Euarchontoglires

Euarchontoglires, also known as Supraprimates, is a major clade of placental mammals comprising the orders Primates (including humans, apes, monkeys, and lemurs), Scandentia (tree shrews), Dermoptera (colugos or flying lemurs), Rodentia (rodents such as mice, rats, squirrels, and beavers), and Lagomorpha (rabbits, hares, and pikas).^[1][2] This superorder represents approximately half of all living mammalian species (around 3,000 of ~6,500 as of 2025), with rodents alone accounting for ~2,300 extant species, making it one of the most diverse groups within Eutheria.^[2][3] Euarchontoglires is defined by shared rare genomic events, including an 18-amino-acid deletion in the SCA1 gene and expansions in certain gene families like bitter taste receptors (TAS2Rs), which distinguish it from other placental clades.^[4][5] Phylogenetically, Euarchontoglires forms one of the four primary lineages of placental mammals, alongside Afrotheria, Xenarthra, and Laurasiatheria, within the larger subgroup Boreoeutheria (Euarchontoglires + Laurasiatheria).^[6][7] The clade originated around 90 million years ago during the Late Cretaceous, with its internal diversification involving key splits: the division into Euarchonta (Primates + Scandentia + Dermoptera) and Glires (Rodentia + Lagomorpha) occurring approximately 75–85 million years ago.^[1][8] This evolutionary history is marked by challenges such as incomplete lineage sorting, particularly at deep nodes like the base of Primates and Glires, which has complicated resolving exact relationships but is supported by genomic analyses of retrotransposons and protein-coding genes.^[1][9] Notable aspects of Euarchontoglires include convergent evolutionary patterns across its orders, such as arboreal adaptations in primates and some rodents, and high encephalization quotients in primates linked to extended longevity and cognitive complexity.^[10][11] The clade's genomic signatures, including variable microsatellite distributions and multigene families like LOX (lipoxygenases) in rodents, underscore its adaptive radiation and utility as a model for studying mammalian evolution, immunology, and disease.^[12][13]

Overview and Definition

Etymology

The name Euarchontoglires combines the terms Euarchonta and Glires, representing a superordinal grouping of placental mammals based on shared evolutionary affinities. Euarchonta derives from the Greek prefix eu- (meaning "true" or "good") and archonta (from archōn, "ruler" or "chief"), translating to "true rulers" and alluding to advanced eutherian mammals such as primates, scandentians (tree shrews), and dermopterans (colugos), which were viewed as higher in the taxonomic hierarchy. The base term Archonta was originally proposed by Gregory in 1910 to denote a similar assemblage emphasizing arboreal and encephalized forms. In contrast, Glires originates from the New Latin plural of Latin glīrēs (from glis, "dormouse"), highlighting the inclusion of rodents and lagomorphs (rabbits, hares, and pikas), orders noted for their cursorial and gnawing specializations reminiscent of dormice.^[14] The superorder Euarchontoglires was formally introduced by McKenna and Bell in their 1997 monograph on mammalian classification, where it served as a cohort uniting Euarchonta and Glires within the subclass Theria, reflecting a synthesis of morphological and emerging molecular insights into placental mammal relationships.^[15] This nomenclature built on earlier supraordinal categories, such as those outlined by Simpson in 1945, which grouped mammals into informal cohorts based on adaptive and anatomical convergences, though McKenna and Bell's system incorporated finer hierarchical distinctions to accommodate fossil and living taxa.^[15] An alternative designation for the clade is Supraprimates, proposed by Waddell, Kishino, and Ota in 2001 to underscore its composition as primates plus their nearest non-primate relatives (including rodents, lagomorphs, tree shrews, and colugos), positioning it "above" the order Primates in a broader phylogenetic context. This synonym gained traction in molecular phylogenetics, where naming conventions shifted toward clade-based terms that prioritize monophyly over traditional morphological cohorts, influencing subsequent revisions in mammalian taxonomy.

