New remarkably complete skeleton of 
Mixodectids are a poorly defined group of enigmatic placental mammals from the Paleocene of western North America whose evolutionary relationships have long been hard to resolve due to their sparse fossil record that consists mostly of dental and gnathic remains , . Taxa classified as mixodectids have been linked to extant mammalian groups including Primates , Rodentia , “Insectivora” , , and Dermoptera . The family Mixodectidae includes two species of Mixodectes from the Torrejonian North American Land Mammal Age (NALMA) of New Mexico (and possibly Wyoming); the monotypic Dracontolestes from the Torrejonian NALMA of Utah; and three species of Eudaemonema from the Torrejonian and Tiffanian NALMAs of Wyoming, Montana, and Alberta , . Similarities between the dentition of mixodectids and those of other extinct Paleogene families such as the Plagiomenidae and Microsyopidae have been recognized and debated for over a century – , , . These extinct families have also often been hypothesized to have affinities with one or more clades of euarchontan mammals (primates, colugos, treeshrews), but fossil evidence to test these hypotheses has been mostly limited to craniodental characters [e.g., ]. Although fragmentary postcrania previously attributed to early Paleocene Mixodectes show similarities to those of other arboreal euarchontans , , , more complete mixodectid specimens are needed to assess their significance regarding the early evolution of this group. Here we describe a new partial skeleton of Mixodectes pungens (NMMNH P-54501; Fig. ) recovered from the West Flank of Torreon Wash (locality NMMNH L-6898) within the Ojo Encino Member [ sensu ], Nacimiento Formation, San Juan Basin, New Mexico (see Supplementary Fig. online). NMMNH L-6898 is stratigraphically located near the middle of the Tj6 fossil horizon [ sensu ]. The fauna from the Tj6 fossil zone is the primary source among faunas that define the early Paleocene late Torrejonian (To3) NALMA Mixodectes pungens interval zone . The age of NMMNH L-6898 is estimated to be 62.4 ± 0.03 Ma based on average sediment accumulation rates between the upper and lower reversal boundaries of a normal polarity zone correlated with Chron C27n . A detrital sanidine age of 62.48 ± 0.02 from within this normal polarity zone from the nearby Escavada Wash provides an additional age constraint for this chron . See – for additional information on this locality and other fossil vertebrates collected at this site. The new dentally associated adult skeleton of M. pungens (NMMNH P-54501) is the most complete specimen of a mixodectid yet recovered, preserving a partial skull and much of the axial skeleton, forelimbs, and hind limbs (Fig. ). Partial disarticulated skeletons of the palaechthonid plesiadapiform Torrejonia , and the cimolestid Acmeodon were recovered from the same horizon, but they are easily distinguished based on their smaller size alone. No repeated tooth loci or skeletal elements have been recognized, which suggests that only one individual of M. pungens was present. Previously described fragmentary postcrania attributed to a smaller species, M. malaris , were mixed with similar-sized postcrania of the arctocyonid archaic ungulate Chriacus orthogonius from the Torrejonian “ Pantolambda zone” (Tj6 fossil horizon sensu ) in the San Juan Basin . The new partial skeleton of M. pungens analysed here could be distinguished from elements attributed to the smaller M. malaris , by its larger size. Here, we describe the best-preserved skeletal elements of M. pungens and discuss the functional implications of this newly documented morphology. We also compared the new M. pungens skeleton to those of other placental mammals including plesiadapiforms, euprimates, treeshrews, and colugos, and we coded it into two morphological data matrices to assess its phylogenetic relationships among eutherian mammals (Crowell et al. modified from Wible et al. , and Chester et al. modified from Silcox et al. ). 

