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Simplicidentata
Simplicidentata
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Simplicidentata
Temporal range: Early Paleocene to present
Marsh rice rat (Oryzomys palustris)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Grandorder: Glires
Mirorder: Simplicidentata
Weber, 1904
Orders

Simplicidentata is a group of mammals that includes the rodents (order Rodentia) and their closest extinct relatives. The term has historically been used as an alternative to Rodentia, contrasting the rodents (which have one pair of upper incisors) with their close relatives the lagomorphs (which have two).[1] However, Simplicidentata is now defined as including all members of Glires (the clade formed by lagomorphs and rodents) that share a more recent common ancestor with living rodents than with living lagomorphs. Thus, Simplicidentata is a total group that is more inclusive than Rodentia, a crown group that includes all living rodents, their last common ancestor, and all its descendants.[2] Under this definition, the loss of the second pair of upper incisors is a synapomorphic (shared derived) feature of Simplicidentata.[3] The loss of the second upper premolar (P2) has also been considered as synapomorphic for Simplicidentata, but the primitive simplicidentate Sinomylus does have a P2.[4]

This sense of Simplicidentata was introduced by Chuankui Li and colleagues in 1987, who ranked Simplicidentata as a superorder including Rodentia and the extinct Mixodontia, contrasted with the superorder Duplicidentata (including Lagomorpha and the extinct Mimotonida).[5] In their 1997 book Classification of Mammals, Malcolm C. McKenna and Susan K. Bell ranked Simplicidentata as a mirorder within the grandorder Anagalida (also including lagomorphs, macroscelideans, and some additional extinct groups). Within Simplicidentata, they recognized the orders Mixodontia (including only the extinct family Eurymylidae from the Paleocene and Eocene of Asia) and Rodentia.[6] McKenna and Bell's decision to use Simplicidentata was criticized by reviewer Frederick S. Szalay, who preferred to simply place the Mixodontia within Rodentia, which would leave Simplicidentata unnecessary.[7] In The Beginning of the Age of Mammals (2006), Kenneth Rose recognized a mirorder Simplicidentata, including Mixodontia, Rodentia, and the genus Sinomylus (not placed in either order), within the superorder Anagalida.[8]

References

[edit]

Literature cited

[edit]
  • Landry, S.O., Jr. 1999. A proposal for a new classification and nomenclature for the Glires (Lagomorpha and Rodentia). Mitteilungen aus dem Museum für Naturkunde in Berlin, Zoologische Reihe 75(2):283–316.
  • McKenna, M.C. and Bell, S.K. 1997. Classification of Mammals: Above the species level. New York: Columbia University Press, 631 pp. ISBN 978-0-231-11013-6
  • McKenna, M.C. and Meng, J. 2001. A primitive relative of rodents from the Chinese Paleocene. Journal of Vertebrate Paleontology 21(3):565–572.
  • Meng, J. and Wyss, A.R. 1994. Enamel microstructure of Tribosphenomys (Mammalia, Glires): Character analysis and systematic implications. Journal of Mammalian Evolution 2(3):185–203.
  • Meng, J. and Wyss, A.R. 2001. The morphology of Tribosphenomys (Rodentiaformes, Mammalia): phylogenetic implications for basal Glires. Journal of Mammalian Evolution 8(1):1–71.
  • Meng, J., Hu, Y. and Li, C. 2003. The osteology of Rhombomylus (Mammalia, Glires): implications for phylogeny and evolution of Glires. Bulletin of the American Museum of Natural History, 275:1–247.
  • Meng, J., Kraatz, B.P., Wang, Y., Ni, X., Gebo, D.L. and Beard, K.C. 2009. A new species of Gomphos (Glires, Mammalia) from the Eocene of the Erlian Basin, Nei Mongol, China. American Museum Novitates 3670:1–11.
  • Rose, K.D. 2006. The Beginning of the Age of Mammals. Baltimore: The Johns Hopkins University Press, 428 pp. ISBN 978-0-8018-8472-6
  • Szalay, F.S. 1999. [Review of] Classification of Mammals: Above the Species Level (subscription required). Journal of Vertebrate Paleontology 19(1):191–195.
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Simplicidentata

