Palaeobiogeographical study of the Late Ordovician brachiopods of Iran based on quantitative analysis

AbstractBrachiopods, as one of the most important benthic fauna in the Late Ordovician, show a great abundance and diversity in many continents. In this study, a quantitative approach of multivariate analyses was conducted in order to study the palaeobiogeography of the brachiopods of Iran and their relationship with the brachiopods from other parts of the world, such as Baltica, Avalonia, Laurentia, South China, Kazakhstan, France, and Portugal, during the Late Ordovician (early Katian). Based on the results of the cluster analysis (CA), six main clusters and two sub-clusters were distinguished. The results obtained from the principal component analysis (PCA) method indicate the differentiation of seven main groups and show a high similarity with the main clusters obtained from the CA method. Based on the results of CA, the brachiopods of Bojnourd, Iran, are placed in a separate cluster together with the Avalonian brachiopods such as Shropshire, Powys (Wales), Anglesey (Wales), and Meath (Ireland). Based on the PCA scatter plot, the brachiopods of Bojnourd are placed in the same group with the Avalonian brachiopods. They are distinguished from the brachiopods of the Zagros and Anarak regions in central Iran, which form a separate group with the brachiopods of France, Portugal, and Morocco. The results of the PCA method indicate that the brachiopods from intracratonic Laurentian Basin such as New York, Manitoulin Island, Kentucky, and Indiana form a distinct group and are differentiated from brachiopods from continental margin such as the Appalachian Basin, Tyrone (Northern Ireland), and Girvan (Scotland).Keywords: Brachiopods, Late Ordovician, Palaeobiogeography, Multivariate analyses, Katian IntroductionDuring the Late Ordovician, much of Laurentia was covered by a shallow epicontinental sea that created carbonate platforms in the intracontinental basins and pericratonic shelves (Finnegan et al. 2012). This marine transgression event and the creation of carbonate structures likely indicate a greenhouse warming episode in the Late Ordovician. In addition, some sedimentological and geochemical data support the hypothesis of a cooling episode in the Late Ordovician, leading to the cold and glacial climate of the Hirnantian (Page et al. 2007; Trotter et al. 2008; Buggisch et al. 2010). Interpretations of the climate change during the Late Ordovician are controversial and, like the Boda event in the late Katian, have been interpreted as both a warming and a cooling episode (Fortey & Cocks 2005; Cherns & Wheeley 2007).As one of the most important groups of marine invertebrates of the Paleozoic, brachiopods have a high diversity and abundance in the Ordovician and are therefore of great importance in palaeobiogeographic studies. According to Webby (2000), major fossil groups such as brachiopods show three global diversity maximum during the “Great Ordovician Diversification Event,”. Each palaeocontinent had different brachiopod diversification trajectories during the Ordovician. Based on Harper & Rong (2001), rhynchonelliform brachiopods diversified during the Dapingian and Darriwilian. The Ordovician brachiopods of Baltica show four diversity maxima in the mid-Darriwilian, late Darriwilian, late Sandbian, and late Katian and are different from the brachiopod diversity curves of Avalonia and Gondwana. The brachiopods of Gondwana had one diversity maxima during the late Sandbian (Hints & Harper 2001; Harper & Mac Nicaill 2002; Harper 2006). In South China, brachiopods had three diversity maxima in the early Floian, late Darriwilian, and late Katian (Zhan & Harper 2006).During the late Darriwilian, the earliest rhynchonellid brachiopods appeared in shallow marine environments of palaeotropical regions, including Laurentia, Siberia, and Kazakhstan (Jin 1996). During the Sandbian and early Katian, the total number of rhynchonellide genera increased from five to fifteen. By the late Katian, rhynchonellide brachiopods became widespread in the epicontinental seas of Laurentia and some of the genera such as Hiscobeccus was endemic to Laurentia (Sohrabi & Jin 2013a).The controlling mechanism for the major changes in biogeographic patterns is not well understood. Because of the importance of the Katian brachiopods in palaeogeographic interpretations, a quantitative approach was conducted to investigate the Katian brachiopods of Iran and other regions of the world. Measuring the faunal similarity of the Iranian brachiopods with those from other regions of the world can provide a comprehensive interpretation of the palaeoclimatic and palaeogeographic control on the brachiopods’ evolution and their changing biogeographic patterns. Material & Methods In this study, the palaeobiogeography of the Late Ordovician brachiopods was investigated. The brachiopod data were compiled from various regions, including Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, Morocco, and