Reexamination of the mandibular and dental morphology of the Early Jurassic mammaliaform Hadrocodium wui

ZHE-XI LUO, BHART-ANJAN S. BHULLAR, ALFRED W. CROMPTON, APRIL I. NEANDER, and TIMOTHY B. ROWE

Luo, Z.-X., Bhullar, B.-A.S., Crompton, A.W., Neander, A.I., and Rowe, T.B. 2022. Reexamination of the mandibular and dental morphology of the Early Jurassic mammaliaform Hadrocodium wui. Acta Palaeontologica Polonica 67 (1): 95–113.

CT visualization of the mandible and dentition of Hadrocodium wui, a stem mammaliaform from the Lower Jurassic Lower Lufeng Formation of Yunnan, China has revealed new features not accessible by previous microscopic study of the fossil. Its mandible shows a postdentary trough with an overhanging medial ridge and a short Meckel’s sulcus. An incomplete part of the ectotympanic and possibly a remnant of Meckel’s element are preserved in the postdentary trough. Thus, Hadrocodium is similar to other mammaliaforms in retaining a mandibular middle ear, contrary to our earlier interpretation. The mandible exhibits a large postcanine diastema from shedding of anterior premolars without replacement, an age-dependent feature better developed in older adults. Another adult feature is the alignment of the ultimate molar to the coronoid process. This is consistent with age-dependent changes in other mammaliaforms where the last molars of the toothrow shift from medial of the coronoid process in the juvenile, to a position in front of the coronoid process in the adult. The mandible has a short mobile symphysis. The dentition consists of I5, C1 (two-rooted), P3 (including P1 position) and M2 (M2 with confluent roots), and i4, c1 (partially two-rooted), p3, and m2 (m2 with partially confluent roots). The two-rooted upper canines are more derived than other Early Jurassic mammaliaforms from the same fauna, although similar to docodontans. Hadrocodium is unique in that the lower m2 cusp a occludes in the embrasure between upper M1–M2, but the posterior part of m2 shows between-cusp occlusion with upper M2 main cusp A. M2 is half the size of the lower m2, and occludes only with the distal half of m2. The upper postcanines show a steep gradient of posteriorly decreasing tooth size, more so than other mammaliaforms. The CT examination corroborates that there are no unerupted teeth in the upper or lower jaws, and the holotype of H. wui is dentally and osteologically mature and capable of independent feeding.

Key words: Mammaliaformes, Hadrocodium, dental morphology, growth pattern, Jurassic, China.

Zhe-Xi Luo [zxluo@uchicago.edu; ORCID 0000-0003-2170-8879] and April I. Neander [aisch@uchicago.edu; ORCID 0000-0001-6816-2872], Department of Organismal Biology and Anatomy, University of Chicago, 1027 East 57th Street, Chicago, Illinois 60637, USA.

Bhart-Anjan S. Bhullar [bhart-anjan.bhullar@yale.edu; ORCID 0000-0002-0838-8068], Department of Earth and Planetary Sciences and Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06511, USA.

Alfred W. Crompton [acrompton@oeb.harvard.edu; ORCID 0000-0001-6008-2587], Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA.

Timothy B. Rowe [rowe@utexas.edu; ORCID 0000-0002-8869-4582], Jackson School of Geosciences and High-Resolution X-ray Computed Tomography Facility, University of Texas at Austin, Austin, Texas 78712, USA.

Received 1 October 2021, accepted 2 March 2022, available online 30 March 2022.

Copyright © 2022 Z.-X. Luo 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

Hadrocodium wui (Luo et al. 2001) is a stem mammaliaform from the Lower Jurassic of China. It is characterized foremost by an expanded braincase with a large brain endocast volume relative to its small skull size, overlapping with the range of brain encephalization of crown Mammalia (Rowe et al. 2011). Given its basal position in mammaliaform phylo­geny, characteristics of Hadrocodium have broader ramifications for understanding the morphological transformation through the descent of mammals from the non-mammaliaform cynodonts (Luo 2007; Rowe and Shepherd 2016; Lautenschlager et al. 2018). In some large-scale phylogenetic studies of mammals, H. wui was included as a comparative taxon for overall interpretation of mammaliaform phylogenies (e.g., Luo et al. 2002, 2017; O’Meara and Thompson 2014; Bi et al. 2014; Zhou et al. 2019). For other studies this fossil was also used as sister taxon for crown Mammalia, and for time-calibration of the mammalian family tree (Tarver et al. 2016; Liu et al. 2017). Its morphologies have been used in other studies on evolution of the brain and cranial muscles (Rowe and Shepherd 2016; Laaß and Kaestner 2017; Lautenschlager et al. 2017, 2018; Rowe 2020).

Hadrocodium wui is a very small mammaliaform. Measured at 12 mm in skull length, it approaches the size range of the smallest-known mammals from the Cenozoic (Bloch et al. 1978; Gebo et al. 2000; Jürgens 2002). It is well established that the evolution from non-mammaliaform cynodonts to modern mammals went through a transformative phase of miniaturization (reduction of body size) with the earliest mammaliaforms (Lautenschlager et al. 2018). Hadrocodium is a major taxon that exemplifies this large-scale evolutionary pattern. The initial systematic paleontology study (Luo et al. 2001) was based on traditional mechanical preparation of the fossil in 1992–1993. Although the skull of H. wui was scanned by Computed Tomography (CT) in 2003, which resulted in studies of its brain endocast (Macrini 2006; Rowe et al. 2011), the rest of the skull has yet to be analyzed from these CT data. Here we provide a detailed description of the mandibles and the teeth from reexamination of this fossil by CT scans, which revealed new information unavailable from previous fossil preparation. This helps to correct some inaccurate interpretations of ours on the mandibular structure and the middle ear (Luo et al. 2001; Luo 2011), as well as the dental occlusal pattern.

Institutional abbreviations.—BMNH, Beijing Museum of Natural History, China; CUP-FMNH, Field Museum of Natural History-Catholic University of Peking collection, Chicago, USA; DMNH, Dalian Museum of Natural History, Dalian, Liaoning, China; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; MCZ, Museum of Comparative Zoology, Harvard University, USA; Gui-Mam, Guimarota mammal fossils, currently housed for study at Institute for Geosciences, Section of Paleontology of the University of Bonn, Germany (the Gui Mam collection belongs to the paleontological collection of the Serviços Geológicos de Portugal, Lisboa, Portugal); UMZC, University Museum of Zoology Cambridge, Cambridge University, UK; YPM, Yale Peabody Museum of Natural History, New Haven, USA.

