Showing papers in "Paleobiology in 1977"

Punctuated equilibria; the tempo and mode of evolution reconsidered

Abstract: We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade—speciation may be random with respect to the direction of a trend (Wright's rule).As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation—not phyletic evolution—must be the dominant mode of evolution.We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation.Phyletic gradualism was an a priori assertion from the start—it was never “seen” in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an “unnecessary difficulty.” We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.

The Mesozoic marine revolution: evidence from snails, predators and grazers

Abstract: Tertiary and Recent marine gastropods include in their ranks a complement of mechanically sturdy forms unknown in earlier epochs. Open coiling, planispiral coiling, and umbilici detract from shell sturdiness, and were commoner among Paleozoic and Early Meso- zoic gastropods than among younger forms. Strong external sculpture, narrow elongate aper- tures, and apertural dentition promote resistance to crushing predation and are primarily as- sociated with post-Jurassic mesogastropods, neogastropods, and neritaceans. The ability to remodel the interior of the shell, developed primarily in gastropods with a non-nacreous shell structure, has contributed greatly to the acquisition of these antipredatory features. The substantial increase of snail-shell sturdiness beginning in the Early Cretaceous has accompanied, and was perhaps in response to, the evolution of powerful, relatively small, shell-destroying predators such as teleosts, stomatopods, and decapod crustaceans. A simul- taneous intensification of grazing, also involving skeletal destruction, brought with it other fundamental changes in benthic community structure in the Late Mesozoic, including a trend toward infaunalization and the disappearance or environmental restriction of sessile animals which cannot reattach once they are dislodged. The rise and diversification of angiosperms and the animals dependent on them for food coincides with these and other Mesozoic events in the marine benthos and plankton. The new predators and prey which evolved in conjunction with the Mesozoic reorganiza- tion persisted through episodes of extinction and biological crisis. Possibly, continental breakup and the wide extent of climatic belts during the Late Mesozoic contributed to the conditions favorable to the evolution of skeleton-destroying consumers. This tendency may have been exaggerated by an increase in shelled food supply resulting from the occupation of new adaptive zones by infaunal bivalves and by shell-inhabiting hermit crabs. Marine communities have not remained in equilibrium over their entire geological history. Biotic revolutions made certain modes of life obsolete and resulted in other adaptive zones becoming newly occupied.

Species richness in marine benthic habitats through the Phanerozoic

Abstract: The distribution of numbers of species and the median number of species from 386 selected fossil communities are tabulated for high stress, variable nearshore, and open marine environments during the Lower, Middle, and Upper Paleozoic, the Mesozoic and the Cenozoic. The number of species always increases from high stress to variable nearshore to open marine environments. Within-habitat variation in number of species is small for long intervals of the Phanerozoic. The median number of species in communities from high stress environments remains fixed at about 8 from the Cambrian to the Pleistocene. In open marine environments, the median is near 30 for the Middle and Upper Paleozoic and almost the same for the Mesozoic. Increases of 50% in median number of species between the Lower and Middle Paleozoic and 2 times between the Mesozoic and Cenozoic occur in open marine en- vironments with parallel, but less pronounced, increases in variable nearshore environments. Conditions controlling overall within-habitat species richness changed at those times. These changes do not correlate directly with evolution of new major taxa, change in physical con- ditions, predation, space availability or oxygen supply. They may be related to changes in resource availability influenced by factors such as the developing terrestrial flora, to lag-time inherent in the evolutionary process of diversification, or to as yet undetermined factors. Al- though provinciality determines total species richness for the biosphere, the within-habitat data suggest that the number of marine invertebrate species in the world has increased since the Middle Paleozoic, contrary to Raup's (1976b) contention, but possibly only by about 4 times, not the order of magnitude or more suggested by Valentine (1970).

The shape of evolution: a comparison of real and random clades

Abstract: The history of life is replete with apparent order. Much of this order may reflect the deterministic causes conventionally invoked, but we cannot be sure until we measure and subtract the order that arises in simple random systems. Consequently, we have constructed a random model that builds evolutionary trees by allowing lineages to branch and become extinct at equal probabilities. We proceed by dividing our simulated tree into clades and by comparing their sizes and shapes with the patterns exhibited by “real” clades as recorded by fossils.We regard the similarity of real and random clades as the outstanding result of this comparison. In both real and random systems, extinct clades arising after an “ecological barrel” had been filled have their maximum diversity at the midpoint of their duration; clades arising during the initial “filling” reach an earlier climax during this preequilibrial period of rapid diversification. However, some potential differences also emerge. Clades still living are much larger than extinct clades. We may attribute this to the morphological superiority of survivors, but we can also simulate it in a model that chooses the originators of clades at random. Real clades undergo greater fluctuations in diversity than do random clades, but the effect is not marked.

