Robert A. Gastaldo

Taphonomy and paleobiology

Abstract Taphonomy plays diverse roles in paleobiology. These include assessing sample quality relevant to ecologic, biogeographic, and evolutionary questions, diagnosing the roles of various taphonomic agents, processes and circumstances in generating the sedimentary and fossil records, and reconstructing the dynamics of organic recycling over time as a part of Earth history. Major advances over the past 15 years have occurred in understanding (1) the controls on preservation, especially the ecology and biogeochemistry of soft-tissue preservation, and the dominance of biological versus physical agents in the destruction of remains from all major taxonomic groups (plants, invertebrates, vertebrates); (2) scales of spatial and temporal resolution, particularly the relatively minor role of out-of-habitat transport contrasted with the major effects of time-averaging; (3) quantitative compositional fidelity; that is, the degree to which different types of assemblages reflect the species composition and abundance of source faunas and floras; and (4) large-scale variations through time in preservational regimes (megabiases), caused by the evolution of new bodyplans and behavioral capabilities, and by broad-scale changes in climate, tectonics, and geochemistry of Earth surface systems. Paleobiological questions regarding major trends in biodiversity, major extinctions and recoveries, timing of cladogenesis and rates of evolution, and the role of environmental forcing in evolution all entail issues appropriate for taphonomic analysis, and a wide range of strategies are being developed to minimize the impact of sample incompleteness and bias. These include taphonomically robust metrics of paleontologic patterns, gap analysis, equalizing samples via rarefaction, inferences about preservation probability, isotaphonomic comparisons, taphonomic control taxa, and modeling of artificial fossil assemblages based on modern analogues. All of this work is yielding a more quantitative assessment of both the positive and negative aspects of paleobiological samples. Comparisons and syntheses of patterns across major groups and over a wider range of temporal and spatial scales present a challenging and exciting agenda for taphonomy in the coming decades.

Taphonomic Trends of Macrofloral Assemblages Across the Permian–Triassic Boundary, Karoo Basin, South Africa

Abstract The terrestrial crisis that reportedly parallels the P/Tr marine mass extinction is based mainly on Northern Hemisphere microfloral assemblages and Southern Hemisphere Gondwanan macrofloral collections. It is well established that taphonomic filters control the ultimate collectable fossil assemblage in any depositional regime. Recognition and comparison of isotaphonomic assemblages are critical before conclusions can be drawn about evolutionary trends over time. Such an approach has been taken in the investigation of pre-boundary, trans-boundary, and post-boundary plant-fossil assemblages in the Karoo Basin, South Africa. Fourteen stratigraphic sections were evaluated in the Balfour and Normandien formations (Lower Beaufort Group), Katberg Formation, and overlying Burgersdorp Formation (Upper Beaufort Group). These include previously published (e.g., Bulwer, Bethulie, Carlton Heights, Wapadsberg, Commando Drift) as well as newly discovered (e.g., Clouston Farm) localities, and span the Late Permian to Middle Triassic. Fossiliferous intervals were characterized with respect to their sedimentology and plant taphonomy, and bulk collections were made at several stratigraphic levels for future evaluation of floristic and plant-insect associational trends. The depositional regimes and plant taphonomic character of assemblages change through time. Much of the Lower Beaufort Group is characterized by parautochthonous assemblages within oxbow-lake channel fills. Below the P/Tr boundary, these are replaced by allochthonous assemblages, poorly preserved in lateral-accretion deposits and barforms of relatively shallow fluvial nature. Allochthonous assemblages within the same fluvial context continue across the boundary into the earliest Triassic (Palingkloof Member and Katberg Formation, and typify the Middle Triassic where scour-and-fill structures preserve plant debris. Based on the literature, parautochthonous assemblages reappear in the Upper Triassic Molteno Formation. Hence, the change in taphonomic regime to poorly preserved allochthonous assemblages (dispersed, fragmentary adpressions) at the critical interval on either side of the P/Tr extinction event, but not coincident with, requires extreme caution when interpreting global patterns from these data. Additionally, the presence of plant fossils in the Early Triassic provides evidence for a vegetated landscape during a time when sedimentation patterns are interpreted to be the result of a land-plant die-off.

Regenerative growth in fossil horsetails following burial by alluvium

Evidence for regenerative growth in fossil plants is limited to reports of wound repair caused by plant‐animal interactions. No paleobotanical data, to date, have documented the regenerative capability of ancient plants following trauma, particularly flood‐generated burial. An erect, in situ equisetalean pith‐cast of Late Carboniferous age has been discovered from which it can be demonstrated that fossil horsetails possessed regenerative capabilities following alluviation. The character and mode of regeneration appears to be a homologue to that of modern Equisetum and is suggestive that this feature evolved early in the history of the group.

