Global assessment of the palaeobiological history of vascular plantsnew frontiers and perspectives from the fossil record

Resumo

This doctoral dissertation has been written in the form of a compendium of publications. In total, seven peer-reviewed papers, which have been published or at least, accepted for publication, have been included between chapters two and eight. Chapter nine corresponds to the final conclusions. The last chapter embraces the complete list of literature cited including all references of the articles and also the references quoted in this chapter. Finally, an appendix list with the original accepted manuscripts has been added. The complete pool of papers have as its main aim to address the theoretical principles revealed by Stephen Jay Gould and Thomas Schopf, and more especially, the Sepkoski's quantitative vision of the fossil record from a whole methodological view. This thesis addresses the fundamental aspects of classical inference in palaeobiology and the use of data from the fossil record, attending to the particularities of the fossil record of vascular plants. In this sense, different inference methods are applied to extract the maximum information from the record, as well as to test biases inherent in it. In short, the main events of the life history of vascular plants on Earth is described and discussed. A complete list of methods and contexts for their proper use are tested to provide improvements in the methodological framework, without neglecting the role of new contributions to the subject, in this case, from one of the most exciting moments of the establishment of plants, in early Devonian times. A complete list of chapters, the corresponding aims, and the main results are summarized below. As a last point, in the discussion of the results and in the subsequent interpretations, I make clear the limitations of the available datasets and also of the quality of the fossil record. The treatment of the sampling biases is a major issue in discussions of global patterns of Phanerozoic taxonomic richness. Furthermore, due to restrictions due to the rock-record, it is universally accepted that diversity estimates are strongly affected by the outcrop areas, taphonomy biases and tectonics. These have been a constant theme in the manuscripts. Chapter II. Cascales-Miñana, B., Muñoz-Bertomeu, J., Ros, R. and Segura, J., 2010. Trends and patterns in the evolution of vascular plants: macroevolutionary implications of a multilevel taxonomic analysis. Lethaia, 43(4): 545-557. The aims of this chapter are the following. i. To present the macroevolutionary patterns of vascular plants from the Silurian to the present-day through a multilevel taxonomic analysis. The disparity concept exposed is also explored. ii. To provide a global view of the evolution of vascular plant life at different taxonomic levels, with special interest on biotic crises by evaluating their impact on the floral turnover patterns and on the temporal distribution of such events. iii. To discuss the concept of mass extinction in vascular plants and the role of competition on a macroevolutionary scale. Summary: Studying the macroevolutionary patterns of vascular plants from the Silurian to the present-day provides a global record of plant life history. Evolutionary rates (origination, extinction and diversification) for families, orders, classes and divisions were analysed, as was abundance and richness for 21 time intervals. An accumulative analysis, based on the total plant fossil record, the accumulated extinctions and relative diversity, was also carried out. The diversification rate shows a uniquely constant and progressive reduction from the end of the Carboniferous to the Permian when the lowest values are registered. Very small peaks seem to reflect a Cretaceous extinction for families. At the family level, only two time intervals present higher extinctions, than originations. Richness and accumulative analyses reveal that only 32% of the families analysed became extinct, and that approximately 90% of them disappeared at the end of the Palaeozoic. Our results indicate that plants did not undergo mass extinction events in the ¿big five¿ sense, but rather, mass ecological reorganization. The absence of important extinction events or evolutionary innovations produced diversification patterns without abrupt changes. Chapter III. Cascales-Miñana, B., 2010. Testing similarity coefficients for analysis of the fossil record using clustering methods: the Palaeozoic flora as a study case. Revista Española de Paleontología, 25(1): 19-34. The aims of this chapter are the following. i. To use similarity and clustering methods to establish a general vision of the Palaeozoic plant fossil record with a comparative methodological approach. ii. To test the potential of Raup-Crick Coefficients for providing robust results and its reliability for studying the consequence of the extinction processes in comparison with a descriptive