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1 doi: /nature14905 Table of Contents Part A. CT Reconstruction of Maxilla and Mandible of Spinolestes xenarthrosus, Video S1, S2 Part B. Extended Systematic Information on Spinolestes xenarthrosus Part C. The Las Hoyas Site Part D. Body Mass Estimate of Spinolestes xenarthrosus Part E. Integumentary Structures and Inner Organs of Spinolestes xenarthrosus Part F. Brief Description of Skeletal Features of Spinolestes xenarthrosus Part G. Character List for Placing Spinolestes xenarthrosus within the Eutriconodont Mammals Part H. Character List for Placing Spinolestes xenarthrosus within the Cynodont- Mammaliaform Clades Part I. References Cited in both the Main Text and the Supplementary Information Part J. PAUP Search Results on the Placement of Spinolestes xenarthrosus within the Eutriconodonts Part K. PAUP Search Results on the Placement of Spinolestes xenarthrosus within the Cynodont-Mammaliaform-Mammalian Clades Part L. Nexus File of PAUP Analysis of Spinolestes xenarthrosus in the Eutriconodont Phylogeny Part M. Nexus File for Phylogenetic Analysis of Spinolestes xenarthrosus in the Mammaliaform Phylogeny 1

2 Part A. CT Reconstruction of Maxilla and Mandible of Spinolestes xenarthrosus Video S1. Right maxilla and right mandible of Spinolestes xenarthrosus with heavily worn deciduous M3 and M4 still in place labially to fully erupted RM3 and RM4. M1 in process of eruption, M2 fully erupted. Video S2. Posterior part of right mandible of Spinolestes xenarthrosus with sulcus for ossified Meckel s cartilage (Meckel s cartilage is fallen off). Part B. Extended Systematic Information on Spinolestes xenarthrosus Clade Mammaliaformes (51 Rowe 1988) Clade (Class) Mammalia (14 Linnaeus 1758) Clade (Order) Eutriconodonta (15 Kermack et al. 1973) Clade (Family) Gobiconodontidae (16 Chow and Rich 1984) Etymology: Spino, from spinosus spiny in Latin; λέστηϛ thief in Greek, lestes in Latin spelling, a common taxon name for mammals; xenarthrosus refers to the xenarthrous dorsal vertebrae with accessorial (ξένος, [Greek] strange) articulation (ἄρϑρον, [Greek] articulation) facets. Holotype specimen: Museo de las Ciencias de Castilla-La Mancha MCCMLH30000, slab A transferred to an artificial matrix of epoxy resin (MCCMLH30000A, Extended Data Fig. 1) for better permanent preservation of the specimen and to expose the under-side of the fossil; counter slab B (MCCMLH30000B, Fig. 2a), left radius and ulna preserved on a separate slab (MCCMLH30000C), as well as a piece of skin (MCCMLH30000D). All four (4) original slabs can be fitted together (total fossil area ca. 30 x 20 cm). The available fossil material thus belongs to the same individual (MCCMLH30000). Life Science Identifier (LSID) for the new taxon: urn:lsid:zoobank.org:act:10b31072-a a7dc-4224fb97e1e3 (ZooBank online registration for the International Code of Zoological Nomenclature [ICZN] LSID). Type Locality and Geological Age: Las Hoyas protected quarry, Calizas de la Huérgina Formation, southwestern Iberian Basin, near the town of Cuenca, Province of Cuenca, eastern central Spain. Latest Barremian age, Early Cretaceous, estimated to be 125 Myr to 127 Myr based on charophytes and ostracodes (52 Diéguez et al. 1995, 53 Vincente & Martín-Closas 2013). Differential Diagnosis: Mammaliaform with typical triconodont molariform pattern of amphilestid type with the central main cusp A/a much higher than anterior and posterior cups B/b and C/c. Differs from Sinoconodon, morganucodontids and docodontans in lacking the postdentary trough and the angular process. Differs from the amphilestids Amphilestes Lydekker 1887 (54) and Phascolotherium Owen 1838 (55) in having fewer premolars, fewer molariforms, or both. The single known specimen of Comodon Kretzoi & Kretzoi 2000 (56), a mandible fragment preserving the last four molariforms is very similar to Phascolotherium and according to 29 Kielan-Jaworowska et al is probably a junior synonym of the latter. Spinolestes differs from Hakusanodon Rougier et al (19) and Juchilestes Gao et al (20) in having fewer molariforms. Differs from 2

3 Aploconodon due to the presence of better developed cusps b and c in the lower molariforms than in the latter (upper dentition of Aploconodon is unknown; 57 Simpson 1925). Klamelia Chow & Rich 1984 (16) and Ferganodon Martin & Averianov 2007 (58) differ from Spinolestes due to the asymmetrical crowns of the lower molars with the main cusp a sitting mesial to the center of the tooth and a very small cusp b. Spinolestes differs from Liaotherium Zhou, Cheng & Wang 1991(59) due to its larger size and having fewer premolars and molars. It differs from Paikasigudodon Prasad & Manhas 2002 (60) as a result of its non-cuspidate cingula and the lack of a large parastylar cusp direct labial to cusp B at the upper molariforms. It differs from Tendagurodon (known by a possibly lower molariform) Heinrich 1998 (61) due to the presence of a lingual cingulid and a slightly angular arrangement of principal cusps. Spinolestes differs from Triconolestes Engelmann & Callison 1998 (62), Dyskritodon Sigogneau-Russell 1995 (63), Ichthyoconodon Sigogneau-Russell 1995 (63), Volaticotherium Meng et al (5), Argentoconodon Rougier et al (64) due to its lack of the posteriorly curved cusps a and b of the latter taxa (33 Gaetano & Rougier 2011). Spinolestes differs from Jeholodens Ji et al (21) and Yanoconodon Luo et al (22) due to the presence of accessory cusps d and e of lower molariforms and a lingual cingulum in the upper molariforms. It differs from Liaoconodon Meng et al (23) in having one less lower incisor and one extra upper molariform; also lacking is the uniform enlargement of incisors and canines of the latter. It differs from the genera of the Triconodontidae in the absence of a vertical tongue-in-groove interlocking of molariforms to the roots, as well as the more uniform heights of the cusps of the latter. Spinolestes is similar to other gobiconodontids in terms of how the upper molariforms are implanted at an oblique angle, and in molariform replacement (24 Jenkins & Schaff 1988), but unique among gobiconodontids in that the erupting (replacing) molariform is lingual (side-by-side) to the deciduous molariform of the same tooth locus. Within the gobiconodontids, Spinolestes