Adaptive radiation of the comatulid crinoids

Abstract: Modern crinoids are dominated by the comatulids (unstalked forms) which range from the intertidal to abyssal depths. Modern stalked crinoids are restricted to depths greater than about 100 m. In the geologic past some stalked crinoids lived at depths of a few tens of meters or less in reef and bank environments. The primary vehicles postulated for the post-Triassic radiation of comatulids are lack of permanent fixation to the substratum and the capacity for mobility. Development of complex muscular articulations has enabled crawling or swimming which serve in habitat selection and avoidance of stress and predators. These and other adaptations may have bestowed on comatulids a higher survival capacity in shallow-water environments compared to stalked crinoids. Modern stalked crinoids lack mobility and complex behavioral adaptations seen in comatulids. Possibly, stalked crinoids in shallow water were unable to cope with the radiation of abundant, predaceous bony fishes in the late Mesozoic and became restricted to greater depths while the more adaptable comatulids gained ascendancy in shallow water.

The poor fossil record of the regular echinoid

Abstract: The fossil record of echinoids is poor because soon after death they break apart into isolated plates. In the present seas regular echinoid species outnumber irregular species; whereas, in the Tertiary only 20% of the known echinoid species are regular. This suggests that regular echinoids are less likely to be preserved than irregular echinoids. The tests of regu- lar echinoids are exposed to scavengers and currents upon their death, but irregular echinoids generally live buried in the sediment and are protected from these destructive forces. Further- more, the tests of the regular echinoids lack the calcareous supports found in some irregular echinoids. The gut is not filled with sediment and its apical system is generally larger and more fragile. Finally, many regular echinoids live in environments less likely to be preserved in the sedimentary record. Although the irregular echinoid is more likely to be fossilized, its record is poor during some periods in the past. Although 1,014 irregular echinoid species are known from the Eocene, only 83 species are known from the Paleocene and only 343 are known from the Oligocene. Is this reduction because fewer species lived then or because they have not been preserved?

Jurassic bivalve biogeography

Abstract: An analysis of the geographic and stratigraphic distribution of nearly 200 Jurassic bivalve genera leads to a number of new discoveries. Similarities between regions reached a maximum in the middle of the period, while the percentage of endemism correspondingly decreased. Diversity increased through the Lower Jurassic to a level which remained more or less stable from Middle Jurassic times onwards, while the origination rate shows the opposite trend. Extinction rate increased early in the period to a maximum in the Pliensbachian and fell thereafter to a low value until the Tithonian, which is marked by a sharp rise. The overall taxonomic composition of the fauna in terms of orders remained substantially stable throughout the period. The relationship with facies is discussed and three major ecological groups distinguished: marginal marine (euryhaline), shallow neritic and deep neritic. Certain pterioids have a very wide distribution and the order as a whole has a significantly higher proportion of cosmopolitan to endemic genera than any other order; the hippuritoids and trigonioids have the highest proportion of endemics. Five faunal provinces are distinguished, and the dominant control on distribution considered to be sea level. Times of high sea level were marked by widespread distribution of taxa and low endemism. High extinction rates were provoked both by regression (in the Tithonian) and by a sharp rise of sea level in the Toarcian, marked by the widespread onset of anaerobic or near-anaerobic conditions in many epicontinental seas. Some latitudinal control is recognised, notably for the hippuritoids and other stenotopic thick-shelled genera, which are confined to low latitudes.

