The Permian-Triassic Extinction Event and Invertebrate Developmental Modes

Home , Extinction event, Permian, Triassic

BULLETIN OF MARINE SCIENCE, 39(2): 607~15, 1986 LARVAL INVERTEBRATE WORKSHOP

THE PERMIAN-TRIASSIC EXTINCTION EVENT AND INVERTEBRATE DEVELOPMENTAL MODES

James W Valentine

ABSTRACT Permian- Triassic extinction removed perhaps 90% or more of marine invertebrate shelf species and permanently altered the higher taxonomic composition of shelf faunas. It is a plausible hypothesis that planktotrophic lineages were disproportionately victimized; so far as can be told, the survivors of some formerly dominant tropical clades were all nonplanktotrophic, and most of the new clades appearing in the Lower Triassic are nonplanktotrophic also. An analysis of the changing patterns of diversity and endemism across the Permian- Triassic boundary suggests that even if planktotrophs had dominated the Permian tropics, a differential extinction of 1.3-1.5 planktotrophs for each nonplanktotroph could account for the inferred pattern. The extinctions may have been associated with change in the pattern of oceanic productivity.

The suspicion has been growing that the Permian-Triassic marine extinctions, whatever their causes, were selective with respect to the life histories of the species involved. It has been suggested that planktotrophic lineages of crinoids (Strath- mann, 1978b), articulate brachiopods (Valentine and Jablonski, 1983), and perhaps archaeogastropods (Erwin and Valentine, 1984) were nearly or quite extinguished across the Permian-Triassic boundary; these groups lack species with planktotrophic stages today. If it is true that these groups suffered extinction of their planktotrophs, then it is plausible that the Permian-Triassic event extinguished planktotrophs preferentially in the fauna as a whole. It is, therefore, reasonable to inquire as to what sorts of conditions, consistent with our knowledge of the geological and biogeographical history ofthose times, would have had such an effect.

THE PERMIAN-TRIASSIC EXTINCTION EVENT Severity and Pattern. - The Permian-Triassic mass extinctions were more severe than any other recorded marine extinction events in terms of percentage of the fauna which was lost. About half of the durably skeletonized families of marine invertebrates disappeared from the fossil record then, and estimates of the toll of species range from just over 91% (calculated from the figures and models in Valentine et al., 1978 but reported as 77% by Raup, 1979) to about 96% (Raup, 1979). A common explanation for these extinctions is that sea level dropped so far as to greatly reduce the area and habitat heterogeneity on the world's shelves, where the bulk of the marine invertebrate fossil species lived. However, Jablonski (1985) has shown that an extinction of this magnitude in today's world cannot be achieved by a major reduction in the shelf faunas alone; indeed, even if the entire fauna of the world's continental shelves were to be extirpated, many more than half of the invertebrate families would remain because they are represented on islands. A pattern of extinction which would reach the same levels as did the Permian- Triassic event would be one that essentially removed the entire tropical fauna, but that left high-latitude faunas largely intact (Jablonski, 1985). A significant drop in sea level near the Permian-Triassic boundary has long been postulated, and it seems likely that the oceans at that time stood below their present levels (Forney, 1975), a record low stand for Phanerozoic time (Hallam,

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1984), Thus, there are few stratigraphic sections available which contain a rea- sonably continuous record of latest Permian and earliest Triassic sediments or fossils, and the biogeography of the extinctions is difficult to reconstruct in any detail. The early Triassic rocks which we do have are on average as variable in terms of depositional environment and lithologic type as rocks of late Permian age (except for a dearth of bioclastics; Kummel, 1973). It is clear, however, that tropical ecosystems were hit very hard indeed, and the early Triassic fossil as- sociations which we do have at whatever paleolatitudes are of low diversity and have the rather plain morphological stamp associated with high-latitude faunas today (Valentine, 1973; Kummel, 1973).