Origin, characteristics, and provenance of plant macrodetritus in a Holocene crevasse splay, Mobile Delta, Alabama

Lower delta plains are primary sites for the deposition, accumulation, and preservation of macrodetrital plant parts. Chacaloochee Bay, an interdistributary estuary in the Mobile Delta, Alabama, is being infilled actively by crevasse-splay sedimentation. Within this system are a variety of sites in which plant macrodetritus has accumulated and has the potential of being preserved. Aerial plant parts constitute laminar leaf-litter beds within various sediments, in addition to extensive accumulations of relatively pure organics (peats). The leaf-litter beds in the crevasse-channel distributary mouth bar and a detrital peat shoal have been examined with respect to the distribution of accumulated plant parts and their size fractionation. Whole leaves recovered from the sediment-water interface of the crevasse channel provide an insight as to the provenance of the accumulated plant litter. The vast majority of leaves being incorporated into the interdistributary bay sediments have not originated from the surrounding vegetational communities. Rather, most recovered leaves originated from communities existing at least 13 km north of the depositional site. The taphonomic processes responsible for the transport, deposition, and alteration of the accumulated macrodetritus include heterochronous leaf fall, fluctuations in bedload and suspension-load transport, rates of periodic flooding, redistribution by tidal and wind-generated surface currents, biodegradation, and bioturbation.

Implications on the paleoecology of autochthonous lycopods in clastic sedimentary environments of the Early Pennsylvanian of Alabama

Abstract Numerous, correlative localities throughout the Upper Cliff Coal Interval (Lower Pennsylvanian), Blount County, Alabama, have been observed in which lycopod forests are preserved. These assemblages are present in lower delta plain facies interpreted to represent (1) the abandonment phase of a distributary channel system with resultant bayou development, (2) non-peat accumulating swamps capping interdistributary bay deposition, (3) possible terminal phase of peat accumulating swamps and (4) crevasse splay deposits. Field observations in combination with sedimentological, preservational and biological evidence support the conclusion that these autochthonous Carboniferous lycopods represent a taphonomically biased assemblage of aquatic or semi-aquatic (swamp) communities. Interpretations derived from the present study in conjunction with published accounts of erect lycopods suggest that these plants could tolerate all waters from fresh to saline conditions. Periodic drying of swampy habitats may have subjected lycopods to subaerial and aerial conditions, but bulk-replaced erect lycopods predominantly inhabited water stressed conditions. The monotypic character of the lycopod forests supports the hypothesis that in the tropical lowlands of the tropical lowlands of the Carboniferous lycopods held a relatively unique, physiologically stressed position in the delta flood basin. These arborescent lycopods became established in any depositional site subject to flooding and standing water when conditions prevailed which allowed for the establishment and development of individuals.

Community heterogeneity of Early Pennsylvanian peat mires

Reconstructions of Pennsylvanian coal swamps are some of the most common images of late Paleozoic terrestrial ecosystems. All reconstructions to date are based on data from either time-averaged permineralized peats or single-site collections. An erect, in situ Early Pennsylvanian forest preserved above the Blue Creek Coal, Black Warrior Basin, Alabama, was sampled in 17 localities over an area of .0.5 km 2 , resulting in the first temporally and spatially constrained Pennsylvanian mire data set. This three-tiered forest was heterogeneous. Lycopsid and calamitean trees composed the canopy, and lepidodendrids, Lepidophloios, and sigillarians grew together at most sites. More juvenile than mature lycopsid biomass occurs in the forest-floor litter, indicating a mixed-age, multicohort canopy. Pteridophytes (tree fern) and pteridosperms (seed fern) dominated as understory shrubs, whereas sphenophyllaleans, pteridophytes, and pteridosperms composed the ground-cover and liana tier. The proportion of canopy, understory, and ground-cover biomass varied across the forest. Low proportions of ground-cover and liana taxa existed where canopy fossils accounted for .60% of the litter. There is a distinct spatial clustering of sites with more or less understory (or ground cover) where canopy contribution was ,60%. Where canopy biomass was low (,50%), understory shrubs contributed more biomass, indicative of light interception and/or competition strategies. Sphenopteris pottsvillea, a ubiquitous ground-cover plant, is abundant in all sites except one, where pteridosperm creepers and lianas dominate the litter, interpreted to indicate total suppression of other ground-cover growth. Ecological wet-dry gradients identified in other Pennsylvanian swamps do not exist in the Blue Creek mire, with the interpreted wettest (Lepidophloios), driest (Sigillaria), and intermediate (Lepidodendron sensu latu) taxa coexisting in most assemblages.

