Published on December 11th, 2013 | by David Marshall0
The Palaeontological Association 57th Annual Meeting
Friday, 13th December 2013
Thematic Symposium “Fossilised ontogenies and evolution”
Welcome and introductory remarks
Dr Christian Klug (University of Zurich)
Reproduction and early development of two plants from the Early Devonian Rhynie chert
Prof. Hans Kerp, Patricia Kearney & Hagen Hass (Universitaet Muenster)
The Rhynie chert is a hot-spring deposit that contains a broad spectrum of excellently preserved fungi, plants and animals. This Pragian (Early Devonian) Lagerstätte is the oldest, completely preserved fossil terrestrial ecosystem. Six land plant species are known from the Rhynie chert and an additional one has been described from another nearby occurrence. Of four species both the sporophyte and gametophyte generations are known in exquisite detail. Aglaophyton major and Rhynia gwynne-vaughanii, had leafless, bifurcated upright axes terminally bearing spindleshaped sporangia. These plants did not have underground parts, but they rested directly on the substrate. Spores are found in situ but also dispersed. Germinating spores are often associated with microbial mats. A series of developmental stages of Aglaophyton, ranging from germinating spores to full-grown gametophytes with antheridia and archegonia, can be documented. Gametangia are standing on top of flattened or in slightly bowl-shaped axis pices and/or laterally just below the apex. The alternation of generations can also be demonstrated for Rhynia. Remarkable is that sporophytes and gametophytes are very similar in having both a vascular strand and stomata, however, they differ in size. Apart from clonal reproduction, a feature shared with all Rhynie chert plants, Rhynia apparently had another mode of asexual reproduction. Rhynia formed small axial propagules with rhizoids, stomata and vascular tissue. Several ontogenetic stages of these propagules have been found and the developmental pattern resembles that of somatic embryogenesis, a form of asexual reproduction known from various modern pteridophytes and angiosperms.
Larval ecology in fossil gastropods
Dr. Alex Nuetzel (Bayerische Staatssammlung fuer Palaeontologie und Geologie Munich)
The shell of marine gastropods conserves and reflects ontogeny to a high degree when compared with other invertebrates. The function of larvae is to transform a small embryo or hatchling into a larger juvenile. Planktotrophy is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multi-whorled shell formed by a free-swimming veliger larva. Basal gastropod clades (e.g., Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic “advanced” gastropods with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophy was largely absent in the Cambrian and evolved at the Cambrian/Ordovician transition mainly due to increasing benthic predation. Early gastropod larval shells were openly coiled and poorly sculptured. During the middle and late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestion, it seems unlikely that the end-Permian mass extinction selected against planktotrophic species.
Trilobites – Evo-Devo on a Roll
Prof. Nigel Hughes (University of California Riverside)
The good record of postembryonic development of the trunk region of some trilobite species permits investigation of how mature body form was constructed. The trunk was built progressively via the expression of new exoskeletal segments in a subterminal generative zone, and by their growth and development in subsequent instars. This permits insight into the relationship between the ontogenies of individual taxa and clade history. The controls of trunk segment growth may be explored in order to understand how segment differentiation was determined in early arthropods. Trilobite trunk segments varied from one another in size, shape, and articulation state, and clade evolutionary history suggests a repeated tendency towards greater morphological differentiation within the trunk, along with allocation of a larger proportion of trunk segments to the mature pygidium. This was paralleled by the evolution of secured, encapsulated enrollment. Enrollment in those individuals with small numbers of articulating segments accommodated considerable flexure at each joint, which show differences in form from those joints within segment-rich thoraces. While the developmental evolution of trilobites retains rich research possibilities, we posit that both biomineralization and encapsulated enrollment enhanced protective capability, and that adaptive response to predation pressure was an important driver of trilobite evolution.
Life histories of early tetrapods: a diversity of ontogenies
Dr Rainer Schoch (Staatliches Museum fuer Naturkunde Stuttgart)
In the largest early tetrapod clade, the temnospondyls, ontogenies were diverse and quite distinct from the life cycles of extant amphibians. Three well-studied genera exemplify the diversity of these long-extinct ontogenies, here analyzed with respect to their bearing on the evolution of important life history traits (plasticity, reaction norm, metamorphosis). The basalmost taxon Sclerocephalus had an especially flexible ontogeny that was readily adjusted by means of developmental evolution to variable lake environments. Plasticity and changes in the reaction norm also played a major role, which is apparent both morphologically and in altered developmental traits. In the branchiosaurid Apateon, a biphasic life cycle was established, with metamorphosis producing a terrestrial morph; truncation of the ontogenetic trajectory produced a sexually mature larva as an alternative morph (neoteny). Plasticity was high only in the larval morphs, permitting the neotenes to adjust to harsh lake conditions. In the non-metamorphosing Triassic Gerrothorax, morphology was extremely conserved, but histology revealed much plasticity at the microscopical level, correlating with environmental fluctuations. This pioneer taxon coped well with changing levels of salinity. Despite their differences, the three temnospondyls were similar in responding to environmental fluctuations by enhanced plasticity, permitting them to populate lakes inhabitable to other taxa, particularly fishes.
Sharks and the deep origin of modern jawed vertebrates
Prof. Coates, Michael
New specimens, methods and trees are transforming our understanding of early shark-like fishes and the early evolution of gnathostomes. Chondrichthyes (sharks, rays and ratfishes) tend to be characterized as primitive, but supporting evidence is elusive: living outgroups (agnathans) are phylogenetically remote, and the fossil record of early sharks is well known to be fragmentary. Changes have been triggered by phylogenies assembled in response to discoveries of remarkable Silurian and early Devonian fishes, revealing new sets of primitive conditions for modern clades. Importantly, such analyses are dismembering long accepted groups of early jawed vertebrates: acanthodians are emerging as primitive chondrichthyans, and placoderms as stem lineage gnathostomes. Such studies have benefitted significantly from the particular value of CT technology for investigations of early chondricthyan morphology. Implications of these new trees and data are still being assessed. The latest branching patterns are far from stable, but they seem to provide a more balanced view of extant clades: the specializations of sharks vs. the primitive retentions of bony fishes. New perspectives are opened on the origins of innovations such as jaws, internal gill skeletons and paired fins, and on the likely influence of Palaeozoic extinctions, re-shaping the roots of the modern vertebrate biota.
