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Phylogenetic position of Phoberomys superimposed on a phylogeny of caviomorph rodents (23) compared in this study. Thirteen characters and 29 character states were examined in 13 caviomorphs. Relationships among taxa were based on a parsimony analysis of molecular data (23, 24), and Phoberomys was added to that tree in several alternative positions, using the scaffold approach (25). The morphological characters were optimized in these different topologies. The most parsimonious placement for Phoberomys was as sister group of Dinomys (47 steps), three steps shorter than the hypothesis of sister group relationship with chinchillas and viscachas and indeed at least three steps shorter than any other hypothesis. Based only on dental traits, Phoberomys has been classified together with the chinchillas and viscachas, with the pakaranas (Dinomys), or as sister group to both. Several postcranial traits support the pakarana hypothesis, represented in this tree: Rectus femoris muscle attachment in pelvis forms an elongated crest; medial and lateral ridges of femoral trochlea convergent proximally; medial condyle wider than lateral condyle in posterior view of femur; medial ridge of astragalar trochlea protrudes posteriorly further than lateral one; anconeal process of ulna extends further cranially than coronoid process. Some of these shared-derived character states of Dinomys and Phoberomys evolved convergently in other caviomorphs. See supplementary information for a complete list of characters, the character matrix, and a list of specimens examined.
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Phoberomys is reported to be the largest rodent that ever existed, although it has been known only from isolated teeth and fragmentary
postcranial bones. An exceptionally complete skeleton of Phoberomys pattersoni was discovered in a rich locality of fossil vertebrates in the Upper Miocene of Venezuela. Reliable body mass estimates yield
∼700 kilog...
Context in source publication
Context 1
... neoepiblemids) has been clas- sified either with the chinchillas and viscachas (8), with the pakarana (9), or as the sister group to both (10). We plotted a set of 13 postcranial characters on a preexisting phylogenetic tree based on molecular data and found that several postcranial features support the association of Phoberomys with the pakarana (Fig. 2). This position for Phoberomys was the one that re- quired the least number of ...
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The capacity of limb bones to resist the locomotor loads they encounter depends on both the pattern of those loads and the material properties of the skeletal elements. Among mammals, understanding of the interplay between these two factors has been based primarily on evidence from locomotor behaviors in upright placentals, which show limb bones th...
Citations
... These indices quantify limb bone thickness relative to body size independent of BM, making them suitable for testing the relationship between body habitus and BM. We analyzed all four measurements to explore potential differences in quantifying body habitus, but since they all produced similar results (see Online Resource 3: Section 11), we focus our discussion on the femur circumference (FC) robustness index (RI); this had the largest sample size among the robustness indices we analyzed and is generally thought to correlate well with BM (e.g., Anderson et al. 1985;Roth 1990;Ruff 1990;Anyonge 1993;Christiansen 1999;Sánchez-Villagra et al. 2003;Campione and Evans 2012). ...
Body mass (BM) is a fundamental variable for many paleobiological investigations that is challenging to accurately infer for species that lack living representatives and/or close morphological analogs. This study explores this issue using notoungulates, a diverse group of extinct South American herbivorous mammals with an extensive fossil record. We use a new dataset of 1,900+ extant mammal species (from ~80,000 specimens) to estimate notoungulate BM based on head-body length and a published dataset of 400+ species (~2,100 specimens) to estimate BM based on occipital condyle width. Condylobasal length, stylopod diameter and circumference, and neck length data are used to explore factors that can confound BM predictions. We estimate the following BM ranges for 10 osteologically well-characterized species and calculate similar ranges for 30 others known from less complete remains: Toxodontia: Thomashuxleya externa (80–120 kg), Homalodotherium cunninghami (250–350 kg), Scarrittia canquelensis (450–550 kg), Adinotherium ovinum (75–90 kg), Nesodon imbricatus (350–400 kg), and Toxodon platensis 1,000–1,200 kg); Typotheria: Interatherium robustum (1.9–2.0 kg), Miocochilius anomopodus (9–14 kg), Protypotherium australe (3.5–4.0 kg), and Pachyrukhos moyani (1.2–1.6 kg). We suggest that species such as these can be used as “calibration points” when inferring BM of species known from more limited remains. Discrepancies between our estimates and previously-published studies are primarily due to the distinctive craniodental morphology of notoungulates and the robust limb bones of toxodontians. There is significant, non-random error correlated with body habitus (i.e., being relatively robust or gracile) in many variables traditionally used to estimate BM, including femur circumference, and new methods are needed to compensate for this.
