The Paleontological Society was founded in 1908, as a section of the Geological Society of America, for the purpose of promoting the science of paleontology. Although disciplinarily inclusive, our founders were an elitist, demographically limited group. Constitutional revisions over the past century trace the evolution of the PS, including increased democratization (with a setback during the McCarthy Era, but accelerating following the turmoil of society as a whole during the 1960s), internationalization, independence from the GSA and our sister organization SEPM, and broadening of our activities and concerns. Comments by Paleontological Society presidents, in their presidential addresses and/or a survey I conducted, reinforce these conclusions. In addition, the presidential addresses demonstrate the shifting concerns and interests of paleontologists over the past century: emphasis on stratigraphic paleontology during the early years of the Society; avoidance of the topic of evolution during the antievolution movement of the 1920s – 1950s; lack of participation in the debates over continental drift; development of paleobiology. Many presidents focused on the identity of paleontology, either praising our potential for contributions to science and society or bemoaning our status and recommending remedies for our situation. Despite some predictions of impending extinction of paleontology and the PS, both our Society and our field remain vigorous as we begin our second century.

Although this paper mentions many specific discoveries and advances it is not intended as a catalog of the “biggest hits” in the sense of public notice, but rather it is an effort to chart how the diversity of paleontological work in the last century fits into the context of the biggest hit of all, the emergence of a “new paleontology” in which conceptual advances have revolutionized every aspect of our profession. When the Paleontological Society was founded no unambiguous fossils were known from the immense stretch of Precambrian time and no hominine fossils were known from Africa. Rigorous phylogenetic analysis and a seat for paleontology at the “high table” of evolutionary biology were in the future. Where once we learned a series of guide fossils and thought we had studied paleontology, now students explore taphonomy, paleoeocology, geobiology and macroevolution in our general courses on paleontology. This paper attempts to take notice of some of the highlights of our evolution from a field focused on cataloging and describing the contents of the fossil record into a complex, multidisciplinary endeavor focused on analytical study of general questions. Some of those hits have been discoveries that document the course of evolution, some have been new conceptual approaches that give us insights that link pattern to process, some are new ways of compiling, analyzing or communicating our knowledge. But with all that the study of the history of life remains at the heart of our profession. The change has been the shift in goal from description to understanding of that history, from “what” to “how.” The greatest hits have been the steps that have opened the way to understanding, that have made following the path possible.

From the beginning of paleontology's existence as a distinct professional community in the early 20th century, paleontologists have argued about ‘where’ the discipline fits among the natural sciences. Long told that paleontologists ought to be content with a subsidiary role as mere documenters of evolutionary change or as stratigraphical ‘handmaidens' to geology, over the past hundred years many paleontologists have actively resisted restrictive pigeonholing and attempted to establish paleontology as an autonomous discipline with status equal to its cousins biology and geology. This essay will survey some of the efforts at paleontological ‘activism’ over the past century, focusing particularly on institutional placement, intellectual contributions, and the use of arguments about the adequacy of the fossil record to bolster claims for disciplinary status.

Finding the beginning of Earth's fossil record is a long-standing palaeontological challenge arising from the quest to understand the origin of life. Research in recent years has necessarily focused on determining the existence (or otherwise) of fossils in the Early Archaean rock record. Nonetheless, despite numerous reports of microfossils(?) and stromatolites, consensus on the existence of life in the Early Archaean has been elusive (e.g. Moorbath, 2005). However, new techniques and approaches are allowing more confident interpretation of the Archaean fossil record, and the nature of the earliest biosignatures can be used to inform our understanding of emergent ecosystems on Earth and perhaps on other terrestrial planets.