Saturday 14th December 2013
Session 1: Proterozoic
Microbial life after the Great Oxidation Event
Schirrmeister, Bettina E., Brasier, Martin, and Donoghue, Philip C. J.
The origin of life dates back 3.45 billion years ago. Among the few bacterial phyla that have been identified from the Precambrian are Cyanobacteria, photosynthesizing prokaryotes responsible for the ‘Great Oxidation Event’ (GOE) over 2.3 billion years ago. Nevertheless, early occurrences of Cyanobacteria are highly debated and far from being resolved. The 1.88 billion year old Gunflint Chert (Canada) depicts one of those locations where presumable cyanobacterial microfossils have been found. But not only have those fossils been questioned, also the ecological context -stromatolite builders- of the Gunflint fossils has been debated. Given the immense importance the Gunflint Formation, with a microfossil abundance and diversity rarely seen in deposits of this age, resolving the life history of those microfossils will give insights into the level of evolutionary complexity shortly after the GOE. Using different resolutions, we have scanned 180 Gunflint samples applying Synchrotron radiation X-ray tomographic microscopy at the Swiss Light Source, Switzerland. So far, we were able to identify over eight distinct morphotypes. Comparisons of morphotype abundance and distribution to modern mat and planktonic environments, suggest that fossils of the Gunflint Chert have been part of planktonic microbial communities. Furthermore, several morphotypes do strongly resemble modern cyanobacterial taxa.
Establishing a Fossil Record of Euglenoids from the 1.1 Ga Nonesuch Formation to the Recent
Wellman, Charles H., Taylor, Wilson A., van de Schootbrugge, Bas, Koutsodendris, Andreas and Strother, Paul K.
The euglenoids are largely freshwater, flagellated protists belonging to the excavate branch of the eukaryotic supertree. Euglenoid cells are covered by a pellicle composed of distinctive, interlocking longitudinal strips of protein-rich heteropolymers. A few workers have noted a similarity between the encysted forms of extant euglenoids and the fossil palynomorph Pseudoschizaea. Similar discoidal palynomorphs marked with concentric striae may represent a Phanerozoic series of euglenoid fossil cysts. The Ordovician-Silurian microfossil, Moyeria cabottii, was previously interpreted as euglenoid based on morphological criteria. We extracted M. cabottii from the Fish Bed Formation (Silurian of Scotland). Its wall ultrastructure shows distinctive frames consisting of Ï€-shaped XX plateaus and shallow U-shaped heels, which are fused to form the pellicle wall. This structure is consistent with the extant euglenoid pellicle. Moyeria cabottii and Moyeria sp. occur in lacustrine shales of the 1.078 Â± 24 Ga XX Nonesuch Formation (USA). This extends back considerably the first occurrence of euglenoids and is consistent with molecular phylogenetic studies that posit their basal position near the root of the eukaryotic supertree. Moyeria now joins a handful of Precambrian fossils with well-established ties to crown groups in the eukaryotic supertree, and provides a useful minimum constraint calibration for molecular clock analyses.
Latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change
Christian Klug, Kenneth De Baets, Björn Kröger, Mark A. Belland Dieter Korn
Since the Cambrian Explosion, giant marine invertebrate species have evolved iteratively in several groups. In the Palaeozoic, marine invertebrate gigantism was heterogeneously distributed through time and space; changes in maximum sizes show no clear relationship with atmospheric or oceanic oxygen and other environmental factors. Although gigantism has found an explanation for Carboniferous land invertebrates in the atmospheric oxygen peak, marine gigantism has not been studied empirically and explained comprehensively.
By quantifying the spatiotemporal distribution of the largest representatives of some major marine invertebrate clades, we assessed links between ecological parameters and giant growth. These occurrence data suggest that temperature and latitude in combination with oxygen played important roles. Marine invertebrate gigantism developed in certain phases and regions with a greater number of extremely large species and their occurrences shifted independently from middle towards low latitudes during the Palaeozoic in all examined groups. This trend roughly coincides with the Late Devonian to Carboniferous cooling and regression as well as with a rise in atmospheric oxygen. This shows how global environmental changes can control the geographical distribution of organisms and that the optimal ecological requirements might differ depending on body size: extremely large organisms might react less flexibly to ecological changes.
Early animals from the Ediacaran Doushantuo Formation?
Cunningham, John A., Donoghue, Philip C. J. and Bengtson, Stefan
Among the fossils recovered from the Ediacaran Doushantuo biota are a number of forms that have been interpreted as animal remains. Reports have been published of stem-metazoans, poriferans, cnidarians, ctenophores and a diversity of bilaterians. However, all of these claims are contentious, with alternative explanations having been proposed in each case. As a result, some of the most widely accepted animal candidates in the biota are a suite of tubular fossils including the genera Sinocyclocyclicus, Ramitubus, Quadratitubus and Crassitubus. These taxa have been interpreted as eumetazoans and compared to cnidarians such as tabulate corals. As this is based largely on comparisons of simple branching patterns and cross-wall structures, more detailed morphological information is needed to test this hypothesis rigorously. Here we present new Synchrotron Radiation X-Ray Tomographic Microscopy (SRXTM) data on Doushantuo tubular fossils, including new taxa. The new data allow detailed anatomical characterization of the fossils at a cellular level, which illuminates the life cycle of the organisms.
