New material of the trechnotherian mammal Lactodens from the Early Cretaceous Jehol Biota: Comparison with Origolestes and implications for mammal evolution

Acta Palaeontol. Pol. 67 (1): 135–153, 2022 https://doi.org/10.4202/app.00918.2021 New material of the trechnotherian mammal Lactodens from the Early Cretaceous Jehol Biota: Comparison with Origolestes and implications for mammal evolution FANGYUAN MAO, CUNYU LIU, and JIN MENG Mao, F., Liu, C., and Meng, J. 2022. New material of the trechnotherian mammal Lactodens from the Early Cretaceous Jehol Biota: Comparison with Origolestes and implications for mammal evolution. Acta Palaeontologica Polonica 67 (1): 135–153. A new specimen of Lactodens sheni, the only known spalacolestine from the Early Cretaceous Jehol Biota, is reported from the Jiufotang Formation, Liaoning, China. The description focuses on the dental and mandibular morphologies from both the new specimen and the holotype, particularly those that were unknown or poorly known from the holotype when the taxon was established. As revealed primarily by high-resolution computed tomography, morphologies and size gradient of the lower molars and detailed features of the mandibles, such as the masseteric foramen, can be unequivocally described. The dental and mandibular morphologies of Lactodens are compared with those of Origolestes lii, also from the Jehol Biota; these two taxa represent by far the best specimens in Spalacotheriidae and Zhangheotheriidae, respectively, and could be used as the representatives of their own groups in future higher-level phylogenetic analysis of mammals. The two taxa display considerable differences in dental and mandibular features, probably indicating a deeper split of spalacotheriids and zhangheotheriids than previously thought. Absence of the Meckelian groove in Lactodens, contrasting to the distinct one that holds a sizable Meckel’s cartilage in adult Origolestes, suggests an independent evolution of the definitive mammalian middle ear within “symmetrodontans”. The morphological gradient in the tooth row and association of the upper and lower dentitions from the same individual animal are also instructive for interpreting molar variations and evolution in “symmetrodontans” and mammals. Key words: Mammalia, Symmetrodonta, Spalacotheriidae, Zhangheotheriidae, dental morphology, mandible, middle ear, Cretaceous, Yixian Formation, China. Fangyuan Mao [[email protected]], Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China; Division of Paleontology, American Museum of Natural History, New York, New York 10024, USA. Cunyu Liu [[email protected]], Beipiao Pterosaur Museum of China, Beipiao County, Liaoning Province, 122100, China. Jin Meng [[email protected]g] (corresponding author), Division of Paleontology, American Museum of Natural History, New York, New York 10024, USA; Earth and Environmental Sciences, Graduate Center, City University of New York, New York, 10016, USA. Received 17 June 2021, accepted 27 September 2021, available online 30 March 2022. Copyright © 2022 F. Mao et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction “Symmetrodonta” is a group of small Mesozoic mammals, characterized by a simple reversed-triangle molar pattern and has been considered as non-monophyletic (Kielan-Jawo rowska et al. 2004). “Symmetrodontans” have been divided into two subgroups: one with “obtuse-angled” molars and the other with “acute-angled” molars. The obtuseangled “symmetrodontans” are those commonly placed in Kuehneotheriidae and Tinodontidae, some of which are among the earliest members of mammals, such as Kuehneotherium (Kermack et al. 1968) and Woutersia (Sigogneau-Russell 1983). The acuteangled “symmetrodontans” include two Cretaceous families: Zhangheotheriidae and Spalacotheriidae; they are characterized by possessing molars with relatively acute triangulation that decreases distally in the tooth row (Fox 1976, 1985; Sigogneau-Russell and Ensom 1998; Cifelli 1999; Cifelli and Gordon 1999; Cifelli and Madsen 1999; Cifelli et al. 2014; Ensom and Sigo gneau-Russell 2000; Averianov 2002; Gill 2004; Rou gier et al. 2003a; Tsubamoto et al. 2004; Hu et al. 1997, 2005a; Lopatin et al. 2005, 2010; Li and Luo 136 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 2006; Sweetman 2008; Cuenca-Bescós et al. 2014; Han and Meng 2016). Because of their small size and delicate skeletons, most “symmetrodontans” are known from isolated teeth and/or fragmentary lower jaws; thus, “symmetrodontans” are commonly ignored in general treatments of mammalian relationships, despite their widely acknowledged importance (Cifelli and Gordon 1999). The taxonomic relationships within Zhangheotheriidae and Spalacotheriidae also remained unstable. In general, zhangheotheriids are considered to be more primitive than spalacotheriids (Cifelli and Madsen 1986, 1999; Tsubamoto et al. 2004; Li and Luo 2006; Sweetman 2008; Averianov and Lopatin 2011; Averianov et al. 2013; Bi et al. 2016; Han and Meng 2016; Mao et al. 2020). Despite of the fragmentary nature of fossils and phylogenetic uncertainties, “symmetrodontans” have been considered as an important group because their reversed-triangle molar pattern has been regarded as being intermediate between the “triconodont” tooth pattern in basal mammaliaforms and the tribosphenic pattern that characterizes therians ancestral to marsupials and placentals (Patterson 1956; Hopson and Crompton 1969; Crompton and Sita-Lumsden 1970; Crompton 1971; Cassiliano and Clemens 1979; Hopson 1994; Cifelli and Madsen 1999; Cifelli and Gordon 1999; Rougier et al. 2003a, b). Functionally, the “symmetrodontan” tooth pattern represents a fundamental step in the evolution of mammalian molar design (Kielan-Jaworowska et al. 2004). Several “symmetrodontans” have been reported from the Jehol Biota. These include Zhangheotherium quinquecuspidens (Hu et al. 1997), Maotherium sinense Rougier et al. 2003b), M. asiaticum (Ji et al. 2009), Akidolestes cifellii (Li and Luo 2006), Anebodon luoi (Bi et al. 2016), Lactodens sheni (Han and Meng 2016) and Origolestes lii (Mao et al. 2020). Of these forms, the genera Akidolestes and Lactodens were placed in Spalacotheriidae and the rest were grouped in Zhangheotheriidae. Lactodens is so far the only spalacolestine known from the Jehol Biota. Spalacolestinae was based on the type genus Spalacolestes as a subfamily within Spa lacotheriidae (Cifelli and Madsen 1999). This subfamily contains about 10 genera of small insectivores that are recovered from the Cretaceous strata of Europa, Asia, and North America. As in all “symmetrodontans”, they are dentally characterized by having the reversed-triangle molar pattern but differ from other “symmetrodontans” in having higher crowned molars with a more acute angle in cusp arrangement in the middle of the tooth series (Cuenca-Bescós et al. 2014). Originally known as a North American endemic group (Patterson 1955; Fox 1976; Cifelli 1990; Cifelli and Madsen 1999), spalacolestines have now been recognized as having a