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 nonplank- totrophic, 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 per- haps archaeogastropods (Erwin and Valentine, 1984) were nearly or quite extin- guished 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 extin- guished 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, 607 608 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986 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 develop- mental 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 plankto- trophs 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 non- planktotrophic 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 fam- ilies that include