Session 2: Proterozoic, Cambrian
A Doushantuo-type acritarch assemblage from phosphorite pebbles of the Ediacaran Biskopås Formation of southern Norway
Adamson, Peter W. and Butterfield, Nicholas J.
The early Ediacaran saw the appearance of remarkably large microfossils succeeding the Cryogenian glaciations. This new microfossil biota, best known from the Doushantuo Formation of South China, includes a diverse assemblage of large (20-800 µm diameter) acanthomorphic acritarchs with potential for biostratigraphically resolving early Ediacaran time.
Doushantuo-type acritarchs have been described from Australia, China, Eastern Europe, India, Siberia, Svalbard, and Urals, but were first described from phosphorite pebbles in the Biskopås Formation, Southern Norway. Restudy of the Biskopås phosphorites has revealed eleven species of acanthomorphic acritarch, numerous colonial and solitary acritarchs, microbial filaments and microbial mats, all in close association. Significantly, the assemblage of Biskopås acritarchs is duplicated in Biozone 2 (Member III) of the Doushantuo Formation in Yangtze Gorges, illustrating close biostratigraphic correlation with South China. The absence of Tianzhushania in the Biskopås, which dominates Biozone 1 in the Doushantuo Formation, is consistent with this observation.
The Biskopås phosphorites were deposited within microbially-dominated – subsequently phosphatized – siliciclastics, which contrast markedly with the carbonate-dominated facies and early diagenetic chert nodules in the Doushantuo Formation. Although the Biskopås does not display the similarities that the Doushantuo and some shale-hosted assemblages do, taxonomic similarities between different environments support their relatively facies independent distribution.
Mathematical models of the morphology of Ediacaran fronds: implications for phylogeny, disparity and the evolution of architectural complexity in an extinct fractal clade
Hoyal Cuthill, Jennifer
The morphology of Ediacaran rangeomorph fronds is characterised by self-similar fractal branching iterated over decreasing size scales. The mathematical rules (or descriptions) for such fractal branching appear to be convergent across a wide range of biological structures and clades (such as plant leaves and vertebrate lungs). This offers an opportunity to apply general mathematical systems for fractal branching (originally developed to describe plant growth and development), to model precisely the morphology of the Ediacaran fronds. I will present a new project which aims to develop a unified mathematical scheme for frond morphology. This will allow computer modelling and visualisation of three dimensional frond morphology and provide new data for phylogenetic and disparity analyses. Preliminary results support a diversification of space-filling strategies and provide new detail regarding changes in body organisation needed to produce such morphologies. Interestingly, there appear to have been several different space-filling derivations that require changes to the fundamental axiom for fractal branching, while others can be modelled by modifications of shallower operations. Implications for the evolution of architectural complexity via modification of self-similarity will be discussed.
Dumbbells, Discs and Inundation: palaeoecocolgical insights from the Ediacaran of Charnwood Forest, UK and Newfoundland, Canada
Kenchington, Charlotte G.
The Avalon Assemblage of Charnwood Forest (UK) and Newfoundland (Canada) collectively comprise the oldest known occurrence of Ediacaran macrofossils. Although long considered the poor cousin of the Newfoundland succession, recent cleaning and silicone rubber moulding of bedding surfaces in Charnwood Forest has revealed a diverse and well-preserved biota, including at least seven previously undescribed taxa. All are preserved as low-relief impressions on upper bedding plane surfaces. One of the new taxa, the colloquially named ‘Dumbbell’, has an unusually large holdfast disc, long stalk and dense, bushy frond.
Detailed sedimentological and petrographical analysis of several fossiliferous localities in Newfoundland and Charnwood Forest has revealed a strong correlation between the nature of the beds directly underlying the fossiliferous horizons and the composition of the hosted biota, particularly in terms of their functional morphology. Stalked forms such as the ‘Dumbbell’ preferentially occur on beds characterized by very thin parallel to wispy lamination, indicating more frequent inundation. In contrast, flat-lying forms such as Fractofusus are dominant on beds with a homogenous, massive character. These subtle differences demonstrate a clear link between functional morphology and palaeoecology in the Ediacaran.
French contribution to the understanding of the emergence of the phoronids-brachiopods total group: Early Cambrian tommotiids and earliest mickwitziid from Montagne Noire, southern France
Devaere, Lea, Clausen, Sebastien, Holmer, Lars E. and Vachard, Daniel
The brachiopods and phoronids have been shown to constitute a monophyletic group. Recent palaeontological discoveries have dramatically improved our conception of the emergence of this major clade. Many recent studies indicate that tommotiids (metazoans bearing an external, multiplated phosphatic scleritome) and mickwitziids (inequivalved organisms with punctate bilaterally symmetrical shell) represent basal to more derived members of the phoronids-brachiopods total group and can shed light on the emergence of their peculiar body-plans. Up to date, the exact phylogenetic relationships within this group and the emergence of the different brachiopods from the basal tommotiids along with its timing are still debated. The study of Lower Cambrian limestones from the Montagne Noire, Southern France, lead to the discovery of new and oldest (Terreneuvian) representative of the mickwitziids, n. gen. et sp., from the Avène-Mendic slice along with an abundant assemblage of sclerites of the camenellan tommotiid Kelanella altaica from the Cambrian Stages 3 to 4 (Minervois nappe). Among the camenellans, Kelanella is interpreted to occupy a transitional position between the Lapworthellidae, characterized by unspecialized conical sclerites, and the Kennardidae and Tommotidae, whose scleritome is constituted of three and two types of sclerites, respectively. Implications for the character evolution along the tommotiids-phoronids-brachiopods lineages are further discussed.
