Heterostracan vertebrates and the Great Eodevonian Biodiversification Event—an essay
The oldest vertebrates are Early Cambrian, cephalized unossified species (craniates) from China. The oldest armoured species (euvertebrates) are Ordovician in age. After Talimaa’s Gap, vertebrates have their first adaptive radiation during the Silurian when jawless species (“ostracoderms”) are dominant and their second radiation during the Devonian when jawed species (gnathostomes), and particularly placoderms (armoured fishes), are dominant. A Lochkovian peak of diversity is registered in various Lower Devonian siliciclastic series all around the Old Red Sandstone Continent and Siberia, for ostracoderms in general, and heterostracan pteraspidomorphs in particular. It occurs at different time slices in the Lochkovian, depending on the localities, and may be followed by another smaller peak in the Pragian. Both events correspond to the rise of the Devonian Nekton Revolution as defined for marine invertebrates. This appears to be the second main biodiversification event in the Palaeozoic, following the Great Ordovician Biodiversification Event or GOBE, when euvertebrates appeared. Taking into account most recent palaeobiological studies on heterostracans that suggest they were microphagous suspension feeders or feeding upon microscopic epiphytes from filamentous algae, the origin of this Great Eodevonian Biodiversification Event (GEBE) of heterostracans is questioned. Both abiotic (sea level, tectonic events, climatic changes—oceanic oxygenation and temperature, continental surface temperature) and biotic (plankton diversity, marine primary productivity, competition with vertebrates and invertebrates, including eurypterids, macroecological turnover) factors are examined. No plausible global evolutionary scenario seems to be presently available.
KeywordsEarly Devonian Nekton Revolution Ostracoderms Palaeobiodiversity Essay in geobiology
Few years ago, at the end of a paper about the origin of tetrapods, George and Blieck (2011) suggested “… that the co-occurrence of a series of bio-events and physical properties of the oceans on Earth during the Early Devonian is not merely a coincidence, but reflects a global re-arrangement of the biosphere. An increase in oxygen is likely to have occurred during the Early Devonian. It would have triggered the emergence of tetrapods in shallow marine environments…” and other biological events. Later, in an online paper concerning the origin of insects and tetrapods, Blieck (2012) considered that “The Early Devonian, between – 416 and – 400 My, seems to be one of the most important period of biodiversification on Earth” characterised by a high atmospheric, and thus oceanic oxygen level eventually correlated to an important increase in (a) the body volume and species richness of marine invertebrates, (b) the diversity of autotrophic corals, (c) the diversity of terrestrial arthropods (including insects), as well as the first occurrence and expansion of tetrapods. “This ‘Great Eodevonian Biodiversification’ might be one of the major episodes of the evolution of life on Earth… As for all other global events, it must be related to the evolution of the Earth. In the Early Devonian indeed, the two main landmasses known as the Old Red Sandstone Continent in the north, and Gondwana in the south, begin to come in contact under plate tectonic activity (Scotese 2002). This tectonic phase is known as the Acadian, the late Caledonian, or the early Hercynian phase. It led to the closing of the Rheic ocean, the decrease in coastal marine areas, and beginning of a global sea level rise that favoured the development of new ecological niches on inundated continental shelves (neritic zone)…” [Blieck 2012, my translation]. This has been renamed the “Great Eodevonian Biodiversification Event” by Blieck (2015). I propose in this paper to evaluate this event through the case of a group of fossil jawless vertebrates known as the heterostracans and to try to relate it to global biotic and abiotic factors.
