Carlton E. Brett

The mid-Paleozoic precursor to the Mesozoic marine revolution

The mid-Paleozoic was punctuated by a rapid radiation of durophagous (shell-crushing) pred- ators. These new predators were primarily placoderm and chondrichthyan fishes but probably also included phyllocarid and eumalacostracan arthropods. Coincident with the radiation of these durophages, beginning in the mid-Devonian, there was an increase in the frequency of predation-resistant morphologies in a variety of marine invertebrate taxa. Among bellerophontid molluscs, disjunct coiling disappeared and umbilici became less common while the frequency of genera with sculpture increased. The abundance of brachiopod genera with spines on one or both valves increased dramatically. Sculpture became more pronounced and common among genera of coiled nautiloids. Inadunate and camerate crinoids showed a marked increase in spinosity, and all three crinoid subclasses tended to develop thicker thecal plates. Trends toward increasing relative frequencies of predation-resistant features were formed in different ways. Bellerophontid genera lacking predation-resistant features tended to go extinct, leaving the sculp- tured, tightly coiled forms as the predominant forms. Among Brachiopoda, the radiation of productids provided the tremendous increase in numbers of spinose genera. Among crinoids, predation-resistant features were acquired through evolution within established clades. These observations suggest that predation by shell-crushing predators has been an important control on the morphology and composition of the marine invertebrate fauna since at least the Middle Devonian. The mid-Paleozoic radiation of durophages and response of the marine fauna was in many respects similar to events of the Mesozoic Marine Revolution, in effect, the Paleozoic precursor to that event.

Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water‐column mixing, and efficient biolimiting nutrient recycling

Integrated geochemical data suggest that black shale deposition in the Devonian Geneseo Formation of western New York was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing. Once established, however, black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity. Recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced N and P regeneration from sediments, sustained high primary productivity by returning nutrients to the photic zone during mixing, and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification. This feedback was ultimately disrupted by rising siliciclastic influx, which diluted organic matter and restored benthic redox stability. The abrupt overturn of diverse, long-standing Appalachian basin marine communities may have been the result of trophic resource destabilization during Geneseo deposition.

Coordinated stasis: An overview

Abstract Coordinated stasis, as defined herein, represents an empirical pattern, common in the fossil record, wherein groups of coexisting species lineages display concurrent stability over extended intervals of geologic time separated by episodes of relatively abrupt change. In marine benthic fossil assemblages, where the pattern was first recognized, the majority of species lineages (60 to more than 80%) are present in their respective biofacies throughout timespans of 3–7 million years. Most lineages display morphological stasis or only very minor, typically non-directional, anagenetic change in a few characters throughout a prolonged time interval; evidence for successful speciation (cladogenesis) is rare, few lineages ( Causes of coordinated stasis and of regional ecological crisis/reorganization remain poorly understood. Tracking of spatially shifting environments appears to be the rule, rather than adaptation to local change. Incumbent species appear to have a very strong advantage and may excluded potential immigrants, as evidenced by temporary incursions of exotic taxa (“incursion epiboles”); this suggests a role for ecological and biogeographic factors in maintaining paleoecological stability. Stabilizing selection may be critical for producing morphological stability in individual lineages. Episodic crises appear to involve environmental perturbations that were too pervasive and/or abrupt to permit local tracking of environment to continue. Some faunal turnovers associated with unconformities may be partially an artifact of stratigraphic incompleteness. Others, however, seem to occur within conformable successions and were evidently rapid. Widespread anoxia, changes in current patterns, and/or climatic change associated with major marine transgression are common correlates of faunal turnovers in marine habitats in the Appalachian Basin. The phenomenon of coordinated stasis has been noted, albeit not fully documented, in a number of ancient marine and terrestrial ecosystems. An important goal for evolutionary paleoecology should be to document the patterns of stability and change in common and rare members of fossil assemblages in order to discern the relative frequency of coordinated stasis in the rock record, to evaluate the mechanisms by which such apparent evolutionary and ecological stability might be produced, and to seek clues (e.g., paleobiological and stratigraphic patterns, geochemical anomalies) as to causes of abrupt pulses of faunal change.