Evidence of Effect on Developmental Modes. -Direct evidence of the developmental modes possessed by late Permian and early Triassic marine invertebrates is largely lacking and must be inferred from indirect evidence. Such inferences usually involve ascription of the developmental mode ofliving taxa to their extinct allies. However, some of the late Paleozoic taxa represented by nonplanktotrophic descendants are suspected of having been planktotrophic. Thus, Strathmann (197 8a) has argued that Paleozoic crinoids, sharing as they likely do a planktotrophic ancestor with other echinoderm clades, may have been planktotrophic themselves. Another line of inference involves the distributional patterns of extinct groups. In today's seas, most of the dominant tropical clades of moderate sized to large invertebrates are planktotrophic. Yet articulate brachiopods, nonplanktotrophic today, dominated many benthic communities during the Paleozoic (as did crinoids for that matter; see Sepko ski, 1981), and were most diverse in low latitudes during those times. It has, therefore, been suggested that many ofthe tropical articulates of Permian times were planktotrophic (Valentine and Jablonski, 1983). Since both articulates and crinoids suffered heavy extinctions across the Permian-Triassic boundary, and as their living descendants are entirely nonplanktotrophic, it is plausible that planktotrophs were eliminated then from these clades. The Archaeogastropoda is another group which is for all practical purposes nonplanktotrophic today (assuming neritaceans are not archaeogastropods). It is not possible to make a strong phylogenetic argument that primitive Paleozoic archaeogastropods were planktotrophic, however (see Chaffee and Lindberg, 1986). Though they were common enough in tropical Paleozoic seas, they were certainly not a dominant group. Today they are relatively depauperate in tropical gastropod faunas; the archaeogastropod/caenogastropod ratio in Puget Sound is 1:4, but in the Panamanian province it is 1:11 (Erwin and Valentine, 1984). This pattern is consistent with a possible loss of planktotrophs but is not convincing in itself. The Permian- Triassic extinction of archaeogastropod families was moderate (38%, although doubtless much higher at the species level; Erwin and Valentine, 1984) and only a single Paleozoic family is extant. There is another line of evidence, also indirect, which suggests that planktotrophs may have been at a disadvantage during the Permian-Triassic event. There was a rise in diversification at the ordinal level during the Triassic (Table 1). Among durably skeletonized taxa, 11 new Triassic orders are found according to the compilation by Sepkoski (1981). One order appears in the early Triassic, the phylloceridan ammonities, presumed to be nonplanktotrophic like the living nau- tiloids (see review in Jablonski and Lutz, 1983 and references therein). In medial Triassic time, five orders appear; a sponge, three articulate crinoid groups, and the scleractinian corals. So far as known, the first four of these clades are nonplanktotrophic today, while planktotrophy is rare among the corals. In the late Triassic, four orders appear, all of which include planktotrophs today (all are VALENTINE: EXTINCTION AND DEVELOPMENT MODES 609

Table I. Orders of durably skeletonized marine invertebrates which first appeared during the Triassic

Living relatives (R) or members (M) Order Status planktotrophic? Early Triassic Spongiomorphida Extinct No (R) Unionida Living No (M)'" Phyllocerida Extinct No (R) Medial Triassic Hexactinosida Living No(M) Scleractinia Living Yes (M) Millericrinida Living No (M) Isocrinida Living No (M) Roveacrinida Extinct No (R) Late Triassic ?Lychniscosidae Living No (M) Diadematoida Living Yes (M) Pedinoida Living Yes (M) Hemicidaroida Extinct Yes (R) Plesiocidaroida Extinct Yes (R)

