Isotopic Studies in South American Mammals: Thirty Years of Paleoecological Discoveries

Abstract

Stable isotope analysis has become a key tool in paleontology, providing insights into ancient diets, ecosystems, climates, and environmental shifts. Despite the growing importance of isotopic studies in South America, no comprehensive bibliometric review has been conducted until now. This study addresses that gap, analyzing the development of the field over the past thirty years. Our results show a rapidly expanding discipline, especially in the last five years, with increasing publication rates and participation from South American researchers, particularly in Brazil and Argentina. However, the analysis also reveals persistent biases: notably, a strong focus on the Quaternary period, which limits broader evolutionary interpretations. Keyword co-occurrence points to dominant themes such as paleodiet, paleoecology, and megafaunal extinction, while highlighting new trends like ecological niche modeling and nitrogen isotope applications. The co-authorship network reflects high levels of collaboration, particularly with Spain and the United States. A marked gender imbalance in authorship is also evident, calling attention to the need for greater inclusivity. This review emphasizes the importance of addressing taxonomic and temporal gaps, strengthening interdisciplinary and international networks, and promoting equity in order to ensure the continued growth and global relevance of isotopic paleontology in South America.

1. Introduction

The Cenozoic fossil record of South American mammals provides remarkable evidence of faunal evolution under conditions of geographical isolation [1]. For millions of years, this continent hosted species that were found nowhere else. These include large ground sloths, armored glyptodonts, native ungulates, and rodents [2,3,4]. The singularity of this fauna makes South America a unique natural laboratory for studying the ecological, environmental, and climatic processes that have shaped the continent’s biodiversity.

Since the 18th century, South American mammalian fauna has been a source of fascination for many renowned paleontologists and naturalists from Argentina (e.g., Florentino Ameghino, Francisco J. Muñiz, Ana Cortelezzi, Rosendo Pascual, Dolores López Aranguren, Andreína Bocchino, and Noemí Violeta Cattoi, among many others), as well as for foreigners such as Charles Darwin, Richard Owen, and George G. Simpson, who made important contributions to the study of these fossils. In fact, the South American fossil record has influenced the development of fundamental disciplines and theories. For example, the Neogene and Quaternary mammal fossils from Argentina and Uruguay played a fundamental role in shaping Darwin’s evolutionary thinking, serving as catalysts for his ideas on species transmutation [5,6,7,8].

Applying biogeochemical techniques to fossils is particularly valuable for providing a detailed understanding of past global change processes, determining the abiotic and biotic mechanisms that drove them, and the responses to these changes, spanning periods beyond the instrumental records [9,10,11,12,13,14,15]. Based on the principle of isotopic fractionation—associated to natural processes such as diffusion, evaporation, and metabolism (respiration, digestion, photosynthesis, etc.), among others—some light stable isotopes (e.g., ¹⁶O, ¹²C, ¹⁴N) allow us to characterize patterns of paleoclimatic, paleoenvironmental, and paleoecological variability. Since the classic paper of Harold Urey [9] laid the foundation for isotope thermometry, in particular, stable isotope analysis applied to fossils has proven to be highly effective and has become a fundamental tool in paleontology over the last few decades [16,17,18,19,20,21,22].

The application of isotopic techniques to the study of South American fossil mammals has seen significant growth in the last decades [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46]. The year 2024 marked an important milestone in the field, as it commemorated three decades since the first studies on this subject were published. In order to contextualize new findings and ensure that current investigations build on a solid foundation of previous work, it is essential to understand the history of isotopic studies applied to South American mammal fossils. Despite the increasing number of studies employing isotopic methods in paleontology, only one recent review [47] has provided a global synthesis of isotopic analyses on mammals. However, this review is limited in scope, focusing exclusively on the Pleistocene and lacking a region-specific emphasis on South America. To address this gap, the present study aims to offer a more comprehensive evaluation of isotopic research on fossil mammals from this region, covering a broader temporal framework and synthesizing the diverse approaches and findings that have shaped the field.

Bibliometric analysis is a powerful tool for assessing scientific production, detecting research trends, mapping collaborative networks, and identifying the most influential contributions within a given discipline. By applying this methodology, we aim to provide a detailed overview of the evolution of isotopic research in South America, highlight key areas of interest, assess the extent of collaboration between researchers and institutions, and identify the most impactful studies in the field. Additionally, this review will serve as a critical resource for guiding future research, ensuring that forthcoming studies are well-informed by past achievements and directed towards addressing persisting knowledge gaps.