Butchering and marrow fracturing as a taphonomic factor in archaeological deposits

Abstract: A major aim of the analysis of faunal remains from archaeological sites is the reconstruction of the palaeoecology of early man. Since taphonomic work must precede palaeoecological work, theories concerning the palaeoecology of early man would benefit from firmly based taphonomic studies of the archaeological deposits. One of the numerous taphonomic factors to be considered is the bone fragmentation produced by early man.The degree of bone fragmentation in a deposit uninfluenced by human factors is an expression of the resistance of the various bones to mechanical and chemical decomposition before and during burial. Thus, the degree of fragmentation where human factors are predominant should provide an illustration of man as a taphonomic factor. In both cases the degree of fragmentation may be used as an indicator of the degree of taphonomic overprint.Examination of bone material from four West European Mesolithic sites (all bog deposits), Star Carr 7200 ± 120 B.C., Kongemosen 6600 ± 100 B.C., Praestelyngen 3200 ± 100 B.C., and Muldbjerg I 2900 ± 80 B.C., reveals differences in marrow fracturing techniques. Furthermore, the different techniques result in different numbers of fragments for the same type of bone. Comparison of faunas from different sites based on the number of bone fragments must therefore be preceded by analysis of the marrow fracturing techniques used at the various sites. The number of fragments per estimated minimum number of individuals should indicate the degree of taphonomic loss.The various types of marrow fracturing found seem to belong to cultures at different levels of technical development. Thus, it may be possible within a limited area such as northern Europe to establish a chronology using marrow fracturing type as one of the ecostratigraphic tools.

Some "laws" of gastropod shell form

Abstract: Five generalizations derived from the shell form of prosobranch gastropods are developed. (1) A univalve of more than one volution whose aperture lies in a plane that passes through the axis of coiling does not live with the aperture parallel to the substrate. (2) A univalve of more than one volution whose aperture lies in a plane that is tangential to the body whorl does live with the plane of the aperture parallel to the substrate. (3) Gastropods with tangential apertures, when extended, support the shell so that the center of mass of the shell and its contents is over the midline of the cephalopedal mass; this balancing of the shell may be accomplished either by regulatory detorsion, by inclination or by a combination thereof, to keep the center of gravity of the shell as low as possible. (4) Angulations or re-entrants in the gastropod aperture are usually indicative of inhalent or exhalent areas; inhalent areas are directed as far anteriorly as possible. (5) Gastropods having elongated apertures possess only a single gill and develop a water current through the mantle cavity from anterior to posterior along the long axis of the aperture; this axis is subparallel to the anterior-posterior axis of the foot.These generalizations are then used as the basis for some deductive interpretations of behavioral modes of Paleozoic Gastropoda.

Brachiopod orientation to water movement; 1, Theory, laboratory behavior, and field orientations

Abstract: Hydrodynamic principles and experiments with empty shells predict that pres- sure distributions around brachiopod shells generated by ambient currents should, depending on the orientation of the shell relative to the current, either augment or oppose the ciliary-driven flow of water through the lophophore. For living articulate brachiopods with plectolophes or spirolophes, orientations where the anterior-posterior axis of the shell is parallel to the cur- rent direction should result in pressure distributions which oppose active pumping. This ef- fect should be strongest when the excurrent region of the shell faces into the current. Orienta- tions where the anterior-posterior axis is perpendicular to the current direction should result in pressure distributions which act in concert with active pumping, most strongly when one of the incurrent regions is directed into the current. These effects are independent of specific shell shape. (2) Laqueus californianus and Terebratulina unguicula actively reorient to currents in the laboratory, preferring orientations where the anterior-posterior axis of the shell is perpendicu- lar to the current and the right-left axis is parallel to the current. Both species may tra- verse an arc as great as 1200 to achieve their final orientation. Hemithyris psittacea also will actively reorient to currents, moving towards orientations where the anterior-posterior axis is perpendicular to the current. The maximum rotation observed for H. psittacea was 450. Terebratalia transversa never reoriented in the laboratory. (3) Using epifaunal hydroid colonies as indicators of current direction, both Hemithyris psittacea and Terebratalia transversa are oriented in nature with the anterior-posterior axis of the shell perpendicular to the prevailing currents. While scuba diving, I confirmed this orientation phenomenon for T. transversa by direct measurement of the orientation of the brachiopods relative to prevailing currents. (4) Larval Terebratalia transversa avoid areas with current speeds greater than about 0.2 cm/s during metamorphosis and show no orientation to the ambient currents immediately after metamorphosis. Post-metamorphic T. transversa can actively reorient on the pedicle. The orientation observed in adults is probably achieved by active reorientation to local cur- rents of post-zygolophe juveniles. (5) Threshold current speeds for reorientation in Laqueus californianus and Terebratulina unguicula are low and approximately equal to the excurrent pumping speeds of each species; dynamic pressure rather than viscous entrainment is probably the relevant factor determining reorientation behavior.

