Javier Echevarría

Southern Hemisphere Palaeobiogeography of Triassic-Jurassic Marine Bivalves

Bivalves have proven to have a great potential for paleobiogeographic analyses due to their relatively complete fossil record, especially for Mesozoic and Cenozoic times. Being mostly benthonic, they have a large variety of life habits which should be taken into account, particularly in detailed paleobiogeographic studies. We will analyze marine bivalve distribution in the Southern Hemisphere during several successive time slices within the Triassic and Jurassic, an epoch marked by critical geologic and biotic events. This period covers both the biotic recovery after the harshest diversity crisis ever (the Permian/Triassic extinction event), and later also the biotic reaction to another severe crisis at the Triassic/ Jurassic boundary. This allows the opportunity to evaluate the response of paleobiogeographic patterns to such events. The Earth’s configuration drastically changed from a concentration of land masses in a unique supercontinent (Pangea) and two oceans (Tethys and Panthalassa), to a fragmented series of continental land masses. These began to disperse, opening sea corridors which largely affected not only the global distribution of biotas but also paleoclimate and sea paleocurrents as well. This dynamic paleogeography adds an interesting ingredient to the study of past distributions of benthic organisms making it possible to frame them into a physically and biologically changing scenario. ‘Every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents... Of late years, ... a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth, and the organisms now inhabiting it, are but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions.’ Alfred Russel Wallace 1855 S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of Triassic-Jurassic Marine Bivalves, SpringerBriefs Seaways and Landbridges: Southern Hemisphere Biogeographic Connections Through Time, DOI: 10.1007/978-94-007-5098-2_1, The Author(s) 2013 1 Alfred Russel Wallace, considered the nineteenth century’s leading expert on the geographic distribution of animal species and sometimes called the ‘‘father of biogeography’’, already recognized the importance of studying the history of biotas long before moving continents and plate tectonics were heard or even thought of. On the other hand Darwin, who initially recognized the importance of geographic isolation to speciation in his unpublished notebooks (see Lieberman 2003, 2008), did not mention this in his later publications. The study of global biodiversity changes is a hot issue these days, as we humans become aware of the fragility of the Earth system and the urgent need to understand it better to keep it going. One of the key aspects of biodiversity is the distribution of organisms, and biogeography is the discipline which tries to recognize and characterize the causes and patterns of distribution. Biogeography is closely linked to ecology, since the distribution of organisms is governed by ecologic factors, but it cannot ignore other matters, such as the origin of species, their dispersal, and extinction, and thus it can be considered a historic science. Biologists are beginning to investigate the causes of the great global biodiversity changes that are now taking place on the Earth. But paleontologists, who possess a much more extensive time perspective, are constantly observing and surveying the changes in biodiversity produced at various times in the past, and they have the most precise access possible to this very important dimension: time. Thus, paleobiogeography, which studies the distribution of organisms in the past, is a very complex subject that combines information from both biology and the Earth sciences (Cecca 2002), and ‘‘paleobiogeographers can actually monitor how the Earth and its biota have evolved and coevolved’’ (Lieberman 2000). The data provided by the fossil record are increasingly being used in combination with other sorts of data in modern biogeographic analysis. The relationship between geology and biogeography is then unavoidable, and should be based on a reciprocal illumination approach (Morrone 2009). Similarly to biogeography, different approaches can be recognized for paleobiogeography (Rosen 1992, 1995): 1. Descriptive paleobiogeography: recognition and description of the distribution of organisms. The outcome is the definition of biogeographic units or biochoremas. Both quantitative (more frequent in neobiogeography) and qualitative (more subjective) methods can be used. 2. Causal paleobiogeography: examines the causes of the observed distributions. There are many arguments related to theoretic biogeography and the philosophic approaches, which will not be discussed further here (for a good synthesis see Cecca 2009). According to the temporal scale of the processes involved, two main viewpoints allow distinction between ecologic biogeography, with a temporal scale related to biologic cycles, and historic biogeography when long-term processes are analyzed. 3. Applied paleobiogeography: the distribution of organisms can be used to infer the role of ecologic factors, the relation between phylogeny and provinciality, or paleogeographic patterns. 2

Bivalves and evolutionary resilience: old skills and new strategies to recover from the P/T and T/J extinction events