pan-Laurasian distribution during the Cretaceous (Nessov 1997; Averianov 2002; Hu et al. 2005a; Tsubamoto et al. 2004; Sweetman 2008; Cuenca-Bescós et al. 2014; Han and Meng 2016). Most spalacolestine species are represented by fragmentary fossils, commonly isolated teeth. Lactodens is the only known spalacolestine that is represented by associated upper and lower teeth, partial cranial and mandibular material, and some postcranial elements from one individual. Here we report a new specimen of Lactodens sheni Han and Meng, 2016, recovered from the Jiufotang Formation at Dapingfang locality, Chaoyang, Liaoning, China, that is close to the type locality Shangheshou. We employed computed tomography to obtain images from the new specimen and the holotype. Our description focuses on the lower molars and other parts that were not preserved or poorly known in the holotype, in the hope that the morphology of Lactodens can be better documented. With the new data, we are able to compare the dental and mandibular morphologies between Lactodens sheni and Origolestes lii, which represent the two main subgroups of “symmetrodontans,” the zhangheotheriids and spalacotheriids, respectively. Because each species is based on relatively complete material from more than one specimen, the characters and variation displayed in the two taxa are better sampled; as such, the two taxa could serve as representatives for “symmetrodontans” in higher-level phylogenetic analyses of mammals. We further discuss implications of the new data to the general evolution of mammals, including the dentition and the middle ear. Institutional abbreviations.—HG-M, Bohai University Pale on tology Center, Jinzhou City, Liaoning Province, China; IVPP, Insti tute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; ZGY, Beipiao Pterosaur Museum of China, Beipiao, China. Other abbreviations.—DMME, definitive mammalian middle ear; OMC, ossified Meckel’s cartilage; TMME, transitional mammalian middle ear. We follow standard convention in abbreviating tooth families as I, C, P, and M, with upper and lower case letters referring to upper and lower teeth, respectively. Material and methods The new specimen (ZGY0053, Fig. 1) is represented by a partial skeleton preserved in a pair of split slabs. While much of the skeletal remains was broken, the mandibles and teeth were preserved in good condition, supplementing additional morphologies to the holotype (HG-M016, Fig. 2A). As in the holotype, it is highly probable that the upper and lower dentitions were preserved roughly in their anatomical positions so that the corresponding pairs of upper and lower teeth and their occlusal relationships can be observed and interpreted with confidence. Unlike the holotype that has been needle-prepared to provide access to the teeth embedded in the matrix, we avoided preparing the new specimen because of its small size and fragile remains. We employed computed tomography (CT) and laminography (CL) to obtain the internal morphologies of the mandibles as well as the morphology of dental and mandibular parts that are still embedded in MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 137 the matrix for both the new specimen and the holotype. We also include the zhangheotheriid Origolestes lii Mao, Hu, Li, Wang, Chase, Smith, and Meng, 2020, in the comparison, with the focus on IVPP V13604, the specimen for which the best images of tooth morphology were provided; other specimens of O. lii were presented in Mao et al. (2020). Imaging and figures.—Optical images of the new specimen (ZGY0053) were taken using a Canon Digital camera with a macro lens. All the high-resolution micro-CT scanning for the two specimens (HG-M016 and ZGY0053) was conducted using a GE v|tome|x m dual tube 240/180kV system (General Electric, Fairfield, CT, USA) in the Yinghua Inspection and Testing (Shanghai) Co., Ltd and the Key Laboratory of Vertebrate Evolution and Human Origins, Chinese Academy of Sciences. The two specimens were scanned using the 240kv microfocus tube at resolution of a 9.53 μm per voxel for the skeleton of HG-M016 using beam energy for 140 kV and 120 μa, 5.204 μm per voxel for the skull of ZGY0053 using beam energy for 140 kV and 140 μa. To improve the signal-to-noise ratio, 1800 projections were collected, for 333–2000 ms and averaged 2 times. Sometimes, to accommodate the length of the specimen, 2 total areas were scanned in the Y-axis (multiscan) of ZGY0053 to produce the final projection stack, reconstructed using Phoenix datos|x (General Electric, Wunstorf, Germany). All of the segmentation and the rendering of the CT scanning data were processed using VG Studio Max 3.4 (Volume Graphics, Heidelberg, Germany). Measurements.—Because the teeth and mandibles are small and partly embedded in the matrix, the measurements Fig. 1. New specimen of Lactodens sheni Han and Meng, 2016 (ZGY0053) from Aptian, Lower Cretaceous Jiufotang Formation of Dapingfang, Caoyang City, Liaoning Province, China. A. Partial skeleton in dorsal view that is preserved in the main slab. B. CT rendered partial skull; mostly ventral portion with teeth in dorsal (B1) and ventral (B2) views and mainly the skull roof in dorsal (B3) and ventral (B4) views. 138 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 were taken using digital methods from the CT images of the specimen. Linear measurements were taken using the Measurements Menu/Coordinate Measurement module in VGStudio Max 3.4 and/or ImageJ. Each measurement was repeated three times, and the average was used. The measurements are listed in Table 1. Terminology.—We follow Kielan-Jaworowska et al. (2004) for the use of “symmetrodontans” as a descriptive term, knowing that “symmetrodontans” probably do not form a monophyletic group. We follow Rougier et al. (2003a) for the use of Zhangheotheriidae as a family, which was defined as the group “formed by the common ancestor of Zhangheotherium and Maotherium and all its descendants”, although the monophyly of this taxon has not been consistently established (Li and Luo 2006; Sweetman 2008; Averianov et al. 2013; Han and Meng 2016; Mao et al. 2020); thus, we place Spalacotheriidae directly under Trechnotheria. Dental terminology follows Han and Meng (2016: fig. 2), which is based on several studies (see references therein). We follow Rougier et al. (2003b) for identification of cusp B (the cusp mesial to cusp A) in the upper molar of Origolestes lii; the homology of this cusp is still uncertain. Geological setting The holotype and the new specimen (ZGY0053) of Lactodens sheni are both from the Jiufotang Formation, with the former from a nearby Shangheshou locality in Caoyang City, Table 1. Measurements (in mm length/width and angular degree of the trigon and trigonid) of the teeth of Lactodens sheni Han and Meng, 2016 (ZGY0053). *, estimated; ?, unknown. Teeth Left Right Upper I3 0.248*/0.174* ? C 0.209*/0.268* 0.47*/0.231* P1 0.564*/0.327 0.853/0.261 P2 0.955/0.362 0.903/0.314 P3 1.19/0.41 1.189/0.446 M1 1.448/0.729 (107°) 1.468/0.756 (125°) M2 