Description and functional assessment of NMMNH P-54501 NMMNH P-54501 preserves a dorsoventrally compressed partial skull (Fig. A). The rostrum includes a right premaxilla with I1 and a partial crown of I2, a right maxilla with P2-4, M1-2, and roots of M3, a left premaxilla with the root of I2, and a left maxilla with P3-4, M1-2, and roots of M3. A left fragmentary dentary is appressed to the ventral surface of the palate and preserves i1-2, p3-4, and m1-2. The premolar and molar morphology of Mixodectes pungens has previously been described – , , but NMMNH P-54501 adds previously unknown tooth crowns for this species (I1-2, i2, and P2-3) that confirm M . pungens has anterior teeth and a dental formula of 2.0.3.3/?2.0.3.3 like its smaller congener, M . malaris . The premaxilla is a dorsoventrally tall and robust bone housing a large I1 and a slightly smaller I2 (Fig. A). The premaxillary-maxillary suture is caudal to the I2 and thus NMMNH P-54501 confirms the presence of two upper incisors as previously inferred . Both maxillae preserve an infraorbital foramen that is dorsal to the P3 like that of plesiadapiforms, the extant treeshrew Tupaia , and the extant colugo Cynocephalus , whereas this foramen is often dorsal to a more caudal tooth position in the extant treeshrew Ptilocercus and adapid primates, and dorsal to a more rostral tooth position in omomyid primates . The rostrum also preserves both nasals, which are displaced ventrally and covered by the maxillae and the paired frontal bones. Due to preservation, it is unclear whether the lacrimal or palatine bones are present in this fragment. Both frontal bones display a caudolaterally projecting triangular postorbital process that arises from prominent anterior temporal lines. In this way, Mixodectes differs from non-microsyopid plesiadapiforms, which lack this process, and is more like that of microsyopid plesiadapiforms and extant euarchontans, although treeshrews and primates have much more developed processes that contact the jugal to form a complete postorbital bar , . The postorbital processes of colugos and microsyopids are triangular but extend far more laterally and are rostrocaudally longer than those of M. pungens . The postorbital process of colugos further differs from that of M. pungens in that the ventral surface is curved and contours to the shape of the eye . Other isolated bones that are associated with this partial cranium and that we tentatively attribute to Mixodectes include a partial left squamosal preserving the glenoid fossa, a partial occipital preserving parts of the nuchal crest and right occipital condyle, and a right petrosal that is too fragmentary to assess beyond the identification of a partial promontorium (Fig. ). The postcranial skeleton of M. pungens consists of many fragmentary cervical, thoracic, lumbar, sacral, and caudal vertebrae, sternebrae, ribs, and many elements of the forelimbs and hind limbs (Fig. ). The vertebral column is represented by over forty vertebrae within the expected size range for M. pungens . These partial vertebrae and/or vertebral bodies are preserved well enough to attribute to different regions of the spine but only a few can be identified to a specific position (e.g., the axis). The sacrum is crushed but appears to include three vertebrae. The number of caudal vertebrae preserved (approximately 20; Fig. ) suggests that M. pungens had a long tail like that of many arboreal mammals. The ribs of M. pungens are narrow (Fig. ) like those of plesiadapiforms and tupaiid treeshrews, whereas the ribs of Ptilocercus and colugos are more craniocaudally expanded, which relates to axial stability , . The shoulder girdle is represented by a complete left and a fragmentary right clavicle, the lateral portions of both scapulae, and nearly complete humeri (Fig. ), which indicate a mobile shoulder like that of arboreal mammals. Both partial scapulae preserve the glenoid fossa, base of the coracoid, and most lateral portion of the scapular spine. The glenoid fossa of Mixodectes (Fig. C) is concave, almost twice as wide inferiorly as superiorly, and pear-shaped in outline like that of extant euarchontans [e.g., ] and known plesiadapiforms [e.g., , ]. The spherical humeral head of M. pungens is greater in diameter than the glenoid fossa and clearly extends superiorly beyond the greater and lesser tuberosities (Fig. C), which indicates a mobile glenohumeral joint . The greater and lesser tuberosities are well developed and provide a large area of insertion for the rotator cuff muscles, which, like that of other arboreal mammals, would provide stability to the shoulder joint of M. pungens during forelimb abduction . The lesser tuberosity is large and projects medially as in other arboreal euarchontans and would have provided a long lever arm for M. subscapularis, which medially rotates the humerus during vertical climbing , . The elbow joint is represented by both humeri, a left complete and right proximal radius, and both proximal ulnae (Fig. ), which indicate a mobile elbow, habitual forearm flexion, and capability for manual grasping like that of arboreal mammals that exhibit vertical positional behaviors. M. pungens has a large, laterally projecting brachioradialis flange (Figs. C and A) for the origin of M. brachioradialis and possibly M. brachialis, which contribute to forearm flexion . The humeral radial fossa is excavated for considerable forearm flexion (Fig. A), whereas the olecranon fossa is