The linear survivorship curve of Van Valen's Law can be generated from stochastic loss of lineages, given a decreasing sample size within an adaptive zone. No special biological explanation is required or called for in order to explain “linearity” in taxonomic survivorship curves.

Consideration of the physics of sinking of hollow, rigid bodies leads to equations of motion for sinking cephalopod shells. We have derived equations of motion for three post-mortem sinking situations: sinking with a fixed amount of water in the phragmocone; rapid phragmocone filling (no siphuncular tube); and slow phragmocone filling (siphuncular tube intact). In all three cases sinking speed can be closely approximated by the terminal velocity calculated from the total weight, buoyancy, and drag parameters of the shell.

Experiments on modern Nautilus shells yield sinking velocities in agreement with calculated values. The experiments also show that orientation of a sinking Nautilus shell varies as the phragmocone fills with water. With small negative buoyancy the shell sinks with its plane of symmetry upright, but as it fills, it begins to rock from side to side and leans over and sinks with its plane of symmetry horizontal when the camerae are about 55% full.

The maximum sinking speed of upright adult Nautilus shells is approximately 30 cm/sec, which appears to be too small for embedding in the bottom upon impact. The maximum depth to which Nautilus sinks in the upright position ranges from about 7 m for rapidly filling shells to as much as 600 m for slowly filling shells. In the latter case, the shell will continue to fill after coming to rest on the bottom, and the stability of the vertical orientation will be removed within 1 or 2 days. Thus, primary vertical preservation of cephalopod shells indicates water depths less than about 10 m.

An unusual isolated specimen representing an immature rhabdosome of Dicellograptus sp. from the Middle Ordovician Athens Shale of Alabama has a flattened sphere attached to the distal end of the nema. The flattened sphere can be interpreted as a float that probably conferred buoyancy on the rhabdosome. Such floats, which became detached in mature rhabdosomes, may be common in the fossil record. The presence of nematophorous siculae in all planktonic graptolites suggests that for immature rhabdosomes a passive, buoyant mode of life, attained by means of a float, might have been widespread taxonomically. This mode of life is compatible with the theory of passive response for the mode of life of mature rhabdosomes, but it is difficult to reconcile with the theory of automobility.

The covariation patterns of the postcranial skeleton in eight species of moas (Aves, Dinornithidae) from the Pleistocene of New Zealand are described using multiple factor analysis. Rotation schemes include an orthogonal Varimax and oblique (direct quartimin) simple structure solutions.

Four major patterns of covariation are resolved: a length factor, primarily that of leg length; a width factor, including pelvis width and long bone widths; a sternal breadth factor; and a sternal length-posterior pelvis length factor. The first two patterns represent a functional separation between body size and those aspects of the skeleton, scaling allometrically, adapted to support body weight. The sternal breadth factor may indicate that features contributing to trunk support are independent from those width measurements of the pelvis and hindlimb bones. The sternal length-posterior pelvis length factor reveals a pattern of covariation that is somewhat independent of other patterns (long bone lengths) determining variability in body size.

We outline a plausible evolutionary sequence that led from prokaryotes to the origin of the first nucleated cell. The nucleus is postulated to evolve after the archaebacterium and eubacterium merged to form the symbiotic ancestor of amitochondriate protists. Descendants of these amitochondriate cells (archaeprotists) today thrive in organic-rich anoxic habitats where they are amenable to study. Eukaryosis, the origin of nucleated cells, occurred by the middle Proterozoic Eon prior to the deposition in sediments of well-preserved microfossils such as Vandalosphaeridium and the spiny spheres in the Doushantou cherts of China.

Significant warming of Earth's climate in the near term seems increasingly likely. If significant enough, this climatic regime could, in the long term, come to resemble previous greenhouse intervals in earth history. Consequently, analysis of the fossil record during periods of extreme warmth may provide important lessons for species biology, including biogeography, in a much warmer world. To explore this issue, we analyzed the biogeographic response of 63 molluscan species to the long-term global warmth in the Late Cretaceous Western Interior Seaway (WIS) of North America, using Geographic Information Systems (GIS) to quantitatively measure changes in range size and distribution throughout this interval. We specifically considered the role that geographic range size played in mediating extinction resistance and invasion potential of these WIS species. We found no relationship between geographic range size and survivorship. However, endemic species with small range sizes were more likely to become invasive. Finally, mollusks did not experience a poleward shift in range out of the tropics during this warm regime. To the extent that these patterns are representative, and the WIS and taxa considered constitute a reasonable ancient analogue to a warmer future world, these results suggest that some biogeographic “rules” may not prevail under greenhouse conditions of long-term, equable warmth. They also suggest that other factors beyond geographic range size, including distinctive niche characteristics, may play quite important roles in species survival and invasion potential. This potentially complicates predictions regarding the future responses of extant species to long-term warming.

