Coiled cephalopods constitute a major part of the Paleozoic nekton. They emerged in the Early Ordovician but nearly vanished in the Silurian. The Emsian appearance of ammonoids started a story of evolutionary success of coiled cephalopods, which lasted until the end-Permian extinction event. This story is investigated by using a taxonomic database of 1346 species of 253 genera of coiled nautiloids and 1114 genera of ammonoids. The per capita sampling diversities, the Van Valen metrics of origination and extinction, and the probabilities of origination and extinction were calculated at stage intervals. The outcome of these estimations largely reflects the known biotic events of the Paleozoic. The polyphyletic, iterative appearance of coiled cephalopods within this time frame is interpreted to be a process of adaptation to shell-crushing predatory pressure. The evolution of the diversity of coiled nautiloids and ammonoids is strongly correlated within the time intervals. Once established, assemblages of coiled cephalopods are related to changes in sea level. The general trends of decreasing mean (or background) origination and extinction rates during the Paleozoic are interpreted to reflect a successive stabilization of the coiled cephalopod assemblages. Different reproduction strategies in ammonoids and nautiloids apparently resulted in different modes of competition and morphological trends. Significant morphological trends toward a stronger ornamentation and a centrally positioned siphuncle characterize the evolution of Paleozoic nautiloids.

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.

Communal hunting allows some modern canids to catch large and powerful prey. As opposed to felids, for example, Recent canids have a limited ability to grapple and subdue prey by using their forelimbs. Instead, they engage in sustained pursuit predation and the success rate during this activity typically increases with the number of individuals participating in the hunt. Clearly, such behaviors do not fossilize directly and have to be inferred from anatomy. This paper focuses on how social pack-hunting in large-bodied fossil canids can be determined and the potential for it among Tertiary canids (Canidae, Carnivora). Craniodental adaptations for handling and killing large prey and forearm utility in running and grappling are investigated by principal components and canonical variates analyses. I also test whether fossil canids responded to predation of large prey by evolving the same morphological traits as their Recent pursuit-type relatives. The analyses show that small and large members of the Recent Caninae share similar craniodental morphologies. However, the same pattern is not present in the fossil subfamilies Borophaginae and Hesperocyoninae. In the latter, large representatives are characterized by being relatively short-faced with reduced anterior premolars and enlarged posterior premolars, thus approaching a “pantherine-like” configuration. These traits are interpreted as an adaptation for killing prey with felid-like canine bites. The elbow joints of large canids also do not converge on a single morphotype. All analyzed species of borophagines and hesperocyonines have retained the ability to supinate their forearms, unlike Recent large Caninae. It is therefore likely that manual manipulation was part of their hunting behavior, thus removing an essential part of the argument for social pack-hunting in these forms, as the benefits of such a strategy become less obvious. An association between the origin of pack-pursuit “wolf avatars” and the origin and evolution of grass-dominated ecosystems is hypothesized. The results presented here clearly suggest that Recent large canids are poor ecological, morphological, and behavioral analogs for their large fossil relatives.

Fossil vertebrate distributions are typically based on body fossils, which are often poorly sampled at the margins of their true temporal and spatial ranges. Because vertebrate ichnofossils can be preserved in great abundance and in different environments than vertebrate body fossils, inclusion of ichnofossil data may improve sampled ranges. However, if ichnofossils are to serve as an independent source of distributional data, then their attribution to a body fossil group (i.e., trackmaker identification) cannot rely on temporal and spatial coincidence. Ichnofossils identified by synapomorphies can act as an independent source of distributional data that can modify spatial, temporal, and character distributions, which in turn may influence hypotheses of locomotor evolution.

In this paper I evaluate the spatial, temporal, and character distributions of early sauropod dinosaurs by using a combined ichnofossil and body fossil data set. Sauropod ichnofossils supplement the spatiotemporal distributions of early sauropods and provide important information on early sauropod foot posture that is rarely preserved or can only be inferred from body fossils. The presence of derived features in early-appearing ichnofossils challenges previous hypotheses of character transformation, implying either parallelism, reversal, or ghost lineages.