Other abbreviations.—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 holotype specimen of Hadrocodium wui (IVPP 8275) was discovered by Xiao-Chun Wu in 1985 near the Dadi locality of the Lufeng Basin, of Yunnan Province, China. The fossil was from the Upper Red Beds of the Lower Lufeng Formation, estimated to be Early Jurassic in age (Sinemurian, about 195 million years) by faunal correlation (Luo and Wu 1994; Kielan-Jaworowska et al. 2004). The specimen was hand-prepared from 1992 to 1993 by the late William W. Amaral, the fossil preparator of the Museum of Comparative Zoology at Harvard University (Cambridge, USA). After the initial study of H. wui (Luo et al. 2001), the type specimen was scanned in 2003 by Rich Ketcham at the CT Lab at the High-Resolution Computed Tomography Facilities in University of Texas (Austin, USA), on the ACTIS scanner with detector panel of 1024 × 1024 dpi, at 150 kv and 0.38 mA. The scan resolution is 0.0191 mm in slice thickness, and also 0.0191 mm in inter-slice spacing.

The dataset used here for studying the teeth and mandible is the same dataset as for the Rowe et al. (2011) study on the brain endocast of H. wui. The segmentation and image analyses from the CT data were completed by co-author AIN using the software Mimics (www.materialise.com) in the Department of Organismal Biology and Anatomy of the University of Chicago.

Systematic palaeontology

Mammaliaformes Rowe, 1988

Genus Hadrocodium Luo, Crompton, and Sun, 2001

Type species: Hadrocodium wui Luo, Crompton, and Sun, 2001; see below.

Hadrocodium wui Luo, Crompton, and Sun, 2001

Fig. 1.

Holotype: IVPP 8275, a nearly complete skull.

Type locality: Dadi Locality, Lufeng Basin, Yunnan Province, China.

Type horizon: Upper Red Beds of the Lower Lufeng Formation, Lower Jurassic.

Material.—IVPP 8275 holotype.

Emended diagnosis.—Dental formula I5/i4 C1/c1 P3/p3 M2/m2. It differs from species of Morganucodon, Erythro­therium, Dinnetherium, Haldanodon, and triconodontids in that primary lower cusp a occludes in the embrasure between the opposite upper molars, instead of the primary cusp a of the lower molar m2 occluding between cusps A and B of the upper M1 and M2. It differs from Megazostrodon rudnerae Crompton and Jenkins, 1968, in lacking prominent labial cingulid cuspules of the upper and from kuehneotheriids in lacking the triangulation of molar cusps. It differs from morganucodontids, eutriconodonts, and kuehneotheriids in the presence of a much larger postcanine diastema. It differs from Sinoconodon and most cynodonts in the one-to-one precise occlusion of the upper and lower molars. It lacks the multicuspate rows on the teeth of Haramiyavia clemmenseni Jenkins, Gatesy, Shubin, and Amaral, 1997, Kalaallitkigun jenkinsi Sulej, Krzesiński, Tałanda, Wolniewicz, Błażejowski, Bonde, Gutowski, Sienkiewicz, and Niedźwiedzki, 2020, multituberculates, and the multiple-­ridged teeth of docodonts.

Description.—Mandibles: The mandibular body (alveolar ramus) is gracile and dorso-ventrally shallow. Its ventral margin is slightly convex, and the tooth-bearing alveolar margin is straight along the postcanine row but noticeably curved upward in the postcanine diastema anterior to the first preserved premolar, and in the symphyseal region of the mandible. The symphyseal region of the mandible is even thinner than the diastema region and the mandibular body along the tooth row (Figs. 1–3). The gracile nature of the mandibular part that bears incisors and canine suggests that there might have been bone resorption after the loss of the anterior premolars. The ventral margin of this mandibular part was slightly damaged by abrasion before the specimen was found. Due to the abrasion, the roots of incisors and canine are partly exposed (Figs. 2–4).

The postcanine diastema is a pronounced feature between the canine and the first preserved premolar, herein designated as p2. Anterior to the p2 there are two dimples on the dorsal margin of the mandible, which clearly represent the plugged alveoli of a two-rooted premolar that was shed at this locus. These plugged (although clearly recognizable) alveoli suggest that there used to be an anterior premolar (p1) in this position. It cannot be ruled out that the postcanine diastema may have accommodated more than one premolar. But here we offer a conservative estimate that H. wui has three premolar positions, and we designate the lost premolar represented by the two plugged alveoli to be p1 (see Discussion).

The coronoid process rises from the alveolar line of the mandibular body, forming a 120–125° angle to the anterior border of the coronoid (Figs. 1–3). The fully developed dentary condyle is preserved on the right mandible, but broken on the left mandible. The condyle is differentiated from the posterior part of the coronoid process by a constricted peduncle, a structure typical of stem mammaliaforms, including haramiyidans (Luo et al. 2002; Gill et al. 2014; Luo et al. 2015a; Schultz et al. 2019; Panciroli et al. 2019; Sulej et al. 2020). The condyle is slightly compressed dorso-­ventrally, less robust than the more bulbous and rounded dentary condyles of Morganucodon species (Kermack and Mussett 1958; Kermack et al. 1973; Gill et al. 2014) and Sinoconodon rigneyi Patterson and Olson, 1961 (Crompton and Sun 1985; Crompton and Luo 1993; Luo 1994; Luo and Wu 1994; Luo et al. 1995).

The anterior margin of the coronoid process is aligned mesio-distally with the postcanine row (including m2). Between the ultimate molar (m2) and the rising base of the coronoid process there is a gap, herein called the retro-molar space (Figs. 3 and 4: arrow). We interpret that the direct alignment of the toothrow in front of the coronoid process is an adult feature, by comparison to the adult pattern of other mammaliaforms with more complete series of ontogenetic stages, and this feature is also apomorphic in a phylogenetic context (details in Discussion).

Lingual aspect: The mandibular symphysis is marked by a distinct area on the lingual aspect of the anterior part of the mandible. Although the symphyseal surface is somewhat damaged on both mandibles, the preserved parts of symphysis show that it is slightly rugose. As preserved, the two hemimandibles are not in direct contact, indicating that the mandibular symphysis was connected by soft tissues that are not fossilized (Figs. 1–3). The symphysis extends posteriorly to below the canine, and this posterior extent of the symphysis is much shorter than the symphyses that extend to the level of the p3 in species of Morganucodon, or to the ultimate premolar (p5) or m1 in docodontans. We interpret that the symphyseal contact likely was mobile, as in all known mammaliaforms, and a vast majority of crown mammals (Kielan-Jaworowska et al. 2004; Scott et al. 2012).

The Meckel’s sulcus is present in the molar region of the mandibular body (Figs. 1–3). The sulcus is short, and extends from below the foramen of the mandibular canal anteriorly toward the ventral mandibular margin; its anterior end fades out below the ultimate molar (m2). However, there is no Meckel’s sulcus on the lingual side of the mandible between the symphysis and the posterior premolars, which is confirmed on the scan slices. The Meckel’s sulcus is present in the premolar and symphyseal region in juvenile or young adult individuals of other mammaliaforms. For example, the Meckel’s sulcus is present in this region in juvenile growth stages of species of Morganucodon, Docodon, and Haldanodon but the sulcus disappears in this region in full adult specimens of the same taxa (Kermack et al. 1973; Krusat 1980; Schultz et al. 2019; see also adult mandible of Borealestes serendipitus Waldman and Savage, 1972; Panciroli et al. 2019).