III Living members non-marine. echinoid groups). There is a clear suggestion here that conditions in early and medial Triassic times were unfavorable for the origin of novel planktotrophic taxa, as if the conditions responsible for the Permian-Triassic extinctions cast a shadow into the early Mesozoic, favoring nonplanktotrophs for millions of years. To pursue this possibility, I examined the families of bivalves and gastropods which first appeared during Triassic time (Table 2). Of the 27 families first known from the early or medial Triassic, 18 are archaeogastropods and may have been nonplanktotrophic (only 4 are extant); 3 are mesogastropods or neritaceans, all extant, which include some planktotrophs; and 7 are bivalves, all but 1 of which is extant and 3 of which are apparently nonplanktotrophic. No archaeogastropod families originated in the late Triassic, but among extant taxa the naticids (mixed planktotrophic and nonplanktotrophic) and four planktotrophic bivalve families appeared, together with several extinct families that belong to clades which are largely though not exclusively planktotrophic (pterioid and veneroid bivalves, for example). Thus, although the evidence is not so clear-cut as with the orders, there does seem to be a tendency for nonplanktotrophs to be important among the newly appearing moluscan families in the early and medial Triassic and for families that include planktotrophs to become more prominent among families orig- inating in the late Triassic (Table 3).

MODELS OF PLANKTOTROPHIC SURVIVORSHIP The data reviewed above certainly do not definitely demonstrate preferential extinction ofplanktotrophs during Permian-Triassic time, but they are consistent with and suggestive of such a history and invite further work. One additional approach is to evaluate the plausibility of the results suggested-loss of most or all planktotrophs in some classes and orders-considering the magnitude of the extinction and assuming a reasonable distribution of planktotrophs among the Permian fauna, with their abundance concentrated then, as now, in the tropics. A crude but serviceable latest Permian marine biosphere model can be postulated to contain two low-latitude provinces, each containing 7,140 species, and two high-latitude provinces, each containing 1,430 species (Valentine et a1., 1978, 610 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986

Table 2. Families of marine bivalves and gastropods which first appear during the Triassic and for which the epoch of first appearance is recorded

Family Status Living planktotrophs? Early Triassic Mysidiellidae Extinct Pachycardiidae Extinct Myophoricardiidae Extinct Middle Triassic Acmaeidae Living No Amberleyidae Extinct Crossostomatidae Extinct Fissurellidae Living No Kittlidiscidae Extinct Laubellidae Extinct Paraturbinidae Extinct Platyacridae Extinct Schizogoniidae Extinct Symmetrocapulidae Extinct Temnotropidae Extinct Trochidae Living No Turbinidae Living No Zygitidae Extinct Neritidae Living Yes Lacunidae Living Yes Yermetidae Living Yes Limopsidae Living No? Cassianellidae Extinct Pectinidae Living Yes Plicatulidae Living Yes Spondylidae Living Yes Trigoniidae Living Yes Cuspidariidae Living No Latumulidae Living No Late Triassic Cirridae Extinct Nododelphinulidae Extinct Stomatellidae Living No Trochotomidae Extinct Naticidae Living Yes Purpurinidae Extinct Buchiidae Extinct Dattiidae Extinct Gryphaeidae Living Yes Monotidae Extinct Pergamidiidae Extinct Arcticidae Living Yes Cardiidae Living Yes Dicerocardiidae Extinct Hippopodiidae Extinct Tancredidae Extinct Burmesiidae Extinct Ceratomyidae Extinct

who, however, used only one high-latitude province). Assume that planktotrophs made up 75% of the tropical and 20% of the high latitude faunas. As a first approximation, let us say that 90% of the species became extinct (the estimates mentioned earlier are not at all precise and this is a conservative rounding-off). VALENTINE: EXTINCTION AND DEVELOPMENT MODES 611

Table 3. Best guess as to larval types of living and extinct molluscan families which first appeared during the Triassic and for which the epoch of first appearance is recorded

Include planktotrophic larvae?