An Early Devonian peak of biodiversity
The analyses of Alroy et al. (2008) and Alroy (2010) are based upon the Palaeobiology Database (PDB) that is not complete for all taxonomic groups, and in particular for Palaeozoic vertebrates (W. Kiessling, pers. comm.). This means that the GEBE is probably more important than shown on their curve (Fig. 1). It is the result of both a high origination rate (Aberhan and Kiessling 2012, fig. 4a) and a low extinction rate (ibid., fig. 4b) which is characteristic for a biodiversification episode. Other global biodiversity curves that have been produced in recent papers show differences to Alroy’s (2010). In these papers, the Ordovician peak appears to be either more important (in number of genera) or equivalent to the Early Devonian one. In Sepkoski’s (2002) paper, the Ordovician peak is more important, in Rohde and Muller’s (2005) paper they are nearly equivalent, while in Smith et al.’s (2012) study the Ordovician peak is again more important, but becomes less important in number of genera when “corrected for variation in sampling using SQS” (Smith et al. 2012, fig. 1). Sepkoski’s (2002) and Rohde and Muller’s (2005) analyses are based upon a larger database than the PDB (see a comparative study by Rasmussen et al. 2016), while Smith et al.’s (2012) analysis is based on the PDB. For Smith and McGowan (2008): “… the Phanerozoic trend in fossiliferousness most likely records the degree to which space is occupied in the shallow marine realm”, and for Smith et al. (2012): “Using the fossil records of North America and Western Europe, we demonstrate that a modelling approach applied to the combined data produces results that are significantly correlated with those derived from subsampling. This concordance between independent approaches argues strongly for the reality of the large-scale trends in diversity we identify from both approaches.” So, as a result, after the presently available fossil record and various analyses of the databases, the GEBE appears not to be a mere artefact.
Vertebrate diversity and trophic relationships
Such a pattern with an apparent replacement of ostracoderms by gnathostomes may be interpreted in terms of trophic relationships, viz. predation of gnathostomes (placoderms, sarcopterygians…) on ostracoderms. The Lower–Middle Devonian event (decrease of diversity) might be interpreted as an ecological turnover from agnathans to placoderms, by comparison with the Devonian–Carboniferous ecological turnover from placoderms to eugnathostomes (Long 1990). However, for Purnell (2001), “the view that diversity patterns and the extinction of most clades of jawless fish reflect competition between agnathans and gnathostomes or between specific clades of jawless and jawed fish … must be regarded as untested, and at present untestable, speculation.” The same author (Purnell 2002), after a study of feeding and oral denticles in heterostracans, concludes that these agnathans were microphagous suspension feeders. He also concludes that “the widespread view that the pattern of early vertebrate evolution reflects a long-term trend towards increasingly active and predatory habits” is contradicted (ibid.). This is due to the fact that he includes conodonts among vertebrates in a basal phylogenetic location (following Donoghue et al. 2000) and that he considers conodonts as predatory animals (Purnell 1995, 2001, 2002) thus leading to the view that “predation is plesiomorphic relative to suspension feeding” (Purnell 2002). However, if we consider that conodonts are not vertebrates (Turner et al. 2010), no basal euvertebrate was predatory, and the long-term trend towards increasingly active and predatory habits of vertebrates may be considered as valid. The transition from agnathan-dominated faunas to gnathostome-dominated faunas has thus occurred during the Early to Middle Devonian period (Fig. 3; also Friedman and Sallan 2012). This trend was already noticeable on Thomson’s (1977) analysis of pattern of diversification among fishes, with its Early Devonian agnathan, Late Devonian placoderm, and Early Carboniferous chondrichthyan peaks (Thomson 1977, fig. 7).
Early Devonian peaks of diversity in heterostracans
Several analyses at different taxic and geographic scales have shown one, or sometimes two peaks of diversity for various groups of heterostracan pteraspidomorphs through the Silurian–Devonian. Before giving the results of those analyses, I note that four clades are included in the Pteraspidomorphi: the Arandaspida, Astraspida, Eriptychiida and Heterostraci (sensu Janvier 1996). Here, I focus on the Heterostraci that are known from the Early Silurian to the Late Devonian (Frasnian) and were mostly living on Laurentia, Avalonia and Baltica in the Silurian, and the Old Red Sandstone Continent (ORSC) and Siberia in the Devonian. More than 300 species have been described to date. Heterostracans inhabited all environments of the Silurian marine platforms, and various environments from shallow marine to intermediate (coastal, estuarian, “brackish”) around the Devonian ORSC. The head carapace of Heterostraci consists of a series of bony plates, the number and morphology of which varies among their different taxa, and enabled to distinguish two main groups, i.e. Cyathaspidiformes (including amphiaspids) and Pteraspidiformes (including psammosteids). Other more problematical groups include traquairaspids, cardipeltids, corvaspids, ctenaspids, Nahanniaspis, and various tessellated forms (tesseraspids, Lepidaspis…) (Janvier 1996). Heterostraci are generally considered as a monophyletic group (see a review by Blieck and Elliott 2016).