defined but shallow, which would have limited full forearm extension . The radii each have a large bicipital tuberosity near the proximal end (Fig. D), which suggests M. biceps brachii was large for forearm flexion and supination that would have assisted in climbing . The humeral capitulum is nearly spherical (Figs. C and A), and the radial central fossa is circular in outline and excavated (Fig. D), both indicating a great ability for forearm supination and pronation ; the zona conoidea clearly separates the capitulum and trochlea (Fig. A) and would have allowed the radius to rotate more freely in relation to the ulna. The medial epicondyle represents over one third of the distal humeral width (Fig. A) and would have provided a large area of origin for the wrist and digital flexors for manual grasping like that of other arboreal euarchontans . As previously reported based on fragmentary postcrania attributed to M. malaris , Mixodectes distal humeri are like those of plesiadapiforms, Ptilocercus , and euprimates, but differ from the derived humeri of colugos, which are much more elongate relative to distal width, have a smaller deltopectoral crest, larger capitulum, narrower medial epicondyle (Fig. ), and a deeply excavated olecranon fossa for full forearm extension . Mixodectes further differs from colugos in having a prominent radial bicipital tuberosity, no evidence of distal fusion between the radius and ulna, a longer olecranon process, a shallower trochlear notch, and no evidence of distal ulnar reduction (Fig. D). The hip joint is represented by both partial innominates and virtually complete femora (Fig. ), which indicate considerable mobility in the hip and a habitually flexed thigh. The innominate has a craniocaudally elliptical acetabulum and a cranially expanded articular surface (Fig. A) as in arboreal euarchontans , . The elliptical acetabulum allows mobility at the hip joint for wide ranges of abduction and lateral rotation, both important for arboreal climbers, and the cranial buttressing likely reflects loads that were incurred during orthograde positional behaviors on vertical supports . The large and nearly spherical femoral head is slightly taller than the greater trochanter (Figs. E and A), which would have also contributed to hip mobility. The long, deep trochanteric fossa (Fig. E) for the insertion of two obturator and two gemelli muscles would have allowed lateral rotation of the thigh when using vertical supports . The femur has a large, dorsomedially projecting lesser trochanter (Figs. E and A) providing a large area of insertion for the hip flexor M. iliopsoas . The third trochanter is small (Figs. E and A) suggesting a less powerful thigh extensor M. gluteus superficialis and a habitually flexed hind limb . The knee joint was evaluated from the two femora and two broken tibiae, as well as muscle origins from the two partial innominates (Fig. ), and suggests Mixodectes had a habitually flexed knee and was not a specialized leaper or a terrestrial runner. The anterior inferior iliac spine of the innominate is small like that of other arboreal euarchontans (Fig. ) and would not have provided a large origin for M. rectus femoris for powerful extension of the knee as in specialized terrestrial runners such as tupaiid treeshrews . The femoral condyles are dorsoventrally shallow, and the patellar groove is proximally restricted and shallow (Fig. A), suggesting a habitually flexed knee unlike the deeper knees of specialized leapers or terrestrial runners in which M. quadriceps femoris powerfully extends their lower legs , . Although the anterior inferior iliac spine, femoral condyles, and patellar groove all suggest lack of powerful extension of the knee, the tibial tuberosity is large (Fig. F), but this appears to be related to the origin of tibialis anterior; the tibial tuberosity has a prominent lateral protuberance for the origin of M. tibialis anterior, which would have contributed to dorsiflexion and inversion of the foot (see below). The upper ankle joint of Mixodectes is represented by both partial tibiae and fibulae and a left astragalus (Fig. ). The distal tibia has a short medial malleolus (Fig. F) and an ungrooved articular facet for the lateral tibial facet of the astragalus. The astragalar lateral tibial facet extends far distally onto the neck (Figs. F and A), which indicates that the foot was habitually dorsiflexed like mammals that cling to vertical supports . The lower ankle joint is represented by the left astragalus and both calcanei (Fig. ), which possess features such as a confluent sustentacular-navicular facet on the astragalus and a corresponding distal continuation of the sustentacular facet on the calcaneal body like that of extant arboreal euarchontans and plesiadapiforms (Fig. ), indicating significant mobility for pedal inversion and eversion , . The calcaneus has a long tuber and a very large, proximodistally long peroneal tubercle like that of plesiadapiforms (Fig. ), the latter of which would have provided leverage for tendons of the peroneal muscles that contribute to pedal eversion and abduction. The calcaneus is not distally elongated as in leaping euprimates (Fig. ), a secondary articulation between the posterior side of the sustentaculum tali and the astragalus is present as in some plesiadapiforms and colugos [see and references therein], and it appears to have a calcaneal-navicular facet like that of colugos , . The transverse tarsal joint