Mass Extinction Interval Research Articles

δ34SCAS and δ18OCAS records during the Frasnian–Famennian (Late Devonian) transition and their bearing on mass extinction models

Many proposed extinction mechanisms for the Late Devonian (Frasnian–Famennian boundary — FFB) mass extinction event include the spread of anoxia in bottom waters during the two Kellwasser anoxic events, which have in turn been linked to changes in continental weathering, volcanic/hydrothermal fluxes, sea-level and climate change. Although there is strong evidence for bottom water anoxia in epicontinental seas at tropical/equatorial palaeolatitudes, the global extent of these changes is poorly understood, largely due to the lack of an oceanic and high-latitude record. In this study, marine δ34Ssulphate and δ18Osulphate records, derived from the analysis of carbonate-associated sulphate (CAS), were constructed from FFB sections in Belgium and Poland in order to examine the effect of the postulated changes in ocean redox conditions on the global sulphur cycle. The results reveal no overall changes in seawater δ34Ssulphate and δ18Osulphate over the FFB interval suggesting that any changes in sulphur cycling, namely relating to rates of pyrite burial and weathering, were not significant on a global scale. This supports the idea that the Kellwasser events were not oceanic phenomena and that they were restricted to epicontinental seas at low latitudes. The exception to the stability of the FFB δ34Ssulphate record is a brief negative excursion during the mass extinction interval in the linguiformis Zone, albeit based on one datapoint at this level in two different sections. This may be attributable to short, sharp marine regression and an increased flux of light sulphur from weathered pyrite.

Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions

A quantitative compilation of carbonate skeletal mineralogy through the Phanerozoic shows a progressive replacement of low-Mg calcite by aragonite. This general trend overrides the subsidiary trend of greenhouse intervals favoring biogenic low-Mg calcite mineralogies (calcite seas), and icehouse intervals facilitating aragonite + high-Mg calcite mineralogies (aragonite seas). The replacement of low-Mg calcite by aragonite was, however, achieved episodically at mass extinction intervals. In particular, the end-Permian extinction both preferentially removed species bearing “unfavorable” low-Mg calcite, and allowed the selective radiation of biota with “favorable” aragonite. This demonstrates the importance of incumbency in the evolution of skeletal mineralogy. We suggest that the broad increase of aragonitic biota has been controlled by changes in atmospheric carbon dioxide partial pressure ( p CO2) via carbonate mineral kinetics. Through the Phanerozoic, broadly decreasing p CO2 levels led to decreasing total alkalinity and dissolved inorganic carbon, and increasing oceanic pH. Superimposed upon this general trend, there are cyclic episodes of relatively high p CO2 and saturation state combined with a lower ratio of magnesium to calcium ions in seawater driven by the relatively slow changes in mid-ocean ridge expansion rates. Mass extinction events, many of which may have been caused by rapid global changes in temperature and/or p CO2, represent major intervals of turnover.

New range data for marine invertebrate species across the early Toarcian (Early Jurassic) mass extinction

Abstract: We have reinvestigated the marine mass extinction interval that occurred during the early Toarcian, which was a time of widespread marine anoxia. The ranges of marine benthic invertebrates are significantly altered using new observations from the Cleveland Basin, UK. Goniomya rhombifera is reported for the first time from the Whitby Mudstone Formation and together with an increased epifaunal bivalve diversity indicates a brief, relatively oxygenated period towards the end of the event. The new data, together with published results, suggest three apparent extinction horizons on a global scale; the first is just above the Pliensbachian–Toarcian boundary, and the following two are in the semicelatum ammonite Subzone. As a result of the Signor–Lipps effect there may be only one, or possibly two, true extinctions. The youngest extinction horizon coincides with the first of the abrupt carbon isotope shifts that characterize this interval, and with increases in sea surface temperature, continental weathering rates, and seawater anoxia. Pseudomytiloides dubius is the only abundant benthic macroinvertebrate during the most hostile environmental conditions but it and all other benthic species are almost entirely absent for many thousands of years immediately after each abrupt negative carbon isotope shift. Supplementary material: A compilation of the Late Pliensbachian and Early Toarcian marine biotic changes and a discussion of the systematics of the key faunal elements are available at http://www.geolsoc.org.uk/SUP18353 .

Stratigraphy, biostratigraphy and C-isotopes of the Permian–Triassic non-marine sequence at Dalongkou and Lucaogou, Xinjiang Province, China

Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ 13C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.

