Special volume ‘Cretaceous tetrapods from South America’

Two fundamental concepts in evolutionary biology, extinction and evolution, are intimately connected to early fossil collections from South America. Cuvier developed the concept of extinction at the turn of the 18th century, based in part on fossils from this continent. The concept of evolution, in turn, was developed by Darwin in the 1850s based in part on his palaeontological, biological, and geological observations during the Beagle voyage. Two centuries later, South America remains central to palaeontological studies, playing a major role in understanding the past biotas from the Southern Hemisphere. In recent decades, South America has not only provided a large number of fossils for research, but has also experienced the development and enlargement of its own palaeontological community. During the past four decades in particular, this growth has resulted in the discovery of diverse new assemblages of fossil tetrapods recorded in the Cretaceous Period (145–66 Mya), which has revealed a pronounced differentiation between tetrapod faunal assemblages of the Northern and Southern Hemispheres. Moreover, Cretaceous fossils have provided valuable insights into how South America’s geographic isolation, driven by plate tectonics, has influenced the evolution and extinction of unique radiations on the continent. Although important Cretaceous marine and terrestrial deposits in South America have long been known, it was only since the 1980s that tetrapod faunas from South America were noted to be markedly different from the better-known faunas from northern landmasses (Bonaparte and Powell 1980, Bonaparte and Novas 1985, Bonaparte et al. 1990). These poorly known ecosystems are the source of multiple new species discovered each year. Especially during the past 20 years, discoveries and scientific papers on Cretaceous tetrapods from South America have increased markedly, reflecting the collecting efforts and research conducted by the growing palaeontological communities in South American countries. The spike in tetrapod discoveries is mainly due to intensive sampling of traditional fossiliferous areas in Argentina and Brazil. Less explored regions are now also contributing to South American vertebrate palaeontology, suggesting that we are still on the steep part of the sampling curve for South American Cretaceous tetrapod assemblages. Increased emphasis on quantitative approaches to macroevolutionary and macroecological questions has led to the development of large-scale palaeontological databases such as the Paleobiology Database (PBDB). This grassroots initiative aggregates spatiotemporal, taxonomic, and taphonomic data from published records of Phanerozoic biota. Nevertheless, the more than 1.26 million fossil occurrences in the PBDB (Peters and McClennen 2016) represent approximately 5% of the unpublished records currently available in museum collections (Marshall et al. 2018). Additionally, the PBDB is subject to a number of collection-related effects, including regional biases, specimen size-related biases, and taxonomic biases. For example, an inquiry to the PBDB for South American Cretaceous tetrapods returns 1533 records, almost 80% of which are from Argentina and Brazil (see Fig. 1; data retrieved 22 September 2024, and including Crocodyliformes, Ichthyosauria, Lissamphibia, Mammalia, Ornithischia, Plesiosauria, Pterosauria, Sauropoda, Theropoda, Sphenodontia, Squamata, and Testudines). Distribution map of South American Cretaceous tetrapods listed in the Paleobiology Database (data retrieved 22 September 2024). Note the concentration of fossil sites in Argentina and Brazil, and the presence of large areas with few or no records at all. Colored dots follow the color coding established by the Commission for the Geological Map of the World (CGMW). Current records in the PBDB reveal that 60% of Cretaceous tetrapod occurrences from South America are predominated by three major dinosaur clades: gigantic sauropods (26%), mid-to-large-bodied theropods (24%), and small-bodied ornithischians (10%). In contrast, crocodyliforms (10%), testudinatans (15%), and other groups, such as lissamphibians, ichthyosaurs, plesiosaurs, pterosaurs, mammals, squamates, and sphenodontians (totalling 15%), are significantly less abundant. There remain important geographical biases in our sampling and knowledge of South American Cretaceous tetrapod biotas, which could be minimized by efforts aiming to provide a broader, synthetic view of those deposits. This Special Volume contributes to these advancements by presenting eight studies from a diverse group of international and local researchers. These studies cover systematics, phylogeny, palaeohistology, palaeoecology, biomechanics, palaeobiogeography, and biodiversity across various taxonomic groups. By incorporating records from lesser-known deposits (e.g. Chile and Colombia) and underexplored areas of Argentina and Brazil, the volume addresses key gaps in the study of South American Cretaceous tetrapods with valuable insights into lissamphibians, turtles, sauropods, theropods, and marine reptiles, offering a clearer understanding of the evolutionary pathways that shaped tetrapod biodiversity during this period. The data used to produce Figure 1 are available in Paleobiology Database, at https://paleobiodb.org/#/, under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike License, available at https://creativecommons.org/licenses/by-nc-sa/4.0/.

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