... At the same time, some enigmatic taxa are not included in such groups and probably represent extinct stem lineages (see Frailey & Campbell, 2004;Antoine et al., 2012;Bertrand et al., 2012;Vucetich et al., 2015;Boivin et al., 2016Boivin et al., , 2017Boivin et al., , 2018Boivin et al., , 2019Arnal et al., 2019Arnal et al., , 2022. Neogene was an interval of time of great importance in the diversification of caviomorphs, evidenced by the disparity in body sizes that marked the evolution of several groups (Ferreira et al., 2020;Rinderknecht & Blanco, 2008;Sánchez-Villagra et al., 2003). Several groups became extinct in this continent during the Late Miocene and Pliocene, possibly associated with drastic environmental and biotic changes (Vucetich et al., , 2015. ...
Here I reviewed the Pleistocene caviomorphs collected by Santiago Roth (collection from Catalog No. 5) and housed at the paleontological collection of the Paläontologisches Institut und Museum, Universität Zürich, Zurich (Switzerland). The fossils were found in Pleistocene strata from Buenos Aires and Santa Fé provinces (Argentina) during the late nineteenth century. The material includes craniomandibular remains assigned to Lagostomus maximus (Chinchilloidea: Chinchillidae), craniomandibular and postcranial (thoracic and sacral vertebra, left scapula, left femur, and right tibia) bones identified as Dolichotis sp. (Cavioidea: Caviidae), and a fragmented hemimandible and isolated tooth of Myocastor sp. (Octodontoidea: Echimyidae). Other rodent specimens from this collection ( Ctenomys sp. and Cavia sp.) are possibly sub-recent materials.
... Despite the limited fossil evidence currently assembled for Borikenomys, the unusual dental traits of the latter (e.g., laminae with heterogeneous thickness of the enamel layer, and narrow inter-lamina space filled with cement), which are otherwise retrieved only in several mainland chinchilloids (extinct and extant dinomyids and chinchillids) and in the West Indian 3 Amblyrhiza, Elasmodontomys and Quemisia, strongly suggest its possible phylogenetic link with these more recent chinchilloid forms of the islands . If this link is proven to be true, it would highlight the long-live insular lineage of the West Indian chinchilloids, including a tendency towards a spectacular island gigantism, as was achieved for some of their South American counterparts (e.g., Mones 1980;Sánchez-Villagra et al. 2003;Rinderknecht and Blanco 2008;Carrillo and Sánchez-Villagra 2015;Álvarez et al. 2017;Rinderknecht et al. 2018;Boivin et al. 2019c;Ferreira et al. 2020). "Giant hutias" unfortunately never reached modern times, although they potentially evolved on the islands for over 30 million years. ...
Analysis of the incisor enamel microstructure of extinct and extant West Indian caviomorph rodents emphasizes a clear microstructural distinction between the Echimyidae (Capromyinae and Heteropsomyinae) among Octodontoidea and the “Heptaxodontidae”, whose phylogenetic affinities are debated. All capromyines and heteropsomyines have a pattern of enamel characterized by a rectangular crystallite arrangement, which is biomechanically strongest in limiting crack propagation most efficiently (subtype [Sbt.] 3 of multiserial Hunter-Schreger bands [HSBs]). This enamel condition is exclusive to all mainland octodontoids. In stark contrast, “heptaxodontids” sampled here exhibit much less derived enamel subtypes of multiserial HSBs, with parallel to acute angular crystallite arrangement (Sbt. 1 [Clidomys], Sbt. 1–2 [Elasmodontomys], and low acute Sbt. 2 [Amblyrhiza]), less well adapted for prevention of crack propagation. The incisor enamel microstructure of Amblyrhiza and Clidomys is consistent with a chinchilloid assignment, as reflected by the anatomy of their auditory region and their unusual dental pattern. For Elasmodontomys, the primitive condition of its incisor enamel is difficult to reconcile with its highly nested phylogenetic position within the Octodontoidea clade (among the Capromyinae), as recently inferred from aDNA analyses. The different enamel patterns among extinct and extant West Indian caviomorphs indicate distinct high-level taxonomic groups, but restricted to the Octochinchilloi (Octodontoidea and Chinchilloidea) among Caviomorpha. The great diversity of caviomorphs on the Caribbean islands resulted from intra-archipelago diversification through time, but their high-level phylogenetic diversity can only be explained by distinct sources, implying de facto multiple (potentially time-staggered) natural colonizations of the West Indies. The chinchilloid-compatible enamel and dental pattern characterizing Borikenomys from lower Oligocene deposits in Puerto Rico strongly suggest a link with some of the recently-extinct “heptaxodontids”, which would substantiate their much greater antiquity in the Caribbean islands.