Stratocladistics can be used to resolve conflicting character and temporal distributions from body fossils and ichnofossils. Stratocladistic analysis of a combined ichnofossil and body fossil data set suggests a richer, more widely distributed diversity of early sauropods than currently recognized in body fossils and suggests that several locomotor characters originated much earlier than implied by body fossils.

One of the enduring puzzles to Stephen Jay Gould about life on Earth was the cause or causes of the fantastic diversity of animals that exploded in the fossil record starting around 530 Ma–the Cambrian explosion. In this contribution, we first review recent phylogenetic and molecular clock studies that estimate dates for high-level metazoan diversifications, in particular the origin of the major lineages of the bilaterally-symmetrical animals (Bilateria) including cnidarians. We next review possible “internal” triggers for the Cambrian explosion, and argue that pattern formation, those processes that delay the specification of cells and thereby allow for growth, was one major innovation that allowed for the evolution of distinct macroscopic body plans by the end of the Precambrian. Of potential “external” triggers there is no lack of candidates, including snowball earth episodes and a general increase in the oxygenation state of the world's oceans; the former could affect animal evolution by a mass extinction followed by ecological recovery, whereas the latter could affect the evolution of benthic animals through the transfer of reduced carbon from the pelagos to the benthos via fecal pellets. We argue that the most likely cause of the Cambrian explosion was the evolution of macrophagy, which resulted in the evolution of larger body sizes and eventually skeletons in response to increased benthic predation pressures. Benthic predation pressures also resulted in the evolution of mesozooplankton, which irrevocably linked the pelagos with the benthos, effectively establishing the Phanerozoic ocean. Hence, we suggest that the Cambrian explosion was the inevitable outcome of the evolution of macrophagy near the end of the Marinoan glacial interval.

Understanding the patterns, causes, and consequences of biotic interchange—the movement of species between neighboring biotas—is crucial for evaluating the effects of human-introduced species in the modern biosphere. Since at least the early Miocene, the tropical and subtropical western Atlantic has comprised two biogeographic provinces, the Gatunian (including the Caribbean) and the Caloosahatchian (North Carolina to Florida and the Yucatán peninsula). Although these adjacent provinces are not separated by a land barrier, exchange of species between them has been limited and intermittent. A synthesis of taxonomic, phylogenetic, stratigraphic, and biogeographic data on six gastropod and two bivalve groups reveals a dramatic shift in the pattern of interchange between these provinces. About 31% of early Miocene Caloosahatchian subgenus- and species-group-level taxa invaded the Gatunian Province by the late Miocene, but no taxa extended their ranges in the opposite direction. Beginning in the early Pliocene and continuing into the early Pleistocene, 40 taxa (roughly one-third of Gatunian diversity) invaded the Caloosahatchian Province from the Caribbean, whereas only four taxa extended their range from Florida into the Caribbean.

Comparisons between the ranked percentage of Gatunian invaders in Florida and the magnitude of regional extinction there for each of four middle Pliocene to early Pleistocene intervals reveal no consistent relation between invasion and prior or concurrent extinction. During the Pliocene, invaders not only compensated for extinctions, but also accounted for almost all the observed increase in standing diversity in Florida. Only after the large extinction event at the end of the Pliocene did invaders from the Gatunian Province not fully compensate for the loss of species.

Although the Miocene interaction between the Gatunian and Caloosahatchian biotas involved two fully tropical entities, the Plio–Pleistocene interaction exemplifies a general pattern in which tropical species often spread to higher latitudes during warm intervals, but warm-temperate or subtropical species rarely become established in the Tropics. Some evidence indicates that tropical Caribbean molluscs are exposed and adapted to more intense competition and predation than their subtropical counterparts in Florida, implying a role for individual-level biotic interactions in determining the predominant direction of interchange. Intensification of north-flowing currents in the western Atlantic may also contribute to the nearly one-way movement of taxa from the Caribbean to Florida during the Pliocene and early Pleistocene. The changing pattern of interchange from the Miocene to the Pliocene further reflects a change in the geography of species richness, with the richer province serving as the chief donor and the province with lower diversity acting as the main recipient of invaders. Diversity, ocean circulation, and the competitive environment thus account for the observed switch in the predominant direction of invasion in the western Atlantic during the Neogene.