1.232/0.827 (74°) 1.203/0.946 (67°) M3 1.051/1.036 (55°) 1.009/1.081 (49°) M4 0.909/1.147 (47°) 0.886/1.137 (40°) M5 0.828/1.109 (45°) 0.788/1.083 (41°) M6 0.564/0.648 (41°) 0.558/0.739 (41°) Lower c 0.351/0.179 0.355/0.157 p1 0.582/0.216 0.541/0.179 p2 0.716/0.216 0.698/0.237 p3 1.087/0.311 1.064/0.326 p4 1.374/0.395 1.361/0.342 p5 1.604/0.611 1.56/0.5 m1 1.258/0.803 (104°) 1.286*/0.65* (?) m2 0.888/0.962 (61°) 0.91/0.813 (63°) m3 0.838/0.984 (49°*) 0.831/0.906 (50°) m4 0.877/1.002 (56°) 0.878/0.75 (62°*) m5 0.643*/0.638* (42°*) 0.756/0.72 (56°) m6 0.481/0.487 (85°) 0.542/0.358 (72°) Fig. 2. CT-images of the trechnotherian mammal Lactodens sheni Han and Meng, 2016, from Aprian, Lower Cretaceous Jiufotang Formation, Caoyang City, Liaoning Province, China. A. Holotype (HG-M016) from Shangheshou area; A1, virtual skull remains with the lingual side of the right mandible exposed (see Han and Meng 2016: fig. 1); A2, virtual skull remains with the lateral side of the right mandible exposed (this side of the skull is embeded in the matrix and not visible except for the part of cheek teeth that were needle-prepared from the back side of the slab (Han and Meng 2016: figs. 1, 5); A3, a CT-slice showing the contrast of bone and matrix and revealing the roots of some teeth. B. ZGY0053 from Dapingfang, a CT-section through the skull (showing the root conditions). Note there is no tooth germ under any cheek teeth in both specimens in A3 and B. MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 139 Fig. 3. CT-rendered lower jaws and dentitions of the trechnotherian mammal Lactodens sheni Han and Meng, 2016 (ZGY0053), from Aptian, Lower Cretaceous Jiufotang Formation of Dapingfang, Caoyang City, Liaoning Province, China. A1–A3, left upper and lower dentition and dentary in medial (only teeth), lateral, and occlusal views, respectively. A4–A6, right upper and lower dentition and dentary in lateral, medial, and occlusal views, respectively. Note: angular end is not the mandibular condyle (see Fig. 6); coronoid tubercle, mis-labelled as “cp” in Han and Meng (2016: fig. 10). Upper and lower teeth in A1, A2, A4, A5 are in the original position that were preserved. Arrows in A2, A4 point to mental foramina (note the difference between the two mandibles). Question mark in A5 points to a faint groove that is not the Meckelian groove (compared to that of Origolestes lii in Fig. 6B). 140 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 Liaoning Province. The Jehol Biota consists of fossils from Jiufotang, Yixian (west Liaoning), and Huajiying (Hebei) formations; these lithological units range chronologically from the youngest to the oldest (Pan et al. 2013; Yang et al. 2020) and the temporal interval was considered to be at least 10 Ma (130–120 Ma) (Pan et al. 2013) or 15 Ma (135–120 Ma) (Yang et al. 2020) with the lower Jiufotang Formation constrained as ~120 Ma (He et al. 2004; Chang et al. 2009). Fossils from the three lithological units were considered representing three evolutionary stages, with the Jiufotang stage being the youngest (Yang et al. 2020). In addition to Lactodens sheni, other mammals from the Jiufotang Formation include the eutriconodontan Liaoconodon hui (Meng et al. 2011) and the multituberculates Jeholbaatar kielanae (Wang et al. 2019) and Sinobaatar pani (Mao et al. 2020). Most recently, the tritylodontid Fossiomanus sinensis (Mao et al. 2021) has been reported from the Jiufotang Formation and was considered to be one of the youngest known tritylodontids, probably younger than the tritylodontid from the Kuwajima Formation (Barremian–Aptian) in central Japan (Matsuoka et al. 2016); thus, F. sinensis represents the most recent relictual member of non-mammaliaform synapsids known to date. Systematic palaeontology Mammalia Linnaeus, 1758 Trechnotheria McKenna, 1975 Spalacotheriidae Marsh, 1887 Spalacolestinae Cifelli and Madsen, 1999 Genus Lactodens Han and Meng, 2016 Type species: Lactodens sheni Han and Meng, 2016, monotypic; see below. Diagnosis.—As for the type species. Lactodens sheni Han and Meng, 2016 Figs. 1–5. Holotype: HG-M016, a partial skeleton with partial cranium and associated dentaries. Type locality: Dapingfang, Caoyang City, Liaoning Province, China. Type horizon: Aptian, Lower Cretaceous Jiufotang Formation. Material.—Holotype and new specimen (ZGY0053), a partial skeleton with partial cranium and associated mandibles from Early Cretaceous (Aptian) Jiufotang Formation, Shangheshou area, Caoyang City, Liaoning Province, China. Emended diagnosis.—A spalacolestine with the dental formula I3-C1-P3-M6/i3-c1-p5-m6, double-rooted upper and lower canine and postcanine teeth, distinct diastemata present between upper premolar series but absent in the lower series, extremely low-crowned and transversely narrow premolars, M1 and m1 with wide angulation that decreases distally, ultimate molars considerably smaller than penultimate ones, cusp C weak in mesial upper molars, mandibular condyle without a neck, presence of the masseteric foramen and lack of the Meckelian groove on the medial rear surface of the dentary bone. Description.—Cranium (Figs. 1, 2): The skull of ZGY0053 was split along the horizontal plane so that the mandible and nearly all teeth were preserved in the main slab and the skull roof, the palate, some dorsal cranial parts, and a few incisors were preserved in the counterpart. Although the cranium was fractured, its general shape in dorsal or ventral view is discernable, similar to that of the holotype. It has a narrow and long rostrum with the facial region gradually flaring toward the zygomatic arch. The arch is slim with its anterior root lateral to M6 and gently curves and ends posteriorly to the glenoid fossa, as shown in the holotype. The glenoid fossa is shallow with its lateral edge more convex than its medial edge and bounded posteriorly by a low postglenoid process or ridge. The occipital condyles are proportionally large, oval-shaped, and widely separated so that the foramen magnum appears large. The anterior edge of the foramen magnum forms a curved notch that intrudes anteriorly in the basioccipital. The rest of the basicranial region in both specimens is too fragmentary to be informative. On the dorsal side (in the counterpart of the slab), the nasals, frontals and maxillae are preserved; the nasals are narrow and long and expand posteriorly. The anterior portions of the frontals form a triangular wedge that inserts between the nasals. In the rostrum the internarial process, a narrow bar formed by the premaxillae, is present and separates the external nares into two openings. It was previously proposed that the reduction of the internarial process and the concomitant coalescence of the nares into a single apertura nasi are new features of eutriconodonts, multituberculates and crown mammals (Ruf et al. 2014; references therein). Thus, if identified correctly, this internarial process represents a primitive condition in mammals. On the ventral side of the rostrum, the alveoli of the incisors show that