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from Grokipedia
Simplicidentata is a of eutherian mammals within the superorder , defined as all gliriform taxa sharing a more recent common ancestor with the order Rodentia than with . It encompasses the crown group Rodentia—comprising all living rodents—and various extinct stem relatives, such as eurymylids (e.g., Matutinia), ischyromyids (e.g., Ischyromys), and other early forms. The clade is distinguished by the simplicidentate condition, featuring a single pair of continuously growing incisors in each of the upper and lower jaws, which contrasts with the duplicidentate (two pairs of upper incisors) morphology of lagomorphs. The name Simplicidentata derives from New Latin roots meaning "simple-toothed," reflecting this key dental synapomorphy. The living representatives of Simplicidentata, the , form the most diverse order of mammals, with approximately 2,800 recognized (as of 2025) accounting for approximately 42% of global mammalian . These animals exhibit remarkable adaptability, inhabiting diverse environments from forests and deserts to urban areas across all continents except , and playing crucial ecological roles as herbivores, seed dispersers, predators, and prey. Within Rodentia, evolutionary diversification has led to distinct suborders based on jaw musculature and locomotion, including the squirrel-like Sciuromorpha, mouse-like , porcupine-like , and African anomalures of Anomaluromorpha. The fossil record of Simplicidentata dates to the early Eocene epoch (approximately 56–33.9 million years ago), with early stem taxa like Matutinia from the Lingcha Formation in China providing insights into primitive cranial and postcranial morphology. Many basal simplicidentates display a protrogomorphous condition, characterized by a long diastema, reduced cheek teeth, and masseter muscles originating anterior to the zygomatic arch, which represents the ancestral state from which more specialized rodent myologies (e.g., sciuromorphy) evolved. This clade's radiation coincides with the Paleogene expansion of forested habitats, facilitating the adaptive success of rodents and underscoring Simplicidentata's pivotal role in mammalian evolutionary history.

Definition and Etymology

Definition

Simplicidentata is a total of mammals within the superorder , encompassing the crown-group —all living —their last common ancestor, and all descendants thereof, as well as extinct stem relatives such as members of the family Eurymylidae (e.g., Matutinia) and Ischyromyidae (e.g., Ischyromys). This phylogenetic grouping represents the total evolutionary lineage stemming from the of modern , incorporating forms that bridge the gap between early and the diversification of proper. The key diagnostic trait of Simplicidentata is the simplicidentate condition, characterized by the presence of a single functional pair of upper s, a derived feature resulting from the evolutionary loss of the second upper incisor pair. This dental specialization contrasts with the duplicidentate condition of the Duplicidentata, which includes lagomorphs (rabbits, hares, and pikas) and their stem relatives that retain the second upper incisor. As part of , Simplicidentata occupies a broader position within the of placental mammals, but its scope is strictly limited to lineages more closely related to than to lagomorphs. In modern , the Simplicidentata was defined by Li et al. (1987) as a superordinal rank to unite Rodentia with select fossil taxa exhibiting the simplicidentate dental morphology, thereby formalizing the clade's conceptual boundaries in early mammalian phylogeny. This emphasizes the clade's unifying apomorphy in reduction, which facilitated adaptive radiations in gnawing and foraging behaviors among its members.

Etymology

The term Simplicidentata is derived from New Latin, combining the Latin stem simplici- (from simplex, meaning "simple" or "single") with dentāta, the neuter plural of dentātus (meaning "toothed"), referring to the characteristic single pair of upper in its members. This nomenclature highlights the simplified dental structure distinguishing these mammals from related groups with more complex arrangements. The term was coined by Swedish zoologist Wilhelm Lilljeborg in 1866 to designate a suborder of characterized by this single pair of upper incisors, excluding those with duplicated incisors. Originally, Simplicidentata was employed as a suborder within the order Rodentia, encompassing all except the Duplicidentata (the group now recognized as , or rabbits and hares, which possess a second pair of small upper incisors behind the primary ones). Over time, its usage has shifted in modern to describe a broader that includes the order Rodentia and various extinct relatives sharing a common , such as early forms like Rhombomylus and Matutinia, reflecting phylogenetic refinements based on fossil evidence. In contrast, Duplicidentata derives similarly from Latin duplic- (from duplex, meaning "double") and dentāta, denoting the duplicated upper incisors of lagomorphs, and was used alongside Simplicidentata to partition rodent-like mammals in 19th-century classifications. The adjective "simplicidentate" is applied to describe the dental condition of having only one functional pair of upper incisors, a key morphological trait defining the group.