The fact that almost 90% of Gatunian immigrants to Florida differentiated taxonomically there indicates that invasion is intermittent. Long-term consequences of this and many other cases of interchange between provinces include enrichment of the regional and global species pool and the spread of adaptations reflecting intense competition and predation.

A global database of gastropod sizes from the Permian through the Middle Triassic documents trends in gastropod shell size and permits tests of the suggestion that Early Triassic gastropods were everywhere unusually small. Analysis of the database shows that no specimens of unambiguous Early Triassic age larger than 2.6 cm have been reported, in contrast to common 5–10-cm specimens of both Permian and Middle Triassic age. The loss of large gastropods is abrupt even at a fine scale of stratigraphic resolution, whereas the return of larger individuals in the Middle Triassic appears gradual when finely resolved. Taphonomic and sampling biases do not adequately explain the absence of large Early Triassic gastropods. Examination of size trends by genus demonstrates that the size decrease across the Permian/Triassic boundary is compatible with both size-selective extinction at the species level and anagenetic size change within lineages. Size increase in the Middle Triassic resulted from the origination of large species within genera that have Early Triassic fossil records and the occurrence of new genera containing large species during the Middle Triassic. Genera recorded from the Permian and Middle Triassic but not the Early Triassic (“Lazarus taxa”) do not contribute to observed size increase in the Middle Triassic. Moreover, Lazarus taxa lack large species and exhibit low species richness during both the Permian and the Middle Triassic, suggesting that they survived as small, rare forms rather than existing at large sizes in Early Triassic refugia. The ecological opportunities and selective pressures that produced large gastropods during most intervals of the Phanerozoic evidently did not operate in Early Triassic oceans. Whether this reflects low predation or competitive pressure, r-selection facilitated by high primary production, or physical barriers to large size remains poorly understood.

Although Darwin was not the first to conceive directional selection as a mechanism of phenotypic change, it is his ideas that were received, and that have shaped population biology to this day. A significant change in his theoretical orientation occurred in the mid-1850s. About then he abandoned environmental selection in favor of competitive selection, and adopted relative adaptation with all its consequences as an alternative. These ideas changed his thinking fundamentally and shaped his argument throughout the writing of his great book. It is still these ideas that predominate today.

Here I examine Darwin's ideas in relation to his principle of divergence, sexual selection, and the nature and origin of species. Finally I suggest that had he not misunderstood the function of sexual communication he might well have understood the nature of species and provided a more penetrating resolution to Herschell's “mystery of mysteries,” with which he opened his book.