is represented by the proximal tarsals described above, both naviculars, and a right cuboid (Fig. ). The calcaneal cuboid facet is subcircular in outline and concave with a plantar pit (Figs. F and A) that accommodates the convex proximal articular surface of the cuboid, which would have contributed to considerable inversion and eversion of the foot. All these features would have enabled an arboreal animal like Mixodectes to navigate uneven and variable branches. The entocuneiform of Mixodectes (Fig. F) resembles those reported for non-carpolestid plesiadapiforms in having a mediolaterally wide dorsal side of the distal facet, which suggests Ptilocercus -like pedal grasping with a divergent but non-opposable hallux , , . The proximal phalanges have pronounced flexor sheath ridges (Fig. B), indicating powerful flexion of the digits . Distal phalanges of M. pungens (Fig. B) are like the previously reported deep distal phalanx that likely belongs to Mixodectes . This deep and mediolaterally narrow ungual morphology is like that of plesiadapiforms and non-euprimate euarchontans [e.g., , , ], but the distal ends are not as deep as those of colugos . This combination of features would have allowed Mixodectes to vertically cling and climb through trees with its claws. To estimate body mass of Mixodectes pungens (NMMNH P-54501), we measured the maximum femoral length (69.4 mm) and anteroposterior midshaft diameter (5.55 mm) and inputted these values in RStudio using a body weight estimation algorithm (appendix III of Boyer and Gingerich ). Body mass was estimated to be 1,370 g (95% CI [375, 4,998]) and 1,270 g (95% CI [668, 2,416]) based on femoral length and midshaft diameter, respectively. Phylogenetic analyses Unconstrained analysis of the modified Crowell et al. matrix recovered 200 most parsimonious trees (MPTs) with lengths of 2,925. Strict consensus of these results supports M. pungens as the sister taxon to the microsyopid Microsyops annectens , and both taxa are supported as the sisters to Cynocephalus (see Supplementary Fig. S2 online). Within Euarchonta, Primatomorpha (Primates + Dermoptera) is recovered with the treeshrew Ptilocercus as the sister to this grouping. Plesiadapiforms are recovered as either stem primatomorphans ( Purgatorius , Foxomomys fremdi , Tinimomys graybulliensis , Dryomomys szalayi , Plesiolestes nacimienti ), stem dermopterans ( Microsyops annectens ), or as stem primates ( Ignacius , Carpolestes simpsoni , Plesiadapis ). The constrained analysis enforced the monophyly of Afrotheria, Xenarthra, Boreoeutheria, Laurasiatheria, and Euarchontoglires, which are generally well-supported in molecular phylogenetic analyses and recovered 2,976 MPTs with lengths of 2,914. Like the unconstrained strict consensus results, Euarchonta is monophyletic and Primatomorpha is supported. However, in the constrained strict consensus tree, Microsyops annectens is recovered as the sister taxon of the extant colugo Cynocephalus , Mixodectes pungens is supported as a stem primatomorphan, and all non-microsyopid plesiadapiforms are recovered as stem primates (Fig. A, Supplementary Fig. S3 online). Unconstrained analysis of the modified Chester et al. matrix recovered two MPTs with lengths of 1,021. The strict consensus of these results supports a monophyletic Euarchonta and Primatomorpha. Within Primatomorpha, M. pungens is recovered as a stem primate sister to a clade consisting of microsyopids, paromomyoids (Paromomyidae + Palaechthonidae), plesiadapoids (Plesiadapidae + Carpolestidae), and crown primates, with a Purgatorius + Micromomyidae clade supported as the most basal primates (Fig. B, Supplementary Fig. S4 online). A constrained analysis was not conducted because this morphological matrix recovered clades that are well-supported in molecular phylogenetic analyses. 

NMMNH P-54501 preserves a dorsoventrally compressed partial skull (Fig. A). The rostrum includes a right premaxilla with I1 and a partial crown of I2, a right maxilla with P2-4, M1-2, and roots of M3, a left premaxilla with the root of I2, and a left maxilla with P3-4, M1-2, and roots of M3. A left fragmentary dentary is appressed to the ventral surface of the palate and preserves i1-2, p3-4, and m1-2. The premolar and molar morphology of Mixodectes pungens has previously been described – , , but NMMNH P-54501 adds previously unknown tooth crowns for this species (I1-2, i2, and P2-3) that confirm M . pungens has anterior teeth and a dental formula of 2.0.3.3/?2.0.3.3 like its smaller congener, M . malaris . The premaxilla is a dorsoventrally tall and robust bone housing a large I1 and a slightly smaller I2 (Fig. A). The premaxillary-maxillary suture is caudal to the I2 and thus NMMNH P-54501 confirms the presence of two upper incisors as previously inferred . Both maxillae preserve an infraorbital foramen that is dorsal to the P3 like that of plesiadapiforms, the extant treeshrew Tupaia , and the extant colugo Cynocephalus , whereas this foramen is often dorsal to a more caudal tooth position in the extant treeshrew Ptilocercus and adapid primates, and dorsal to a more rostral tooth position in omomyid primates . The rostrum also preserves both nasals, which are displaced ventrally and covered by the maxillae and the paired frontal bones. Due to preservation, it is unclear whether the lacrimal or palatine bones are present in this fragment. Both frontal bones display a caudolaterally projecting triangular postorbital process that