Evidence is mounting that microbial ecosystems may have had a firm foothold as early as ~3.5 Ga (Tice and Lowe, 2004; Schopf, 2006; Hofmann et al., 1999; Allwood et al., 2006, 2007b; Westall et al., 2006; Westall and Southam, 2006). Significantly, there is now also evidence that the Early Archaean record may not be as meager and cryptic as previously thought. For example, the 3.43 Ga Strelley Pool Chert of the Pilbara Craton of Western Australia contains kilometer-scale tracts of a fossilized stromatolite (microbial?) reef (Allwood et al., 2006, 2007b) and provides a large suite of evidence that is consistent with life's existence. Moreover, the rapidity with which the Strelley Pool reef established itself on a newly-submerged landmass suggests that life was well established by that time, waiting in the wings in planktonic form until conditions favored sessile biofilm formation. The rich vault of information in such rocks as the Strelley Pool Chert may shed light not only upon life's antiquity, but also on the nature of early organisms and ecosystems, the environments that nurtured them, the processes that aided preservation of biosignatures and the palaeontological approaches needed to interpret them. This in turn will be a valuable guide in the search for—and interpretation of—ancient microbial biosignatures in the geologic record of other planets or moons.

Catalytic hydropyrolysis (HyPy) is a powerful analytical technique for fragmenting macromolecular organic matter, such as kerogen (insoluble sedimentary organic matter), and releasing covalently-bound molecular constituents including branched and cyclic biomarker hydrocarbons. Here we illustrate our molecular approach to paleobiology with lipid biomarker data collected from rock bitumens and kerogens hosted within sedimentary units of the Neoproterozoic Huqf Supergroup, South Oman Salt Basin, Sultanate of Oman. We emphasize that parallel analyses of free and bound biomarker pools affords more confidence that we have correctly identified syngenetic compounds. One enigmatic class of compounds that is prominent in many late Proterozoic and Cambrian sedimentary rocks and oils, including from the Huqf Supergroup, is a series of C14-C30 mid-chain methylalkanes which were originally denoted X-peaks. Despite their abundance in the Precambrian rock record, little is known about the organisms responsible for their biosynthesis. Here we propose a possible origin of X-peak methylalkanes from colorless sulfur bacteria (a very heterogeneous group of chemolithotrophic γ-proteobacteria). In modern marine settings, these bacteria are abundant mat formers wherever a sedimentary sulfide-rich horizon intersects the seafloor producing a steep geochemical redox gradient. This condition may have been met more commonly on shallow marine shelves in late Neoproterozoic basins and these benthic mats may have acted as environmental buffers consuming hydrogen sulfide. If our hypothesis is correct, proliferation of sulfide-oxidizing benthic microbial mats, commencing in the late Cryogenian in South Oman Salt Basin, implies unique and specific benthic conditions during the evolution of the earliest metazoans.

The Ediacaran Period represents a critical transition in Earth history. Major perturbations and innovations occurred in the Ediacaran climate, ocean, and biosphere systems. This paper reviews recent advances in Ediacaran glaciations, oxidation events, and biological evolution. There were one or more glaciations in the Ediacaran Period. Ediacaran successions also record multiple negative δ13Ccarb excursions in addition to the excursion associated with basal Ediacaran cap dolostones. These negative δ13Ccarb excursions possibly represent pulses of ocean oxidation events. The Ediacaran Period is also distinguished by two unique biotas—the Doushantuo-Pertatataka acritarchs and classical Ediacara biota—that characterize, respectively, the early and late part of the period. These two biotas appear to be separated by a glaciation and by a major negative δ13Ccarb excursion, although the exact temporal relationship among the climatic, geochemical, and biotic events is far from resolved. Future research should focus on improving geochronological, paleoenvironmental, and paleontological data from key Ediacaran successions in order to test the apparent and tantalizing couplings between evolutionary and environmental events.