Chemical variability of apatite in the shells of Cambrian lingulate brachiopod Ungula ingrica (Eichwald)
Lang, Liisa, Kirsimäe, Kalle and Vahur, Signe
The skeletal apatite is known to have a very variable nature, with biochemical, environmental and diagenetic signatures merged in the shell composition. Several investigations have demonstrated the existence of two discrete apatite phases in the lingulate shells that are thought to be related with different types of lamellae in the shell structure. We show for the first time using combined infrared (ATR FT-IR) and energy dispersive spectroscopic (EDS) mapping of the cross sections of Ungula ingrica shells the compositional variation of apatite that follows the lamination of brachiopod shell. The compact laminae are rich in PO43-, Na, Mg and poor in F and Ca, the less compact (baculate) laminae are rich in carbonate, F and Ca, but contain relatively less Na and Mg. The differences in chemical composition of the apatite in different types of laminae suggest different origin of the apatite phases that can be interpreted as the original skeletal apatite and secondary apatite, respectively. It is likely that the compact laminae have the highest preservation potential, and are composed of original apatite secreted by the organism, whilst the less compact laminae are mostly composed of secondary apatite.
Session 3B: Evolution
The phylogeny of the Coleoidea – the major ‘constructing areas’
Many conflicting ideas about the higher-level phylogeny of the Coleoidea are currently circulating in the literature. This is considered to be mainly due to a bulk of new discoveries in the past few years. Apart from taxonomical and terminological inconsistencies, this disagreement largely arose from real homology problems, which are mainly based on shell characteristics.
It is the aim of the present talk to summarize the most important conflicts, to differ between terminological confusions and ambiguous character polarizations, and to perform different phylogenetic scenarios. In doing so, evolution-relevant questions such as ‘Is Recent Vampyroteuthis a living fossil?’, ‘Are belemnites really extinct?’, or ‘How conservative is Recent Spirula?’ will be considered.
Visualizing developmental constraints and the anisotropy of phenotype space
Morphological spaces (morphospaces) are quantitative representations of phenotype space that have proved particularly useful in the broad field of evolutionary morphology. Yet, do current conceptualizations and uses of morphospaces appropriately echo the evolutionary dynamics of organisms depicted in such spaces? Many studies implicitly assume that the phenotype space is an isotropic state-space, but two main lines of evidence suggest that such a view is inadequate: First, large-scale palaeontological analyses have shown that order and discontinuity are distinctive features of morphospace occupation; Second, advances in evolutionary developmental biology have shed light on the statistical properties of the genotype-phenotype map and their consequences for the structure of variation and patterns of evolutionary change. Here, I use a well-understood trilobite case study to illustrate the effect of developmental constraints on the directionality of evolutionary transitions and on the accessibility structure of phenotype space. The morphospace obtained is strongly anisotropic and reveals the discordance between the apparent range of possible phenotypes in the geometric neighbourhood of an evolving lineage and their actual accessibility. It is advised that geometric measures of distance in morphospace should be taken with caution and if possible replaced or complemented with more realistic and developmentally meaningful measures of evolutionary accessibility.
Do ‘living fossil’ fishes exist? And does it matter?
Cavin, Lionel and Guinot, Guillaume
A strict ‘tree-thinking’ approach led some neontologists to reject the concept of ‘living fossil’. Here we present three case-studies among piscine vertebrates, and we show why they deserve to be qualified of ‘living fossils’.
1: Morphological disparity observed in the coelacanth lineage since the Devonian, expressed in number of osteological transformations, is more reduced than the morphological disparity observed in any vertebrate clades during the same time interval.
2: Based on a phylogeny of dipnoans including fossil taxa, the extant Australian lungfish lies at the tip of the longest temporal isolated branch of the vertebrate tree of life and gets the highest “Evolutionary Distinctiveness” value for all vertebrates.
3: The discovery of teeth of a Palaeozoic shark lineage in the Early Cretaceous of Southern France represents the longest gap in the fossil record of a marine vertebrate lineage and indicates evidence of deep-sea refuge environments during biotic crises.
Although ‘living fossils’ is not etymologically satisfactory (a fossil cannot be alive), the expression is suitable to define a group of organisms with specific evolutionary characteristics. Recognition of their particularisms is essential for understanding episodes in the history of life that could not be otherwise detected by a time-free ‘pattern reconstruction’ approach.
Tooth development in batoids (skates and rays)
Underwood, Charlie J., Smith, Moya M. and Johanson, Z.
The batoids, including the skates, stingrays, sawfish and guitarfish, are a diverse group today, comprising over half of all living chondrichthyan species. They form a sister group to all living sharks, although their origins are still poorly known. Like sharks and many extinct shark-like groups, rays produce teeth continuously throughout their life, teeth forming lingually, in association with a dental lamina, before moving labially and being shed at the outer margins of the jaws. Teeth in each sequential row are added to even, then odd, alternate positions on the jaw; in every other row a tooth is at the jaw symphysis. Study of embryos of several species of modern batoids has revealed that there are several modes of structural pattern within the group. In some taxa, the first formed teeth are present along the length of the jaw with little or no subsequent increase in tooth numbers during ontogeny. In others, only two teeth are present in the first row with additional teeth added further from the symphysis with each new row. Unravelling the phylogenetic and evolutionary significance of these patterns is necessary for a better understanding batoid origins among the chondrichthyans.
The dinosaur-bird evolutionary transition: phylogeny and patterns of morphological evolution
Brusatte, Stephen L., Lloyd, Graeme T., Wang, Steve C. and Norell, Mark A.