Psychological evidence suggests that the visual outline of an object is the most important character for discriminating differences in external morphology. External morphology is an important taxonomic character in describing living and fossil species, for example, the blastoid Pentremites. Comparison of different views of the same specimen utilizing Fourier series suggests that the skeleton of Pentremites commonly is not rotationally symmetrical and that the asymmetry is not associated with any specific ray. Analysis of a growth series indicates that the amplitudes of the second and third harmonics are significantly correlated with growth, which is demonstrated to be anisometric. Definable changes in the lateral outline can be attributed to changes in harmonic amplitudes and a wide range of morphological forms comparable to known taxa can be generated by systematically varying the amplitudes of the first four harmonics. The population used in this study probably represents Pentremites robustus Lyon and is from Bangor Limestone in the abandoned Moulton Quarry, Lawrence County, Alabama.

Fish scales accumulating in marine laminated sediments can provide a record of population variability of small pelagic fishes. Although some studies have noted signs of scale degradation that could affect estimates of population variability, there are presently no well-developed means to evaluate degradation. We developed several indices as indicators of fish scale preservation in two box-cores that we collected off Pisco (14°S), one at 301 m near the center of the oxygen minimum zone (OMZ), and the other at 201 m near the upper limit of the OMZ. These indices include (1) an index of fish scale integrity (estimate of scale wholeness relative to fragmentation), (2) the fungi-free area of fish scales and vertebrae, (3) the ratio of fish scales to vertebrae (as well as fish scales to vertebrae and bones), and (4) the ratio of whole scales to fragments. We address whether lower numbers of anchovy scales occurring in association with reduced total organic carbon fluxes and higher bottom-water oxygen concentrations are due entirely to lower abundances of anchovy or whether differential preservation of the fish scales in the sediments plays an important role in reduced scale abundances. Comparison of temporal sequences between the two cores provides the means to assess whether there are differences in the preservation of fish scales. The combined indices indicate that the lower numbers of fish scales in the earliest period have been affected by degradation, and to a greater degree in the box-core from 201 meters, which can be subject to higher oxygen concentrations. On the other hand, decadal-scale variations in fish scale abundance within the period of better preservation are unlikely to be caused by degradation. We discuss the utility and drawbacks of different indices of preservation for reconstructing past changes in fish population sizes with fluxes of fish debris and also briefly discuss the utility of these indices to other paleobiological systems.

Wolpoff's (1984) recent discussion of evolutionary rates in Homo erectus deserves careful study. Whether Homo erectus or other hominid species exhibit gradual, continuous change in key characters or whether instead there is evidence for morphological stasis in the fossil record is an important question. By allocating all Homo erectus specimens to three groups of early, intermediate, and later geological age and by comparing group means for 13 measurements, Wolpoff attempts to show that gradualism is the rule for this mid-Pleistocene taxon. This method is straightforward, but it is crucial that the samples be composed in a manner which is biologically reasonable. I argue here that Wolpoff has not done this. While there may be legitimate doubt concerning sorting of the fossils, especially where specimens are incomplete, several of the individuals said to be representative of Homo erectus are simply inappropriate for use in this analysis. Wolpoff insists that he has employed a “conservative” definition of the species, but instead he has measured everything in sight. This approach to the record does influence his results.

Changes in morphological variability within fossil populations of planktonic protists provide insight into the processes responsible for morphological change. Evolution in the radiolarian genera Theocorythium and Lamprocyclas is documented biometrically in Pliocene and Pleistocene core material from the equatorial Pacific and Indian Oceans. The patterns of morphological change in Theocorythium within a single Pacific core could be interpreted as indicating the in situ evolution of T. trachelium trachelium from T. vetulum via intermediate forms during the Pleistocene. However, consideration of biogeographic data shows that this is not the case and supports instead two alternate interpretations: (1) immigration coupled with extensive hybridization and introgression, or (2) multiple polymorphs of a single species changing in frequency along a cline. Analyses of evolutionary change in Theocorythium and the closely related genus Lamprocyclas during the Pliocene suggest that hybridization may be a significant factor in the production of new species. Evolutionary models and phylogenetic analyses should consider that barriers to gene flow may vary significantly with time and permit the exchange of genetic information between populations previously reproductively isolated.

