Paleobiogeography of Mass Extinction

Study of the diversity, distribution patterns, and endemism of the Late Tithonian ammonites is especially important because soon, at the end-Tithonian, the majority of them died. Arguably, the end of the Jurassic marks a mass extinction event (Raup and Sepkoski, 1986; but for an opposite view, see Hallam, 1986; and Hallam and Wignall, 1997). Hallam (1986) claimed that family level extinction data of ammonites are not impressive, but he admitted that taxonomic resolution down to lower level along with stratigraphic precision can only help in understanding the extent of extinction. Since then, floods of information on ammonite diversity, pale-obiogeography, and refinements of stratigraphic ranges across the Jurassic-Cretaceous boundary have emerged (e.g., see Fatmi and Zeiss, 1994; Tavera et al., 1986; Riccardi, 1991; Enay and Cariou, 1997; Cecca, 1999; Oloriz et al., 1999;

Zakharov and Rogov, 2003; and many others) and a new treatise on Cretaceous ammonites has appeared (Wright et al., 1996). Literature scanning reveals that out of 84 ammonite genera (excluding leiostracans) of the Late Tithonian, 67 did not pass the Jurassic-Cretaceous boundary, thus showing a significant extinction signal (80%). Hallam (1986) tried to show that the end-Jurassic extinction event is of moderate scale and regional in nature. But many other workers also have investigated the extinctions of other organisms during the end-Tithonian and found significant extinction peaks. For example, Kelly (1984-92) analyzed bivalve data of England and Zakharov and Yanine (1975) also demonstrated extinction on the basis of Central Russian bivalves. In both areas, bivalve diversity was reduced significantly. Ager (1975, in Hallam, 1986), Davis (1975, in Hallam, 1986) and Sandy (1988) attempted to assess brachiopod mortality across the Jurassic-Cretaceous boundary, while others studied foraminifers (e.g., Jenkins and Murray, 1981), coc-coliths (Hamilton, 1982) and ostracods (Ascoli et al., 1984; Whatley, 1988). All of them noted a major turn over of taxa at the boundary. More significantly, like the K-T boundary extinction, the Jurassic-Cretaceous extinction event also affected terrestrial and taxonomically different biota. Bakker (1993) recognized near-total extinction of plesiosaur reptiles and Benton (1995) claimed greater mortality of continental faunas at family level than of marine groups at the end of the Tithonian. It would be interesting to know, therefore, the biogeographic distribution patterns of the Late Tithonian ammonites which may, in turn, help in understanding the nature and causes of the extinction (also see Raup and Jablonski, 1993).

Late Tithonian ammonite genera have wide geographic distribution. Longitudinally, they are found from the boreal eastern Pacific to Austral Indonesia and the latitudinal distribution ranges from the Arctic regions in the north to Antarctica in the southern hemisphere. There are at least 60 major fossil occurrences in the world from where different assemblages of Late Tithonian ammonites have been recorded (see Cecca, 1999; Zakharov and Rogov, 2003). Since the number of genera in many localities is few and the Late Jurassic ammonites overall show strong endemism, we have plotted diversity data on a provincial, subprovincial, and regional basis (Fig. 17.3). Geographic and stratigraphic distributions have been drawn from the literature cited above and are also subjected to an updating and evaluation procedure. Stratigraphic ranges of some genera have been shown to be different in different literature, but we follow the data provided by the new Treatise (Wright et al., 1996).

The distribution of ammonite genera shows a broad latitudinal gradient with diversity decreasing towards at least the South Polar Region (see also Tables 17.1-3). Diversity is maximum in the subtropics on either side of the equator. The Mediterranean diversity is represented by 44 genera and in the south, the Indo-Madagascan Province and the Himalayas include 29 genera (see Fig. 17.3 and Tables 17.1-2). Similar trends of different taxa also existed during other mass extinction events such as the K-T (see Stanley, 1984, 1988; Jablonski, 1986; Zinsmeister et al., 1989). High latitude Tithonian ammonites were more diverse in some provinces in the northern hemisphere than in the southern hemisphere. This is due to latitudinal shifts towards the north shown by many Tethyan genera during the Late Tithonian (see Zakharov

Fig. 17.3 Diverisity of ammonite genera in different regions during the Late Tithonian. The base map is modified after Enay and Cariou (1997). A = Kutch; B = Madagascar; C = Northwest Pakistan; D = Himalayas; E = Indonesia; F = New Zealand; G = Antarctica; H = Southern South America; I = Central-West South America; J=Northwestern South America; K= Mexico; L= Cuba; M= North Africa; N=Iraq; O = Southern Spain; P = Boreal Pacific, shown at Primorje; Q = Boreal East Europe, shown at Crimea; R = Boreal West Europe, shown at England; S = Boreal Eastern Pacific, California. Sources are mentioned in the text.