differs from Gobiconodon (24 Jenkins & Schaff 1988, 25 Kielan-Jaworowska & Dashzeveg 1998, 26 Li et al. 2003) in having more incisors and fewer premolars (but see 65 Lopatin & Averianov 2015 for an alternative interpretation of tooth loci in Gobiconodon). It differs from Meemannodon in having one more lower incisor (upper dentition of Meemannodon is unknown; 66 Meng et al. 2005) and much larger i2. It differs from Hangjinia due to the marked enlargement of i1 and the presence of four molariforms (instead of the two present in Hangjinia; 67 Godefroit & Guo 1999). Of all of the gobiconodontids, Spinolestes is most similar to Repenomamus in terms of dentition, but differs from Repenomamus species in having one fewer molariform for both the upper and lower jaws, an enlargement of the lower first incisor, as well as having well-developed lingual cingulum, in addition to the labial cingulum of the upper molariforms. Notes on Tooth Morphology: The dental formula of Spinolestes xenarthrosus is I3-C1-P2-M4/i3-c1- p2-m4. The upper incisors are smaller than the lower and are separated by interdental spaces. Slightly recurved I1 is conical and somewhat laterally compressed with a posterior cutting edge. I2 is in the process of eruption. It has a similar shape but is slightly smaller than I1, with a worn distal side. I3 is fully erupted, of similar size to I2 but more strongly laterally compressed and with a distal cutting edge. The upper canine is similar in shape to the incisors (incisiform) and small. The right upper C is exposed in apical view and is in the process of eruption. Only parts of the root were preserved of the left upper C. Both P1s were detached from their alveoli and both are also single-rooted, as the right P1 alveolus preserves a single broken root. P2 is double-rooted and larger, and has a tricuspate crown with an enlarged and recurved cusp A. The upper molariforms are double-rooted and have a distinctive and line-like lingual cingulum. The main cusp A is much higher than cusps B and C, which sit at the same level. The upper deciduous molariforms are heavily worn and the entire crowns are beveled down to the roots near the end of the teeth s functional duration. The deciduous molariform DM3 is still present immediately labial to the molariforms RM3 (replacing/erupting) and DM4 is still present labial to RM4 (fully erupted) (see video S1 of maxilla, Part A). The wear facets on M2 are oriented at an oblique angle to its crown and these facets are facing lingually. 3

4 The lower incisors are larger than the upper incisors. The first incisor (i1) is the largest and is procumbent, with a round cross section. The second and third incisors (i2 and i3) are smaller than i1 and are semi-procumbent. The canine is incisiform and smaller than the posterior incisors. The lower i1 to lower canine form a posteriorly decreasing size gradient. After the canine follow two lower premolars which are almost completely obscured in the ventral view of the skull because the mandibles are preserved in occlusion. However, these lower premolars are clearly detected by CT scanning (see Part A). Posterior to the premolars are four lower molariforms of the amphilestid type with main cusp a much higher than cusps b and c which are of equal size. Accessorial cuspules d (distal) and e (mesial) are present, cuspule e is slightly larger than d. Cusps b and e are shifted somewhat lingually leading to a slight angulation of the main cusps of the crown in the occlusal view. On the lingual side is a sharp continuous cingulid that connects cuspules e and d. At the base of cusp a, the cingulid bend in an apical direction. The labial side of the molariforms is gently convex without cingulid. Molariform measurements in mm (L=length, W= width): M2, L=1.99; RM4, L=1.81; rm3, L=2.06, W=1.14; m4, L=1.77, W=1.04. Notes on Tooth Replacement Pattern: Spinolestes exhibits replacement at various tooth positions including molariforms. In the maxilla, erupting teeth are present at the I2, C, M1, and M3 positions. The heavily worn deciduous molariforms are still in place labially to RM3 and RM4 (fully erupted) (Video S1). The successive generation(s) of erupting postcanines tends to be positioned lingual (medial) to the deciduous predecessor of the same locus in the majority of premammalian cynodonts. Thus, the erupting tooth positioned lingual to the deciduous predecessor of the same tooth locus is a plesiomorphic feature of premammalian cynodonts. However, as Spinolestes is placed in the gobiconodontids, which is a part of the crown Mammalia and far more derived than premammalian cynodonts, this positional relationship between the erupting molariform and its deciduous predecessor would represent an atavistic condition to premammalian cynodonts. The lingual placement of the erupting molariforms appears to be less complex than the vertical replacement of the premolars of crown mammals. In the mandible, the replacement at the same molariform locus is vertical, the erupting tooth is directly below its deciduous predecessor. In the lower antemolariform dentition, the replacement situation is unclear because the teeth are partially obscured. At m1 and m2 positions replacing teeth have been detected below the deciduous molariforms. Replacing m1 is more advanced and the tooth crown is better developed than the replacing m2; dm3 is already shed, and rm3 is nearly fully erupted. The last molariform (m4) is fully erupted in place. Thus the lower tooth replacement pattern of Spinolestes is alternating among the consecutive tooth positions, as is the case in many Mesozoic mammaliaforms (68 Luo et al. 2004, 29 Kielan-Jaworowska et al. 2004). We would suggest that both the upper and the cheek teeth were replaced in an alternating pattern. We can observe that Spinolestes has two generations of teeth in the same locus of most molariforms. However, study is limited by the fact that only a single type specimen is available. At present the number of generations of replacements which may have occurred over the life-span of an individual is unknown. Replacements of molariforms have also been reported for the anterior molariforms of Gobiconodon ostromi with likely three generations of molariforms (24 Jenkins & Schaff 1988). 65 Lopatin & Averianov (2015) observed two generations of replacement for molariforms in Gobiconodon hoburensis and three generations in larger Gobiconodon borissiaki from the Early Cretaceous of Mongolia. 4