During the Early Jurassic, cyst-forming dinoflagellates began a long-term radiation that would portend ecological importance of these taxa in the pelagic plankton community throughout the rest of the Mesozoic era. The factors that contributed to the evolutionary success of dinoflagellates are poorly understood. Here we examine the relationship between oceanographic and climatic conditions during the Hettangian–Toarcian interval in relation to the radiation of dinoflagellates and other organic-walled phytoplankton taxa in the Tethys Ocean. Our analysis is based on two data sets. The first includes δ13Ccarb, δ13Corg, total organic carbon (TOC), and quantitative palynological observations derived from the Mochras Core (Wales, U.K.), which spans the complete Early Jurassic. The second is a coupled Mg/Ca and δ18O record derived from analyses of belemnite calcite obtained from three sections in northern Spain, covering the upper Sinemurian to Toarcian. From these two data sets we reconstructed the influence of sea level, trophism, temperature, and salinity on dinoflagellate cyst abundance and diversity in northwest Europe. Our results suggest that organic-walled phytoplankton (acritarchs, prasinophytes, and dinoflagellates) diversity increased through the Early Jurassic. The radiation coincides with a long-term eustatic rise and overall increase in the areal extent of continental shelves, a factor critical to cyst germination. On shorter timescales, we observed short bursts of dinoflagellate diversification during the late Sinemurian and late Pliensbachian. The former diversification is consistent with the opening of the Hispanic Corridor during the late Sinemurian, which apparently allowed the pioneer dinoflagellate, Liasidium variabile, to invade the Tethys from the Paleo-Pacific. A true radiation pulse during the late Pliensbachian, with predominantly cold-water taxa, occurred during sea level fall, suggesting that climate change was critical to setting the evolutionary tempo. Our belemnite δ18O and Mg/Ca data indicate that late Pliensbachian water masses cooled (ΔT ≈ −6°C) and became more saline (ΔS ≈ +2 psu). Cooling episodes during generally warm and humid Early Jurassic climate conditions would have produced stronger winter monsoon northeast trade winds, resulting in hydrographic instability, increased vertical mixing, and ventilation of bottom waters. During the late Pliensbachian, dinoflagellates replaced green algae, including prasinophytes and acritarchs, as primary producers. By producing benthic resting cysts, dinoflagellates may have been better adapted to oxidized ocean regimes. This hypothesis is supported by palynological data from the early Toarcian ocean anoxic event, which was marked by highly stratified anoxic bottom water overlain by low-salinity, warm surface waters. These conditions were advantageous to green algae, while cyst-producing dinoflagellates temporarily disappeared. Our results suggest that the rise in dinoflagellate diversity later in the Jurassic appears to correspond to deep water ventilation as a result of the opening of the Atlantic seaway, conditions that appear to have simultaneously led to a loss of prasinophyte dominance in the global oceans.

Batches of freshly fallen Metasequoia glyptostroboides litter were subjected to one of 12 degradation regimes varying in temperature, pH, and oxygen availability for a three-month decay period. The regimes were intended to simulate possible conditions prevailing during the first winter following the autumnal leaf fall for the Metasequoia dominated forests of Axel Heiberg Island ca. 45 Ma. The decayed leaves were examined by transmission electron microscopy to quantify the extent and quality of chloroplast preservation. The best preservation of chloroplast membranes was observed at pH 8.5 and at 10°C, although it was found that rapidly freezing samples also reliably preserved ultrastructural chloroplast features with a high degree of fidelity to the living state. The results from this study suggest that the ambient water chemistry of the depositional environment in the Eocene floodplain forests of Axel Heiberg Island could have been basic, and maintained by a natural carbonate buffering system, but they also demonstrate that the preservation of chloroplast features can occur under varying environmental conditions.

A theoretical morphologic model defining patterns of shell sculptures in Bivalvia is introduced. It is based on the displacement of sculptural elements along the growing shell margin and introduction of new sculptural elements. The kinematics of the sculptural elements are defined in terms of the following parameters: maximum speed of displacement of sculptural elements, position along the growing shell margin where a migrating element attains maximum speed, and position of the divergence axis of the riblets. Computer models successfully mimicked most of the diverse patterns of bivalve shell sculptures. Morphometric analysis revealed that the displacement speed of a sculptural element is not constant but depends on the relative position of the element on the shell margin. It was revealed that the primary component of variation in bivalve shell sculptures could be explained by variation in the displacement speed of sculptural elements around the divergence axis of riblets.

Improvements in our understanding of green plant phylogeny are casting new light on the connection between character evolution and diversification. The repeated discovery of paraphyly has helped disentangle what once appeared to be phylogenetically coincident character changes, but this has also highlighted the existence of sequences of character change, no one element of which can cleanly be identified as the “key innovation” responsible for shifting diversification rate. In effect, the cause becomes distributed across a nested series of nodes in the tree. Many of the most conspicuous plant “innovations” (such as macrophyllous leaves) are underlain by earlier, more subtle shifts in development (such as overtopping growth), which appear to have enabled the exploration of a greater range of morphological designs. Often it appears that these underlying changes have been brought about at the level of cell interactions within meristems, highlighting the need for developmental models and experiments focused at this level. The standard practice of attempting to identify correlations between recurrent character change (such as the tree growth habit) and clade diversity is complicated by the observation that the “same” trait may be constructed quite differently in different lineages (e.g., different forms of cambial activity), with some solutions imposing more architectural limitations than others. These thoughts highlight the need for a more nuanced view, which has implications for comparative methods. They also bear on issues central to Stephen Jay Gould's vision of macroevolution, including exaptation and evolutionary recurrence in relation to constraint and the repeatability of evolution.

