Systematic Paleontology

Class Mammalia Linnaeus, 1758

Superlegion Trechnotheria McKenna29

Family Zhangheotheriidae Rougier, Ji and Novacek25

Anebodon luoi, gen. et sp. nov.

Holotype: A partial skull and associated dentaries with nearly complete upper and lower dentition (STM 38-4, Tianyu Museum of Nature, Shandong Province, China. Figs 1, 2, 3, Figs S1 and 2 and Table 1).

Table 1 Measurements (in mm) of the dentition of the zhangheotheriid Anebodon luoi, gen. et sp. nov. L-left side; R-right side. Full size table

Figure 1 Stereophotographs of the skull of the zhangheotheriid Anebodon luoi, gen. et sp. nov., in dorsal (a), ventral (b), right lateral (c) and left lateral (d) views; illustrations of the skull in dorsal (e), ventral (f) and left lateral (g) views. Illustrations are enlarged to show detail and are not to same scale as photographs. Dotted fill represents matrix. Abbreviations: al, anterior lamina of petrosal; as, alisphenoid; ax, axis; bs, basisphenoid; ef, ethmoidal foramen; fdv, foramen for frontal diploic vein; fr, frontal; if, incisive foramen; iof, infraorbital foramen; lac, lacrimal; lf, lateral flange of petrosal; mapf, major palatine foramen; mpf, minor palatine foramen; mx, maxilla; na, nasal; os, orbitosphenoid; otc, anterior opening of orbitotemporal canal; pa, parietal; pal, palatine; pmx, premaxilla; smx, septomaxilla; spf, sphenopalatine foramen. Full size image

Figure 2 Right (a–f) and left (g–l) dentaries of the zhangheotheriid Anebodon luoi, gen. et sp. nov., in lateral (a,d,g,j), dorsal (b,e,h,k) and medial (c,f,i,l) views. Abbreviation: mg, Meckel’s groove. Full size image

Figure 3 Left upper (a–d) and left lower (e–h) dentition of the zhangheotheriid Anebodon luoi, gen. et sp. nov., in occlusal (a,c,f,h), buccal (b,d) and lingual(e,g) views, showing canine and postcanine teeth. The lower m4 is reflected from the right side. Full size image

Etymology: From the Greek anebos, young, in reference to the relatively late replacement of the last premolar position and the Greek odontos, tooth, a common suffix for Mesozoic mammals. The species name honours Zhe-Xi Luo for his contribution to the study of Mesozoic mammals.

Horizon and Locality: Yixian Formation, Lower Cretaceous, dated about 123 Mya; Beipiao, Liaoning Province, China.

Diagnosis: Zhangheotheriid differing from Maotherium and Zhangheotherium in having a postcanine dental formula of P5/4 M3/4 and in more obtuse triangulation of distal molars. Further differs from Zhangheotherium in having upper molars with deeper ectoflexus, subequal cusp B and metacone and well-developed lingual cingulum and lower molars with paraconid taller than metaconid. Differs from Maotherium and Kiyatherium in the absence of upper molar parastyle and less reduction of ultimate molar. Most closely resembles Kiyatherium but differs in having double-rooted upper canine, molariform P3/p3 and weaker upper molar ectocingulum and lingual cingulum.

Description and Comparisons

Skull

A detailed description and additional images of the skull, dentary and dentition of Anebodon can be found in the Supplementary Information. Anebodon is now the third zhangheotheriid known by cranial material. Unfortunately, most of the descriptions of Maotherium and Zhangheotherium are focused on the character-rich petrosal, which is not preserved in our new taxon. However, some comparisons with these taxa can be made from the limited overlapping described skull regions and from gleaning morphological information from the character dataset of Ji et al.26. The skull of Anebodon is relatively low-vaulted and generally primitive in construction (Fig. 1), as in other zhangheotheriids. While generally well preserved in three dimensions, the skull is distorted such that the right side is sheared anteriorly relative to the left side (most apparent along the posterior margin of the palate, Fig. 1b,f). This distortion affects to some degree the appearance and relative sizes of the bones in the orbitotemporal region and what remains of the sidewall of the braincase and the associated crushing has made some sutural boundaries difficult to interpret. The slender appearance of the snout (in dorsal and ventral views) is due to the collapse of most of the right half of the palate. In general proportions, the skull of Anebodon would likely have resembled that reconstructed for Maotherium sinensis25.