Triassic epoch yes no Total

Early 2 I 3 Medial 8 17 25 Late 14 4 18

Table 4 gives figures for two cases of differential extinctions. In the first case, 95% ofplanktotrophs are lost, which requires that about 75% ofnonplanktotrophs are lost in order to reach a final figure of 90% extinctions. In the other case, 98% of planktotrophs and therefore 66% of nonplanktotrophs are lost. Thus, these two cases have differential extinction ratios of planktotrophs over nonplanktotrophs of about 1.3: I and 1.5: I respectively, which seems conservative enough. In the first case, the resulting extinctions leave nearly twice as many nonplanktotrophs as planktotrophs in the surviving fauna, and in the second case, over six times as many. If we imagine a clade which had 500 species before the extinction and 50 afterwards, there would have been 281 low- and 25 high-latitude planktotrophic species in a world conforming to the Permian model. A 95% extinction would have reduced the planktotrophs to 15 species, a 98% extinction to six species. If for some reason the nonplanktotroph advantage had been a bit higher in this clade, from deterministic or even stochastic reasons, all the planktotrophs might have been lost. Furthermore, there is some evidence (reviewed above) that planktotrophy was not favored for some time following the extinctions, so that any surviving planktotrophic lineages might have been lost later without diversifying to produce any surviving daughter planktotrophs. In short, the extinction was so vast that a relatively moderate differential could have plausibly resulted in the loss of all planktotrophs in some important clades, even if they formed the major developmental type in those clades during the latest Permian. Since the articulate brachipods were hit harder than the average clade, and the Paleozoic crinoid clades disappear entirely, the loss of planktotrophs in these classes during the Permian-Triassic extinctions is possible even if they were at only a relatively modest disadvantage.

ENVIRONMENTAL CHANGES ASSOCIATED WITH PERMIAN- TRIASSIC EXTINCTIONS Literally scores of possible causes of the Permian-Triassic extinctions have been suggested. It is not appropriate to survey all of these notions here, but a few of the environmental changes which have been postulated are supported by obser- vational data. First, the close of the Permian was accompanied by marine deprovincialization (Valentine and Moores, 1970) which has proven to be even more extensive than originally believed; in middle Permian time, there were between 8 and 12 provinces, while the early Triassic fauna was cosmopolitan (Erwin, 1985). Such a reduction in provinces indicates a significant reduction in endemism and greatly lowers the capacity of the marine biosphere to contain species. As marine provinces are defined partly by geographic and partly by climatic barriers, the deprovincialization during the assemblage of continents to form Pangaea, with the accompanying reduction in geographic barriers, is expectable. On the other hand, 612 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986

Table 4. Numbers of species in marine biospheres modeled after Latest Permian (pre-extinction) and three early Triassic (post-extinction) situations. The late Permian model contains one low latitude province with 75% planktotrophs and two high latitude provinces with 20% planktotrophs each. The low latitude province contains 14,280 species, and high latitude provinces contain 1,430 species each

PlanklO- NonpLanklo- TOlal Ralio of non planks. lrophs lrophs species 10 planks.

Late Permian 10,996 4,714 15,710 2.3/1 Early Triassic 1,100 471 1,571 2.3/1 90% of species lost No developmental bias Early Triassic 549 1,022 1,571 1/1.9 90% of species lost (95% planktotrophs) Early Triassic 220 1,351 1,571 1/6.2 90% of species lost (98% planktotrophs) the creation of Pangaea occurred gradually over several geological periods, and probably cannot account for the extent of the deprovincialization unless accompanied by climatic change; marine temperatures probably were relatively monotonous geographically during earliest Triassic time. Second, the lowest sea level stand during the entire Phanerozoic is associated with this greatest of all extinctions, suggesting a connection. Three possible con- necting principles are a reduction in shelf habitat heterogeneity, a lowering in shelf area, and a decrease in the stability of the trophodynamics of the marine shelf ecosystems. The faunal change across the extinction involves a reduction in the number of fossil community types, suggesting that a reduction in habitat heterogeneity did indeed occur. Also, diversity within the few shelf communities present in the early Triassic was greatly lowered. In fact, the structure of the early Triassic communities is reminiscent of boreal or subarctic communities today, although there is no evidence the temperature was low (merely monotonous). This suggests that seasonality or irregularity in productivity comparable to that in high latitudes today may have played a significant role in structuring the communities (Valentine, 1973; 1983). Finally, evidence of a bolide impact near the Permian-Triassic boundary has been reported for Chinese sections (Sun et aI., 1984; Xu et aI., 1985). If these reports are confirmed and if the impact event proves to have been general rather than local, it will raise the possibility that the impact was associated with the extinctions, such as has been postulated for the end Cretaceous extinction events (Alvarez et aI., 1980). However, there is evidence that the Permian-Triassic extinctions began millions of years before the end of the Permian (Erwin, 1985 for review) and the long lag in rediversification, coupled with the suppression of planktotrophy if that is a real phenomenon, suggest that conditions remained deleterious for millions of years after the boundary as well. In summary, the extinctions accompanied the assembly of Pangaea, a major fall in sea level, and probably a reduction in the range of marine climates. So far as we can tell, the reduction in diversity began long before the end ofthe Permian but probably climaxed near that time, diversity then recovering only gradually.