A number of challenges face us paleontologists as we head into the 21st century. None is more difficult than explaining the Cambrian explosion, the dramatic differentiation of most metazoan animal phyla and classes about 545–530 million years ago. Molecular paleobiology, the experimental and theoretical integration of the geologic and the genetic historical records of life, holds promise to help elucidate the causality of the Cambrian explosion, especially as it relates to understanding how so many animal body plans appeared in such a relatively short period of time, and why these body plans were so stable over the subsequent 500 million years. Three discoveries made over the last few years suggest that the answers to these problems might be right around the corner. First, the notion that phenotypic plasticity was higher early, as compared to later, in a clade's history has finally been confirmed and quantified for trilobites. Second, it has been shown that a recently discovered group of genes, microRNAs, regulate the precision of genic output, turning what is a fairly sloppy process (the number of transcripts derived from any genetic locus) into a precise number of protein molecules. And third, microRNAs are continuously being added to metazoan genomes, with their first major influx occurring at the base of the protostomes and deuterostomes, the very animals that so dramatically make their first appearances in the Early Cambrian. I propose that because phenotypic variation decreases through geologic time, that because miRNAs decrease genic variation in output levels, and because the number of miRNAs found within a genome increases through geologic time, miRNAs might be instrumental in the canalization of development, sacrificing phenotypic variation for developmental precision, and ultimately allowing for increases to morphological complexity. Hence, part of the solution to the Cambrian conundrum might be microRNAs, how they control development through ontogenetic time, and how they evolve through geologic time.

Patterns of phenotypic change documented in the fossil record offer the only direct view scientists have of evolutionary transitions arrayed over significant durations of time. What lessons should be drawn from these data, however, has proven to be rather contentious. Although we as paleontologists have made great progress in documenting the geological record of phenotypic evolution with greater thoroughness and sophistication, these successes have been limited by the use of verbal models of how phenotypes change. Descriptive terms such as “gradual” have been understood differently by different authors, and this has led to completely incompatible summary statements about the fossil record of morphological evolution. Here I argue that the solution to this ambiguity lies in insisting that different evolutionary interpretations be represented as explicit, statistical models of evolution. With such an approach, the powerful machinery of likelihood-based inference can be help resolve long-standing paleontological questions.

Here I first review this approach and some aspects of its implementation. Then, I show how this approach leads to new traction on important issues in evolutionary paleobiology, including: understanding modes of evolution and determining their relative importance, separating evolutionary mode from tempo, assessing the evidence for hypotheses of punctuated change, and detecting adaptive evolution in the fossil record.

Plants provide unmatched opportunities to evaluate long debated evolutionary patterns in terms of the detailed biology of the fossil organisms. Leaves serve here as an example of how those advantages can be exploited. Over the history of vascular plants, three important transitions in leaf evolution—the origin of laminate leaves, the progressive loss of seed plant morphological diversity, and the evolution of more angiosperm-like leaves—also represent major shifts in leaf development and physiology. These transitions often occurred in parallel in different lineages, such as the evolution of marginal growth in each of at least four independent origins of laminate leaves during the Devonian and Carboniferous. Each also entailed dramatic reorganizations of leaf hydraulics. For example, the length of the finest distributary vein order varies from up to tens of centimeters down to hundreds of microns in successive groups of dominant seed plants. Angiosperms impose an additional trend upon these patterns with the evolution of their uniquely high vein densities. Vein density strongly influences and can provide a proxy for other physiological characteristics, such as assimilation and transpiration rates. The large increase in transpiration capacity accompanying the evolution of angiosperm leaf traits may even play an important role in feeding precipitation and thereby altering local climate.

How Earth and organisms interacted in the past is one of the largest questions in paleobiology. Observed histories of organisms and Earth environments need to be linked under a set of uniformitarian assumptions to address this question. Functional morphology, which studies how organismal body parts interact with their physical environments, is an important tool in establishing the link. Being uniformitarian or ergodic, functional morphology is most robust when directly incorporating physical (or mechanical) principles into hypotheses and their tests. Such ‘physical functional morphology’ may not be always possible, but the number of examples is slowly increasing. Once a series of robust functional inferences are made, it may be possible to study its correlation or correspondence with the historical record of environmental proxies. This framework was applied to the Mesozoic marine reptiles Ichthyopterygia, which is known for the evolution of fish-shaped body profiles in the derived clade Parvipelvia. A suite of evidence suggests that parvipelvians had advanced cruising ability and dark-adapted vision that were lacking in the more basal forms, which they replaced during the major marine transgression between the latest Anisian (Middle Triassic) and the middle Norian (Late Triassic). The ability to forage in broader expanses of and deeper water may have enabled parvipelvians to survive when shallow water environments became reduced during the major regression phase, but much more study is needed to test such an inference.

