Despite exhaustive investigation of present-day Nautilus, the phylogenetic relationships of the five or six recognized species within this genus remain unclear. Mitochondrial and nuclear DNA sequence data plus a suite of morphological characters are used to investigate phylogenetic relationships. Systematic analysis of the morphological variation fails to characterize described species as independent lineages. However, DNA sequence analysis indicates that there are three geographically distinct clades consisting of western Pacific, eastern Australian/Papua-New Guinean, and western Australian/Indonesian forms. The morphologically and genetically distinct species Nautilus scrobiculatus falls outside the three geographically recognized assemblages. Members of the genus Nautilus also exhibit low levels of sequence divergence. All these data suggest that Nautilus is currently undergoing diversification, which may have begun only several million years ago. These data also suggest that some of the morphological features used to define Nautilus species may simply represent fixed variations in isolated populations within the same species.

Bottom site remote camera photosequences at depths of 73–538 m on forereef slopes in Palau show that Nautilus belauensis is a highly mobile, chemosensitive, epibenthic scavenger and opportunistic predator. The overall depth range of this species is ca. 70–500 m, but photosequences indicate a preferred range of 150–300 m. Nautilus is active both nocturnally and diurnally, locating bait sites within 1–2 h. Associated macrofauna includes caridean shrimps, crabs, and eels; teleosts are rare below 100 m, but sharks are recorded in most photosequences below 250 m. Summarily, Nautilus exhibits a combination of characters that typify deep-sea strategy, including reproductive tactics, growth rate, and population dynamics. This and other evidence suggest that fossil Nautilidae may have been deep-water forms, in contrast to the typically shallower water ammonoids, and that Nautilus is a normal component of the deep forereef rather than a late Cretaceous refugee from shallow water.

The mass, wingspan, and wing area of pterosaurs were reconstructed. Mass was estimated by determining volume and multiplying by avian density. This method was considered appropriate only for smaller pterosaur species because there is evidence for lower density in larger species. These reconstructions were used to compare the wing shapes of Triassic and Jurassic pterosaurs with those of birds. Pterosaurs had wings of below-average loading and above-average aspect compared to the avian mean. This wing design was compatible with relatively slow and highly efficient flight, with high maneuverability. Wing area depends on the reconstruction model adopted; wings attached to the hindlimb principally reduced aspect, and secondarily reduced loading, which would improve take-off performance at the expense of efficiency. The wing shape and cranial feeding adaptations of pterosaurs were most compatible with a marine or aerial predatory adaptive zone. The reconstructed pterosaurs show a limited range of wing shape compared to birds. This may partly reflect preservational bias favoring species living in marine or lagoonal environments, but this is not a complete explanation because there is a lack of pterosaurs with wings of high loading like the marine ducks and auks. Structural, physiological, or adaptive factors may have limited pterosaur wing shape.

The extinct shark Carcharocles megalodon is one of the largest marine apex predators ever to exist. Nonetheless, little is known about its body-size variations through time and space. Here, we studied the body-size trends of C. megalodon through its temporal and geographic range to better understand its ecology and evolution. Given that this species was the last of the megatooth lineage, a group of species that shows a purported size increase through time, we hypothesized that C. megalodon also displayed this trend, increasing in size over time and reaching its largest size prior to extinction. We found that C. megalodon body-size distribution was left-skewed (suggesting a long-term selective pressure favoring larger individuals), and presented significant geographic variation (possibly as a result of the heterogeneous ecological constraints of this cosmopolitan species) over geologic time. Finally, we found that stasis was the general mode of size evolution of C. megalodon (i.e., no net changes over time), contrasting with the trends of the megatooth lineage and our hypothesis. Given that C. megalodon is a relatively long-lived species with a widely distributed fossil record, we further used this study system to provide a deep-time perspective to the understanding of the body-size trends of marine apex predators. For instance, our results suggest that (1) a selective pressure in predatory sharks for consuming a broader range of prey may favor larger individuals and produce left-skewed distributions on a geologic time scale; (2) body-size variations in cosmopolitan apex marine predators may depend on their interactions with geographically discrete communities; and (3) the inherent characteristics of shark species can produce stable sizes over geologic time, regardless of the size trends of their lineages.