the upper incisors are separated by small diastemata and I2 is the largest one of the three upper incisors. A few small loose teeth are presumably upper incisors except for a double-rooted one that may be the canine. On the palate, which is heavily fractured, there are signs of pits lingual to the upper molars; these are likely the pits for receiving the tall cusps of the lower molars, a feature observed in some other Mesozoic taxa, such as species of Origolestes (Mao et al. 2020), Morganucodon (Jäger et al. 2019), and Repenomamus (Hu et al. 2005b). Mandible (Figs. 1–3, 6): In ZGY0053 the two mandibles and most teeth are preserved in the main slab. Because the skull roof was peeled off (preserved in the counterpart slab), the exposed skull fragments and roots of upper teeth are exposed in dorsal view. The two mandibles were flipped down medially in preservation and their lingual sides are exposed in the main slab (Fig. 1). The buccal sides of all mandibles in the holotype and the new specimen are embedded in the matrix so that the morphology in this view was little known when Lactodens sheni was first reported (Han and Meng 2016); it became available owing to CT-imaging in this study. MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 141 Fig. 4. Upper molars of the trechnotherian mammal Lactodens sheni Han and Meng, 2016, from Aptian, Lower Cretaceous Jiufotang Formation, Caoyang City, Liaoning Province, China. A. Holo type (HG-M016) from Shangheshou area; right ultimate premolar and M1–M6 in buccal (A1) and occlusal (A2) views, showing the corresponding cusps on different teeth. B. ZGY0053 from Dapingfang; left upper molars (M1–M6) in occlusal (B1), buccal (B2), and lingual (B3) views. Image in B3 is horizontally flipped for convenience of comparison. 142 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 Compared to a slender and long horizontal ramus the masseteric fossa is broad; it is delimited by a blunt anterior ridge and a ventral ridge that is blunt anteriorly and crest-like posteriorly. The ultimate lower molar is mesial to the anterior ridge. On the medial surface of the dentary, there is no Meckelian groove, as noted in the holotype (Han and Meng 2016). There is a faint groove when the mandible is viewed with the light coming in certain angles (see Fig. 3A6), but this is certainly not the Meckelian groove. At the root of the anterior ridge the masseteric foramen opens in the anterior end of the fossa. The posterior end of the fossa is squared up and the posteroventral corner is bounded by thickened bone that may be misinterpreted as the condyle if only this part was preserved (Fig. 3A4). We term this thickened area as the “angular end” (Figs. 2, 6). The condyle is dorsal to the angular end and does not have a neck; it is transversely widened and has a convex articular surface. The coronoid process is broad and tilts at about 64° (measured as the smaller angle between the anterior ridge of the process and the horizontal axis through the alveoli; or the larger angle is 116°; Han and Meng 2016). The mandibular notch, the space between the coronoid process and the condyle, is broad and shallow. There are multiple mental foramina on the buccal surface of the mandible and the number varies even in the two mandibles of the ZGY0053, in the left mandible there are at least six mental foramina. The posterior-most one, also the largest one, is consistently below p5 in the ZGY0053 mandibles and in the holotype. Each mental foramen leads anteriorly to a narrow groove. CT-scans revealed that there are two roots for each upper or lower postcanine tooth, arranged mesiodistally. The roots are short relative to the height of the tooth crown and flare at the distal ends. The roots of the lower teeth end the roof of the mandibular canal. There is no evidence for tooth germs of erupting teeth in the dentary bone under the upper and lower dentition. The rest of the mandible of Lactodens sheni has been described by Han and Meng (2016). Upper teeth (Figs. 3, 4, 7): In the ZGY0053 the tip of the skull was broken so that the upper incisors were poorly preserved in the counterpart slab, whereas only fragments of lower incisors were preserved. The canines and postcanine teeth in both the upper and lower jaws are in good condition and show the same tooth formula and similar tooth morphologies as in the holotype, which allow us to assign the new specimen to Lactodens sheni (Han and Meng 2016). The new specimen confirms some unusual features observed in the holotype, such as the relatively large diastemata between the upper premolars, different upper and lower premolar numbers, and the low crown premolars that have the shape of deciduous teeth. In addition to the dental morphology and diastema that were used to distinguish the molars from premolars, we add here that the two roots of the molars are more closely positioned mesiodistally, each being transversely widened, compared to those of the premolars. However, the identification of the tooth as M1 instead of P4 is based on our understanding of the tooth morphologies (Han and Meng 2016; this study); this tooth may prove to be a P4 in future study. With the CT-scan rendered tooth crowns, we provide correlated cusp labels along the tooth row from the ultimate premolar to the molars to illustrate homologous cusp identification on different teeth. For instance, cusp B in P3 is considered as equivalent to the stylocone in the molars. The ultimate premolar can be considered as having a typical triconodont tooth pattern with the cusps aligned in a row on a transversely compressed tooth crown. M1 shows a transitional cusp arrangement, which is “obtuse-angled” and lies morphologically in between the premolars and the distal molars. Following the convention of tribosphenic and tribosphenic-like mammals, as well as their putative sister groups (Crompton and Jenkins 1968; Davis 2011, but see an alternative hypothesis in Rougier et al. 2003b), we consider cusp A to be equivalent to the paracone, cusp B to the stylocone, and cusp C to the metacone. In occlusal view the outline of the upper molar has a skewed triangular shape. The embrasure between adjacent upper molars has a similar but reversed shape of a skewed triangle that matches the shape of the lower molar. The preparacrista is transversely straight, extending from cusp A to B. The postparacrista is longer and extends diagonally to cusp C or the metastyle (or C plus D) at the distobuccal corner in distal molars. This configuration is most characteristic of M2 and M3. Cusp B may be traced in all molars. Cusp C is discernable on M1–M3 of the holotype but fades away on M4–M6. Cusp C is not discernable on M2–M6 in the ZGY0053, which is partly attributable to the lower resolution of the CT-san of that specimen. From M1 to M3 of the holotype, cusp C migrates buccally away from cusp A. Cusp C may be interpreted as either lost or merged with cusp D on M4–M6 in Lactodens sheni. If this interpretation is correct, it implies that the metastyle may be homologized to