basic binary (presence-absence) index such as the Jaccard similarity measure. iii. To bring additional evidence that the similarity measures are an excellent indicator of the proccesses those modify the taxonomic composition of the plant record. Summary: This paper reports a global methodological approach based on the similarity and clustering methods of the Pal¬aeozoic plant fossil record, using a comparative approach between two similarity measures: the Jaccard and Raup-Crick Coefficients. The results show that although the Raup-Crick Coefficients clearly have the potential for providing more robust results, the consequences of the extinction processes are better reflected in the simi¬larity analysis based on the Jaccard Coefficients. On the other hand, the cluster analysis based on UPGMA algo¬rithm shows four robust clusters and reveals new evidence for the distinctiveness of the Mississippian flora. Finally, the results reveal that similarity and cluster analyses are powerful tools to interpret the consequence of the processes modifying the taxonomic composition of the several analyzed Palaeozoic time units. Chapter IV. Cascales-Miñana, B., 2011. New insights into the reading of Paleozoic plant fossil record discontinuities. Historical Biology, 23(2-3): 115-130. The aims of this chapter are the following. i. To produce a global assessment of Palaeozoic plant life using multivariate statistics. ii. To emphasise the heterogeneity of the plant fossil record over time iii. To understand the consequences of discontinuities in the history of Palaeozoic plant life in terms of taxonomic composition by taking into account the geological constraints in interpretaing the plant fossil record. Summary: Studying the discontinuity patterns of Palaeozoic vascular plants using multivariate methods provides a global vision of these key events. Non-metric multidimensional scaling, detrended correspondence analysis and cluster analysis have been employed together with a set of diversity and abundance measures, and an evaluation of the geologic constraints from the plant fossil record data. The results reveal four clear significant discontinuities in terms of taxonomic composition and record representativeness, during the early-middle Devonian, Devonian-Carboniferous, Mississippian-Pennsylvanian and early-late Permian. Due to the controversial character of plant fossil record data and the effect of mass extinction events, the results can be explained in terms of taxonomic turnover and ecological reorganisation, which emphasise the crucial role of the geologic constrains in palaeobiological inference. Chapter V. Cascales-Miñana, B. and Cleal, C.J., 2012. Plant fossil record and survival analyses. Lethaia, 45(1): 71-82. The aims of this chapter are the following. i. To report the first global interpretation of the history of plant life based on survival analysis from Silurian to the present time, mainly using the cohort and polycohort methods of analysis. ii. To analyse the heterogeneous longevity observed in the plant fossil record and its implication for the interpretation of taxonomic survivorship curves. iii. To discuss the role of the nature and quality of the plant fossil record data in palaeobilogical inference for extracting extinctions patterns. Summary: Survival analysis is a classic palaeobiological method widely used on the animal fossil record. This study reports the first application of survivorship analyses on the plant fossil record from a global viewpoint and provides a new comparative approach of this methodology. The results reveal three important plant extinction events in the history of plant life at a global scale. The results also clearly suggest that the origination events are more intensive than extinction processes and that the origination moment of several lineages of vascular plants is an important factor that conditions their longevity. This study supports the general idea that vascular plants tended to be less affected by the environmental changes that caused mass extinction in other groups of organisms. Chapter VI. Cascales-Miñana, B., Martínez-Perez, C. and Botella, H., 2011. Discovery of a Lochkovian flora (LowerDevonian) in the Iberian Peninsula. Geodiversitas, 33(1): 33-41. The aims of this chapter are the following. i. To describe new plant macrofossil remains from the upper Lochkovian (Lower Devonian) of Spain. ii. To increases our understanding of the early evolution and diversification of vascular land plants. iii. To illustrate with an example of Devonian floras, how new discoveries can modify the temporal information and stratigraphical ranges of taxa at a local scale, contributing new data for future palaeobiological analyses. Summary: The early Devonian represents an important episode in plant life history, which was marked by the diversification of land plants. Unfortunately, remains of early Devonian plants in the Iberian Peninsula are scarce. In the present paper, we describe a small