RECENT PATTERNS OF DEVELOPMENTAL TYPES It is a time-honored method of paleontology to search recent patterns for clues to past events. Knowledge of the major distributional patterns of developmental VALENTINE: EXTINCTION AND DEVELOPMENT MODES 613 modes today has been reviewed recently by Jablonski and Lutz (1983). So far as known, nonplanktotrophs are represented in disproportionately large percentages among small-bodied species, offshore shelf and deep-sea species, and high-latitude species. Planktotrophy is most common among the inner sublittoral invertebrates of the trophics. It is clear, however, that both nonplanktotrophy and planktotrophy have been present among marine invertebrates for hundreds of millions of years, and that both occur together in most environments and probably have for geo- logically long periods. However, the distributions of these developmental types are by no means random, as outlined above. What we have to explain, then, is why there is more of one type in some situations than in others, even though each type may persist at some level, for millions of years, in just about any marine environment. The most general explanation of the dichotomy between planktotrophy and nonplanktotrophy is that it represents a tradeoff between strategies emphasizing fecundity and survival, respectively. For an excellent account of this view, see Christiansen and Fenchel, 1979, who conclude that the strategies may be evo- lutionarily stable. Fecundity and viability are the two major components of natural selection. It is difficult to estimate accurately the relative effects of these components, but workers have been increasingly confident that fecundity is usually the more important (Feldman and Lieberman, 1985 for review). For example, Lewontin (1974) estimated that the contribution of fecundity to the average fitness exceeded the contribution of viability by about 2 to 1. Subsequent work has supported the importance offecundity, and indeed some studies have shown that the contributions of fecundity to average fitness may exceed those of viability by more than 5 to 1 (see Seagar et al., 1982 and references therein). Since a major attribute of species with planktotrophic development is high fecundity, it is clear that a shift from nonplanktotrophic to planktotrophic modes can be driven by important fitness differentials and requires no special explanation, although other adaptive advantages may, of course, be involved. Certainly, the forms and func- tions of planktotrophic larvae, as of other larvae, are shaped by selection for adaptive advantages, but those are issues separate from the origin of the strategy of planktotrophy. It is nonplanktotrophy, with its lowered fecundities, which is the more difficult to account for of the two strategies. Therefore, one would expect to find that nonplanktotrophy is most common where larval survival is at a premium. For example, it has been argued that small-bodied forms cannot produce vast numbers of eggs and, therefore, must often resort to nonplanktotrophy (Jablonski and Lutz, 1983 and references; Chaffee and Lindberg, 1986). Perhaps the low levels of trophic supplies in the deep sea (Banse, 1964; Sanders and Hessler, 1969; Rowe, 1971) may help to account for the abundance of nonplanktotrophs there. For shallow water, among species of the sort which were extinguished during the Permian- Triassic event, nonplanktotrophy seems to be more common where pelagic productivity is most seasonal (Valentine, 1983), and trophic supplies are, therefore, temporally limited. There are theoretical reasons for expecting survival in the plankton to be lowered in such seasonal situations (Boyce, 1979; Valentine, 1983).

