Palaeobiodiversity and Palaeoenvironments

, Volume 97, Issue 3, pp 375–390 | Cite as

Heterostracan vertebrates and the Great Eodevonian Biodiversification Event—an essay

Original Paper

Abstract

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.

Keywords

Early Devonian Nekton Revolution Ostracoderms Palaeobiodiversity Essay in geobiology 

Introduction

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

Since the pioneering work of Phillips (1860), it has become traditional to distinguish various periods of diversification of life on Earth. Phillips (1860) is indeed well known for having introduced the three major divisions of the Phanerozoic Era, viz. Palaeozoic, Mesozoic and Cenozoic, separated from each other by a strong decrease in biodiversity, and in particular of marine organisms (see a narrative and references in Scott 2015). A much more detailed view of this fact appeared with the development of analyses of large data bases such as the one developed by Sepkoski (1981) and followers. Later authors distinguished three main ecological assemblages of marine organisms, called respectively Cambrian, Palaeozoic and Modern Evolutionary Faunas, a much simpler version of which was already present in Phillips’ (1860) book. Concerning the Phanerozoic Eon, a series of biodiversifications and mass extinctions is now recognised through all recent biodiversity analyses of, mostly, marine organisms. It includes several peaks of biodiversity from the Ordovician to the Paleogene. In this series, the Early Devonian peak appears to be the second main Palaeozoic biodiversification event (Fig. 1). On Alroy’s (2010) curve, it is even more important than the Ordovician peak, a.k.a. the Great Ordovician Biodiversification Event or GOBE. The Early Devonian peak, renamed as the Great Eodevonian Biodiversification Event or GEBE (Blieck 2015), is mostly due to the development of the Palaeozoic Evolutionary Fauna (sensu Sepkoski 1981), that includes invertebrates such as trilobites, crinoids, bivalvs and “worms”, and also the oldest known ossified vertebrates such as ostracoderms, placoderms, and osteichthyans. It is followed by a massive drop during the Middle Devonian (sensu Alroy et al. 2008).
Fig. 1

Alroy’s (2010, fig. 3) Phanerozoic diversity curve for the three marine evolutionary faunas (sensu Sepkoski 1981; metazoans less tetrapods), viz. Cambrian (Cm), Palaeozoic and Modern. Slightly modified after a figure from GEOL 331 Principles of Paleontology, Department of Geology, University of Michigan (http://theresilientearth.com/files/images/phanerozoic_genera-alroy2010-500.jpg). After a slight decrease during the Silurian, it clearly shows an Early Devonian peak as the continuation of the Cambrian–Ordovician biodiversification events, followed by a large Mid-Devonian drop (Alroy et al. 2008, p. 98)

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.

This GEBE is the marker of the beginning of the Devonian Nekton Revolution (DNR) as defined by Klug et al. (2010), who have shown Early Devonian peaks of diversity for a series of marine organisms such as acritarchs, dacryoconarids + homoctenids, orthocerids, as well as for vertebrates (osteostracans, galeaspids, pteraspidomorphs, thelodonts, and acanthodians). This corresponds to the predominance of jawless fish (ostracoderms) over jawed fish (gnathostomes) in the Early Devonian, although the latter undergo a radiation from the Middle to the Late Devonian, with peaks either in the Frasnian or the Famennian (placoderms and osteichthyans in the Frasnian, chondrichthyans in the Famennian; except for acanthodians which have a first peak in the Early Devonian; Klug et al. 2010, fig. 2). This turnover in vertebrate faunas is one element of the DNR, interpreted as a major macroecological change where benthic animals declined while nektonic ones radiated (ibid., fig. 1). Concerning fish indeed, the earliest, Ordovician forms were demersal (see e.g. Astraspis and Sacabambaspis), the Silurian forms were still mostly demersal (ostracoderms) while nektonic acanthodians became more abundant, and we observe a radiation of fully nektonic forms in the Devonian and later (ostracoderms in part, placoderms, chondrichthyans, osteichthyans). However, demersal vertebrates still existed in the Devonian (see e.g. the psammosteid pteraspidomorphs, the galeaspids, osteostracans, some of the thelodonts, the antiarch placoderms). We even know of supposed partly buried species such as representatives of Arctictenaspis or Doryaspis if Dineley’s (1976) and Pernègre’s (2005) interpretations are correct (Fig. 2). However, arguments refuting a half-buried habitus for Arctictenaspis (Ctenaspis in Dineley 1976) were developed by Blieck and Heintz (1983) and Elliott and Blieck (2010) who interpret it as animals that were nektonic and swam near the surface. The DNR for vertebrates can thus be seen as having begun in the Silurian, when the Devonian period (a.k.a. the “Age of Fishes”) is more likely interpreted as a progressive domination of gnathostomes over ostracoderms, and corresponds to the Predation Revolution of vertebrates (Blieck 2011). This “domination” may have been a mere ecological shift, with gnathostomes progressively invading the nekto-benthic and nektonic domains (Klug et al. 2010; see discussion about the trophic relationships here below).
Fig. 2