The evolution of birds from theropod dinosaurs was one of the great evolutionary transitions in the history of life. Although there has been much recent work on avian origins, there remains little consensus on the phylogenetic relationships of birds and their closest relatives (coelurosaurian theropods), which hinders work on large-scale macroevolutionary patterns during the dinosaur-bird transition. We present a new comprehensive species-level phylogenetic analysis of Mesozoic coelurosaurs (150 taxa, 853 discrete characters), which is the latest iteration of the Theropod Working Group dataset. The character dataset was used to study trends in morphological disparity (anatomical variability) and the phylogeny was used as a framework to assess rates of character change across the dinosaur-bird transition. Basal avialans (birds) overlap in morphospace with their closest non-avialan relatives, without any significant statistical separation between them. This indicates that, in their overall anatomy, birds were merely part of a continuum of coelurosaurian morphological evolution and not a highly distinctive group. However, Avialae and proximal nodes and branches on the phylogeny are associated with significantly elevated rates of character change, suggesting that the origin of major groups such as birds may involve a burst in the pace of morphological change.
Are there rules underlying body shape evolution in ray-finned fishes? Regionalization of the axial skeleton in the ambush predator guild
Maxwell, Erin E., Wilson, Laura A. B. and Sanchez-Villagra, Marcelo
Elongation in fishes is strongly correlated with increased numbers of vertebrae. The addition of vertebrae can occur in the abdominal region, the caudal region, or both, and suggests that these axial regions are evolving independently. We tested whether unrelated groups of fishes in the elongate ‘ambush predator’ guild are characterized by the preferential addition of vertebrae to the abdominal region, predicted based on the posterior displacement of the median fins in this ecomorphotype. We examined changes in regional vertebral counts related to elongation in fossil and living representatives of seven clades of teleost and non-teleost fishes [Saurichthyidae, Lepisosteidae (gars), Belonidae (needlefish), Esocidae (pikes), Sphyraenidae (barracudas), Fistulariidae (cornetfishes), Aulostomidae (trumpetfishes)], using weighted squared change parsimony reconstruction and phylogenetic independent contrasts. We observed a preferential addition of vertebrae to the abdominal region in five of the seven clades surveyed. The two clades not characterized by preferential lengthening of the abdominal region, Saurichthyidae and Aulostomidae, showed a novel vertebral region anterior to the anal fin, distinct from both the abdominal and more posterior caudal vertebrae. Based on these results, it appears that regional changes in the axial skeleton of ray-finned fishes can be predicted from changes in median fin position.
Discordance between evolutionary rates and disparity in carnivoran cranial evolution
Goswami, Anjali , Smaers, Jeroen , Soligo, Christophe and Polly, P. David
Rapid morphological evolution is often thought to contribute to the generation of organismal diversity, but increased evolutionary rates are not necessarily correlated with increased disparity. We analysed evolutionary rates and disparity for a 3-D morphometric dataset of cranial landmarks representing a sample of living and extinct carnivorans (Mammalia, Placentalia). Evolutionary rates were reconstructed along phylogenetic branches of a fully resolved tree using an adaptive peak model. Disparity was estimated as landmark variance, and rates and disparity were compared across the cranium and within previously-identified. Results indicate that evolutionary rates and disparity are not significantly correlated (Spearman’s r = 0.23, p = 0.09) across the entire skull. Modules that display the greatest disparity in carnivorans (orbit and zygomatic regions) do not display significantly higher rates of evolution than other modules. The basicranial module shows strong integration and low disparity, but one of the highest rates of evolution, suggesting that integration does not necessarily constrain rate of morphological evolution, although it may constrain morphological diversity. Discordance between evolutionary rates and morphological disparity suggest that carnivorans evolve rapidly but within a relative limited area of morphospace, consistent with previous qualitative and quantitative assessment of repeated convergences in morphology across distantly-related carnivorans.
Session 4B: Palaeozoic
Anomalocaridids had two sets of lateral flaps
Van Roy, Peter, Daley, Allison C. and Briggs, Derek E. G.
While our understanding of anomalocaridids has made great strides over the past decades, several aspects of their morphology remain poorly understood. Their lateral flaps have been considered as derived from the flaps of taxa like Kerygmachela, Pambdelurion and Opabinia, making them potentially homologous to the exopods of euarthropods, but the arrangement of the flaps and apparent absence of any trunk limbs in anomalocaridids remains problematic. A newly discovered complete, three-dimensionally preserved anomalocaridid specimen from the Early Ordovician Fezouata Biota of Morocco reveals that it possessed two sets of lateral flaps, a feature that can also be identified in at least one other anomalocaridid fossil from the middle Cambrian Burgess Shale of Canada. The lower, largest set of flaps is ventral in origin while the second, smaller set arises dorsally, completely separate from the ventral set. The dorsal flaps are closely associated with the setal blades. These findings indicate that the dorsal flaps are homologous to the flaps of ‘gilled lobopodians’ and the exopods of euarthropods, while the ventral flaps correlate to lobopodian limbs and euarthropod endopods. Hence, anomalocaridids represent a stage preceding the fusion of endo- and exopod into the euarthropod biramous limb, securing their position within the arthropod stem.
Origins and early evolution of nacre
Vendrasco, Michael J. and Checa, Antonio G.
Evidence from modern animals and the fossil record indicate that nacre originated independently in different groups of mollusc. There are plentiful data on original inner shell microstructures of molluscs from the Early and Middle Cambrian, but nacre is absent. We have confirmed that this shell microstructure is preserved in cephalopods–and probably also occurs in bivalves and gastropods–from the Ordovician of the Midwestern USA, and thus we infer that nacre originated during the Great Ordovician Biodiversification Event or just prior to it. We show that the preservation of this earliest known nacre is unique and the original texture was diagenetically replicated, but the structure can nevertheless be reliably inferred to have been mother-of-pearl. By the middle to late Palaeozoic, nacre was common in bivalves, gastropods, and especially cephalopods, correlating with increasing predation pressure through that interval. Our current work has revealed that nacre in these cases appears modern to a great degree and in some instances shows ultrastructural details important in understanding the origins of this shell microstructure.
Ordovician-Silurian Chileida – first post-Cambrian records of an enigmatic group of Brachiopoda
Holmer, Lars E., Popov, Leonid and Bassett, Michael G.