Organic geochemistry and palynofacies of the Early–Middle Frasnian transition (Late Devonian) of the Holy Cross Mountains, Southern Poland

Geochemical and palynological investigation of the Lower to Middle Frasnian reef-to-basin succession at the well-known Kowala Quarry (Holy Cross Mountains) demonstrates the variable character of organic matter (OM) deposited in different oxidative–reductive conditions. During sedimentation of the basinal deposits, anoxic conditions in the water column prevailed. In the organic-rich sediments deposited during the Palmatolepis punctata Zone (set D), anoxia occurred in the photic zone, as inferred from the presence of isorenieratane, aryl isoprenoids and gammacerane. In the same kind of deposits, but occurring during the Palmatolepis hassi Zone (set F), anoxia apparently occurred mainly in the deeper parts of the basin and only sporadically reached the photic zone. This interpretation is based on the lack of isorenieratane, traces of aryl isoprenoids and the presence of gammacerane in the investigated samples. The basinal clay-enriched sediments of sets D and F are divided by an organic-poor set E, typical for the carbonate platform (fore-reef) environment characterized by oxic conditions. A palynostratigraphical study of the Frasnian section from the Kowala Quarry allowed recognition of two miospore zones: Geminospora semilucensa– Perotrilites donensis (SD) and Archaeoperisaccus ovalis–Verrucosisporites grumosus (OG). Relatively small quantitative and qualitative changes of the organic particles (miospores, tracheids, prasinophytes and acritarchs) are interpreted primarily in terms of sea-level fluctuations and enhanced productivity. In particular, the appearance of abundant, abnormally large mature leiospheres in set D can be the result of progressive eutrophication, related to environmental changes recorded in the recently detected major positive carbon isotope anomaly. As during the Frasnian–Famennian mass extinction interval, this eutrophication could have been stimulated by a rapid increase in primary production and subsequently expanding anoxic conditions, but it is evident only in the early phase of the Middle Frasnian δ 13C excursion. The processes of the secondary hydrothermal fluid migration, taking place after the lithification of the sediments, caused partial OM oxidation. The products of these processes are such phenyl derivatives of aromatic hydrocarbons as phenylphenanthrenes, terphenyls, phenyldibenzothiophenes and phenyldibenzofurans. Moreover, an oxygen compound rarely described from sedimentary OM, dibenzo- p-dioxin and its methyl- and dimethyl-derivatives, seems to be the additional product of hydrothermal OM oxidation.

Size variation of conodont elements of the Hindeodus–Isarcicella clade during the Permian–Triassic transition in South China and its implication for mass extinction

Based on the analysis of thousands of conodont specimens from the Permian –Triassic (P–T) transition at Meishan (the GSSP of P–T Boundary) and Shangsi sections in South China, this study investigates the size variation of Hindeodus and Isarcicella P1 elements during the mass extinction interval. The results demonstrate that Hindeodus– Isarcicella underwent 4 episodes of distinct size reduction during the P–T transition at the Meishan Section and 2 episodes of size reduction in the earliest Triassic at Shangsi. The size reductions at Meishan took place at the junctions of beds 24d/24e, 25/26, 27b/27c and 28/29, and at the junctions of beds 28/29c and 30d/31a at Shangsi. The two earliest Triassic size reduction episodes were correlative between the two sections. These changes coincide with some important geological events such as eustatic sea-level changes, anoxic events, carbon isotope oscillations, miniaturization of brachiopods and microbial changes. Through detailed investigation of the palaeoenvironment and the palaeoecology of Hindeodus– Isarcicella, the authors propose that the main causes of the size reduction was a sharp decline of food availability because of the mass extinction and the anoxic event during the P/T transition. The pattern of size reduction supports suggestions that the end-Permian mass extinction was multi-episodal, consisting of 3 extinction events rather than a single catastrophic event.

Bryozoan diversity in Southern Siberia at the Devonian–Carboniferous transition: New data confirm a resistivity to two mass extinctions

Two spectacular mass extinctions, namely the Kellwasser and the Hangenberg events, affected the Earth's biota during the Late Devonian–Early Carboniferous. Diverse assemblages of bryozoans are known from the Fasnian–Tournaisian deposits of Southern Siberia, which includes such regions as the Kuznetsk Basin, the Kolyvan'–Tom' Zone, the Rudny Altay, and the Gorny Altay. Our new data establish the stratigraphic ranges of 154 species, which belong to 56 genera and 21 families. These data have been analyzed quantitatively to measure the patterns of diversity dynamics and the changes in the taxonomic diversity structure. The species diversity increased at the Frasnian–Famennian transition. After a short-term, but dramatic decrease in the middle Famennian, the total number of species accelerated in the late Famennian and the Tournaisian. A similar pattern occurred for both genera and families, although the radiation at the Frasnian–Famennian transition was not as large, and total family diversity decreased slightly in the Tournaisian. Changes in total species diversity differed between orders of bryozoans. Changes in taxonomic diversity structure were moderately rapid. The Tournaisian species diversity was governed by the other genera comparing to the early time intervals. Thus, our regional data suggest that bryozoans generally survived both the Frasnian/Famennian and the Devonian/Carboniferous mass extinctions, confirming similar conclusions made earlier with global data. However, a mid-Famennian regional crisis (diversity drop) in the evolution of bryozoans is documented within Southern Siberia. Bryozoan radiations coincided with pulses of basin deepening that occurred during both mass extinction intervals, perhaps explaining why bryozoans were resistant to extinction. The mid-Famennian crisis might have been caused by a rapid regression coupled with basin shallowing.

Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery

Modern study of the end-Permian mass extinction in the marine realm has involved intensive documentation of the fossil content, sedimentology, and chemostratigraphy of individual stratigraphic sections where the mass extinction interval is well preserved. These studies, coupled with innovative modeling of environmental conditions, have produced specific hypotheses for the mechanisms that caused the mass extinction and associated environmental stress. New paleobiological studies on the environmental distribution and ecological importance of brachiopods, benthic molluscs, and bryozoans support the hypothesis that stressful ocean conditions—primarily elevated H2S levels (euxinia) but also heightened CO2 concentrations—were the prime causes of the end-Permian mass extinction. These studies also further demonstrate that both the Late Permian interval preceding this mass extinction and the Early Triassic interval that followed were times of similar elevated environmental stress. In the low-diversity Early Triassic biosphere, huge numbers of benthic molluscs, in particular four cosmopolitan genera of bivalves, typically covered the seafloor. That a few marine genera thrived during this time indicates a greater than usual tolerance to some combination of marine anoxia, as well as elevated CO2 and/or increased H2S concentrations. Research focusing on experiments with modern organisms similar to those that died, as well as those that thrived, in microcosms where levels of O2, CO2, and H2S can be experimentally manipulated will enable an even more detailed understanding of the nature of this greatest Phanerozoic biotic crisis.

Early Triassic recovery of echinoderms

The Permian–Triassic mass extinction interval was an important time in the evolutionary history of the echinoderms. Details of the extinction and, in particular the immediate post-extinction recovery in the Early Triassic, are seldom addressed because of a perception that the Permian–Triassic echinoderm fossil record is too poor. However, only the Holothuroidea and Asteroidea lack any Early Triassic fossil representatives. Even in these groups, details of the extinction and recovery can be inferred from recent cladistic analyses. The Holothuroidea are unique amongst the echinoderms in showing no family level extinction through the Permian–Triassic interval, possibly due to their deposit-feeding lifestyle. In contrast, the Echinoidea, Crinoidea and probably the Asteroidea underwent severe evolutionary bottlenecks during that time. In the echinoids, significant post-Permian radiation occurred from the Late Triassic (Carnian), although it may have begun in the Early Triassic. In the Crinoidea, fossil diversity increases dramatically from the Late Ladinian, although cladistic analyses suggest that initial diversification took place in the Earliest Triassic (Induan). Many undescribed crinoid remains from Lower Triassic strata worldwide also imply that the post-Permian radiation in this group may have been more rapid than currently thought. Locally in the Spathian, crinoid ossicles may approach rock-building densities. The presence of at least seven Early Triassic fossil ophiuroid species may indicate rapid post-Permian radiation in the Ophiuroidea, although the higher level affinities of these taxa are presently unresolved and the Late Permian record is poorly known. Ophiuroid remains are the most diverse echinoderm fossils during the Early Triassic, comprising both complete body fossils and disarticulated ossicles. Holothuroids possibly radiated in the Early Triassic, but current evidence from cladistic analysis favours a largely Anisian age for the post-Permian radiation in this group. All known Early Triassic echinoderms were small-sized animals that inhabited very shallow, oxygenated, low palaeolatitude environments within wave base. To cite this article: R. Twitchett, T. Oji, C. R. Palevol 4 (2005) .