Phylogenetic systematics is the dominant form of taxonomy for most biologists, vertebrate paleontologists and to a lesser degree invertebrate paleontologists. Taxonomies are based strictly on evolutionary relationships with traits of organisms such as morphology and sequence data, being used as evidence for relationships. Two types of taxa are recognized - species that may be monophyletic or paraphyletic and clades that must be monophyletic. The phylogeny is an hypothesis of relationships that can be used to illuminate many areas of paleobiology including: unsampled morphology from incompletely preserved organisms, temporal distribution of taxa, and evolutionary patterns and mechanisms. Consequently, an understanding of phylogenetic assumptions, experimental design, and language are critical for the incorporation of phylogenetic taxonomies into larger studies of paleobiology.

Paleontologists rely on information from modern organisms to understand fossils, but fossils can in turn be used to more completely understand the living. This is facilitated when the fossil record is understood from a phylogenetic context. Phylogenetic analyses allow the identification of robust calibration points for molecular dating analyses, and in the absence of phylogeny, “conflicts” between fossils and molecules may arise that are based not on the data, but on methodology or taxonomic philosophy. More importantly, phylogenetic analyses using fossils can overturn evolutionary scenarios based solely on living taxa, and they can direct researchers in more appropriate directions. This is necessary if paleontology is to be fully integrated with both the Earth and life sciences.

Macrostratigraphy is the study and statistical analysis of sediment packages that formed continuously at a specified scale of temporal resolution and that are bound by gaps recognizable at that same scale. The temporal ranges of gap-bound packages, compiled separately for different geographic locations, permit area-weighted, survivorship-based measures of rock quantity and spatio-temporal environmental continuity to be measured. Analytical basin fill models suggest that the parameters controlling sedimentation and sequence stratigraphic architecture, such as base level and sediment supply, can be detected quantitatively by macrostratigraphy.

Macrostratigraphic analysis of the marine sedimentary rock record in the United States at a temporal resolution of ~106 years reproduces most of the well-known Sloss sequences, but it also identifies two prominent megasequences, the Paleozoic and Modern megasequences, which are separated by a Permian-Triassic discontinuity and Phanerozoic minimum in rock quantity. Many short- and long-term features of the macroevolutionary history of marine animals are reproduced by macrostratigraphy, including 1) many patterns in genus richness, 2) patterns in rates of genus extinction and, to a lesser degree, rates of origination, and 3) patterns of extinction selectivity and the shifting relative richness of Sepkoski's Paleozoic and Modern evolutionary faunas. The extent to which macrostratigraphy reproduces the macroevolutionary history of marine animals transcends what is expected by geologically-controlled sampling biases. Instead, the processes which control the spatio-temporal dynamics of shelf sedimentation, including expansions and contractions of shallow epicontinental seas, have probably exerted a consistent influence on the macroevolutionary history of marine animals. Exploring the common cause hypothesis by putting fossils back into rocks and rocks into a new quantitative framework for physical environmental change holds considerable promise for paleobiology.

