The paradox of predictability provides a bridge between micro- and macroevolution.

The relationship between the evolutionary dynamics observed in contemporary populations (microevolution) and evolution on timescales of millions of years (macroevolution) has been a topic of considerable debate. Historically, this debate centers on inconsistencies between microevolutionary processes and macroevolutionary patterns. Here, we characterize a striking exception: emerging evidence indicates that standing variation in contemporary populations and macroevolutionary rates of phenotypic divergence is often positively correlated.

Late Ordovician eurypterid preserves oldest euchelicerate musculature in pyrite.

Pyritization of soft tissues of invertebrates is rare in the fossil record. In New York State, it occurs in black shales of the Lorraine Group (Late Ordovician), the best-known example of which is Beecher's Trilobite Bed. Exceptional preservation at the quarry where this bed is exposed allowed detailed examination of trilobite and ostracod soft-tissue anatomy.

Gill function in an early arthropod and the widespread adoption of the countercurrent exchange mechanism.

Rising but fluctuating oxygen levels in the Early Palaeozoic provide an environmental context for the radiation of early metazoans, but little is known about how mechanistically early animals satisfied their oxygen requirements. Here we propose that the countercurrent gaseous exchange, a highly efficient respiratory mechanism, was effective in the gills of the Late Ordovician trilobite . In order to test this, we use computational fluid dynamics to simulate water flow around its gills and show that water velocity decreased distinctly in front of and between the swollen ends, which first encountered the oxygen-charged water, and slowed continuously at the mid-central region, forming a buffer zone with a slight increase of the water volume.

Shared patterns of segment size development in trilobites and vertebrates.

The relative sizes of body segments are a major determinant of the shape and functionality of an animal. Developmental biases affecting this trait can therefore have major evolutionary implications. In vertebrates, a molecular activator/inhibitor mechanism, known as the inhibitory cascade (IC), produces a simple and predictable pattern of linear relative size along successive segments.

Long-term clade-wide shifts in trilobite segment number and allocation during the Palaeozoic.

Arthropods are characterized by having an exoskeleton, paired jointed appendages and segmented body. The number and shape of those segments vary dramatically and unravelling the evolution of segmentation is fundamental to our understanding of arthropod diversification. Because trilobites added segments to the body post-hatching which were expressed and preserved in biomineralized exoskeletal sclerites, their fossil record provides an excellent system for understanding the early evolution of segmentation in arthropods.

Modularity in the trilobite head consistent with the hypothesized segmental origin of the eyes.

The trilobite head served multiple functions and was composed of several fused segments. Yet, the underlying organization of the trilobite head, and whether patterns are conserved across trilobites, remains unclear. Modeling the head as being composed of modules, or subunits that vary and thus have the potential to evolve semi-independently can reveal underlying patterns of organization.

Selectivity and the effect of mass extinctions on disparity and functional ecology.

Selectivity of mass extinctions is thought to play a major role in coupling or decoupling of taxonomic, morphological, and ecological diversity, yet these measures have never been jointly evaluated within a single clade over multiple mass extinctions. We investigate extinction selectivity and changes in taxonomic diversity, morphological disparity, and functional ecology over the ~160-million-year evolutionary history of diplobathrid crinoids (Echinodermata), which spans two mass extinctions. Whereas previous studies documented extinction selectivity for crinoids during background extinction, we find no evidence for selectivity during mass extinctions.

The trilobite upper limb branch is a well-developed gill.

Whether the upper limb branch of Paleozoic "biramous" arthropods, including trilobites, served a respiratory function has been much debated. Here, new imaging of the trilobite shows that dumbbell-shaped filaments in the upper limb branch are morphologically comparable with gill structures in crustaceans that aerate the hemolymph. In , the upper limb's partial articulation to the body via an extended arthrodial membrane is morphologically comparable to the junction of the respiratory book gill of and differentiates it from the typically robust exopod junction in Chelicerata or Crustacea.

Incorporating Hierarchical Characters into Phylogenetic Analysis.

Popular optimality criteria for phylogenetic trees focus on sequences of characters that are applicable to all the taxa. As studies grow in breadth, it can be the case that some characters are applicable for a portion of the taxa and inapplicable for others. Past work has explored the limitations of treating inapplicable characters as missing data, noting that this strategy may favor trees where internal nodes are assigned impossible states, where the arrangement of taxa within subclades is unduly influenced by variation in distant parts of the tree, and/or where taxa that otherwise share most primary characters are grouped distantly.

Air Breathing in an Exceptionally Preserved 340-Million-Year-Old Sea Scorpion.

Arachnids are the second most successful terrestrial animal group after insects [1] and were one of the first arthropod clades to successfully invade land [2]. Fossil evidence for this transition is limited, with the majority of arachnid clades first appearing in the terrestrial fossil record. Furthermore, molecular clock dating has suggested a Cambrian-Ordovician terrestrialization event for arachnids [3], some 60 Ma before their first fossils in the Silurian, although these estimates assume that arachnids evolved from a fully aquatic ancestor.

Disparities in the analysis of morphological disparity.

Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation.

Ignoring stratigraphic age uncertainty leads to erroneous estimates of species divergence times under the fossilized birth-death process.

Fossil information is essential for estimating species divergence times, and can be integrated into Bayesian phylogenetic inference using the fossilized birth-death (FBD) process. An important aspect of palaeontological data is the uncertainty surrounding specimen ages, which can be handled in different ways during inference. The most common approach is to fix fossil ages to a point estimate within the known age interval.