Fig. 17.3 Diverisity of ammonite genera in different regions during the Late Tithonian. The base map is modified after Enay and Cariou (1997). A = Kutch; B = Madagascar; C = Northwest Pakistan; D = Himalayas; E = Indonesia; F = New Zealand; G = Antarctica; H = Southern South America; I = Central-West South America; J=Northwestern South America; K= Mexico; L= Cuba; M= North Africa; N=Iraq; O = Southern Spain; P = Boreal Pacific, shown at Primorje; Q = Boreal East Europe, shown at Crimea; R = Boreal West Europe, shown at England; S = Boreal Eastern Pacific, California. Sources are mentioned in the text.

and Rogov, 2003; and references therein). Climatic equability during the entire Jurassic was envisaged by Hallam (1969). Since the Arctic taxa are not found south of 30° N, Zakharov and Rogov (2003) believed in the existence of a temperature gradient during the Late Jurassic - Early Cretaceous period. Jeletzky (1984) also suggested a cooling event during the terminal Tithonian on the eastern coast of the Paleopacific. In addition to fossil data, recent general circulation model simulations of the Jurassic climate also reveal latitudinal thermal gradients at certain times and a semi-arid climate in many tropical areas especially during the Late Jurassic - Early Cretaceous (Page, 2005). Our study on Tithopeltoceras also shows latitudinal control on ammonite distribution (Shome et al., 2005). Surprisingly, high latitude southern hemisphere Austral faunas are marked by low endemism (12%).

The Majority of Late Tithonian ammonite genera became extinct at the Jurassic-Cretaceous boundary (80% as mentioned earlier). We intend here to study the extinction intensity of ammonites in different areas and to know whether there is any geographic control on extinction intensity. During the K-T extinction, some workers believe (e.g., Copper, 1977; Stanley, 1988; Banerjee and Boyajion, 1996; but see Raup and Jablonski, 1993, for a different view), that tropical areas were more severely affected than temperate or polar regions. This is envisaged because tropical taxa show greater endemism than cold water forms (Jablonski, 1986). We have followed Raup and Jablonski (1993) in choosing the extinction metric for our analysis. According to them "extinction was quantified as the proportion of genera found in an assemblage, or local group of assemblages, that suffered global extinction in the final stage" This is adopted here mainly because of the rarity of truly continuous stratigraphic sections across the Jurassic-Cretaceous boundary, and direct comparison of assemblages immediately below and above the boundary would result in only a limited analysis. However, unlike Raup and Jablonski (1993), we have included only Late Tithonian genera and thus have avoided genera which became extinct during the Early Tithonian (cf. Hallam, 1986). Otherwise, extinction

Extinction Intensity
Fig. 17.4 Extinction intensity (percentage) in important fossil-bearing occurrences shown in Fig. 17.3.

values at the Jurassic-Cretaceous boundary would be artificially elevated. Extinction percentages of different provinces/regions are plotted in Fig. 17.4.

Extinction appears to be high and uniform throughout the world except in the Panboreal Superrealm. Here, the mean extinction is 50%. This is mainly for two reasons. First, some Boreal realms have low diversity (see Fig. 17.3). Second, the Panboreal Superrealm was invaded by many Tethyan genera during the Late Tithonian (see Table 17.3), some of which survived the Jurassic-Cretaceous boundary crisis (for details see below). In the boreal west Pacific Realm, diversity is less; of seven genera, three are Tethyan forms which crossed the Jurassic-Cretaceous boundary, but surviving genera are inevitably pooled in the global data, thus decreasing the regional extinction percentage. The majority of Boreal endemics (12 out of 15, i.e., 80%) as well as all immigrant genera died out in these areas during the J-K transition. If the Tethyan genera are removed from the count, the "sanctuaries" of extinction in northern higher latitudes disappear and the mean extinction becomes 86%. Thus, extinction patterns show no gradient across either latitude or longitude. The mean extinction is 73% and the median is 77%. This indicates a quite high extinction signal all over the world.