Brains of early carnivores

Abstract: It is commonly believed that the brains of the ancestors of modern carnivores (miacids) were superior to (e.g., larger than) those of other early carnivores (creodonts and mesonychids). Examination of the fossil record of brains of early carnivores reveals no evidence to support that belief. Moreover, evolutionary trends towards increasing relative brain size and an expansion of neocortex are seen in both miacids and creodonts. The neocortex expanded in a different way in miacids than in creodonts and mesonychids (evidenced by different sulcal patterns), but the biological significance of the observed differences is unknown.

Recruitment, growth, and mortality of a living articulate brachiopod, with implications for the interpretation of survivorship curves

Abstract: An intertidal sample of 118 to 155 Terebratalia transversa on Saltspring Island, B.C., was censused in 1974, 1975, and 1976. Growth in length was a maximum of 7.8 mm per year and declined with size (age). As in fossil articulates, mortality rate was independent of age (size). Recruitment was patchy in time and space, was multi-annual or continuous, and was concentrated near conspecific adults. There were as many as 800 individuals/m 2 . None moved or changed orientation. Morphologic variation of Terebratalia valves is not directly controlled by the intensity of waves or currents. Paleontological survivorship curves based on size-frequency data are subject to ambiguous interpretation because two critical assumptions (direct relationship of size and age, constant population structure) are difficult to justify.

Notes on animal weight, cameral fluids, swimming speed, and color polymorphism of the cephalopod Nautilus pompilius in the Fiji Islands

Abstract: Forty-six specimens of Nautilus pompilius Linnaeus were captured in depths varying between 100 and 500 m outside of the fringing reef near Suva, Fiji Islands. Thirty- eight of the specimens were male. Air weight per individual varied between 347 and 630 g. Sexual dimorphism in size is indicated, since mature shell modifications (approximated septa, blackened aperture) were present in two females weighing about 350 g (soft parts plus shell) and one weighing slightly over 400 g; the smallest male showing mature shell mod- ifications weighed 496 g. All newly captured specimens were heavier than seawater, with mean weight in seawater of 1.87 g determined for twenty-five specimens. Total volumes of cameral liquid ranged between 13.5 and 0 ml. Thirteen of twenty-five sampled specimens showed less than 1.0 ml of cameral liquid from all chambers. Average cameral liquid os- molarity was lower than that observed in sampled populations of N. macromphalus from New Caledonia and N. pompilius from the Philippine Islands. Maximum swimming rates were 0.25 m/sec. N. pompilius exhibits two common color polymorphs.

Form and function of orthoconic cephalopod shells with concave septa

Abstract: Models of "ideal" orthoconic shells having simple concave septa with minimal weight and maximal strength and analysis of 72 species of fossil orthocones and cyrtocones yield important insights into the physical principles underlying cephalopod shell design. The ideal septum is a spherical cap weighing only 77% of a hemispherical septum of equal strength. The septa of most longicones approximate this ideal shape while those of brevicones are less curved, probably owing to buoyancy problems. Increase in septal strength leads to weight increase unless the shell becomes more logiconic or septal spacing increases or both. However, increased spacing requires more cameral liquid for septum formation, thus reducing buoyancy. In ideal longicones, septal spacing resembles the cone radius for thick, strong septa but de- clines to half of the cone radius for thin, weak septa. In ideal intermediates and brevicones, spacings are respectively reduced by factors of about 2 and 4, with similar additional de- pendence on septal thickness. Most real septa resemble these ideal models. The relative length of the body chamber to the phragmocone varies greatly between about 0.2 and 1.5, depending mainly on the wall thickness and to a lesser degree on the septal thickness, apical angle and body density. Removal of cameral liquid in the adult must be compensated for by additional growth to retain neutral buoyancy. The conditions for neutral equilibrium calculated for longicones with different "counterweights" indicate that: (1) cameral liquid only is least feasible; (2) half-and-half calcium carbonate and liquid results in one-third length and one-quarter volume reduction of the body chamber; (3) with calcium carbonate only, body chamber reduction is minimal. Real 'counterweights' appear to be in- termediate between (2) and (3), providing the animal with horizontal stability, which is missing in (3). Most uncalcified siphuncles reduce the body chamber only slightly although they improve horizontal stability. If the wall attains full thickness only at the apical end of body chamber, the liquid-only 'counterweight' becomes feasible.