Quantitative analysis of growth rings in pre-Quaternary fossil woods is commonly used as a paleoclimatic indicator. In this paper, a global analysis of the relationship between climate and growth ring parameters in modern trees is presented that, in part, invalidates the use of fossil woods in this way. Data reprocessed from the International Tree-Ring Data Bank are used to analyze three parameters, mean ring width, mean sensitivity, and percentage latewood, from 727 sites across a global climatic range. Results allow the complex relationship between climate and growth ring parameters to be quantified at the global scale for the first time. They reveal the enormous variability in tree response to climate-forcing, which is influenced by disparate factors such as taxonomy, ontogeny, ecology, and environment. Quantitative analysis of fossil growth ring data in light of the modern results indicates that even the largest and most detailed fossil studies conducted to date are probably inadequate in distinguishing a paleoclimate signal from the background noise of variability. The validity of using quantitative growth ring parameters as indicators of Pre-Quaternary climates is therefore questionable. Only in well-constrained studies where paleoclimatic, ontogenetic, and taxonomic sources of variability can be controlled, and data sets are very large, may fossil growth ring analysis provide useful paleoecological data. The findings of this paper do not invalidate in any way the use of growth rings in fossil woods as qualitative paleoclimatic indicators.

Multivariate analysis of shell characters and quantification of morphological diversity (morphospace occupation and disparity) are used here to investigate the modes of morphological diversification of ammonites. We define five events in early cardioceratid history that connect geographical changes causing emigration or immigration phases with biodiversity dynamics: (1) the initial colonization of the Arctic Basin by the Cardioceratidae at the end of the Bajocian, Middle Jurassic; (2) the first appearance of the Kosmoceratidae clade in the Boreal Realm during the Bathonian; (3) the ensuing expansion phase of this clade in the Boreal Realm; (4) the first phase of migration of the Cardioceratidae (early Callovian) through Eastern Europe, Western Britain and the Yukon corridor; and (5) the second unrelated migration phase in the Western Interior only. Analysis of spatial occupation shows that acquisition of this field occurs essentially by replacement or subdivision of preexisting peaks of occupation. These replacements seem to follow different patterns: progressive trend, saturation, iteration, and apparent preferential extinction. We describe these patterns and suggest different factors that may have shaped them, including a morphological differentiation that has been interpreted by various authors as sexual dimorphism. Another factor that could cause disparity modification is fluctuations in the ammonites' proximal environment. The effect of immigrating faunas is a third (and preponderant) factor that is prominent in the studied example: immigration phases of the Cardioceratidae lead to increased morphological diversity, whereas the spread of nonindigenous species reduces it and is contemporaneous with a morphological shift in the native clade. We thus demonstrate here that geographical constraints play a significant role in the expression of innovation and may be seen as a major factor in macroevolutionary dynamics.

The evolution of scutes in thyreophoran dinosaurs, based on Scutellosaurus, Scelidosaurus, Stegosaurus, and several ankylosaurs, began with small rounded or ovoid structures that typically had slight, anteroposteriorly oriented keels. These scutes were elaborated in two general and overlapping ways: they could flare laterally and asymmetrically beneath the keels that mark the anteroposterior axis, and they could be hypertrophied in their distal growth to produce plates, spikes, and other kinds of ornamentation. Stegosaurus plates and spikes are thus primarily hypertrophied keels of primitive thyreophoran scutes, sometimes with elaboration of dermal bone around their pustulate bases. Histologically, most thyreophoran scute tissues comprise secondary trabecular medullary bone that is sandwiched between layers of compact primary bone. Some scutes partly or mostly comprise anatomically metaplastic bone, that is, ossified fibrous tissue that shows incremental growth.

