This summer's meeting season saw five gatherings devoted specifically to the topic of extinction: Chicago's Field Museum symposium in May (see Diamond 1983); a field conference to examine the Cretaceous-Tertiary transition in Montana; a symposium on molluscan extinction at the American Malacological Union meeting in Seattle; a special session at the annual meeting of the Society for the Study of Evolution in St. Louis; and a research conference on the campus of Northern Arizona University in Flagstaff (see also Lewin 1983a,b,c). Several recent books consider the topic (Ehrlich and Ehrlich 1981; Nitecki 1981; Silver and Schultz 1982; Martin and Klein, 1984) and plans are underway to publish the papers presented at the Field Museum, Seattle, and Flagstaff meetings. Our review of the topic is based principally on the themes explored at the St. Louis and Flagstaff meetings.

Time is so fundamental to the everyday thinking of paleobiologists and geologists that it is seldom given close critical attention. Many of the currently debated issues in evolutionary history—catastrophic extinctions and punctuated vs. gradual morphological change, for instance—include assumptions about time and rate which are seldom made explicit. Methods now exist for calibrating evolutionary patterns through estimates of time components in the fossil and sedimentological records. There is a growing realization that increased precision in defining time frameworks can greatly clarify evolutionary problems. This has helped to stimulate renewed interest in the traditional topic of time and how it is represented in the biological and geological record.

The Precambrian-Cambrian Boundary traditionally has been viewed as a fundamental discontinuity in the historical record of life. Almost from the birth of stratigraphic geology, it was recognized that the richly fossiliferous strata of Cambrian and younger periods were underlain by sedimentary rocks largely devoid of macroscopic fossils. Darwin (1872) was well aware of this dichotomy and of the “sudden manner in which several groups of species first appear” in the rock record. In the sixth edition of the Origin of Species (1872), he wrote, “The case [for this sudden appearance] at present must remain inexplicable … and may be truly urged as a valid argument against the views [on evolution] here entertained.” Careful biostratigraphic work by Walcott and others in the closing decades of the nineteenth century accentuated rather than dispelled the problem. Diverse Early Cambrian invertebrate assemblages were documented from North America and Scandinavia, but repeated searches for comparable fossils in Precambrian strata produced only a few, enigmatic remains (see review by Yochelson [1979]).

From a geological point of view, life is an integrated part of the exogenic cycle, a mere elaboration of the physical and chemical processes operating on earth. The origin of life marks the transition from a physical and chemical world into one where physical, chemical, and biological processes form an integrated continuum. Life is at work in a big way, and one may regard the biosphere as a laminar, highly activated global envelope, energized by solar radiation, modeling the terrestrial physiognomy, and catalyzing major geochemical reactions. There can be no doubt that the biota have exerted a profound influence on the development of our planet. The history of life and the earth is one of coevolution (Dubos 1979).

For paleobiologists interested in extinct carnivorous reptiles, publication of Walter Auffenberg's (1981) 13–month field study of the Komodo dragon (Varanus komodoensis), or ora, as he prefers to call it, provides a useful portrait of a reptile as the top predator of a terrestrial community. A great deal of information about the ora's habitat utilization, activity patterns, diet, social behavior, and reproduction is presented.

I. Many believe that sooner or later molecular biology will mechanistically provide the precise genotypic basis for phenotypic change. This in turn will provide the basis for attacking several major evolutionary problems posed by paleontology, including the following:

1. — There are approximately 30 phylum-level taxa in the animal kingdom, and yet in the “fast” version of evolutionary history, many of then arose 600 Myr ago in a very short period of time (of order <30 Myr). In terms of basic body plan and inferred ecology, these sponges, clams, arthropods, etc. were seemingly as different then as they are now. It is as though the rate of morphologic change was much more rapid during that interval than during the nearly 600 Myr since then. Even the “slow” version of evolutionary history (which allows for an additional unfossilized record of about 500 Myr) demands an answer to what has controlled the development of major evolutionary Bauplans.