Trilobite taphonomy and Middle Devonian taphofacies

Detailed study of Middle Devonian trilobites from New York State indicates that taphonomic attributes are distributed nonrandomly among the strata and therefore may be used to characterize different taphofacies (i. e., facies defined on the basis of diagnostic taphonomic traits). Environments near normal wave base are represented by bioclastic deposits in which sclerites are disarticulated and fragmented, and display hydrodynamically stable postures (Taphofacies 1). Slightly deeper water environments, characterized by high sedimentation rates and pervasive deep bioturbation, generated assemblages of dominantly disarticulated remains with isolated occurrences of articulated trilobite remains (Taphofacies 2). Shelly concentrations form thin but prominent layers that alternate with barren mudstone interbeds (Taphofacies 3). Facies 3A is characterized by scavenger disarticulation and an abundance of articulated remains (sediment starvation). Facies 3B displays disarticulated remains in hydrodynamically stable postures and a general absence of articulated remains (sediment bypass). The most distal (deep-water) environments are represented by assemblages dominated by articulated trilobite remains (Taphofacies 4). Differences in the taphonomy of articulated remains may be related to inferred sedimentation rates as follows: Taphofacies 4A assemblages are characterized by a predominance of moult remains (commonly as intact ensembles) and display evidence for low rates of sedimentation. Taphofacies 4B assemblages indicate moderate rates of background sedimentation and yield abundant enrolled trilobites, typically associated with syngenetic pyrite. Taphofacies 4C reflects high rates of sedimentation; articulated remains are relatively common, but skeletal concentration is minimal. Taphonomic features provide criteria by which relative rates of sedimentation and bathymety may be inferred. The taphofacies concept, therefore, constitutes a useful tool in the recognition and understanding of environmental processes and products.

Sequence stratigraphy, paleoecology, and evolution; biotic clues and responses to sea-level fluctuations

Paleoecology has a dual relationship with sequence stratigraphy. On one hand, body and trace fossils, together with their taphonomy, may provide sensitive indicators of environmental parameters, including depth, substrate consistency, sedimentation rate/turbidity, and benthic oxygenation, which are critical in recognizing and interpreting parasequences and sequences. Fossils may provide some of the best guides to identifying key surfaces and inferring sedimentation dynamics within sequences. Conversely, the sequence stratigraphic paradigm and its corollaries provide a predictive framework within which to examine biotic changes and interpret their probable causes. Such changes include ecological epiboles (short-term, widespread proliferation of normally rare species), outages (absence of normally common species), ecophenotypic changes, and long-term (tens to hundreds of Ka) community replacement. Community replacement should be carefully distinguished from short-term (10 to a few hundred years) ecological succession, rarely resolvable at the scale of single beds, although replacement series through shallowing-to-deepening cycles may display some features that parallel true succession. Replacement in marine communities may be relatively chaotic, but, more commonly in offshore settings, it appears to involve lateral, facies-related shifting of broad biofacies belts, or habitat tracking. Tracking patterns may be nearly symmetrical in areas of low sediment input. However, replacement cycles are commonly asymmetrical. The asymmetries involve both apparent and real effects; deletion of portions of facies transitions at sequence boundaries or condensed sections leads to artifactual asymmetries. Alternatively, in areas proximal to siliciclastic sources, tracking asymmetries arise from the markedly higher sedimentation rates during regressive (late highstand) than transgressive phases. Replacements may also involve immigration of species into a sedimentary basin, either as short-lived events (incursion epiboles) or as wholesale faunal immigrations. The latter will typically follow intervals of extinction/emigration of the indigenous faunas. Both large and small immigration events appear most commonly during highstands (transgressive peaks), which may be associated with altered water-mass properties, and may open migration pathways for nekton and planktonic larvae. At least in isolated basins, allopatric speciation may also occur during fragmentation of habitats associated with regressions. Finally, there are predicted and empirical correlations between sequence-producing sea-level fluctuations and macroevolution. Major extinctions may be associated with habitat reduction during major regressions (lowstands), or with anoxic events during major transgressions. Generally, rising sea level may be correlated with evolutionary radiations. Hence, some ecological-evolutionary unit boundaries may correlate either with sequence boundaries or maximum flooding surfaces. However, in other cases, no correlation has been found between macroevolutionary patterns and sequence stratigraphy. The situation is obviously complex, but sequence stratigraphy at least provides a heuristic framework for developing and testing models of macroevolutionary process.