Criteria to distinguish parautochthonous leaves in tertiary alluvial channel-fills

The plant taphonomic character of fluvial channels in the Upper Oligocene strata of the Weiselster basin, near Borna, Germany, has been studied to determine if concentrated macrofloral-bearing intervals represent allochthonous or parautochthonous accumulations. Single bedding planes or leaf accumulations that represent a single cluster of leaves, the taphonomic signature of leaf fall, were examined representing 1.6 m2 of surface area. Unequivocal autochthonous elements, such as the floating water fern, Salvinia, were excluded from the data set. Each assemblage was evaluated separately. Criteria useful in identifying parautochthonous leaf assemblages include: (1) a heterogenous mixture of plant parts with phytoclast concentration increasing upsection in the fill; (2) a log-normal distribution of leaf sizes in the assemblage; (3) a gaussian or log-normal distribution of leaf size in all dominant taxa; (4) a random directional orientation of leaves or, in the case of taxodiaceous conifers, short shoots on any particular bedding surface; and (5) a biased leaf-surface orientation (one leaf surface predominantly facing upwards). Actualistic data are provided as justification for establishing these criteria.

NO MAJOR STRATIGRAPHIC GAP EXISTS NEAR THE MIDDLE-UPPER PENNSYLVANIAN (DESMOINESIAN-MISSOURIAN) BOUNDARY IN NORTH AMERICA

Abstract Interregional correlation of the marine zones of major cyclothems between North America and eastern Europe does not support assertions that a major stratigraphic gap exists between the traditional regional Desmoinesian and Missourian stages in North America. Such a gap was previously proposed to explain an abrupt change in megafloral assemblages in the northern Appalachian Basin and by extension across all of North America. Conodont-based correlation from the essentially complete low-shelf Midcontinent succession (distal from the highstand shoreline), through the mid-shelf Illinois Basin, to the high shelf of the Appalachian Basin (proximal to highstand shoreline) demonstrates that all major ∼400 kyr cyclothem groupings in the Midcontinent are recognizable in the Illinois Basin. In the Appalachian Basin, however, the grouping at the base of the Missourian is represented only by paleosols and localized coal. The immediately preceding grouping was removed very locally by paleovalley incision, as is evident at the 7–11 Mine, Columbiana County, Ohio, from which the original megafloral data were derived. At the few localities where incised paleodrainage exists, there may be a gap of ∼1000 kyr, but a gap of no more than ∼600 kyr occurs elsewhere in the Appalachian Basin at that level and its magnitude progressively decreases westward into the Illinois (∼300 kyr) and Midcontinent (<200 kyr) Basins. Thus, while a gap is present near the Desmoinesian–Missourian boundary in North America, it is typically more than an order of magnitude smaller than that originally proposed and is similar to the gaps inferred at sequence boundaries between cyclothems at many horizons in the Pennsylvanian of North America.

Are the most durable shelly taxa also the most common in the marine fossil record

This paper tests whether the most common fossil brachiopod, gastropod, and bivalve genera also have intrinsically more durable shells. Commonness was quantified using occurrence frequency of the 450 most frequently occurring genera of these groups in the Paleobiology Database (PBDB). Durability was scored for each taxon on the basis of shell size, thickness, reinforcement (ribs, folds, spines), mineralogy, and microstructural organic content. Contrary to taphonomic expectation, common genera in the PBDB are as likely to be small, thin-shelled, and unreinforced as large, thick-shelled, ribbed, folded, or spiny. In fact, only six of the 30 tests we performed showed a statistically significant relationship between durability and occurrence frequency, and these six tests were equally divided in supporting or contradicting the taphonomic expectation. Thus, for the most commonly occurring genera in these three important groups, taphonomic effects are either neutral with respect to durability or compensated for by other factors (e.g., less durable taxa were more common in the original communities). These results suggest that biological information is retained in the occurrence frequency patterns of our target groups.

A mechanism to explain the preservation of leaf litter lenses in coals derived from raised mires

Abstract Leaves and other non-woody canopy parts are rarely preserved in coals. Although the pH of pore waters within peat swamps is usually relatively low (≤3.5), providing geochemical conditions that would promote preservation after burial, shed canopy parts may remain at the air–soil interface for periods of up to several months prior to burial by additional organic detritus. Leaf half-life in tropical forests has been reported to range from several weeks to months, depending upon species histology. During this time of exposure on the forest floor, catabolic (internal enzymatic), fungal, bacterial and root degradation, as well as saprophagous scavenging, act upon the least resistant parts to promote decay into hemic and sapric macerals. It is unusual, then, to find well-preserved leaves in peats or coals. When such accumulations are encountered, either permineralized in coal balls or duripartically preserved in lignites, the bedded leaves generally are spatially isolated. Several explanations have been proposed to account for such fossil Lagerstatten that require temporal changes in accumulation or degradation rates. Neither of these mechanisms is required to account for such accumulations. Bedded leaves, showing minimal evidence of subaerial exposure and degradation, have been recovered at depth from a vibracore taken 1 km into the interior of a peat swamp in the Rajang River delta, Sarawak, East Malaysia. Evidence is provided to indicate that such accumulations form within peat substrate depressions resulting from the displacement of rootstocks as trees either die and fall over, or are blown down in severe storms. These localized, acidic water-filled pools act as a natural buffer to the degradation of fallen canopy parts that accumulate therein.