In all species, geographic range is constrained by a combination of ecological and historical factors. Ecological factors relate to the species' niche, its environmental or biotic limits in multidimensional space, while historical factors pertain to a species' ancestry, specifically the location at which a species evolved. Historical limitations are primary during speciation, while ecological factors control the subsequent expansion and contraction of species range. By assessing biogeographic changes during the lifespan of individual species, we can assess the relationship between paleobiogeography, paleoecology, and macroevolution. Quantitative paleobiogeographic analyses, especially those using GIS-based and phylogenetic methods, provide a framework to rigorously test hypotheses about the relationship between species ranges, biotic turnover, and paleoecology. These new tools provide a way to assess key questions about the co-evolution of life and earth. Changes in biogeographic patterns, reconstructed at the species level, can provide key information for interpreting macroevolutionary dynamics–particularly speciation mode (vicariance vs. dispersal) and speciation rate during key intervals of macroevolutionary change (biodiversity crises, widespread invasion events, and adaptive radiations). Furthermore, species ranges can be reconstructed using ecological niche modeling methods to examine the effects of environmental controls on geographic range shifts. Particularly fruitful areas of investigation in future paleobiogeographic analysis include (1) the relationship between species ranges and speciation events/mode, (2) relationship between shifting ecological regimes and range expansion and contraction, (3) the impact of interbasinal species invasions on both community structure and macroevolutionary dynamics, (4) the mechanics of transitions between endemic to cosmopolitan faunas at local, regional, and global scales, (5) how ecology and geographic range impacts species extinction during both background and crisis intervals.

Three case studies are presented to illustrate both the methods and utility of this theoretical approach of using paleobiogeographic patterns to assess macroevolutionary dynamics. The first case study examines paleobiogeographic patterns in shallow marine invertebrates during the Late Devonian Biodiversity Crisis. During this interval, speciation by vicariance declined precipitously and only species exhibiting expanding geographic ranges survived the crisis interval. Patterns of biogeographic change during the Late Ordovician Richmondian invasion (Cincinnati Arch region) reveal similar patterns; speciation rate declines during invasion intervals and widely distributed endemic species are best able to survive in the new invasive regime. Phylogenetic biogeographic patterns during the Miocene radiation of North American horses suggest climatic parameters were important determinants of speciation and dispersal patterns.

The past century has witnessed a number of significant breakthroughs in the study of extinction in the fossil record, from the discovery of a bolide impact as the probable cause of the end-Cretaceous (K/T) mass extinction to the designation of the “Big 5” mass extinction events. Here, I summarize the major themes that have emerged from the past thirty years of extinction research and highlight a number of promising directions for future research. These directions explore a central theme—the evolutionary consequences of extinction— and focus on three broad research areas: the effects of selectivity, the importance of recovery intervals, and the influence of spatial patterns. Examples of topics explored include the role that trait variation plays in survivorship, the comparative effects of extinctions of varying magnitudes on evolutionary patterns, the re-establishment of macroevolutionary patterns in the aftermath of extinction, and the extent to which spatial autocorrelation affects extinction patterns. These topics can be approached by viewing extinctions as repeated natural experiments in the history of life and developing hypotheses to explicitly test across multiple events. Exploring the effects of extinction also requires an interdisciplinary approach, applying evolutionary, ecological, geochronological, geochemical, tectonic, and paleoclimatic tools to both extinction and recovery intervals.

Symbiotic associations are a poorly studied aspect of the fossil record, owing largely to the taphonomic biases that inhibit direct observation that two organisms shared an intimate association in life. A symbiosis between an infesting animal and a skeleton-producing host can form a bioclaustration cavity that directly preserves the association and has a high preservation potential. Identification of ancient mutuals and parasites must reject the null hypothesis of commensalism by demonstrating that the symbiosis correlates with a positive or negative change in host fitness as compared to a non-symbiotic relative of the host taxon. Reviews of the Paleozoic record of marine symbionts show that the majority are hosted by colonial animals, especially corals and calcareous sponges. These hosts include structural forms that have moderate to high levels of integration and can support bioclaustrations between clonal units, mitigating the negative effects of symbionts, and perhaps facilitating the symbiosis.

The fossil record is biased toward recording long-lasting, widespread, equilibrated associations. By contrast, parasitisms that are especially negative to the host are expected to be fossilized rarely. The symbiotic associations that form bioclaustrations may also represent an endolithic adaptive strategy in response to biological antagonisms, such as predation and spatial competition. The Late Ordovician rise in symbiotic bioclaustrations joins burrows and borings as trace fossil examples of infaunalization strategies that accompany the Ordovician faunal radiation.