Brachiopods of the order Chileida have been recorded previously only from rocks of Early to Mid-Cambrian age (Botomian – Amgaian). They are typified by having a calcareous strophic shell with a delthyrium and colleplax, and these characters are shown to be present in species of the two new genera n. gen. 1 and n. gen. 2, from the Late Ordovician of Kazakhstan and the Silurian of Sweden and Britain, respectively. In specimens of n. gen. 2, the triangular colleplax is phosphatized secondarily by bacterial activity. It is suggested that the phosphatized colleplax represents an organic pad that served as the original attachment structure of n. gen. 2 by encrustation. N. gen. 1 and n. gen. 2 represent the first post-Cambrian record of chileids from the Ordovician and Silurian; the new family forms the first phylogenetic link between Cambrian chileids and Carboniferous – Permian isogrammids.
Biodiversity, biogeography and phylogeography of Ordovician rhynchonelliform Brachiopoda
Harper, David A. T, Rasmussen, Christian M. Ø, Liljeroth, Maria, Blodgett, R. B, Candela, Yves, Jin Jisuo, Percival, Ian G, Rong Jiayu, Villas, Enrique and Zhan Renbin
Phylogeographic evolution and changing distribution and diversity of rhynchonelliforms through the Ordovician are linked to its dynamic palaeogeography. Early Ordovician (Tremadocian and Floian) low-diversity faunas were punctuated by local species pumps, notably on the South China Palaeoplate. During the Early to Mid-Ordovician (Dapingian–Darriwilian), marine life experienced an unprecedented hike in diversity at species, genus and family levels, firmly installing suspension-feeding benthos as the main component of the Palaeozoic fauna. The continents were widely dispersed together with a large number of microcontinents and volcanic arcs related to intense magmatic and tectonic activities. Climates were warm and sea levels high. Pivotal to the diversification is the role of gamma (inter-provincial) diversity and by implication the spread of continents and frequency of island arcs and microcontinents. Provincial patterns were disrupted during the Sandbian and early Katian with the migration of elements of the benthos into deeper-water regimes, enjoying more cosmopolitan distributions. Later Katian faunas exhibit partition between carbonate and clastic environments. During the latest Katian, patterns were disrupted by poleward migrations of warm-water taxa in response to changing climate, facilitating low-latitude species pumps in carbonate settings. The Hirnantian was marked by severe extinctions across orthide-strophomenide clades within few, but well-defined, climatically-controlled provinces.
The third-dimension of the puzzle: morphofonctional and phylogenetic applications of X-ray scanning of Cambrian stemmed-echinoderms
Clausen, Sebastien, Porez, Elise, Monnet, Claude and Debeauvais, Laurence
Echinoderms challenge the understanding of body-plan appearance and evolution. The emergence and relationships of Palaeozoic taxa with stalk and food-gathering appendages used for filter feeding, traditionally classified within the Pelmatozoa, have been particularly debated during the last decades. The monophyly of this group has been strongly questioned and it is generally split into two sub-phyla based on important differences in structure and insertion of their feeding-appendages: Blastozoa (former cystoids) and Crinozoa (crinoids). For some authors, crinozoans are not rooted in blastozoans but in edrioasteroids, as suggested by the Ordovician protocrinoids. For others, crinoids may have emerged from a blastozoan body-plan, as suggested by “chimeric” taxa. Recent discoveries are controversial and, instead of resolving this phylogenetic jig-saw, brought new disconnected pieces in the puzzle. This study was aimed at analyzing microanatomical structures of some mid to late Cambrian debated taxa and remains with exceptional preservation by means of CT-scanning and synchrotron radiation. The 3D reconstruction of internal microstructures of feeding-appendages enabled testing possible relationships or convergences within pelmatozoan-echinoderms. It shows that a blastozoan rooting of crinoids cannot be rejected, although a high plasticity of blastozoan appendages complicate the phylogenetic enigma and raised questions about homologies used to define the different sub-phyla.
Soft-part preservation in a solutan echinoderm from the Fezouata Biota (Lower Ordovician, Morocco)
Lefebvre, Bertrand, Guensburg, Thomas E., Martin, Emmanuel, Milne, Claire H., Mooi, Rich, Noailles, Fleur and Vannier, Jean
In the Zagora area (central Anti-Atlas, Morocco), the Fezouata Shales have yielded successive highly diverse assemblages of marine invertebrates comprising both abundant remains of skeletonized taxa typical of the Great Ordovician Biodiversification Event (e.g., articulate brachiopods, bivalves, cephalopods, crinoids, gastropods, graptolites, ostracods, starfish), and numerous exquisitely preserved remains of lightly sclerotized or soft-bodied organisms (e.g., aglaspidids, anomalocaridids, machaeridians, marrellomorphs). Solutans constitute a small clade of controversial, asymmetric Palaeozoic echinoderms (Middle Cambrian – Lower Devonian). They have been interpreted either as basal deuterostomes (“calcichordates”), as hemichordate-like, pre-radiate echinoderms (“carpoids”), or as highly derived blastozoan echinoderms (“cystoids” s.l.). The morphology of solutans invariably consists of a single feeding appendage (brachiole), a flattened body (theca), and a bipartite, stem-like appendage (homoiostele). However, their soft-part anatomy remained completely unknown. A single specimen from the Fezouata Biota (late Tremadocian) partly bridges this gap, and provides the first evidence of soft-part preservation (hind-gut) in this long extinct class of echinoderms.