2. — Some morphologic traits (though probably not individual species) persist for immense periods of geologic time—the general shape of the horseshoe crab, for example, or the bristle patterns of taxa of meiofaunal “worms” living among sand grains. What is the developmental rationale for these persistent traits? In short, what is the molecular basis for genetic and morphologic canalization?

3. — The general pattern of species specialization proceeds unabated overtime, but every once in a while the developmental clock is reset. Organisms which are sexually mature but with simplified or “primitive” morphology arise de novo, and these lineages then form the stock of unspecialized forms from which future specializations evolve. What is the molecular basis for resetting the developmental clock?

4. — Reproductive isolation may arise “accidentally” through genomic changes unrelated to changes in the local environment. To the extent this is true, speciation has a largely accidental component. Knowledge of the rate and type of genomic change is critical for an evaluation of this idea.

At present there are many animal phyla that contain only a few species. The existence of these small phyla can be used to test assumptions about speciation and extinction in multicellular animals.

We first model the number of species in a monophyletic clade with a birth and death process that assumes rates of speciation and extinction are constant. If no new phyla have evolved since the Cambrian and speciation and extinction rates for minor phyla are similar to or greater than those estimated from fossils, then our model shows that the probabilities of minor phyla surviving to the present are small. Random variation in extinction and speciation rates does not improve the chances for persistence. If speciation rates exceed extinction rates at the initial radiation of the clade, but before the clade becomes large, speciation rates come to equal extinction rates and both are low, persistence from before the Ordovician up to the present becomes likely. These models show that if speciation and extinction rates are independent of the number of species in a clade, then conditions before the Ordovician strongly influence today's distribution of species among taxa.

We also discuss a model in which speciation and extinction rates depend on the number of species in a clade. This alternative model can account for the persistence of phyla with few species to the present and predicts a short duration for phyla that did not exceed a threshold number of species.

In earlier and simpler times in North American geology, there was an extraordinarily large amount of unexplored ground per available geologist to study it. State and territorial geological surveys, which undertook the first comprehensive mapping programs in the nineteenth century, were at first able to make only the grossest descriptions of outcrops in order to complete their assignments to map an area. In these pioneer surveys, black shales stood out as very easily identifiable strata; characteristically colored, fine grained, with distinctive lamination, and well known to lack fossils found so commonly in adjacent strata. “Black shale, undivided” is a very familiar legend in older outcrop descriptions; the lack of comparability of black shales to other strata effectively prevented their division, for both conceptual and technical reasons. This is one of the many paradoxes we can recognize in relation to black shales; they are as difficult to interpret as they are easy to recognize.

The most popular theoretical propositions are often those which spell the final doom to a notion that has already fallen into disrepute. This is as true today as it was when August Weismann (1892) published his seminal volume, Der Keimplasma, which far more than Cuvier's vituperative attacks, put the ideas of Lamarck to final rest.

The commonly observed fact that the principal biologic groups appear early in an adaptive radiation is shown to be an artifact of the geometry of the evolutionary tree. For marine invertebrates of the Phanerozoic, total diversity and species turnover rates combine to make early origins of major groups inevitable. This is demonstrated through an analysis of the theoretical probability distribution of divergence time (time to most recent common ancestor) in comparison with observed distributions. Whereas the fact of early origins does not require ecological explanations, the interpretation of the morphologic distinctness of major groups when they first appear remains an ecological problem.

The concept of punctuated equilibria was introduced by Eldredge and Gould (1972) as an interpretation of the sequential occurrence of species as actually observed in the fossil record. The pattern of sudden appearance of a new species followed by little or no morphological change during the remainder of the species' existence contrasts with phyletic gradualism, the pattern of slower, more evenly distributed change which was long considered to be the principal mode of species formation (Simpson 1944). The question of which is the dominant pattern of evolutionary change has continued as an actively debated theme in much of the paleontological literature of the past decade.