Dorsal sails supported by hyperelongate neural spines of dorsal vertebrae were evolved by various tetrapods, but most work on their function has centered on the pelycosaur Dimetrodon, in which the sail has generally been interpreted as a thermoregulatory structure that would permit rapid warming in the morning and cooling during the hot midday. The pelycosaur Edaphosaurus differed from other sailed tetrapods in that the neural spines supporting the sail had laterally directed tubercles or cross-bars. Past interpretations of Edaphosaurus suggested that the cross-bars were embedded in a thick fat-storage structure or extended from a thin sail to enhance its utility for intraspecific display. However, wind tunnel modeling of air flow over a thin sail with laterally projecting cross-bars supports a thermoregulatory interpretation of the sail of Edaphosaurus. The cross-bars would produce a turbulent flow, which would increase the effectiveness of convective cooling. Measurements of heat flow in an instrumented model show that cross-bars increase heat loss from the sail. The cross-bars may have enabled Edaphosaurus to thermoregulate effectively with a smaller and lower dorsal sail than would have been required without them.

The emergence of Phanerozoic global diversity as a central theme of investigation has resulted from a confluence of factors, including the assembly by several researchers of global taxonomic databases; the advent of computers, which permitted construction and analysis of global Phanerozoic diversity trajectories; and the recognition that Phanerozoic diversity trends are important bellwethers of the evolutionary processes that cause biotic transitions. Despite the enormous progress in the measurement and interpretation of Phanerozoic diversity over the past quarter century, much of which has been reported in Paleobiology, these studies have collectively generated at least as many new questions as they have answered—arguably the mark of an area of inquiry that continues to be vital. In this essay, I discuss several outstanding issues in the investigation of Phanerozoic diversity, ranging from the viability of literature-derived databases for investigating global diversity trends, to the biological significance of the myriad biotic transitions that have taken place throughout the Phanerozoic.

Both molecular clocks and the first appearances of major groups in the fossil record suggest that most of the range of diatom morphologies observed today had evolved by the end of the Cretaceous Period. Despite this, a canonical reading of the Cenozoic fossil record suggests a dramatic rise in taxonomic diversity that can be interpreted as an explosion of morphological variety. We investigated this apparent discrepancy by using a discrete-character-based, empirical diatom morphospace, resolved by molecular phylogeny and by fossil occurrences through time. The morphospace shows little correspondence to phylogeny and little Cenozoic change in disparity as measured by mean pairwise distance. There is, however, an increase in the total volume of morphospace occupied. Although the increase in occupied volume through time ostensibly supports a conclusion of increasing morphological variety, sampling biases and other data suggest an underlying stationary pattern more consistent with molecular clock data.

Although much natural historical evidence has been adduced in support of the occurrence of several mass extinctions during the Phanerozoic, unambiguous statistical confirmation of the mass extinction phenomenon has remained elusive. Using bootstrapping techniques that have not previously been applied to the study of mass extinction, we have amassed strong or very strong statistical evidence for mass extinctions (see text for definitions) during the Late Ordovician, Late Permian, and Late Cretaceous. Bootstrapping therefore verifies three of the mass extinction events that were proposed by Raup and Sepkoski (1982). A small amount of bootstrapping evidence is also presented for mass extinctions in the Induan (Triassic) and Coniacean (Cretaceous) Stages, but high overall turnover rates (including high origination) in the Induan and uncertain estimates of the temporal duration of the Coniacean force us to conclude that the evidence is not compelling.

We also present the results of more liberal X2 tests of the differences between expected and observed numbers of familial extinctions for stratigraphic stages. In addition to verifying the mass extinctions identified using bootstrapping, these analyses suggest that several stages that could not be verified as mass extinction stages using bootstrapping (including the last three in the Devonian, and the Norian Stage of the Triassic) should still be regarded as candidates for mass extinction. Further analysis will be required to test these stages in more detail.

The silicoflagellates are a class of enigmatic chrysophytes characterized by netlike skeletons composed of opaline silica. Other major groups of siliceous plankton—the diatoms and radiolarians—exhibit evidence of decreasing size or silicification over the Cenozoic. We investigated trends in the silicoflagellate fossil record by constructing a species-level database of diversity and morphological metrics. This new database reveals a proliferation of silicoflagellate species with spined skeletons along with an increase in the mean number of spines per species over the Cenozoic. Although there is little change in skeleton size or silicification among species with spines, those without spines are larger than species with spines and exhibit a decrease in size toward the present. Increased grazing pressure combined with declining surface silicate availability may have shifted the costs and benefits of silicification, causing divergent responses in skeletal morphology between these different morphological lineages of silicoflagellates over time. We postulate that diminishing Cenozoic surface silicic acid availability may have predisposed large spineless silicoflagellate species to extinction, whereas increased grazing pressure may have contributed to the extinction of all remaining spineless species within the edible size range of grazers.