Taxonomy

Historical Classification

The term Simplicidentata was coined by Swedish zoologist Wilhelm Lilljeborg in as a suborder within the order , specifically to separate possessing a single pair of upper incisors from the Duplicidentata, which at the time encompassed lagomorphs alongside certain other forms. This distinction emphasized dental morphology as a primary classificatory criterion, reflecting the era's focus on observable anatomical traits to delineate mammalian orders. In the early , Simplicidentata was frequently equated with the bulk of , often excluding the hystricognathous in classificatory schemes that prioritized jaw and muscle adaptations. Albert E. Wood's influential 1955 revision formalized Simplicidentata as a suborder of , incorporating subordinal divisions based on structure and myology while maintaining its core alignment with non-hystricognathous . Mid-20th-century discussions featured debates over fossil inclusions, with Frederick S. Szalay critiquing in the 1970s the assignment of certain taxa to Simplicidentata due to uncertainties in their dental and postcranial affinities relative to modern rodents. In their comprehensive 1997 , Malcolm C. McKenna and Susan K. Bell elevated Simplicidentata to mirorder status within the grandorder Anagalida, encompassing Rodentia and the extinct Mixodontia to better accommodate early forms. A pivotal shift occurred in when Chuankui Li and colleagues proposed Simplicidentata as a superorder to integrate Asian fossils such as Sinomylus, arguing that these primitive anagalids shared key synapomorphies with beyond subordinal rank.

Modern Taxonomy

In contemporary taxonomy, Simplicidentata is recognized as a stem-based within the larger group , comprising all taxa more closely related to crown-group than to . This total group includes the order Rodentia sensu stricto and various stem lineages, effectively equivalent to Rodentia sensu lato in broader classifications, though it lacks a formal Linnaean rank and is treated as an unranked . The defining synapomorphy is the presence of a single pair of continuously growing upper incisors, distinguishing it from the duplicidentate condition in lagomorphs. Key inclusions encompass the crown Rodentia along with Paleogene stem groups such as the families Eurymylidae, Reomyidae, and Ischyromyidae, which exhibit primitive dental and cranial features bridging early Glires to modern rodents. For instance, Eurymylidae from the and Eocene of are often placed in the extinct order Mixodontia as basal simplicidentates. The genus Sinomylus is similarly regarded as a primitive member near the base of Simplicidentata, though its exact position—whether as a stem taxon or within Rodentia—remains debated due to mosaic traits combining rodent-like incisors with lagomorph affinities. These assignments draw from comprehensive reviews emphasizing morphological evidence from dental and postcranial remains. The taxonomic hierarchy positions Simplicidentata subordinate to within , with the structure > > Simplicidentata (encompassing stem and crown Rodentia). This framework reflects a cladistic consensus prioritizing phylogenetic relationships over traditional ordinal ranks. Ongoing debates center on the affinities of certain taxa, such as Rhombomylus, which some analyses include within Simplicidentata based on shared cranial and dental features like hypocone development, while others propose it as part of a distinct anagalid lineage outside the clade due to discrepancies in jaw mechanics and incisor morphology. Such uncertainties highlight the challenges of resolving stem-group boundaries with fragmentary fossils. Furthermore, earlier schemes like that of McKenna and Bell () have faced criticism for relying on paraphyletic groupings and underemphasizing cladistic definitions, prompting revisions in favor of total-clade approaches in recent works.

Phylogeny

Relationships within Glires

The superorder comprises two primary clades: Simplicidentata, which includes crown-group Rodentia and various stem taxa, and Duplicidentata, encompassing crown-group along with stem groups such as Mimotonidae (e.g., Mimotona) and, in certain phylogenies, Rhombomylus. These clades form groups within Glires, supported by both morphological and molecular evidence that positions Simplicidentata as more closely related to than to lagomorphs, while Duplicidentata aligns with lagomorphs and their extinct relatives. This dichotomy reflects the early divergence of gnawing mammals adapted to diverse herbivorous niches, with Simplicidentata characterized by the loss of the second upper , distinguishing it from the duplicidentate condition retained in its clade. Key synapomorphies uniting Simplicidentata and Duplicidentata within include enlarged, ever-growing incisors for gnawing, loss of canines, reduced premolars, and a toothless , alongside postcranial features such as a calcaneal canal in the tarsus and adaptations like enlarged hindlimbs for leaping locomotion. Certain cranial features, including a simplified and enlarged auditory bullae, further support this close relationship, though Simplicidentata uniquely exhibits the reduction to a single pair of upper incisors. These shared traits highlight the common ancestry of , with adaptations for folivory and rapid movement evolving early in the lineage. Fossil evidence for early originates primarily from deposits in , with taxa like Tribosphenomys minutus from the late of representing potential basal members that exhibit primitive rodentiaform features and outgroup status to crown Rodentia. estimates, calibrated against divergences, place the origin of Glires around 75-80 million years ago in the , preceding the diversification of its subclades. This timeline aligns with the appearance of stem glires in Asian faunas, suggesting an Asian center of origin for the group. Ongoing debates center on the precise placement of genera like Rhombomylus from the early Eocene of Asia, which some analyses position as basal to both Simplicidentata and Duplicidentata due to its mix of primitive gliroid traits, while others affiliate it with Duplicidentata based on mandibular features such as an elongated diastema and duplicidentate-like incisor morphology. Mandibular studies of early glires, including Eocene forms like Gomphos and Paleocene taxa like Heomys, reveal intermediate characteristics blending simplicidentate and duplicidentate conditions, complicating stem-group assignments and underscoring the mosaic evolution of chewing mechanisms in basal Glires.