Scope and membership

Euarchontoglires encompasses five living orders of placental mammals: Primates, with approximately 540 species (as of 2025) including humans, monkeys, apes, and lemurs; Scandentia, comprising 23 species of tree shrews; Dermoptera, consisting of 2 species of colugos; Rodentia, with approximately 2,870 species (as of 2025) of rodents such as mice, rats, squirrels, and beavers; and Lagomorpha, including approximately 110 species of rabbits, hares, and pikas.^{[16][17][18][19]} This superorder accounts for over 3,500 species in total, representing approximately 55% of all extant placental mammals (as of 2025) and highlighting its significant contribution to mammalian biodiversity.^[2][3] Certain extinct families are sometimes associated with Euarchontoglires as potential early members or stem groups, including Apatemyidae from the Eocene, Anagaloidea from the Paleocene to Eocene, and Arctostylopidae from the Late Paleocene to Early Eocene.^[20] Membership in Euarchontoglires is determined by shared derived anatomical traits, such as specific dental and cranial features, alongside molecular data from DNA sequences and retrotransposon markers that robustly confirm its monophyly as a clade.^[6][21]

Shared Characteristics

Anatomical features

Euarchontoglires exhibit several distinctive anatomical features in the skull and postcranium that set them apart from other placental mammals, reflecting adaptations to diverse ecological niches such as arboreality and enhanced sensory capabilities. In the postcranium, many members of Euarchontoglires, particularly within Euarchonta, display reduced claws or flattened nails rather than sharp claws, which enhance grasping ability on branches or substrates. For instance, primates possess flat nails on all digits, promoting precise manipulation and secure arboreal locomotion, a trait that contrasts with the more pointed claws typical of many laurasiatherians. Dental morphology varies across the clade but shows notable patterns: Glires often feature hypsodont (high-crowned) molars adapted for grinding abrasive vegetation, as seen in rodents and lagomorphs where crowns exceed root length to withstand wear from fibrous diets. In contrast, Euarchonta display more generalized, lower-crowned dentition suited to omnivory or frugivory, with reduced emphasis on extreme hypsodonty. The brain in Euarchontoglires shows relative enlargement compared to basal placentals, particularly in the neocortex, which supports advanced sensory integration and processing. This neocortical expansion, evident in higher encephalization quotients across lineages like primates and some rodents, likely evolved convergently within the clade to accommodate visual and olfactory demands, though basal forms retain a smaller overall brain volume.^[22] Limb adaptations in arboreal Euarchontoglires include a flexible shoulder girdle and elongated digits, enabling enhanced mobility and prehensility. For example, colugos and primates have glenoids positioned for greater scapular rotation, paired with extended phalanges that facilitate clinging and leaping in forested environments.^[23]

Genetic markers

Euarchontoglires is characterized by conserved signature indels (CSIs) in protein-coding sequences that are unique to its members, providing molecular synapomorphies for the clade. A study examining four nuclear genes—SCA1, PRNP, TNFα, and HSPB3—identified 17 such indels exclusively present in Euarchontoglires species, including primates, tree shrews, colugos, rodents, and lagomorphs, with their absence in outgroups confirming clade specificity.^[24] These CSIs, along with similar markers in additional proteins, underscore the shared evolutionary history of the group, though comprehensive surveys indicate at least 25 conserved proteins bearing indels diagnostic for related subclades like Glires within Euarchontoglires.^[25] Mitochondrial genes, such as cytochrome b, also exhibit sequence patterns and indels that align with Euarchontoglires monophyly when analyzed phylogenetically.^[26] Retrotransposon insertions further delineate Euarchontoglires through shared orthologous short interspersed nuclear elements (SINEs) in genomic loci across its taxa. SINE1 family elements, derived from 7SL RNA, show parallel evolutionary patterns but are prominently distributed in Euarchontoglires lineages, including primates and rodents, with specific insertions serving as homoplasy-free markers of common ancestry.^[27] Genome-wide surveys of transposed elements reveal hundreds of shared presence/absence patterns that bolster clade cohesion, such as orthologous retrotransposon sites unique to this group compared to other boreoeutherians.^[1] Karyotypic features include a reconstructed ancestral diploid chromosome number of 2n=46 for the clade, with conserved synteny blocks between rodents and primates, as evidenced by comparative chromosome painting. Genome-wide phylogenomic analyses provide strong statistical support for Euarchontoglires monophyly, with bootstrap values often exceeding 95% across concatenated datasets from thousands of loci. For example, analyses of over 1.4 million aligned nucleotides from multiple nuclear genes yielded 95% bootstrap support for the clade uniting Euarchonta and Glires.^[6] Whole-genome sequences from representative species further affirm this topology, with posterior probabilities near 1.0 and minimal conflict in gene trees.^[28] Key studies establishing these markers range from early ribosomal RNA analyses, such as 18S rRNA sequences that first suggested affinity between primates and Glires in the 1990s, to contemporary multi-gene phylogenomics incorporating nuclear and mitochondrial data for robust validation.

