Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact email@example.com with any questions.
Sponges [Porifera] are the phylogenetically oldest metazoan phylum still extant today; they share the closest relationship with the hypothetical common metazoan ancestor, the Urmetazoa. During the past 8 years cDNAs coding for proteins involved in cell-cell- and cell-tissue interaction have been cloned from sponges, primarily from Suberites domuncula and Geodia cydonium and their functions have been studied in vivo as well as in vitro. Also, characteristic elements of the extracellular matrix have been identified and cloned. Those data confirmed that all metazoan phyla originate from one ancestor, the Urmetazoa. The existence of cell adhesion molecules allowed the emergence of a colonial organism. However, for the next higher stage in evolution, individuation, two further innovations had to be formed: the immune- and the apoptotic system. Major defense pathways/molecules to prevent adverse effects against microbes/parasites have been identified in sponges. Furthermore, key molecules of the apoptotic pathway(s), e.g., the pro-apoptotic molecule comprising two death domains, the executing enzyme caspases, as well as the anti-apoptotic/cell survival proteins belonging to the Bcl-2 family have been identified and cloned from sponges. Based on these results—primarily obtained through a molecular biological approach—it is concluded that cell-cell- and cell-matrix adhesion systems were required for the transition to a colonial stage of organization, while the development of an immune system as well as of apoptotic processes were prerequisites for reaching the integrated stage. As the latter stage already exists in sponges, it is therefore likely that the hypothetical ancestor, the Urmetazoa, was also an “integrated colony.”
We consider three issues that appear to be important in the interpretation of developmental genetics in an evolutionary context. The three issues under discussion are 1) evolutionary loss as applied to evo-devo data; 2) the limits on our ability to infer ancestry based on tree reconstruction; and 3) “type 2” errors in the assessment of homology of developmental gene expression data. Lack of consideration of any or all of these disparate issues narrows the set of hypotheses under consideration. We examine these issues through examples drawing on new data on POU domain genes as well as through reference to published work on Distal-less, engrailed and Nk2 genes.
Hexactinellid sponges are metazoans in which the major tissue component is a multinucleated syncytium. The preferred deepwater habitat of these sponges makes collection of hexactinellids in good condition difficult, and has hindered extensive examination of their body plan. Nonetheless, over the last three decades a number of studies have explored their ecology, histology and physiology. It has been shown that hexactinellids are extremely long-lived animals. Their cytoplasm consists of a giant, multinucleated tissue, the trabecular syncytium, which is connected via open and plugged cytoplasmic bridges to cells such as archaeocytes, choanoblasts, and cells with spherical inclusions. Because all of the sponge is cytoplasmically interconnected, electrical signals can propagate through the animal. The effector response is arrest of the feeding current. The perforate plugged junction apparently allows tissues to specialize in different ways while maintaining limited cytoplasmic continuity. Larvae of hexactinellid sponges are already largely syncytial. Although it is not known when the first syncytial tissues are formed or when perforate plugged junctions first appear during embryogenesis, evidence that embryos are cellular until gastrulation suggests that hexactinellid sponges may have evolved from cellular sponges and that syncytial tissues are not a primitive trait of the Metazoa.
Our inability to answer many questions regarding the development of metazoan complexity may be due in part to the prevailing idea that most eukaryote “phyla” originated within a short period of geologic time from simple unicellular ancestors. This view, however, is contradicted by evidence that larger groups of eukaryotes share characters, suggesting that these assemblages inherited characters from a common ancestor. Because molecular analyses have had limited success in resolving the relationships of higher eukaryote taxa, we have undertaken a phylogenetic analysis based primarily on morphological characters. The analysis emphasizes characters considered to have a high probability of having evolved only once. Transitions between taxa are evaluated for the likelihood of character-state transformations. The analysis indicates that the evolutionary history of the clade containing the Metazoa has been complex, encompassing the gain and loss of a secondary and perhaps a primary photosynthetic endosymbiont with accompanying changes in trophic level. The history also appears to have included a hetero-autotrophic ancestor that possessed a “conoid” feeding apparatus and may have involved a transformation from a flagellate to an amoeboid body form, a trend toward increased intracellular compartmentation, and the development of complex social behavior. Such changes could have been critical for establishing the underlying complexity required for a rapid diversification of cell and tissue types in the early stages of metazoan evolution.