Taphofacies models for epeiric sea environments: Middle Paleozoic examples

Abstract Skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur. Aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and, therefore, are useful in discriminating taphonomic facies. For purposes of taphonomic comparisons, skeletonized marine taxa are reduced to five generally defined types: massive, arborescent, univalved bivalved, and multielement. Different skeletal types display different attributes which reflect specific responses to physical, chemical, and/or biological processes. The taphonomic properties of disarticulation, reorientation and sorting, fragmentation, corrosion and abrasion, skeletal dissolution, and early diagenesis display patterns which are predictable with respect to a variety of inter-related physico-chemical parameters. These can be quantified through indices which evaluate relative frequencies of occurrence. Sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event-related properties, thereby generating composite taphofacies. Such events produce taphonomic signature which in themselves correspond to specific environmental conditions; i.e., water depth and event intensity. Indeed, background and event-related processes interact to generate complex, but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments. The comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties. Deductively-derived taphofacies models, corroborated by empirical data from Paleozoic strata, illustrate the distribution of taphonomic properties with respect to environmental energy (turbulence), background sedimentation rate, and sediment oxygenation. These parameters also indirectly influence other, less obvious factors including bioturbation and sediment chemistry. Trends in the distribution of taphonomic properties (taphonomic gradients) permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore-offshore transect within a generalized epeiric sea setting. Taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena. Moreover, taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies.

Sequences, cycles, and basin dynamics in the Silurian of the Appalachian Foreland Basin

Abstract Field stratigraphic analysis of the Early and medial Silurian strata of Ontario, New York and Pennsylvania permits recognition of six major sequences, reflecting 3rd-order cycles, in the Niagaran (Llandoverian-Ludlovian) of New York and adjacent regions. These sequences correspond approximately to the Medina, lower Clinton, middle Clinton, upper Clinton (two sequences) and Lockport Groups. Each is bounded by widespread unconformities produced by the interplay of eustatic sea-level drop and local tectonic uplift. In general, erosional sequence boundaries and transgressive surfaces are merged, with no record of the lowstand wedge. Each sequence is subdivisible into two to five sub-sequences that are marked by sharp, though generally non-erosional basal discontinuities. Investigation of the Clinton and Lockport Groups corroborates earlier suggestions that carbonate and siliciclassic tongues extended basinward from opposite margins synchronously in response to sea-level drops. Analysis of the thickness and basinward extent of the progradational tongues reveals the existence of multi-scale sedimentary cycles. Sequences and component sub-sequences are correlative basinwide and are synchronous within the resolution of bio- and event-stratigraphy. Major drops in relative sea-level are represented by the sequence-bounding discontinutiies. These surfaces are overlain by siliciclastic wedges near terrigenous source areas, or winnowed carbonate pack- and grainstones on the tectonically passive margin. Overlying highstand deposits, typically shales, calcareous mudstones, and limestones, are separated from lowstand or transgressive deposits by thin condensed intervals commonly marked by phosphatic or glauconitic material and/or surfaces of maximum sediment starvation. The relative highstand deposits of sequences and sub-sequences are developed in relatively deep-water facies and are, at most, slightly progradational. Hence, most sequences and sub-sequences are sharply bounded, roughly upward deepening (to slightly shallowing) successions of strata. In turn, sub-sequences are further divisible into widespread minor parasequences and parasequence sets, probably corresponding to 6th- and 5th-order cycles, respectively. The smaller scale units (0.2–5 mthick) commonly display an upward shallowing motif. The nested relationship of these cycles supports the hierarchical model of transgressive-regressive allocycles proposed by Busch and Rollins (1984). Niagaran cycles discussed herein are equivalent to third-, fourth- and fifth-order cycles and are traceable circumbasinally. Depocenters of successive sequences and sub-sequences display a pattern of eastward-westward-eastward basin-axis migration through the Silurian. During early Llandoverian, the Taconic (Queenston) deltaic complex began to subside, and the basin axis, depocenter, and eastern shoreline shifted eastward approximately 200 km over a 6 million year period. As the basin axis migrated eastward, the western basin ramp (forebulge) was repeatedly upwarded and eroded, resulting in the generation of regionally angular unconformities at the bases of sequences II, IV and VI. The pattern of unconformity (greatest to the west) suggests rise of the Algonquin Arch concomitant with eastward basin-axis migration. During the late Llandoverian to early Wenlockian, the Algonquin Arch subsided slightly, concurrently with a reversal in direction of basin-axis migration. During deposition of upper Clinton and Lockport-Vernon strata (sequences V, VI), the basin axis shifted approximately 300 km back to the west. Eastward migration of the basin axis corresponds with a time of tectonic quiescence and probable thrust-load relaxation. The abrupt reversal of migration in the late Llandoverian may be a signal of renewed thrusting in the hinterland at the onset of the Salinic Disturbance.