5 Part C. The Las Hoyas Site The Las Hoyas site is located in the southern Serranía de Cuenca in the southwestern Iberian Mountain Range. The site is part of the La Huérgina Limestone Formation (69 Sanz et al. 1988, 17 Fregenal-Martínez & Meléndez 2000). The sedimentary environment of the Las Hoyas deposits that yielded the fossil of Spinolestes was a carbonate-rich subtropical wetland with ponds, small lakes and/or a flood plain within a low-relief terrain. The fossil plants of this palaeoenvironment are primarily small coniferous plants which were concentrated within patches on hummocks within the wetland setting (72 Buscalioni & Fregenal-Martínez 2010). A latest Barremian age is indicated for the Las Hoyas fossil site based on combined data from ostracodes, charophytes, palynomorphs, and the megaflora (52 Diéguez et al. 1995). The Konservat- Lagerstätte of Las Hoyas has produced a wealth of exquisitely preserved fossils of plants, arthropods, fishes, amphibians, turtles, lizards, crocodylomorphs, non-avian dinosaurs, birds, and a triconodont mammal (70 Sanz et al. 1996, 71 Meléndez 1995, 72 Buscalioni & Fregenal- Martínez 2010). Spinolestes lived close to the water, in a wetland or flood plain, in association with a flora dominated by conifers and a great diversity of insects. The fossiliferous sediments of Las Hoyas are finely laminated limestones with a small fraction of clay and organic matter. The periphyton-dominated wetland was covered by thick microbial mats and was influenced by climatically driven cyclical oscillations of the water level (72 Buscalioni & Fregenal-Martínez 2010). The microbial mats played an important role in preservation of soft tissues such as mineralized muscle tissue (75 Briggs et al. 1997). Part D. Body Mass Estimate of Spinolestes xenarthrosus We estimated the body mass of Spinolestes according to the method of 73 Gingerich & Smith (1984) which determined the body mass based on the skull size by the scaling relationship of skull length and body mass. This empirical regression is based on a dataset of extant placental insectivores and primates, and has been applied previously to estimating the body mass of Mesozoic mammals (e.g., 3 Ji et al. 2006; 74 Luo et al. 2001): Lg 10 (Body-Mass [g]) = 3.68 x Lg 10 (skull-length [mm]) On the basis of a skull length of 35.8 mm, the body mass was herein estimated at 77 grams for MCCMLH A second and alternative method to estimate body mass is based on the regression between body mass to mandibular length, as developed by 48 Foster (2009), from a dataset of placental insectivores and marsupials: Ln (body-mass [g] = x Ln (mandible-length [mm]) By this mandible size to body mass scaling, we estimated that the body mass is about 73 grams for Spinolestes, based on the mandibular length of mm. 5

6 Our third estimate of the body mass was based on the scaling relationship of femoral length and body mass of all mammals, as developed by 47 Campione & Evans (2012): Log 10BodyMass = x Log10 FemurLength By the femoral length to body mass regression from a dataset of all mammals (47 Campione and Evans 2012), the body mass of Spinolestes is about 52 grams on a femoral length of mm. Our fourth, and final, estimate of the body mass is based on the scaling relationship of humeral length and body mass of all mammals, also according to 47 Campione & Evans (2012): Log 10BodyMass = x Log10 HumerusLength Based on the humeral length to body mass ratio of all mammals dataset (47 Campione & Evans 2012), the body mass of Spinolestes is ca. 72 grams for a humeral length of 19.7 mm. With these body mass estimations, Spinolestes falls within the size range of the didelphid marsupials Monodelphis brevicaudata and Marmosa murina (47 Campione & Evans 2012). Table S1. Skeletal measurements of Spinolestes xenarthrosus MCCMLH30000 in mm. Skull, length Skull, width at zygomatic arch Skull, occipital width Skull, intercondylar width 9.13 Mandible, length Dorsal vertebrae 1-23, length Sacrum, length Caudal vertebrae 1-22, length Scapula, length Humerus, length Radius, length Radius, mid-shaft diameter 1.48 Ulna, length Ulna, mid-shaft diameter 1.79 Olecranon, length above pivot 3.60 Femur, length Femur, mid-shaft diameter 2.31 Tibia, length Tibia, mid-shaft diameter 1.90 Fibula, length Fibula, mid-shaft diameter

7 Table S2. Body mass estimates for Spinolestes xenarthrosus MCCMLH Spinolestes Estimate: mean (minimummaximum) 72 g (56 g 94 g) 52 g (38 g 72 g) Anatomical Measurement Humerus Length = 19.7 mm Femur Length = mm 73 g Mandible length = g Scaling Equations BM = Body Mass L = length (mm) Log 10BM=2.8626xL og 10L Log 10 BM=2.993xLo g 10 L Ln e BM= x LneL Comparative Taxa All mammals All mammals Placental insectivores & marsupials Sources of equations 47 Campione & Evans Campione & Evans Foster 2009 Part E. Integumentary Structures and Internal Organs of Spinolestes xenarthrosus General Pelage Features The pelage of Spinolestes consists of longer guard (cover) hairs as well as shorter hairs of the denser underfur closer to the skin, in addition to patches of spines or spine-like structures and some dermal scutes in the lumbar and sacral region. Hairs are partially fossilized with histological structures preserved, most probably due to phosphatization; this process has been previously noted in soft-tissue preservation of other vertebrates from Las Hoyas (75 Briggs et al. 1997). The halo of the fur shows a nearly continuous outline of the pelage, on both the dorsal and ventral aspects of the torso, and from the scapular region up to the skull scalp. Several patches of skin with relatively sparse hairs are preserved around, although slightly separated from, the skeleton. Of these, four large patches of well preserved skin are located near the separated left scapula and cervical atlas (C1), next to the mid-section of thoracic ribcage, near the thoraco-lumbar transition, and on the end of the tail on the counter part. A fragment of skin (near the thoracolumbar transition) shows a wave pattern of skin folds separated by irregular depressions (Extended Data Figure 4c). This pattern is commonly found in modern hairy mammals (36 Lovell & Getty 1957: fig. 