Functional significance of spines in the Pennsylvanian horseshoe crab Euproops danae

Abstract: Euproops danae, a xiphosuran from the Mazon Creek (Braidwood) Fauna, had two sets of spines—genal and ophthalmic—extending from the posterior margin of its prosoma. In young individuals, the genal spines were longer than the ophthalmic spines. During growth, the relative length of each type of spine increased, but the rate of increase was much greater for ophthalmic spines. In order to explain these morphological and ontogenetic features, I have studied the hydrodynamic behavior of whole-animal models representing the actual morphology of E. danae, and various modifications thereof, at different body sizes. It can be argued that passive settling, while in an enrolled posture, was probably the primary defensive reaction that a swimming individual would have exhibited in response to an encounter with a potential predator. Experiments show that the array of spines on E. danae was an important control on the style of settling experienced by an enrolled individual. Models of the actual morphology settle steadily, while models with either longer or shorter prosomal spines tend to undergo lateral oscillations induced by turbulence in the wake. Steady fall would have rendered the horseshoe crab less perceptible to either the visual or lateral line systems of contemporary aquatic vertebrates, and thus a morphology capable of producing it would have been an important adaptation for reducing the risk of predation. This minimization of oscillatory movement would actually obtain for a variety of conceivable spine morphologies, but actual morphologies represent those ‘solutions’ which simultaneously optimize other aspects of spine function, such as mechanical protection. This optimum design changes during ontogeny because settling dynamics scale non-linearly with size. This work has both specific applications to the interpretation of similar structures in other arthropods and more general implications for the study of evolutionary functional morphology.

Biological interaction between fossil species; character displacement in Bermudian land snails

Abstract: Poecilozonites (Gastrelasmus) is an important component of the endemic land snail fauna of Pleistocene Bermuda. The type species P. circumfirmatus Redfield usually oc- curs in sympatry with its congener P. discrepans Pfeiffer, though each species is found alone at several localities. The species are less alike morphologically where they occur together than where they are allopatric. This allopatric convergence and sympatric divergence strongly suggests the biological interaction known as character displacement, often documented for living populations. The relatively complete fossil record of Bermuda offers advantages for studying this phenomenon. Collections can be made from a variety of microhabitats occupied through time. Statistical analysis of 1,600 individuals collected from more than 100 localities indicates that interspecific variation is primarily a function of the presence or absence of a congener and depends to a lesser degree on microhabitat. P. circumfirmatus undergoes a smaller morphological change between allopatry and sympatry than does P. discrepans. Study of relative abundances suggests that P. discrepans may have been competitively inferior, though no functional bases for differences between the species are known.

Genic heterozygosity and rate of speciation

Abstract: The hypothesis is proposed that mean level of heterozygosity is functionally re- lated to rate of speciation in evolutionary phylads. Under this hypothesis, phylads which speciate more rapidly do so because of increased level of within-species genetic variability which is then available to conversion to species differences under appropriate ecological or environmental conditions. An important corollary is that rate of speciation could be limited in phylads with low genetic variability, irrespective of environmental considerations. This hypothesis has been tested with respect to electrophoretically detectable variation in products of structural genes in two families of North American fishes characterized by grossly different rates of speciation. Totals of 69 species of the highly speciose Cyprinidae, and 19 species of the relatively depauperate Centrarchidae, were assayed for mean level of hetero- zygosity at 11-24 genetic loci. Since Cyprinidae and Centrarchidae exhibit on the average nearly identical levels of genic variation (H = 0.052 ? 0.004, and H = 0.049 + 0.009, respectively), the hypothesis that level of heterozygosity affects rate of speciation in these fishes is not supported. Nonetheless, the amount of genic variability in both Cyprinidae and Centrarchidae is large, comparable to mean levels in previously studied vertebrates. The great wealth of genome variability, reflected in the electrophoretic variation present in virtually all outcrossing organisms, apparently can accommodate considerable flexibility in rate and pattern of evo- lutionary response to the various environmental regimes challenging organisms.