Session 6: Triassic, Jurassic
Evidence for an intra-Early Triassic crisis of benthic ecosystems
Hofmann, Richard, Hautmann, Michael, Schneebeli-Herman, Elke and Bucher, Hugo
The recovery from the end-Permian mass extinction is traditionally described as highly protracted. However, recent studies have shown that, in addition to nektonic groups such as ammonoids and conodonts, benthic ecosystems also show a remarkably volatile recovery pattern. Based on quantitative palaeoecological analysis of Pangean shelf settings of the Palaeotropics, we show that benthic ecosystems experienced incipient recovery and a subsequent decline within the first million years of the Early Triassic. The evidence includes (i) comparatively diverse associations around the Griesbachian-Dienerian transition, (ii) decreasing diversity and evenness towards the end of the Dienerian, and (iii) notable turnover and extinction around the Dienerian-Smithian boundary. The integration of these results with published geochemical data from the eastern Panthalassa margin and palynofacies data of the eastern Tethys suggests that this demise correlates with widespread oxygen-depletion. The interregional recovery of benthos during the Griesbachian contradicts the long held hypothesis of persistent shallow marine anoxia in the aftermath of the end-Permian mass extinction as a main driver of the delayed recovery. Our data rather suggest that a subsequent diversity setback during the Dienerian mimics a sluggish recovery of benthic ecosystems after the greatest crisis in the Phanerozoic history of life.
Systematics, origins and palaeoecology of placodont marine reptiles (Sauropterygia, Placodontiformes)
Neenan, James M. and Scheyer, Torsten M.
Placodonts are a plesiomorphic clade of durophagous sauropterygians from the eastern and western Tethys Ocean of the Middle to Late Triassic. The evolutionary relationships and origins of the Placodontia remain unclear, particularly in the light of new taxa from China. In order to resolve this, micro-computed tomography was used on crania from all valid taxa and, for the first time, a comprehensive phylogeny including all eastern and western placodonts is presented. Among the Chinese forms, Placodus inexpectatus clusters with European unarmoured ‘placodontoids’, Sinocyamodus within Cyamodontida, while Glyphoderma and Psephochelys fall within the highly nested Placochelyidae, thus pulling this node back into the late Middle Triassic. This indicates that all placodont clades originated during a period of intense speciation during the Middle Triassic, with armoured cyamodontoid taxa diversifying into the Late Triassic on both sides of the Tethys.
Additionally, a skull of a new sauropterygian from Winterswijk, the Netherlands, Palatodonta bleekeri, has provided evidence concerning both the palaeogeographic and evolutionary origins of Placodontia. Characters such as single row of, conical palatine teeth place Palatodonta as sister taxon to Placodontia, indicating an origin of the clade in the western Tethys and suggesting their characteristic palatine dentition did not initially evolve for durophagy.
Taphonomy and palaeoecology of a Jurassic ichthyosaur: similarities and differences to modern whale falls
Danise, Silvia, Matts, Katie and Twitchett, Richard J.
Bones of an ichthyosaur (Ophthalmosaurus sp.) from the Oxfordian (Upper Jurassic) Sandsfoot Formation and the associated invertebrate fauna were investigated to test the hypothesis that carcasses of Mesozoic marine reptiles hosted similar communities to modern and fossil whale falls. Bite marks on the bone surfaces indicate initial scavenging by fishes. Echinoid grazing traces (Gnathichnus pentax) indicate colonisation of the defleshed bones by algal or microbial mats. This is confirmed by the presence of micro-borings in the bones and microbially induced carbonates on the bone surface (clotted micrite). Pyrite framboids and peloidal fabrics within the bone spaces possibly indicate microbial decay of the bone organic compounds by sulphate reduction. Finally, numerous suspension feeding macro-invertebrates, free living close by or directly attached to the bones, indicate prolonged exposure and colonization of the carcass on the sea floor prior to final burial. In this marine reptile fall, as in whale falls, we recognise an initial “mobile scavenger” and a final “reef stage”. The microbial signatures (clotted micrite, pyrite framboids and peloids) may indicate development of a “sulphophilic stage”, as in modern whale falls, even though no direct evidence for a chemosynthetic-based ecosystem (e.g. chemosynthetic bivalves such as lucinids and thyasirids) was found.
Large scale environmental gradients as a control on the skeletal taphonomy of Ichthyopterygia
Beardmore, Susan R., Orr, Patrick J. and Furrer, Heinz
Taphonomic studies using datasets with large “n” often neglect to resolve palaeogeographic and stratigraphic contexts of specimens, preventing temporal and/or spatial variation in preservation from being tested. We illustrate this via analysis of the taphonomy of Ichthyopterygia (n=173) from the Besano (Middle Triassic), Posidonienschiefer (Lower Jurassic) and Blue Lias (Lower Jurassic) formations of Europe. A semi-quantitative methodology was used to confirm a general taphonomic pathway for the clade that was also apparent at locality level. A division between trends in peripheral and medial regions, and a tendency for the shoulder/pelvic joint to disarticulate first but loss of completeness to initiate in the distal part of each limb, implies carcasses routinely sank quickly after death. The “floating” phase within the water column was limited and “residence” at the sediment, relatively extended; this model contradicts those proposed previously. Overall, loss of articulation and completeness is highest for the Besano, and lowest for the Blue Lias, Formation. As ichthyopterygian morphology is generally conservative, this primarily reflects a depositional context, specifically variation in current activity, with the fidelity of preservation decreasing from the edge of an epicontinental basin (Blue Lias), across its centre (Posidonienschiefer) to an intra-platform basin on a carbonate shelf (Besano).
Session 7B: Silurian to Triassic
Colonisation of the water column by ostracods in the late Silurian
Perrier, Vincent, Williams, Mark and Siveter, David J.
Ostracods are the most prolific arthropods in the fossil record, ranging from at least the Ordovician onward. They are present in a wide range of ecosystems, mostly as part of the (nekto)benthos. They are also an important component of modern marine zooplankton, from near surface to abyssal depths. Often very abundant in sub-thermocline ocean waters, most ostracod zooplankton are micro-predators or detritus feeders. Ostracods made the ecological shift from the benthos during the Silurian, as evidenced by a comprehensive fossil record of their environmental distribution and details of their soft anatomy. Coupled with information on Recent ostracods, there is an extensive dataset by which to assess ‘when’ (temporally), ‘how’ (physiology and functional anatomy), and ‘why’ (environmental and biological feedback) ostracods colonized the water column, a major event in the ecological radiation of the group and a model for the study of benthic to zooplanktonic ecological shifts.