Classification History

Pre-molecular era

In the pre-molecular era, mammalian classifications were predominantly based on morphological traits derived from anatomical dissections and fossil evidence, often resulting in groupings that emphasized superficial similarities over deep evolutionary relationships. Early 20th-century schemes frequently united Primates and Insectivora in higher taxa due to shared primitive features like cursorial locomotion, pentadactyl limbs, and insectivorous habits. A foundational contribution came from George Gaylord Simpson's 1945 monograph, which established the grandorder Archonta to include the orders Primates, Chiroptera (bats), and Insectivora (encompassing lipotyphlan insectivores and scandentians such as tree shrews), justified by common arboreal or aerial adaptations and relatively enlarged brains compared to other ungulaticulates. Rodents were distinctly separated from these groups and placed in the superorder Glires together with Lagomorpha, reflecting convergences in continuously growing incisors adapted for gnawing, though Simpson noted significant divergences in occlusal patterns and jaw mechanics that precluded closer affinities with primates. Simpson explicitly rejected proposals for a rodent-primate clade, attributing the dismissal to stark dental disparities, including the sciurognathous skull and hystricomorphous masseter in rodents versus the more generalized primate dentition lacking rootless incisors. This arrangement highlighted ongoing debates, as some earlier workers had speculated on ungulate-rodent links, but fossil evidence from Paleogene deposits reinforced rodents' isolation within herbivorous clades. The concept of Glires gained further traction through Albert E. Wood's 1955 revision of rodent taxonomy, which underscored morphological ties to lagomorphs via shared subordinal features like the myomorphous zygomatic arch and specialized cheek teeth for folivory, drawing on Eocene fossils to argue for a common origin despite adaptive radiations. Tree shrews, meanwhile, occupied an ambiguous position, initially aligned with insectivores in early schemes but reassigned by Simpson to a primitive suborder within Primates (Tupaioidea) based on encephalization and orbital orientation, though critics contested this due to their shrew-like dentition and lack of primate-specific traits like forward-facing eyes. Overall, these classifications depended heavily on incomplete fossil records, fostering polyphyletic assemblages such as the paraphyletic Insectivora, which masked the true branching patterns later illuminated by genetic analyses.