2. Materials and Methods

2.1. Bibliographic Data Acquisition

A bibliometric network analysis was performed to review the scientific literature related to isotopic studies of fossil mammals from South America. The first step was the selection of a bibliographic database, an indispensable tool that contains information about scientific activities [47]. Currently, the most widely used databases for bibliometric studies are Web of Science (WoS) and Scopus [48]. Studies comparing both databases have shown certain advantages of Scopus over WoS, such as indexing approximately 66% of exclusive journals [49], and Scopus also has more citations across all research fields [50]. Although both are robust sources, our preliminary searches revealed that all of the records retrieved from WoS were also indexed in Scopus without any additional unique publications. Therefore, Scopus was chosen as the main source for the literature search in this work.

Scientific papers were collected from database on February 10th, 2025. Scopus is a database that allows the search for publications based on pre-defined key terms that are present in titles, abstracts, or subject headings. A strategy based on four criteria was used for the key terms entered in the search: (i) technical scope (isotop*), (ii) temporal scope (Cenozoic* OR Paleocen* OR Eocen* OR Oligocen* OR Miocen* OR Pliocen* OR Pleistocen* OR Quaternary OR Paleogen* OR Neogen* OR Cuaternario), (iii) study scope (*mammal* OR megafauna* OR fossil* OR bone* OR t**th OR mamifero* OR fosil* OR diente* OR Proboscidea OR Cingulata OR Pilosa OR Primates OR Rodentia OR Carnivora OR Peryssodactyla OR Artiodactyla OR Cetartiodactyla OR Sparassodonta OR Pyrotheria OR Litopterna OR Notoungulata OR Didelphimorphia OR Paucituberculata OR Polydolopimorphia OR Chiroptera OR Astrapotheria OR Lagomorpha OR Eulipotyphla OR Monotremata), and (iv) geographic scope (South America OR Argentin* OR Bolivia* OR Brasil* OR Chil* OR Colombia* OR Ecuador* OR Guyana* OR Peru* OR Surinam* OR Uruguay* OR Venezuela* OR Guiana*). Each block of key terms was separated by “AND”. Therefore, at least one term from each criterion was always included in the search results. The use of “*” in some of the terms allows multiple options to be included; for example, isotop* includes isotope, isotopes and isotopic, and t**th includes tooth and teeth. This sequence of terms generated an initial database of 212 publications. The results were exported to a csv file containing basic publication information, including title, authors, year of publication, and total citations.

2.2. Database Processing

The next step was to process the data by filtering and selecting only publications that were strictly related to the research field. To achieve this, publications that did not focus on isotopic studies of mammalian fossils were discarded, as some papers focused on invertebrates or reptiles. The fact that the third criterion used in the search (scope of study) includes terms for mammalian taxa and study material (e.g., tooth) separated by the order “OR” may explain why the search results included these taxa. Publications focusing on regions outside South America (e.g., studies on South American taxa from North or Central American localities), duplicates, and those with incomplete or irrelevant information were also excluded. This data processing resulted in a final database of 80 papers.

Spelling errors were carefully corrected, and duplicate data was removed for each bibliometric entity, including author, keyword, and journal. For example, authors occasionally used different versions of their signatures, which were standardized. In the case of keywords, there were certain criteria for the standardization of terms:

• All terms were standardized to American English (e.g., palaeoecological to paleoecological).
• Terms were written in the singular (e.g., isotopes to isotope).
• Terms referring to animals were written using the scientific name (e.g., horses to Equidae; mammals to Mammalia).
• Hyphens were removed (e.g., radiocarbon-dating to radiocarbon dating).
• Geographical regions were renamed to country names (e.g., North Argentina to Argentina).

2.3. Bibliometric Network Analysis

Bibliometric network analysis is a methodology that integrates bibliometrics and social network analysis to explore and visualize relationships and structures within the specific scientific literature [51,52,53]. Network and graph theory enable social network analysis to explore the relationships and social structures of different scientific fields [54,55,56,57].

VOSviewer is a software specialized in the construction and visualization of bibliometric maps. Authors, documents, keywords, countries, organizations, and documents can be the objects of interest of network maps [58]. This analysis utilized VOSviewer software version 1.6.20 [58] to perform a bibliometric analysis of co-authorship and co-occurrence of keywords. In network analysis, the nodes represent the elements of interest in the network (e.g., authors or keywords); their size represents the frequency of appearance in documents; the links between two nodes indicate the relationship between them (e.g., co-authorship or appearance in the same paper); and the weight of the links, shown by their thickness, reflects the intensity or frequency of the relationship between two nodes (e.g., number of co-authored publications or number of times that two keywords appear together).

The elements—in this case, authors or keywords—were uniquely assigned to clusters, which comprise a set of closely related elements that are distinguished by their varying colors [59]. Clusters were identified using the VOS (Visualization of Similarities) clustering technique, which employs an optimization algorithm to group related nodes based on connection strength. Initially, connection weights were assigned according to co-authorship frequency or keyword co-occurrence. Subsequently, the algorithm optimized modularity by prioritizing stronger internal connections within clusters over external ones. The resulting cluster visualization was generated by assigning different colors to each cluster and spatially positioning nodes within the same cluster closer together. In co-authorship networks, these clusters typically reflect research groups or collaborative communities, while in keyword co-occurrence networks, they reveal thematic structures and related research topics within the field.

