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Khosla, A. and Lucas, S.G., eds., 2016, Cretaceous Period: Biotic Diversity and Biogeography. New Mexico Museum of Natural History and Science Bulletin 71.

STRATIGRAPHIC RESOLUTION AND THE K-PG OSTRACODE RECORD

DAIANE CEOLIN*, CRISTIANINI TRESCASTRO BERGUE AND GERSON FAUTH

Instituto Tecnológico de Micropaleontologia, itt Fossil, Universidade do Vale do Rio dos Sinos, Avenida Unisinos, n° 950, 93022-750, Rio Grande do Sul, Brazil; * Corresponding author. Daiane Ceolin, Email:daianeceolin@yahoo.com.br

Abstract—The paleontological mass extinction event at the end of Mesozoic (the so-called K-Pg mass extinction) has generated several, and partly controversial or even sensational hypotheses as to its cause. Podocopida ostracodes, which are benthic or nektobenthic, were affected in a non-catastrophic mode. Not withstanding, reviewing the studies on this subject it becomes clear that stratigraphic resolution and taxonomic accuracy exerted a strong influence on the significance attributed to this event in previous studies. Although in this event no suprageneric taxon became extinct, as was the case in the Permo-Triassic event, the richness of such genera as Paleocosta Benson, Hysterocythereis Ceolin and Whatley, Orthrocosta Ceolin and Whatley, Keijia Teeter, Actinocythereis Puri, and Protocosta Bertels increased in some of the studied K-Pg sections. Some ecological peculiarities are briefly commented on in an attempt to explain this selective pattern of Podocopida ostracode extinction.

INTRODUCTION Ostracodes are tiny crustaceans (usually ~ 0.4 mm-1.0 mm) that inhabit most marine and nonmarine environments, and whose fossil record ranges from Ordovician to Quaternary. During their life span podocopid ostracodes present up to eight instars before reaching adulthood (see Rodriguez-Lázaro and Ruiz-Muñoz, 2012 for more details). Due to their widespread occurrence and preservation potential of the carapace, ostracodes provide one of the most complete and abundant fossil records among animals. Their evolutionary success is demonstrated by the diversity of aquatic environments they thrive in, ranging from the abyssal depths of oceans to the phytotelmata of bromelids, hot springs, caves, etc. The richness resulting from such diversity of habitats makes them both good paleoenvironmental indicators and biostratigraphic markers. During the Phanerozoic, five big mass extinction events (the so called “Big Five”) took place (Ausich and Lane, 1999). The sensitivity of the organisms to these events depends on how the ecosystems are affected as well as on the habitats and habits of the different species. The K-Pg Mass Extinction (KPME) at the end of the Cretaceous (66 Ma) is not the most extensive of the mass extinction events recognized, but undoubtedly is the most widely known, even to non-academics. During this biological crisis the dinosaurs and the ammonoids became extinct, as also the belemnites, etc. Approximately 26% of the families and 75% of the species in both marine and terrestrial environments became extinct between the Maastrichtian and the Danian (Alvarez et al., 1980; Surlyk, 1990; Ridley, 2006; Gradstein et al., 2012). Some workers advocated an instantaneous event (Tappan, 1979) others a more gradual one (Gamper, 1977; Keller et al., 1993; Coccioni and Galeotti, 1994; Skelton, 2003) and this remains a controversial subject. The major theories argue climatic changes (e.g., Bramlette, 1965; Tappan, 1979), asteroid impacts (e.g., Alvarez et al., 1980; Kent, 1981; Alvarez, 1987; Smith, 1990; Canudo et al., 1991; Olsom et al., 1997), volcanism (e.g., Zoller et al., 1983; Hallam, 1987; McLean, 1985; Baksi et al., 1994; Keller et al., 2008, 2011; Punekar et al., 2014, 2015) or multiple causal agents (e.g., Ward et al., 1995; Hallam, 2005; Alegret et al., 2012; Renne et al., 2015; Mateo et al., 2015). The stratigraphic record indicates the coincidence of the asteroid impact evidenced by metamorphic quartz and microtectites. Moreover, this event is associated with millimetric iridium-rich layers, which is an element usually associated with both extraterrestrial bodies and volcanism (Alvarez et al., 1980; Officer and Drake, 1985; Alvarez, 1987). This study discusses the relationship between the stratigraphic resolution and the response of Ostracoda to the KPME based on the fossil record of the order Podocopida in outcropping K-Pg sections, both marine and nonmarine worldwide (Fig. 1). The podocopids comprise the vast majority of fossil ostracod assemblages and have benthic or nektobenthic habits.