The lateral margin of the nasal cavity is formed by a relatively large septomaxilla (Fig. 1d,g), a primitive condition shared with other zhangheotheriids. This bone bears a medially-directed horizontal process, though it is unclear if a septomaxillary foramen is present due to matrix infill. The vertical portion of the bone tapers posterodorsally but is damaged, making it impossible to determine if it contacted the maxilla (as in Vincelestes30) or if the facial process of the premaxilla separated it from the maxilla (as in docodonts31). The premaxilla is similarly damaged, so contact between it and the nasals cannot be determined. A moderate-sized infraorbital foramen is present on the facial portion of the maxilla above the ultimate premolar, but this region is damaged and no additional foramina are visible. The nasals are anteriorly narrow in dorsal view and broaden considerably posteriorly towards their relatively straight contact with the frontals (Fig. 1a,e), as in Maotherium sinensis25. It should be noted that the nasals are scored as posteriorly narrow by Ji et al.26 in both Maotherium and Zhangheotherium. No nasal foramina are visible.

The hard palate extends posterior to the last upper molar and palatal vacuities and postpalatine torus are lacking (Fig. 1b,f). Alveoli for four subequal incisors, I1–I4, are contained entirely within the premaxilla. The I4 alveolus is concave laterally, contouring a pronounced groove on the root of the tooth. The canine alveolus is entirely within the maxilla. A thickened posterior prong of the palatine bears a well-defined minor palatine foramen opening into the floor of the orbit (Fig. 1f,g).

The frontal and lacrimal have limited orbital exposure, with most of the anteromedial wall of the orbit formed by a large perpendicular process of the palatine (Figs 1 and S1), the condition in mammaliaforms such as Morganucodon and Haldanodon31,32 and differing from that in multituberculates which have large orbital contributions from the frontal and maxilla33. While not described, the zhangheotheriids Maotherium and Zhangheotherium were scored as having expansive orbital exposure of the maxilla26. The ethmoidal foramen is positioned at the suture between frontal and palatine and the frontal forms the entire anterior opening of the orbitotemporal canal (Fig. S1). A well-defined, anterodorsally-directed foramen is present at the presumed posterior extent of the palatine where this bone would have met the sphenoid complex. A possible identification for this foramen is the transverse canal, a feature common among extant metatherians and in some eutherians34,35, but so far limited among non-therians to the multituberculate Kryptobaatar33. The orbitosphenoid is comparable in development to Morganucodon31 and Kryptobaatar. The alisphenoid is interpreted as a relatively large and broad element, contacting the frontal at its anterodorsal margin. Much of the inferior portion of the alisphenoid is missing; accordingly, other major openings in the sidewall of the braincase cannot be identified. A thickened vertical ridge is interpreted as the suture between alisphenoid and anterior lamina of the petrosal. The posterior extent of the latter element is missing, but it appears likely that the relative proportions of these two bones were similar to the condition in the zatherian Vincelestes36.

The alveolar portions of both dentaries are well preserved, but each is missing most or all of the posteroventral portion behind the tooth row. From available evidence, the dentary of Anebodon differs little from other zhangheotheriids; the coronoid process ascends at a low angle (~150°) and a well-defined Meckel’s groove is present on the medial side of the dentary (Fig. 2).