A new family of dissimilarity metrics for discrete character matrices that include inapplicable characters and its importance for disparity studies.

The use of discrete character data for disparity analyses has become more popular, partially due to the recognition that character data describe variation at large taxonomic scales, as well as the increasing availability of both character matrices co-opted from phylogenetic analysis and software tools. As taxonomic scope increases, the need to describe variation leads to some characters that may describe traits not found across all the taxa. In such situations, it is common practice to treat inapplicable characters as missing data when calculating dissimilarity matrices for disparity studies.

The oldest known digestive system consisting of both paired digestive glands and a crop from exceptionally preserved trilobites of the Guanshan Biota (Early Cambrian, China).

The early Cambrian Guanshan biota of eastern Yunnan, China, contains exceptionally preserved animals and algae. Most diverse and abundant are the arthropods, of which there are at least 11 species of trilobites represented by numerous specimens. Many trilobite specimens show soft-body preservation via iron oxide pseudomorphs of pyrite replacement.

Development, Trait Evolution, and the Evolution of Development in Trilobites.

Trilobites offer one of the best fossil records of any arthropod group. This is due to a number of factors, most notably the combination of (1) having inhabited areas where organisms are more likely to be buried and ultimately fossilized; and (2) having had a highly biomineralized exoskeleton more likely to survive the stresses of fossilization. This biomineralized exoskeleton was also morphologically complex, bearing traits that had ecological significance, and was present throughout postembryonic development, from larval to adult stages.

Phylogenetic Paleoecology: Tree-Thinking and Ecology in Deep Time.

The new and emerging field of phylogenetic paleoecology leverages the evolutionary relationships among species to explain temporal and spatial changes in species diversity, abundance, and distribution in deep time. This field is poised for rapid progress as knowledge of the evolutionary relationships among fossil species continues to expand. In particular, this approach will lend new insights to many of the longstanding questions in evolutionary biology, such as: the relationships among character change, ecology, and evolutionary rates; the processes that determine the evolutionary relationships among species within communities and along environmental gradients; and the phylogenetic signal underlying ecological selectivity in background and mass extinctions and in major evolutionary radiations.

Simple versus complex models of trait evolution and stasis as a response to environmental change.

Previous analyses of evolutionary patterns, or modes, in fossil lineages have focused overwhelmingly on three simple models: stasis, random walks, and directional evolution. Here we use likelihood methods to fit an expanded set of evolutionary models to a large compilation of ancestor-descendant series of populations from the fossil record. In addition to the standard three models, we assess more complex models with punctuations and shifts from one evolutionary mode to another.

Dynamic evolutionary change in post-Paleozoic echinoids and the importance of scale when interpreting changes in rates of evolution.

How ecological and morphological diversity accrues over geological time has been much debated by paleobiologists. Evidence from the fossil record suggests that many clades reach maximal diversity early in their evolutionary history, followed by a decline in evolutionary rates as ecological space fills or due to internal constraints. Here, we apply recently developed methods for estimating rates of morphological evolution during the post-Paleozoic history of a major invertebrate clade, the Echinoidea.

Differential niche dynamics among major marine invertebrate clades.

The degree to which organisms retain their environmental preferences is of utmost importance in predicting their fate in a world of rapid climate change. Notably, marine invertebrates frequently show strong affinities for either carbonate or terrigenous clastic environments. This affinity is due to characteristics of the sediments as well as correlated environmental factors.

Extinction space--a method for the quantification and classification of changes in morphospace across extinction boundaries.

Three main modes of extinction are responsible for reductions in morphological disparity: (1) random (caused by a nonselective extinction event); (2) marginal (a symmetric, selective extinction event trimming the margin of morphospace); and (3) lateral (an asymmetric, selective extinction event eliminating one side of the morphospace). These three types of extinction event can be distinguished from one another by comparing changes in three measures of morphospace occupation: (1) the sum of range along the main axes; (2) the sum of variance; and (3) the position of the centroid. Computer simulations of various extinction events demonstrate that the pre-extinction distribution of taxa (random or normal) in the morphospace has little influence on the quantification of disparity changes, whereas the modes of the extinction events play the major role.

Evolutionary mode routinely varies among morphological traits within fossil species lineages.

Recent studies have revitalized interest in methods for detecting evolutionary modes in both fossil sequences and phylogenies. Most of these studies examine single size or shape traits, often implicitly assuming that single phenotypic traits are adequate representations of species-level change. We test the validity of this assumption by tallying the frequency with which traits vary in evolutionary mode within fossil species lineages.

How species longevity, intraspecific morphological variation, and geographic range size are related: a comparison using late Cambrian trilobites.

Phenotypic variation is fundamental to evolutionary change. Variation not only evinces the connectivity of populations but it is also associated with the adaptability and evolvability of taxa. Despite the potential importance of morphological variation in structuring evolutionary patterns, little is known about how relative differences in intraspecific morphological variation and its geographic structure are linked to differences in species longevity.

Mosaic heterochrony and evolutionary modularity: the trilobite genus Zacanthopsis as a case study.

Logical connections exist between evolutionary modularity and heterochrony, two unifying and structuring themes in the expanding field of evolutionary developmental biology. The former sees complex phenotypes as being made up of semi-independent units of evolutionary transformation; the latter requires such a modular organization of phenotypes to occur in a localized or mosaic fashion. This conceptual relationship is illustrated here by analyzing the evolutionary changes in the cranidial ontogeny of two related species of Cambrian trilobites.