The diversity of pteraspidiforms has been evaluated at the genus level, from the Ludlow to the Givetian of the ORSC as a whole (Blieck 1984). It clearly shows two peaks, a first one in the “middle” Lochkovian (Rhinopteraspis crouchi Zone or its equivalents in the Arctic), and a second one in the “early/middle” Pragian (Rhinopteraspis dunensis Zone or its equivalents in the Arctic and the USA). The Lochkovian peak is the result of a period of rather high origination rate beginning with the Devonian, and a low extinction rate, while the Pragian peak correlates directly with an origination peak and a low extinction rate (Blieck 1984, fig. 75). Novitskaya (2007, fig. 3) obtained the same pattern at both genus and species levels for Přidoli to Early Devonian pteraspidiforms of the ORSC, with a first peak of 18 genera (37 species) in the Lochkovian, and a second peak of 10 genera (20 species) in the Pragian. However, it must be noted that this diversity is underevaluated for pteraspidiforms because both analyses (Blieck 1984 and Novitskaya 2007) do not take anchipteraspids and psammosteids into consideration (both taxa being included in the monophyletic order Pteraspidiformes in most recent reviews: Janvier 1996, fig. 4.9; Pernègre and Elliott 2008). Cyathaspidiforms of the ORSC also show a peak of diversity in the Lochkovian with 14 genera (and 36-40? species, depending of the number of species included in the genus Poraspis which needs to be revised; Novitskaya 2007, fig. 1), but none in the Pragian. However, again, this diversity of cyathaspidiforms is underevaluated because Novitskaya (2007) did not include amphiaspids in this taxon, contrary to other authors as, e.g. Janvier (1996 fig. 4.8). The diversity of amphiaspids has been evaluated separately by Novitskaya (2008), at genus level, for both regions of Siberia where they have been collected, i.e. the Taimyr Peninsula and the NW Siberian Platform. In both regions, they show a peak of diversity in the “middle” Lochkovian, Uryum Beds of the Ust’Tareya Horizon of Taimyr (with seven genera), and Kureika Horizon of the NW Siberian Platform (with 10 genera) (Novitskaya 2008, fig. 2–3).
This short review of Early Devonian sequences of the ORSC and Siberia shows that they all have a peak of heterostracan diversity (specific or generic richness) in the Lochkovian, plus a second one in the Pragian for pteraspidiforms as a whole, and two other smaller ones in the upper Lochkovian and lower Pragian of Podolia that are mostly due to pteraspidiforms as well. However, depending on the marine epicontinental platform where the heterostracan assemblages have been originally living, the Lochkovian peak may occur either in the “middle” Lochkovian (ORSC, N. France, East Baltic, Podolia, Spitsbergen, Siberia) or the upper Lochkovian (Severnaya Zemlya), thus showing that the events of biodiversification were not exactly contemporaneous (at the geological time scale) from one continental margin to another one for heterostracans in the Lochkovian. This can also be shown for other groups of Palaeozoic fossils as, e.g. for Ordovician chitinozoans (Paris et al. 2004).
The geobiological context
Sea level changes? After Haq and Schutter (2008, fig. 2 and chart), the long-term trend for the Late Silurian to Early Devonian is a decline of sea level (eustatic low); this is strange if we consider that a period of increase in marine biodiversity should be related to an increase in submerged epicontinental platforms, thus of marine niches, in relation with a rise in sea level; Blieck et al. (1995, p. 455) indeed hypothesised that the mid-Lochkovian (Rhinopteraspis crouchi Zone) peak of diversity observed for agnathan vertebrates in northern France and surrounding regions (Anglo-Welsh area and the Ardenne) might be correlated with the short-term high stand of the uppermost eurekaensis Conodont Zone.
Tectonic events? The Old Red Sandstones (ORS) are late- and post-orogenic molassic lithofacies of the late- and post-Caledonian Orogeny, or the late- and post-Eohercynian Phase; most pteraspidiform heterostracans have been collected in such facies, and it has been shown that their peaks of diversity are correlated to the ORS deposition phases (Blieck 1984, fig. 75).
Climatic changes? It is now known that all physical changes of planet Earth are under control of its plate tectonic activity, including tectonic phases, sea level changes, and thus climatic changes (see e.g. Cowen 2003; Gornitz 2009); what climatic changes are we able to identify for the Early Devonian period through various recent geochemical and modelling analyses?