Interpretation of the ecological niches of some Early Devonian heterostracan pteraspidomorphs. Half buried: aArctictenaspis (sensu Elliott and Blieck 2010; after Dineley 1976, fig. 8: Ctenaspis), bDoryaspis after Pernègre (2005, fig. 10); demersal: cGabreyaspis after Novitskaya (2004, fig. 45), dDrepanaspis after Gross (1963, fig. 11A); nektonic: eTorpedaspis after Broad and Dineley (1973, fig. 30), fRhinopteraspis after Gross (1937, fig. 2). Note (1) the different location and orientation of the oral aperture, which is antero-dorsal in the half-buried species (and above a pseudo-rostrum in Doryaspis), subterminal in the demersal ones, and ventral in the nektonic ones; (2) that the superficial ornamentation of the head carapace of half-buried and demersal species is mostly tuberculated, although it is made of dentine ridges in the nektonic ones; and (3) that the ventral shield of the head of half-buried species is much more vaulted downward than the dorsal shield. Not to scale. [See contradictory interpretation of Arctictenaspis in Blieck and Heintz 1983, and in Elliott and Blieck 2010.]

Vertebrate diversity and trophic relationships

The general diversity curve of vertebrates in the Early and Middle Palaeozoic shows a series of events of either biodiversification (radiation) or decrease. At the family level, after a mild diversification in the Late Ordovician followed by the Talimaa’s Gap (sensu Turner et al. 2004), the first long-term episode of diversification is met with in the Silurian and Early Devonian (Fig. 3a). It culminates in the Early Devonian (Lochkovian–Pragian) with a Lochkovian peak for agnathans (ostracoderms) followed by a diversification of gnathostomes, that is, mainly placoderms, but also acanthodians and sarcopterygians. However, the total count of families shows a drop from the Emsian to the Givetian, followed by a diversity peak in the Frasnian. “There is a specific problem with the Emsian record. The Emsian is a comparatively long stage of the Devonian (−407,6 to −393,3 My, thus a duration of more than 14 My), but also comparatively poor in vertebrate remains. One reason may be the paucity of terrigenous sediments, but the few Emsian terrigenous formations (e.g. in Spitsbergen) do not provide much better information. This may indeed not be an artefact, but does indicate a drop in vertebrate diversity. Yet, a number of phylogenetic events probably took place during the Emsian, notably the beginning of the rise of advanced tetrapodomorphs.” (P. Janvier, e-mail, 20/09/2016). By comparison, the Frasnian peak is most probably partly due to a taphonomic bias, owing to a great richness of vertebrates in Fossil-Lagerstätten such as Steinbruch Schmidt (a.k.a. as Bad Wildungen) in Germany or the Gogo Formation in Australia, followed by a drop in the Famennian whose vertebrate diversity is certainly underevaluated (Carr and Jackson 2009; Blieck 2011). Interestingly, an analysis of their diversity at the genus level by Friedman and Sallan (2012) for all marine fishes, and by Sansom et al. (2015) for ostracoderms shows the same pattern with an agnathan peak in the Lochkovian that follows the Silurian radiation, and a gnathostome (mostly placoderm) peak in the Frasnian that follows the Emsian to Givetian drop (Fig. 3b). Sansom et al. (2015) have tested the quality of the fossil record of ostracoderms in order to separate the biological and geological signatures of their diversity curve. They conclude that the predominance of the geological (stratigraphical) signal is due to the fact that ostracoderms were mostly tied to shallow water environments. Concerning ostracoderms proper, their diversity gradually declines towards the end-Devonian. They all come extinct before or at the Frasnian–Famennian boundary, except for a single known species of thelodonts that survived in the Famennian of Iran and Australia (North and East Gondwana) (Hairapetian et al. 2015).
Fig. 3