The postdentary trough is present on the medial side of the mandible, and a part of this trough is dorsally bordered by the medial ridge (Figs. 1–3). The trough is shallow along its entire length and its anterior part contains the mandibular foramen. Posterior to the foramen the trough is open dorsally and lacks a border, likely for the passage of the inferior alveolar nerve (from the mandibular branch of the trigeminal nerve) into the mandibular canal. The posterior end of the trough becomes slightly wider and opens in a shallow embayment posteriorly below the posterior part of the medial ridge (Figs. 1–3).

Some incomplete parts of middle ear elements are detected by CT scans, and can be visualized (Fig. 4). The preserved parts certainly include a part of the ectotympanic (or “angular”) bone. Another preserved element may be a fragment of the Meckel’s element (or the “prearticular”). However, the surangular as interpreted for Morganucodon oelheri Rigney, 1963, by Kermack et al. (1973), cannot be identified with certainty. The identities of these postdentary elements are difficult to establish and remain uncertain due to poor preservation. These elements cannot be fully resolved until better specimens of H. wui are recovered in the future.

On the medial side and near the base of the coronoid process, there is an oval and elongate depression, which we interpret to be the coronoid facet. Near the coronoid fossa of the right mandible, a small piece could belong to the coronoid bone but this identification remains tentative because this element is difficult to trace in coronal CT slices, and must be corroborated by additional specimens of H. wui recovered in the future. Although smaller, the fossa for the coronoid is identical in shape and position to the coronoid facet of Morganucodon watsoni Kühne, 1949 (Kermack et al. 1968). The coronoid fossa is less similar to that in Kuehneotherium praecursoris D.M. Kermack, K.A. Kermack, and Mussett, 1968 (Gill 2004; Gill et al. 2014) than to M. watsoni (Pamela Gill, personal communication 2021).

Hadrocodium shares plesiomorphic features related to the postdentary trough with basal mammaliaforms. Not­withstanding the minor differences of a shorter and thinner Meckel’s sulcus, a shallower trough, and much weaker medial ridge, Hadrocodium shares these plesiomorphic features with Sinoconodon, morganucodontans, and docodontans. This new revelation by CT scanning necessitates a major revision of the interpretation of these features by Luo et al. (2001).

The alveolar margin of the mandible is nearly straight and horizontal. The posterior part of this margin lacks the dental lamina groove (Crompton’s groove sensu Parrington 1971); this groove is a plesiomorphic feature present in species of Morganucodon and Sinoconodon rigneyi, and a wide range of non-­mammaliaform cynodonts on the lingual side near the last molars (Kermack et al. 1973; Crompton and Luo 1993). The absence of this feature is an apomorphy of Hadrocodium and is otherwise known in docodontans (Schultz et al. 2019; Panciroli et al. 2019) and in the Late Triassic haramiyidans (Jenkins et al. 1997; Luo et al. 2015a; Sulej et al. 2020).

Labial aspect: On the labial side of the mandible, the coronoid process has a shallow concave area, identified as the masseteric fossa. The anterior border of the fossa lacks a distinct border and grades into the mandibular body as in species of Morganucodon (Kermack et al. 1973; Crompton and Luo 1993; Lautenschlager et al. 2017). The ventral extent of the masseter muscle attachment is not a clearly demarcated border in H. wui; this is similar to Dinnetherium nezorum Jenkins, Crompton, and Downs, 1983 (Jenkins et al. 1983; Crompton and Luo 1993) and to docodontans (Krusat 1980; Ji et al. 2006; Schultz et al. 2019; Panciroli et al. 2019). There is no foramen in the masseteric fossa, as in some derived zatherian mammals (Davis 2012). The lateral ridge extending from the dentary condyle and peduncle is distinctive. We interpret that the lateral ridge is a landmark boundary, approximately, between the concave surface for M. masseter profundus dorsally and a narrow, flat surface for the M. masseter superficialis on the moderately inflected angular process of the mandible, as reconstructed for other mammaliaforms, by comparison to extant mammals (Turnbull 1970; Lautenschlager et al. 2017; Schultz et al. 2019).

Two mental foramina are present on the labial side of the anterior (symphyseal) part of the mandible (Figs. 1–3). The anterior of these foramina is small and located between i4 and the canine on the better preserved left mandible. The posterior mental foramen is oval shaped and much larger than the anterior-most foramen. It is located in the postcanine diastema region on both mandibles, just anterior to the level of the p1 position indicated by plugged alveoli of this tooth. These foramina are connected to the mandibular canal inside the mandibular body (Fig. 5).

The mandibular canal on the left mandible is exposed by erosion of the thin cortical bone. From the exposed width of the canal, and from the cross section of the right mandible, the canal diameter is measured to be 0.12–0.13 mm, in a mandibular body that is 0.8–0.9 mm in height (Figs. 2, 5). The canal is oval in cross section (Fig. 5) and is similar in H. wui and M. oehleri. The canal is positioned lateral to the root alveoli in cross section and is in the laterally bulging part of the mandibular body (Fig. 5A). The lateral placement of this canal relative to the postcanine alveoli is similar in H. wui and M. oehleri, but this placement is slightly different from the canal’s position in docodontans. In all docodontans examined so far (Schultz et al. 2019; Panciroli et al. 2019), the mandibular canal is ventrolateral to the tooth root apices (Fig. 5C, D). The latter pattern of docodontans is more closely comparable to those of extant therians than in morganucodontans and Hadrocodium, and therefore more derived. In crown mammals of the Mesozoic, such as the eutriconodontans Juchilestes and Repenomamus, the mandibular canal is located directly ventral to the root apices (Fig. 5E).

The mandibular canal diameter is 14.5 to 15% the size of the mandibular body depth. If scaled to the width or the lingual side depth (= height) of the mandibular body, the mandibular canal shows about the same proportion to the width and height of the mandibular body (Fig. 5) as the canals of docodontans and morganucodontans. The mandibular body of H. wui shows a distinct pattern in that the height of the mandibular body is lower on the labial side of the postcanine alveoli (Fig. 5A2: asterisk), than on the lingual side of the alveoli (Fig. 5: arrow). The mandibular canal diameter appears to be slightly larger if it is scaled to the labial height of the mandible. Overall, the mandibular canal diameter is about the same relative to the mandibular body in H. wui, as in species of Morganucodon and docodontans. The mandibular canal of mammaliaforms (including H. wui) is much smaller than those of Cretaceous Teinolophos trusleri Rich, Vickers-Rich, Constantine, Flannery, Kool, and Van Klaveren, 1999, and the extant monotremes, than previously interpreted by Rowe et al. (2008).