Molecular evidence

The concept of Euarchontoglires as a monophyletic clade grouping Euarchonta (primates, tree shrews, and colugos) and Glires (rodents and lagomorphs) was first proposed based on molecular data in the late 1990s. Waddell et al. (1999) analyzed complete mitochondrial genomes from multiple placental mammals and identified a close relationship between these groups using likelihood-based methods, marking an early molecular signal for the clade despite limitations of mtDNA, such as high substitution rates. This proposal gained robust support in the early 2000s through nuclear gene analyses. Murphy et al. (2001) examined sequences from 19 nuclear and three mitochondrial genes across 42 placental species, employing Bayesian phylogenetics to resolve the early placental radiation; their results strongly confirmed Euarchontoglires as one of four major superordinal clades within Placentalia, with high posterior probabilities excluding alternative groupings. Concurrently, Madsen et al. (2001) used a multi-locus dataset of 18 nuclear genes from 91 species to delineate Boreoeutheria as the sister clade to Afrotheria and Xenarthra, positioning Euarchontoglires alongside Laurasiatheria and resolving prior morphological inconsistencies, such as the misplaced association of insectivores with primates. Throughout the 2000s, expanded multi-locus studies further solidified these findings, shifting placental classification toward molecularly defined clades. By the 2010s, phylogenomic approaches incorporating over 100 genes across dozens of taxa provided deeper resolution, accounting for processes like incomplete lineage sorting via coalescent models; for instance, Song et al. (2012) analyzed 23 nuclear loci with multispecies coalescent methods, reinforcing Euarchontoglires monophyly and confirming the monophyly of Euarchonta (comprising Primatomorpha—primates plus dermopterans—and Scandentia) as the sister group to Glires, while placing eulipotyphlans (formerly "insectivores") firmly within Laurasiatheria. The position of Scandentia within Euarchonta remains somewhat contentious, with some analyses supporting it as sister to Primatomorpha and others suggesting alternative placements due to incomplete lineage sorting.^[29] Recent whole-genome analyses in the 2020s have refined divergence estimates and addressed remaining uncertainties. Building on earlier work like McCormack et al. (2012), which used ultraconserved elements from multiple loci to probe placental relationships, studies such as Doronina et al. (2022) integrated transposable elements and genomic data from representative species, estimating the Euarchontoglires crown divergence at approximately 90 million years ago during the Late Cretaceous, with the Boreoeutheria split around 100 Ma; these analyses highlight the clade's ancient origins amid rapid radiations.^[30][1] The molecular evidence prompted a taxonomic paradigm shift from traditional superordinal ranks to clade-based nomenclature, as reflected in influential references like Wilson and Reeder (2005) and subsequent editions, which adopted Euarchontoglires as a core placental division based on genomic congruence.^[31]

Evolutionary History

Origins and divergence

Euarchontoglires, as a major clade within the placental mammals, traces its origins to the mid-Cretaceous period, with Boreoeutheria (Euarchontoglires + Laurasiatheria) diverging from other stem placentals approximately 100 million years ago during the Albian stage.^[32] This early divergence reflects the initial radiation of placental orders amid the breakup of Pangaea and the diversification of eutherian mammals in a world dominated by dinosaurs. Molecular clock analyses, incorporating fossil calibrations from the Cretaceous-Paleogene (K-Pg) boundary, support this timeline, highlighting the deep roots of Boreoeutheria within placental evolution.^[32] The defining split within Boreoeutheria, separating Euarchontoglires from Laurasiatheria, occurred around 90 million years ago in the Late Cretaceous, based on Bayesian relaxed molecular clock estimates calibrated to the K-Pg extinction event at 66 Ma.^[33] These estimates derive from extensive genomic datasets, including nuclear genes, and account for rate variations across lineages to provide robust divergence dates.^[34] This bifurcation set the stage for independent evolutionary trajectories, with Euarchontoglires developing unique adaptations distinct from the carnivoran and ungulate-dominated Laurasiatheria. The internal division into Euarchonta (Primates + Scandentia + Dermoptera) and Glires (Rodentia + Lagomorpha) occurred approximately 75–85 million years ago.^[1] Following the K-Pg mass extinction at approximately 66 Ma, Euarchontoglires underwent a significant early radiation, capitalizing on vacated ecological niches left by the demise of non-avian dinosaurs and other reptilian groups.^[35] This post-Cretaceous diversification marked a pivotal phase in mammalian evolution, enabling rapid occupancy of terrestrial, arboreal, and subterranean environments across Laurasia. Paleobiogeographic reconstructions indicate that Euarchontoglires likely originated on the northern supercontinent of Laurasia, with initial diversification centered in regions corresponding to modern North America and Asia.^[36] During the Paleogene epoch (66–23 Ma), key adaptive events shaped the subclades of Euarchontoglires: Glires experienced rapid evolution toward specialized herbivory, exemplified by the rodent radiation at the Paleocene-Eocene transition, which facilitated exploitation of plant-based diets through dental and digestive innovations.^[37] Concurrently, Euarchonta adapted to arboreal lifestyles, with enhanced grasping limbs and visual acuity supporting tree-dwelling habits in forested Paleogene ecosystems.^[38] These developments underscore the clade's versatility in responding to post-extinction opportunities.