In the case of the co-authorship analysis, a mapping of the temporal evolution of the network was also carried out. In the temporal representation, nodes and links were colored according to the year of publication, providing a dynamic view of the temporal evolution of the scientific network. In addition to the cluster map, a density map was created for the keyword co-occurrence analysis. Density maps are visual representations that show the distribution and concentration of elements. They facilitate the identification of high-activity areas or patterns within bibliometric data [60].

3. Results and Discussion

3.1. Temporal Trend Analysis

The bibliographic search yielded 80 papers published between 1994–2024 (Table S1). The number of documents is relatively low compared to other disciplines such as taxonomic paleontology, rhodolith beds, geoheritage and geosites, zooarcheology, or biodiversity [47,57,61,62,63,64,65,66,67]. A preliminary search in Scopus using the search criteria described in the Material and Methods section, yielded a database of 406 and 587 articles for Africa and Europa, respectively. Compared to the 212 publications in South America, there were twice as many and almost three times as many documents in Africa and Europe, respectively. This suggests that the isotopic paleontology applied to South American fossil mammals is a relatively emerging field compared to other fields of knowledge and other geographical areas. Several factors, including the novelty of the technique and the uneven historical development of scientific research in South America, contribute to the scarcity of research in this area. Developing isotopic techniques requires specialized equipment and advanced technical expertise that has not been uniformly available throughout the world. Therefore, another factor that we consider crucial is the historical inequality in the development of scientific research between the Global North and the Global South. While North American and European countries have traditionally led scientific production due to their access to financial resources, infrastructure, and established academic networks, South American countries have faced significant challenges, including limited funding or reduced access to advanced technology. This has limited their ability to conduct cutting-edge research [68,69]. Until relatively recently, the majority of paleontological research in South America has been focused on more traditional studies (e.g., taxonomy) for which there was already an established knowledge base and readily accessible techniques. Therefore, the delay in the implementation of isotopic techniques may be related to the need to prioritize limited resources towards lower-cost research. However, in recent years, the capacity to adopt and develop more advanced techniques, including isotopic analysis, has increased due to economic growth and the strengthening of scientific institutions in some South American countries, which has also enhanced international collaboration.

The aforementioned trend was reflected in the increasing number of publications, from 1 paper in the early years to 11 papers in 2023 and 12 papers in 2024 (Figure 1). The 80 papers accumulated a total of 4416 citations in Scopus, and the citations per year analysis indicated an upward trend (Figure 1). Overall, the trends in the number of publications and citations highlight growing interest in the field and suggest that the number of publications will increase significantly in the future.

To assess the impact of the citations received by the articles included in this review, two metrics were used: the Field-Weighted Citation Impact (FWCI) and the percentile of each article in relation to the number of citations in Scopus. The FWCI adjusts the number of citations of an article compared to the average number of citations in its field, allowing a more equitable assessment of impact. Meanwhile, the percentile indicates the position of an article within the total number of publications in its field, providing a perspective on its relative relevance. Of the 73 papers cited at least one time, 47 (~65.3%) had FWCI scores above 1, indicating that a substantial portion of the literature reviewed had a higher-than-average impact in its area. This finding indicates that these papers have not only been cited but have also influenced research in this field. Additionally, eight papers (11.1%) had an FWCI greater than 0.7, indicating that a notable number of works also received a relevant amount of citations citations, although below the overall average. In terms of percentile, the results were equally positive.

The work of Tejada-Lara et al. [70] ranked in the 99th percentile, indicating exceptionally high impact compared to other papers in the same field. This highlights the relevance and significant contribution of this article to the literature, which will be discussed in more detail below. Similarly, 14 papers ranked in the 90th percentile, suggesting that they represent the top 10% of the most cited papers in their field. Furthermore, 30 papers were between the 90–75% percentile and 24 papers were between the 75–50% percentile. This result also demonstrates strong citation performance. However, four papers ranked between the 25th and 50th percentiles, suggesting that, while they have a positive impact, their influence is less significant compared to the other analyzed articles. The high proportion of articles with an FWCI greater than 1 and the remarkable distribution in the percentiles reflect the importance and relevance of the papers published to date.