Preservation and paleoecology of a Middle Ordovician hardground community

-Limestone beds in the Middle Ordovician (Trentonian) Bobcaygeon Formation, exposed near Kirkfield, Ontario, exhibit irregular, bored and encrusted surfaces indicative of early lithification. These hardgrounds were formed in extensively burrowed carbonate sediments and their hummocky surface topographies were inherited, in part, from a pre-existing pattern of burrow tunnels. A diverse community, including bryozoans, brachiopods, crinoids, edrioasteroids, and paracrinoids, colonized these hard substrates. In addition, most surfaces are riddled with small, cylindrical boreholes (Trypanites) which represent dwellings of softbodied organisms. Some hardgrouind surfaces were inhabited by multiple generations of organisms. Remains of the older generations of encrusters were strongly abraded and nearly obliterated. Superimposed upon these worn remnants are well-preserved remains of rather fragile organisms, e.g. complete hybocystitid crinoids and edrioasteroids. Evidently, certain hardground surfaces were rapidly buried by muds, resulting in the in situ preservation of the last generation of attached organisms. Slightly differing subcommunities of organisms inhabited various microhabitats provided by the irregular hardground surfaces. Thus, the relative abundance of bryozoans and echinoderms encrusting the roofs of small crevices differs from that on the exposed upper surfaces of the hardgrounds. This is the geologically oldest known hardground community in which microhabitat subdivision can be recognized. However, polarity between the subcommunities is not as pronounced as in geologically younger hard substrate faunas. Carlton E. Brett. Museum of Paleontology, University of Michigan, Ann Arbor, Michigan 48109 W. David Liddell. Museum of Paleontology, Univer.sity of Michigan, Ann Arbor, Michigan 48109 Accepted: March 18, 1978

Morphology, faunas and genesis of Ordovician hardgrounds from Southern Ontario, Canada

Abstract We have used associations of different microfacies to define facies (or microfacies associations) which form reasonably well-defined sequences, which we infer, from analogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scale topographic differentiation into shoal, slope and basinal environments. Shoal environments are characterized by typically cross-bedded, well-sorted bioclastic sands, with intershoal areas consisting of interbedded bioclastic sands and heavily bioturbated finer-grained carbonates. Slope and “basinal” environments are typically represented by “proximal” and “distal” cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water clastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed to migration of channels on the fans and the effect of funnelling of storm surges down the channels. Although hardground morphology and faunas are mostly related to local effects such as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renewed sedimentation. Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biotas. Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flow fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assemblages formed after redeposition.

Predation in the Paleozoic: Gastropod-Like Drillholes in Devonian Brachiopods

Middle Devonian articulate brachiopods (Ludlowville and Moscow Formations, Hamilton Group, New York) have external tapered holes with a central boss that are indistinguishable from drillholes of naticid gastropods that are known from the Triassic and later. Drillholes are specific to prey (ribbed shells were avoided) and specific to sites on prey. Healed drillholes suggest penetration of live prey. As many as 44 percent of the preferred prey are drilled, indicating a level of predation that has been reported only from post-Paleozoic strata.