cusp D or C plus D in L. sheni. On the preparacrista there is no cusp. The parastyle is small; it may also be lost or merged with cusp B in distal molars. The ectoflexus is shallow and the distal stylar cusp is low, extending mesiodistally along the buccal edge of the tooth crown; it is distinct on M2–M4 but nearly absent in distal molars. The cingulum is not strong but complete; in several teeth it was broken away. In buccal or lingual view, cusp A is not symmetrical; its mesial side is convex and steep, whereas its distal side is concave and more gently sloping. As revealed by CT-images for both the holotype and the new specimen, the upper molars have two roots, arranged mesiodistally. The roots are transversely wide (mesiodistally compressed), with the distal one being narrower than the mesial one; this is particularly distinct in M3–M5. A detailed description of the upper teeth of Lactodens sheni has been provided by Han and Meng (2016). Lower teeth (Figs. 3, 5, 7): In the lower dentition, the incisors, canine, p2 and p4–m1 were preserved in the holotype of Lactodens sheni and described by Han and Meng (2016); the other teeth were not preserved or broken. In the ZGY0053 all the lower teeth (c, p1–p5, m1–m6) were MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 143 Fig. 5. Lower teeth of the trechnotherian mammal Lactodens sheni Han and Meng, 2016 (ZGY0053) from Aptian, Lower Cretaceous Jiufotang Formation of Daping fang, Caoyang City, Liaoning Province, China), right c–m6 in lingual (A1) and occlusal (A2) views (with the dentary bone segmented away) and in posterobuccal view (A5), left p5–m6 in buccal (A3) and occlusal (A4) views. See also Figs. 3 and 7. 150 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 Finally, in addition to the tooth-to-tooth occlusal relationship that has been examined in detail in interpreting tooth evolution toward the tribosphenic pattern (Crompton 1971; Davis 2011; Schultz and Martin 2014), the dentitions as a whole also provide information about tooth occlusion. In Lactodens (and Origolestes as well) the upper and lower dentitions from a single individual are available and preserved in nearly original occlusal relationship. Although the upper and lower molars are equal in number (6 to 6), the ultimate lower molar is a half tooth distal to its upper counterpart when the dentitions are in occlusion, or the ultimate upper molar fits in the embrasure between the ultimate and penultimate lower molars. This is in contrast to the dentition of the tribosphenic pattern, in which the ultimate upper molar is always a half tooth distal to the lower one because the protocone of the former must fit in the talonid basin of the ultimate lower molar; thus, the entire ultimate upper molar is distal to the trigonid of the ultimate lower molar. The same relationship is present in Origolestes lii and other “symmetrodontans” where the upper and lower dentitions are known from a single individual. In most cases there is at least one more lower molar compared to the upper molar count; in these forms the ultimate lower molar would be the distalmost tooth when the dentitions are in occlusion. Regardless the complicated evolutionary pattern in the upper molar, the trigonid of the tribosphenic lower molar is presumably homologous to the entire lower molar of “symmetrodontans” and the main shearing facet (facet 1) in relation to the protoconid (cusp a) and metaconid (cusp c) remains continuously traceable throughout holotherians (Crompton and Jenkins 1968; Crompton 1971; Rougier et al. 2003b; Davis 2011; Schultz and Martin 2014). Thus, during the evolution from the reversed-triangular tooth to the tribosphenic pattern within trechnotherians, the ultimate lower molar would be the one that had to be lost in the dentition; or with the development of the talonid on the lower molar, the presence of the ultimate lower molar distal to the ultimate upper one became functionally meaningless. This perhaps indicates that the occlusal relationship of upper and lower teeth and the number of upper vs. lower teeth are coupled. A similar condition (at least one additional lower molar, compared to the upper molar count) may also be present in eutriconodontans, as shown in several taxa where the upper and lower dentitions from a single individual are unequivocally known (Ji et al. 1999; Hu et al. 2005b; Meng et al. 2011; Hou and Meng 2014). The ultimate lower molar as the distalmost tooth may be regarded as a primitive feature for trechnotherians. Meckelian groove.—The evolution of the mammalian middle ear consists of at least two steps that involve complicated changes of the postdentary bones, jaw joint and basicranial region (Meng et al. 2011). The first step is from the mandibular middle ear to the transitional mammalian middle ear (TMME), which marks the separation of the postdentary bones from the dentary bone. The second step is from the TMME to the definitive mammalian middle ear (DMME; Allin 1975; Allin and Hopson 1992), which is characterized by the resorption of the Meckel’s cartilage and full separation of the auditory bones from the mastication process. Fossils of “symmetrodontans” document changes in the second evolutionary step in which the postdentary trough is absent, but the Meckelian groove is present in some taxa and held an OMC (Wang et al. 2001; Meng et al. 2003). The Meckelian groove on the medial surface of the dentary bone is observed in relatively early branching “symmetrodontans” (Simpson 1925a; Cassiliano and Clemens 1979; Hu et al. 1997; Cifelli and Madsen 1999; Averianov 2002; Meng et al. 2003; Ji et al. 2009). Recently, Mao et al. (2020) reported several specimens of Origolestes lii that have well-preserved lower jaws and OMCs. These specimens convincingly demonstrate that the groove accommodated a sizable OMC that was probably held in the groove by soft tissue in life. However, as a close relative of O. lii in the Jehol Biota, Lactodens sheni does not have the Meckelian groove. Absence of the groove has also been recognized in other spalacotheriids where the dentary bone is adequately preserved, such as species of Spalacolestes (Cifelli and Madsen 1999), Heishanlestes (Hu et al. 2005a), and Yaverlestes (Sweetman 2008). The non-grooved condition, as represented by L. sheni, suggests lack of the OMC in adults, which further implies acquisition of the DMME in spalacotheriids. A Meckelian groove similar to that of Origolestes lii is present in many stem-group therian mammals that are dentally more advanced than “symmetrodontans”, such as species of Palaeoxonodon (Close et al. 2016; Panciroli et al. 2018), Amphitherium (Prothero 1981; Butler and Clemens 2001), Nanolestes (Martin 2002), peramurans (Davis 2012), Kielantherium (Dashzeveg and Kielan-Jaworowska 1984), and Prokennalestes (Kielan-Jaworowska and Dashzeveg 1989). It may be inferred that an OMC similar to that of Origolestes was present in these forms and that the DMME of therians could have been derived from the configuration documented in O. lii. Thus, the