The “plumbing” of Stegosaurus plates was not apparently built to support a “radiator” system of internal blood vessels that communicated with the outside of the plates and coursed along their external surfaces to return heated or cooled blood to the body core. Possibly a purely external system supported this function but there is no independent evidence for it. On the other hand, many of the vascular features in stegosaurian plates and spikes reflect bautechnisches artifacts of growth and production of bone. Surface vascular features likely supported bone growth and remodeling, as well as the blood supply to a keratinous covering. When the gross and microstructural features of the plates and spikes are viewed in phylogenetic context, no clear pattern of thermoregulatory function emerges, though an accessory role cannot be eliminated in certain individual species. It seems more likely, as in other groups of dinosaurs, that the variation of dermal armor form in stegosaurs was primarily linked to species individuation and recognition, perhaps secondarily to inter- and intraspecific display, and rarely to facultative thermoregulation.

This paper investigates trends in the evolution of body size and shape in the Plesiosauria, a diverse clade of Mesozoic marine reptiles. Using measures from well-preserved plesiosaur specimens, we document and interpret evolutionary patterns in relative head size, body size, and locomotor variables. Size increase is a significant trend in the clade as a whole, and in constituent clades. The trend in relative head size is of variance increase; observed head sizes are both smaller and larger than ancestral values. In the locomotor system, changes in propodial and girdle proportions appear concomitant with body size increase and are interpreted as allometric responses to the physical constraints of large body size. Other trends in the locomotor system are significantly correlated with both body size and relative head size. These locomotor trends evolved convergently in several clades of plesiosaurs, and may have had an ecomorphological basis, although data are lacking to constrain speculation on this point. The evolution of the locomotor system in plesiosaurs sheds new light on the response of aquatic tetrapods to the physical constraints of foraging at large body size.

Previous studies have shown that secular variation in the Mg/Ca ratio of seawater throughout the Phanerozoic would have subjected the aragonite-producing codiacean algae to at least three transitions between the low-Mg calcite (molar Mg/Ca <2) and aragonite + high-Mg calcite (molar Mg/Ca >2) nucleation fields in the oceans, since their origin in the Ordovician. These studies have asserted that major sediment production by codiacean algae in Recent tropical seas is permitted by the Mg/Ca ratio of modern seawater (molar Mg/Ca ∼5.2) remaining within the aragonitic/high-Mg calcite nucleation field (molar Mg/Ca >2). Here I present the results of experiments conducted to determine the effects of ambient Mg/Ca on the mineralogy, growth rate, primary productivity, calcification rate, and biomechanics of the codiacean alga Penicillus capitatus.

P. capitatus specimens were grown in three artificial seawater treatments that mimic ancient seawater of differing Mg/Ca ratios, corresponding to the low-Mg calcite nucleation field (molar Mg/Ca ∼1.0), a “boundary field” (molar Mg/Ca ∼2.5), and the aragonite + high-Mg calcite nucleation field (molar Mg/Ca ∼5.2). Significantly, P. capitatus specimens maintained a mostly aragonitic mineralogy in all three seawater treatments. However, linear growth rates, primary productivity, calcification, and thallus stiffness decreased with reductions in ambient Mg/Ca. That P. capitatus precipitates approximately three-quarters of its CaCO3 as aragonite in the seawater treatment that favors the inorganic precipitation of low-Mg calcite suggests that the alga dictates the precipitation of that polymorph, either by pumping cations to create an internal aragonite nucleation field (molar Mg/Ca >2) or by employing organic templates that specify the nucleation of the aragonite polymorph (Borowitzka 1984). However, the alga's precipitation of one-quarter of its CaCO3 as low-Mg calcite suggests that its mineralogical control is limited and can be partially overridden by the Mg/Ca ratio of ambient seawater. The reduced calcification of P. capitatus specimens grown in the low-Mg calcite and boundary nucleation fields is probably due to the inherent difficulty of precipitating aragonite from seawater which does not naturally support its nucleation. The decreased rates of linear growth and primary production are probably caused by reductions in CO2 available for photosynthesis due to the reduction in calcification (Borowitzka and Larkum 1977). The observed decrease in thallus stiffness is probably due to the reductions in calcification and primary productivity observed in P. capitatus specimens grown in the low-Mg calcite and boundary nucleation fields.