... Extant semi-aquatic beavers are the second largest rodent in the world today; the largest extant rodent, the capybara, is amphibious; the largest beavers to have existed were semi-aquatic; the largest known rodents ever found had amphibious ecologies [8,31,77,78]. Yet, there is evidence for many small semi-aquatic beavers in the fossil record, including now Microtheriomys, the fifth smallest amphibious beaver to date. Interestingly, the previously oldest semi-aquatic North American beaver, Monosaulax, is also among the smallest five amphibious beavers; Eucastor tedi, another Miocene-aged amphibious beaver, is also small [52]. ...
The North American rodent fossil record includes hundreds of species representing both an incredible taxonomic diversity and great ecological disparity. Although it is during the Oligocene that taxonomic diversity first peaks, it is not until the Miocene, almost 10 Myr later, that many ecologies, particularly locomotory ecologies, are recorded. Here, I present a new Oligocene-aged species of beaver from Montana, Microtheriomys articulaquaticus sp. nov., which represents the oldest semi-aquatic rodent in North America and the oldest amphibious beaver in the world, pushing the advent of semi-aquatic ecology in beavers by 7 Myr. I also provide morphological data supporting a terrestrial ecology for the sister taxon to Castoridae. Together with existing data, these findings lead to a new hypothesis for the evolutionary ecology of castorids whereby swimming was exapted from burrowing during the Oligocene. This evolution of semi-aquatic locomotion may have taken place in North America instead of Eurasia. It started in small beavers with gigantism achieved only much later. Indeed, body size evolution in castoroids follows a directional drift. Beavers obey Cope's rule, a selection for larger size over time that appears associated with semi-aquatic ecology and may well explain their low modern diversity.
... However, even larger rodent species are known to exist in the fossil record, most notably Josephoartigasia monesi (Dinomyidae) and Phoberomys pattersoni (Neoepiblemidae) (figure 1). Both Josephoartigasia and Phoberomys are represented by well-preserved cranial remains [10,11], and well-preserved postcranial remains are also known for P. pattersoni [12]. Josephoartigasia and Phoberomys pertain to separate but closely related clades among Chinchilloidea that appear to have evolved large body sizes independently [13,14] and are more broadly related to chinchillids and the modern pacarana (Dinomys branickii, Dinomyidae) among living rodents. ...
... Josephoartigasia and Phoberomys were much larger than the extant Hydrochoerus, but exactly how large is a matter of controversy. Initial studies proposed body masses of approximately 700 kg (range of 436-741 kg) for Phoberomys [12] based on measurements of the humerus and femur and 1211 kg (range of 468-2586 kg) for Josephoartigasia [11] based on measurements of the skull, with these animals regarded as comparable to large bovids in size [15,16]. However, later studies have suggested smaller sizes of 234-250 kg [17,18], 420-580 kg [19] or 444-655 kg [13] for P. pattersoni (some of which are based on estimates from craniodental variables rather than postcranial ones) and sizes of 622-917 kg [13] or potentially as low as approximately 350 kg for J. monesi [8], respectively. ...
... However, it is much lower than the average mass of 159 kg estimated using the diameter of the humerus or femur in Kerber et al. [21]. This is very similar to the pattern seen in Phoberomys: previous studies estimating body mass using limb bone diaphyseal dimensions [12,18,19] produce much higher estimates than other skeletal proxies such as tooth row length (e.g. [17,18], OCW and CBL in this study), often producing estimates that are more than twice the size of estimates based on other skeletal variables. ...