possible acquisition of the DMME in spalacotheriids implies at least another independent evolution of the fully suspended middle ear that took place within “symmetrodontans”, provided that spalacotheriids form a monophyletic group. Independent evolution of the DMME refers to the processes in which the postdentary bones became separated from the dentary, and which involve two steps, as mentioned above. In O. lii, the bony contact between the OMC and auditory bones was lost but the OMC is still present, perhaps functioning as a support structure for the auditory bones through a connection of soft tissue (Mao et al. 2020). This could be viewed as either the end state of the TMME or the beginning of the DMME, which is different from the condition present in eutriconodontans, such as Liaoconodon hui; in the latter, a substantial contact existed between the OMC on the one hand and the long anterior processes of the malleus and ectotympanic on the other hand (Meng et al. 2011). Thus, MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 151 the evolutionary change from the O. lii condition to that of Lactodens sheni can be narrowed down to the loss of the OMC in adult and securely housing of the auditory bones at the basicranial region. A similar process could have taken place independently in therians. Morphological disparity.—As already noted, spalacotheriids differ from zhangheotheriids in having higher crowned molars with a more acute angle in cusp arrangement in the middle of the tooth series (Cuenca-Bescós et al. 2014). As also noted by Cifelli and Madsen (1999), species of Zhangheotherium (Maotherium as well) is strikingly atypical in having features such as the complete lack of cingula on the lower molars. Primitively, Zhangheotherium quinquecuspidens has rounded, conical molar cusps that lack connecting crests (Hu et al. 1997). These features show that zhangheotheriids have not achieved the dental specialization characteristic for Spalacotheriidae (Averianov 2002). Our comparison shows that the two families are similar mainly in the development of main cusps (cusp a, b, and c) on the lower molars, but they differ in nearly all other aspects of dental morphology. Furthermore, the dental structures are consistently associated with the mandibular structures. The size, shape and orientation of the coronoid process, the mandibular condyle, and the masseteric and pterygoid fossae are distinctive between the two families. It appears that the mandibular configuration in each species has been specialized in its own way. In particular, the mandibular structures of Origolestes lii are quite unusual in that the mandibular condyle has a long and curved peduncle and that the coronoid process is a narrow strip that extends posteriorly over the condyle and defines a narrow mandibular notch. These features appear unique among mammals. Lactodens sheni is the only spalacolestine from a stratigraphically younger unit and the several species of zhangheotheriids are from the Yixian Formation in the Jehol Biota, but the morphological differences between them may not be explained by the temporal difference. If spalacotheriids were derived from zhangheotheriids, as suggested by some phylogenetic analyses (Han and Meng 2016), the evolutionary changes of the teeth and mandibles would be remarkable. The observed differences probably indicate a deep split between the two groups, perhaps deeper than previously thought, and such a deep split has been shown by the phylogenetic analysis of Averianov et al. (2013). The general tooth morphology and small body size of Lactodens sheni and Origolestes lii suggest an insectivorous lifestyle. Han and Meng (2016) speculated on the dietary adaptation of L. sheni based on its dental and mandibular morphologies and body size. Compared to O. lii, whose conical teeth appear specialized for piercing or puncturing, the dentition with well-developed crests in L. sheni suggests embrasure shearing. These two kinds of dental morphologies, along with the mandibular features and orientations of the jaw muscles (inferred from the bony structures), form two complexes that show different biomechanical dynamics of mastication in food processing, which would argue for different habitat preferences between these two species and between spalacotheriids and zhangheotheriids. A more sophisticated analysis about the biomechanics of the teeth and mandibles, as done for species of Morganucodon and Kuehneotherium (Gill et al. 2014), may better reveal dietary preferences between the two forms. It has also been suggested that the long-term survival of spalacotheriids is linked to their ability to consume food resources not readily available to other mammals in the ecosystem (Averianov 2002). Although this is difficult to test, the coexistence of L. sheni and zhangheotheriids with other mammals (multituberculates, eutriconodontans, therians; Meng 2014) in the Jehol Biota, as well as their high masticatory disparities, are consistent with partitioning of the food resources. Acknowledgements We greatly appreciate the opportunity to contribute to this special volume honoring Richard L. Cifelli for his contribution to the study of Mesozoic mammals. We thank Shuhua Xie (IVPP) for specimen preparation; Shuyan Hou (Yinghua Inspection and Testing Co., Ltd., Shanghai, China), Yemao Hou, Pengfei Yin (IVPP) for CT scanning of the specimens; Wei Gao (IVPP) for photograph; Zhonghe Zhou and XiaolingWang (both IVPP) for discussions of localities. We are grateful to Julia A. Schultz (Universität Bonn, Germany), Alexander O. Averianov (Zoological Institute RAS, Saint Petersburg, Russia), and Brian M. Davis (University of Louisville School of Medicine, USA) for constructive comments and Olivier Lambert (Institut royal des Sciences naturelles de Belgique, Brussels, Belgium) for editorial comments that have improved the science and writing of the work. This work was supported by the National Natural Science Foundation of China (42122010; 41404022; 41688103); the Youth Innovation Promotion Association CAS (2019076); and the Kalbfleisch Fellowship, Richard Gilder Graduate School, American Museum of Natural History. References Allin, E.F. 1975. Evolution of the mammalian middle ear. Journal of Morphology 147: 403–437. Allin, E.F. and Hopson, J.A. 1992. Evolution of the auditory system in Synapsida (“mammal-like reptiles” and primitive mammals) as seen in the fossil record. In: D.B. Webster, A.N. Popper, and R.R. Fay (eds.), The Evolutionary Biology of Hearing, 587–614. Springer, New York. Archibald, J.D. and Averianov, A.O. 2006. Late Cretaceous asioryctitherian eutherian mammals from Uzbekistan and phylogenetic analysis of Asioryctitheria. Acta Palaeontologica Polonica 51: 351–376. Averianov, A.O. 2002. Early Cretaceous symmetrodont mammal Gobiotheriodon from Mongolia and the classification of Symmetrodonta. Acta Palaeontologica Polonica 47: 705–716. Averianov, A.O. and Lopatin, A. 2008. “Protocone” in a pretribosphenic mammal and upper dentition of tinodontid “symmetrodontans. Journal of Vertebrate Paleontology 28: 548–552. Averianov, A.O. and Lopatin, A.V. 2011. Phylogeny of triconodonts and symmetrodonts and the origin of extant mammals. Doklady Biological Sciences 436 (1): 32–35. Averianov, A.O., Martin, T., and Lopatin, A.V. 2013. A new phylo geny for basal Trechnotheria and Cladotheria and affinities of South 152 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022 American endemic Late Cretaceous mammals. Naturwissenschaften 100: 311–326. Bi, S.