Fossil record

The fossil record of Euarchontoglires begins shortly after the Cretaceous-Paleogene (K-Pg) boundary, with the earliest known representatives appearing in the latest Cretaceous or earliest Paleocene of North America. Purgatoriidae, a family of small, arboreal plesiadapiforms, are among the oldest euarchontan fossils, dating to approximately 66 million years ago (Ma) from sites in the western United States and Canada. These mammals, known primarily from dental and postcranial remains, exhibit primitive primate-like features such as grasping capabilities in the tarsals, suggesting early adaptations for life in trees, and are considered potential stem euarchontans or stem primates.^[39][40][41] Stem glires are represented by several enigmatic Paleogene groups, including Anagalidae (formerly part of Anagaloidea), which are primitive euarchontoglires from the Eocene of Asia, particularly Mongolia and China, showing lagomorph-like dental specializations but basal to the rodent-lagomorph clade. These fossils, dating to around 50-40 Ma, provide evidence of early glires diversification in Asia, with mandibular and cranial features indicating insectivorous or omnivorous diets. Another debated group is Arctostylopidae, known from the late Paleocene to early Eocene (approximately 60-50 Ma) of Asia and North America, with Asian origins; their affinities remain uncertain, but dental and postcranial traits suggest possible links to basal glires or other euarchontoglires, though not undisputedly within the clade.^[20][42][43] Within Euarchonta, early fossils include dermopterans such as Dermotherium from the late Eocene of Southeast Asia (Thailand), around 37 Ma, representing one of the oldest Asian records of gliding mammals with specialized patagia for arboreal locomotion. The extinct family Apatemyidae, spanning the Paleogene (Paleocene to Oligocene, 66-23 Ma) in Europe and North America, features elongated snouts and limbs suggestive of gliding or scansorial habits, with phylogenetic analyses placing them as possible euarchontans, though their exact position remains debated due to mosaic morphologies.^[44][45] Significant gaps characterize the early fossil record of Euarchontoglires, particularly for rodents, whose oldest undisputed fossils date to the early Eocene (around 55 Ma) in North America and Asia, despite molecular estimates indicating divergence from other euarchontoglires in the late Cretaceous. This discrepancy implies the existence of "ghost lineages"—unpreserved evolutionary branches—extending back before the K-Pg boundary, as supported by total-evidence dating that reconciles sparse Paleocene records with genomic data suggesting crown euarchontoglires originated around 80-90 Ma. Debates persist over the completeness of this record, with some analyses highlighting potential sampling biases in early Cenozoic deposits, underscoring the role of post-K-Pg ecological opportunities in the clade's radiation.^[46][47][48]

Subclades

Euarchonta

Euarchonta is a clade of placental mammals comprising the orders Primates, Scandentia (tree shrews), and Dermoptera (colugos), positioned as the sister group to Glires within the larger Euarchontoglires superorder.^[41][1] This grouping reflects molecular phylogenetic analyses that consistently recover Euarchonta as monophyletic, diverging from Glires (the sister clade within Euarchontoglires) around 80 million years ago during the Late Cretaceous.^[1] The internal phylogeny of Euarchonta centers on the mirorder Primatomorpha, which unites Primates and Dermoptera as sister taxa, with Scandentia as the outgroup to this pair.^[49] This topology is primarily supported by molecular data, including genomic sequences and retrotransposon insertions, although morphological evidence sometimes favors alternative arrangements such as Scandentia sister to Primates alone.^[49] Shared morphological synapomorphies bolstering the Primatomorpha-Scandentia split include specialized tarsal bones, such as a mobile ankle joint (hallux) adapted for grasping, and enhanced eye anatomy featuring forward-directed orbits for stereoscopic vision.^[50][49] Euarchonta encompasses approximately 565 extant species (as of 2025), with the vast majority belonging to Primates (~540 species across 16 families).^[51] Scandentia includes about 23 species in two families (Tupaiidae and Ptilocercidae), all confined to tropical forests of Southeast Asia and southern China. Dermoptera is represented by just two species in a single family (Cynocephalidae), the Sunda colugo (Galeopterus variegatus) and Philippine colugo (Cynocephalus volans), both gliding arboreal mammals endemic to Southeast Asian rainforests.^[51] Characteristic traits uniting Euarchonta include large, forward-facing eyes that enhance depth perception for navigating complex arboreal environments, and grasping extremities with elongated digits and opposable halluces suited for clinging to branches.^[52][53] These features underpin predominantly arboreal lifestyles across the clade, facilitating locomotion and foraging in forested canopies.^[52][53] The clade originated approximately 80 million years ago in the Late Cretaceous, with the earliest definitive fossils appearing shortly after the Cretaceous-Paleogene boundary around 66 million years ago.^[54] Plesiadapiforms, a diverse group of Paleogene mammals, are widely regarded as stem euarchontans, exhibiting transitional traits like claw-like nails and dental specializations that bridge archaic insectivores to modern forms; key examples include Purgatorius, the geologically oldest known member from the earliest Paleocene.^[41][50] This fossil record underscores Euarchonta's rapid diversification in post-extinction North American ecosystems.^[54]

