Palaeocast lunchtime lecture - The third-dimension of the puzzle: morphofonctional and phylogenetic applications of X-ray scanning of Cambrian stemmed-echinoderms - Sebastien Clausen
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.
Ediacaran matground ecology persisted into the earliest Cambrian
The beginning of the Cambrian was a time of marked biological and sedimentary changes, including the replacement of Proterozoic-style microbial matgrounds by Phanerozoic-style bioturbated mixgrounds. Here we show that Ediacaran-style matground-based ecology persisted into the earliest Cambrian. Our study in the type section of the basal Cambrian in Fortune Head, Newfoundland, Canada reveals widespread microbially induced sedimentary structures and typical Ediacaran-type matground ichnofossils. Ediacara-type body fossils are present immediately below the top of the Ediacaran but are strikingly absent from the overlying Cambrian succession, despite optimal conditions for their preservation, and instead the microbial surfaces are marked by the appearance of the first abundant arthropod scratch marks in Earth evolution. These features imply that the disappearance of the Ediacara biota represents an abrupt evolutionary event that corresponded with the appearance of novel bilaterian clades, rather than a fading away owing to the gradual elimination of conditions appropriate for Ediacaran preservation.
Palaeocast lunchtime lectures - Origins and early evolution of nacre - Vendrasco & Checa
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.
Palaeocast lunchtime lectures - Anomalocaridids had two sets of lateral flaps - Van Roy
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.
Palaeocast lunchtime lectures - Development of tooth plates in the earliest jawed vertebrates - Martin Rücklin
The first jawed vertebrates, the “placoderms”, exhibit an immense variety of jaw and dental morphologies including both teeth and tooth plates. Tooth plates of ptyctodont “placoderms” have been compared with dental structures in crown-group gnathostomes including the holocephalan chondrichthyans, but interpreted to have evolved in parallel with gnathostome teeth. Little is known about the internal morphology of these plates both in terms of the tissues from which they are comprised and their mode of development. We examine the dental structures and development in the ptyctodont “placoderm” Kimbryanodus williamburyensis from the Upper Devonian of Australia. Synchrotron tomography revealed primary mineralised structures building a framework and pleromic dentine infilling primary spaces, but also cutting primary growth lines, evidencing remodelling. Our data support an interpretation of sequential growth of the primary structure away from the oral surface of the tooth plate. Secondarily initial structures are infilled and partially remodelled with pleromic hard tissues. In spite of their outward differences, teeth and tooth plates of “placoderms” and tooth plates of holocephalan chondrichthyans share developmental processes like infill of primary structures with dentine and sequential growth.
Palaeocast lunchtime lectures - Visualizing developmental constraints and the anisotropy of phenotype space - Sylvain Gerber
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.
Palaeocast lunchtime lectures - Chemical variability of apatite in the shells of Cambrian lingulate brachiopod Ungula ingrica (Eichwald) - Liisa Lang
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.
I thought I’d stopped with the Palaeocast lunchtime lectures, but I’ve noticed some more. I’ve been trying to pick out the ones which might be of interest to people who love the Ediacaran biota, even though they are not on that specifically (so basically things like other Precambrian palaeontology, the Cambrian, and interesting perspectives on evolution.