By digitizing the enamel configuration and calculating the overall directional tendencies of the enamel edges in complex equid dental patterns, quantitative changes in functional attributes were traced through a phylogenetic series. Pronounced changes both in the directional emphasis of the enamel edges and in the chewing motion occurred and were interrelated. Two distinct trends were found. From early Eocene to Oligocene, edge alignment became increasingly confined to a single direction, but the teeth retained the phase I and phase II chewing angles characteristic of many primitive herbivorous and omnivorous mammals. The chewing direction in the horizontal plane also remained essentially unchanged. During the Oligocene and the early Miocene, both edge direction and chewing direction remained stable. However, the middle Miocene genus Merychippus represents a stage in which the enamel directional emphasis was lost in transition to a new anteroposterior emphasis, a trend that continued into the late Tertiary equine taxa. Concurrent with this transformation was a loss of all but vestiges of the phase I and II chewing angles in the vertical plane and a shift of the chewing direction in the horizontal plane to an almost transverse movement of the jaw.

In the Oligocene equids, the edges of the labial cusps did attain perpendicularity to the phase I chewing direction, which was oblique to the horizontal plane, but the chewing direction in the horizontal plane was far from perpendicular to the lingual enamel edge direction because the lingual cusps were configured for efficiency in compressive, not shearing, motion that dominated the phase II surfaces. In the later Tertiary the chewing direction and edges attained essentially a perpendicular relationship, which represents the theoretical optimum for a shearing system. The radical change in edge direction in the Miocene probably occurred as a result of an increase in translative movement (longer glide) between the surfaces, making the lingual edges subject to selection for perpendicularity and flattening of the occlusal surface, resulting in a condition where a single edge direction became optimal for all cusps. The extended chewing stroke translation was perhaps the key event initiating the other changes and would have resulted from increased chewing effort accompanying a shift to a diet of mainly grass. The loss of the dual (shearing and compressive) function of the occlusal surface suggests that the earlier diet may have contained significant proportions of other plant parts, such as fruits, seeds, or tender leaves that might be optimally chewed by compressive movements.

An investigation of shell morphometrics of the prosobranch genus Paramelania from Lake Tanganyika shows striking contrasts between some live and dead populations from identical localities. Living populations and surficial dead shells were collected from 15 sublittoral-profundal localities along the east side of the lake. Interpopulation variability in this endemic gastropod is clinal (N-S) for several shell characters. Intrapopulation variability of dead shell populations frequently exceeds that of live populations. Distinctive morphs may be present in a local dead shell population which are absent in the live population from the same locality. However, the phenodeviant dead shell morphology may occur among live snails elsewhere in the lake. Phenodeviant shells may be encrusted, but are unabraded and show no preferred orientation.

Radiocarbon ages of >2000 yr b.p. on dead phenodeviants suggest that surficial shell accumulations in Lake Tanganyika are strongly time-averaged. Although wave activity and biogenic concentration are important processes in Lake Tanganyika, neither is sufficient under modern lake conditions to account for these admixed assemblages. Winnowing and in situ stratigraphic condensation during periods of lowered lake levels is the most likely explanation for the occurrence of phenodeviant surficial shells. This hypothesis is consistent with the observation that extensive shell lags are exposed on the lake floor in areas currently inhabited by the same snails.

A model of biogeographic range fluctuations for particular Paramelania morphs during the Holocene, in concert with lake level fluctuations, can account for the shell assemblages seen on the lake floor today. The complex history of these accumulations suggests that taphonomic admixtures may obscure the interpretation of evolutionary sequences in the lacustrine stratigraphic record.

Tectonic deformation is an important part of the taphonomic histories of many fossils. Although the effects of deformation, and methods to remove those effects, have been a subject of inquiry for over a century, systematic testing under known parameters has never been used to determine how the effects of deformation and the performance of retrodeformation techniques might vary. Comparative studies of morphology depend on the accurate estimation of variance-covariance structure, so an understanding of the effects of retrodeformation on covariance structure is important in assessing the utility of these methods. Here we address these issues by using geometric morphometric simulations. Nondeformed data sets were generated from specimens of the extant turtle Emys marmorata, which were known by definition to be nondeformed, and which possess a known ontogenetic signal. Deformation was simulated by applying a combination of uniform shear and uniform compression/dilation to the data. Data were retrodeformed by reflection and averaging of bilaterally symmetric landmarks, use of a principal components analysis to identify a deformation component of shape variation, and removal of the affine component of shape variation among specimens. Deformation increased the amount of variance in the data, as well as altering the variance structure. However, low to moderate levels of deformation did not prevent the confident recovery of the known ontogenetic signal in some cases. The tested retrodeformation techniques did not work well. They either removed too little or too much variance from the data, and provided little improvement in variance structure. Retrodeformation often did not improve our ability to extract the ontogenetic signal from the data, and in some cases introduced an arti-factual relationship between size and shape. All of the scrutinized methods showed some properties, such as reducing variance or producing visually appealing images of specimens, that could make them appear to be working in cases where the correct biological signal is not known. This emphasizes the need for simulation testing in the development and evaluation of retrodeformation techniques.