The top ten most cited articles are listed in Table 1. The work of Cerling et al. [17], published in Nature, accumulated the highest number of citations with a total of 1737, followed by Latorre et al. [25] (250 citations) and Cerling [71] (212 citations). Although these three articles were published before the year 2000, and thus have nearly 30 years of citation history, their quality and influence on subsequent research are clear. In terms of the topics of these articles, they all used carbon (δ¹³C) and oxygen (δ¹⁸O) isotopic analyses of fossil mammal tooth enamel to investigate changes in vegetation. Cerling et al. [17] focused on global changes in vegetation during the Miocene–Pliocene boundary and showed a significant transition to C₄ plants in several regions, including South America. Cerling et al. [71] highlighted the transition to C₄ grasslands in South America by examining the relationship between declining atmospheric CO₂, the development of C₄ ecosystems, and the evolution of mammals. Latorre et al. [25] examined C₄ grass expansion and global change during the Late Miocene, using isotopic evidence from fossil mammals to show C₄ plant expansion in several regions of the Americas, including South America. The three studies share a common objective, distinct from the other works: evaluating vegetation changes (C₄ plant expansion) and their impact on ecology and mammalian evolution at large geographic scales (global or continental). This explains their high citation rate, since they are key contributions that have established important concepts not only in the field of isotopic paleontology, but also for paleontology in general. Furthermore, because they are not exclusively focused on data from South America, they are also relevant in other regions of the world.

The majority of the remaining top ten most cited articles focused on the paleoecology of fossil mammals, utilizing stable isotope analysis to primarily assess diet, habitat use, trophic interactions, and the evolution of various South American taxa during the Cenozoic. Nevertheless, the geographic scope of these studies varied; some focused on a single region/country (e.g., Argentina, or Bolivia; [23,24,72], while others were more broadly based [32,73,74]. The same applies to the temporal context; four studies addressed the Pleistocene [32,72,73,74], while others [23,24] evaluated changes in mammalian diets from the Oligocene to the Pleistocene in Bolivia and Argentina, respectively. Additionally, some studies focused on specific taxonomic groups, such as toxodontids [73] and gomphotheres [74], while others examined a broader set of taxa or even the entire mammalian fauna (e.g., [32]). Among these, three studies were published before the year 2000 [23,24,72]. All of the papers published before the year 2000 were included in the top ten most cited papers due to the longer period for citations to accumulate. Tejada-Lara et al. [70] was the most recently published paper of the top ten and had only six years to accumulate citations. However, it was a highly cited work, ranking sixth on the list, and as previously mentioned, it was in the 99th percentile. This paper differed from the others in its aims, as it evaluated how the body mass of herbivores influenced the isotopic fractionation of carbon and nitrogen in their tissues (e.g., tooth enamel). Consequently, it is a more methodological work that provides a solid foundation for studies in any spatio-temporal context, contributing to its high citation count.

Table 1. Top 10 most cited articles.

	Author/s	Year	Title	n	Reference
1	Cerling et al.	1997	Global vegetation change through the Miocene/Pliocene boundary.	1737	[17]
2	Latorre et al.	1997	The expansion of C4 grasses and global change in the late Miocene: Stable isotope evidence from the Americas.	250	[25]
3	Cerling et al.	1998	Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution.	212	[71]
4	MacFadden	2005	Diet and habitat of toxodont megaherbivores (Mammalia, Notoungulata) from the late Quaternary of South and Central America	137	[73]
5	MacFadden and Shockey	1997	Ancient feeding ecology and niche differentiation of Pleistocene mammalian herbivores from Tarija, Bolivia: Morphological and isotopic evidence.	125	[72]
6	Tejada-Lara et al.	2018	Body mass predicts isotope enrichment in herbivorous mammals.	117	[70]
7	Sánchez et al.	2004	Feeding ecology, dispersal, and extinction of South American Pleistocene gomphotheres (Gomphotheriidae, Proboscidea)	107	[74]
8	MacFadden et al.	1994	South American fossil mammals and carbon isotopes: a 25 million-year sequence from the Bolivian Andes	103	[23]
9	MacFadden et al.	1996	Cenozoic terrestrial ecosystem evolution in Argentina: Evidence from carbon isotopes of fossil mammal teeth	95	[24]
10	Domingo et al.	2012	The effect of paleoecology and paleobiogeography on stable isotopes of Quaternary mammals from South America	83	[32]

Table 1. Top 10 most cited articles.

	Author/s	Year	Title	n	Reference
1	Cerling et al.	1997	Global vegetation change through the Miocene/Pliocene boundary.	1737	[17]
2	Latorre et al.	1997	The expansion of C4 grasses and global change in the late Miocene: Stable isotope evidence from the Americas.	250	[25]
3	Cerling et al.	1998	Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution.	212	[71]
4	MacFadden	2005	Diet and habitat of toxodont megaherbivores (Mammalia, Notoungulata) from the late Quaternary of South and Central America	137	[73]
5	MacFadden and Shockey	1997	Ancient feeding ecology and niche differentiation of Pleistocene mammalian herbivores from Tarija, Bolivia: Morphological and isotopic evidence.	125	[72]
6	Tejada-Lara et al.	2018	Body mass predicts isotope enrichment in herbivorous mammals.	117	[70]
7	Sánchez et al.	2004	Feeding ecology, dispersal, and extinction of South American Pleistocene gomphotheres (Gomphotheriidae, Proboscidea)	107	[74]
8	MacFadden et al.	1994	South American fossil mammals and carbon isotopes: a 25 million-year sequence from the Bolivian Andes	103	[23]
9	MacFadden et al.	1996	Cenozoic terrestrial ecosystem evolution in Argentina: Evidence from carbon isotopes of fossil mammal teeth	95	[24]
10	Domingo et al.	2012	The effect of paleoecology and paleobiogeography on stable isotopes of Quaternary mammals from South America	83	[32]