assemblage of early land plants from the Lochkovian (Lower Devonian) of the Teruel Province in Spain. The main element consists of Taeniocrada-like sterile stems that are predominantly dichotomous and ribbon-like with a narrow central strand. An unidentified fossil formed by dichotomous axes together with an uncertain globular structure were also observed. This finding increases our knowledge of the palaeogeographical distribution of early land plants. Chapter VII. Cascales-Miñana, B., 2012. Disentangling temporal patterns in our perception of the fossil history of gymnosperms. Historical Biology. In press. The aims of this chapter are the following. i. To re-evaluate the main episodes of the fossil history of gymnosperms by covering current tendencies in palaeobiology through the newly compiled taxonomic data on gymnosperms by Anderson et al. (2007). ii. To provide a framework to evaluate how our perception of plant life history can be biased due to the taxonomy of plant fossils and its time-dependent aspects. iii. To provide a set of comparative approaches to determine the crucial role of the timescale factor within our perception of large-scale evolutionary patterns and diversification. Summary: By taking gymnosperms as a case study, this article evaluates the perception of plant life history from the fossil record to test the biases associated with the time-dependent aspects of the taxonomy, following a stepwise modelling procedure based on two divergent sets of time units. The idea that the effects of the temporal component of palaeobiological inference need to be evaluated to remove any possible bias in our interpretation and perception of plant evolution based on analyses of large-scale data sets is investigated. The results reveal important differences in our perception of the tempo of gymnosperm evolution and how it is biased in terms of time unit length due to the loss of information as a consequence of the timescale resolution. Despite singletons representing real morphological diversity translated into independent taxonomic categories, these taxa can distort perceptions of the intensity of the long palaeofloristic diversification moments of gymnosperms if their effect is not considered. This study shows a complete overview of the evolutionary profiles of gymnosperms with significant discrepancies in the function of how singletons are quantitatively processed in palaeobotanical data analyses, and it provides new evidence about how the ¿zoom effect¿ can magnify our perception of extinction events. Chapter VIII. Cascales-Miñana, B. and Diez, J.B., 2012. The effect of singletons and interval length on interpreting diversity trens from the palaeobotanical record. Palaeontologia Electronica. In press. The aims of this chapter are the following. i. To embrace modern methodologies for reading of the temporal component of the fossil record data using the the plant record in a global perception. ii. To test the role of singleton taxa and the length of the time units employed as a source of ¿distortion¿ (variability) in the interpretation of plant diversification patterns. iii. To re-evaluate the evolutionary patterns of vascular plants analysed in previous works using more recent methodologies to check the robustness of the palaeobotanical data. Summary: The reliability and the identification of potential biases are central aspects to providing an accurate robust palaeontological data analysis. In the last decade, older concepts of evolutionary processes and patterns have been revised in the light of new methods and hypotheses. Moreover, new mathematical algorithms have been developed to better interpret evolution and to reduce several biases inherent in the animal fossil record. This new global study based on a complete overview of the plant fossil record uses comparisons with previously established palaeofloristic patterns, and employs a set of regression, accumulative and evolutionary analyses to test the influence of two important variables in the inference methods: the role of singleton taxa and the duration of the time units. Regression analyses reveal that the duration of the time units employed does not distort our perception of the number of singletons at the family level. Likewise, duration as single factor also does not affect the measures of taxonomic diversity, and does not influence representation of the main evolutionary patterns of vascular plants. The analysis reveals that the per-capita origination and extinction rates used provide global diversity patterns that diminish the effects of the possible taxonomic problems and preservational biases relating to the controversial nature of singleton taxa due to its restricted record. They reveal that the main trends and plant turnover is characterized by slight abrupt changes, thus providing a more realistic vision about the dimension and magnitude of the observed evolutionary processes and diversification patterns from the plant fossil record.