Reviewed by Carlton E. Brett (University of Cincinnati)

Hartenfels, S., C. Hartkopf-Fröder, and P. Königshof, eds. 2022 and 2024. The Rhenish Massif: More than 150 years of research in a Variscan mountain chain, parts I and II. Palaeobiodiversity and Palaeoenvironments 102 (3) & 104 (3).

The Rhenish Massif of western Germany and adjacent regions is a structural block within the Variscan (Carboniferous) mountain terrane. The thick stratigraphic succession comprises shallow shelf siliciclastics and reefal carbonates (Rhenish facies) to deep water and typically hypoxic shale-prone sediments (Hercynian facies), largely derived from northern Caledonian-Acadian source areas and deposited along the northern margin of the Rheic Ocean on the southern continental shelf-slope. Despite structural complexities and low-grade metamorphism of some rocks, the Rheno-Hercynian sediments are typically highly fossiliferous and record a series of environments, from coastal swamps with some of the oldest (Eifelian) fossil forests to shallow, storm-dominated siliciclastics, carbonate platforms, and a variety of dysoxic-to-deeper-marine mud-bottom settings. The presence of strong Acadian to early Variscan volcanism adds to the diversity of environments: small volcanic islands often mantled with small carbonate banks and reefs formed an intricate mosaic of varied environments. The intercalation of tephra (bentonite) beds allows for correlation and assignment of absolute dates. The carbonates yield rich conodont assemblages, as well as shelly faunas, while deeper Hercynian facies yield an abundant record of ammonoids as well.

Together, these factors make the Rhenish strata among the most dynamic and well-studied Devonian successions in the world. Yet, as these paired publications document, there is still a wealth of data to be tapped in these rocks and, together with new methods and models, there has been something of a renaissance of study in the past few decades. This special issue on the stratigraphy and paleoecology of the Rhenish Massif, published in two installments in 2022 and 2024, represents an outstanding compilation of new research primarily on the Devonian to Mississippian of western Germany and adjacent regions. In a total of 20 major papers (19 original papers and one extensive review of the Famennian conodont stratigraphy and evolution and global bioevents), more than 40 authors, including most of the major researchers on German Devonian, have explored a broad array of topics in the general areas of paleontology, microfacies analysis, biostratigraphy (particularly conodont and ammonoid zonation), tectonics, and biogeography. Many of these papers are quite extensive and present, in addition to a wealth of new data, also broad-ranging reviews, syntheses and novel interpretations. This set forms an excellent complement to the recently published synthesis on the stratigraphy, facies, and paleoenvironments of another classic Devonian area: the Acadian foreland basin of New York State (Ver Straeten et al., 2023).

The guest editors, Sven Hartenfels, Christoph Hartkopf-Fröder, and Peter Königshof, have done an excellent job of providing succinct overviews of each special issue. In addition, the second issue includes a touching and informative obituary of leading German researcher Hans-Georg Herbig, an author of a paper in the first issue who’s untimely passing in 2024 shortly preceded publication of the second.

The range of topics covered in the two special issues can be roughly divided into three broad themes: a) systematic paleontology and phylogeny of Devonian-Mississippian marine invertebrates, plants, and fresh-water invertebrates, b) carbonate petrology and paleoecology of Devonian reefs, and c) stratigraphic syntheses and depositional environments of Upper Devonian-Mississippian stratigraphic intervals in the Rhenish Massif, including potential stratotypes.

In the first category, papers include systematics, biogeography, and paleoecology of disparate marine invertebrates, including: a restudy and description of fenestellid bryozoans from the Middle Devonian of the Eifel region (Ernst, 2022: 521–540); two papers on bivalves: Middle and Upper Devonian cardiolids that appear to have been early siphonate filter feeders (Nagel-Myers, 2022: 541–555) and Mississippian deeper water parallelodont arcoids and their Devonian predecessors (Friedel and Amler, 2024: 511–535); a new Givetian, reef-associated discosorid and associated nautiloids (Afhüppe and Becker, 2022: 613–697); new Famennian ammonoids with implications for the base of the German Hemberg Stage (Becker, 2024: 683–707); and two new species of the trilobite Gondwanaspis with implications for a cosmopolitan distribution near the Givetian-Frasnian boundary (Helling and Becker, 2022: 697–710); finally, Pauly and Haude (2024: 571–628) give a well-illustrated systematic account of exceptionally preserved large echinoids which, though flattened, preserve detailed structures, such as very delicate pedicellaria. The authors suggest that there was a faunal turnover of echinoids associated with the Hangenberg extinctions at the end of the Devonian, presumably associated with the Hangenberg biocrisis.