Muscle tissue may have played a central role in the early evolution of mesoderm. The first function of myocytes could have been to control swimming and gliding motion in ciliated vermiform organisms, as it still is in such present-day basal Bilateria as the Nemertodermatida. The only mesodermal cells between epidermis and gastrodermis in Nemertodermatida are myocytes, and conceivably the myocyte was, in fact, the original mesodermal cell type. In Nemertodermatida as well as the Acoela, myocytes are subepithelial fiber-type muscle cells and appear to originate from the gastrodermal epithelium by emigration of single cells. Other mesodermal cells in the acoels are the peripheral parenchyma (connective tissue) and tunica cells of the gonads, and these also arise from the gastrodermis. Musculature in many of the coelomate protostomes and deuterostomes, on the other hand, is in the form of epitheliomuscular (myoepithelial) cells, and this cell type may also have been an early form of the mesodermal myocyte. The mesodermal bands in the small annelid Polygordius and in juvenile enteropneusts have cells intermediate between mesenchymal and epithelial in their histological organization as they develop into myoepithelia. If acoelomates were derived from coelomates by progenesis, then the fiber-type muscles of acoelomates could be products of foreshortened differentiation of such tissue. The precise serial patterning of circular muscle cells along the anterior-posterior axis during embryonic development in the acoel Convoluta pulchra provides a model for early steps in the gradual evolution of segmentation from iterated organ systems.
In simplest terms, the complexity of the metazoan body arises through various combinations of but two tissue types: epithelium and mesenchyme. Through mutual inductions and interactions, these tissues produce all of the organs of the body. Of the two, epithelium must be considered the default type in the Eumetazoa because it arises first in embryonic development and because mesenchyme arises from it by a switching off of the mechanisms that underly differentiation and maintenance of epithelial cells. In the few model metazoans whose epithelia have been studied by molecular techniques (largely Drosophila, Caenorhabditis, mouse), the molecular mechanisms underlying differentiation of epithelia show remarkable similarity. Extrapolating from these studies and from comparisons of the morphology of epithelia in lower metazoans, I propose how epithelia arose in the stem metazoan. Steps in epithelial differentiation include 1) establishment of cell polarity by molecular markers confined to either apical or basolateral domains in the plasma membrane; 2) aggregation of cells into sheets by localization of cell-adhesion molecules like cadherin to the lateral membrane; 3) formation of a zonula adherens junction from the cadherins by their localization to a discrete belt; 4) cell-to-cell linking of certain transmembrane proteins (primitively in the septate junction) to produce gates that physiologically isolate compartments delimited by the cells; and 5) synthesis of a basal lamina and adaptation of receptors (integrins) to its components. Despite morphological differences in the variety of cell junctions evident in various epithelia, the underlying molecular markers of these junctions are probably universally present in all eumetazoan epithelia.
The basic problem in an evolutionary transition is to understand how a group of individuals becomes a new kind of individual, possessing the property of heritable variation in fitness at the new level of organization. During an evolutionary transition, for example, from single cells to multicellular organisms, the new higher-level evolutionary unit (multicellular organism) gains its emergent properties by virtue of the interactions among lower-level units (cells). We see the formation of cooperative interactions among lower-level units as a necessary step in evolutionary transitions; only cooperation transfers fitness from lower levels (costs to group members) to higher levels (benefits to the group). As cooperation creates new levels of fitness, it creates the opportunity for conflict between levels as deleterious mutants arise and spread within the group. Fundamental to the emergence of a new higher-level unit is the mediation of conflict among lower-level units in favor of the higher-level unit. The acquisition of heritable variation in fitness at the new level, via conflict mediation, requires the reorganization of the basic components of fitness (survival and reproduction) and life-properties (such as immortality and totipotency) as well as the co-option of lower-level processes for new functions at the higher level. The way in which the conflicts associated with the transition in individuality have been mediated, and fitness and general life-traits have been re-organized, can influence the potential for further evolution (i.e., evolvability) of the newly emerged evolutionary individual. We use the volvocalean green algal group as a model-system to understand evolutionary transitions in individuality and to apply and test the theoretical principles presented above. Lastly, we discuss how the different notions of individuality stem from the basic properties of fitness in a multilevel selection context.
The maximum degree of hierarchical structure of organisms has risen over the history of life, notably in three transitions: the origin of the eukaryotic cell from symbiotic associations of prokaryotes; the emergence of the first multicellular individuals from clones of eukaryotic cells; and the origin of the first individuated colonies from associations of multicellular organisms. The trend is obvious in the fossil record, but documenting it using a high-resolution hierarchy scale reveals three puzzles: 1) the rate of origin of new levels accelerates, at least until the early Phanerozoic; 2) after that, the trend may slow or even stop; and 3) levels may sometimes arise out of order. The three puzzles and their implications are discussed; a possible explanation is offered for the first.