Generating and testing hypotheses is an integral part of any science, and some of the most stimulating paleobiological hypotheses of the past few decades relate to the ecological properties of fossils or fossil assemblages. Here, we outline recent methods for framing paleoecological questions that should facilitate the further quantitative evaluation of paleoecological hypotheses. First, we describe theoretical ecospaces, which are frameworks for classifying the ecologic properties of individuals or species based on multiple characters. We discuss the utility of theoretical ecospace in understanding evolutionary constraints and biodiversification, among other topics. Second, we discuss the reconstruction of high-resolution paleoecological gradients using ecological ordination techniques. Ordination can help uncover the paleoenvironmental factors that controlled fossil assemblage composition, track these factors through time, and evaluate the environmental and ecological context of major biotic changes. As an example, we present a new gradient analysis of the Yorktown Formation (Pliocene) of Virginia in which substrate and disturbance controlled molluscan assemblage composition. As a further example, we ordinate samples of mid-Paleozoic and late Cenozoic marine fossil assemblages based on their ecological content (as determined using a theoretical ecospace) to test whether the same environmental and ecological factors controlled the distribution of ecological lifestyles in both time intervals, despite the many differences between them. Although depth-related variation is evident in both data sets, the Cenozoic samples show stronger evidence of environmental control on ecologic content within depth zones. In contrast, Paleozoic gradients are consistent with a more random component in assemblage content. These analyses are quite preliminary, however, and should be verified with more extensive data.

The great bulk of the angiosperm fossil record consists of isolated fossil leaves that preserve abundant shape and venation (leaf architectural) information but are difficult to identify because they are not attached to other plant organs. Thus, poor taxonomic knowledge has tempered the tremendous potential of fossil leaves for constructing finely resolved records of biodiversity through time, extinction and recovery, past climate change and biotic response, paleoecology, and plant-animal associations. Moreover, paleoecological and paleoclimatic interpretations of fossil leaves are in great need of new approaches. Recent work is rapidly increasing the scientific value of fossil angiosperm leaves through advances in traditional paleobotanical reconstruction, phylogenetic understanding of both leaf architecture and the response of leaf shape to climate, quantitative plant ecology using measurable, correlatable leaf traits, and improved understanding of insect leaf-feeding damage. These emerging areas offer many novel opportunities to link paleoecology and neoecology. Increased collaboration across traditionally separate research areas is critical to continued success.

An outstanding challenge with broad implications for an ecologically sustainable future is to understand how living systems—whether natural or social—balance opportunity and constraint in a given environment. In this paper, I compare the proposed mechanics of a heuristic developed to explain transformational change in systems ecology with various paleontological patterns and hypotheses for its conceptual homology and thus explanatory power in causal terms. The adaptive cycle heuristic, which has potential to influence current environmental and natural resources law and policy, has two components: 1) cycles that alternate between long periods of growth and shorter periods that create opportunities for innovation (new structures or conditions that become economically successful), and 2) the interaction of nested sets of such cycles (panarchies) across space and time scales. I critically evaluate three basic underlying tenets of the adaptive cycle related to the circumstances of innovation—empty niche space, competition and availability of resources—because of their importance to the development of a theoretical framework for understanding the ecological dimension of opportunity in biological evolution. I conclude that not all of the proposed mechanics and observed phenomenology of the adaptive cycle are appropriate in biological evolution. I draw insight, however, from the hierarchical nature of the heuristic to outline a “panarchical” conceptualization of the escalation hypothesis; I identify self-organization, emergence, selection and adaptation, and feedback as phenomena that are held in common across systems and scales, which influence how entities in the economic hierarchy of life arise, interact and evolve.

Two roads diverged in a wood, and I, I took the one less traveled by, and that has made all the difference. Robert Frost.

Every system either finds a way to develop or else collapses. Aleksander Solzhenitsyn