Two interesting papers deal with biotas of marginal marine-fresh water and adjacent ecosystems on the southern side of the Old Red Continent. The Devonian terrestrial facies of the Rhenish Massif document important events in the early evolution of land plants, including some of the World’s oldest forest deposits in the Eifelian, as well as early non-marine invertebrates. This issue includes a paper discussing fossil material still older (Pragian): vascular plants called zosterophylls from the vicinity of Lahrbach Valley, a region that has yielded both spectacular early invertebrates (eurypterids, scorpions, and trigonotarbids) together with well-preserved plant material (Gossmann et al., 2022, po. 503–521). The new zosterophylls are partially pyritized material, which shows well preserved cone-like sporangia-strobili with seven rows of scales. A second paper (Poschmann et al., 2024: 535–569) documents the systematics, paleoecology and associations of normally very rare diplostracan clam shrimps from marginal marine–fresh water sections in the Lower Devonian (Emsian) of the Eifel region.

A theme of four papers in the two special issues is the detailed stratigraphy, microfacies and paleoecology of Middle Devonian reefs of the Rhenish Massif. Tabulate-rugose coral and stromatoporoid reefs were globally most extensive during the Eifelian-to-Frasnian interval but they were decimated by the volatile events of the Late Devonian. Reefs in the Rhenish Block occur in two major areas: a) large Givetian-to-Frasnian reef tracts tens of kilometers wide and stretching west-east for several hundred kilometers in the northern portion of the massif, the so-called Hagen-Balve reef complex in the northern Remscheid-Altena Anticline exposed in the Hönne River Valley, and b) local reef complexes associated with volcanic seamounts in the vicinity of the Lahn and Dill synclines in western Germany. Reefs in both areas are treated in a series of papers. All share an emphasis on detailed description, excellent illustration, and interpretation of microfacies.

An exceptionally complete 200-m core through a Givetian reef at Hahnstätten in the southern Lahn Syncline provided the opportunity for a detailed look at reef growth in a tectonically active region. Königshof et al. (2024: 461–492) describe and beautifully illustrate each of nine major facies and microfacies with color photographs of polished slabs and thin sections and give documentation of varying environments and biotas including frame-building corals and stromatoporoids through this thick buildup. An important conclusion is that changes in reefal growth reflect local tectonics and that eustatic fluctuations played a secondary role.

In contrast, equally detailed microfacies studies by Löw et al. (2022: 573–612) and Stichling et al. (2022: 622–692) of the Hagen-Balve reef complex demonstrates that reefs in this more stable area responded primarily to extrinsic forces, primarily eustatic fluctuations. Löw et al. (2022) detail the succession of changes in carbonate microbiofacies/environments in the initial, late-early-to-middle Givetian phases of the Hönne Valley reef near Binolen. In additional to detailed petrographic study of slabbed and thin-sectioned samples of facies, the authors provide a unique log of relative abundance of microfossils, including foraminifera, sponge spicules, ostracodes, scolecodonts, holothurian and ophiuroid sclerites, conodonts, fish bone fragments, and others. The authors document two cycles of deepening and shallowing within this succession, recognizable by changes between coarse crinoidal and skeletal packstones and rudstones to micritic wackestone facies. Microfacies document a more detailed high-resolution record of fluctuations at meter scale. This pioneering study establishes evidence for response to environmental fluctuations driven by climatic/eustatic changes.

Stichling et al. (2022) document the longer history of a massive reef (part of the Hagen-Balve complex) with detailed study of two drill cores and cliff outcrops adjacent to the enormous Asbeck Quarry, near Beul. The authors tested the influence of environmental changes in shaping the buildup and, particularly, in its eventual drowning as well as in local post-reefal developments of a condensed deeper water carbonate bank. What particularly stands out is the persistence of reef building through a significant portion of the Givetian and early Frasnian. It is remarkable that global bioevents/anoxic events can be recognized in the reef and post-reef facies. The reef grew strongly during the early–middle Givetian, was restricted during the Taghanic biocrisis, and more-so during the hypoxic Frasne event. Final drowning of the reef occurred during the early Frasnian Middlesex anoxic event. Post-reefal condensed carbonate bank facies show lithologic signatures of the lower and upper Kellwasser events, the Condroz regressive events, and a sharp change to through-going middle Famennian Hemberg marine red facies at the top of the succession. Building on these studies, it would be important to see if the series of eustatic and biofacies turnover events documented in the Givetian of eastern North America, Morocco, and elsewhere could be identified within the detailed frameworks provided by these authors.

Königshof and Flick (2024: 493–510) discuss an intriguing case study of the Balduinstein reefs situated on the fringes of volcanic islands/seamounts that emerged along the southern, Rheic margin of the Rhenish Massif. The interplay of eustasy and local tectonism-volcanic centers makes this an important contribution to the study of Paleozoic reefs and synsedimentary volcanics. It is further intriguing that terrestrial plant remains have been observed in beds intercalated with reefal and volcanic rocks pointing to a complex of marine and terrestrial environments on a volcanic island.