Colonial hydrozoans represent some of the most diverse and complex body plans within the Metazoa. Complex hydrozoans colonies are more physiologically and structurally integrated than their simple colonial relatives. Colonial integration is commonly associated with the regulation of the general structural plan of the colony, the division of labor, and the physiological integration of the colony. In the hydrozoan Hydractinia, these features are manifested through evolutionary innovations involving the spatial regulation of polyps within the colony, the development of polyp polymorphs, and the acquisition of a stolonal mat. These innovations all involve evolutionary changes in the regulation of polyp and colony-wide patterning systems. In Hydractinia, the ParaHox gene, Cnox-2, is expressed in a spatially restricted manner along the axes of stolons and polyps, suggesting that changes in the regulation of this gene may be in part responsible for the evolutionary innovations important for colonial complexity.
Growth in colonial organisms by iteration of modules inherently provides for an increase in available morpho-ecospace relative to their solitary relatives. Therefore, the interpretation of the functional or evolutionary significance of complexity within groups that exhibit modular growth may need to be considered under criteria modified from those used to interpret complexity in solitary organisms. Primary modules, corresponding to individuals, are the fundamental building blocks of a colonial organism. Groups of primary modules commonly form a second-order modular unit, such as a branch, which may then be iterated to form a more complex colony. Aspects of overall colony form, along with their implications for ecology and evolution, are reflected in second-order modular (structural) units to a far greater degree than by primary modular units (zooids). A colony generated by modular growth can be classified by identifying its second-order modular (structural) unit and then by characterizing the nature and relationships of these iterated units within the colony. This approach to classifying modular growth habits provides a standardized terminology and allows for direct comparison of a suite of functionally analogous character states among taxa with specific parameters of their ecology.
Three features contribute to the complexity of an entity: number of parts, their order, and their iteration. Many functional biological entities are complex when measured by those attributes, and although they are produced in tree-like architectures, the organizational structures that permit them to function are in the form of hierarchies. Natural hierarchies can be thought of as organizing structures that are emergent properties of complex functional entities, and which are transformed from trees by process networks. For example, hierarchies are observed in the architecture of metazoan bodies (the somatic hierarchy) and in the biotic structure of ecogeographic units (the ecological hierarchy). As the metazoan developmental genome is quite complex and has been evolved through tree-like processes, it must harbor at least one hierarchy, which is most clearly indicated in the developmental processes that create the somatic hierarchy. For multicellular organisms, the processes that serve to transform trees of gene expression events into a somatic hierarchy have produced complicated signaling networks whose histories can probably be recovered in general outline.
The “Ediacaran organisms,” which preceded and overlapped the Cambrian radiation of metazoans, include many fossils whose systematic positions remain contentious after over fifty years of study. It might seem that nothing particularly useful can be learned from a biota full of oddballs. However, analyses of the distribution of the Ediacaran organisms in time and space can be carried out without having to guess at the systematic position of the organisms. Combining these results with data on paleotectonics, paleoenvironmental parameters, and the ages of various assemblages sheds light on the origins, ecology, and even the systematic positions of the Ediacaran organisms. Parsimony Analysis of Endemism (PAE) confirms earlier studies in grouping Ediacaran biotas into three major clusters: the Avalon, White Sea, and Nama Assemblages. The available radiometric and stratigraphic data suggest that the Avalon is the oldest, the White Sea is next oldest, and the Nama extends to the base of the Cambrian. The “frondlike” Ediacaran taxa, and to a lesser extent the “medusoids,” collectively show significantly longer stratigraphic ranges, broader geographical and paleoenvironmental ranges, and less provinciality than “bilaterian” and tubular taxa. Almost all tubular Ediacarans appear to be confined to equatorial areas, whereas other Ediacaran organisms show weak or no latitudinal diversity gradients. I conclude that the Ediacaran organisms show a diverse range of responses to various environmental parameters. There is no basis for classifying them all as having a single body plan and mode of life, as has often been done in the past.