Broader Position in Euarchontoglires

Simplicidentata forms part of the clade within the superorder , where —comprising Simplicidentata and its Duplicidentata (lagomorphs and relatives)—is positioned as the to , which includes , scandentians (tree shrews), and dermopterans (colugos). This structure reflects the of as one of the four major placental clades, supported by extensive genomic analyses that resolve deep divergences among eutherians. Molecular phylogenies provide robust evidence for this arrangement, with studies using nuclear genes such as , , and alpha-2B adrenergic receptor confirming the of and its sister relationship to . Similarly, mitochondrial 12S rRNA sequences have corroborated across diverse and lagomorph taxa, integrating with nuclear data to affirm the topology. Fossil-calibrated molecular clocks place the divergence of from other boreoeutherians at approximately 85–90 million years ago during the , aligning with Bayesian estimates that incorporate paleontological constraints. Cladistically, Simplicidentata is defined as the clade of all more closely related to crown-group than to crown or crown , encompassing both extant and their stem-lineage relatives such as eurymyloids. This stem-based definition highlights Simplicidentata's position basal to Duplicidentata within while excluding euarchontan lineages. Seminal genomic work by Murphy et al. (2001) provided the foundational support for , using concatenated nuclear and mitochondrial sequences from 18 mammals to recover with high . Fossil integrations further contextualize this phylogeny, with early represented by purgatoriids and other plesiadapiform-like forms in deposits of and , suggesting an initial diversification in Laurasian landmasses.

Evolutionary History

Origins and Early Forms

The earliest potential records of Simplicidentata precursors appear in the fossil record of , around 75 million years ago (Ma), with taxa such as Zalambdalestes lechei and Barunlestes butleri from the in . These zalambdalestids exhibit dental features, including enlarged lower incisors, that suggest a close relationship to , the broader encompassing Simplicidentata and , though their exact position as direct precursors to Simplicidentata remains debated due to the absence of fully incisors typical of later forms. Phylogenetic analyses place them within a gliriform linking them to early Tertiary , indicating that the lineage leading to Simplicidentata may have diverged by the late . The confirmed origins of Simplicidentata are firmly established in the , approximately 66–60 Ma, primarily in , following the Cretaceous-Paleogene (K-Pg) mass extinction. Fossils such as Sinomylus lii from the early of represent primitive stem simplicidentates, characterized by a dental pattern featuring a single pair of upper incisors and bunodont molars adapted for omnivorous or insectivorous diets. Early members of the Eurymylidae, including genera like Heomys and Tribosphenomys minutus from late deposits in (e.g., Bayan Ulan locality), further illustrate this basal radiation, with their molars showing transverse crests and reduced premolars that bridge primitive eutherian traits and those of crown . These Asian forms dominated the initial post-K-Pg recovery among small mammals, diversifying rapidly in forested environments before spreading to other Laurasian landmasses. A pivotal event in Simplicidentata evolution was the immediately after the K-Pg , which eliminated non-avian dinosaurs and opened ecological niches for small, herbivorous or omnivorous mammals. This radiation favored the development of ever-growing incisors for gnawing, a hallmark of the , allowing early simplicidentates to exploit seeds, nuts, and soft in recovering ecosystems. Molecular and fossil clock estimates place the origin of crown Rodentia around 60 Ma, aligning with the late appearance of early crown taxa such as ischyromyids. Transitional traits in these early forms, such as the retention of a functional second upper (P2), distinguish them from crown , which typically lack this tooth and exhibit more specialized cheek teeth for herbivory. For instance, Sinomylus retains a simple, peg-like P2, reflecting an intermediate stage between generalized mammals and the derived dental reduction seen in later . This morphology underscores the gradual evolution of key simplicidentate adaptations during the , setting the stage for Eocene diversification.