arises from prominent anterior temporal lines. In this way, Mixodectes differs from non-microsyopid plesiadapiforms, which lack this process, and is more like that of microsyopid plesiadapiforms and extant euarchontans, although treeshrews and primates have much more developed processes that contact the jugal to form a complete postorbital bar , . The postorbital processes of colugos and microsyopids are triangular but extend far more laterally and are rostrocaudally longer than those of M. pungens . The postorbital process of colugos further differs from that of M. pungens in that the ventral surface is curved and contours to the shape of the eye . Other isolated bones that are associated with this partial cranium and that we tentatively attribute to Mixodectes include a partial left squamosal preserving the glenoid fossa, a partial occipital preserving parts of the nuchal crest and right occipital condyle, and a right petrosal that is too fragmentary to assess beyond the identification of a partial promontorium (Fig. ). The postcranial skeleton of M. pungens consists of many fragmentary cervical, thoracic, lumbar, sacral, and caudal vertebrae, sternebrae, ribs, and many elements of the forelimbs and hind limbs (Fig. ). The vertebral column is represented by over forty vertebrae within the expected size range for M. pungens . These partial vertebrae and/or vertebral bodies are preserved well enough to attribute to different regions of the spine but only a few can be identified to a specific position (e.g., the axis). The sacrum is crushed but appears to include three vertebrae. The number of caudal vertebrae preserved (approximately 20; Fig. ) suggests that M. pungens had a long tail like that of many arboreal mammals. The ribs of M. pungens are narrow (Fig. ) like those of plesiadapiforms and tupaiid treeshrews, whereas the ribs of Ptilocercus and colugos are more craniocaudally expanded, which relates to axial stability , . The shoulder girdle is represented by a complete left and a fragmentary right clavicle, the lateral portions of both scapulae, and nearly complete humeri (Fig. ), which indicate a mobile shoulder like that of arboreal mammals. Both partial scapulae preserve the glenoid fossa, base of the coracoid, and most lateral portion of the scapular spine. The glenoid fossa of Mixodectes (Fig. C) is concave, almost twice as wide inferiorly as superiorly, and pear-shaped in outline like that of extant euarchontans [e.g., ] and known plesiadapiforms [e.g., , ]. The spherical humeral head of M. pungens is greater in diameter than the glenoid fossa and clearly extends superiorly beyond the greater and lesser tuberosities (Fig. C), which indicates a mobile glenohumeral joint . The greater and lesser tuberosities are well developed and provide a large area of insertion for the rotator cuff muscles, which, like that of other arboreal mammals, would provide stability to the shoulder joint of M. pungens during forelimb abduction . The lesser tuberosity is large and projects medially as in other arboreal euarchontans and would have provided a long lever arm for M. subscapularis, which medially rotates the humerus during vertical climbing , . The elbow joint is represented by both humeri, a left complete and right proximal radius, and both proximal ulnae (Fig. ), which indicate a mobile elbow, habitual forearm flexion, and capability for manual grasping like that of arboreal mammals that exhibit vertical positional behaviors. M. pungens has a large, laterally projecting brachioradialis flange (Figs. C and A) for the origin of M. brachioradialis and possibly M. brachialis, which contribute to forearm flexion . The humeral radial fossa is excavated for considerable forearm flexion (Fig. A), whereas the olecranon fossa is defined but shallow, which would have limited full forearm extension . The radii each have a large bicipital tuberosity near the proximal end (Fig. D), which suggests M. biceps brachii was large for forearm flexion and supination that would have assisted in climbing . The humeral capitulum is nearly spherical (Figs. C and A), and the radial central fossa is circular in outline and excavated (Fig. D), both indicating a great ability for forearm supination and pronation ; the zona conoidea clearly separates the capitulum and trochlea (Fig. A) and would have allowed the radius to rotate more freely in relation to the ulna. The medial epicondyle represents over one third of the distal humeral width (Fig. A) and would have provided a large area of origin for the wrist and digital flexors for manual grasping like that of other arboreal euarchontans . As previously reported based on fragmentary postcrania attributed to M. malaris , Mixodectes distal humeri are like those of plesiadapiforms, Ptilocercus , and euprimates, but differ from the derived humeri of colugos, which are much more elongate relative to distal width, have a smaller deltopectoral crest, larger capitulum, narrower medial epicondyle (Fig. ), and a deeply excavated olecranon fossa for full forearm extension . Mixodectes further differs from colugos in having a prominent radial bicipital tuberosity, no evidence of distal fusion between the radius and ulna, a longer olecranon process, a shallower trochlear notch, and no evidence of distal ulnar reduction (Fig. D). The hip joint is represented by both partial innominates and virtually complete femora (Fig. ), which indicate considerable mobility in the hip and a habitually flexed