3.2. Topics of Study

There was a predominance of 80% of works analyzing enamel compared to the ones that included bone (47.5%), which is not surprising due to the higher chemical stability and better preservation of the original isotopic composition of enamel over time [18,75]. Moreover, δ¹³C was analyzed in the majority of studies (97.5%), reflecting the crucial role of this isotopic ratio in paleoecological and paleoenvironmental reconstructions [18,76,77]. Conversely, δ¹⁸O values were reported in 65% of the papers, and only 17.5% included δ¹⁵N. The reduced focus on nitrogen isotopic composition may be attributed to the specific requirements for its analysis (i.e., collagen preservation). Interestingly, the first paper to include nitrogen was from 2012 [78], while carbon and oxygen have been utilized in papers since 1994 and 1997, respectively. Until recently, nitrogen analysis relied heavily on the preservation of collagen, a protein susceptible to degradation in unfavorable environmental conditions, such as arid climates or with materials older than 100,000 years [19]. Consequently, nitrogen studies are limited to contexts with an exceptional collagen preservation of Late Pleistocene–Holocene materials.

An evaluation of the studied periods and epochs revealed that 77% of the papers presented isotopic data from the Quaternary, 16% from the Neogene, and only 7% from the Paleogene (Figure 2A). Hence, there is a clear bias towards the study of fossils from more recent periods such as the Pleistocene (61%) compared to others such as the Oligocene (5%), Eocene (2%), or Paleocene, the last of which lacks isotopic studies (Figure 2A). Several factors contribute to this trend. Firstly, Pleistocene deposits are more abundant in South America, resulting in a greater number of fossil localities. Additionally, fossils from this relatively recent geological period are more likely to exhibit less diagenetic alteration. Moreover, the Pleistocene has attracted a considerably higher percentage of research because several key events occurred during this time interval, such as the Great American Biotic Interchange (GABI), glacial/interglacial events alternation, and the extinction of megafauna [79,80,81,82]. Conversely, older periods such as the Paleogene and Neogene present challenges including poorer material preservation—which makes isotopic analyses more difficult—the possible lack of deposits, and lower general investment in research for these periods.

The geographical distribution of the studies indicates that Brazil and Argentina were placed as the main countries of origin of the analyzed fossil localities (Figure 2B). This reflects not only their large size but also the long tradition held by these countries on studying their rich fossil record, with pioneers such as such as Francisco Javier Muñiz (18th century), Florentino Ameghino (19th–20th century) and Dolores López Aranguren (20th century) in Argentina, among others, and Llewellyn Ivor Price and Carlos de Paula Couto in Brazil (20th century). This also reflects the scientific capacity and infrastructure available for research in these countries. On the contrary, countries like Paraguay and Venezuela still lack isotopic studies. This situation may lead to an imbalance in the representation of South American past diversity, potentially underrepresenting contributions from other countries.

Finally, the most studied mammalian orders during the last three decades were Notoungulata, Pilosa, Artiodactyla, and Perissodactyla (Figure 3A). At the family level, Equidae was the most studied, followed by Camelidae, Gomphotheriidae, and Toxodontidae (Figure 3B). The remarkable study of Toxodontidae, an endemic family of South America and one of the most abundant and diversified groups, underscores its ecological and evolutionary importance, especially during the Pleistocene. However, three of the top four families were taxa of North American origin. This aligns with the previously mentioned interest in events like the GABI, reflected in the increased study of immigrant families in South American isotopic research.

3.3. Leading Journals

Efficiently identifying the leading journals within a specific scientific discipline is crucial for researchers, particularly early-career researchers, as it helps to maximize the impact, visibility, and reach of a paper [47]. The analyzed publications in this work were distributed across 37 journals.

Table 2. Top three journals with the largest number of publications (n = total number of publications; % = percentage; and the number of papers published in each decade).

Journal	n	%	1st Decade 1994–2004	2nd Decade 2005–2014	3rd Decade 2015–2024
Quaternary Science Reviews	10	12.8	-	1	9
Journal of South American Earth Sciences	6	7.7	-	-	6
Palaeogeography, Palaeoclimatology, Palaeoecology	6	7.7	1	1	4

presents the top three journals, which all together published 22 articles, representing a significant portion of the total dataset (28.2%).