Increase of planktonic diversity
Purnell (2002), after a thorough analysis of various hypotheses advocated for the feeding mode of heterostracans, concluded that “heterostracans were microphagous suspension feeders”. If pteraspids in particular, and heterostracans in general, were microphagous suspension feeders, and thus, if they were feeding upon planktonic organisms (phyto- and/or zoo-plankton), what is the biodiversity curve of the Silurian–Devonian phyto- and zoo-plankton, i.e. of acritarchs, chitinozoans, radiolarians, and planktonic ostracods? Klug et al. (2010, fig. 2) published a Devonian biodiversity curve of acritarchs: the mean standing diversity shows a first peak (less than 100 species) in the Lochkovian and a second more important one (less than 150 species) in the Frasnian. For the Silurian–Lower Devonian, Le Hérissé et al. (2009, fig. 16) show a peak for acritarchs and chlorophytes in the late Early Lochkovian of the sub-Andean zone of Bolivia. So, this pattern is similar to the vertebrate one, with a Lochkovian peak, an Emsian to Givetian strong decline, a Frasnian (and Famennian) peak, followed by a decrease in the Tournaisian.
Concerning the Devonian diversity curve of chitinozoans, various contradictory results have been obtained. Paris and Nõlvak (1999, fig. 3) show a peak of diversity in the Lower Devonian at the series level, but a peak in the Pridoli at the stage level, following a Wenlock peak as for agnathans (Paris and Nõlvak 1999, fig. 5). However, for Grahn and Paris (2011, fig. 1), the main peak occurred in the Ludlow (not the Pridoli), followed by a decline in the Lower Devonian (when agnathans increase), while for Nestor (2009), the East Baltic, Swedish and global biodiversity curves of chitinozoans show different patterns with a peak in the early Wenlock (Sheinwoodian), a low level in the early Ludlow (Gorstian), and again a high level in the Pridoli. The Devonian biodiversity curve of radiolarians is quite different, with the highest genus diversity in the Famennian–Tournaisian and a low genus diversity during the Early Devonian to Frasnian (Klug et al. 2010, fig. 2). Concerning other potential planktonic prey of vertebrates, we must consider planktonic ostracods for which a Devonian diversity curve is not yet available (V. Perrier, pers. comm., 9 July 2016), as well as cyanobacteria and eventually viruses whose Palaeozoic biodiversity is still unknown. So, it appears that comparing diversity curves of various groups of planktonic organisms with the heterostracan one is not really conclusive, except perhaps for acritarchs that show a pattern similar to that of the heterostracans.
Increase of primary productivity and algae
Elliott et al. (2004), while describing a new Allocryptaspis from the Lower Devonian of Utah (A. sandbergi), showed that (i) previous reconstructions of the oral cover of Allocryptaspis were incorrect, (ii) the anterior margins of the plates of A. sandbergi bear delicate prongs, eventually worn away by abrasion against the substrate (Elliott et al. 2004, fig. 5–6), and hypothesised that oral plates were used as scraping or combing devices to remove microscopic epiphytes from filamentous algae. We can thus hypothesise that the trends in biodiversity of heterostracans (at least of some of them) were following the trends of changes in the marine primary production and of algae. Again, we are faced with contradictory and non-conclusive results found in the literature. Martin (1996) concluded that “Cambrian-to-Devonian seas were characterized by extremely low nutrient ("superoligotrophic") conditions”, although May (1996), when analysing the relationships between sea-level changes, biogeography and bioevents in the Devonian, was convinced that “The best explanation for the majority of Devonian bioevents is the effect of oceanic anoxia” correlated to a rapid transgression, a direct consequence of which was “the increased primary biomass production (caused by both increased influx of nutrients and increased total surface of shallow seas).” Allmon and Martin’s (2014) review “confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak.” For Holland and Sclafani (2015), “the most likely … is an increase in the spatial density of marine invertebrates over the Phanerozoic, an interpretation supported by previous studies of fossil abundance. This, coupled with a Phanerozoic rise in body size, suggests that an increase in primary productivity through time is the primary cause of Phanerozoic increases in θ [fundamental biodiversity number], global richness, local richness, local evenness, abundance, and body size.” These assertions are not precise enough to conclude that there is a correlation between the marine primary productivity and heterostracan palaeobiodiversity.