Glires

Glires is a monophyletic clade within the superorder Euarchontoglires, comprising the mammalian orders Rodentia (rodents) and Lagomorpha (rabbits, hares, and pikas), and serving as the sister group to Euarchonta (primates, treeshrews, and colugos).^[21] The monophyly of Glires is robustly supported by both molecular and morphological evidence, with key synapomorphies including enlarged, ever-growing incisors adapted for gnawing and associated mandibular modifications such as a reduced angular process and loss of certain jaw muscle attachments.^[55] These dental and cranial features facilitate efficient processing of tough plant material, distinguishing Glires from other placental mammals. The divergence between Rodentia and Lagomorpha is estimated to have occurred in the late Cretaceous, approximately 66–75 million years ago (Ma), near the Cretaceous-Paleogene (K-Pg) boundary.^[21][56] Glires exhibits remarkable species diversity, encompassing over 2,800 extant species (as of 2025) that represent approximately 41% of all living mammals. Rodentia alone includes about 2,700 species distributed across three major suborders—Myomorpha (e.g., mice, rats, and hamsters), Hystricomorpha (e.g., porcupines, guinea pigs, and New World cavies), and Sciuromorpha (e.g., squirrels and beavers)—with a global range spanning all continents except Antarctica.^[51] Lagomorpha, in contrast, comprises around 93 species in two families: Leporidae (rabbits and hares, approximately 60 species) and Ochotonidae (pikas, about 33 species), primarily inhabiting temperate and boreal zones in Eurasia, North America, and Africa, with limited presence in South America and Australia due to human introductions.^[51] This disparity underscores the explosive diversification within Rodentia compared to the more constrained adaptive radiation of lagomorphs. Shared morphological and ecological traits unite Glires, including hypsodont or hypselodont (continuously growing) molars suited for grinding abrasive vegetation, predominantly herbivorous diets focused on grasses, leaves, and roots, and elevated reproductive rates that enable rapid population recovery and colonization.^[57][58] These adaptations, particularly the ever-growing dentition, support a lifestyle centered on foraging and burrowing in terrestrial environments, contrasting with the more arboreal and visually oriented forms in the sister clade Euarchonta. The evolutionary history of Glires is marked by an explosive radiation following the K-Pg mass extinction event approximately 66 Ma, which eliminated non-avian dinosaurs and opened ecological niches for small, nocturnal mammals. This post-extinction diversification led to the rapid emergence of crown-group Glires in the Paleocene and Eocene, with early rodent fossils such as Reithroparamys from the early Eocene (around 50 Ma) exemplifying primitive ischyromyid forms with squirrel-like adaptations for arboreal or scansorial habits.^[59][60] By the Eocene, Glires had achieved significant morphological and ecological disparity, laying the foundation for their modern dominance in small-mammal communities.^[61]