Ashgillian graptolite fauna of the Yangtze region and the biogeographical distribution of diversity in the latest Ordovician

Abstract Ashgillian graptolites have been described and recorded globally from 15 different paleoplates. The most diverse graptolite faunas are from the Yangtze region, South China, including 28 genera and 96 species. Among them, 25 genera and 73 species belong to the DDO fauna (Dicranograptidae – Diplograptidae – Orthograptidae fauna), and only three genera but 23 species belong to the N fauna (Normalograptidae fauna). Among the Yangtze graptolite fauna there are six endemic genera and 51 endemic species, which represent 21% and 52%, respectively, of the Ashgillian fauna in the region. This is an unusually high level of endemism. Endemic species are present in both the endemic and the cosmopolitan genera. A late Ashgillian stepwise extinction event has recently been recognized, based on graptolite diversity changes and graphic correlation. This begins with a major extinction from the Diceratograptus mirus Subzone to the middle Normalograptus extraordinarius-N. ojsuensis Zone, and ends with a minor extinction prior to the end of the Normalograptus persculptus Zone. Graptolite faunal replacement of the DDO fauna by the N fauna occurred throughout this interval. A comparison of Ashgillian graptolite diversity between the Yangtze region and other regions indicates that two different biogeographical realms existed in mid-Ashgillian time. A moderate-diversity graptolite fauna is present in the low-middle latitude realm, which includes South Scotland, Kazakhstan, Kolyma, Nevada, Yukon, Canadian Arctic, SE Australia. The Yangtze region was located in this realm, but was characterized by a very high-diversity fauna. Some other localities, including eastern Avalonia (Wales and England), the Argentina Precordillera, and Bohemia, which mainly represent the mid- to high-latitude realm, contain the lower-diversity mid-Ashgillian assemblages. This biogeographical distribution suggests a latitudinal diversity gradient, which may be controlled mainly by water temperature. This climate gradient becomes much less evident by late Hirnantian time in which most parts of the world have a relatively low diversity fauna totally dominated by normalograptid species, many of which appear to have been eurytopic. Throughout the Ashgillian, however, the Yangtze platform shows a high diversity and long persistence of DDO taxa the mass extinction interval. This may be a consequence of the semi-restricted nature of the basin in which conditions relatively favorable to graptolite survival and speciation existed throughout all or most of the Hirnantian.

Determining the role that ecological and developmental constraints play in controlling disparity: examples from the crinoid and blastozoan fossil record.

It is widely believed that morphological constraints are responsible for the observed pattern of decreasing major morphological innovation in both the Metazoa and Metaphytes over geological time. This is readily seen as the decreasing trend of origination of higher taxa: phyla, classes, and orders. Currently, there are two competing evolutionary hypotheses that have been proposed to explain this phenomenon: (1) the empty ecospace hypothesis and (2) the developmental constraint hypothesis. To distinguish between hypotheses 1 and 2, the change of morphological innovation before and after several mass extinction events was measured in the Crinoidea and Blastozoa. Mass extinction intervals provided a means in which to remove ecospace limiting constraints and allow the developmental constraint hypothesis to be thoroughly tested. Within the Crinoidea, disparity was measured before and after three mass extinctions. Within the Blastozoa, disparity was measured before and after two mass extinctions. For each taxon, three suites of characters were analyzed: ecological, nonecological, or "developmental" and a combination of the two previous suites plus 50 additional characters. Four different measures of disparity were used to analyze each character suite. In the majority of the cases investigated, disparity rebounds to comparable levels or in some cases higher levels in both the Crinoidea and Blastozoa. The results indicate that developmental constraints are not responsible for the decrease in disparity throughout the geologic range of the taxa. The more likely scenario is that increasingly structured ecological guilds have made it much more difficult to allow large increases in disparity.

Trans-Atlantic dinoflagellate cyst stratigraphy across the Cenomanian–Turonian (Cretaceous) Stage boundary

Abstract. The principal palynological proxy for the Cenomanian–Turonian Stage boundary, the top of consistent/common Litosphaeridium siphoniphorum (a dinoflagellate cyst), occurs in Greenhorn Bed 73 at the international stratotype section, west of Pueblo, Colorado, USA. This datum occurs in the same position, as indicated by planktonic foraminifera (a few beds higher than the range top of R. cushmani), ammonites (upper part of the S. gracile/M. geslinianum Zone) and geochemistry (immediately below maximum δ13C values), at Pueblo (Western Interior Basin) and localities in southern England (Wessex–Paris Basin) and northern Germany (Lower Saxony Basin). Of over 100 dinoflagellate cyst taxa recorded from Pueblo and a correlative section at Lulworth, southern England, possibly as few as six do not range into the Turonian. In the uppermost Cenomanian – lowermost Turonian succession at Pueblo, there are no consistent absences of any common taxa (with four exceptions) and there is no evidence for a collapse in cyst-forming dinoflagellate populations during the Cenomanian–Turonian boundary mass extinction interval/‘oceanic anoxic event’. However, the composition of palynological assemblages from the Upper Cenomanian appears to reflect palaeoenvironmental stress and/or an increase in the supply of land-derived and relatively nearshore palynomorphs.