The third broad theme of these collected papers is the chronostratigraphy, bioevents, and tectonics of the Upper Devonian-Mississippian interval in the Rhenish Massif and correlatives elsewhere. Not surprisingly, given their great importance in Devonian chronostratigraphy, the systematics, biostratigraphy, and biogeographic distribution of conodonts feature in nearly all papers.

A fascinating study of reworked sands and volcaniclastic sediments intercalated in a carbonate succession and a remarkable angular unconformity (Ribbert and Piecha, 2022: 557–572) provides evidence of local tectonic uplifts and erosion during the late Frasnian to early Famennian interval, well dated by conodont assemblages. Based on previous studies and detailed examination of spectacular, but temporary, outcrops exposed during highway (A-44) construction near Hülsbeck in 2015, the authors give a detailed chronology of events constrained by conodont biostratigraphy. Quartz grains, quartzites, and volcanic debris, apparently derived from an uplifted region near Krefeld, were deposited in late Frasnian sandstones and subsequently uplifted and reworked during an erosional interval roughly encompassing the Frasnian–Famennian boundary (rhenana to triangularis conodont zones). These reworked siliciclastic grains were mixed with skeletal limestone during early mid-Famennian time. Subsequently, the area was uplifted, tilted, and then beveled to a peneplane during a long hiatus, roughly the middle Famennian, and was finally covered at an angular unconformity by later Famennian crinoidal limestones. This paper nicely illustrates the synsedimentary tectonics of the Rhenish Massif.

These two issues also contain extensive reviews of biostratigraphy and chronostratigraphy in the Upper Devonian to Mississippian (Tournaisian). Very extensive comprehensive syntheses of revised conodont and ammonoid biostratigraphy of critical boundaries, with proposals of possible GSSPs (Global Stratotype Section and Point), are provided for the middle Frasnian (Saupe and Becker, 2022; Becker, 2024), the late Famennian and Devonian-Carboniferous boundary (Hartenfels et al., 2022; Hartenfels, 2024; Korn et al., 2024), and the mid Tournaisian (Esteban Lopez, 2024). These studies are outstanding compilations of old and new data and provide exemplars of the multidisciplinary approach that characterizes the work of the International Subcommission on Devonian Stratigraphy (SDS) in its efforts to establish global reference areas (GSSPs) and global correlations using updated biostratigraphy, litho- and sequence stratigraphy, and chemostratigraphy. These boundaries are critical intervals, including first- and second-order mass extinctions (see Becker et al., 2016 for classification) and these papers also address the detailed patterns, biotic extinctions and turnovers. By their nature, these studies go far beyond the focus area of the Rhenish Massif with reviews of data from most key outcrop areas in the world.

Saupe and Becker (2022: 711–762) update and document the conodont biostratigraphy and microfacies of the mid-Frasnian interval at a classic Martenberg locality in the eastern Rhenish Massif. The authors revise the systematics of Palmatolepis jamieae and subdivide the interval into two subzones, suggest abandonment of the jamieae Zone, and propose a revised conodont zonation for a prospective middle–upper Frasnian substage boundary. This boundary coincides with the base of a global transgressive interval associated with entry of the distinctive Palmatolepis semichatovae, commonly within black shale or deep-water nodular facies, the so-called semichatovae (upper Rhinestreet) Event. The authors review 20 sections globally that demonstrate similar patterns of eustatic deepening at this time.

The Martenberg section is a strong contender for a GSSP of a middle–upper Frasnian substage boundary. Although at present substages are only informal, the SDS and Thomas Becker, in particular, have been very proactive in proposing subdivisions of stages and providing extensive documentation of potential stratotype sections. This paper is an excellent example of this approach and illustrates attempts to align multiple proxies including global events with biostratigraphy.

In recent years, there has been a strong emphasis on critically redefining the Devonian–Carboniferous (D–C) boundary, in part because of problems with the bounding conodont-based boundary; in addition, the present GSSP at La Serre trench E’ in Montagne Noire, France, has proven to have inadequacies, including the apparent cut-out of a critical interval. This has led to concerted efforts by a Devonian–Carboniferous Working group to locate and carefully document a new GSSP locality and perhaps to modify the biostratigraphic definition.