Broader Relationships

Within Boreoeutheria

Boreoeutheria comprises the two major superorders Euarchontoglires and Laurasiatheria, forming a monophyletic clade that excludes the southern-hemisphere-derived Atlantogenata, which unites Xenarthra and Afrotheria.^[6] This grouping represents the northern-hemisphere-dominant branch of placental mammals, with Euarchontoglires encompassing primates, rodents, lagomorphs, tree shrews, and colugos, while Laurasiatheria includes carnivorans, cetaceans, bats, ungulates, and insectivores.^[6] The monophyly of Boreoeutheria is robustly supported by shared retroposon insertions, such as LINE-1 elements and SINEs, which serve as phylogenetic markers indicating common ancestry, as well as concordant nuclear gene trees from large-scale genomic analyses.^[62] Key evidence includes multiple independent retroposon loci unique to Boreoeutheria, confirming its distinction from Atlantogenata.^[63] The divergence of Boreoeutheria from Atlantogenata occurred approximately 90–100 million years ago during the mid-Cretaceous, as estimated in 2023 genomic analyses.^[35] Distinct evolutionary traits highlight the divergence within Boreoeutheria: Euarchontoglires exhibit notable brain expansion through neocorticalization and enhanced manual dexterity, particularly in arboreal primates and rodents adapted for grasping and manipulation.^[64] In contrast, Laurasiatheria features adaptations like specialized carnivory in orders such as Carnivora, with dentition and digestive systems optimized for meat consumption, and fully aquatic modifications in cetaceans, including streamlined bodies, echolocation, and blubber layers for thermoregulation.^[65][66] Boreoeutheria likely originated in the Northern Hemisphere on the Laurasian supercontinent, with subsequent diversification driven by continental rifting.^[67] Euarchontoglires, in particular, radiated in forested environments, leveraging angiosperm expansion and arboreal niches for evolutionary success.^[68]

Comparison with Laurasiatheria

Laurasiatheria, the sister clade to Euarchontoglires within the magnorder Boreoeutheria, encompasses several diverse orders including Carnivora (such as cats, dogs, and bears), Perissodactyla (horses, rhinoceroses, and tapirs), Cetartiodactyla (even-toed ungulates like deer, pigs, and cetaceans including whales and dolphins), Chiroptera (bats), Pholidota (pangolins), and Eulipotyphla (shrews, moles, and hedgehogs).^[69] This superorder comprises a substantial portion of placental mammal diversity, with approximately 2,700 species across its lineages.^[70] Key differences between Euarchontoglires and Laurasiatheria manifest in dietary adaptations and sensory emphases. Members of Euarchontoglires, such as primates, rodents, and lagomorphs, predominantly exhibit omnivory or herbivory, often linked to specialized plant-based feeding like granivory, frugivory, or grazing, supported by expansions in olfactory receptor genes that enhance scent detection for foraging in complex vegetation.^[71] In contrast, Laurasiatheria features a higher proportion of carnivorous forms in orders like Carnivora and piscivorous adaptations in cetaceans, alongside herbivorous ungulates in Perissodactyla and Cetartiodactyla, with sensory specialization including echolocation in many Chiroptera species for navigating and hunting in low-light or aerial environments.^[65][72] Despite these divergences, Euarchontoglires and Laurasiatheria share ancestral Boreoeutherian traits, including specific cranial features such as a relatively high position of the lateral semicircular canal in the inner ear and an early distribution centered in Laurasia during the Late Cretaceous.^[73][74] These shared morphological elements in the basicranium and postcranial skeleton underscore their common origin before the breakup of Laurasia facilitated separate radiations. Phylogenetic analyses provide robust support for the sister relationship between Euarchontoglires and Laurasiatheria, with molecular clock estimates indicating a symmetric divergence around 90 million years ago during the Late Cretaceous, and no evidence of intermixing between the clades in comprehensive genomic trees.^[75][76] Ecologically, Euarchontoglires largely dominate small-mammal niches in forested habitats, where primates, rodents, and treeshrews exploit arboreal and understory resources through enhanced vision and grasping adaptations.^[49] In comparison, Laurasiatheria occupies more open terrestrial habitats with ungulates grazing in grasslands and savannas, as well as fully aquatic realms in oceans via cetaceans, reflecting broader exploitation of expansive and marine environments.^[77][78]