Diversification and Extinctions

The diversification of Simplicidentata accelerated markedly during the Eocene epoch (approximately 56–34 Ma), with an explosive radiation of stem groups such as the Ischyromyidae across and . This period witnessed the emergence of crown-group featuring specialized gnawing adaptations, including sciuromorph and hystricomorph forms, marking a transition from more generalized ancestors. Eocene fossils indicate a period of rapid evolutionary diversification following precursors in . In the and (34–5 Ma), Simplicidentata expanded globally via emerging land bridges and continental configurations, enabling widespread colonization. Numerous stem lineages, including the Eurymylidae, underwent by the late Eocene, while crown persisted and diversified further. A pivotal event was the Miocene of caviomorph in , where isolated evolution fostered ecological specialization amid changing vegetation. Throughout the , climate oscillations, tectonic shifts like , and biotic pressures from competitors drove these patterns of radiation and loss. The Pleistocene (2.6 Ma–11.7 ka) saw megafaunal extinctions that affected certain larger simplicidentate forms, such as the giant beaver , yet the clade as a whole expanded, resulting in approximately 2,700 extant species as of 2023.

Characteristics

Dental and Cranial Features

Simplicidentata is defined by the key synapomorphy of a reduced featuring a single pair of ever-growing upper s, with enamel covering only the anterior (labial) surface, which facilitates self-sharpening through differential against the softer dentine on the posterior side and enables efficient gnawing behaviors essential for processing tough plant material. This configuration contrasts with the ancestral mammalian condition of multiple incisor pairs and contrasts notably with the Duplicidentata (lagomorphs), which retain a peg-like second upper incisor alongside the primary pair. The lower incisors are similarly ever-growing, with enamel covering the labial surface and the lingual surface consisting of dentine, facilitating self-sharpening through differential . Cranially, Simplicidentata exhibit an elongated rostrum that accommodates the hypertrophied s and supports enhanced olfactory capabilities, often with the loss of the second upper and reduction or absence of premolars to streamline the dental arcade for herbivory. In many subgroups, particularly sciuromorph and hystricomorph , the is enlarged to allow passage of the medial onto the rostrum, enhancing jaw adduction force for mastication—a less pronounced in stem lineages. Early mandibular forms often display an inflected angular process, providing expanded attachment for the and digastric muscles to support powerful biting. Dental variation within Simplicidentata reflects evolutionary diversification, with stem groups like Eurymylidae possessing low-crowned, lophate molars characterized by transverse crests and weak conules for grinding softer vegetation. In contrast, crown-group Rodentia have evolved highly diverse occlusal patterns, including the prismatic enamel structure in arvicoline molars, where alternating triangular prisms form sharp cutting edges adapted to abrasive grasses and forbs, often with hypsodont or ever-growing cheek teeth. These molar adaptations underscore the clade's radiation into varied ecological niches, from arboreal to fossorial habitats.

Postcranial Anatomy

The postcranial skeleton of Simplicidentata exhibits a range of adaptations reflecting their diverse locomotor habits, from to and arboreal lifestyles. A key shared feature with other is the hindlimb morphology, characterized by elongated tarsals that enhance stride length and stability during rapid terrestrial locomotion. For instance, in early Eocene ctenodactyloid , the astragalus and show pronounced elongation and a calcaneal canal, stabilizing the ankle joint for efficient cursoriality while allowing some capability in smaller forms. Many burrowing simplicidentates, such as mole-rats, display reduced or vestigial clavicles, which permit greater mobility and facilitate powerful strokes during excavation without interference from a rigid pectoral attachment. The in Simplicidentata is notably flexible, with a compliant vertebral column that supports agility in navigating complex environments. This spinal flexibility, observed in modern like mice, enables lateral bending to improve turning radius and balance during high-speed maneuvers, contributing to evasion from predators. Appendicular adaptations build on the pentadactyl limb plan, with claws varying by : curved and sharp in arboreal species for gripping branches, robust and flattened in forms for digging, and partially webbed in semi-aquatic taxa for . These limb modifications allow simplicidentates to exploit niches from canopies to underground burrows, underscoring the clade's ecological versatility. Stem simplicidentates, such as Matutinia from the early Eocene, retain more primitive postcranial features akin to basal , including generalized tarsals indicative of ambulatory "slow " locomotion without extreme elongation. In contrast, crown-group show further specialization, with enhanced robustness in some lineages to support weight-bearing postures. Tail morphology varies widely, serving functions from balance to prehensility; for example, in hystricognath like , the tail acts as a fifth limb for stability during arboreal movement. These postcranial traits collectively provide biomechanical support for activities like gnawing, where hindlimbs anchor the body and the flexible spine maintains posture under sustained force.