To this end, Hartenfels et al. (2022: 763–829) provide a detailed analysis of a potential new GSSP at Borkewehr near the town of Balve in the northern Rhenish Massif. This locality was selected following a survey by Thomas Becker and co-authors of some 30 potential sections, most of which proved inadequate for multiple reasons. The paper on Borkewehr exemplifies the multi-disciplinary approach and gives considerable detail on the patterns of facies and faunal change associated with the latest Devonian Hangenberg crisis near its type-area. A forestry track and small quarry expose every bed of a ~3 m condensed latest Devonian succession, including the pre-event Wocklum Limestone, which yields a series of clymeniid ammonoids and conodonts and the widespread Hangenberg Black Shale at which the first phase of a global first-order mass extinction takes place. The succession continues with Hangenberg Shale and Sandstone equivalents, recording a global glacioeustatic lowstand, a second less-severe phase of the extinction, and finally the thin Stockum Limestone with basal dark shale recording renewed transgression in the final phases of the biocrisis. The traditional Devonian–Carboniferous boundary at the entry of the conodont Siphonodella (Eosiphonodella) sulcata lies near the top of this limestone. This succession is overlain by the post-event micritic nodular Hangenberg Limestone of earliest Tournaisian age. The authors document not only the lithostratigraphy and conodont and ammonoid occurrences within this succession, but also the sequence stratigraphic framework, chemostratigraphy, and cyclostratigraphy/astrochronology to produce a quantitative time scale for boundary interval and the Hangenberg biocrisis. Again, this chapter illustrates best practices in chronostratigraphy and the efforts to make critical boundaries identifiable through multiple proxies, biologic events, sequence stratigraphy (sea level), time-specific facies, and geochemistry.

The contribution of Esteban Lopez (2024: 735–752) gives important documentation of conodonts from a section of allochthonous Mississippian turbiditic sandstones and shales in the Hörre Nappe, which was thrust northward in the later Carboniferous Variscan Orogeny. This paper documents rich conodont faunas that provide a window into the diversification of conodont animals during the “mid” Tournaisian interval, in deep-water Kulm environments of the Armorican terranes south of the Rhenish Block.

The paper by Korn et al. (2024: 707–734) provides an exemplar of the developing use of C-isotopic excursions to aid in correlation. Using a series of profiles of δ13C from carbonates and organic matter, the authors find striking similarities in patterns of negative and positive excursions in the vicinity of the Devonian–Carboniferous boundary. Such studies are increasingly applied in integrated stratigraphic correlations. Recognition of patterns of isotopic excursions near critical boundaries and their documentation in GSSPs and supplementary sections provides a critical new tool in global correlation that makes location of boundaries accessible to a broader suite of geoscientists beyond biostratigraphic specialists. Thus, such studies are invaluable in moving global stratigraphy forward.

Finally, Hartenfels (2024: 629–682) provides an overview of the several Famennian events, including the Kellwasser (Franian-Famennian boundary) and Hangenberg biocrises, as recorded in the Rhenish Massif and elsewhere. The paper reviews an enormous amount of previous work and summarizes details of the lower and upper Kellwasser, Nehden, Condroz, Annulata, Dasberg, and Hangenberg events (with 64 points of conclusion!). A key summary point of this major review is that the entire 12-million-year interval of the Famennian is characterized by a series of intermediate and large-scale (first-order) bioevents, most of them associated with brief pulses of anoxia and black, organic rich facies. The author concludes that bioevents of different scales show similarities in patterns, suggesting common causal factors that were subdued or amplified to varying degrees depending upon additional factors. This will be an essential reference in ongoing discussions of this critical interval in life history.

In summing up, this two-issue set represents an outstanding compendium of new research primarily on Devonian-to-Mississippian stratigraphy and paleoecology of a classic area. All papers in these two special issues are uniformly excellent in technical quality, well edited, and I particularly applaud the effort to standardize the way in which stratigraphic sections and faunal data are presented, providing excellent illustrations of microfacies and fossils. The editors are to be congratulated on pulling together and vetting so many contributions and wealth of new data, all well documented, and superbly illustrated. Beyond the data on individual taxa and stratigraphic sections, there are critical themes and important syntheses that build toward a broader understanding of the geology and paleontology of a classic region, and the Devonian world as a whole. The authors, in their varied and well produced papers, promote the Rhenish Massif as a natural laboratory for understanding ecosystems, chronostratigraphy, and Earth system processes in the middle Paleozoic Era.

References

Becker, R. T., J. E. A. Marshall, and A.-C. DaSilva. 2020. The Devonian Period. Pp. 733–809 in F. M. Gradstein, J. G. Ogg, M. D. Schmitz, and G. M. Ogg, eds. The Geologic Time Scale 2020, Vol. 2. Elsevier BV, Amsterdam, The Netherlands, 1176 pp.

Becker, R. T., pp. Königshof, and C. E. Brett. 2016. Devonian climate, sea level and evolutionary events: An introduction. Pp. 1–11 inR. T. Becker, pp. Königshof, P., and C. E. Brett, eds. Devonian Climate, Sea Level and Evolutionary Events. Geological Society, London, Special Publications 423.

Ver Straeten, C. A., D. J. Over, and D. Woodrow, eds. 2023. Devonian of New York, v. 1–3. Bulletins of American Paleontology 403–408, 1016 pages.