A SHORT ACCOUNT OF K-PG OSTRACODE RESEARCH Among the five biggest Phanerozoic mass extinction events, only the most representative of them, the Permian-Triassic one, significantly

affected the ostracodes. The Beyrichiocopina, Binodicopina and Carbonitoidea were completely extinguished, while Cytheroidea and Cypridoidea became more diversified (Horne et al., 2002). The rare studies on the KPME, however, demonstrate a distinct scenario. The paucity of studies on K-Pg ostracodes results mostly from the scarcity of good sedimentary sections recording this boundary. The vast majority of the sections are marine, the study by Bressière et al. (1980) on lacustrine deposits from the Hautes Corbières (France) being the only one carried out on nonmarine assemblages. After the pioneering study by Salahi (1966), new contributions appeared only in the 1980s and 1990s (Fig. 1). These studies are mostly concentrated in the equatorial region, mainly North Africa at the K-Pg stratotype (El Kef, Tunisia). According to Donze et al. (1982), El Kef records many Maastrichtian species continuing in the Danian. In the adjacent area of Djebel Dry (Algerie), a similar pattern occurs (Damotte and Fleury, 1987). Studies by Damotte (1991, 1993) and Colin et al. (1998) north of Mali and in other Algerian sections (Djebel Dyr and South Oran) also did not show a catastrophic extinction pattern. In South America, Guernet and Danelian (2006) observed in the Demerara Rise, Suriname (ODP Sites 1260 and 1261), a selective ostracode extinction attributed to a drop in oceanic productivity. The Poty section, in the Pernambuco Basin, is the only K-Pg section cropping out in Brazil. Taxonomic, biostratigraphic and geochemical studies carried out there revealed non-catastrophic ostracode extinction (Fauth et al., 2005; Rodrigues et al., 2014). In the Neuquén Basin (Argentina), Bertels (1973) recorded rich and well preserved ostracode assemblages and a possible hiatus in the Upper Maastrichtian. In some Neuquén locations, there are paraconformity surfaces not evident in the lithology, however, their importance to the fossil record was not discussed in that work.

THE OSTRACODS AND THE K-PG MASS EXTINCTION EVENT

Mass extinction events are global ecological crises with episodes of extinction, survival and origination that cause, to variable degrees, biodiversity turnovers at either a gradual or catastrophic tempo. The magnitude of a mass extinction is assessed by the amount of species extinguished in a time interval (Signor and Lipps, 1982; Smith, 1994; MacLeod et al., 1997) (Fig. 2). The taxa react differently by: i) disappearing during the catastrophic event; ii) exploring resources and habitats vacated and increasing their populations (opportunists); iii) escaping from adverse conditions and returning when stabilized (Lazarus effect), or iv) resisting the modified environment (survivors) (Kauffman and Harries, 1996; MacLeod et al., 1997). The assessment of the influence of mass extinction events on a particular group depends on its biological characteristics and the nature of the fossil record (Robertson et al., 2013). Maddocks (1985), for instance, claims that the ostracode feeding strategy does not influence the species survival as observed in Bivalvia. The stratigraphic resolution, sedimentation processes and sampling must also be taken into account (Holland and Patzkowsky, 2015). Studies focused exclusively on taxonomy and with poor sampling resolution might result in a distorted chronostratigraphic distribution of the taxa and false horizons of origin

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and extinction, since the local last occurrence of a species might be a consequence of erosive processes or the absence of sedimentation. Prior studies demonstrated that the last occurrences or the faunal turnovers during the K-Pg were actually environmentally induced local events (Maddocks, 1985; Damotte and Fleury, 1987; Brouwers and De Deckker, 1993), reinforcing the hypothesis of Holland and Patzkowsky (2015). The K-Pg, as with other mass extinction events, represents an opportunity to assess the effect of global environmental crises on biological evolution. Its influence on ostracodes is particularly fascinating not only because of the good record in some sections, but because of the existence of genera with living representatives, which allows modern comparisons. A representative example of the influence of the sampling in assemblage interpretation is given by the Neuquén Basin, where one of the best records of K-Pg ostracodes is found. During its sedimentological history a transgressive event from the Atlantic Ocean caused the deposition of the Jagüel (Maastrichtian-Danian) and Roca (Danian) formations (Camacho, 1992; Malumián, 1999; Del Río, 2011), which constitute the only marine deposits of this basin during this time. According to Bertels (1975a), in the Huantrai-co section the Maastrichtian faunas of ostracodes and foraminifers were totally replaced in the Danian. However, in a study at Cerro Azul, a new K-Pg section described by Musso et al. (2012), some species previously identified only in the Danian by Bertels, were also recorded in the Maastrichtian (Ceolin et al., 2015) (Fig. 3). The first stratigraphic interpretations proposed for the Neuquén Basin were influenced by the regressive processes identified in the Roca Formation, which indicate a shallowing trend with the absence of hiatuses (Barrio, 1990). Moreover, the K-Pg boundary in the Neuquén Basin proposed by Bertels (1975b) was in the Roca Formation, where the Upper Maastrichtian was absent in the studied section (Bertels, 1968). Later, sedimentological and biostratigraphic studies carried out in different areas of that basin identified the K-Pg boundary in the Jagüel Formation and positioned the Roca Formation in the Upper Danian (Barrio, 1990; Rodriguez, 2011). In synthesis, two general deductions are applicable from the worldwide K-Pg ostracode record. First, no suprageneric taxon became extinct, which means that all the families, superfamilies, orders and so on, registered in the Danian were already present in the Maastrichtian. Second, a significant number of Maastrichtian species continued through the Danian, demonstrating a selective extinction pattern (Fig.