Ordovician to earliest Carboniferous diversity of vertebrates. a Family-level diversity of all vertebrate higher taxa after Blieck (2011, fig. 2, modified; database of Benton 1993). b Genus-level diversity of ostracoderms (in colours) and placoderms (in grey) after Sansom et al. (2015, fig. 1, slightly modified). Both level diversity curves show the same pattern with an agnathan peak in the Lochkovian that follows the Silurian radiation, and a gnathostome (mostly placoderm) peak in the Frasnian that follows the Emsian to Givetian drop. Abbreviations: aARG Arenig, LLN/LLO Lanvirn/Llandeilo, CRD Caradoc, ASH Ashgill, LLY Llandovery, WEN Wenlock, LUD Ludlow, PRD Přidoli, LOK Lochkovian, PRA Pragian, EMS Emsian, EIF Eifelian, GIV Givetian, FRS Frasnian, FAM Famennian, TOU Tournaisian, VIS Viséan, Chondr. Chondrichthyes, Actinopt. Actinopterygii, Sarcopt. Sarcopterygii; bnon-ps. Heterostraci non-psammosteid Heterostraci, psam. Heterostraci psammosteid Heterostraci, San Sandbian, Kat Katian, LLa Llandovery, Wen Wenlock, Lud Ludlow, Pri Přidoli, Loc Lochkovian, Pra Pragian, Ems Emsian, Eif Eifelian, Giv Givetian, Fra Frasnian, Fam Famennian

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).

Coming back to the predator–prey relationships, it has been classically considered that eurypterid arthropods were vertebrate predators (Romer 1933 and refs. in Lamsdell and Braddy 2010). This has been re-analysed by Lamsdell and Braddy (2010) who conclude that “the suborder Eurypterina begins to decline steadily through the Early Devonian, coinciding with the radiation of Placodermi. Stylonurina remain relatively unaffected, but undergo a drop in diversity as part of the end-Devonian mass extinctions, along with agnathans and placoderms” (Fig. 4). Nevertheless, predation of vertebrates on eurypterids or contrarily of eurypterids on vertebrates is difficult to demonstrate. Even if we know many Silurian and Early Devonian localities with co-occurring vertebrates and eurypterids (e.g. Plotnick 1999; Lamsdell and Braddy 2010), very scarce examples of supposed direct evidence of predation of eurypterids on agnathans are known: Lebedev et al. (2009, fig. 1) interpret a double trace, made of a smaller and a bigger tip, on the dorsal shield of the heterostracan Larnovaspis kneri (figured by Blieck 1984, fig. 44B) as the result of biting by a sarcopterygian or acanthodian tooth, or more likely by an eurypterid distal denticle; Elliott and Petriello (2011, fig. 7) figure a trace made of three aligned pits (two smaller and one bigger) on the dorsal shield of the heterostracan Lechriaspis patula as a damage most likely caused by an eurypterid. However, agnathans may also have been the prey of gnathostomes. Several examples are now known for the Devonian: Mark-Kurik (1966, fig. 1) illustrated examples of predation damage on psammosteid heterostracan bony plates that would have been caused by the co-occurring sarcopterygian Glyptolepis; similar evidence is seen on Middle and Late Devonian psammosteids by Lebedev et al. (2009) and Elliott (in Elliott and Petriello 2011: 527), where most probable predators were porolepiform and osteolepiform sarcopterygians (including the large sarcopterygian Laccognathus). This can also be illustrated by psammosteid branchial plates whose ventral side shows bite marks in the lower Frasnian, Yam-Tesovo locality of the Leningrad Region (NW Russia), where sarcopterygians co-occur, including a tetrapod (V. Glinskiy, St Petersburg State University, work in progress, e-mail Oct. 1, 2015). A more obvious case of predation is a small osteostracan swallowed by an acanthodian from the Lower Old Red Sandstone (Dittonian≈Lochkovian) of the Wayne Herbert quarry, Hereforshire, England (Miles 1973, pl. 1, fig. 2: “rim of cephalaspid” similar to Pattenaspis in the ventral part of Ptomacanthus anglicus; P. Janvier, e-mail 21/09/2016). Similar to psammosteids, several cases of bite marks on placoderm bony plates are attributed to predatory sarcopterygians by Lebedev et al. (2009). Small osteolepiform sarcopterygians could even be the prey of larger sarcopterygians (ibid.). The increase in taxonomic richness of sarcopterygians in the Middle and Late Devonian, contemporaneous with the decline and disappearance of agnathans and placoderms (Fig. 4), may be interpreted in the sense of predation by the former on the latter. Other possible predators of vertebrates in general, and agnathans in particular, are cephalopods (orthocerid nautiloids and ammonoids) that are elements of the Devonian Nekton Revolution (Klug et al. 2010). This hypothesis has been recently reinforced by the description of Late Devonian mandibles from Hangenberg type shales of Ma’der (E. Anti-Atlas, Morocco) by Klug et al. (2016) who concluded that “chitinous normal-type jaws were likely to have already been present at the origin of the whole clade Ammonoidea, i.e. in the early Emsian (or earlier)” (see their fig. 9). These authors, however, interpret the appearance of ammonoid jaws as, possibly, the result of “selection pressure of the intensifying gnathostome radiation in the Late Silurian to Early Devonian” (Klug et al. 2016, p. 14). We may contrarily imagine predation of ammonoids over agnathans in the Silurian and Early Devonian, when gnathostomes became predators of ammonoids in the Middle and Late Devonian…. However, to my knowledge, no predator–prey relationship between Devonian representatives of these groups has ever been mentioned; this is perhaps simply due to the fact that they occupied different environments. Alternatively, predator–prey relationships have been described by Hansen and Mapes (1990, fig. 170–173) between Early Pennsylvanian sharks and cephalopods where sharks were the predators of nautiloids, at a time when ostracoderms had long been extinct [for a more general interpretation of predation in Palaeozoic marine environments, see Brett and Walker 2002].
Fig. 4