Iran. Most of the brachiopod data in the present study are from the formations of the early Katian.The Laurentian brachiopods data were obtained from various regions of North America, including the Ottawa Valley, Lake Simcoe, northern Rocky Mountains, western Newfoundland, Manitoulin Island, New York, Kentucky, Indiana, Oklahoma, South Dakota, Nevada, California, Mississippi Valley, Champlain Valley, Hudson Valley, Appalachian Valley, Girvan (Scotland), and Tyrone (Northern Ireland). The brachiopods of Baltica are from the eastern Baltic (Estonia and Lithuania) of Keila and Oandu age (Rõõmusoks 2004; Hints 1998, 2010). The brachiopods of southern Norway are from the Oslo-Asker area, which are related to deep-water facies (Hansen 2008). The Late Ordovician brachiopods of Avalonia are from Waterford and Wexford (southeast Ireland), Meath (east Ireland), and Powys and Anglesey (Wales) (Cocks 2008). In Britain and Ireland, the late Sandbian–early Katian strata include Scotland, Shropshire, Wales, and Ireland. The brachiopods of Scotland are from the Caradoc aged formations in the Girvan area including the Craighead Limestone, Myoch Formation, Whitehouse Bay, and Albany Mudstone Formation. The brachiopods of the Shropshire region include the Caradoc aged formations such as the Acton Scott, Onny Shale, Cheney Longville, Spy Wood Grit, Horderley Sandstone, Whittery Shale, Hagley Shale, Whittery Volcanic, Hoar Edge Grit, Woolston, Smeathen Wood Beds, and Cheney Longville formations. The brachiopods of northwest Wales, Anglesey, include Sandbian–early Katian formations such as the Garn Formation, Llanbabo Formation, and Crewyn Formation. In northern Ireland, the brachiopods are from the Tyrone region including the Bardahessiagh Formation of the Burrellian age. The brachiopods of southern and southeastern Ireland are from Meath, Wexford, and Waterford and include the Burrellian age formations of the Duncannon Group, such as the Annestown Formation, Lower Tramore Volcanic Formation, Grange Hill Formation, Upper Tuffs and Shales of Grangegeeth Volcanic Series, Collon Formation, and Clashford House Formation.The early Katian brachiopods of Kazakhstan are related to the Chu–Ili, Ishim–Selety, and Boshchekul terrains and include the Anderken and Dolankara formations (Popov et al. 2002; Nikitin et al. 2006).In South China, the early Katian brachiopods include the Pagoda Formation (Zhan & Jin 2007; Bergstrӧm et al. 2009). The early Katian brachiopods of Portugal are related to the Cabeço do Peão and Ferradosa formations (Henry & Romano 1978; Cooper 1980; Romano 1980, 1982, 1991; Young 1985, 1988). In Morocco, the Late Ordovician brachiopods are from the Khabt-el-Hajar Formation (Fortey & Cocks 2005). Most of the Katian rhynchonelliform brachiopods of Gondwana are from the western Mediterranean regions such as France, Spain, and Portugal (Torsvik & Cocks 2011). In Iran, the Late Ordovician brachiopods include the Bojnourd region (Ghelli Formation), the Anarak region in Central Iran (Chah Gonbad Formation), and the Zagros region (Siahu Formation). Multivariate analyses: The data used in this study include a large number of genera in general, as well as a large number of endemic genera from different regions. In this study, multivariate analyses were conducted based on the early Katian brachiopods dataset to investigate the palaeobiogeography of the brachiopods of Iran and their relationships with the other brachiopods from Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, and Morocco.The dataset was generated based on binary data that includes 261 brachiopod genera of the early Katian from 30 geographical regions. In this dataset, the geographic regions were selected as locations and the brachiopod genera as variables (presence or absence). The dataset was subjected to multivariate analyses using PAST software (Hammer et al. 2001; Hammer & Harper 2006), which was developed for analyzing paleontological data.In order to distinguish the distribution patterns of the brachiopods in time and space, cluster analysis (CA) and principal component analysis (PCA) methods were employed. To perform cluster analysis (CA), a dendrogram algorithm was generated based on the paired group method by using the Raup-Crick similarity coefficient. The Raup-Crick similarity coefficient shows fully segregated clusters.The dataset was also subjected to PCA by using the variance-covariance algorithm in PAST software. The result was plotted in the PCA scatterplot, based on principal components 1 (X-axis) and 2 (Y-axis), which correspond to eigenvalues 1 and 2, respectively. Discussion of Results & Conclusions In this study, the results of multivariate analyses of the Late Ordovician (early Katian) brachiopods from 30 geographical regions including Laurentia, Baltica, Avalonia, South China, Kazakhstan, France, Portugal, Morocco, and Iran indicate several palaeogeographic patterns. Based on the results of CA and PCA, several clusters and gr