FIGURE 1. Location map of the K-Pg boundary sections with studies on Ostracoda: 1, Alaska (Brouwers and De Deckker, 1993); 2*, Caribbean Sea (Aumond et al., 2010); 3, Poty quarry (Fauth et al., 2005); 4*, Pelotas Basin (Ceolin et al., 2011); 5, Neuquén Basin (Bertels, 1975b); 6, Djebel Dry, Algeria (Damotte and Fleury, 1987); 7, Texas (Maddocks, 1985); 8, Ellès, Tunisia, (Said-Benzarti, 1988); 9, El Kef, Tunisia (Donze et al., 1982); 10, Tilemsi, Mali (Colin et al., 1998); 11, Suriname (Guernet and Danelian, 2006); 12, Libya; 13, Corbières, France. The locations with asterisks correspond to studies based on coring and are not used in the interpretations herein proposed.

FIGURE 2. Depositional hiatus: the presence of a hiatus is suggestive of a catastrophic event: a, hypothetical distribution of biostratigraphical ranges for taxa A-N across an event horizon; b, distribution of the same datum as they would appear in the stratigraphical record with a hiatus surface. Adapted from MacLeod et al. (1997).

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FIGURE 3. Some of the most significant species of the Maastrichtian and Danian ostracodes from Cerro Azul section, Neuquén Basin, Argentina. Figures 1-12 are Maastrichtian species; figures 13-32 are Danian species: 1, Keijia flexuosa (Bertels, 1975a); 2, Keijia huantraicoensis (Bertels, 1969); 3, Aleisocythereis polikothonus Ceolin and Whatley, 2015; 4, Actinocythereis tuberculata Bertels, 1974; 5, Cythereis stratios Ceolin and Whatley, 2015; 6, Cythereis trajectiones Ceolin and Whatley, 2015; 7, Castillocythereis albertoriccardii Ceolin and Whatley, 2015; 8, Hysterocythereis coinotes Ceolin and Whatley, 2015; 9, Hysterocythereis attenuata (Bertels, 1975a); 10, Orthrocosta phantasia Ceolin and Whatley, 2015; 11, Sthenarocythereis erymnos Ceolin and Whatley, 2015; 12, Petalocythereis venusta (Bertels, 1975a); 13, Paracypris bertelsae Ceolin and Whatley, 2015; 14, Bythoceratina cheleutos Ceolin and Whatley, 2015; 15, Phelocyprideis acardomesido Ceolin and Whatley, 2015; 16, Cytheropteron hyperdictyon Ceolin and Whatley, 2015; 17, Hemingwayella verrucosus Ceolin and Whatley, 2015; 18, Heinia prostratopleuricos Ceolin and Whatley, 2015; 19, Keijia circulodictyon Ceolin and Whatley, 2015; 20, Keijia kratistos Ceolin and Whatley, 2015; 21, Ameghinocythere archaios Ceolin and Whatley, 2015; 22, Actinocythereis indigena Bertels, 1969; 23, Actinocythereis rex Bertels, 1973; 24, Actinocythereis biposterospinata Bertels, 1973; 25, Castillocythereis multicastrum Ceolin and Whatley, 2015; 26, Cythereis clibanarius Ceolin and Whatley, 2015; 27, Hysterocythereis paredros Ceolin and Whatley, 2015; 28, Hysterocythereis diversotuberculatus Ceolin and Whatley, 2015; 29, Hysterocythereis inconnexa (Bertels, 1973); 30, Orthrocosta decores Ceolin and Whatley, 2015; 31, Orthrocosta atopos Ceolin and Whatley, 2015; 32, Petalocythereis shilleri (Bertels, 1973). Scale bars are 100μm.