The final 10 Myr of the Paleozoic saw two of the biggest biological crises in Earth history: the middlePermian extinction (often termed the Guadalupian–Lopingian extinction [GLE]) that was followed 7–8 Myr later by Earth's most catastrophic loss of diversity, the Permian–Triassic mass extinction (PTME). These crises are not only manifest as sharp decreases in biodiversity and—particularly for the PTME—total ecosystem collapse, but they also drove major changes in biological morphological characteristics such as the Lilliput effect. The evolution of test size among different clades of foraminifera during these two extinction events has been less studied. We analyzed a global database of foraminiferal test size (volume) including 20,226 specimens in 464 genera, 98 families, and 9 suborders from 632 publications. Our analyses reveal significant reductions in foraminiferal mean test size across the Guadalupian/Lopingian boundary (GLB) and the Permian/Triassic boundary (PTB), from 8.89 to 7.60 log10 μm3 (lg μm3) and from 7.25 to 5.82 lg μm3, respectively. The decline in test size across the GLB is a function of preferential extinction of genera exhibiting gigantism such as fusulinoidean fusulinids. Other clades show little change in size across the GLB. In contrast, all Lopingian suborders in our analysis (Fusulinina, Lagenina, Miliolina, and Textulariina) experienced a significant decrease in test size across the PTB, mainly due to size-biased extinction and within-lineage change. The PTME was clearly a major catastrophe that affected many groups simultaneously, and the GLE was more selective, perhaps hinting at a subtler, less extreme driver than the later PTME.

Periodicity has recently been reported in the extinction rates of fossil marine families since the Permian. The analysis used appears particularly sensitive to parameter estimation techniques, particularly in the definition of mass extinctions. It also fails to incorporate autocorrelation in the fossil record into its null hypothesis and rests on an inappropriate a posteriori comparison to the null hypothesis. An alternative analysis, examining the time-lags between periods of high extinction rates, produces no evidence of a cycle.

We have devised a simple model for assessing the role of development in shaping the evolution of morphological disparity. Disparity of a clade at any given time is expressed in terms of the developmental dynamics that lead to the variety of adult morphotypes observed. We use assumed phenotypic manifestations of developmental processes, as they could be detected from allometric characterizations, to distinguish a few, nonexclusive types of evolutionary changes in ontogeny. On the basis of this formalization, we describe the diversification of hypothetical clades, using the standard curve of adult morphological disparity, the curve of juvenile disparity, and the curve of allometric disparity, the latter quantifying the diversification of clades in allometric space. Contrasts of these curves reflect the underlying developmental scheme that drives temporal changes in disparity. We then vary the parameters of the model to assess the expected signature of each metric under specific conditions: changes in the relative frequencies of the types of evolutionary developmental changes, changes in the transition magnitude attached to each of them, and effects of temporal variation in average adult size on disparity curves and patterns of morphospace occupation. Results emphasize the potential contribution of these proxies for developmental dynamics—juvenile morphological disparity, allometric disparity, and average adult size—in enriching the interpretation of standard disparity curves and the description of clade histories, with possible process-oriented inferences.

The geometric regularity of oblique sculptures in pectinids and some other bivalves was examined both theoretically and empirically. Camptonectes microsculpture occurs exclusively on the external surface of foliated calcite, and is well characterized by an invariably orthogonal relation to growth increments and almost uniform density over the surface. It is genetically unrelated to radial and other sculpturing. According to our SEM observations, its divergent orientation seems to be controlled primarily by the growth of calcite laths. In contrast, the shagreen microsculpture of pectinids is characterized by a quincunx arrangement of scales on the diagonal network. Shagreen microsculpture can be regarded as a variant of diagonal sculpture together with the simple oblique, divaricate, V-shaped, and zigzag patterns that are widespread in other bivalves. The geometric regularity of these variations as well as their mutual relationship can be readily recognized by computer simulations. All of these diagonal sculptures may result from interference between radial and commarginal elements. The formation of diagonal sculptures seems to occur by the periodic activation of regularly arranged sculpture-producing cells on the mantle margin.