Models for biotic survival following mass extinction

Abstract Mass extinction intervals are characterized by three dynamic processes: extinction, survival, and recovery. It has been assumed that the taxa surviving a mass extinction are composed predominantly of eurytopic groups and opportunistic/disaster species. However, high-resolution stratigraphic and palaeontological analyses of several mass extinction intervals show that the repopulation of the global ecosystem takes place among ecologically and genetically diverse and complex taxa and occurs far too rapidly to be solely attributed to rapid radiation from a few ecological generalists. We suggest a number of potential survival mechanisms or strategies ( sensu Fryxell 1983) which have evolved in diverse taxa and which could have allowed them to survive mass extinction intervals. These mechanisms consist of: rapid evolution, preadaptation, neoteny/progenesis, protected and/or unperturbed habitat, refugia species, disaster species, opportunism, broad adaptive ranges, persistent trophic resources, widespread and rapid dispersion, dormancy, bacterial-chemosymbioses, skeletonization requirements, reproductive mechanisms, larval characteristics and chance. Because of the wide variety of potential survival mechanisms, the range of survivors may be far higher than previously hypothesized. This would account, in part, for the diversity and evolutionary state of Lazarus taxa and for the rapid re-establishment of some complex ecosystems following many mass extinction intervals, without calling on “explosive” radiation from generalist/opportunist stocks following a mass extinction interval.

Common Patterns of Mass Extinction, Survival, and Recovery in Marine Environments: What Do They Tell Us About the Future?

Mass extinction is characterized by the loss of more than 50 percent of the world's species within a short interval of geologic time - months to as much as 3 million years (My). In the fossil record, these events have primarily been recorded from the marine realm. Three patterns of mass extinction have been described - catastrophic, stepwise, and graded extinction. Many well-studied extinction intervals contain elements of more than one pattern, suggesting that these biotic crises were caused by varied forcing mechanisms linked by complex environmental feedback loops. This hypothesis is supported by the discovery that the four well-studied Phanerozoic mass extinctions (Late Devonian, middle and terminal Cretaceous, Eocene-Oligocene boundary extinctions) share a number of physical, chemical, and biological characteristics in common. They consistently show stepwise extinction patterns linked to intervals of extraordinary fluctuations in the temperature, chemistry and structure of ocean-climate systems, at rates and magnitudes well above background levels. In addition, tropical ecosystems were the first and most severely affected, and more poleward, temperate biotas were mainly stressed during the later phases of the extinction interval. Evidence for these unusual environmental changes is derived from high-resolution (cm-scale) paleobiological, sedimentological, trace-element and stable-isotope analyses spanning mass extinction intervals. These dramatic environmental fluctuations were the immediate causes of mass extinction, as they progressively exceeded the survival limits of global biotas largely adapted to warm, equable, ice-free climates which characterized over 90 percent of Phanerozoic time. These environmental fluctuations probably represented feedback phenomena from more powerful, short-term forcing mechanisms which abruptly perturbed the structure of ocean-climate systems. Multiple impacts of extraterrestrial objects within short (<1-3 My) time intervals - so-called meteorite/comet showers - are the most logical candidates. This hypothesis is supported by physical and chemical evidence for impacts clustered around most, but not all, Mesozoic and Cenozoic mass extinctions.

Taxon age and selectivity of extinction

Taxon-age distributions were compiled for families of marine animals surviving or becoming extinct in each stage of the Phanerozoic. I demonstrate, through the use of a modified bootstrap analysis, that there is no difference between the longevity of families becoming extinct during times of background extinction and times of mass extinction. In both mass and background extinction intervals the mean age of families that become extinct is 2 standard deviations below the geometric mean taxon age of families available for extinction. Young families are more susceptible to extinction, perhaps as the result of lower species richness or of occupying a smaller geographic range. There is no tendency during mass extinctions toward loss of families with different taxon ages other than those that become extinct during background times. Thus, in terms of family survival, mass extinction appears to be an exaggeration of processes of background extinction.

Phanerozoic trends in background extinction: Consequence of an aging fauna

Changes in the rate of background extinction are analyzed as a function of changing age distributions of families. Declining background-extinction rates have previously been attributed to evolution at the family or community level. It is demonstrated, however, that if the probability of extinction for a family decreases through its own history, then overall changes in background-extinction rate are a function of a changing familial age distribution. In geologic stages where there is a preponderance of young families, extinction rates can be predicted to be higher than in times where there is a plethora of older families. In addition, it is demonstrated that a difference exists between the age distributions of families that become extinct in background- and mass-extinction intervals.