Dentition: Here we are revising the dental formula of H. wui to I5/i4–C1/c1–P3/p3–M2/m2 (Figs. 6–9). This change includes the alveoli of the first upper and lower premolars that were shed in the P1/p1 position. The new formula differs from the previous I5/i4–C1/c1–P2/p2–M2/m2 (Luo et al 2001: fig. 1), but the previous study only counted the intact permanent teeth and excluded the upper P1 and lower p1 positions indicated by the visible (but plugged) alveoli of the shed premolars. In the following description we use a dental formula that includes the tooth positions represented by empty and plugged alveoli. We also follow Crompton and Jenkins (1968) and Crompton (1974) for their scheme of cusp designations.

Upper dentition: There are five upper incisors in the better-­preserved right premaxilla (Luo et al. 2001; Rowe et al. 2011). Crowns of I2–I4 are intact but I1 and I5 are broken, although their alveoli can be clearly identified (Figs. 6, 7, 9). The left premaxilla has preserved only I4 (incomplete) and I5; the left I1–I3 and their corresponding left premaxillary part are damaged and lost. The I5 is separated from the upper canine by a gap that accommodates the tall and trenchant lower canine locking into a deep canine pit (the paracanine fossa) in the premaxilla when in full occlusion (Fig. 6). This is visible from cross sections of the CT scans.

The upper canine shows a triangular outline in lateral profile and is slightly compressed medio-laterally (Figs. 6, 7, 9). The canine has two strong roots divergent from each other at an angle of 20–25°. The conical shape of the crown as previously illustrated by Luo et al. (2001: fig. 1) was not correct. The posterior root of the two divided roots was not detected by manual fossil preparation in previous study (Luo et al. 2001), as the root was covered by a mixture of frail bone and matrix in the specimen. The upper postcanine diastema is a prominent gap between the two-rooted canine and the preserved premolars (P2 and P3 on the right side, but only P3 on the left side). The right maxillary alveolar margin in the postcanine diastema shows a surface dimple corresponding to a single plugged alveolus (Fig. 6). This is identified by CT visualization, and also recognizable on the specimen under the microscope. This plugged alveolus is interpreted to have held a single-rooted premolar that was shed. Its position is herein designated as “P1-position” (Figs. 5–7).

The upper P2 is preserved only on the right maxilla, but this tooth on the left maxilla was lost without being replaced, and its two alveoli are plugged though still recognizable. The right P2 is two-rooted but its crown is broken. The tooth is separated anteriorly from the plugged P1 alveolus by a small interdental gap. It is also separated posteriorly from P3 by a very small interdental gap. P3 is the largest of all teeth in height and in length (Table 1). Its large main cusp (cusp A) is slightly asymmetrical with a straight anterior crest and a slightly concave posterior crest. At the cingular level, the tooth shows no distinctive cingular line on either the lingual or the labial aspect although in the cingular positions there are slight bulges. There are two miniscule cingular cuspules on the mesial and the distal ends of the crown. However, the large mesial cuspule illustrated in previous study (Luo et al. 2011: fig. 1) is an artefact, as shown by CT visualization. The left and right P3s show an asymmetrical variation on two sides: the left P3 has a weakly developed cusp on the posterior crest, which is entirely absent on the right P3 (Fig. 7). We could not rule out that this difference is caused by preservation differences of the left P3 versus the right P3.

The upper M1 is better preserved on the right side than the left. This tooth bears three main cusps: the primary cusp A in the middle, a lower but pointed cusp B on the mesial end of the crown, and a low posterior cusp C (worn off on the left M1). Additionally, a miniscule cingular cusp D is on the distal end of the crown (Figs. 6, 7). The base of the crown has a slightly bulging labial cingulum although lacking any cingular line or cuspules. The lingual aspect of the tooth exhibits a weak cingulum, which is otherwise featureless. M1 has two fully divided roots and the posterior root is curved or slanted mesially, in both the left and the right M1. The distal part of left M1 appears to be either heavily worn or damaged in fossilization, such that cusp C and cuspule D are lost.

M2 is characterized by the taller cusp A and shorter cusps B and C located respectively at the mesial and distal ends of the crown. But it lacks distal cuspule D. The tooth crown is antero-posteriorly symmetrical with respect to the middle cusp A. M2 has two fully confluent roots connected by dentine over the root length. The M2 roots are much shorter than those of M1. The gradient of variation of the successively more confluent and shorter roots in the more posterior molars, as seen here in H. wui (Fig. 7), is also well documented in specimens of Morganucodon watsoni, in which the last upper molar (M4) has the smallest (also the shortest) crown of all postcanines, and often with a fusion of the two roots (Parrington 1971; Kermack et al. 1981; Jäger et al. 2019). The upper molars of Kuehneotherium praecursoris also show a gradient of variation in root fusion of molars (Mills 1984; Gill 2004). In the latest reconstruction of the toothrow in Kuehneotherium praecursoris, Gill (2004) placed the smaller to smallest upper molars with more fused roots in the ultimate toothrow position(s), although the degree of fusion of the two roots can be variable in the posterior molars (Gill 2004, personal communication 2021).

The upper M2 is very small, its crown is much shorter than the M1 crown, and lower in height than P3 and M1 (50% of P3 length and 65% of the M1 length). Taken together, P3, M1, and M2 form a posteriorly decreasing size (length) and height gradient. The posteriorly decreasing size of molars is also present between m2 and the ultimate molar (m4 or m5) in morganucodontans and docodontans, but the steep gradient of size reduction in P3–M1–M2 is an unusual and distinct feature of H. wui.

Thus the size reduction of the ultimate molar crowns (M2/m2), and the confluence and fusion of the roots of M2/m2, as seen in H. wui are similar to the ultimate molars of Morganucodon watsoni (Parrington 1971; Kermack et al. 1981; Gill 2004; Jäger et al. 2019). Kuehneotherium praecursoris is also interpreted to have very small ultimate upper and lower molars (Gill 2004). The comparative morpho­logy across mammaliaforms is a strong indication that the small upper M2 with fused roots in H. wui type specimen is the terminal tooth in the entire dentition.

Lower incisors and canine: The left incisors are well preserved and are the basis for this description. Of the four lower incisors, i1 is twice the thickness of other incisors. The elongate i1 root extends posteriorly and medial to the roots of i2 through i4. Although the apical end of the long i1 root is not preserved (as far as can be assessed by CT), the alveolar canal of this root reaches posteriorly to a point medial to the i4 root. The i2, i3, and i4 are peg-like, and they form a size gradient with i2 being the shortest and i4 being the tallest.