Carbon (δ13C) and oxygen (δ18O) isotopic results are presented from 42 tooth enamel carbonate samples of rhinos (Family Rhinocerotidae) from a sequence of Florida Neogene localities between 9.5 and 4.5 Ma. These data are used to interpret ancient diets and test previous hypotheses of terrestrial/aquatic adaptations of two sympatric rhinos, Aphelops and Teleoceras. The long-limbed, shorter-crowned Aphelops traditionally has been reconstructed as an open-country browser (similar to the modern black rhino), whereas short-limbed, higher-crowned Teleoceras traditionally has been reconstructed as an amphibious grazer (similar to the modern hippo). Between about 9. 5 and 7 Ma the δ13C values (all <−11.0‰) from Florida Aphelops and Teleoceras indicate that both rhinos were feeding on C3 plants. This diet probably included a combination of browse and C3 grasses, although the exact proportions for each genus cannot be distinguished isotopically. In contrast, after the late Miocene global carbon shift as represented at 4.5 Ma in Florida, Aphelops was a browser (mean δ13C = −11.9‰), whereas Teleoceras was a mixed feeder/C4 grazer (mean δ13C = −7.0‰). Oxygen isotopic values indicate that neither Aphelops nor Teleoceras was principally aquatic. Given these new isotopic data, more plausible modern analogs for these two extinct rhinos are, respectively, the terrestrial browsing black rhino (Diceros bicornis) and the terrestrial grazing white rhino (Ceratotherium simum), which are sympatric today in Africa.

In the fossil record, biogeographic data at the species level are normally not adequately preserved to be utilized in the reconstruction of marine paleoprovinces; consequently, higher taxa are employed. Thus it is cogent to question whether modern marine provinces, which are generally recognized by their species content, are still distinctive at higher taxonomic levels. Similarity coefficients computed from the bivalve and gastropod faunas of six contiguous and five spatially separated modern provinces indicate that the present-day marine provinces are in fact recognizable at the generic and in most cases at the familial level. The deficiencies and biases of the fossil record obscure the distinctiveness and reduce the precision with which ancient provinces can be delimited. Nevertheless, paleoprovinces that are recognized from the distribution patterns of genera and families are clearly comparable to modern provinces that are determined by species distributions.

Taphonomic analysis of several late Pleistocene mastodon (Mammut americanum) skeletons excavated in southern Michigan provides compelling evidence of mastodon butchery by Paleo-Indians. The occurrence of butchery and details of butchering technique are inferred primarily from patterns of bone modification. An important aspect of butchering practice was production and use of tools fashioned from bones of the animal being butchered. Evidence for butchery and bone tool use includes matching marks on the conarticular surfaces of disarticulated pairs of bones; cutmarks on bones; green bone fracturing; use wear, secondary flaking, and impact features on bone fragments; and burned bone. Interpretation of these features is facilitated by information on patterns of bone distribution and disarticulation preserved in a primary depositional context. Preliminary comparisons among nine sites indicate that putative butchering sites differ consistently and in a variety of ways from sites that appear to record no human involvement. Although based on a small sample of sites, the apparent frequency of butchered individuals relative to those that were not butchered is unexpectedly high. These findings provide new evidence of a well-developed “bone technology” employed by the late Pleistocene human inhabitants of eastern North America. In addition, these data offer circumstantial support for the hypothesis that human hunting was an important factor in the late Pleistocene extinction of mastodons.

Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

Lengths of crinoid stalk segments consisting of multiple columnals (pluricolumnals) from the Fort Payne Formation of south-central Kentucky (Mississippian) were tested for uniformity using simulations. The results indicate that stalk segment lengths are too uniform to have been produced by a random process operating on homogeneous stalks.

The most parsimonious explanation for the uniformity of pluricolumnal lengths is based on analogy with stalks of living isocrinids. The stalks of these crinoids are organized into multicolumnal segments of approximately uniform length: columnals within each segment are connected by “through-going” ligament and “intercolumnal” ligament. The articulation between segments, however, consists of only intercolumnal ligament. Preliminary results from experiments on Recent carcasses indicate that the latter is the primary plane of disarticulation following death.

The preservational pattern of the Mississippian stalks suggests that their ligamentary organization was similar to that of the isocrinids. Since the plane of primary disarticulation in isocrinids corresponds to the autotomy plane, Mississippian crinoids also may have been capable of autotomizing their stalks.