SUMMARY AND CONCLUSIONS When dealing with an extinction event so disastrous that survivors number fewer than one in ten, it is difficult to identify causes of differential extinctions, even if they are not simply stochastic. In assessing the possible selective aspects of such an extinction, it is more useful to look at the survivors than at the victims- 614 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986 there are so many victims, and at least in the case of the Permian-Triassic extinctions, we know so little about them compared with the survivors. We can say, with about as much confidence as is possible when dealing with remote events of this sort, that the survivors were nonplanktotrophic in a greater proportion than survivors of a similar extinction in today's biosphere would be if the extinction were random with respect to developmental type, and that nonplanktotrophy seems to have been favored during the early post-extinction rediversifications. However, confidence in interpretations of the developmental strategies of the victims is much lower; they have no descendants by definition, and there is little direct evidence as yet as to their reproductive biologies. The postulated differential extinction patterns can be modeled plausibly, based on analogy with modern patterns. Indeed, the high-latitude faunas of the Permian probably faced much milder climates than high-latitude faunas today, and the Permian tropical provinces were latitudinally more extensive than those of today (Stehli, 1970). There- fore, if anything, we might expect the Permian faunas to contain relatively more planktotrophs than modern faunas, and nonplanktotrophs would have to have been even more heavily favored by the extinction than modeled. At any rate, if we do accept such a differential extinction as a possibility, what can we say about the likely extinction mechanisms? It is doubtful that selection for small body size per se or for physiological adaptation to low temperatures played significant roles in creating bias in developmental types during the Permian-Triassic extinctions, although some of the survivors were rather small. However, if planktotrophs are at any disadvantage when productivity patterns are perturbed, then most of the environmental changes which are most likely to have accompanied the extinctions could have selected against planktotrophy. Near-collapse of pelagic productivity as suggested by some proponents of the bolide impact hypothesis would certainly affect planktotrophic larvae most. However, since the possible long time scale of Permian diversity reduction and of the conditions generally inclement for planktotrophy argue against a sudden extinction event, it is more plausible on present evidence to suggest the productivity changed, perhaps gradually, to a state favoring nonplanktotrophs, and that this state lasted for some millions of years. High seasonality, accompanying the assemblage of Pangaea and the concomitant fall in sea level, might do the trick.

ACKNOWLEDGMENTS

Research supported in part by NSF Grants EAR 81-21212 and EAR-l70Il. D. Hedgecock, R. Strathmann, D. Jablonski and an anonymous reviewer made helpful criticisms of the manuscript. Thanks are due to D. Jablonski and to R. Lutz for data on larval types and for other courtesies.

LITERATURE CITED

Alvarez, L. W., W. Alvarez, F. Asaro and H. Michel. 1980. Extraterrestrial cause for the Cretaceous- Tertiary extinction. Science 208: 1095-1108. Banse, K. 1964. On the vertical distribution of zooplankton in the sea. Progr. Oceanogr. 2: 53-125. Boyce, M. S. 1979. Seasonality and patterns of natural selection for life histories. Am. Nat. 114: 569-583. Chaffee, C. and D. R. Lindberg. 1986. Larval biology of early Cambrian molluscs: the implications of small body size. Bull. Mar. Sci. 39: 536-549. Christiansen, F. G. and T. M. Fenchel. 1979. Evolution of marine invertebrate reproductive patterns. Theoret. Pop. BioI. 16: 267-282. Erwin, D. H. 1985. The Cerithiacea, Subu1itacea, Pyramidellacea and Acteonacea of the Permian Basin, West Texas and New Mexico with a consideration of Permo-Triassic gastropod dynamics. Ph.D. Dissertation, University of California, Santa Barbara. 289 pp. VALENTINE:EXTlNCfIONANDDEVELOPMENTMODES 615