Various modes of preservation of Ediacaran fossils in different sediments, quartz sand at Zimnie Gory in northern Russia and lime mud at Khorbusuonka in northern Yakutia, show that the sediment was liquid long after formation of the imprints and that its mineralogy did not matter. A laminated 2 mm thick microbial mat is preserved intact at Zimnie Gory. It stabilized the sediment surface allowing formation of imprints on it. The soft body impressions on the under surface of the sand bed and within it developed owing to formation of a less than 1 mm thin “death mask” by precipitation of iron sulfide in the sediment. Fossils of the same species or even parts of the same organism may be preserved differently. Internal organs either collapsed, their cavities being filled with sediment from above, or resisted compression more effectively than the rest of the body. This allows restoration of the original internal anatomy of Ediacaran organisms. At Zimnie Gory numerous series of imprints of Yorgia on the clay bottom surface with the collapsed body at their end represent death tracks. The environment of formation of the Ediacaran fossils was thus inhospitable to most organisms. Those adapted to it, namely the radially organized frondose Petalonamae (of possible ctenophoran affinities), anchored in the mat with their basal bulbs. They evolved towards sessile life possibly in symbiosis with photo- or chemoautotrophic microorganisms. Vagile Ediacaran organisms belong mostly to the Dipleurozoa (somewhat resembling chordates and nemerteans), characterized by a segmented dorsal hydraulic skeleton, intestine with metameric caeca, and serial gonads. Only a fraction of the actual Precambrian faunal diversity is represented in the Ediacaran biota.
THE CAMBRIAN EXPLOSION: PUTTING THE PIECES TOGETHER
We propose that some of the more conspicuous Ediacaran fossils from the Avalon Peninsula of Newfoundland, including Aspidella, Charnia, and Charniodiscus, were biologically similar to members of the Kingdom Fungi. These organisms were multicellular or multinuclear, lived below the photic zone, could not move or defoul themselves, did not exhibit taphonomic shrinkage, and were not transported or moved. Aspidella, in particular, appears to exhibit indeterminate growth without a maximum size constraint, and appears to show growth zonations similar to modern mycelia. Other fossils from this deposit exhibit a fractal-like growth pattern. Together, these features falsify algal, lichen, and metazoan interpretations of these fossils, yet reflect characteristics of modern fungal mycelia. We emphasize that although no Mistaken Point fossil appears to be a metazoan, not all of the Mistaken Point taxa, and not all of the Ediacaran organisms in general, can reasonably be interpreted using a fungal analogy. Furthermore, the hypothesis that these fossils were functionally fungus-like need not imply that the organisms were members of the crown-group Fungi. We propose further tests for evaluating both this functional hypothesis and the phylogenetic hypothesis that these organisms were members of the total-group Fungi.
The idea that the last common ancestor of bilaterian animals (Urbilateria) was segmented has been raised recently on evidence coming from comparative molecular embryology. Leaving aside the complex debate on the value of genetic evidence, the morphological and developmental evidence in favor of a segmented Urbilateria are discussed in the light of the emerging molecular phylogeny of metazoans. Applying a cladistic character optimization procedure to the question of segmentation is vastly complicated by the problem of defining without ambiguity what segmentation is and to what taxa this definition applies. An ancestral segmentation might have undergone many complex derivations in each different phylum, thus rendering the cladistics approaches problematic. Taking the most general definitions of coelom and segmentation however, some remarkably similar patterns are found across the bilaterian tree in the way segments are formed by the posterior addition of mesodermal segments or somites. Postulating that these striking similarities in mesodermal patterns are ancestral, a scenario for the diversification of bilaterians from a metameric ancestor is presented. Several types of evolutionary mechanisms (specialization, tagmosis, progenesis) operating on a segmented ancestral body plan would explain the rapid emergence of body plans during the Cambrian. We finally propose to test this hypothesis by comparing genes involved in mesodermal segmentation.
Molecular data is ideal for exploring deep evolutionary history because of its universality, stochasticity and abundance. These features provide a means of exploring the evolutionary history of all organisms (including those that do not tend to leave fossils), independently of morphological evolution, and within a statistical framework that allows testing of evolutionary hypotheses. In particular, molecular data have an important role to play in examining hypotheses concerning the tempo and mode of evolution of animal body plans. Examples are given where molecular phylogenies have led to a re-examination of some fundamental assumptions in metazoan evolution, such as the immutability of early developmental characters, and the evolvability of bauplan characters. Molecular data is also providing a new and controversial timescale for the evolution of animal phyla, pushing the major divisions of the animal kingdom deep into the Precambrian. There have been many reasons to question the accuracy and precision of molecular date estimates, such as the failure to account for lineage-specific rate variation and unreliable estimation of rates of molecular evolution. While these criticisms have been largely countered by recent studies, one problem has remained a challenge: could temporal variation in the rate of molecular evolution, perhaps associated with “explosive” adaptive radiations, cause overestimation of diversification dates? Empirical evidence for an effect of speciation rate, morphological evolution or ecological diversification on rates of molecular evolution is examined, and the potential for rate-variable methods for molecular dating are discussed.