4b, 38 Evans & de Lahunta 2012). Although the skull and cervical vertebrae are displaced from the torso, the body outline is continuous from the torso, around the scapular region, up to the skull. The outlines are closely matched on the main part (transferred from the original sedimentary slab onto the translucent resin plastic), and on the counter-part (still intact). We suggest that the left external ear pinna is preserved and is connected to the skull scalp associated with the disarticulated left scapula. The external ear pinna is sub-oval in shape (17.5 and 8.5 mm in height and width, respectively) (Extended Data Fig. 7a, b). A very similar situation was observed in an actuo-taphonomic experiment conducted by Bastian Mähler (Bonn) with a rat carcass (Rattus norvegicus) floating in water. During the decay process, the skull became detached from the scalp and sank to the bottom, while the scalp with the left pinna remained intact and connected to the floating carcass (Extended Data Fig. 7c, d). In the Spinolestes specimen, the scalp with the left pinna is dorsally completely connected to the neck and thorax region. The scalp is 7

8 thicker and more densely covered by hairs than the small isolated patches of skin that are scattered around the skull. There is no skin or hair preserved at the displaced skull, and the partly visible soft part outline most likely derives from adhering soft tissue of cheeks and chin below the skin (compare to Extended Data Figure 7c). The pinna is a complex and rigid skinfold which is reinforced by cartilage and dense connective tissue (76 Csillag 2005). Arguably the thicker and almost hairless tissue (pinkish in color) that defines and fills the entire inner surface of the alleged pinna in Spinolestes is a remnant of cartilaginous tissue (Fig. 2a and Extended Data Fig. 7a, b). Longer guard hairs, made up of fine brownish filaments up to 5 mm long, are present in dense patches in the parietal region of the skull, and the cervical and scapular regions of the body. There appears to be a continuous patch of longer hairs in the head and neck regions than observed for the rest of the body, and this most probably was a mane. Similar filaments (1 to 3 mm long) are more sparsely arranged in the dorsal region, where they form a median hairy crest. Long and fine guard hairs are also present along most of the tail. Impressions and outlines of guard hairs can also be seen under light microscope in the thoracic and abdominal region of the animal. Scanning Electron Microscope examination reveals the histological structures of the hairs, the keratinized epidermal cells on skin surface, skin pores, and also associated hair roots, hair bulbs and hair follicles. While a full description of the skin s morphological and histological structure requires further study, we will herein briefly describe each of these integumentary features of Spinolestes xenarthrosus. Histological Structure of Hairs In many regions (i.e., lumbar region, abdomen, hindlimbs, and tail) separated patches of skin have preserved the histological structure of individual hairs, which can be examined by both light microscope and SEM (Fig. 2 and Extended Data Figs 4-6). Primary (Guard) Hairs The basal shafts of the primary hairs (the largest hair from each follicle orifice) have diameters from 20 µm to more than 35 µm, a similar size range as the diameter of guard hairs of small rodents, small lagomorphs and insectivorans (77 Sessions et al. 2009). The primary guard hairs are estimated to reach 3 to 5 mm in length in the impression (albeit without histological structures). The cuticles (scales) on primary hair shafts are imbricated (partially overlapping). The individual scales have an ovate outline, simple (non-serrate) free edges and smooth external surfaces. The imbricated scales have an irregular mosaic to terrace arrangement (Extended Data Fig. 5a, b) (sensu 78 Chernova & Kuznetsov 2001), which is known from guard hairs of extant placentals (9 Chernova 2006, 79 Hausman 1924) and marsupials (80 Lyne & McMahon 1950). The cuticles are not large enough to encircle the relatively large shaft to form the annular (coronary) pattern as in the secondary hairs. The interior of hair shaft is exposed in many primary (guard) hairs that are broken longitudinally (Fig. 2g). These show the basic histological structure of a mammalian hair with an inner medulla, the intermediate cortex, and the outer layer of cuticles (scales) (9 Chernova 2006, 79 Hausman 1924, 81 Hausman 1920). Of the primary (guard) hairs, the medullary core of the hair shaft is of the discontinuous type with medullary chambers separated by interstitial structures (sensu 78 Chernova & Kuznetsov 2001, 79 Hausman 1924). Based on SEM examination, we would suggest that the hair shafts have relatively thick cortices made of typical fusiform, spindle-shaped cells that form the cutical scales (sensu 40 Hausman 1932: fig. 3). Secondary Hairs (hairs of underfur) The basal shaft of secondary hairs (the multiple thinner and shorter hairs from each follicle orifice) range in diameter from 5 to 15 µm. For the majority of the secondary hairs where the surface is intact, the shaft surface shows an annular (coronary) arrangement 8

9 of cuticles (Extended Data Fig. 5b, c) where individual cuticles are large enough to encircle the shaft. Cuticles have simple (nonserrate) free margins and smooth external surfaces. In several hairs where the interior structure is exposed, the medulla would appear to be continuous. The secondary hairs appear to have simpler medullary structure and a mainly coronary (annular) pattern of cuticles (scales). These features are quite different from those of the primary (guard) hairs. In addition they are also thinner, with much smaller diameters, as well as being shorter. Based on these differences, we suggest that the secondary hairs are comparable with the downy hairs of the underfur of extant mammalian pelage. The secondary hairs, or underhairs tend to form bundles. Such bundles can be formed exclusively by the secondary hairs, as in the abdominal region, but on most of the body surface secondary hairs can be bundled together with one or two primary hairs, as well represented in extant mammals, such as dogs (36 Lovell & Getty 1957; see the literature reviewed by 38 Evans & de