Several extinct chinchilloid rodents in the clades Dinomyidae and Neoepiblemidae grew to sizes much larger than any living rodent species. However, the exact size of these rodents is a matter of controversy, with authors disagreeing due to issues over extrapolation and model selection. Prior estimates for the two largest extinct rodents, Phoberomys pattersoni and Josephoartigasia monesi , range from 230 to 700 kg for P. pattersoni and 350 to 2600 kg for J. monesi . Here, I estimate body mass in large, extinct rodents using occipital condyle width (OCW), a strong predictor of body size in mammals, using a dataset that circumvents many of the issues faced by previous studies of species. Body masses under shape-corrected OCW are much lower than previous studies: 108–200 kg for P. pattersoni and 480 kg for J. monesi . Mass estimates for other rodent taxa ( Neoepiblema , Telicomys , Dinomys ) agree with previous studies. Estimates using skull length, corrected condyle width and head-body length are similar, suggesting estimates of 150 kg for Phoberomys and 480 kg for Josephoartigasia , and that larger estimates of 700 and 1200 kg are unlikely. High estimates in previous studies appear to be due to the unrecognized, nonlinear relationship between certain skeletal measurements (skull size) and body mass.
... instead. In the early 2000s, an almost complete but poorly preserved skeleton of Phoberomys pattersoni was found in the Urumaco Formation of Venezuela, including postcranial remains associated with the skull (Sánchez-Villagra et al. 2003;Horovitz et al. 2006; see also Geiger et al. 2013). More recently, Muñoz et al. (2019) described the forelimb bones of Perimys from the Santa Cruz Formation, Argentina. ...
... In recent contributions, the morphology of the skull and teeth and their implications for taxonomy, phylogeny, and paleobiology of Brazilian Late Miocene neoepiblemids have been studied (Kerber et al. 2017a, b;2019a, b;Ferreira et al. 2020). Inferences, mainly based on the paleobiogeographic and sedimentary patterns of the neoepiblemid fossils, but also based on osteological traits, have suggested that they probably inhabited water-related environments, like swamps, lakes, or rivers, being possibly semiaquatic or using these places to forage (Sánchez-Villagra et al. 2003;Vucetich et al. 2010;Nasif et al. 2013;Wilson and Geiger 2015;Kerber et al. 2017b). In this paper, we analyze the morphology of postcranial bones of Neoepiblema (with an emphasis on N. acreensis, the best-sampled neoepiblemid from Brazil). ...
... Solimões Formation,Upper Miocene,Brazil). Phoberomys pattersoni (UNEFM-VF-20; Urumaco Formation, Upper Miocene, Venezuela) was studied from additional images provided by the authors of the specimen description (Sánchez-Villagra et al. 2003;Horovitz et al. 2006). Analysis of the Miocene neoepiblemid Perimys was based on data provided by Muñoz et al. (2019). ...
In this paper, we study the postcranial morphology (humerus, ulna, innominate, femur, tibia, astragalus, navicular, and metatarsal III) of Neoepiblema, a giant Late Miocene South American rodent, searching for evidence about its paleobiology based on unpublished specimens from Solimões Formation (Upper Miocene, Brazil). The study includes a morphofunctional analysis of the postcranial bones and a comparison with extant and extinct rodents, especially Phoberomys. The morphofunctional analysis of the postcranial bones suggests that Neoepiblema (as well as Phoberomys) would have a crouched forelimb that was not fully extended, with powerful pectoral and triceps musculature, and able to produce movements of pronation/supination and possibly with a hand able to grasp. The combination of characters of the innominate bone, femur, and tibia indicates a predominance of parasagittal movements and a thigh with powerful musculature used during propulsion. In sum, the analyzed postcranial features are consistent with the limb morphology of ambulatory rodents, but with faculty to dig or swim. The sedimentary evidence of the localities in which fossils of neoepiblemids have been found suggests that these rodents lived in wet and water-related environments (near swamps, lakes, and/or rivers).