thigh. The innominate has a craniocaudally elliptical acetabulum and a cranially expanded articular surface (Fig. A) as in arboreal euarchontans , . The elliptical acetabulum allows mobility at the hip joint for wide ranges of abduction and lateral rotation, both important for arboreal climbers, and the cranial buttressing likely reflects loads that were incurred during orthograde positional behaviors on vertical supports . The large and nearly spherical femoral head is slightly taller than the greater trochanter (Figs. E and A), which would have also contributed to hip mobility. The long, deep trochanteric fossa (Fig. E) for the insertion of two obturator and two gemelli muscles would have allowed lateral rotation of the thigh when using vertical supports . The femur has a large, dorsomedially projecting lesser trochanter (Figs. E and A) providing a large area of insertion for the hip flexor M. iliopsoas . The third trochanter is small (Figs. E and A) suggesting a less powerful thigh extensor M. gluteus superficialis and a habitually flexed hind limb . The knee joint was evaluated from the two femora and two broken tibiae, as well as muscle origins from the two partial innominates (Fig. ), and suggests Mixodectes had a habitually flexed knee and was not a specialized leaper or a terrestrial runner. The anterior inferior iliac spine of the innominate is small like that of other arboreal euarchontans (Fig. ) and would not have provided a large origin for M. rectus femoris for powerful extension of the knee as in specialized terrestrial runners such as tupaiid treeshrews . The femoral condyles are dorsoventrally shallow, and the patellar groove is proximally restricted and shallow (Fig. A), suggesting a habitually flexed knee unlike the deeper knees of specialized leapers or terrestrial runners in which M. quadriceps femoris powerfully extends their lower legs , . Although the anterior inferior iliac spine, femoral condyles, and patellar groove all suggest lack of powerful extension of the knee, the tibial tuberosity is large (Fig. F), but this appears to be related to the origin of tibialis anterior; the tibial tuberosity has a prominent lateral protuberance for the origin of M. tibialis anterior, which would have contributed to dorsiflexion and inversion of the foot (see below). The upper ankle joint of Mixodectes is represented by both partial tibiae and fibulae and a left astragalus (Fig. ). The distal tibia has a short medial malleolus (Fig. F) and an ungrooved articular facet for the lateral tibial facet of the astragalus. The astragalar lateral tibial facet extends far distally onto the neck (Figs. F and A), which indicates that the foot was habitually dorsiflexed like mammals that cling to vertical supports . The lower ankle joint is represented by the left astragalus and both calcanei (Fig. ), which possess features such as a confluent sustentacular-navicular facet on the astragalus and a corresponding distal continuation of the sustentacular facet on the calcaneal body like that of extant arboreal euarchontans and plesiadapiforms (Fig. ), indicating significant mobility for pedal inversion and eversion , . The calcaneus has a long tuber and a very large, proximodistally long peroneal tubercle like that of plesiadapiforms (Fig. ), the latter of which would have provided leverage for tendons of the peroneal muscles that contribute to pedal eversion and abduction. The calcaneus is not distally elongated as in leaping euprimates (Fig. ), a secondary articulation between the posterior side of the sustentaculum tali and the astragalus is present as in some plesiadapiforms and colugos [see and references therein], and it appears to have a calcaneal-navicular facet like that of colugos , . The transverse tarsal joint is represented by the proximal tarsals described above, both naviculars, and a right cuboid (Fig. ). The calcaneal cuboid facet is subcircular in outline and concave with a plantar pit (Figs. F and A) that accommodates the convex proximal articular surface of the cuboid, which would have contributed to considerable inversion and eversion of the foot. All these features would have enabled an arboreal animal like Mixodectes to navigate uneven and variable branches. The entocuneiform of Mixodectes (Fig. F) resembles those reported for non-carpolestid plesiadapiforms in having a mediolaterally wide dorsal side of the distal facet, which suggests Ptilocercus -like pedal grasping with a divergent but non-opposable hallux , , . The proximal phalanges have pronounced flexor sheath ridges (Fig. B), indicating powerful flexion of the digits . Distal phalanges of M. pungens (Fig. B) are like the previously reported deep distal phalanx that likely belongs to Mixodectes . This deep and mediolaterally narrow ungual morphology is like that of plesiadapiforms and non-euprimate euarchontans [e.g., , , ], but the distal ends are not as deep as those of colugos . This combination of features would have allowed Mixodectes to vertically cling and climb through trees with its claws. To estimate body mass of Mixodectes pungens (NMMNH P-54501), we measured the maximum femoral length (69.4 mm) and anteroposterior midshaft diameter (5.55 mm) and inputted these values in RStudio using a body weight estimation algorithm (appendix III of Boyer and Gingerich ). Body mass was estimated to be 1,370 g (95% CI [375, 4,998]) and 1,270 g (95% CI [668, 2,416]) based on femoral length and midshaft diameter, respectively. 