Quaternary Science Reviews emerged as the leading journal in this field, featuring 10 publications. Its broad scope, spanning paleontology, paleoclimatology, archaeology, and various dating methodologies relevant to the Quaternary, makes it a suitable outlet for studies employing isotopic analyses. The journal’s relevance is further underscored by the fact that over 75% of the analyzed works incorporated Quaternary fossil samples, aligning with the predominant focus of this journal. The next two most frequently used journals were Journal of South American Earth Sciences and Palaeogeography, Palaeoclimatology, Palaeoecology (“Palaeo3”), both with six publications. The first one stands out for its regional focus, providing a platform for research specifically related to South America’s paleontological record, while the second one has long been recognized for its contributions to multidisciplinary paleoenvironmental research, including studies using isotopic techniques. Its relevance is probably due to the fact that it shares a similar scope with Quaternary Science Reviews but extends to other periods.

Beyond the top three, Quaternary International ranked fourth with five publications. Due to its emphasis on Quaternary research, including those utilizing isotopic analyses, it shares a similar role with Quaternary Science Reviews in disseminating research belonging to this period. Moreover, several other journals sampled made notable contributions to the diffusion of isotopic studies in South America, including Ameghiniana, Earth and Planetary Science Letters, Historical Biology, Journal of Quaternary Science, Paleobiology, Quaternary Research, and Revista Brasileira de Paleontologia, each publishing three articles.

A key finding of this analysis was the absence of journals specializing solely in isotope or geochemical research. Instead, the identified journals predominantly featured studies in paleontology, paleoecology, and Quaternary science. This observation supports the findings of França et al. [47], who noted a similar trend towards publication in journals with broader paleoenvironmental scopes, rather than those focused solely on geochemical methodologies. This distribution emphasizes the interdisciplinary nature and global impact of isotopic applications in the paleo-sciences.

3.4. Co-Ocurrence Keywords Networks

Authors, documents, keywords, countries, organizations, or documents can be the objects of interest of network maps [58]. Keywords are extremely useful in bibliometric analyses because they allow the identification of topics of interest and the relationships between them, as well as identifying emerging fields. The keyword co-occurrence analysis was performed using the total number of author keywords for a threshold of two co-occurrences (each keyword must appear in at least two papers). The number of keywords meeting this condition was 55. The co-occurrence analysis identified seven clusters (Figure 4A).

The largest cluster (cyan, 19 items) included a wide variety of terms related to isotopes (“Carbon” and “Oxygen”), dentition (“Tooth”, “Enamel”, “Hypsodonty”), regions (“Argentina”, “Bolivia”, “Brazil”), time (“Miocene”, “Pliocene”, “Holocene”) or paleoenvironment conditions (“C₄ plants”, “Precipitation”, “CO₂”), among others. Therefore, this first cluster grouped terms that were more detailed in relation to the field of study either methodologically, geographically, or temporally. Terms related to the main objective of the studies, such as “Isotope”, “Diet” and “Paleoenvironment”, were concentrated in the second largest cluster (green, nine items). This cluster also included keywords like “Pleistocene”, “Toxodontidae”, “Gomphoteriidae”, and “Stegomastodon”, directly related to the main themes identified in the previous section. Concerning the orange cluster (eight items), the majority of terms were related to macroevolution and the faunal changes of the Late-Pleistocene–Early Holocene boundary, such as “Megafauna”, “Extinction”, “Hippidion”, and “Equus”. The blue cluster (six items) grouped terms like “Niche”, “Nitrogen”, and “Collagen”, which are related to mammalian diet in relatively recent times, analyzing nitrogen composition in bones. The yellow cluster (five items) included “Radiocarbon Dating”, “Quaternary”, “Fossil”, “Mammalia”, and “Amazonia”, indicating that most of the works that involved radiocarbon-dated mammal fossils focused on the Brazilian region. The purple cluster (four items) grouped terms such as “Proboscidea”, “Perissodactyla”, “Habitat”, and “South America”, indicating that there was a line of research focusing on the paleoecology of these immigrant taxa in South America, a result consistent with the topics of study. Finally, the pink cluster (four items) included keywords such as “Niche”, “Mixing Models”, and “Paleoecology”, terms directly related to study methodologies.