The lower canine is mediolaterally compressed and the lateral profile of its crown has a lanceolate outline; the crown is slightly asymmetrical with the anterior crest being more convex and the posterior crest more concave. The canine has mediolaterally compressed roots, which are incipiently divided, with a shallow groove on the medial aspect along the root length, and a figure 8 shape in horizontal cross-section. Internally the canine roots have two divided root canals, as visualized in CT slices. The canine roots are long and penetrate the entire depth of the mandible. The lower canine of H. wui shows similarity to lower canines of docodontans in having incipiently divided roots with side grooves (Meng et al. 2015; Schultz et al. 2019; Panciroli et al. 2019). It is almost identical to the isolated lower canines of Kuehneotherium praecursoris identified from the Lower Jurassic fissure-fill deposits of the UK (Gill 2004: fig. 3.16), and bears some resemblance to the lower canines of Morganucodon watsoni (Mills 1971; Parrington 1971). But the root structure and crown shape differ from the more conical crown and single-rooted canine (with singular root canal) of Morganucodon oehleri Rigney, 1963 (BMNH2858) (Luo et al. 1995) and of Sinoconodon rigneyi (Fig. 9).

Lower premolars: There is a long postcanine diastema posterior to the canine and anterior to p2 in both mandibles. On the right mandible, there are two partially plugged alveoli, indicating that a two-rooted premolar was shed (Figs. 4, 6, 8). Although this anterior premolar is no longer present, its position is marked by a vestige of its alveoli. We designated this as p1 position, and we propose that the first premolar position, despite that the tooth is already lost, should be included in the tooth count and the dental formula. The comparison of the postcanine diastema will be further detailed below (see Discussion section).

The lower p2 crown, as preserved on right p2, shows a triangular outline in lateral view with a central cusp a and a posterior cusp c. The p2 is separated anteriorly by an interdental gap from the plugged p1 alveoli, also posteriorly separated by an interdental gap from p3. The tooth has two roots. The lower p3 is the tallest and most trenchant tooth of all lower teeth. Its crown is triangular in lateral outline. Its main cusp is asymmetrical in tilting anteriorly with a nearly vertical anterior crest that is shorter than the slanting posterior crest. The posterior crest bears a small cuspule, separated from the distal cingulid cuspule. The distal end of the p3 crown shows no interlocking with m1.

Overall, the ultimate premolar of H. wui more closely resembles the posterior premolar type of Kuehneo­therium praecursoris (Gill 2004; Gill et al. 2014) rather than Mor­ganucodon watsoni (Kermack et al. 1973). The largest premolar (p3) of IVPP8275 is nearly identical to premolars that were interpreted to be the posterior premolars of Kuehneotherium praecursoris by Gill (Gill 2004: figs. 3–18; Gill et al. 2014) and by Kermack and co-workers (Kermack et al. 1968: text-fig. 3).

Lower molars: Lower m1 has a tall cusp a, and a much shorter cusp b on the mesial end of the tooth but no anterior cingulid cusp e. The posterior main cusp c is also a short cusp, separated from a distinctive distal cusp d. Cusps a–d are aligned in a straight line. The base of the crown shows no cingulid line and lacks cusps on either the labial or lingual side of the tooth.

The lower m2 has a tall cusp a and two well-developed cusps b and c. Both cusps b and c are distinctive in rising higher from the base, and different from their counterparts on m1. The main cusps a–c of m2 are aligned in a straight line similar to the straight-line cusp pattern of m1. Cusp b is closely twinned with a small (although discernible) anterolingual cuspule e. The posterior main cusp c is lower than the anterior cusp b, and c is more divergent posteriorly from the primary cusp a, such that cusp a and cusp c are farther apart from each other than cusp a and cusp b. The distal cusp d is recognizable but not well developed. The crown shows no lingual or labial cingulid structure, unlike the ultimate molars of species of Morganucodon that show a distinct cingulid line in m4 of M. oehleri and distinctive lingual cingulid plus strong cingulid cusps (such as cusp g) (sensu Mills 1971; Kermack et al. 1973; Crompton 1974; Butler 1988; Fig. 10). The lower m2 has much more confluence of its two roots than m1. Hadrocodium wui has a molar interlocking mechanism between the distal cusp d of m1 and the flat surface between cusp b and the very small cingulid cuspule e of m2. But there is not tooth-to-tooth interlock between p3 and m1.

The lower m2 is the smallest tooth of the p3–m1–m2 series. The left m2 is smaller than the left m1, and about 84% of the latter in length. The left m2 is also smaller than the left p3, and about 93% of the latter in length. The lower p3–m2 constitute a gradient in decreasing crown height in the successively more posterior teeth. There is a greater degree of fusion of roots in the most posterior molar as compared to the preceding postcanines.

The posteriorly decreasing size gradient of the posterior lower molars, as seen here in H. wui, has long been recognized for M. watsoni and M. oehleri (Parrington 1971; Kermack et al 1973, 1981; Crompton and Luo 1993; Jäger et al. 2019). This plesiomorphic pattern is also typical of docodontans, many of which are now known by complete lower molar series (Krusat 1980; Ji et al. 2006; Luo and Martin 2007; Averianov et al. 2010; Meng et al. 2015; Rougier et al. 2015; Schultz et al. 2019; Panciroli et al 2019; Zhou et al. 2019). A similar gradient in posterior decrease of molar size has been interpreted for Kuehneotherium praecursoris (Gill 2004), and has also been documented for Haramiyavia (Jenkins et al 1997; Luo et al. 2015a). Therefore, we interpret that the posteriorly decreasing gradient of lower molars is a plesiomorphic feature of perhaps all mammaliaforms of Late Triassic to Early Jurassic age, including Hadrocodium. The eruption of the posterior-most molar, the m2 in the case of H. wui, which is also the smallest of the postcanine size gradient, indicates the end of tooth development.

The two roots of the lower m2 in H. wui are fused along most of their length (Figs. 8, 10); the roots are fused by more than two-thirds of the root length in right m2 (Figs. 8, 10), or by half of the root length in left m2. This is structurally comparable to the terminal lower molar (m4) in morganucodontans with roots fused either partially (as in M. oehleri), or entirely (as in some m4s of M. watsoni) (Fig. 10). These fused roots of the m4 of Morganucodon correspond to a singular alveolus, instead of two alveoli as in preceding molars (Fig. 11; see Parrington 1971; Kermack et al. 1973).

The pattern of the smallest ultimate molar (m4) with partially or entirely fused roots is now documented in many specimens of M. watsoni (Parrington 1971; Kermack et al. 1973; Jäger et al. 2019). The same pattern is also present in the full-grown adult specimens of the docodontan Borealestes serendipitus, in which the last two lower molars show tightly compressed and partially fused roots (Panciroli et al. 2019). Here we also corroborate this pattern in an unerupted m4 of M. oehleri (Fig. 10). Although complete lower jaws with intact teeth are not available for kuehneotheriids, the best available postcanine reconstruction for this group (Gill 2004) suggests similarly that the last molars have tightly compressed or fused roots in Kuehneotherium praecursoris (Kermack et al. 1968; Gill 2004). A similar pattern also occurs in Kuehneotherium stanislavi Debuysschere, 2017 (Debuysschere 2017). Based on this comparison, we interpret that the lower m2 of the H. wui type specimen, with its fused roots, is the last and terminal molar to erupt, and equivalent to the terminal molar (m4) of species of Morganucodon (Fig. 8) and last molars (m5 and m6) of B. serendipitus (Panciroli et al. 2019).