--- and J. W. Valentine. 1984. Differential gastropod extinctions: possible role oflarval strategies. Geol. Soc. Amer. Abstr. with Progr. 16: 503. Feldman, M. W. and U. Lieberman. 1985. A symmetric two-locus fertility model. Generics 109: 229-253. Forney, G. G. 1975. Permo-Triassic sea level change. J. Geol. 83: 773-779. Hallam, A. 1984. Pre-Quaternary sea-level changes. Ann. Rev. Earth Planet. Sci. 12: 205-243. Jablonski, D. 1985. Marine regressions and mass extinctions: a test using the modern biota. Pages 335-354 in J. W. Valentine, ed. Phanerozoic diversity patterns: profiles in macroevolution. Prince- ton University Press, Princeton, New Jersey. --- and R. A. Lutz. 1983. Larval ecology of marine benthic invertebrates: paleobiological implications. Bio. Rev. 58: 21-89. Kummel, B. 1973. Lower Triassic (Scythian) molluscs. Pages 225-233 in A. Hallam, ed. Atlas of palaeobiogeography. Elsevier Sci., Amsterdam. Lewontin, R. C. 1974. The genetic basis of evolutionary change. Columbia Univ. Press, New York. 346 pp. Raup, D. M. 1979. Size of the Permo-Triassic bottleneck and its evolutionary implications. Science 206: 217-218. Rowe, G. T. 1971. Benthic biomass and surface productivity. Pages 441-454 in J. D. Costlow, ed. Fertility of the sea, Vol. 2. Gordon and Breach, New York. Sanders, H. L. and R. R. Hessler. 1969. Ecology of the deep-sea benthos. Science 163: 1419-1424. Seager, R. D., F. J. Ayala and R. W. Marks. 1982. Chromosome interactions in Drosophila mela- nogaster. II. Total fitness. Genetics 102: 485-502. Sepkoski, J. J., Jr. 1981. A factor analytic description of the Phanerozoic marine fossil record. Paleobiology 7: 36-53. Stehli, F. G. 1970. A test of the earth's magnetic field during Permian time. J. Geophys. Res. 75: 3325-3342. Strathmann, R. R. 1978a. The evolution and loss of feeding larval stages of marine invertebrates. Evolution 32: 894-906. ---. 1978b. Progressive vacating of adaptive types during the Phanerozoic. Evolution 32: 907- 914. Sun, Y., Z. Chai, S. Ma, X. Mao, D. Xu, Q. Zhang, Z. Yang, J. Sheng, C. Chen, L. Rui, X. Liang, J. Zhao and J. He. 1984. The discovery of iridium anomaly in the Permian-Triassic boundary clay in Changxing, Zhejiang, China and its significance. Pages 235-245 in Academia Sinica, Developments in geoscience. Science Press, Beijing. Valentine, J. W. 1973. Evolutionary paleoecology of the marine biosphere. Prentice-Hall, Englewood Cliffs, New Jersey. 511 pp. ---. 1983. Seasonality: effects in marine benthic communities. Pages 121-156 in M. J. A. Tevesz and P. L. McCall, eds. Biotic interactions in recent and fossil benthic communities. Plenum Press, New York and London. --- and D. Jablonski. 1983. Larval adaptations and patterns of brachiopod diversity in space and time. Evolution 37: 1052-1061. --- and E. M. Moores. 1970. Plate-tectonic regulation of faunal diversity and sea level: a model. Nature 228: 657-659. ---, T. C. Foin and D. Peart. 1978. A provincial model of Phanerozoic marine diversity. Paleo- biology 4: 55-66. Xu, D.-Y., S.-L. Ma, Z.-F. Chai, X.-Y. Mao, Y.-Y. Sun, Q.-W. Zhang and Z.-Z. Yang. 1985. Abun- dance variation of iridium and trace elements at the Permian/Triassic boundary at Shangsi in China. Nature 314: 154-156.

DATEACCEPTED: February 4, 1986.

ADDRESS: Department of Geological Sciences. University of California. Santa Barbara. California 93106.