Fossiomanus Mao et al., 2021

Genus Fossiomanus Mao et al., 2021

Type locality: Lamadong , Jianchang County, Liaoning province, China; Aptian , Lower Cretaceous .

Fossiomanus sinensis Mao et al., 2021 Fig. 1 View Fig .

Type material: ZGY0052, a nearly complete skeleton.

Type locality: Lamadong , Jianchang County, Liaoning province, China . Type horizon: Aptian , Lower Cretaceous Jiufotang Formation .

Material.— Holotype and the new specimen ( ZMNH M31795 View Materials ), a partial mandible discovered from the Jiufotang Formation at Lamadong site.

Description (mandible only).—The mandible, ZMNH M31795, is mainly represented by the dentaries, and the tooth row is badly damaged except for the lower incisors and one displaced left postcanine ( Fig. 1A 1 View Fig , A 2). The dentaries are slightly compressed dorsoventrally. The length of the mandible is 51.3 mm. The mandibular symphysis is transversely broad and not fused with a longitudinal suture in the anterior portion between the left and right dentaries. The suture exhibits an interlocking connection on the posteroventral portion of the symphysis, likely suggesting a partial fusion between the dentaries. On the ventral side of the symphysis, a zig-zag suture is present ( Fig. 1A View Fig 4). The posterior margin of the symphyseal facets is anteriorly concave and extends posteriorly to the level of the anterior-most part of the tooth row. The ventral margin of the dentaries is rounded and broad in the symphysis region. In lateral view, a mental foramen is visible on the lateral surface of the left dentary dorsal to the anterior part of the tooth row ( Fig. 1A View Fig 3).

Only one lower incisor is present in each dentary of the mandible, characterized by an enlarged and procumbent crown Fig. 1A View Fig 2, B). No additional small or developing lower incisors are present near this tooth locus on each side. The apical portion of the lower incisors is linguolabially compressed, featuring a longitudinal ridge along the midline of the crown, with sharp medial and lateral margins ( Fig. 1B View Fig 4). The basal portion of the lower incisors is more robust and rounded, and the roots appear to be open as indicated by the incomplete outline observed in the distal region. The lower canine is absent, as in all tritylodontids, and a distinct diastema separates the lower incisor from the postcanine row on each side. The diastema has a ridged dorsal margin. The postcanine rows run relatively straight along the length of each dentary with minimal twisting between the anterior and posterior teeth. A distinct internal groove is present on the medial surface of each dentary ( Fig. 1A View Fig

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One left lower postcanine is preserved, though slightly displaced ( Fig. 1A View Fig 2, C). The tooth measures 2.5 mm in length, 1.6 mm in width, and 4.4 mm in height (including both the crown and root). The postcanine tooth exhibits a cusp formula of 2:2, consisting of two longitudinal rows of equal-sized cusps Fig. 1C 1 View Fig ). The cusps are distally concave and separated by a deep, open groove. The postcanine tooth has two roots, mesial and distal, which are fused in their proximal half and only separate in their distal half ( Fig. 1C View Fig 2, C 3). The tip of the mesial root curves mesially, while the tip of the root shows damage to the distal region.