Molar occlusion: The upper and lower molars, now visualized from CT scans (Figs. 6–8), show several features previously hidden by occlusal association of upper and lower teeth. This is helpful to rectify several incorrect interpretations of molar occlusion in our earlier study (Luo et al. 2001: fig. 1). The new pattern of correspondence between cusps of upper and lower molars is different by half of a cusp from the previous incorrect reconstruction (Luo et al. 2001: fig. 1).

The upper postcanines show more of a lingual inclination than the more vertically oriented lower molars. The upper molar lingual inclination is common for the triconodont-like dentition of other mammaliaforms, such as Morganucodon, and of eutriconodontans (Jäger et al. 2019, 2020, 2021). This pattern is obvious from the better preserved right side of the skull, which shows more lingually inclined upper teeth in CT sections (Figs. 4, 5).

The lower m1 cusp a is aligned with the embrasure between the upper P3 and M1 (Fig. 6). The anterolabial surface of the large cusp a of m1 occludes with the postero-lingual aspect of the cusp A of the upper P3. Because the cusp b on m1 is relatively low, the entire anterolabial aspect of m1 likely forms a continuous contact facet to P3. The posterolabial aspect of cusp a of m1 occludes with the anterolingual surface of upper M1 across both cusp A and cusp B. As cusp c of the m1 is relatively small (better preserved on the left m1), the m1 crown forms a continuous wear surface between cusps a and c. It also appears that m1 is more strongly worn than m2, and this is consistent with the greater degree of wear of m1 as the first erupted molar of the molar row, as in other mammaliaforms with diphyodont tooth replacement. The lower m2 primary cusp a occludes into the embrasure between the posterior cusp C of M1 and the anterior cusp B of M2, while m1 cusp b can contact the lingual surface of cusp C of the upper M1 (Fig. 6).

The upper M1 primary cusp A occludes into the embrasure between the cusp c of lower m1 and cusp b of m2. The lingual surface of cusp C of the M1 has a discernible facet that matches the labial facet across cusps a and b of the lower m2.

A striking pattern of the molar occlusion is the unique occlusal match of M1–M2 vs. m2 as a consequence of size disparity of the very small upper M2 vs. larger lower m2. Because of the much reduced size of the upper M2 relative to m2, cusp A of the M2 directly corresponds to the shallow notch between cusp a and cusp c of m2. On the labial side of the lower m2, there are wear facets on the shallow valley. Overall the lower p3–m1–m2 occlude in the embrasure of the upper P3–M1–M2, but due to the size difference of M2, the M2 cusp A occludes between cusps a and c of the lower m2.

In extant mammals, wear facets developed in the ultimate molars would indicate the terminal stage of dental ontogeny (Anders et al. 2011), as the ultimate molars are the last to erupt in the full permanent dentition in extant marsupials (van Nievelt and Smith 2005a; Astua and Leiner 2008) and placentals (Smith 2000; Anders et al. 2011). Thus, the presence of a wear pattern on M2/m2 suggests that the H. wui holotype specimen is an adult, or approaching the adult stage (see Discussion for further details on growth stage assessment).

Stratigraphic and geographic range.—Upper Red Beds of Lower Lufeng Fromation, Yunnan, China; Dadi Locality in Lufeng Basin.

Discussion

For the mammaliaforms of the Late Triassic and Early Jurassic, Hadrocodium wui is very small (rostro-condylar length of the skull equals 12 mm), about 40% the length of the smallest-known specimen of Morganucodon oehleri (Luo et al. 2004). In our redescription, we provide additional support for an adult ontogenetic age of the type specimen despite its small size (Crompton and Luo 1993; Luo et al. 2001). Using several dental and mandibular features that are known to change with developmental stages in other mammaliaforms, we polarize the variation of these characters along a continuum of juvenile-adult growth, for a comparative assessment of the growth stage as a proxy to the age estimate of H. wui.

Absence of un-erupted replacement teeth.—There is no un-erupted replacement tooth in any of the incisor, canine, or premolar loci of H. wui. In extant shrew species, which are some of the smallest placental mammals, the incisors and canines have typical diphyodont (two-generational) replacements. Neonatal or juvenile individuals frequently have an un-erupted replacement tooth below the deciduous teeth, or the erupting replacement teeth are juxtaposed with the deciduous teeth (Ziegler 1971, 1972). In some shrews, the primary generation of deciduous teeth is developed embryonically, but these teeth are resorbed before eruption. Only the secondary generation of (permanent) incisors or canine erupts (Kindahl 1959; Bloch et al. 1998). In a more recent study on tooth development in the shrew Sorex araneus, it was demonstrated that only the last premolar locus develops both the deciduous and the permanent replacement tooth (Järvinen et al. 2008).

By comparison to these small extant placentals, the absence of un-erupted replacement teeth in any incisor or canine loci in H. wui suggests one of two developmental scenarios: (i) the H. wui specimen had grown beyond the juvenile stages of replacing incisors and canines, or (ii) it was similar to adults of those small placentals in which the deciduous predecessors of diphyodont loci were present as embryonic tooth germs but resorbed prior to birth of the animal, and only adult permanent teeth developed to full eruption. Either scenario would suggest H. wui type specimen is an adult, or a young adult at a late growth stage.

M2/m2 are the terminal molar loci.—The upper M2 and lower m2 have already developed occlusal wear, as the much reduced M2 occludes with m2 (Fig. 6). The roots of the molar are extensively fused in m2 and completely fused in M2. The ultimate molars are the smallest teeth of the posteriorly decreasing gradient of m2 and more posterior molars in all stem mammaliaform clades. In Morganucodon species, both the upper and lower ultimate molars have stronger fusion of their roots than the more anterior molars (Fig. 8) (Parrington 1971; Jäger et al. 2019). In the docodontan Borealestes serendipitus, the lower ultimate molar has partially fused and tightly compressed roots (Panciroli et al. 2019). This is strong evidence that the M2/m2 are the ultimate molars because they have identical characteristics of ultimate molars of other mammaliaforms.