A critical point of view may be that heterostracans were not marine organisms, but rather fresh water inhabitants. However, several recent analyses showed that they were most probably living in near-shore marine environments of the ORS Continent and Siberia (see a short historical review in Blieck 1985; and references in Blieck 2011, and Blieck and Elliott 2016). The most recently published pteraspid, Mitraspis cracens Elliott et al. (2015), from the Lochkovian of Prince of Wales Island, Canada, is viewed as having “inhabited a variety of environments ranging from deltaic to shallow marine.” So, comparing biodiversity curves of heterostracans with those of marine micro-organisms and with trends in marine primary production is not nonsensical.
Biotic factors—critical points of view
Several authors critically considered the trophic relationships between agnathans and gnathostomes on the one hand and the function of vertebrates in the global diversity of Palaeozoic life on the other. Purnell (2001), who evaluated the “… hypothesis that jawless vertebrates were driven to almost complete extinction by competition with gnathostomes”, and that the “Rates of family extinctions in jawless vertebrates were highest in the Early Devonian, but gnathostome diversity peaked in the Late Devonian”, concluded that “The hypothesis that the pattern of early vertebrate diversity reflects competition between agnathans and gnathostomes or between specific clades of jawless and jawed fish must be regarded as untested, and at present untestable, speculation.” For Butterfield (2011), “Fish are unquestionably powerful geobiological agents in the modern oceans, both directly and through their coevolutionary impact on zooplankton and phytoplankton dynamics”, and “their Devonian radiation is just as likely to have been the cause as the consequence of mid-Paleozoic shifts in oceanic redox.” For Sansom et al. (2015), “Early jawless vertebrates (ostracoderms) exhibit restriction to shallow-water environments. The distribution of their stratigraphic occurrences therefore reflects not only flux in diversity, but also secular variation in facies representation of the rock record.” All these opinions should thus be kept in mind when evaluating the evolution of agnathan diversity through time. If “demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturated benthic habitats together with the availability of abundant planktonic food”, the Devonian Nekton Revolution “can be interpreted as reflecting an escalation at the bottom, forcing an invasion of benthic or demersal organisms into the free water column” (Klug et al. 2010). This scenario is not so different from the one of Sallan and Carlton (2015) for whom “the transition to the Devonian ‘age of fishes’ was triggered by opening of ecospace following mass extinction”. The Silurian–Early Devonian biodiversification period is indeed following the end-Ordovician mass extinction; the latter having been possibly due to: metal-induced poisoning of the plankton in relation to generalised anoxia (Vandenbroucke et al. 2015); and/or a severe selenium depletion in the oceans (Long et al. 2016); and/or eutrophication with microbial-sulphate reduction (Schobben et al. 2016). We can probably hypothesise that a series of oceanic chemical factors triggered the Silurian–Early Devonian biodiversification that followed the end-Ordovician mass extinction.
As said here above, the activity of planet Earth is responsible of the development of life, through causal events such as tectonic activity, eustatic changes, and in fine climatic changes. I focus on latter aspect here.
Ocean and atmosphere oxygenation
It is classically considered that the evolution of biota is intimately related to the oxygenation of the oceans and atmosphere. Several methods have been developed in order to test this hypothesis. After the study of the oxygen isotope composition of apatite phosphate of Devonian conodonts from Europe (Germany, France, Czech Republic), North America and Australia, Joachimski et al. (2009) concluded that the Early Devonian, and in particular the Lochkovian, was a period of warm tropical conditions at ca. 30 °C, followed by a cooling episode in the Pragian to Middle Devonian (23–25 °C), contrary to the classical point of view that the Middle Devonian was “a supergreenhouse interval”. They compared this trend to the development of Devonian coral reefs, for both cnidarian corals and stromatoporoids, and suggested that “Middle Devonian coral–stromatoporoid reefs flourished during cooler time intervals whereas microbial reefs dominated during the warm to very warm Early and Late Devonian” (Joachimski et al. 2009, figs 7–8). The Early Devonian trend is questioned by Slavik et al. (2015) who used magnetic susceptibility and gamma-ray spectrometry on Silurian–Devonian rock sequences from the Pyrenees and the Prague basin. They concluded that, if the Lochkovian was possibly an “ extremely hot” stage, the Pragian was still “hot and humid”. Nevertheless, the net decrease in oceanic surface water temperatures observed for the Early to Middle Devonian, related to the increase of oxygen curve of conodont apatite (Joachimski et al. 2009), is in agreement with Dahl et al.’s (2010) results. The latter authors studied the isotopic composition and concentration of molybdenum in sedimentary rocks to evaluate global oceanic oxygenation. They recognised two main episodes for the Neoproterozoic and Phanerozoic, the second one in the late Early Devonian at ca. –400 Ma, as a component of a long-term increase of several tens of millions of years, beginning in the Late Silurian and covering the whole Early Devonian (Dahl et al. 2010, fig. 3). This episode “correlates with the diversification of vascular plants” [see e.g. Gensel 2008] and “with a pronounced radiation of large predatory fish” [see e.g. Bambach 2002; Payne et al. 2009; see Fig. 3a where placoderms show their first peak of diversification in the Emsian = late Early Devonian]. It may be the greatest oxygenation event in Earth history (Dahl et al. 2010)—but see Butterfield (2011) who thinks that the “Devonian radiation [of fish] is just as likely to have been the cause as the consequence of mid-Paleozoic shifts in oceanic redox”.