Classification

Crown Group Rodentia

The Rodentia comprises all extant species and their descendants from the last common , which molecular and paleontological dates to approximately 60 million years ago in the early , shortly after the Cretaceous-Paleogene boundary. This represents the living core of the order, distinct from earlier diverging stem lineages, and encompasses the vast majority of diversity observed today. Modern classifications divide crown Rodentia into five major suborders based on phylogenetic analyses of morphological and molecular data: Sciuromorpha, which includes squirrels (Sciuridae) and mountain beavers (Aplodontiidae) characterized by sciuromorphous jaw musculature; , encompassing beavers (), pocket gophers (Geomyidae), and kangaroo rats () with specialized burrowing adaptations; , featuring diverse forms like mice, rats, voles, and hamsters with myomorphic masseter muscles; , including porcupines, guinea pigs, chinchillas, and other hystricognaths (such as caviomorphs in and the family Ctenodactylidae of gundis in North African and Asian arid regions) with enlarged infraorbital foramina; and , restricted to African springhares () and scaly-tailed squirrels () with specialized gliding adaptations. Crown Rodentia exhibits remarkable diversity, with approximately 2,500 extant species organized into 34 families, for about 40% of all mammalian (as of 2024). Key families include (muroids such as mice and rats), with around 800 representing the largest family; ( rats, voles, hamsters, and lemmings), with over 600 ; and Sciuridae (squirrels and relatives), comprising about 285 adapted to arboreal and terrestrial lifestyles. Rodents in the crown group are distributed worldwide across all continents except , inhabiting diverse ecosystems from forests and grasslands to deserts and urban areas. A notable biogeographic pattern is the radiation of caviomorphs (a within ) in , where they diversified extensively following the continent's isolation from around 100 million years ago, with ancestral hystricognaths likely arriving via transatlantic dispersal in the late Eocene or . This isolation facilitated their adaptive success in filling ecological niches, including large herbivores like the .

Stem and Extinct Groups

The stem groups of Simplicidentata consist of extinct mammals that form the total group beyond the crown Rodentia, sharing key synapomorphies such as a single pair of enlarged upper incisors but exhibiting more primitive dental and cranial features. These taxa, primarily known from and , document the early radiation of simplicidentates following their divergence from duplicidentates in the late . Among the key stem families, Eurymylidae represents one of the earliest lineages, known from the to Eocene of , with primitive molars featuring low crowns and bunodont cusps that prefigure rodent cheek teeth. Specimens from , such as those in the genus Eurymylus, exhibit a mix of gliriform and -like traits, positioning the family as basal to all modern within Simplicidentata. Remyidae, from the Eocene of and , includes early myomorph-like forms with elongated snouts and specialized zygomasseteric structures adapted for gnawing, bridging primitive ischyromyids to more derived rodent clades. Ischyromyidae, spanning the Eocene to across and , comprises squirrel-like with robust postcrania suited for arboreal or scansorial habits, often regarded as stem taxa due to their protrogomorphous musculature. Other notable early Glires taxa include Anagale from the Eocene of , which exhibits anagalid affinities with simplified and mandibular features showing some convergences with early simplicidentates, though its exact placement within remains debated. Mixodontia, recorded from the Eocene of , features taxa with multi-cusped premolars and enamel patterns that have sparked debate over their status as true or a separate simplicidentate offshoot. Sinomylus, a basal form from the of , retains multiple premolars and a second upper in some specimens, highlighting transitional morphology near the root of Simplicidentata. The extinct diversity of Simplicidentata stems encompasses approximately 50 genera across the , distributed among families like Eurymylidae, Ischyromyidae, and others, reflecting a peak in the Eocene before most lineages vanished by the early . No stem taxa survive beyond the , with post- fossils largely confined to crown rodent transitions. Placement debates persist for certain genera; for instance, Rhombomylus, once considered a simplicidentate stem, has been reassigned as a sister to Duplicidentata based on dental and mandibular evidence.

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