Lahunta 2012). Hair Follicles, Hair Bulbs, and Related Structures A consistent pattern of hair distribution of Spinolestes is that hairs are bundled in compound follicles, a pattern documented in excellent detail in carnivores and lagomorphs among placental mammals (36 Lovell & Getty 1957, 37 Whitely 1958), also known in monotremes and marsupials (39 Spencer & Sweet 1899). Each compound follicle usually has one thicker and longer primary hair(s), and several thinner secondary hairs, which can number up to 6 or 7. For Spinolestes, we interpret that the skin orifice where multiple hairs are bundled represents the common orifice of compound hair follicles. In several locations where the follicles are exposed and can be examined by light microscopy and SEM, enlarged ends of roots of both primary and secondary hairs are preserved. In some of these, even the hair papilla can be recognized (Extended Data Fig. 4e). Around the bundled hair roots, we can observe on the transferred slab with the embedding resin plastic, which is translucent under light microscope, an ovoid structure at the bases of primary and secondary hairs. This type of ovoid structure surrounding the hair root is made of polygonal cells and we interpret it to be the fossilized hair bulb. The hair bulbs range from 180 µm to over 200 µm in size (Extended Data Fig. 6: HB). In extant mammals such as the dog (reviewed by 38 Evans & de Lahunta 2012), during normal growth of mammalian hairs in compound follicles, there is usually a single primary hair from the follicle oriface in the newborn dog fetus. The primary hair follicle gives rise to additional secondary hair follicles in dog neonates of 3 to 4 weeks. At 8 to 10 weeks, the secondary hairs become arranged around the primary hairs (Extended Data Fig. 4b). The secondary hairs appear externally through the same orifice as the primary hairs of the same compound follicle (36 Lovell & Getty 1957: fig. 3, 38 Evans & de Lahunta 2012: fig. 3-8). The thicker primary hair(s) and the smaller secondary hairs in Spinolestes are bundled through the orifice of the compound follicle, in a similar manner as compound follicles of carnivorans. Skin Surface Several areas of preserved skin surface show flat keratinzed cells that are fossilized in situ. These epidermal cells (keratinocytes) generally have polygonal outlines of various proportions, with their largest dimension between 25 and 35 µm. The preserved skin surface has skin pores filled with amorphous matrix. The pores have circular to oval shapes, 150 to 180 µm in their longest dimension (Extended Data Fig. 4f: P). Alterated Hairs The longer guard hairs preserved as filament-like fine impressions can be 3 to 5 mm long, on the dorsal side of the animal, in the tail, and local areas of the abdomen. Some hairs preserved in the several skin patches show short shafts of hairs (less than 1 mm) (Fig. 2c-d). These hairs have truncated distal ends which are dark in color (and is darker than the rest of the shaft). Such features are known as pathological block hairs and i-hairs (41 Rudnicka et al. 2012: fig. 2.50). This kind of truncated hair is caused by a fungal infection, known as dermatophytosis, and recognised 9

10 in the hair pathology of extant mammals (41 Rudnicka et al. 2012). However, the assessment of this putative pathological condition in some (although not all) hair patches requires further corroboration given the fact that fungi can degrade hair during the biostratinomic phase (82 Tridico et al. 2014). Protospines Cylindrical micro-tubules are present in the sacral region of Spinolestes. These structures are best preserved on the transferred slab (MCCMLH3000A). The thicker ones are ca. 130 µm in diameter while the smaller ones are ca. 80 µm in diameter. The structures appear to be mainly circular or oval in cross section. These spines are also accompanied by some tubules which are ca. 40 µm in diameter. When present as independent structures, the tubules show a scaly external surface, a medullary cavity and a cortex. The histological structure of the hair-like tubules corresponds to the structures of hairs. We can determine that the protospines are formed by the confluence or merger of at least two or more tubules. The protospines and isolated tubules appear to be clustered or bundled, but the orientation of such bundles appears to be somewhat random (Fig. 2e, f). In the embryogenesis of mammals, a spine is formed later than hairs and it is developed from the fusion of several hair follicles (9 Chernova 2006). This is a similar developmental process to that of the embryogenesis of the hairs by bundling multiple hairs in a compound follicle (9 Chernova 2006). A spine can develop from the merger of multiple hair follicles of similar size, or from one single merging episode with several smaller surrounding hair follicles. Therefore, the spines formed by the merger of several hair-like tubules in Spinolestes is similar to the spine-forming mechanism based on the fusion of hair follicles in extant mammals (9 Chernova 2006). Given the phylogenetic basal position of gobiconodontids in crown Mammalia, the fusion of hair-like tubules into spine-like structures suggests that the morphological development of protospines, as seen in extant mammals with spines (9 Chernova 2006), was already established in the Mesozoic. Dermal Scutes Some keratinized structures with oval to almost circular outlines are present in the thoracic, lumbar and pelvic regions (Fig. 2c, h and Extended Data Fig. 8). These plate-like structures are up to 4 mm across (in major axis length) (Fig. 2h). Given that they seem to be keratinized and their plate-like characteristics, we identify these as dermal scutes. The scutes appear to be located among a random arrangement of numerous tubules merging to a homogeneous keratinized matrix (Extended Data Fig. 8c). Dermal scales are also known in the Jurassic mammaliaform Castorocauda (3 Ji et al. 2006). Therefore, the presence of dermal scutes and protospines in Spinolestes indicates that diverse integumentary structures occurred in more than one clade of basal mammals. Function of Spines Among extant mammals, the spiny mouse (Acomys) is an excellent analogue for the function of spines in Spinolestes. As in Spinolestes, the spines of Acomys are concentrated in the lower