Graphical abstract
... In particular, Millien and Bovy [31] found that extinct giant caviomorphs like Phoberomys pattersoni have hindlimb bones that are disproportionately robust to their body size even relative to extant caviomorphs, which according to these authors may have produced inaccurate body mass estimates for this taxon. This is demonstrated in the fact that, due to their [33]), hyaenodont "creodont" (B, Hyaenodon horridus; modified from Scott and Jepsen [34]), and canid carnivoran (C, Canis lupus, public domain from Wikimedia Commons), scaled to the same thorax length (not head-body length, due to differences in relative neck length in the three taxa), illustrating the proportionally larger heads of Borhyaena and Hyaenodon unusually robust hindlimbs, body mass estimates for glyptodonts and extinct giant caviomorphs like Phoberomys based on the femur range from 70 to 380% higher than estimates based on the humerus [74,75]. Compounding problems with the influence of ecology or phylogenetic signal is the fact that most extant large mammals, such as artiodactyls, equids, many carnivorans, and even rhinocerotids to some degree [76,77], are cursorial and have relatively gracile limbs. ...
Background
Body mass estimation is of paramount importance for paleobiological studies, as body size influences numerous other biological parameters. In mammals, body mass has been traditionally estimated using regression equations based on measurements of the dentition or limb bones, but for many species teeth are unreliable estimators of body mass and postcranial elements are unknown. This issue is exemplified in several groups of extinct mammals that have disproportionately large heads relative to their body size and for which postcranial remains are rare. In these taxa, previous authors have noted that the occiput is unusually small relative to the skull, suggesting that occiput dimensions may be a more accurate predictor of body mass.
Results
The relationship between occipital condyle width (OCW) and body mass was tested using a large dataset (2127 specimens and 404 species) of mammals with associated in vivo body mass. OCW was found to be a strong predictor of body mass across therian mammals, with regression models of Mammalia as a whole producing error values (~ 31.1% error) comparable to within-order regression equations of other skeletal variables in previous studies. Some clades (e.g., monotremes, lagomorphs) exhibited specialized occiput morphology but followed the same allometric relationship as the majority of mammals. Compared to two traditional metrics of body mass estimation, skull length, and head-body length, OCW outperformed both in terms of model accuracy.
Conclusions
OCW-based regression models provide an alternative method of estimating body mass to traditional craniodental and postcranial metrics and are highly accurate despite the broad taxonomic scope of the dataset. Because OCW accurately predicts body mass in most therian mammals, it can be used to estimate body mass in taxa with no close living analogues without concerns of insufficient phylogenetic bracketing or extrapolating beyond the bounds of the data. This, in turn, provides a robust method for estimating body mass in groups for which body mass estimation has previously been problematic (e.g., “creodonts” and other extinct Paleogene mammals).
Paper currently available as open access at https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-021-01224-9.
... The pacarana (Dinomys branickii) is the sole living dinomyid, but the group includes numerous extinct members (e.g., Kerber et al. 2018), including the largest rodent known, Josephoartigasia monesi (Rinderknecht and Blanco 2015). Other extinct chinchilloids include neoepiblemids, which have a temporal range from the late Oligocene to late Miocene and also attained extremely large body size (e.g., Bondesio et al. 1975;Sánchez-Villagra et al. 2003;Ferreira et al. 2020), and cephalomyids, a rather poorly known late Oligocene to early Miocene group (e.g., Kramarz 2001b;Busker et al. 2020). ...
The fossil record of chinchillid rodents (Hystricomorpha: Caviomorpha) begins in the early Miocene. However, nearly all remains have thus far been limited to the Lagostominae, which includes the extant plains viscacha (Lagostomus maximus). Here, we describe the first Neogene remains referable to Pan-Chinchillinae (otherwise including extant chinchillas and mountain viscachas, Chinchilla and Lagidium), and refer them to Miochinchilla gen. nov. Miochinchilla differs from extant chinchillines in having obliquely oriented upper molar laminae, sinuous lower molar laminae, a laminar distal lobe of M3, a medial neurovascular sulcus on the floor of the infraorbital foramen delineated by a low lateral crest, a larger and more robust coronoid process, and a complete entepicondylar foramen, thereby significantly adding to the group’s recorded morphological diversity. At present, Miochinchilla is only known from the central Andes (southern tropics). The oldest known pan-chinchilline (and possibly chinchilline), Miochinchilla surirense gen. et sp. nov., is represented by many well-preserved remains from Chucal, northern Chile (late early Miocene). More limited material of Miochinchilla plurinacionalis sp. nov. derives from middle Miocene sites in south-central Bolivia (Nazareno and Quebrada Honda). Intralaminar transverse crests, structures not previously recognized in fossil chinchillids, occur in M. surirense but not M. plurinacionalis. M. plurinacionalis further differs from M. surirense in its more oblique laminae and mesiodistally narrower molars. Although Miochinchilla is exclusively closely related to extant chinchillines, its precise relationship to them remains unclear. We report intralaminar transverse crests in some specimens of the lagostomine Prolagostomus, which suggests the parallel origin of this dental feature in pan-chinchillines and lagostomines.