-D., Zheng, X.-T., Meng, J., Wang, X.-L., Robinson, N., and Davis, B. 2016. A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution. Scientific Reports 6: 1–9. Butler, P.M. and Clemens, W.A. 2001. Dental morphology of the Jurassic holotherian mammal Amphitherium, with a discussion of the evolution of mammalian post‐canine dental formulae. Palaeontology 44: 1–20. Cassiliano, M. and Clemens, W. 1979. Symmetrodonta. In: J. Lillegraven, Z. Kielan-Jaworowska, and W. Clemens (eds.), Mesozoic Mammals: The First Two-thirds of Mammalian History, 150–161. University of California Press, Berkeley. Chang, S.C., Zhang, H., Renne, P.R., and Fang, Y. 2009. High-precision 40Ar/39Ar age for the Jehol Biota. Palaeogeography, Palaeoclimatology, Palaeoecology 280: 94–104. Cifelli, R.L. 1990. Cretaceous mammals of southern Utah. III. Therian mammals from the Turonian (early Late Cretaceous). Journal of Vertebrate Paleontology 10: 332–345. Cifelli, R.L. 1999. Therian teeth of unusual design from the mid-Cretaceous (Albian–Cenomanian) Cedar Mountain Formation of Utah. Journal of Mammalian Evolution 6: 247–270. Cifelli, R.L. and de Muizon, C. 1997. Dentition and jaw of Kokopellia juddi, a primitive marsupial or near-marsupial from the medial Cretaceous of Utah. Journal of Mammalian Evolution 4: 241–258. Cifelli, R.L. and de Muizon, C. 1998. Marsupial mammal from the Upper Cretaceous North Horn Formation, central Utah. Journal of Paleontology 72: 532–537. Cifelli, R. and Gordon, C. 1999. Symmetrodonts from the Late Cretaceous of Southern Utah, and comments on the distribution of archaic mammalian lineages persisting into the Cretaceous of North America. Geology Studies, Brigham Young University 44: 1–16. Cifelli, R.L. and Madsen, S.K. 1986. An Upper Cretaceous symmetrodont (Mammalia) from southern Utah. Journal of Vertebrate Paleontology 6: 258–263. Cifelli, R.L. and Madsen, S.K. 1999. Spalacotheriid symmetrodonts (Mammalia) from the medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local fauna, Cedar Mountain Formation, Utah, USA. Geodiversitas 21: 167–214. Cifelli, R.L., Davis, B.M., and Sames, B. 2014. Earliest Cretaceous mammals from the western United States. Acta Palaeontologica Polonica 59: 31–53. Cifelli, R.L., Rowe, T.B., Luckett, W.P., Banta, J., Reyes, R., and Howes, R.I. 1996. Fossil evidence for the origin of the marsupial pattern of tooth replacement. Nature 379: 715–718. Close, R.A., Davis, B.M., Walsh, S., Wolniewicz, A.S., Friedman, M., and Benson, R.B. 2016. A lower jaw of Palaeoxonodon from the Middle Jurassic of the Isle of Skye, Scotland, sheds new light on the diversity of British stem therians. Palaeontology 59 155–169. Crompton, A. 1971. The origin of the tribosphenic molar. In: D. Kermack and K. Kermack (eds.), Early Mammals. Zoological Journal of the Linnean Society 50: 65–87. Crompton, A. and Jenkins, F.A. 1968. Molar occlusion in Late Triassic mammals. Biological Reviews 43: 427–458. Crompton, A. and Sita-Lumsden, A. 1970. Functional significance of the therian molar pattern. Nature 227: 197–199. Cuenca-Bescós, G., Canudo, J.I., Gasca, J.M., Moreno-Azanza, M., and Cifelli, R.L. 2014. Spalacotheriid “symmetrodonts” from the Early Cretaceous of Spain. Journal of Vertebrate Paleontology 34: 1427–1436. Dashzeveg, D. and Kielan-Jaworowska, Z. 1984. The lower jaw of an aegialodontid mammal from the Early Cretaceous of Mongolia. Zoological Journal of the Linnean Society 82: 217–227. Davis, B.M. 2011. Evolution of the tribosphenic molar pattern in early mammals, with comments on the “dual-origin” hypothesis. Journal of Mammalian Evolution 18: 227–244. Davis, B.M. 2012. Micro‐computed tomography reveals a diversity of Peramuran mammals from the Purbeck Group (Berriasian) of England. Palaeontology 55: 789–817. Ensom, P. and Sigogneau-Russell, D. 2000. New symmetrodonts (Mammalia, Theria) from the Purbeck Limestone Group, Lower Cretaceous, southern England. Cretaceous Research 21: 767–779. Fox, R.C. 1976. Additions to the mammalian local fauna from the upper Milk River Formation (Upper Cretaceous), Alberta. Canadian Journal of Earth Sciences 13: 1105–1118. Fox, R.C. 1984. A primitive, “obtuse-angled” symmetrodont (Mammalia) from the Upper Cretaceous of Alberta, Canada. Canadian Journal of Earth Sciences 21: 1204–1207. Fox, R.C. 1985. Upper molar structure in the Late Cretaceous symmetrodont Symmetrodontoides Fox, and a classification of the Symmetrodonta (Mammalia). Journal of Paleontology 59: 21–26. Gill, P.G. 2004. A new symmetrodont from the Early Cretaceous of England. Journal of Vertebrate Paleontology 24: 748–752. Gill, P.G. 1974. Resorption of premolars in the early mammal Kuehneotherium praecursoris. Archives of Oral Biology 19: 327–328. Gill, P.G., Purnell, M.A., Crumpton, N., Brown, K.R., Gostling, N.J., Stampanoni, M., and Rayfield, E.J. 2014. Dietary specializations and diversity in feeding ecology of the earliest stem mammals. Nature 512: 303–305. Han, G. and Meng, J. 2016. A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian “symmetrodontans”. Zoological Journal of the Linnean Society 178: 343–380. He, H.Y., Wang, X.L., Zhou, Z.H., Wang, F., Boven, A., Shi, G.H., and Zhu, R.X. 2004. Timing of the Jiufotang Formation (Jehol Group) in Liaoning, northeastern China, and its implications. Geophysical Research Letters 31: L12605. Hopson, J.A. 1994. Synapsid evolution and the radiation of non-eutherian mammals. In: R.S. Spencer (ed.), Major Features of Vertebrate Evolution, 190–219. The Paleontological Society, Knoxville. Hopson, J.A. and Crompton, A. 1969. Origin of mammals. In: T. Dobzhansky, M.K. Hecht, and W.C. Steere (eds.), Evolutionary Biology, 16–72. Appleton-Century-Crofts, New York. Hou, S. and Meng, J. 2014. A new eutriconodont mammal from the Early Cretaceous Jehol Biota of Liaoning, China. Chinese Science Bulletin 59: 546–553. Hu, Y.-M., Fox, R.C., Wang, Y.-Q., and Li, C.-K. 2005a. A new spalacotheriid symmetrodont from the Early Cretaceous of northeastern China. American Museum Novitates 3475: 1–20. Hu, Y.-M., Meng, J., Wang, Y.-Q., and Li, C.-K. 2005b. Large Mesozoic mammals fed on young dinosaurs. Nature 433: 149–152. Hu, Y.-M., Wang, Y.-Q., Li, C.-K., and Luo, Z.-X. 1998. Morphology of dentition and forelimb of Zhangheotherium. Vertebrata PalAsiatica 36 (2): 102–125. Hu, Y.-M., Wang, Y.-Q., Luo, Z.-X., and Li, C.-K. 1997. A new symmetrodont mammal from China and its implications for mammalian evolution. Nature 390: 137–142. Jäger, K., Luo, Z.