Palaeocontinent geography and temperature
Continents and oceans are coupled in the global climatic system. So, we can also explore the Devonian continent temperature in order to trace possible triggers of the Early Devonian peak of diversity. In this field, contradictory results have been obtained by different groups of scientists. Nardin et al. (2011) obtained a global trend for the mean annual continental surface temperatures in the Early Palaeozoic (as simulated by the “coupled climate-carbon model GEOCLIM”), where they show a continuous increase from the middle Silurian to the end Early Devonian, starting after the late Llandovery glacial interval (Nardin et al. 2011, fig. 4). This is in contradiction with Joachimski et al.’s (2009) for the Early Devonian. However, in Nardin et al.’s (2011) study, the model is constrained by relative locations of palaeocontinents without considering the role of the vegetation (otherwise temperature would decrease; E. Nardin, e-mail, 17/07/2015). Le Hir et al. (2011) used “a coupled climate/carbon/vegetation model to investigate the biophysical impacts of plant colonization on the surface climate”. They obtained a “significant atmospheric CO2drop” with the GEOCLIM/SLAVE-JR model from the Early to Late Devonian (Le Hir et al. 2011, fig. 4). Their scenario is as follows: a continuous decrease of pCO2, triggered by the drifting of Laurussia and Siberia into more humid latitudinal belts over the course of the Devonian, led to the spread of land plants during the Devonian that has been an important controlling factor of the Palaeozoic climatic evolution. This long-term decrease in Devonian continent temperatures is in contradiction with Nardin et al. (2011) for the Early Devonian, and with Joachimski et al. (2009) for the Mid-Late Devonian.
It appears that the Early Devonian peak of biodiversity in the marine realm is not purely artefactual, but corresponds to an important episode of life on Earth. Plate tectonic activity and palaeogeographic changes at the Silurian–Early Devonian transition with increased deposits of Old Red Sandstone favoured, at least locally, a sea level rise with widening of near-shore and transitional land-sea niches where heterostracans (and other ostracoderms such as osteostracans, galeaspids, and some thelodonts) flourished. Those changes correlate with a warm tropical climate (at least at the acme of the diversification peak in the Lochkovian) and other chemical conditions of the ocean that favoured a peak of primary production and of planktonic diversity and abundance (at least acritarchs). However, we have no detailed pertinent scenario to propose for explaining this episode. In summary, the Lochkovian was a greenhouse world without polar ice caps, continents without important forests, warm seas and oceans with a flourishing life, including abundant early vertebrates among an exuberant invertebrate fauna during a time of rise of predators.
This paper is based on a series of oral communications that have been given in front of IGCP 596–SDS Symposium: Climate change and biodiversity patterns in the Mid-Palaeozoic (Sept. 20–22, 2015, Brussels, Belgium), the 2016 meeting of Association Française de Paléontologie (30 March–2 April 2016, Elbeuf, France), and IGCP 591 Closing meeting: The Early to Mid Palaeozoic Revolution (6–9 July 2016, Ghent, Belgium). The organisers of these meetings are warmly thanked. Several colleagues helped during the making of this paper, viz., Lauren C. Sallan (University of Pennsylvania at Philadelphia, PA, USA), Elise Nardin (Université Paul Sabatier, Toulouse, France), and some others who are cited in the text. They are thanked for this. Both reviewers David K. Elliott (Northern Arizona University, Flagstaff, USA) and Philippe Janvier (CNRS, Muséum National d’Histoire Naturelle, Paris, France) made detailed comments on the “manuscript”, that greatly improved its quality.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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