Remarks.—We assign this specimen to the recently reported species Fossiomanus sinensis based on the similarities of dental morphology and shared locality of discovery. The curvature, fusion of roots, and the cusp formula of lower postcanines closely resembles that of the holotype of F. sinensis, JZMP 2107500093. The preserved lower postcanine of ZMNH M31795 ( 2.5 mm in length) is much smaller than that of the holotype (approximately 4.5 mm in length, as measured from figures) ( Mao et al. 2021: extended fig. 3E). The replacement pattern of postcanines in tritylodontids is featured by teeth moving anteriorly, with oldest teeth lost at the anterior end of the postcanine row and newly erupted teeth being added at the posterior end of the postcanine row, so-called the “conveyor belt” system ( Kühne 1956; Matsuoka and Setoguchi 2000; Panciroli et al. 2017). Given that intraspecific variation in postcanine size is distinct in tritylodontids due to ontogenetic differences (e.g., Panciroli et al. 2017), the size difference of lower postcanines between these two specimens suggests that ZMNH M31795 likely represents a younger individual of F. sinensis . The lower postcanine in ZMNH M31795 is likely an anteriorly implanted, soon-to-be lost tooth. Fossiomanus sinensis with the preserved dentary length of 51.3 mm in ZMNH M31795, represents a medium-sized tritylodontid. This size of the mandible within the clade ranges from small taxa like Lufengia ( CVEB 12001, dentary length about 23 mm, cranium length estimated about 31 mm) ( Liu et al. 2022) to large-sized taxa like Kayentatherium (skull length over 220 mm) (Sues 1986b). The mandibular symphysis in Fossiomanus is transversely wider than in most Jurassic taxa, such as Oligokyphus , Dinnebitodon , and Kayentatherium ( Kühne 1956; Sues 1985a, 1986a, b). A zig-zag suture of the mandibular symphysis was noted in the holotype of F. sinensis by Mao et al. (2021), whereas the new specimen exhibits a longitudinally oriented suture in the anterior portion of the symphysis. In addition, the posteroventral portion of the symphysis tends to be partially fused with an interlocking suture in dorsal view. Recent studies in fossorial rodents suggest that compared to the cranium, mandible morphology is more adaptable to functional selective pressures ( McIntosh and Cox 2019). The fossorial adaptation to scratch-digging of Fossiomanus , as reflected by its postcranial morphology ( Mao et al. 2021), might also be enhanced by the shovel-like lower incisor and the broad, partially fused mandibular symphysis, which likely contributes to the stability of the mandible and the efficiency of penetrating and soil-grasping of the lower incisors, a similar mechanism as shown in fossorial rodents ( Kley and Kearney 2007; Van Wassenbergh et al. 2017; Mclntosh and Cox 2019).

Only one pair of lower incisors is present in Fossiomanus ( Fig. 1A View Fig 2, B), similar to Bienotherium (Young 1947; Cui and Sun 1987), and Kayentatherium (Sues 1986b) , but different from Bienotherodes ( Sun 1984; Cui and Sun 1987; Watabe et al. 2007) and Lufengia (two pair of lower incisors; Liu et al. 2022), and Oligokyphus (three pair of lower incisors; Kühne 1956). In lower incisors, the tooth crown is more laterally compressed, and the root is open posteriorly in Fossiomanus , different from that of the Early Cretaceous Montirictus from central Japan ( Matsuoka et al. 2016). Tritylodontids exhibit a pattern of horizontal tooth replacement, with the addition of newly erupting postcanines at the distal end of the tooth row ( Kühne 1956; Matsuoka and Setoguchi 2000; Jasinoski and Chinsamy 2012). Differences in tooth morphology are an important aspect in evolutionary morphology of tritylodontids, and distinct variations have been observed in height, number, fusion, and curvature of postcanine roots (e.g., Cui and Sun 1987; Averianov et al. 2017; Panciroli et al. 2017; Liu et al. 2022). The curvature of the roots in the lower postcanines increases anteriorly, whether the roots of lower postcanines are fused or not. In Fossiomanus , the double-rooted and partially fused condition of the lower postcanine roots differs from the long, separate, and distinctly curved condition in Bienotherium ( Cui and Sun 1987) and from the slighter shorter and less curved condition in Lufengia ( Liu et al. 2022) . The fusion of the proximal root in the lower postcanine of Fossiomanus is intermediate between that seen in the Middle Jurassic Stereognathus and Early Cretaceous Montirictus ( Matsuoka et al. 2016; Averianov et al. 2017). The fusion of lower postcanine roots tends to be more developed in some specimens of the Middle Jurassic derived Stereognathus sp. (with nearly fully fused roots; Averianov et al. 2017) and Bienotheroides single-rooted with complete fusion; Cui and Sun 1987).

Stratigraphic and geographic range.—Aptian, Lower Cretaceous Jiufotang Formation in Liaoning Province, China.