Diversity dynamics among bivalves during the Triassic and Early Jurassic provides the opportunity to analyse the recovery patterns after two mass extinctions: Permian/Triassic and Triassic/Jurassic (T/J). The results presented here are based on a newly compiled worldwide genus-level database and are contrasted to the main morphological characters of the different taxonomical (orders and their constituent families and genera) and ecological groups. Many of such morphological characters are innovations appearing during the time span considered. Diversity and evolutionary rates were assessed and compared between these groups. During the Early Triassic there was a slow recovery, dominated by epifaunal taxa, the order Pectinida being the most diverse. The major post-Permian radiation took place during the Anisian, with several morphological and ecological innovations appearing and/or diversifying. The Late Triassic was a time of great diversification and ecological specialisation. Although the T/J was a true mass extinction for bivalves, it was not indiscriminate as its impact was stronger on specialised orders and not all ecological categories were equally affected. Recovery during earliest Jurassic was fast, confirming the high-evolutionary resilience of bivalve molluscs, except for groups with thick shells and tropical distribution, probably because of a biocalcification crisis.

Ecological signature of the end-Triassic biotic crisis: what do bivalves have to say?

In order to understand the causes underlying the Triassic–Jurassic (T/J) mass extinction, we tested different bivalve features for extinction selectivity, i.e. shell mineralogy, age at the Rhaetian and three main autoecologic traits (feeding mechanism, tiering and motility/attachment). Also, diversity and turnover rates throughout the Triassic and the Early Jurassic were analysed in detail. The dataset employed for this analysis was a precise database at genus level including data from Induan to Sinemurian times. Results point to a true mass extinction for bivalves around the T/J boundary. This extinction was not age-selective at the boundary. Certain analyses suggested that shell mineralogy was a character significantly increasing survival odds, but this relationship seems to reflect selectivity on autoecologic traits. There was no difference in extinction proportions between both feeding types (i.e. deposit feeders and filter feeders); among the other traits, deep burrowers, epifaunal-motile and endobyssate forms seem to have been favoured, while shallow burrowers (and probably reclined forms) were more heavily affected. This pattern suggests an environmental stress at the boundary with some particular issues affecting the different life modes. Models linking magmatism in the Central Atlantic Magmatic Province with the end-Triassic mass extinction are a plausible scenario for this kind of perturbation.

Recovery of Scleractinian Morphologic Diversity During the Early Jurassic in Mendoza Province, Argentina

Abstract. After a diversity peak during the Late Triassic, corals were severely affected by the end-Triassic extinction. The study of their recovery is fundamental for a better understanding of the ecological rearrangement undergone by Early Jurassic marine invertebrate faunas. In this contribution we analyze the morphologic recovery shown by scleractinians in southern Mendoza Province, which is the only place in the Neuquén Basin with marine outcrops spanning the Triassic/Jurassic boundary. A two-stage recovery pattern was recognized. During the first stage (Hettangian—Sinemurian) only solitary corals, most of them discoidal, could be found. After a hiatus encompassing the latest Early Sinemurian and the Late Sinemurian, the second stage (Pliensbachian) developed. A sharp increase in morphological diversity of solitary corals is then recorded, with discoidal, cupolate, patellate, turbinate, trochoid/turbinate, trochoid/ceratoid and maybe cylindrical morphologies. Additionally, colonial forms with low degree of corallite integration (phaceloid and cerioid colonies) appeared in the basin. The diversification trend hereby described provides useful insight regarding the scleractinian recovery after the end-Triassic mass extinction event within southern basins of South America. Furthermore, this recovery pattern is comparable with the one recognized for other regions (Chile, western North America, central Asia) yet it differs from that observed in some European basins. The trend outlined herein for Early Jurassic corals from the Neuquén Basin may reflect a large-scale phenomenon and/or the action of local adverse conditions (such as fluvial influence), which is open to further testing.

A Bivalve Perspective

Mesozoic bivalves have been the subject of many paleobiogeographic studies, either with the aim of recognizing units, to argue about the proposal of opening of seaways and exotic terranes movements, or even to relate biogeography with extinction and evolution. With a few notable exceptions, Northern Hemisphere data were used and frequently conclusions extrapolated worldwide. In the analysis of bivalve geographic distribution, some special issues should be taken into account, such as larval type, mode of life, and tolerance to certain environmental factors, which are here briefly discussed for Southern Hemisphere bivalves. Special attention is paid to the proposed pseudoplanktonic habit as an aid to dispersal, to reef-building bivalves, and to those with special low-oxygen tolerance. For some of the various analyses performed, Triassic-Jurassic bivalve genera were classified according to their paleobiogeographic affinities in truly cosmopolitan, low-latitude (Tethyan), high-latitude (austral or bipolar), trans-temperate (Pacific), and endemic.