back (43 Montandon et al. 2014). Here they form a tuft of spiny hair that gives the animal a larger appearance and acts as protection against predators. The Acomys spines are easily shed, and when a spiny mouse is bitten in the back, it can escape leaving behind the predator with its mouth full of spines (43 Montandon et al. 2014). Acomys also has the unusual ability of being able to easily shed and regenerate dorsal skin as a predator escape behavior (83 Seifert et al. 2012). The concentration of the dermals scutes in the lower dorsal region of Spinolestes can also be interpreted as a protective device. Remnants of Lung, Liver and Muscular Diaphragm Lung Within the thoracic ribcage of Spinolestes, a patch of soft tissue (ca. 11 mm long), can be identified as remnants of lung tissue preserved in situ as a natural cast (sedimentary infillings) of the peripheral conductive and acinar airways (Extended Data Fig. 9). Our interpretation of this feature as 10

11 lung tissue is based on its location in the thoracic ribcage, corresponding to its expected anatomical position. Moreover, this tissue shows dichotomous branching and acinus-like tubular structures. They resemble the bronchioles of the mammalian lung that conduct airflow, for the gas-exchange in pulmonary alveoli (38 Evans & de Lahunta 2012, 44 Weibel et al. 2005). These features appear to be soft tissue tubules and acinar spaces in approximately the same size range as the bronchioles and acini of small extant mammals (84 Vasilescu et al. 2012). Remnant of Liver A large oval area of reddish-brown soft-tissue (length ca. 20 mm) can be observed posteriorly to the lung tissue (Extended Data Fig. 9a). This is interpreted as a remnant of the liver on the basis of its anatomical position and color. Fossilized liver tissue appears to be reddish because this organ is rich in iron. The presence of iron has been shown to play a major role in the fossilization of soft-tissue structures, especially vessels (85 Schweitzer et al. 2014). Fossilized liver has been documented for the dinosaur Scipionyx from the Early Cretaceous (Albian) of Italy (45 Dal Sasso & Signore 1998). The liver of another dinosaur, Sinosauropteryx, has also been tentatively identified (86 Ruben et al. 1999). Evidence on Diaphragm - The boundary between the inferred lung tissue and the liver tissue extends, obliquely, from the distal tip of the 3 rd rib to near the proximal ends of the 14 th and 15 th ribs, and is more concave near the the sternal series, but less concave in the more dorso-posterior parts near the vertebral column (Fig. 2a and Extended Data Fig. 9a). In extant mammals, the muscular diaphragm is packed between the lung in the thoracic cavity, and the liver in the abdominal cavity. It extends from the mid-sternal area to the posterior-most thoracic vertebrae, and the posterior-most part of the diaphragm extends to the mid-lumbar region of the vertebral column (38 Evans & de Lahunta 2012). Therefore, the shape, size, and anatomical position of the lung and liver boundaries correspond to the location of the muscular diaphragm, indicating that Spinolestes shares this apomorphy with extant mammals. The diaphragm is a distinctive respiratory structure, which evolved in concert with the development of active and sustained movement, such as running (46 Bramble & Jenkins 1991). The presence of the diaphragm in Spinolestes suggests that the functions related to this structure were already developed in Mesozoic mammals. Part F. Brief Description on Skeletal Features of of Spinolestes xenarthrosus The skeleton is embedded in a lateral-view position with the right side exposed on the transferred slab (MCCMLH30000A). The preserved skull has been turned during fossilization to a dorso-ventral position and is exposed with its ventral side on the transferred slab (MCCMLH30000A) (Fig. 1b, Extended Data Fig. 1). The skull is moderately elongated with a robust, rounded snout. The robust zygoma is relatively straight (plesiomorphic) and unlike the strongly-arched zygoma of triconodontids (30 Kielan-Jaworowska et al. 2004), and is formed mostly by the jugal. The jugals are fully developed, and extend from the maxillary zygomatic root to the squamosal glenoid, and are thus unlike the reduced and internally placed jugal of the multituberculates and triconodontids (87 Hopson et al. 1989). The glenoid facets of the squamosals are transversely expanded, although not troughed. The petrosal has a cylindrical promontorium, with a narrow lateral trough completely flanked by the lateral flange as in other gobiconodontids (Luo, personal observation) and eutriconodonts (88 Rougier et al. 1996). The mandibles are robust, and have a rounded angular region but no angular process, as in all of the eutriconodontans. The occipital condyles are large and oval shaped. The mandibular body is relatively short and robust. The mandibular ventral and posterior margins around the angular have a flattened surface. The ventral border of the angular region has prominent medial and lateral crests, which form the ventral border of the pterygoid fossa medially, and the masseteric fossa laterally, respectively. The condyles of the mandibles are broadly oval and bent and skewed laterally from the plane of the coronoid process and mandibular body. As preserved, they are still in articulation with the glenoid facets. The Meckel s cartilages are ossified and preserved, and they are 11