The present study suggests that aragonitic algae would have been handicapped in oceans characterized by Mg/Ca ratios that did not support their inherent mineralogy. Producing aragonite in seawater outside of the aragonite + high-Mg calcite nucleation field would probably have reduced the competitiveness of these algae, made them more susceptible to predation, and reduced their contribution to carbonate sedimentation. These findings support earlier assertions that the dominant ecological and sedimentological roles of codiacean algae in Recent tropical seas is permitted by a Mg/Ca ratio of seawater that supports the algae's aragonitic mineralogy during this time.

Finite element modeling (FEM) has been used to evaluate microstructure-controlled stability of selected eggshells of Indian dinosaurs. Our study suggests that under static load the eggshell microstructure of Megaloolithus cylindricus displays a low magnitude of tensile stress over most of the spherolith. The magnitude of this tensile stress is lower than that displayed in M. jabalpurensis, M. baghensis, and Subtiliolithus kachchhensis. In M. cylindricus, a shell thickness matching the length of the spheroliths prevents the failure of eggshells, whereas in M. jabalpurensis and M. baghensis, which have thinner shells, the development of additional subspheroliths compensates for the relatively higher magnitude of tensile stresses. Extremely thin eggshell in S. kachchhensis shows a still higher magnitude of tensile stresses, thereby making it prone to cracking, but the propagation of cracks is apparently checked and stability reinforced by wider spacing of pore canals.

Global information on Paleozoic, Mesozoic, and extant non-angiosperm leaf morphologies has been gathered to investigate morphological diversity in leaves consistent with marginal growth and to identify likely departures from such development. Two patterns emerge from the principal coordinates analysis of this data set: (1) the loss of morphological diversity associated with marginal leaf growth among seed plants after sharing the complete Paleozoic range of such morphologies with ferns and (2) the repeated evolution of more complex, angiosperm-like leaf traits among both ferns and seed plants. With regard to the first pattern, morphological divergence of fern and seed plant leaf morphologies, indirectly recognized as part of the Paleophytic-Mesophytic transition, likely reflects reproductive and ecological divergence. The leaf-borne reproductive structures that are common to the ferns and Paleozoic seed plants may promote leaf morphological diversity, whereas the separation of vegetative and reproductive roles into distinct organs in later seed plant groups may have allowed greater functional specialization—and thereby morphological simplification—as the seed plants came to be dominated by groups originating in more arid environments. With regard to the second pattern, the environmental and ecological distribution of angiosperm-like leaf traits among fossil and extant plants suggests that these traits preferentially evolve in herbaceous to understory plants of warm, humid environments, thus supporting inferences concerning angiosperm origins based upon the ecophysiology of basal extant taxa.

Gould's Wonderful Life (1989) was a landmark in the investigation of the Cambrian radiation. Gould argued that a number of experimental body plans (“problematica”) had evolved only to become extinct, and that the Cambrian was a time of special fecundity in animal design. He focused attention on the meaning and significance of morphological disparity versus diversity, and provoked attempts to quantify disparity as an evolutionary metric. He used the Burgess Shale as a springboard to emphasize the important role of contingency in evolution, an idea that he reiterated for the next 13 years. These ideas set the agenda for much subsequent research. Since 1989 cladistic analyses have accommodated most of the problematic Cambrian taxa as stem groups of living taxa. Morphological disparity has been shown to be similar in Cambrian times as now. Konservat-Lagerstätten other than the Burgess Shale have yielded important new discoveries, particularly of arthropods and chordates, which have extended the range of recognized major clades still further back in time. The objective definition of a phylum remains controversial and may be impossible: it can be defined in terms of crown or total group, but the former reveals little about the Cambrian radiation. Divergence times of the major groups remain to be resolved, although molecular and fossil dates are coming closer. Although “superphyla” may have diverged deep in the Proterozoic, “explosive” evolution of these clades near the base of the Cambrian remains a possibility. The fossil record remains a critical source of data on the early evolution of multicellular organisms.