Rancho La Brea is unquestionably the most famous fossil deposit in North America, and its fauna occupies a key position in vertebrate paleontology as the basis for the Rancholabrean Land Mammal Age. The 23 acres now known as Hancock Park yielded the bulk of fossil specimens from this locality. A minimum of 518 taxa have been identified to date including 79 algae (all but one of which are diatoms), 72 vascular plants, 9 nonmarine mollusks (13 taxa of marine mollusks were carried to the site by early Indians), 135 arthropods, 27 lower vertebrates, 138 birds, and 58 mammals. A detailed list of the biota, too lengthy for inclusion here, is available from the authors on request.

The punctuated equilibrium hypothesis, largely derivable from Mayr's peripheral isolate model of speciation, is but one of many possible paleontologic patterns of speciation which may be tested against the deep-sea fossil record of planktonic microfossils. The nature of reproduction and variation within a species has a profound effect on its expected tempo and mode of evolutionary change. Biologic data on pelagic plankton and on protistan genetics and reproduction suggest that speciation in pelagic holoplanktonic protists may also be parapatric, “equal” allopatric, or the result of hybridization. Each of these models is shown to make testable predictions of paleontologic pattern. Published records of speciation in planktonic deep-sea microfossil data are compatible with these alternative models. Existing data sets are not yet sufficiently complete to provide strong tests. Future work needs to integrate taxonomic, paleoecologic, biogeographic, and biometric data in studies of individual speciation events. Micropaleontologic studies of morphologic change through time may thereby contribute to our understanding of speciation mechanisms in the pelagic protistan plankton.

The geometry of the simpler types of graptolite stipes can be quantified by assuming them to consist of stacked cylinders representing the thecal tubes. It can be shown that specification of any three characters, for example, thecal inclination, thecal spacing, and stipe width, effectively defines the others (thecal overlap, thecal length and width, thecal “density”); hence the parameters commonly used in the definition of species are not independent. Variation within and between species can be represented by three-character plots. The effects of altering the apertural angle from 90° on such characters as thecal spacing can also be represented geometrically. If thecae are considered as cones rather than cylinders, it follows that as stipe width increases, thecal curvature will describe a sine curve if the thecae are to remain in contact; this observation is matched on real graptolites. Thecal curvature beyond 90° to the dorsal wall is generally impossible without thecae detaching from their neighbors to produce thecal isolation. Growth of thecae along a stipe can be represented graphically. Computer fits to growth curves for five dichograptids show that they can be closely described by an exponential decrease in growth increments along the stipe, and that this model is more accurate than two possible alternatives.

An analysis of the structure and kinematics of the forelimbs and hindlimbs of pterosaurs, and functional analogy with recent and fossil vertebrates, supports a reappraisal of the locomotory abilities of pterosaurs. A hypothesis of structural, aerodynamic, and evolutionary differences distinguishing vertebrate gliders from fliers is proposed; pterosaurs fit all the criteria of fliers but none pertaining to gliders. The kinematics of the reconstructed pterosaur flight stroke reveal a down-and-forward component found also in birds and bats; structural features of the shoulder girdle and sternum unique to pterosaurs may be explained in light of this motion. The recovery stroke of flight was accomplished, in birdlike fashion, by a functional reversal of the action of the M. supracoracoideus by the pronounced enlargement of the acrocoracoid process, which acted as a pulley. The wing membrane was supported and controlled through a system of stiffened, intercalated fibers, which were oriented like the main structural elements in the wings of birds and bats.

The hindlimbs of pterosaurs were independent of the wing membrane, and articulated like those of other advanced archosaurs and birds, not like those of bats. The gait was parasagittal and the stance digitigrade. Because of limitations on the motion of the forelimb at the shoulder, pterosaurs could not have walked quadrupedally. However, bipedal locomotion appears to have been normal and quite sufficient in all pterosaurs. There is nothing batlike about pterosaur anatomy; on the other hand, pterosaurs bear close structural resemblances to birds and dinosaurs, to which they are most closely related phylogenetically.