Unconstrained analysis of the modified Crowell et al. matrix recovered 200 most parsimonious trees (MPTs) with lengths of 2,925. Strict consensus of these results supports M. pungens as the sister taxon to the microsyopid Microsyops annectens , and both taxa are supported as the sisters to Cynocephalus (see Supplementary Fig. S2 online). Within Euarchonta, Primatomorpha (Primates + Dermoptera) is recovered with the treeshrew Ptilocercus as the sister to this grouping. Plesiadapiforms are recovered as either stem primatomorphans ( Purgatorius , Foxomomys fremdi , Tinimomys graybulliensis , Dryomomys szalayi , Plesiolestes nacimienti ), stem dermopterans ( Microsyops annectens ), or as stem primates ( Ignacius , Carpolestes simpsoni , Plesiadapis ). The constrained analysis enforced the monophyly of Afrotheria, Xenarthra, Boreoeutheria, Laurasiatheria, and Euarchontoglires, which are generally well-supported in molecular phylogenetic analyses and recovered 2,976 MPTs with lengths of 2,914. Like the unconstrained strict consensus results, Euarchonta is monophyletic and Primatomorpha is supported. However, in the constrained strict consensus tree, Microsyops annectens is recovered as the sister taxon of the extant colugo Cynocephalus , Mixodectes pungens is supported as a stem primatomorphan, and all non-microsyopid plesiadapiforms are recovered as stem primates (Fig. A, Supplementary Fig. S3 online). Unconstrained analysis of the modified Chester et al. matrix recovered two MPTs with lengths of 1,021. The strict consensus of these results supports a monophyletic Euarchonta and Primatomorpha. Within Primatomorpha, M. pungens is recovered as a stem primate sister to a clade consisting of microsyopids, paromomyoids (Paromomyidae + Palaechthonidae), plesiadapoids (Plesiadapidae + Carpolestidae), and crown primates, with a Purgatorius + Micromomyidae clade supported as the most basal primates (Fig. B, Supplementary Fig. S4 online). A constrained analysis was not conducted because this morphological matrix recovered clades that are well-supported in molecular phylogenetic analyses. 

For over a century, the enigmatic Mixodectidae has been hypothesized to be closely related to members of Euarchonta , , , , but the skeleton of Mixodectes pungens described here provides the most compelling evidence for euarchontan affinities to date. This craniodentally associated postcranial skeleton is very similar to that of arboreal euarchontans, including plesiadapiforms, which supports inferences based on less complete material attributed to M. malaris that mixodectids are euarchontan mammals . Mixodectids have long been considered a possible ancestral group to extant colugos based on several features of their dilambdodont dentition (e.g., large upper molar conules, crest-like lower molar paracristid), but this hypothesis has been challenged more recently given that some of these similarities appear to evolve within the genus Eudaemonema , suggesting they evolved independently from colugos (e.g., ). Similarly, dental features have long linked Mixodectidae to the contemporary plagiomenids, which in turn were considered extinct dermopterans, but more recent non-dental discoveries of plagiomenids including a partial skull and postcrania suggest they are neither dermopterans nor euarchontan mammals. Another extinct family that may represent the closest known relatives of mixodectids based on dental similarities is the Microsyopidae, but comparisons are limited because dentally associated postcrania of microsyopids have not yet been reported. All results from phylogenetic analyses incorporating novel data from NMMNH P-54501 support Mixodectes as a primatomorphan mammal (Fig. ). Although differences among resulting topologies presented here are clearly due to character and taxon sampling, and because one matrix was not designed (nor were characters modified) to test interrelationships among plesiadapiforms , all three hypotheses of mixodectid relationships are worth considering. The unconstrained results of the modified Crowell et al. matrix support M. pungens and the microsyopid M. annectens as sister taxa and as stem dermopterans, whereas the constrained results support M. annectens as the sister taxon to the extant colugo Cynocephalus, and Mixodectes as a stem primatomorphan (Fig. A, Supplementary Figs. S2-3 online). The unconstrained analysis of the modified Chester et al. matrix supports Mixodectes as a stem primate sister to a clade of microsyopids, paromomyoids, plesiadapoids, and euprimates (Fig. B, Supplementary Fig. S4 online). Mixodectes postcrania are very similar to those of plesiadapiforms, but many of these similarities are likely retained euarchontan synapomorphies and therefore not informative for evaluating the alternative tree topologies presented here. If mixodectids are stem dermopterans, some of the proposed dental (e.g., dilambdodonty) and postcranial (e.g., calcaneonavicular facet) similarities between Mixodectes and colugos