Whilst the “Cambrian Explosion” continues to attract much attention from a wide range of earth and life scientists, the detailed patterns exhibited by the terminal Proterozoic–Early Cambrian biotas remain unclear, for reasons of systematics, biostratigraphy and biogeography. In particular, recent changes in absolute dating of the Cambrian have refined the period of time that the fossil record might be of most help in revealing the dynamics of the undoubted radiation taking place at this time. The famous exceptionally preserved faunas seem to be rather close temporally, and as yet reveal little about the earliest and critical period of evolution, deep in the Cambrian. Nevertheless, the most parsimonious interpretation of the Cambrian fossil record is that it represents a broadly accurate temporal picture of the origins of the bilaterian phyla.
Exceptionally preserved, non-biomineralizing fossils contribute importantly to resolving details of the Cambrian explosion, but little to its overall patterns. Six distinct “types” of exceptional preservation are identified for the terminal Proterozoic-Cambrian interval, each of which is dependent on particular taphonomic circumstances, typically restricted both in space and time. Taphonomic pathways yielding exceptional preservation were particularly variable through the Proterozoic-Cambrian transition, at least in part a consequence of contemporaneous evolutionary innovations. Combined with the reasonably continuous record of “Doushantuo-type preservation,” and the fundamentally more robust records of shelly fossils, phytoplankton cysts and trace fossils, these taphonomic perturbations contribute to the documentation of major evolutionary and biogeochemical shifts through the terminal Proterozoic and early Cambrian.
Appreciation of the relationship between taphonomic pathway and fossil expression serves as a useful tool for interpreting exceptionally preserved, often problematic, early Cambrian fossils. In shale facies, for example, flattened non-biomineralizing structures typically represent the remains of degradation-resistant acellular and extracellular “tissues” such as chaetae and cuticles, whereas three-dimensional preservation represents labile cellular tissues with a propensity for attracting and precipitating early diagenetic minerals. Such distinction helps to identify the acuticular integument of hyolithids, the chaetae-like nature of Wiwaxia sclerites, the chaetognath-like integument of Amiskwia, the midgut glands of various Burgess Shale arthropods, and the misidentification of deposit-feeding arthropods in the Chengjiang biota. By the same reasoning, putative lobopods in the Sirius Passet biota and putative deuterostomes in the Chengiang biota are better interpreted as arthropods.
There was a major diversification known as the Ordovician Radiation, in the period immediately following the Cambrian. This event is unique in taxonomic, ecologic and biogeographic aspects.
While all of the phyla but one were established during the Cambrian explosion, taxonomic increases during the Ordovician were manifest at lower taxonomic levels although ordinal level diversity doubled. Marine family diversity tripled and within clade diversity increases occurred at the genus and species levels. The Ordovician radiation established the Paleozoic Evolutionary Fauna; those taxa which dominated the marine realm for the next 250 million years. Community structure dramatically increased in complexity. New communities were established and there were fundamental shifts in dominance and abundance.
Over the past ten years, there has been an effort to examine this radiation at different scales. In comparison with the Cambrian explosion which appears to be more globally mediated, local and regional studies of Ordovician faunas reveal sharp transitions with timing and magnitudes that vary geographically. These transitions suggest a more episodic and complex history than that revealed through synoptic global studies alone.
Despite its apparent uniqueness, we cannot exclude the possibility that the Ordovician radiation was an extension of Cambrian diversity dynamics. That is, the Ordovician radiation may have been an event independent of the Cambrian radiation and thus requiring a different set of explanations, or it may have been the inevitable follow-up to the Cambrian radiation. Future studies should focus on resolving this issue.