Historical biogeographers are divided over the major process whereby extant species, or their ancestors, arrived in their present geographical locations. One faction promotes the idea of discrete “centers” of evolutionary radiation, from which species have been supplied to other areas by long-distance dispersal. A second faction advances the notion of widespread ancestral biotas, which have been fractionated into descendant biotas over time (vicariance). A seemingly endless polemic has elevated these rather simple alternatives to causes célèbres, and many workers have responded by dismissing virtually the entire vicariance literature (see Pielou 1981). We believe that the important distinction between vicariance and center-of-origin/long-distance-dispersal can be made clear to those interested in questions of paleobiogeography. In our opinion, the recent contribution by Briggs (1981) in this journal did not do so, and we now take the opportunity to try.

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.

Macerations of fragmented plant compressions of Silurian (Llandoverian) age yield sheets of organic material bearing ridges and depressions (interpreted as corresponding to surficial dimensions of cells), fragments of smooth walled tubular elements, and fragments of tubular elements with differentially thickened walls (“banded tubes”). A fragment of tissue (1.2 mm long), consisting of smooth walled tubes (17 ± 6.9 μm in diameter), surrounding 2–3 larger (18.8 ± 2.1 μm in diameter) banded tubes, was isolated from an irregularly shaped, relatively large (1 × 3 mm) compression. Comparisons between (1) fragments of tubular cell types, not organized into strands, isolated from 122 compressions, and (2) dispersed tubular cell types previously reported from the Massanutten Formation and other Silurian formations (Tuscarora and Clinton), reveal no significant morphologic differences. Comparisons between the organization of smooth-walled and banded tubular cell types found in the tissue strand and the organization of cell types in nematophytic plants (Nematothallus, Nematoplexus, Prototaxites) indicate a similarity in construction (tubular) but a lack of correspondence in organization. The strand of tissue is interpreted as representing part of the internal anatomy of a nonvascular land plant of unknown taxonomic affinity. On the basis of analogy with present-day embryophytes, the strand of tubular cell types is inferred to have functioned as a conductive tissue. The significance of “banded tubes” in Silurian strata is discussed, and it is concluded that, until more is known about the anatomy, morphology, and biochemistry of the parent plant(s), the habitat and systematic affinity of these organisms are conjectural.

Stratigraphic range data are used to derive time scales on which taxonomic survivorship curves for genera and families are as nearly as possible independent of their times of origin. These time scales correct for temporal variations in overall extinction rates caused by major extinctions and the pull of the Recent. Survivorship curves for genera and families on their respective time scales are well fit by Pareto distributions differing only in their scale parameter.

Many cheilostome bryozoans of diverse phylogenetic origin grow as erect, arborescent colonies with branches of modified planar form composed of two layers of zooids back to back. Regular branching enables a growing colony to expand in surface area, and hence in the number of zooids that feed, reproduce, and perform other vital functions, at an accelerating rate. During growth, branches first all diverge, then increasingly converge, and in late stages of growth begin to interfere with each other's growth and function. Interference can set limits to the width and thickness of branches and hence to the number and size of zooids.

Simulation of growth using a 3–dimensional mathematical model shows that a narrow range of possible values of branching angles minimizes branch interference in late growth stages. These values are prevalent in fossil and modern species. Branch spacing at later growth stages is correlated with the distance between branches at first crossing, providing room for feeding organs of the two facing layers of zooids to protrude and function. Interbranch distances dwindle as branches increasingly converge, so emphasis on minimizing interference at a late stage sets a practical limit to growth beyond that stage. To gain this long-term benefit requires adhering to a regular pattern throughout growth. The considerable variation in branching properties in fossil and modern species, and a variability in spacing inherent in the growth pattern itself, limit the amount of usable interbranch space. Despite a higher intraspecific variability, branching properties are as distinctive interspecifically as zooidal properties, and variability is randomly distributed through the colony. A small reduction in variability between fossil and modern species suggests that increasing regularity may provide a selective advantage in the utilization of interbranch space.