Shell strength of Nautilus as a depth limiting factor

Abstract: Implosion of 47 epoxy-sealed shells of Nautilus pompilius indicates that very young shells (20-30 mm diameter at last septum) can withstand 136 atm pressure (= 1360 m equivalent depth), but intermediate and mature shells (30-145 mm diameter) implode at approximately 30-70 atm (= 300-700 m). Strain gages attached to the shells during implosion tests show that the flank and venter are compressed by external pressure, but the septum undergoes tension and structurally should be the most vulnerable to hydrostatic pressure. Overall shell -strength is substantially less than reported values of tensile strength (approximately 364 atm) and compressive strength (approximately 1561 atm) of the con- stituent shell material, nacre. Shell strength as a depth limiting factor appears to be con- trolled by shell and septal architecture and by stress concentrators such as flaws and microstructural inhomogeneity. The wide range in implosion values and lack of strong correlation between such parameters as septal thickness and implosion pressure indicate that determination of depth ranges for fossil cephalopods may be difficult.

Functional interpretation of lyre-shaped Bryozoa

Abstract: Colonies of Late Paleozoic lyre-shaped fenestrate bryozoans began growth on rigid substrates which were on the sea floor. After production of a basal disc, a fenestrate network developed in the shape of an erect bowed fan. The fenestrate fan was initially concave on the apertural side, but the curvature reversed a few millimeters above the base. Colonies were then separated along or just above the basal disc, and rested directly on the sea floor. The zooecial apertures were directed upward. The lateral margin of the fan grew to form the supporting struts of the soon-to-be lyre-shaped colony. An open vault formed below the upwardly bowed fan. Lyre-shaped colonies are interpreted to have lain on the sea floor so that each had the proximal end on the upcurrent side. Feeding zooids were on the upper surface and pulled water through their expanded tentacles. The filtered water passed through the fenestrules, into the vault, and out the open end of the vault, which was on the downcurrent side.

Community relations of Silurian crinoids at Dudley, England

Abstract: Silurian crinoids reached maximum abundance on shallow shelf bottoms of skeletal-rich carbonate. In-place crinoid preservation in the Wenlock Limestone of Dudley, England, provides a community model for these occurrences, showing a pattern among crinoids of high taxonomic diversity, conspecific clustering, relatively robust morphology, and numerical dominance of other invertebrates. Densities of about 40 crinoid individuals per m2 occupied a feeding level between 10 cm and 1 m above the bottom and may have effected trophic control over other fauna. Beneath the crinoids lived a second level of bryozoa, a third level of tabulates, and a sparse fauna of brachiopods and other solitary organisms. Crinoids also exercised significant biologic control of substrate. In contrast to the Dudley assemblage, Silurian communities of terrigenous, soft-bottom environments contained crinoids of low abundance, low taxonomic diversity, and small size. In the terrigenous communities, crinoids appear to have had no major effect on either trophic structure or substrate. High sedimentation rates limited the role of crinoids in such communities, and the dominant organisms were deposit feeders and solitary, low-level suspension feeders.

Formation and paleontologic recognition of structures caused by marine annelids

Abstract: Laboratory and field collected sediments were x-rayed to document the array of biogenic sedimentary structures produced by the burrowing and feeding behavior of six species of marine intertidal annelid (Glycera robusta, Nephtys caecoides, Pectinaria californi- ensis, Notomastus magnus, Eupolymnia crescentis, and Cirriformia spirabrancha). Polychaete burrows were found to vary greatly in structural complexity with both errant (N. nagnus) and relatively sessile forms (C. spirabrancha) producing a variety of biogenic structures. Sediment mixing by the tentacle-feeding polychaete C. spirabrancha was observed by se- quentially x-raying an experimental field enclosure stratified with an opaque substance. The experiment demonstrates that tentacle-feeding polychaetes can influence the topography of the sediment-water interface and transport substantial amounts of near surface material downward. Criteria by which fossil biogenic sedimentary structures, presumably produced by soft- bodied organisms, can be assigned a feeding function have been advanced by Walker (1972). Some of the assumptions inherent in feeding function analysis were applied, with varying degrees of success, to the biogenic structures of modem soft-sediment polychaetes.

A test for evolutionary equilibria: Phanerozoic brachiopods and Cenozoic mammals

Abstract: The fossil record of Phanerozoic brachiopod genera and Late Cenozoic New World mammal genera is examined for evidence of evolutionary equilibria One necessary (but insufficient) condition is met: within temporal intervals, numbers of originations correlate with numbers of extinctions Eliminating temporally short-ranging brachiopods, however, reduces the correlation so that it explains only 16% of the variation More decisive tests of the equilibrium hypothesis appear impossible with available data Difficulties of temporal and geographic scale, taxonomic level, and ecological consistency must be resolved before equilibrium models can be applied in paleontology for other than inspiration

Quantification of shape by use of Fourier analysis; the Mississippian blastoid genus Pentremites

Abstract: 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.