-X., and Martin, T. 2020. Postcranial skeleton of Henkelotherium guimarotae (Cladotheria, Mammalia) and locomotor adaptation. Journal of Mammalian Evolution 27: 349–372. Ji, Q., Luo, Z.-X., and Ji, S.-A. 1999. A Chinese triconodont mammal and mosaic evolution of the mammalian skeleton. Nature 398: 326–330. Ji, Q., Luo, Z.-X., Zhang, X., Yuan, C.-X., and Xu, L. 2009. Evolutionary development of the middle ear in Mesozoic therian mammals. Science 326: 278–281. Kermack, D.M., Kermack, K., and Mussett, F. 1968. The Welsh pantothere Kuehneotherium praecursoris. Zoological Journal of the Linnean Society 47: 407–423. Kielan-Jaworowska, Z. and Dashzeveg, D. 1989. Eutherian mammals from the Early Cretaceous of Mongolia. Zoologica Scripta 18: 347–355. Kielan-Jaworowska, Z., Cifelli, R.L., and Luo, Z.-X. 2004. Mammals From the Age of Dinosaurs: Origins, Evolution, and Structure. 630 pp. Columbia University Press, New York. MAO ET AL.—TRECHNOTHERIAN MAMMALS FROM CRETACEOUS JEHOL BIOTA 153 Li, G. and Luo, Z.-X. 2006. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439: 195–200. Lopatin, A.V. and Averianov, A.O. 2006. Revision of a pretribosphenic mammal Arguimus from the Early Cretaceous of Mongolia. Acta Palaeontologica Polonica 51: 339–349. Lopatin, A.V. and Averianov, A.O. 2017. The stem placental mammal Prokennalestes from the Early Cretaceous of Mongolia. Paleontological Journal 51: 1293–1374. Lopatin, A.V., Averianov, A.O., Maschenko, E., and Leshchinskiy, S. 2010. Early Cretaceous mammals of Western Siberia: Zhangheotheriidae. Paleontological Journal 44: 573–583. Lopatin, A.V., Maschenko, E., Averianov, A.O., Rezvyi, A.S., Skutschas, P., and Leshchinskiy, S. 2005. Early Cretaceous mammals from Western Siberia: Tinodontidae. Paleontological Journal 39: 523–524. Luo, Z.-X., Chen, P., Li, G., and Chen, M. 2007. A new eutriconodont mammal and evolutionary development in early mammals. Nature 446: 288–293. Mao, F., Hu, Y., Li, C., Wang, Y., Chase, M.H., Smith, A., and Meng, J. 2020. Integrated hearing and chewing modules decoupled as shown in a new Cretaceous mammal. Science 367: 305–308. Mao, F., Zhang, C., Liu, C., and Meng, J. 2021. Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature 592: 577–582. Martin, T. 2002. New stem-lineage representatives of Zatheria (Mammalia) from the Late Jurassic of Portugal. Journal of Vertebrate Paleontology 22: 332–348. Matsuoka, H., Kusuhashi, N., and Corfe, I.J. 2016. A new Early Cretaceous tritylodontid (Synapsida, Cynodontia, Mammaliamorpha) from the Kuwajima Formation (Tetori Group) of central Japan. Journal of Vertebrate Paleontology 36 (4): e1112289. Meng, J. 2014. Mesozoic mammals of China: implications for phylo geny and early evolution of mammals. National Science Review 1: 521–542. Meng, J., Hu, Y., Wang, Y., and Li, C. 2003. The ossified Meckel’s cartilage and internal groove in Mesozoic mammaliaforms: implications to origin of the definitive mammalian middle ear. Zoological Journal of the Linnean Society 138: 431–448. Meng, J., Wang, Y.-Q., and Li, C.-K. 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472: 181–185. Nessov, L. 1997. Cretaceous Non Marine Vertebrates of Northern Eurasia. 218 pp. Institute of Earth Crust, University of Sankt Petersburg, Saint Petersburg. Pan, Y., Sha, J., Zhou, Z., and Fürsich, F.T. 2013. The Jehol Biota: definition and distribution of exceptionally preserved relicts of a continental Early Cretaceous ecosystem. Cretaceous Research 44: 30–38. Panciroli, E., Benson, R.B., and Butler, R.J. 2018. New partial dentaries of amphitheriid mammal Palaeoxonodon ooliticus from Scotland, and posterior dentary morphology in early cladotherians. Acta Palaeontologica Polonica 63: 197–207. Patterson, B. 1955. A symmetrodont mammal from the Early Cretaceous of northern Texas. Fieldiana: Zoology 37: 689–693. Patterson, B. 1956. Early Cretaceous mammals and the evolution of mammalian molar teeth. Fieldiana-Geology 13: 1–105. Prothero, D.R. 1981. New Jurassic mammals from Como Bluff, Wyoming, and the interrelationships of non-tribosphenic Theria. Bulletin of the American Museum of Natural History 167: 277–326. Rougier, G.W. 1993. Vincelestes neuquenianus Bonaparte (Mammalia, Theria), un primitivo mamífero del Cretácico inferior de la Cuenca Neuquina. 720 pp. Ph.D. Dissertation Thesis, Universidad Nacional de Buenos Aires, Buenos Aires. Rougier, G.W., Ji, Q., and Novacek, M.J. 2003a. A new symmetrodont mammal with fur impressions from the Mesozoic of China. Acta Geologica Sinica‐English Edition 77: 7–14. Rougier, G.W., Spurlin, B.K., and Kik, P.K. 2003b. A new specimen of Eurylambda aequicrurius and considerations on “symmetrodont” dentition and relationships. American Museum Novitates 3398: 1–16. Ruf, I., Maier, W., Rodrigues, P.G., and Schultz, C.L. 2014. Nasal anatomy of the non‐mammaliaform cynodont Brasilitherium riograndensis (Eucynodontia, Therapsida) reveals new insight into mammalian evolution. The Anatomical Record 297: 2018–2030. Schultz, J.A. and Martin, T. 2014. Function of pretribosphenic and tribosphenic mammalian molars inferred from 3D animation. Naturwissenschaften 101: 771–781. Sigogneau-Russell, D. 1983. A new therian mammal from the Rhaetic locality of Saint-Nicolas-de-Port (France). Zoological Journal of the Linnean Society 78: 175–186. Sigogneau-Russell, D. and Ensom, P. 1998. Thereuodon (Theria, Symmetrodonta) from the Lower Cretaceous of North Africa and Europe, and a brief review of symmetrodonts. Cretaceous Research 19: 445–470. Simpson, G.G. 1925a. Mesozoic Mammalia, II: Tinodon and its allies. American Journal of Science 10: 451–470. Simpson, G.G. 1925b. Mesozoic Mammalia, III: Preliminary comparison of Jurassic mammals except multituberculates. American Journal of Science 10: 451–470. Simpson, G.G. 1928. A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum. 215 pp. Trustees of the British Museum, London. Sweetman, S.C. 2008. A spalacolestine spalacotheriid (Mammalia, Trechnotheria) from the Early Cretaceous (Barremian) of southern England and its bearing on spalacotheriid evolution. Palaeontology 51: 1367–1385. Tsubamoto, T., Rougier, G.W., Isaji, S., Manabe, M., and Forasiepi, A.M. 2004. New Early Cretaceous spalacotheriid symmetrodont mammal from Japan. Acta Palaeontologica Polonica 49: 329–346. Wang, Y., Hu, Y., Meng, J., and Li, C. 2001. An ossified Meckel’s cartilage in two Cretaceous mammals and origin of the mammalian middle ear. Science 294: 357–361. Wang, H., Meng, J., and Wang, Y. 2019. Cretaceous fossil reveals a new pattern in mammalian middle ear evolution. Nature 576: 102–105. Wible, J.R., Rougier, G.W., Novacek, M.J., and Asher, R.J. 2009. The eutherian mammal Maelestes gobiensis from the Late Cretaceous of Mongolia and the phylogeny of Cretaceous Eutheria. Bulletin of the American Museum of Natural History 327: 1–123. Yang, S., He, H., Jin, F., Zhang, F., Wu,Y., Yu, Z., Li, Q., Wang, M., O’Connor, J. K., and Deng, C. 2020. The appearance and duration of the Jehol Biota: Constraint from SIMS U-Pb zircon dating for the Huajiying Formation in northern China. Proceedings of the National Academy of Sciences 117: 14299–14305.