in Eurypterina; swimming forms). It is assigned to the superfamily Moselopteroidea, along with Moselopterus and Vinetopterus, that share an oval metastoma with anterior notch, anterior ‘ears’ on the coxa of appendage VI, a pediform appendage VI with a podomere 7a, and serrate posterior margins of the postabdominal segments. This new genus and Vinetopterus are the most primitive Eurypterina known, and show that both basal Eurypterina and Stylonurina possess a three-segmented genital operculum and a pediform appendage VI. Eurypterina are distinguished by the presence of a podomere 7a and the lack of transverse sutures on the prosoma. Furthermore, some ‘derived’ characters, such as genital spatulae and epistomal sutures are now found in the most basal Eurypterina. This also suggests that one of the earliest (Ordovician) eurypterids, Megalograptus, considered by some workers as a basal member of the Eurypterina, actually belongs within the Mixopteroidea.
Restudy of the Silurian Pentland Hills eurypterid fauna helps to resolve the poorly understood ea... more Restudy of the Silurian Pentland Hills eurypterid fauna helps to resolve the poorly understood early phase of eurypterid evolution. ‘Drepanopterus’ bembycoides and ‘D’. lobatus are not stylonurines, but identified as synonyms of ‘Nanahughmilleria’ conica, representing a distinct new genus with an intriguing set of characters; an epistoma, appendages II-IV with poorly-expressed paired spines, and a pediform appendage VI with a modified podomere 7a (as in Eurypterina; swimming forms). It is assigned to the superfamily Moselopteroidea, along with Moselopterus and Vinetopterus, that share an oval metastoma with anterior notch, anterior ‘ears’ on the coxa of appendage VI, a pediform appendage VI with a podomere 7a, and serrate posterior margins of the postabdominal segments. This new genus is the most primitive Eurypterina known, and shows that both basal Eurypterina and Stylonurina possess a three-segmented genital operculum and a pediform appendage VI. Eurypterina are distinguished by the presence of a podomere 7a and the lack of transverse sutures on the prosoma. The new genus shows some adaptation towards a sediment-grubbing mode of life which suggests that the ‘swimming paddle’ of Eurypterina initially developed for digging in sediment much as in modern Limulus. Furthermore, some ‘derived’ characters, such as genital spatulae and epistomal sutures are now found in the most basal Eurypterina, which comprises Moselopteroidea, Eurypteroidea and an unnamed clade consisting of Onychopterella and Tylopterella. Megalograptus, previously considered to be a basal taxon, resolves as a member of the Mixopteroidea, suggesting a rapid diversification event early in eurypterid evolution, probably during the late Ordovician.