Dental morphology changes dramatically across the artiodactyl-cetacean transition, and it is generally assumed that this reflects the evolutionary change from herbivory and omnivory to carnivory. To test hypotheses regarding tooth function and diet, we studied size and position of wear facets on the lower molars and the stable isotopes of enamel samples. We found that nearly all investigated Eocene cetaceans had dental wear different from typical wear in ungulates and isotope values indicating that they hunted similar prey and processed it similarly. The only exception is the protocetid Babiacetus, which probably ate larger prey with harder skeletons. The closest relative of cetaceans, the raoellid artiodactyl Indohyus, had wear facets that resemble those of Eocene cetaceans more than they do facets of basal artiodactyls. This is in spite of Indohyus's tooth crown morphology, which is unlike that of cetaceans, and its herbivorous diet, as indicated by stable isotopes. This implies that the evolution of masticatory function preceded that of crown morphology and diet at the origin of cetaceans.

Specific conductance was calculated for secondary xylem in seven Carboniferous stem taxa utilizing an equation derived from the Hagen-Poiseuille relation. Arborescent and lianoid representatives of major pteridophytic (Calamitaceae, Lepidodenraceae, Sphenophyllaceae) and gymnospermous (Cordaitaceae, Medullosaceae) groups were examined. In the calamite Arthropitys communis and the seed plant Cordaites (Cordaixylon sp. and Mesoxylon sp.), conductance corresponded approximately to the low end of the range for both extant conifers and angiosperms. A substantially higher conductance was determined for the wood of Arthropitys deltoides, conforming to the high end of the range for conifers and the low-middle part of the range for angiosperms. The highest conductance values were found in Sphenophyllum plurifoliatum, Medullosa noei, and Paralycopodites brevifolius and corresponded to the middle-high portion of the range for vessel-containing angiosperms. This outcome is particularly significant in light of the fact that tracheary elements in the fossils are imperforate. The results indicate that conductance in secondary xylem of some of the most ancient, woody groups was comparable to that in extant plants and that highly effective conducting tissue developed relatively early in plant evolution. Moreover, it is suggested that the general relationship between wood anatomy, growth habit, and ecology demonstrated for living plants can also be extended back in time to include fossil plants.

It is shown that the abundant Jurassic oyster Gryphaea provides material adequate to test biogeographic and evolutionary hypotheses involving speciation. Phyletic size increase is widespread if not characteristic and tends to be marked by a sharp initial increase followed by a longer period of stability. The larger size of phyletically younger forms is apparently because of greater age of individuals rather than a higher growth rate. Species durations fall within the range of the majority of Jurassic bivalve species. The detailed study of European Gryphaea indicates a pattern of punctuated equilibria allied with morphological trends, some of which are paedomorphic. Gradualistic, species selection and cladistic models of change are not supported. World-wide analysis of Gryphaea species supports centres of origin and migration rather than vicariance, and a relationship exists between migrational and evolutionary events and changes in the physical environment.

Recent Nautilus pompilius from the Fiji Islands and N. macromaphalus from New Caledonia show decreasing cameral liquid volumes relative to total phragmocone volume during ontogeny. A maximal value of 32% of the phragmocone filled with cameral liquid was measured from a 190 g N. pompilius. No specimens of over 500 g total weight of either species exceeded 12%. These figures are in contrast to values derived for seven ammonoid species by Heptonstall (1970), who found values ranging between 19 and 52%.

The relationship between cameral liquid volume and salinity within single chambers engaged in the emptying process are examined in N. pompilius and N. macromphalus. Both species start with newly formed chambers filled with cameral liquid isotonic to seawater. Ionic removal by the siphuncular epithelium rapidly reduces the cameral liquid osmolarity, producing osmotic movement of the cameral liquid into the blood spaces of the siphuncle. In both species the lowest cameral liquid salinities occur when the chamber is slightly over half emptied. After this point, which coincides with decoupling of the cameral liquid from the siphuncle, cameral liquid volume continues to decrease, but cameral liquid salinity increases, indicating that the rate of ionic removal slows relative to liquid removal. In N. macromphalus decoupled cameral liquid salinity rises until it is nearly isotonic to seawater when the chamber is nearly emptied. In N. pompilius, however, the rate of ion removal in decoupled cameral liquid is not slowed as much as in N. macromphalus, since it rarely exceeds 40% seawater osmolarity even when the chamber is nearly emptied. The differences in emptying methods demonstrated in these two species are probably related to their different habitat depths: N. pompilius from Fiji is found in much deeper water and must employ more physiologic work to empty chambers at greater depth.