12 relatively robust, curved and truncated at the posterior ends. The left Meckel s cartilage is in the intact position and still attached to the left mandible. The right Meckel s cartilage became detached from the right mandible, exposing a wide Meckel s groove on the mandible (see Part A, Video S2). The middle ear bones themselves are lost, but given that the Meckel s cartilage is curved away from the posterior part of the mandible, we infer that the ear was mediolaterally separate from the mandible but still connected antero-posteriorly to the mandible via the ossified Meckel s cartilage, as in Yanoconodon and Liaoconodon, other gobiconodontids preserved with this structure (22 Luo et al. 2007, 23 Meng et al. 2011, 26 Li et al. 2003, 28 Luo 2011). Spinolestes has 16 thoracic vertebrae, seven lumbar vertebrae (with transverse processes), three fused sacrals and 22 caudals. The atlas (C1) with a completely preserved neural arch is separated from the vertebral column and is preserved next to the separated left scapula. The axis (C2) is preserved on the transferred slab (MCCMLH30000A) above the vertebral column (between dorsals 7 and 8). The dens of the axis (C2) is preserved as a plastic cast of the natural mold on slab A and points anteriorly. At least four post-axial cervical vertebrae are preserved in partial articulation, and these can be seen close to the separated left tibia and fibula and are partially covered by the fibula. The anteriormost preserved cervical (most likely C3) shows two cervical pedicles. The ventral pedicle is well developed as the inferior lamella of the cervical, as identified in the gobiconodontid Repenomamus (27 Hu 2006: pp , fig. 3-5). These pedicles are for the cervical ribs but the ribs themselves are not fused to the centrum. The triangular dorsal part of its spinous process is preserved on slab B (nontransferred MCCMLH30000B). Between the scapulae three isolated cervical ribs are present. We tentatively suggest that Spinolestes has seven cervicals, typical of mammaliaforms, and with un-fused post-axial cervical ribs, just like other eutriconodontans (21 Ji et al. 1999, 27 Hu 2006). There are 23 dorsal vertebrae, of which 16 are thoracals and seven lumbars (with transverse processes). Dorsals 1-11 have the longest spinous processes, from D11 to more posterior dorsals, the spinous process becomes descreasingly shorter, and its end becomes dorso-ventrally flattened and plate-like. There is no anticlinal vertebra present because all of the dorsal vertebrae have posteriorly leaning spinous processes. The spinous process of dorsal 22 is only slightly bent caudally, that of dorsal 23 is vertical, in transition to the likely similarly oriented spinous processes of the sacral vertebrae. Spinolestes exhibits the lower (ventral) xenarthrous articulation, in addition to typical pre- and postzygapophyses for inter-vertebral articulation, beginning with dorsal vertebrae 9/10 through dorsals 16/17 (Fig. 1e, f, Extended Data Fig. 2d). Among extant mammals, fully developed xenarthry is restricted to South American xenarthrans (armadillos, anteaters, and sloths). In extant xenarthrans, an extra intervertebral articulation is developed, in addition to the pre- and post-zygapormophyses (12 Rose & Emry 1993, 30 Gaudin 1999, 31 Lessertisseur & Saban 1967, 89 Grassé 1955). This complex extra-articulation, known as xenarthry, consists of a dorsal (upper) xenarthrous articulation and a lower (ventral) xenarthrous articulation (sensu 12 Rose & Emry 1993), collectively known as lateral zygapophyseal vertebral articulation (sensu 30 Gaudin 1999). Extant xenarthran mammals also have a prominent metapophysis a process rising between the plesiomorphic prezygapophysis and the derived upper (dorsal) xenarthrous process. In the early Cenozoic mammals, the accessory xenarthrous articulation, in varying degree of development, is known from Eocene palaeanodonts (10 Storch 1981, 12 Rose & Emry 1993), and Ernanodon (11 Ting 1987, 12 Rose & Emry 1993). For Mesozoic mammals, this kind of accessory articulation (lateral zygapophyseal vertebral articulation sensu 30 Gaudin 1999) has also been reported for the Jurassic mammaliaform Fruitafossor (13 Luo & Wible 2005). Spinolestes (MCCMLH30000) shows extra vertebral articulation (Fig. 1e, f, Extended Data Fig. 2d) similar to that of Fruitafossor. In Spinolestes, the lateral zygapophyseal vertebral articulation (Fig. 2f: alz/plz) is present. This structure is equivalent to the dorsal xenarthrous process of extant xenarthrans (12 Rose & Emry 1993: fig. 7.3). However, the ventral xenarthrous process (sensu 12 Rose & Emry 12

13 1993) is present but very underdeveloped on contacts of D15/D16 and D16/D17 in Spinolestes, but not in D10-D14, or D17/D18. Similarly, in Fruitafossor, only the dorsal xenarthrous articulation (a part of the lateral zygapophyseal articulation of 30 Gaudin (1999) is present, but not the ventral xenarthrous articulation (sensu 12 Rose & Emry 1993). Among other eutriconodontans, the lateral zygapophyseal articulation is also present. For example, this feature can be seen in the gobiconodontid Repenomamus (27 Hu 2006: fig. 3-8; also personal observations on additional gobiconodontid specimens from the Cretaceous Yixian Formation). The xenarthrous articulations in gobicondontids including Spinolestes and in Fruitafossor differ somewhat from that of Xenarthra in that they lack the typical metapophysis and the ventral part of the two accessorial articulation facets (90 Starck 1982: fig. 49). Xenarthry is commonly associated to the strengthening of the vertebral column in the thoraco-lumbar transition (12 Rose & Emry 1993, 91 Hildebrand & Goslow 2001). In the case of fossorial adaptation of armadillos and anteaters, this conveys functional advantage for digging (91 Hildebrand & Goslow 2001), but it also strengthens the vertebral column for arboreal xenarthrans. There are 20 pairs of ribs, of which 16 belong to the thoracic and four belong to the lumbar vertebrae. The ultimate and penultimate lumbars have fused ribs. The thoracic ribs are double-headed, and have robust shafts with round cross sections. The proximal parts of the rib shafts have lateral crests. The thoracic vertebrae have elongate and recurved spinal processes which devlop a flat and expanded top surface beginning with dorsal vertebra 11. The robust spinous processes indicate that Spinolestes possessed a strong dorsal musculature. Spinolestes has three co-ossified sacral vertebrae. The ilio-sacral connection is now detached (as preserved) and was not co-ossified. The tail has 22 caudal vertebrae and is equal to the body length. Caudals 1-3 are very short, 4-6 somewhat longer, and beginning with caudal 7 they are elongated and slender. Caudals 7-9 have double transverse processes and the anterior transverse process has a lateral foramen at its base. The haemal arches of the caudals are very long and oriented anteriorly and these arches appear to have a ventral keel. The shape of the caudal vertebrae as well as the slightly curved embedding posture in the fossil resembles the situation in the Eurotamandua specimen (10 Storch 1981: fig. 7). This preserved posture was interpreted