might be synapomorphies or parallelisms even though colugo-like specializations for folivory, suspension, and gliding are not present in Mixodectes . If mixodectids and plesiadapiforms are stem primatomorphans or stem primates, the dental specializations of mixodectids are likely autapomorphic. Also, mixodectids and microsyopids may be more closely related to each other than to non-microsyopid plesiadapiforms (as supported by the unconstrained results of the modified Crowell et al. matrix; Supplementary Fig. S2 online), but we question phylogenetic results that support microsyopids nested among non-microsyopid plesiadapiforms (e.g., Fig. B) given that they are distinct from other plesiadapiforms in their dental and inferred bullar morphology , , , . Again, regardless of which phylogenetic hypothesis is correct, all analyses conducted here support Mixodectes as a primatomorphan. Postcranial evidence indicates that Mixodectes pungens was a claw climbing arborealist that often clinged to vertical supports like tree trunks. Body mass of NMMNH P-54501 based on femoral length and midshaft diameter is estimated to be 1,370 g and 1,270 g, respectively, which confirms that M. pungens was a very large primatomorphan mammal for the early Paleocene of North America . Based on estimated body mass and molar features, such as the presence of a hypocone and strong wear on crests , M. pungens was omnivorous and likely consumed leaves but was not as specialized for shearing as extant or extinct colugos. Regardless of whether mixodectids are stem dermopterans, which is further complicated by these clades being geographically restricted to western North America and southern Asia, respectively, Mixodectes serves as a model for colugo origins as an arboreal primatomorphan that increased body mass dramatically and started moving away from a more frugivorous diet like that common among many plesiadapiforms. In this way, mixodectids appear to have exploited a new ecological niche in North America, potentially driven by sympatric plesiadapiforms such as Torrejonia wilsoni , also documented at this locality, which were also arboreal but smaller bodied and more frugivorous. Therefore, new insights from this skeleton of Mixodectes add to understanding not only where this enigmatic family fits in the tree of life but also how Euarchonta and Placentalia diversified ecologically in the early Cenozoic following the Cretaceous-Paleogene mass extinction. 

Phylogenetic analyses The new skeleton of M. pungens was coded into two complementary character-taxon matrices to assess relationships among eutherian and euarchontogliran mammals, respectively (see Supplementary Information). The first matrix was modified from Crowell et al. , which was originally designed to test relationships among Cretaceous eutherians and crown placental mammals and provides a broad selection of eutherian mammals and rigorous character sampling . The second matrix was modified from Chester et al. , which was originally designed to test interrelationships within Euarchontoglires . Together, these matrices provide independent tests of the phylogenetic position of Mixodectes pungens within Eutheria and, more specifically, among euarchontogliran mammals. Cladistic analyses using maximum parsimony were conducted in TNT (v. 1.5) . Multistate characters were unordered and New Technology Search was executed using 100 replications as the starting point for each hit, 200 iterations of ratcheting, 10 rounds of tree drifting, 10 rounds of tree fusing, and sectorial searching. The resulting MPTs were used as starting trees in a Traditional Heuristic Search that was executed using tree-bisection and reconnection. 

The new skeleton of M. pungens was coded into two complementary character-taxon matrices to assess relationships among eutherian and euarchontogliran mammals, respectively (see Supplementary Information). The first matrix was modified from Crowell et al. , which was originally designed to test relationships among Cretaceous eutherians and crown placental mammals and provides a broad selection of eutherian mammals and rigorous character sampling . The second matrix was modified from Chester et al. , which was originally designed to test interrelationships within Euarchontoglires . Together, these matrices provide independent tests of the phylogenetic position of Mixodectes pungens within Eutheria and, more specifically, among euarchontogliran mammals. Cladistic analyses using maximum parsimony were conducted in TNT (v. 1.5) . Multistate characters were unordered and New Technology Search was executed using 100 replications as the starting point for each hit, 200 iterations of ratcheting, 10 rounds of tree drifting, 10 rounds of tree fusing, and sectorial searching. The resulting MPTs were used as starting trees in a Traditional Heuristic Search that was executed using tree-bisection and reconnection. 

Below is the link to the electronic supplementary material. Supplementary Material 1 