A case study of the supposed pelagic entomozoid Richteria migrans from the Ludlow Series will be detailed in the light of new discoveries in the Russian Arctic and central Asia. Its morphology, palaeogeographic distribution, facial and environmental range, and relationship to water chemistry and climate will be assessed to evaluate its mode of life.
Devonian lycopod forests in Svalbard
Berry, Christopher M., Marshall, John E.A.
The presence of Devonian in situ lycopsid trees (as Archaeosigillaria) is highlighted on the Geological Excursion Map of Billefjorden, Central Spitsbergen. These occur in the Plantekløfta conglomerate and are currently dated as Famennian (late Late Devonian). New palynological data shows that they are, in fact, Mid Devonian to earliest Late Devonian in age and the oldest-known record of in situ arborescent lycopsids showing the spatial structure of the vegetation.
The standing lycopsid tree fossils are internal mudstone to sandstone casts, buried by mudstones, sandstones or cobble conglomerates. They are spaced on the order of 10-20 cm apart. New observations show the trees to have bulbous, cormose bases up to 20 cm wide and narrow roots. Incomplete trunks were up to 133 cm in length. The trunks are predominantly preserved as internal casts, and the surfaces show a pattern of diamond-shaped leaf bases and can be referred to Protolepidodendropsis pulchra. In comparison with the cladoxylopsid-aneurophytalean progymnosperm forests at Gilboa, New York, the Svalbard lycopsids grew much closer together, and were monospecific at least locally. The in situ microspore and megaspore have been identified and enable us to map the global distribution of these lycopod forests in both time and space.
Tabulate corals on crinoids: unusual biocoenosis from the Lower Devonian of Morocco
Berkowski, Błażej and Zapalski, Mikołaj K.
Hamarilopora minima is a favositid tabulate coral occurring in the Lower Devonian (Pragian) of Hamar Laghdad (SE Morocco). These corals commonly encrust crinoid stalks belonging to the crinoid genera: Amurocrinus, Cyclocharax (?) and Schyschcatocrinus. Most of the hosts belong to S. breviatus. Encrustation of long pluricolumnals (up to 30 elements) evidences in vivo encrustation.
Most coral-hypobiont associations show signs of interactions, such as anatomical changes and deformations. In the described case crinoid hosts do not display any evident anatomical changes of the skeleton. As coral epibionts were changing mechanical properties of the crinoid stalk it can be inferred that the coral influence on the crinoid was negative. On the other hand, the coral was elevated over the seafloor and profited from nutrient-bearing water currents. Therefore the interaction can be classified as faint parasitism. Recent crinoids possess a variety of epibionts, but the majority of them are crustaceans; no analogous association is known from recent seas.
Diversity among new Tournaisian tetrapods from Scotland
Clack, Jennifer and Smithson, Timothy R.
New discoveries from the earliest Carboniferous have the potential to change our understanding of the evolution of terrestrial tetrapods. Two specimens from Burnmouth and another from Willie’s Hole variously include skull and postcranial material. One Burnmouth specimen shows a jugal with a long posterior extension and a shallowly embayed orbital margin (primitive). A presumed tabular with a posterior “horn-like” process bears overlap surfaces for two more medial bones. They suggest a tabular-parietal contact (derived). The postcranium includes a lozenge-shaped interclavicle (?primitive), both cleithra, clavicles and humeri, a radius, scapulocoracoid, ribs, notochordal centra and neural arches. The other specimen consists of a disarticulated skull with a broad, denticulated pterygoid (primitive), a jugal with a deeply excavated orbital margin (derived), a distinctive lower jaw, teeth with folded enamel, and a parasphenoid with posterior wings (derived). Specimens differ greatly from each other, neither resembles any previously known taxon. The Willie’s Hole skull superficially resembles that of a temnospondyl in outline, although key features cannot be seen. The pterygoid appears broad suggesting small or absent interpterygoid vacuities (primitive). Newly assigned postcrania include limb and girdle elements. These specimens illustrate conflicting character combinations, unsuspected diversity, and possible explosive radiation of tetrapods in the Tournaisian.
Evolutionary dynamics of fishes through the Late Permian mass extinction
Romano, Carlo and Koot, Martha B.
Mass extinction events near the Palaeozoic/Mesozoic boundary had a major impact on life on Earth. Here we present an updated analysis of the diversity dynamics and size changes of Chondrichthyes and Osteichthyes during the Permian/Triassic. We show that chondrichthyan genus diversity declined during the Middle-Late Permian. Many Palaeozoic groups (e.g. Petalodontiformes) were largely replaced by hybodonts and modern sharks (Neoselachii). A significant overall decrease in tooth size and body length observed across the Permian/Triassic boundary suggests a selective loss of large-sized chondrichthyans. The largest extinction occurred amongst marine groups, with benthic and pelagic groups suffering most, but selectivity for these palaeoecological traits is not evident. Osteichthyes show a general pattern of low diversity during the Permian to higher levels in the Triassic, leading to increased diversity among the modern ray-finned fishes (Neopterygii). Palaeopterygii experienced a significant increase in body size across the Middle/Late Permian boundary and remain among the larger fishes during the Triassic. Neopterygians mostly remained smaller and, thus, mostly occupied lower positions within aquatic food webs. Our data indicates an important evolutionary turnover among fishes, changing from chondrichthyan-dominated communities of the Carboniferous-Permian to osteichthyan (actinopterygian)-dominated associations of the Meso- and Cenozoic.