Dentition

The dental formula of Anebodon is interpreted as I4/3 C1/1 P5/4 M3/4, with the left side of the specimen preserving evidence of the entire upper and lower dentition (Figs 1,2 and S2). Anebodon is referred to the Zhangheotheriidae based primarily on features of the dentition. Anebodon can be distinguished from tinodontids in that the upper and lower molars are better triangulated (Fig. 3). It differs from spalacotheriids in the strong development of cusp B and retention of a large metacone, as well as the presence of a strong lingual cingulum and the lack of mesiodistal compression of the molars. Both upper and lower canines are small in Anebodon, a feature shared among tinodontids and zhangheotheriids. The upper canine crown is premolariform and double-rooted, while the lower crown is incisiform and single-rooted.

The first two upper and lower premolars in Anebodon are premolariform. The P2 is very small and has a single, heavy root though it is grooved buccally and may divide deeper in the alveolus (Fig. 3b,d). Due to its small size, it is possible that this tooth could be interpreted as a retained DP1, but based on the very limited evidence for replacement at this position in Mesozoic mammals we prefer a conservative approach. The lower p2 bears a distinct but damaged metaconid. The remaining premolars are strongly molariform, with the upper premolars having a progressively better-developed lingual cingulum (complete on the P5). The P4 is the tallest tooth in the postcanine series, even accounting for the incomplete eruption of the P5. The crowns of the P5/p4 lack wear and are not fully erupted, as the distal base of the crowns has yet to clear the alveolar margin (Fig. 3b,e); evidence of replacement at these tooth positions establishes a minimum upper and lower premolar count which we interpret to be the full adult condition (see Discussion below). Among zhangheotheriids, Anebodon differs most strikingly from the well-represented Zhangheotherium and Maotherium in postcanine dental formula. These two genera, known by multiple nearly complete skeletons, have postcanine counts of P2/3 M5/5-6 and P1-2/1-3 M4-5/6, respectively23,24,25,26. These differ considerably from the count of P5/4 M3/4 in Anebodon. We consider the increase in number of molars in some zhangheotheriids to be derived (but see discussion below), with our new taxon more closely resembling tinodontids in this regard.

The mesial molars in Anebodon are more acutely triangulated than the ultimate premolars (M1 = 95°, m1 = 85°). The main accessory cusps are well developed, though the metacone/metaconid and metastylar region decrease in size distally (Figs 3 and S2). The upper molars have a complete and rather crenulated lingual cingulum. Molars of Anebodon differ from Zhangheotherium and Maotherium in lacking progressively more acute triangulation of the three main cusps distally through the molar series, a feature which these other taxa share with more derived spalacotheriids22. Instead, the cusp angle in Anebodon increases slightly distally and by the ultimate lower molar (m4) the main cusps are nearly in-line (Figs 3h and S2k). Anebodon further differs from Zhangheotherium in the strong development of the upper molar lingual cingulid and greater depth of the ectoflexus; upper molars of the new taxon differ from those of Maotherium in lacking a parastyle and development of a mesially-projecting parastylar hook.

Among known zhangheotheriids, comparisons to Kiyatherium cardiodens28 are most favourable. The two partial maxillae referred to Kiyatherium preserve evidence of the entire postcanine dentition, originally interpreted as P1–4 and M1–4 with the first upper molar undergoing replacement (designated as RM128). However, molars are by definition never replaced37 and this fifth postcanine should be identified as the P5. Contrary to the original interpretation, it is likely that both maxillae referred to Kiyatherium bear the entire molar series, with the M3 more reduced in size in the holotype specimen28. Though damaged, it is clear that the P5 of the referred specimen is less worn than the M1 and belongs to a different tooth generation. Under our interpretation, Anebodon and Kiyatherium share a postcanine dental formula of P5/4 M3/4. These two taxa are also broadly similar in molar morphology and differ only in that Anebodon possesses a relative more molariform the third premolar, double-rooted upper canine, weaker development of upper molar cingula and less reduction of the ultimate upper and lower molars.