Density maps allow researchers to identify areas of high activity or patterns within the bibliometric data [60]. There was a total of 13 words with five or more occurrences that correspond to the high-density areas of the map (Figure 4B). These areas indicate the most explored and recurring themes within the study topic. As expected, the term “Isotope” was the keyword with the most occurrences (38), followed by “South America” (27) and “Pleistocene” (19) (Figure 4C). Other keywords obtained were “Niche”, “Diet”, “Paleoecology”, and “Carbon”. All of them focused on the same topic—the evaluation of the paleodiet of the taxa—indicating that this was a recurrent theme within the evaluated studies. As observed in the analysis of study topics, carbon was the most commonly used isotope and the only one that appeared as a keyword in more than five articles; additionally, it was the oldest keyword. The same happened with the temporal range studied; most of the isotopic papers focused on the Quaternary, and this was reflected in the most used keywords, since “Quaternary” and “Pleistocene” were the only terms referring to geological times that were used in more than five documents. Interestingly, during the last decade, there has been a growing interest in a broader view of mammalian paleoecology, with an emphasis on studying ecological niches of megafauna, although still within the Quaternary time frame.

3.5. Mapping Diversity in Science

Over the past 30 years, 264 authors have published isotope studies on South American mammals. Of these, only 27.3% (72 authors) have published at least two papers, including 13 authors (4.9%) with at least five articles and only 4 (1.5%) with ten or more documents. A bibliometric network analysis of the 72 recurring authors revealed that Mário André Trindade Dantas had the highest number of papers (18), the highest number of connecting links from other authors (19), and the highest total link strength (53). Alexander Cherkinsky ranked second in connecting links from other authors (17) and total link strength (39), appearing in 10 papers. Finally, Laura Domingo, Lidiane Asevedo, and Hervé Bocherens ranked third, each with 16 collaborators but with different link strength (37, 28, and 25, respectively) and number of papers (8, 7, and 6, respectively).

Ten co-authorship clusters were revealed by the network analysis (Figure 5A); two of the authors (Adolfo Gil and Augusto Tessone) were not connected to any of the others and, therefore, were excluded from the network map. The network shows a highly collaborative discipline, with 90% of the clusters involving researchers from different countries. The countries with the highest presence in a larger number of clusters were Brazil and Argentina (each with four clusters) and Spain and the United States (each with three clusters). Moreover, the United States was the country with the highest number of collaborations with others (six), followed by Argentina and Brazil (five each).

The cyan cluster was the largest group (13 authors), showing a close relationship between Chilean, Brazilian, and Spanish authors. On one side, Lidiane Asevedo linked this cluster with the lime and green ones, while on the other, there was a strong connection with the blue cluster, in which Laura Domingo acted as the main link. The lime cluster was the second-largest group, also with 13 authors, including Mário André Trindade Dantas, among others. This group was composed mainly of Brazilian authors, except for Alexander Cherkinsky from the United States. This author, along with Lidiane Asevedo, served as the connection to the green cluster (Figure 5A). Although both the lime and green clusters were predominantly composed of Brazilian authors, the yellow cluster was the only one showing a research group composed exclusively of Brazilian authors (Figure 5B). The orange cluster was the third largest group (nine authors), and Hervé Bocherens was the link with the lime group (Figure 5A). This cluster was mostly represented by Argentinian authors except for two Germans. The fourth largest cluster (eight authors) was the blue one, based on the collaboration between Argentinian, Uruguayan, and Spanish researchers (Figure 6B). Besides its already mentioned connection to the cyan cluster, it was also linked to the purple cluster, in which Mª Teresa Alberdi acted as the main connector (Figure 5A). This purple cluster showed the collaboration between Spanish, Argentinian, and Mexican authors and connected with the US pink cluster through the link of Jose Luis Prado, who also acted as a connection to the orange cluster. Finally, there were two clusters that appeared isolated from the rest of the network (Figure 5A): the exclusively Uruguayan brown cluster and the red cluster, which united one Argentinian and two US authors.

The temporal evolution of the co-authorship clustering provides a dynamic view of the scientific network’s temporal evolution (Figure S1). Bruce J. MacFadden, Jay Quade, and Thure E. Cerling (pink cluster) constituted the initial group of authors in this field of study (Figure 5C). Notably, until the third lustrum (2005–2009), only authors from the United States had participated in the papers. This changed when Jose Luis Prado, Mª Teresa Alberdi, and Begoña Sánchez (purple cluster) began their collaboration on isotopic studies of South American faunas. In the subsequent lustrum (2010–2014), these two clusters linked through Jose Luis Prado. During the following lustrum (2015–2019), the number of authors quadrupled to 28, with the initial development of the cyan, blue, lime, orange, and red clusters. During this lustrum, Laura Domingo was not yet connected with the author from the blue cluster, having only collaborated with authors from the cyan and purple clusters. Additionally, the orange and lime clusters and Renato Lopes (yellow cluster) were not yet connected to the other groups, as Lidiane Asevedo, who served as the linking author, did not appear until the following lustrum. In the last five years of the analysis (2020–2024), a steady growth to 72 authors and the development of the previous clusters, along with the emergence of new ones (brown cluster), occurred (Figure 5C). Consequently, the number of authors participating in two or more papers increased in the last lustrum.