Kutch and the tropical Mediterranean southern Spain have higher extinction percentages (91% and 94%, respectively) and can be considered as "hot spots" of extinction. In Kutch, although the lowermost Cretaceous includes marine strata, the basin became extremely shallow (Biswas, 1977). Few gastropod and bivalve genera survived the Jurassic-Cretaceous boundary, but they also show species level turnover (Das, 2002; authors' personal observation). All ammonite genera including leiostracans suffered regional extermination (Bardhan et al., 1989). Admittedly, the present extinction metric has the disadvantage that it fails to locate a local hot spot because the genera that died in that area may have survived elsewhere. In Kutch it so happened that all the genera also went extinct globally, so that this local hot spot was not hidden by this effect. This is perhaps due to the fact that the Kutch Sea harbored shallow water faunas which suffered most during the extinction.

The Mediterranean Province has an unusual number of endemics. A total of 14 of the 33 genera in Spain are endemic (42%) and none of them crossed the J-K boundary. This has greatly increased the extinction value. Kutch and southern Spain were located in the subtropical latitudes, but we do not believe that these areas were worst hit during the extinction. The high latitude Austral faunas (in spite of low endemism) have higher extinction values than other tropical areas.

Analysis of 17 genera surviving the extinction indicates that they belong to six families out of nine which persisted up to the Late Tithonian. Neocomitidae were most diverse (14 genera at the J-K boundary of which seven genera entered into the Cretaceous). Unfortunately, we do not have at present the species level database of all genera and, therefore, cannot tell whether any selectivity of survival of species-rich genera existed. However, three successful genera, i.e., Berriasella, Lytohoplites, and Spiticeras were diverse during the Late Tithonian (see Collignon, 1960; Tavera, 1985).

The surviving tropical genera quickly diversified immediately after the boundary in the earliest Berriasian and belonged to two important families (Neocomitidae and Olcostephanidae) of the Early Cretaceous (see Wright et al., 1996, Table 17.2). It is tempting to suggest that like background extinvction, cosmopolitan genera are resistant to mass extinction (see Jablonski, 1989) at the Jurassic-Cretaceous boundary. However, 13 of the 19 cosmopolitan genera (with at least distribution in more than two provinces, see Table 17.1) went extinct. One selective element, however, becomes apparent; of 15 Tethyan genera which show climatic shifts towards northern high latitudes (see Table 17.3), eight survived the J-K boundary crisis. This perhaps suggests their ability to maintain range expansion in times of environmental stress. We have excluded leiostracan data from our analysis. Leiostracans have a higher rate of survival (only 30% became extinct) and were present in the Early Cretaceous both in the tropical and extratropical areas (Arkell et al., 1957; Wright et al., 1996; and Shome and Roy, 2006). Leiostracans, which are believed to be offshore dwellers (not necessarily deeper water, see Kennedy and Cobban, 1976; Bardhan et al., 1993), have lower extinction rates throughout the Jurassic (House and Senior, 1981).

The Late Tithonian ammonites are in general less diverse or poorly known from the whole Mediterranean Tethys (Cecca, 1999). This is due to large-scale destruction of the southern European platform and epicontinental habitat attributed to marine regression (Fourcade et al., 1991). But in southern Spain the situation was quite different. Here the ammonites were the most diverse in the world (33 genera, see Table 1, and Tavera et al., 1986) and belong to the Ammonitico Rosso facies, which represented the pelagic, epioceanic environments including submarine highs (see Cecca, 1992; Cecca, 1999). This area is one of the rare sites where continuous sections across the Jurassic-Cretaceous boundary and the earliest Cretaceous ammonite assemblages are found (Tavera, 1985; Cecca, 1998). Berriasella, which suffered regional extinction elsewhere, crossed the boundary here and diversified quickly afterwards (see Tavera, 1985; Tvera et al., 1986). There is a general agreement that the Jurassic-Cretaceous boundary interval witnessed a major sea level lowering and regression took place in all provinces (e.g., Cecca, 1999; Zakharov and Rogov, 2003). This may be one of the causul factors of large-scale extinction of the Jurassic ammonites and other groups. A few genera survived, perhaps due to chance, with areas like Subbetic Spain serving as an "island refugia" (see Kauffman and Erwin, 1995) where Berriasella and several other Mediterranean genera survived.

In conclusion, analysis of the biogeography of the end-Jurassic ammonites reveals that extinction is homogeneous throughout the world. It shows no selectivity by latitude (except a few extinction "hot spots") and geographic range (both endemic and pandemic groups suffered equally). Shallow water forms suffered most, while pelagic leiostracans have significantly lower extinction intensity, but this is due to low extinction rate in this group during the entire Jurassic. The Jurassic-Cretaceous ammonite extinction pattern is consistent with that observed in bivalves during the K-T mass extinction event (see Raup and Jablonski, 1993; Jablonski and Raup, 1995).

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