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4). The biological principles that determined the latter are not easily understood, because the autoecologic knowledge of ostracode species (even the living ones) is very limited. However, some biological characteristics of the ostracodes that could have influenced their adaptativeness during ecological crisis are: (1) Phytal and animal substrates: species of some marine genera (e.g., Aurila Pokorny, Loxoconcha Sars and Paradoxostoma Fisher) live on algae and depend on them not only as a substrate, but also as a food resource. Paradoxostoma has the mandibles transformed into a device for sucking the juices of the algae they live on (Morkhoven, 1963). In nonmarine environments, there are species that are associated with aquatic macrophytes. There are also some marine species living on other animals. It is plausible, therefore, that the extinction of aquatic algae and plants would have affected the ostracodes living on them. (2) Epibenthic and interstitial habits: In those highly diversified genera there are species adapted to different substrates (Frenzel and Boomer, 2005). Krithe Brady, Crosskey and Robertson, is a typical marine ostracode with infaunal species (Coles et al., 1994). Some species of Parapolycope Sars with this habit were also recorded in Japan (Tanaka and Tsukagoshi, 2010). Carbonel et al. (1986) comment on the transition from the epifaunal to the interstitial habit in the nonmarine genus Kovalevskiella Carbonel, Colin, Danielopol and Londeix during an environmental crisis in the Oligocene-Miocene. The genera Comontocypris Wouters and Danipussella Wouters also have interstitial species. Possibly, the epibenthic/nektobenthic and interstitial species responded differently to the K-Pg ecological changes, though it is arbitrary to say which one took advantage. Another example is Aratrocypris Whatley, Ayress, Downing, Harlow and Kesler, which has a plug-like anteroventral projection that is speculated to be to forward movements, or a device for feeding habit of this species that can live in an infaunal or epifaunal environment. (3) Reproductive strategies: The occurence of brood-rearing of either eggs (e.g., Platycopina) or juvenile instars (e.g., Xestoleberis Sars and Cyprideis Jones) might have supplied different ecological responses to ostracods during environmental crises. (4) Food strategies: Besides the widespread detritivorous habit, there are also ostracod predators (Sohn and Kornicker, 1972), commensal and even parasitic on other crustaceans (Mckenzie, 1972). These different strategies result from structural modifications in appendages that allowed suctorial feeding. Ventrally flattened carapaces are also common in these species. Morphological peculiarities in the mandibles are present in Platycopida and Darwinuloidea, whose filter combs improve food capture. Besides the occurrence of survival species, we also observe a significant increase in the post-event diversity (Fig. 5). Genera such

as Paleocosta Benson (El Kef), Hysterocythereis Ceolin and Whatley, Orthrocosta Ceolin and Whatley, Keijia Teeter, Cythereis Jones and Actinocythereis Puri (Cerro Azul) and Protocosta Bertels (Poty) become richer in the Danian. This observation is neatly exemplified by the Cerro Azul assemblages, where 21 species became extinct in the Maastrichtian, 18 survived and 28 new ones appeared in the Danian. On the other hand, in El Kef the assemblages are less diversified in the Danian than in the Maastrichtian. The explanation for this contrasting pattern might involve both methodological procedures and the local environmental conditions.

CONCLUSION Changes in pre- and post-K-Pg assemblages were observed in all studies revised, with variable incidence of surviving species. In the Brazos River (Maddocks, 1985), Algeria (Damotte and Fleury, 1987), Neuquén Basin (Bertels, 1973) and North Alaska (Brouwers and De Deckker, 1993) the occurrence of hiatuses, transgressive and regressive processes induced a false interpretation of Maastrichtian-Danian faunal turnovers. Different from the pattern observed in planktic organisms, the extinction in the podocopid ostracodes was gradual and usually followed by richness increases in the Danian. Probably, the benthic habit weakened this event as seen in foraminifers, where the planktic species where severely affected, but not the benthic ones (Culver, 2003). Moreover, ecological peculiarities of the Maastrichtian communities are to some degree probably responsible for the selective ostracode extinction recorded. The Danian diversification demonstrates adaptations to new environmental conditions resulting from this ecological crisis, as seen in events of lesser magnitude (e.g., Paleocene-Eocene, Eocene-Oligocene) (Benson, 1983, 1990; Zarikian, 2015).

ACKNOWLEDGMENTS This chapter is partly based on the doctoral thesis of the senior author. We are grateful to Dr. Andrea Concheyro (Universidad de Buenos Aires, UBA, Argentina) for the samples and help during the doctoral thesis. Michele Goulart (itt Fossil, UNISINOS) did the careful SEM work and Guilherme Krahl (itt Fossil, UNISINOS) gave valuable advice on the creation of some figures. The authors would like to thank Dr. Spencer Lucas and Dr. Robin Whatley for their helpful reviews.

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