In 2005–2006, only one of the six authors was from a South American country, whereas in 2024, authors from South American countries represented 50% of the total. A similar trend was detected when analyzing the first authors of the articles in each of the decades. In the first decade, of the nine papers published, only 22.2% of the first authors were affiliated with Ibero-American institutions. In contrast, in the second and third decades, this figure rose to 55.6% (18 papers) and 81.1% (53 papers), respectively. This pattern suggests that researchers from South American countries are increasingly assuming leadership roles in the production of isotopic research, rather than being included solely as providers of local fossil samples. This indicates a significant increase in researchers from this region focusing their investigations on the isotopic analysis of fossil mammals over the last decade. Similar to the increase in publications, improvements in macroeconomic and geopolitical conditions in the region are reflected in the number of South American researchers. An increase in research investment, which facilitates access to this technique, as well as an increase in the international collaborative network may explain this trend.

Analyzing the gender distribution of authors allows us to identify the participation of women in the field and the presence of gender bias. This gender analysis revealed a significant disparity in the authorship of isotopic papers from South America among the 72 authors previously evaluated (Figure 6). When analyzing the individual countries, Spain and Brazil stood out as the only ones with a gender ratio within current international standards for gender balance [83,84], with 55.6% and 43.5% of authors being women, respectively. Additionally, in France, 50% of the authors were women; however, it is important to note that this country was represented by only two authors (Figure 5). The authorship imbalance was particularly pronounced in Chile, the United States, and Argentina, considering the significant sample of researchers analyzed from these nations (total number of authors exceeding five), where women represented only 16.7%, 14.3%, and 7.1% of the authors, respectively. These results highlight the existence of strong gender biases against female participation within this research topic and emphasize the need to promote inclusion and gender equity practices to ensure fair and diverse authorship representation, since ensuring equity positively impacts and enriches scientific studies [85].

4. Conclusions

Bibliometric analysis provided a novel, comprehensive assessment of the current state of isotopic paleontology research on South American mammals. The results demonstrated a significant growth trend in this field, evidenced by increases in both publication volume and researcher participation. Notably, there has been a substantial increase in contributions from South American authors over the last five-year period. Currently, they represent 70% of the 2024 publications, a marked contrast to earlier periods, where they were largely absent. This trend underscores a growing regional focus within the field and underscores the increasing importance of local expertise.

The high proportion of articles with a FWCI higher than 1, coupled with their distribution across high citation percentiles, demonstrates the relevance and impact of current research in the isotopic paleontology of South American mammals. However, identified taxonomic and chronological biases highlight the necessity for a broader, more inclusive research approach. The limited data on the older faunas (e.g., Paleogene) hinder the reconstruction of long-term paleobiological and paleoenvironmental trends, emphasizing the need for investigations encompassing the early Cenozoic. In terms of keyword co-occurrence analysis, the most recurrent topics were related to isotopic analysis for reconstructing past environments and diets, with a strong emphasis on the Quaternary period. The most frequent keywords (“Isotope”, “Pleistocene”, “Diet”, “Paleoecology”, and “Carbon”) highlight a primary focus on evaluating the paleodiets and ecological roles of extinct taxa, especially using carbon isotopes. This is further supported by the dominance of keywords related to methodology, geographic scope, and specific taxa or periods. The presence of keywords like “Niche” and “Mixing Models” in recent studies further highlights the rising interest in integrating stable isotope data with ecological modeling approaches. This shift reflects a widening paleoecological perspective, with increased emphasis on ecological niche modeling and nitrogen isotope analysis.

Beyond thematic, temporal, and taxonomic biases, gender analysis revealed significant imbalances in female participation. Promoting equitable representation in scientific authorship is crucial not only for social justice but also to enrich research by incorporating diverse perspectives. Increased participation from women and underrepresented groups will enhance the quality and impact of future studies.

Finally, network analysis using clustering methods provided valuable insights into collaborative structure trends. Of the 264 researchers that contributed to this field over the past three decades, only 27.3% published at least two papers at the time of this study, indicating a core group of specialists driving much of the research. Co-authorship analysis revealed a highly collaborative landscape, with 90% of identified clusters involving international partnerships. Nevertheless, the presence of exclusively regional clusters, such as a Brazilian-dominated cluster and an Uruguayan cluster, suggests that, while international collaborations are prevalent, certain research groups maintain strong national identities.

This systematic literature analysis offers a comprehensive understanding of the field’s progress over the past three decades, highlighting areas requiring further investigation and providing a valuable foundation for guiding future research directions and fostering a more structured and comprehensive development of the field. The identified emerging thematic areas provide a roadmap for future research and collaborative opportunities. Expanding the geographic, temporal, and taxonomic breadth of the studies will be crucial for shaping the future of isotopic paleontology in South America.