Family-level diversity of Eurypterida and vertebrates through the Palaeozoic, combined after Lamsdell and Braddy (2010, fig. 2) and Purnell (2001, fig. 12.4 pro parte) from Benton’s (1993) data, slightly modified. Abbreviations: stratigraphical log—see Fig. 3; F/F Frasnian–Famennian boundary, P/T Permian–Triassic boundary. Note the co-occurrence (correlation) of a strong decrease in agnathan (ostracoderm) diversity (up to extinction at the F/F boundary) and increase in placoderm and sarcopterygian diversities (and secondarily of other gnathostome groups)

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).

At a wider taxonomic scale, the species-level diversity of heterostracans has been evaluated for the Late Silurian to Late Devonian sequence of the October Revolution Island of Severnaya Zemlya, in Russian Arctic. It shows a strong peak in the late Lochkovian Pod’emnaya Formation, with 26 different species out of a total of 48 species for the whole group (cyathaspidiforms, including amphiaspids; tesseraspids, corvaspids, traquairaspids, ctenaspids, Lepidaspis, and pteraspidiforms, including psammosteids) (Blieck et al. 2002, fig. 4) (Fig. 5). This trend is to be compared with the pattern observed for heterostracans in the Lower Devonian sequence of Spitsbergen, in the European Arctic. The stratigraphical distribution of heterostracans in both the Red Bay Group and the Wood Bay Formation of NW Spitsbergen, at genus level, shows a series of three peaks, the main one being in the “middle” Lochkovian vogti horizon of the Red Bay Group, with 11 different genera (provisional evaluation of Blieck et al. 1987, fig. 2) (Fig. 6). In this sequence, other diversified vertebrates are mostly osteostracans (with a peak in the upper Lochkovian) and placoderms (with a peak in the Pragian).
Fig. 5

Species-level diversity of heterostracans in the Upper Silurian to Upper Devonian of October Revolution Island, Severnaya Zemlya, Russia (after Blieck et al. 2002, fig. 4). It clearly shows a peak in the Upper Lochkovian and extinction before the Famennian. Abbreviations: Ludl. Ludlow, Prid. Přidoli, Eifel. Eifelian, Famen. Famennian

Fig. 6

Genus-level diversity of heterostracans in the Lower Devonian of NW Spitsbergen (after Blieck et al. 1987, fig. 2 for the Fraenkelryggen and Ben Nevis formations, and Pernègre and Blieck 2016, fig. 4 for the Wood Bay Formation). It shows the ‘middle’ Lochkovian peak of the vogti horizon