In extant didelphid marsupials, the end of cranio-dental growth is marked by full eruption of the ultimate upper molar (M4), which occurs after eruption of the ultimate lower molar (m4) (van Nievelt and Smith 2005a, b; Astua and Leiner 2008). By comparison, the ultimate upper M2 has erupted and is in full occlusion with the lower m2 in H. wui, and this occlusal function generated wear facets on the lower m2 between cusps a and c (Fig. 6). From this we infer that the dentition of the type specimen of H. wui was fully erupted. In extant placentals, the dental development approaches the end stage when the ultimate upper and lower molars are occluding (Anders et al. 2011) as these are the last of the adult teeth to erupt (Smith 2000). By this comparison, the skull of H. wui is either a fully grown adult, or at least a young adult that is dentally mature and capable of independent feeding.

Postcanine diastema.—By our conservative estimate of premolar count, the prominent diastema on the mandible shows two plugged alveoli of p1. This suggests that a two-rooted premolar had been shed. In the upper jaw, there is a single dimple in the diastema between the canine and the upper P2, indicating that an anterior upper premolar (P1) was lost and its alveolus was plugged (Figs. 6, 7). Taking into account these lost premolars, we interpret, conservatively, that H. wui had three upper and three lower premolar positions. A less likely alternative is that the postcanine diastema may correspond to more than one premolar, and if so H. wui would have had four premolar loci. However, there are no traces of tooth alveoli in the dentary anterior to p1 position and in the maxilla anterior to P1 position, thus no evidence that more than one premolar was lost in the existing diastemata. Thus we interpret that H. wui has only three premolars.

The timing for shedding premolars appears to differ between the left and right upper jaws. Further, the loss of both P1 and P2 on the left upper jaw suggests that the loss of premolar 2 occurred earlier on the upper jaw than on the mandibles. The developmental loss of some premolars in H. wui suggests that the animal already experienced tooth replacement process, which again indicates it was dentally mature, but not a juvenile, despite its small skull size.

A postcanine diastema that enlarges in successively older individuals has been well characterized for other mammaliaforms. Sinoconodon rigneyi has a postcanine diastema, which first appears in the youngest-known juveniles, and then enlarges with age in successively older individuals as more anterior premolars were shed without being replaced (Crompton and Luo 1993; Zhang et al. 1998). In larger (presumably older) individuals the postcanine diastema is so much enlarged that it can be as long as the length of the entire remaining postcanine row (Crompton and Luo 1993). Morganucodon watsoni is also known for a similar age-related development of this diastema (Fig. 11), and older individuals of M. watsoni can lose up to two lower premolars (Mills 1971; Parrington 1971; Kermack et al. 1973). This type of growth-related loss of up to two premolars is also documented in Dinnetherium nezorum (Fig. 1; see also Crompton and Luo 1993) and in Kuehneotherium praecursoris (Gill 1974, 2004). But this feature is absent in older individuals of Docodon victor and Borealestes serendipitus among docodontans (Schultz et al. 2019; Panciroli et al. 2019, 2021).

Non-mammaliaform cynodonts, such as diademodontids, traversodontids, and tritylodontids, are also characterized by prominent development of age-dependent postcanine diastemata (Hopson 1971; Crompton and Luo 1993), or post-incisor diastemata (Hoffman and Rowe 2018). The age-related growth of this diastema in cynodonts is similarly formed by shedding of teeth without replacement of a succeeding tooth in the same locus (Hopson 1971; Hoffman and Rowe 2018). Based on the consistent developmental pattern of the postcanine diastema in other mammaliaforms and cynodonts, the presence of a sizable tooth gap in the type specimen of H. wui is an indication that the individual already had significant alterations to the dental arcade related to aging.

Molar–coronoid alignment and retro-molar space.—The ultimate molar (m2) is aligned directly in front of the coronoid process, and there is a retro-molar space between m2 and the rising base of the coronoid process (Fig. 11A: ­arrow). These features are interpreted to be mandibular characteristics of adult individuals, by comparison to the more complete growth-related variation of the mandibles in other mammaliaforms (Fig. 11). In an earlier study of the growth variation of Morganucodon watsoni (= “Eozostrodon parvus”), Par­rington (1971) recognized an age-related shift of the posterior molars relative to the coronoid process. In juveniles and early young adults, the successively more posterior molars erupt medial to the anterior margin of the coronoid process. But in the oldest individuals of this growth series, the ultimate molars have shifted anteriorly relative to the coronoid process, and the ultimate molar is in front of the coronoid process (Fig. 11B, C). However, in some rare instances, the ultimate molar remains antero-medial to the base of the coronoid process in M. watsoni (Pamela Gill, personal communication 2021). It is possible that these individuals have a complete dentition but have not yet completed the growth of the lower jaw.

This pattern is also present in the growth series of Doco­don victor Schultz, Bhullar, and Luo, 2019 (Schultz et al. 2019: fig. 4). In the youngest-available individual in the growth series of D. victor, the last molar (m5) has just erupted medial to the coronoid process. In an adult mandible, m5 and m6 have already shifted anteriorly to the front of the coronoid, while the last molar (m7) is still medial to the coronoid. In the oldest-available individual of this growth series, the ultimate molar (m8) and penultimate m7 have both shifted anteriorly, while the coronoid process is shifted posteriorly relative to the toothrow, such that the toothrow including the last m8 is aligned with the coronoid process (Schultz et al. 2019: fig. 4). The position of the ultimate molar anterior to the base of the coronoid process in the oldest-available adult specimen of D. victor is similar to that of H. wui. By this growth-related coronoid-molar re-alignment, the H. wui type specimen is most likely an adult, despite its minute size.

To summarize, the above-mentioned characters indicate that the specimen of H. wui is at least a young adult, by the absence of any un-erupted replacement teeth. Its status as a young or fully grown adult is also supported by growth-related tooth or mandibular features that can be clearly differentiated along the juvenile-adult continuum of growth in other mammaliaforms with more complete growth stages, as seen in morganucodontans (Parrington 1971; Kermack et al. 1973, 1981; Crompton and Luo 1993; Jäger et al. 2019), Sinoconodon (Crompton and Luo 1993; Zhang et al. 1998), and docodontans (Schultz et al. 2019; Panciroli et al. 2019).

Taxonomic characteristics of Hadrocodium.—Hadroco­dium wui can be easily distinguished from Morganucodon oehleri and Sinoconodon rigneyi from the Lower Lufeng Formation by a list of discrete characters:

(i) Upper canine of H. wui has fully divided roots, instead of the single root of the canine of species of Morganucodon and Sinoconodon rigneyi (Parrington 1973; Kermack et al. 1973, 1981; Zhang et al. 1998). The fully divided canine roots of H. wui are more similar to those of Haldanodon and Docofossor (Fig. 9) than Morganucodon and Sinoconodon.

(ii) Incipient division of the lower canine roots has two root canals, as in the canine of Kuehneotherium praecursoris and Agilodocodon scansorius Meng, Ji, Zhang, Liu, Grossnickle, and Luo, 2015, instead of single, tubular root of the lower canine of Morganucodon and Sinoconodon (Fig. 9).