to show a rigid tail which was possibly used to support the body during scratch-digging for other extinct Cenozoic mammals (10 Storch 1981). Based on this comparison, we would suggest that the tail of Spinolestes had a similar function. The shoulder girdle is therian-like (Fig. 1a, b). The scapula is broad and has a triangular outline. It has a high scapular spine that ends ventrally in the acromion, as seen in the partially preserved specimen on the transferred slab. The posterior border of the scapula is strongly bent laterally and forms a straight secondary spine. The acromion on the scapular spine is medium-sized, and oriented cranially. It appears to reach below the level of the glenoid. The glenoid fossa is oval (antero-posteriorly) and oriented almost perpendicular to the scapular plate. The left scapula is preserved in medial view on slab B. The clavicle is robust and curved. Its distal end is enlarged and has a large oval facet for mobile articulation with the acromion. The proximal end of the clavicle is not fused to the interclavicle, which is disarticulated on slab B as preserved.the small coracoid is fused to the scapula without procoracoid. In the antero-ventral region of the thorax there are five sternal elements (manubrium, sternebrae 2-4, and the xiphoid) and the partially preserved interclavicle. As in other eutriconodonts (21 Ji et al. 1999, 22 Luo et al. 2007, 27 Hu 2006), the joints between the clavicula and the interclavicula and between the clavicula and scapular acromion were mobile. The humerus is robust and has a hemispherical head that sits directly on the shaft without neck. The distal end is about twisted anti-clockwise relatively to the proximal head. Radial and ulnar condyles are separate which is a plesiomorphic character shared by other eutriconodontans. The large, bulbous radial condyle of the right humerus is exposed, as is the large ectepicondyle. There is a shelf- 13

14 like extension proximally of the ectepicondyle. These features point to a strong musculature and indicate that Spinolestes was potentially an accomplished scratch-digger. The radius and the ulna are stout and are not fused. The radius has a medio-laterally oval proximal articulation facet. The distal end of the radius is medio-laterally much wider than at the proximal end. The ulna has a longitudinal groove along the lateral side of its shaft. The olecranon is stout and not elongated. The semilunar notch is semicircular. The olecranon is stout and not elongated, and by the olecranon length ratio to the ulna ratio, we can unambiguously rule out that Spinolestes was a fossorial mammal with a subterraneous lifestyle. The ratio of the olecranon process to the length of the ulna distal to the pivot of the elbow joint is 19.7% in Spinolestes. This is below the lower end of the range of fossorial mammals (23% in the ground squirrel Spermophilopsis, 92 Hildebrand 1985). Other fossorial mammals have ratios between 1/3 (gopher, African mole-rat), ½ (aardvark, pangolin, mole), 2/3 (Mediterranean mole-rat, armadillos), or ¾ (marsupial mole, golden moles) (91 Hildebrand & Goslow 2001). While the aardvark and pangolins are much larger than Spinolestes and are the most powerful scratch-diggers among mammals (91 Hildebrand & Goslow 2001), have most of the others above mentioned a subterraneous lifestyle which can be excluded for Spinolestes with certainty. The carpal bones are robust and densely packed. The metacarpals and phalanges are stout and relatively short. The claws are moderately laterally compressed and are only slightly curved (Fig. 1d); they have well developed lateral grooves for the insertion of the horny sheaths although the sheaths are not preserved. The phalangeal index (PI [combined length of proximal + intermediate phalanges]/length of metartarsal x 100) for manual digit ray 3 is 121, which is within the middle of the top quantile of terrestrial mammals (49 Kirk et al. 2008). The phalangeal slenderness ratio (PSR - [sum lengths/sum widths of the same autopodial elements] x 100) for assessment of manual and pedal patterns for locomotorial adaptation of diprotodontian marsupial mammals (93 Weisbecker & Wharton 2006) for the intermediate phalanges of all digits is 183% in Spinolestes. This is in the upper range of terrestrial taxa. PSR of all proximal phalanges is 192, which is in the lower half of terrestrial diprotodontian marsupials (93 Weisbecker & Wharton 2006: fig. 7). The hand proportions and shape of the distal phalanges suggest a terrestrial adaptation for Spinolestes. Special digging adaptations such as bony stops at the phalanges for protection against hyperextension are not present. Both the ilium and the ischium are preserved on slab A. The ilium, ischium, and pubis are not coossified at the acetabulum and the acetabular suture is open. The contact area of the ilium to the three fused sacrals is relatively short and is not co-ossified. Both broad triangular epipubes are preserved on slab B. The femur is exposed from the anterior side and is still in articulation with the acetabulum. The femoral proximal head is spherical and set off from the shaft by a neck. The greater trochanter is well developed and is only slighty lower than the femoral head. The lesser trochanter is not exposed. The shaft of the femur has an oval cross section. The distal end is wide and bears a broad and shallow patellar groove. Tibia and fibula are straight and both have rounded shafts; they are not fused. The fibula is only slightly thinner than the tibia. At the proximal end the fibula is slightly enlarged. The parafibula is present, but not fused to the fibula. It is located in the space between the lateral femoral condyle and the proximal articulation facet of the fibula with the femur. The tibia has a proximal lateral process to articulate with the proximal end of the fibula, and a lateral malleolus at its distal end. Both calcanei are preserved. The right calcaneus, which is exposed in dorsal view, is preserved near the distal ends of the right tibia and the fibula. The left calcaneus is isolated from the skeleton and is exposed in ventral view. It has an elevated astragalar facet, and a sustentacular facet on its medial side. Both the astragalar and the sustentcular facets are oriented medially. The cuboid facet is facing 14