On the opposite (southern) side of the ORSC, several regions have delivered a good fossil record of Early Devonian vertebrates, including the classical sequences of Artois in northern France, the East Baltic States, and Podolia in Ukraine. In N. France, the Přidoli to Emsian sequence of both subsurface and surface localities shows a peak of diversity of all vertebrates at the base of the “middle” Lochkovian Rhinopteraspis crouchi Zone in the Pernes Formation (nine species including three of heterostracans; Blieck et al. 1995, fig. 3). This peak was recently confirmed with the description of the vertebrate fauna from the slag heap of Liévin shaft n° 8, from the Pernes Formation, which yielded heterostracans and other vertebrates (an osteostracan and an arthrodire) of the lower Rhinopteraspis crouchi Zone, the heterostracans including five different species (Blieck and Styza 2014). In the East Baltic States, the Lower Devonian is known in the subsurface of Lithuania, Latvia and southernmost Estonia. It is represented by three lithostratigraphic units separated from each other by important gaps (Kleesment and Mark-Kurik 1997; Paškevičius 1997; Mark-Kurik and Põldvere 2012), the age of which is still difficult to establish and correlate with international standards (see e.g. Blieck et al. 2000, fig. 12). Nevertheless, its vertebrate fauna has been studied for a long time, and in particular its heterostracans (see a review by Blieck et al. 1988), whose diversity shows a peak of 11 species in the “middle” Lochkovian, Stoniškiai Regional Stage that yielded the index-species Rhinopteraspis crouchi (Blieck et al. 2000, fig. 11–12). In Ukraine, the Silurian–Devonian sequence along the Dniestr river and its tributaries of Podolia is well-known for its stratigraphy (see e.g. Nikiforova 1977) and vertebrate fauna. Recent reviews of both its ostracoderm and placoderm components lead to a re-evaluation of both the diversity and age of its heterostracans (Dupret and Blieck 2009; Dupret 2010; Voichyshyn 2011). There is, however, a disagreement between various authors about the location of the Lochkovian–Pragian boundary (L/P). Dupret and Blieck (2009) and Dupret (2010) define this boundary between the Ustechko Member and the Khmeleva 1 Member of the Dniestr Formation (that is between the Old Red I and Old Red II in previous nomenclature), while Voichyshyn (2011, Table 1 and p. 182) keeps a more conservative view with a lower L/P boundary, somewhere in the lower Ustechko Member (Fig. 7). The Lochkovian–Pragian part of the Podolian sequence yielded 52 different species of heterostracans (pteraspidiforms, cyathaspidiforms, including ctenaspids; corvaspids, “Lepidaspis”, tesseraspids) according to Voichyshyn’s (2011, Table 1) review, making probably this series the richest one for this group of ostracoderms. This should be compared to the 34 species of Lochkovian–Pragian heterostracans of Severnaya Zemlya (Blieck et al. 2002) and the 18 genera of Lochkovian–Pragian heterostracans of Spitsbergen (Blieck et al. 1987), the specific richness of which has to be re-evaluated. Three peaks of diversity are observed in the Podolian sequence, with 24 species in the “middle” Lochkovian lower Ivanie Formation, 15 (16?) species in the late Lochkovian (sensu Dupret and Blieck 2009) lower Ustechko Member of the Dniestr Formation, and 10 species in the early Pragian Khmeleva 1 Member of the Dniestr Formation (Fig. 7). The acme of heterostracan diversity is thus again reached in the “middle” Lochkovian.
Fig. 7

Species-level diversity of heterostracans in the Upper Silurian and Lower Devonian of Podolia, Ukraine (after Voichyshyn 2011, Table 1, p. 16–17). Ludlow–Přidoli and Přidoli–Lochkovian (Silurian–Devonian) boundaries after Blieck et al. (2000, fig. 13). Lochkovian–Pragian boundary after Dupret and Blieck (2009). Abbreviations: Fm. formation, Gp. group, Ludl. Ludlow, Prid. Přidoli

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

The question is thus: What may have triggered this Early Devonian peak of biodiversity among marine life and among early vertebrates? Considering heterostracans, we must first question the cause of their diversification through the Silurian and Lower Devonian, and thus of its Lochkovian peak. According to Mallatt (1984), “Feeding modes which on the basis of extant fish are closely related to benthic microphagous suspension feeding include deposit feeding, epilithic algal scraping, and macrophagous suspension feeding; early jawless vertebrates are predicted to have included all these feeding types.” Increase of the oceanic primary production would have led to an increase of diversity of the phytoplankton, thus of the zooplankton, and of algae diversity, and finally of their eaters including heterostracans. What relations between all sets of factors are we able to hypothesise? What may have triggered the increase of primary production and the plankton diversity in the Early Devonian?
  • 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?

Biotic factors

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.

Abiotic factors

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 coralstromatoporoid 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.

Conclusion

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.

Notes

Acknowledgements

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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© Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Université de Lille–Sciences et TechnologiesVilleneuve d’Ascq cedexFrance

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