(iii) Upper and lower molars of H. wui have no distinct cingulum or cingulid as opposed to the condition in M. ­oehleri (Fig. 10). The ultimate molar (m2) is much smaller than the ultimate molar (m4) of M. oehleri. Molars of Hadrocodium are also distinguishable from those of Dinnetherium in lacking the cingulid structure including cingulid cusps.

(iv) A steep decrease in size of posterior upper postcanines (P3–M1–M2) is observed in Hadrocodium. This is more conspicuous than the size decrease in other mammaliaforms, such as M2–M3–M4 of morganucodontans and docodontans.

(v) Hadrocodium has a unique combination of embrasure occlusion of M1–M2 with m1–m2, with M2 cusp A occluding between cusp b and cusp c of m2 (Fig. 6). This is an autapomorphic character of Hadrocodium.

Size disparity among the earliest mammaliaforms.—Mammaliaform taxa from the Late Triassic and Early Jurassic are already known from a range of body sizes, and Hadrocodium further expands the scope of this disparity. Although the stem mammaliaforms went through an evolutionary bottle-neck of size reduction in phylogeny as compared to non-mammaliaform cynodonts (Lautenschlager et al. 2018), this does not preclude the basalmost mammaliaforms showing size differentiation among themselves. For example, the largest specimen of Sinoconodon rigneyi has a skull length over 60 mm, and is estimated to have a body mass over 500 grams (Kielan-Jaworowska et al. 2004; Luo et al. 2004). Among the best sampled mammaliaforms in the Lower Jurassic fissure-fill sediments of the United Kingdom (reviewed by Parrington 1971; Kermack et al. 1973; Evans and Kermack 1994; Gill 2004; Clemens 2011), several morganucodontan taxa, such as Bridetherium dorisae and Pacyodon davidi, are now recognized by differences in cusp morphology and also by differences in tooth sizes (Clemens 2011). Elsewhere in the world, mammaliaform faunas of the Early Jurassic age display a wide generic diversity especially among docodontans and morganucodontans through the Triassic–Jurassic transition (Clemens 2011; Debuysschere et al. 2015). Parrington (1971, 1973) and Mills (1971) were the first to recognize that the posterior molars of Morganucodon-related taxa can be classified into different size groups, and the Morganucodon-like molars from fissure-fill sediments are polymorphic. Of these, some taxa of small tooth sizes have been further recognized as different taxa of morganucodontans (Clemens 2011). The posterior-most molars in these small-sized taxa (less than 1 mm in tooth length) are different from the m4s of Morganucodon watsoni (Clemens 2011). Mammaliaform taxa from the Late Triassic of Continental Europe have also long been known to show different tooth morphology and size disparity (Clemens 1980; Sigogneau-Russell and Hahn 1994; Debuysschere et al. 2015; Debuysschere 2017). Implicit in the discussion of size disparity and distinctive morphological differences is that the tooth size can differentiate these mammaliaform taxa if corroborated by morphological differences. Importantly, the small size of a fossil does not necessarily mean it would be a juvenile individual of a larger species (Clemens 2011; Debuysschere et al. 2015).

Hadrocodium wui adds to the size disparity of the Early Jurassic Lufeng mammaliaforms. The Lower Lufeng Formation mammaliaform assemblage has three distinctive size groups: Sinoconodon rigneyi is the largest (skull size ranges from the smallest at 22 mm in length to the largest at 62 mm in length); Morganucodon oehleri is intermediate in body size (skull size ranges from the smallest at 27 mm in length up to the largest around 38 mm in length) (Luo et al. 2004); H. wui at 12 mm in skull length is the smallest (Luo et al. 2001). The size difference between these taxa is now also corroborated by differences of Hadrocodium from other mammaliaforms in postcanine formula, canine structure, and occlusion of molar crowns. The posterior molars may also show variable root structure.

Implications for middle ear evolution of mammals.—This new CT study has revealed that the middle ear elements were still attached to the postdentary trough in H. wui. Given that Hadrocodium is close to the ancestral node of the crown Mammalia, this adds to the evidence that stem mammaliaforms all had middle ear elements connected to the mandible (Luo et al. 2016, 2017; Luo and Manley 2020). The detached middle ear evolved separately in monotremes, multituberculates, and crown therians.

Conclusions

The CT scans and visualization show that Hadrocodium wui has plesiomorphic mandibular and middle ear characters. The new dental characters of H. wui indicate that it differs from Morganucodon oehleri and Sinoconodon rigneyi from the same fauna of the Lower Jurassic Lower Lufeng Formation. Additionally, the newly characterized features of the holotype of H. wui suggest that it is an adult, or a young adult, capable of independent feeding. It is not a juvenile of an existing mammaliaform taxon of the same fauna.

Acknowledgements

For the many contributions by our esteemed colleague Professor Richard L. Cifelli (Oklahoma Museum of Natural History, Norman, USA) to the study of early mammalian evolution and his extensive field exploration of Mesozoic mammal fossils, it is our privilege to be able to contribute this new study on a Jurassic mammaliaform for the festschrift issue of Acta Palaeontologica Polonica in his honor. We are grateful for the invitation by Brian M. Davis (University of Louisville, USA). This study would not be possible without the discovery the fossil of Hadrocodium by Xiaochun Wu (Canadian Museum of Nature, Ottawa, Canada), and we are much indebted to Ai-Lin Sun (IVPP) and Xiaochun Wu for making this precious specimen available for our studies. The studies of Hadrocodium would have been impossible without the supreme skills of the late William A. Amaral (MCZ) who patiently prepared out the external morphologies of this very small fossil. We like to thank Richard Ketcham (Jackson School of Geosciences, University of Texas, Austin, USA) for scanning the fossil, and Thomas E. Macrini (University of Texas, Austin, USA and St. Mary’s University, San Antonio, USA) for helping to process the CT data. We also thank Qing-Jin Meng and Jianjun Li (both BMNH), for allowing us to access comparative specimens during this study, and Pamela Gill (University of Bristol, UK) and Kai R.K. Jäger (University of Bonn, Germany) for providing images of Morganucodon teeth for our study. During the course of this work, we benefitted from discussion with Richard L. Cifelli, James A. Hopson (The University of Chicago, USA), Thomas Martin (University of Bonn, Germany), Yuan-Qing Wang (IVPP), Xiao-Chun Wu, Kai R.K. Jäger and the late Professor William A. Clemens. For interpretation of mammaliaform mandibles and teeth, we benefited from the extensive feedback and comment by Pamela Gill, Stephan Lautenschlager (University of Birmingham, UK), Elsa Panciroli (University of Oxford, UK), and Julia A. Schultz (University of Bonn, Germany). The authors are very grateful for the thoughtful critiques and many suggestions for improvement by Brian Davis, Pamela Gill, and Thomas Martin, although any errors or insufficiency remain the authors’ own.

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Acta Palaeontol. Pol. 67 (1): 95–113, 2022

https://doi.org/10.4202/app.00949.2021