... Neogene neoepiblemid rodents from the Neotropics include two late Miocene genera: †Neoepiblema and †Phoberomys (Sánchez-Villagra et al. 2003;Horovitz et al. 2006;Kerber et al. 2019), both recorded in the upper Miocene sequence of Urumaco . Adult specimens of Neoepiblema show the lower premolar and molars with three laminae, while B1, C1, D1, E1, F1, G1, J1). ...
The Pliocene–Pleistocene transition in the Neotropics is poorly understood despite the major climatic changes that occurred at the onset of the Quaternary. The San Gregorio Formation, the younger unit of the Urumaco Sequence, preserves a fauna that documents this critical transition. We report stingrays, freshwater bony fishes, amphibians, crocodiles, lizards, snakes, aquatic and terrestrial turtles, and mammals. A total of 49 taxa are reported from the Vergel Member (late Pliocene) and nine taxa from the Cocuiza Member (Early Pleistocene), with 28 and 18 taxa reported for the first time in the Urumaco sequence and Venezuela, respectively. Our findings include the first fossil record of the freshwater fishes Megaleporinus, Schizodon, Amblydoras, Scorpiodoras, and the pipesnake Anilius scytale, all from Pliocene strata. The late Pliocene and Early Pleistocene ages proposed here for the Vergel and Cocuiza members, respectively, are supported by their stratigraphic position, palynology, nannoplankton, and 86Sr/88Sr dating. Mammals from the Vergel Member are associated with the first major pulse of the Great American Biotic Interchange. In contrast to the dry conditions prevailing today, the San Gregorio Formation documents mixed open grassland/forest areas surrounding permanent freshwater systems, following the isolation of the northern South American basin from western Amazonia. These findings support the hypothesis that range contraction of many taxa to their current distribution in northern South America occurred rapidly during at least the last 1.5 million years.
... South American rodents of the infraorder Caviomorpha represent one of the most spectacular evolutionary radiations among living New World mammals (Hershkovitz 1972), and fossil data suggest the independent evolution of gigantism in at least three lineages (Vucetich and Deschamps 2015). Caviomorphs have the broadest range in body size within rodents, and make Rodentia one of the mammalian orders with the largest range in body size (Sánchez-Villagra et al. 2003). Most notable among the caviomorphs is the largest living rodent, the capybara, Hydrochoerus hydrochaeris (Linnaeus, 1776), family Caviidae (Fig. 1C). ...
Gigantism results when one lineage within a clade evolves extremely large body size relative to its small-bodied ancestors, a common phenomenon in animals. Theory predicts that the evolution of giants should be constrained by two tradeoffs. First, because body size is negatively correlated with population size, purifying selection is expected to be less efficient in species of large body size, leading to increased mutational load. Second, gigantism is achieved through generating a higher number of cells along with higher rates of cell proliferation, thus increasing the likelihood of cancer. To explore the genetic basis of gigantism in rodents and uncover genomic signatures of gigantism-related tradeoffs, we assembled a draft genome of the capybara (Hydrochoerus hydrochaeris), the world’s largest living rodent. We found that the genome-wide ratio of non-synonymous to synonymous mutations (ω) is elevated in the capybara relative to other rodents, likely caused by a generation-time effect and consistent with a nearly-neutral model of molecular evolution. A genome-wide scan for adaptive protein evolution in the capybara highlighted several genes controlling post-natal bone growth regulation and musculoskeletal development, which are relevant to anatomical and developmental modifications for an increase in overall body size. Capybara-specific gene-family expansions included a putative novel anticancer adaptation that involves T cell-mediated tumor suppression, offering a potential resolution to the increased cancer risk in this lineage. Our comparative genomic results uncovered the signature of an intragenomic conflict where the evolution of gigantism in the capybara involved selection on genes and pathways that are directly linked to cancer.