Phenetic variability and functional morphology of erect cheilostome bryozoans from the Danian (Palaeocene) of Denmark

Abstract: Fragments of cheilostome and cyclostome bryozoans are the most common fossils in the mound structures commonly found in the Lower Danian limestones of southern Scandinavia Sedimentological investigations and measurement of colony morphology combined with flume channel experiments indicate that: 1) The mounds were affected by unidirectional currents which created relatively high current velocities on the upstream flanks and summits, and low velocities on the downstream flanks and in the basins 2) Colonies with slender stems and thin walls dominated in areas with low current velocities and those with thick stems and thick walls dominated in areas with high velocities 3) The specific variability of the stem diameter was mainly controlled by the geometrical arrangement of the zooids In general, species with a bilamellar arrangement of the zooids show high variability, whereas species with a radial arrangement show small variability 4) Variation in the wall thickness is primarily related to the mode of growth of the individual zooids In the ascophorans deposition was confined to the external surface of the stems On the other hand, in the coilostegoid anascans the secondary skeletal material was deposited on all the internal surfaces of the zooids as well as on the external side of the cryptocyst Cuticles do not seem to have been present in the secondary thickened part of the frontal wall of either the coilostegoid anascans or the ascophorans 5) Calcareous material was deposited predominantly on the lower and older portions of the colony

Natural selection in a Cretaceous oyster

Abstract: A population of shells of the Cretaceous oyster Agerostrea mnesenterica was ob- tained from a single locality. The age at death for each shell could be determined, and the population was deemed to be essentially undisturbed and capable of providing a mortality record of the local population averaged over a span of time. We measured four morphometric characters which, on the basis of a functional morphologic model, could be expected to have adaptive value. The frequency distribution of each character was analyzed separately for the three year old and for the six year and older specimens in order to determine which indi- viduals died young and which survived to old age. Ontogenetic effects were separated from those of differential mortality by independent means. The results indicated that there was (1) directed selection for maximum curvature, (2) centripetal selection for a plica number of about 8, and (3) a net centripetal effect on both arc length and plica height, composed of selection against largest values coupled with a possible selection against smaller values that cannot be separated from ontogenetic effects. Only the selection for maximum curvature and optimal plica number can be reconciled with the predictions of the model.

On the measurement of size-independent morphological variability; an example using successive populations of a Devonian spiriferid brachiopod

Abstract: More than 500 undistorted, unfragmented pedicle valves of Ambocoelia umbonata (Conrad) (Spiriferida, Brachiopoda) were recovered from each of four levels within a Middle Devonian fossil cluster. The fossil cluster, an ellipsoidal shell accumulation measuring one meter in diameter and 2 cm in thickness, was exhumed from an exposure of the Ludlow- ville Shale (Hamilton Group) of western New York. Size frequency histograms indicate that the brachiopod experienced vely high levels of juvenile mortality, due, probably, to the effects of high bottom turbidity. Sedimentological and paleontological evidence indicates that the cluster represents a sequence of in situ benthic associations. Size-independent variation was estimated by calculating the eigenvalue of the minor axis on a log-transformed plot of pedicle valve lengths and widths. The eigenvalue technique eliminates the effect of allometrically induced shape changes and is applicable to multi- character analyses of morphological variability. Size-independent variability among the larger individuals of Ambocoelia decreases in suc- cessively younger cluster populations. The decrease is not correlated with any observed or inferred change in substratum, diversity, equitability or turbidity.

A comparison of diversity and trophic nuclei of live and dead molluscan faunas from the Essex chenier plain, England

Abstract: The rarefaction graphs and trophic nuclei of dead molluscan shell cumulates and their associated live molluscan fauna from seven ‘ecological’ habitats in the Essex Chenier Plain facies are examined and compared. The results show that while the trophic nuclei of the live and dead faunas tend to be dissimilar, the rarefaction graphs indicate that the dead fauna will be more diverse than the live, though changes in the diversity of the live fauna tend to be mirrored in the dead fauna.