The photosynthetic gas exchange capacities of early angiosperms remain enigmatic. Nevertheless, many hypotheses about the causes of early angiosperm success and how angiosperms influenced Mesozoic ecosystem function hinge on understanding the maximum capacity for early angiosperm metabolism. We applied structure-functional analyses of leaf veins and stomatal pore geometry to determine the hydraulic and diffusive gas exchange capacities of Early Cretaceous fossil leaves. All of the late Aptian—early Albian angiosperms measured possessed low vein density and low maximal stomatal pore area, indicating low leaf gas exchange capacities in comparison to modern ecologically dominant angiosperms. Gas exchange capacities for Early Cretaceous angiosperms were equivalent or lower than ferns and gymnosperms. Fossil leaf taxa from Aptian to Paleocene sediments previously identified as putative stem-lineages to Austrobaileyales and Chloranthales had the same gas exchange capacities and possibly leaf water relations of their living relatives. Our results provide fossil evidence for the hypothesis that high leaf gas exchange capacity is a derived feature of later angiosperm evolution. In addition, the leaf gas exchange functions of austrobaileyoid and chloranthoid fossils support the hypothesis that comparative research on the biology of living basal angiosperm lineages reveals genuine signals of Early Cretaceous angiosperm ecophysiology.

The depositional history of Upper Miocene through Recent sediments from DSDP Site 214 (Ninetyeast Ridge, Indian Ocean) is reexamined. Samples of the Globorotalia tumida planktic foraminiferal lineage, originally obtained from these sediments by Malmgren et al. (1983), serve as the empirical basis for the recognition of punctuated anagenesis as a distinct mode of phenotypic evolution and have been the subject of numerous additional investigations. However, conclusions reached by previous authors depend strictly on the validity of the original chronostratigraphic interpretation of these sediments. Graphic correlation analysis of first- and last-appearance datum levels for a total of 41 planktic foraminiferal, radiolarian, and calcareous nannoplankton taxa provides evidence for a more complex depositional history at this deep-sea site than originally believed. Based on a conservative model of variation in the pattern of sediment accumulation rates, the lowermost portion of the studied section (6.5-4.3 Ma) represents an interval of temporally condensed sediment accumulation (1.88 cm/1,000 yr) followed by an interval (4.3-2.8 Ma) of temporally expanded sediment accumulation (3.97 cm/1,000 yr). This interval, in turn, is followed by a depositional hiatus or an extremely condensed interval, at least 800,000 yr in duration, which is followed by another relatively condensed (1.36 cm/1,000 yr) interval from 2.0 Ma-Recent. Although this chronostratigraphic reinterpretation deviates substantially from the original, which recognized Site 214 as being both temporally continuous and exhibiting a constant sediment accumulation rate from the Upper Miocene through the Upper Pliocene, it is more consistent with expectations based on Neogene eustatic sea-level fluctuations and global surveys of Neogene hiatus distributions. Age assignments for samples of the Gr. tumida lineage based on the revised chronostratigraphic model reverse some findings of previous investigators with respect to the distinctiveness of phenotypic evolutionary rates characterizing the transition from Gr. plesiotumida to Gr. tumida. Finally, a brief survey of similar marine invertebrate lineage studies shows that changes in the rate of phenotypic evolution often appear to coincide with major physical changes in the paleoceanographic environment. Such correspondences may be due, at least in part, to the effect of these environmental changes on sediment accumulation rates. Paleobiologists who seek to understand patterns of phenotypic change over time must remove the effects of variations in sediment accumulation rates from their data before evolutionary hypothesis testing and remain aware of the limitations imposed on their interpretations by the uncertain nature of chronostratigraphic inference.

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.

The Lower Tertiary Gulf Coast was subjected to repeated transgressive-regressive cycles of varying rate and extent which had a marked effect on species longevities. In some neogastropods dispersal and geographic distribution played important roles in species longevities. Species with planktic larval stages that inhabited broad areas of the Gulf Coast could maintain their distributions even during regressive phases when delta building was strong. Wide distributions, free gene flow, and resistance to isolation gave planktic species an evolutionary stability that resulted in greater species longevity. Species with nonplanktic development could attain wide geographic ranges when barriers were minimized, but they were unable to maintain their distributions in the face of regressive progradation. Thus their populations were isolated, and extinction and speciation accelerated, giving them lesser evolutionary longevity. Although planktic volutids tend to be widespread, planktic fasciolariids and mitrids were often restricted in distribution and therefore probably stenotopic. If dispersal and environmental tolerance are considered as separate forces, they seem to act in combination to produce very long, very short or intermediate species longevities.