The Trilobita were characterized by a cephalic region in which the biomineralized exoskeleton showed relatively high morphological differentiation among a taxonomically stable set of well defined segments, and an ontogenetically and taxonomically dynamic trunk region in which both exoskeletal segments and ventral appendages were similar in overall form. Ventral appendages were homonomous biramous limbs throughout both the cephalon and trunk, except for the most anterior appendage pair that was antenniform, preoral, and uniramous, and a posteriormost pair of antenniform cerci, known only in one species. In some clades trunk exoskeletal segments were divided into two batches. In some, but not all, of these clades the boundary between batches coincided with the boundary between the thorax and the adult pygidium. The repeated differentiation of the trunk into two batches of segments from the homonomous trunk condition indicates an evolutionary trend in aspects of body patterning regulation that was achieved independently in several trilobite clades. The phylogenetic placement of trilobites and congruence of broad patterns of tagmosis with those seen among extant arthropods suggest that the expression domains of trilobite cephalic Hox genes may have overlapped in a manner similar to that seen among extant arachnates. This, coupled with the fact that trilobites likely possessed ten Hox genes, presents one alternative to a recent model in which Hox gene distribution in trilobites was equated to eight putative divisions of the trilobite body plan.
The accumulation of multiple phylogenetic hypotheses for the Metazoa invites an evaluation of current progress in the field. I discuss three case studies from the recent literature to assess how cladistic analyses of metazoan morphology have contributed to our understanding of animal evolution. The first case study on cleavage cross patterns examines whether a decade of unanimous character scoring across different cladistic studies can be considered a reliable indicator of accumulated wisdom. The two remaining case studies illustrate how the unique strength of cladistic analyses to arbitrate between competing hypotheses can be crippled when insufficient attention is directed towards the construction of the data matrix. The second case study discusses a recent morphological cladistic analysis aimed at providing insight into the evolution of larval ciliary bands (prototrochs) in the Spiralia, and the third case study evaluates how four subsequent morphological cladistic analyses have contributed to our understanding of the phylogenetic placement of a problematicum, the Myzostomida. I conclude that current phylogenetic analyses of the Metazoa have not fully exploited the power of cladistics to test available alternative hypotheses. If our goal is to generate genuine progress in understanding rather than stochastic variation of opinions through time, we have to shift our attention from using cladistics as an easy tool to generate “novel” hypotheses of metazoan relationships, towards employing cladistics more critically as an effective instrument to test the relative merit of available multiple alternative hypotheses.
The increase in trace fossil diversity across the Neoproterozoic-Cambrian boundary often is presented in terms of tabulations of ichnogenera. However, a clearer picture of the increase in diversity and complexity can be reached by combining trace fossils into broad groups defined both on morphology and interpretation. This also focuses attention on looking for similarites between Neoproterozoic and Cambrian trace fossils. Siliciclastic sediments of the Neoproterozoic preserve elongate tubular organisms and structures of probable algal origin, many of which are very similar to trace fossils. Such enigmatic structures include Palaeopascichnus and Yelovichnus, previously thought to be trace fossils in the form of tight meanders.
A preliminary two or tripartite terminal Neoproterozoic trace fossil zonation can be be recognized. Possibly the earliest trace fossils are short unbranched forms, probably younger than about 560 Ma. Typical Neoproterozoic trace fossils are unbranched and essentially horizontal forms found associated with diverse assemblages of Ediacaran organisms. In sections younger than about 550 Ma a modest increase in trace fossil diversity occurs, including the appearance of rare three-dimensional burrow systems (treptichnids), and traces with a three-lobed lower surfaces.
The Cambrian radiation is that key episode in the history of life when a large number of animal phyla appeared in the fossil record over a geologically short period of time. Over the last 20 years, scientific understanding of this radiation has increased significantly. Still, fundamental questions remain about the timing of the radiation and also the tempo of evolution. Trilobites are an excellent group to address these questions because of their rich abundance and diversity. Moreover, their complex morphology makes them readily amenable to phylogenetic analysis, and deducing the nature of macroevolutionary processes during the Cambrian radiation requires an understanding of evolutionary patterns. Phylogenetic biogeographic analysis of Early Cambrian olenellid trilobites, based on a modified version of Brooks Parsimony Analysis, revealed the signature of the breakup of Pannotia, a tectonic event that most evidence suggests is constrained to the interval 600 to 550 Ma. As trilobites are derived metazoans, this suggests the phylogenetic proliferation associated with the Cambrian radiation was underway tens of millions of years before the Early Cambrian, although not hundreds of millions of years as some have argued.
Phylogenetic information from Early Cambrian olenellid trilobites was also used in a stochastic approach based on two continuous time models to test the hypothesis that rates of speciation were unusually high during the Cambrian radiation. No statistical evidence was found to support this hypothesis. Instead, rates of evolution during the Cambrian radiation, at least those pertaining to speciation, were comparable to those that have occurred during other times of adaptive or taxic radiation throughout the history of life.