Reinvestigation of Yuanotherium minor and its implications for the cuspal homology and maxillary-palatal evolution of tritylodontids

doi:10.19615/j.cnki.2096-9899.250331

Abstract

Abstract:

Several tritylodontid taxa have been reported from the Upper Jurassic of the Wucaiwan area in the Junggar Basin of Xinjiang, northwestern China, including Yuanotherium minor. The original study described the partially preserved postcanine teeth in the middle of the left upper maxilla. After detailed re-examination of the specimen and by CT scanning, 3D reconstruction, and scanning electron microscopy observations, we provided a more detailed description of the osteology, neurosensory, and tooth wear pattern for all the bones preserved in this specimen and clarified some characters. Based on new information about the cusp wear pattern, the chewing movement pattern of the dentition and detailed cusp morphology, we discussed the cuspal homology of upper cheek teeth of tritylodontids and postulate a standardized method for cusp identification. We hypothesize that the unique maxilla characteristics furnish the evidence for transitional stages about the evolution of the upper jaw-palate structure in tritylodontids.

Key words: Upper Jurassic, Tritylodontidae, Yuanotherium, cuspal homology, maxillary-palatal evolution

摘要：

新疆准噶尔盆地五彩湾地区上侏罗统已经报道了数种三列齿兽类化石标本，其中包括小袁氏兽(Yuanotherium minor)。最初的研究主要描述了该标本左上颌中部部分保存的3颗不完整的颊齿。通过对标本的再研究，结合CT扫描、三维重建以及扫描电镜观测，对该标本保存的所有骨骼形态、牙齿磨蚀方式和神经通路进行了更详细的描述和重建，并对一些重要特征进行了厘定。基于牙齿咬合、磨痕、形态等信息，对上颊齿齿尖的同源性进行了讨论，提出了齿尖式的建议确认标准。根据小袁氏兽独特的上颌骨特征，提出了关于三列齿兽类上颌-腭部结构演化假说，并给出了过渡阶段的形态证据。

关键词: 上侏罗统, 三列齿兽类, 袁氏兽, 齿尖同源性, 上颌-腭部演化

CLC Number:

Q915.873

LIU Lu, REN Ji-Cheng, MAO Fang-Yuan. Reinvestigation of Yuanotherium minor and its implications for the cuspal homology and maxillary-palatal evolution of tritylodontids. Vertebrata Palasiatica, 2025, 63(2): 81-101.

刘璐, 任纪澄, 毛方园. 2025, 63(2): 81-101, 小袁氏兽的再研究及三列齿兽齿尖同源性和上颌-腭部演化的探讨. 古脊椎动物学报.

Figures/Tables 10

Fig. 1 The fossil locality and geological map in the Wucaiwan area, Xinjiang, northwest China

Fig. 2 CT-scan rendered images of Yuanotherium minor (IVPP V15335) with cranial elements segmented in different colors, in ventral (A), dorsal (B), medial (C), lateral (D), anterior (E), and posterior (F) views Abbreviations: eld. exit of the lacrimal duct; gpf. greater palatine foramen; iof. infraorbital foramen; La. lacrimal; laf. lacrimal foramen; mas. maxillary sinus; mlc. maxilla-lacrimal cavity; Mx. maxilla; ozf. orbital zygomaticofacial foramen; Pal. palatine; palp. palatal process of maxilla; zff. zygomaticofacial foramen; zyp-Mx. zygomatic process of maxilla

Fig. 3 The segmented left maxilla of Yuanotherium minor (IVPP V15335) in lateral (A), medial (B), anterior (C), posterior (D), dorsal (E), and ventral (F) views Abbreviations: avp. alveolar platform; palp. palatal process; PC1-3-av. alveolus for PC1-3; zyp-Mx. zygomatic process of maxilla. Scale bars=1.5 mm

Fig. 4 The segmented palatine of Yuanotherium minor (IVPP V15335) in anterior (A), posterior (B), dorsal (C), ventral (D), medial (E), and lateral (F) views Abbreviations: arp. air passage within palatine; gpf. greater palatine foramen;hp. horizontal plate of palatine; mxr. maxillary recess of palatine

Fig. 5 The segmented lacrimal of Yuanotherium minor (IVPP V15335) in anterior (A), posterior (B), dorsal (C), ventral (D), medial (E), and lateral (F) views Abbreviations: eld. exit of lacrimal duct; fap. facial process of lacrimal; iof. infraorbital foramen; laf. lacrimal foramen; mas. maxillary sinus; mlr. middle lacrimal ridge; mxr-La. maxillary recess of lacrimal; obp. orbital process of lacrimal; ozf. orbital zygomaticofacial foramen; vlr. ventral ridge of lacrimal; zff. zygomaticofacial foramen; zyp-La. zygomatic process of lacrimal

Fig. 6 Virtual 3D reconstruction of the maxillary canal system of Yuanotherium minor (IVPP V15335) in left oblique (A), transparent left oblique (B), left oblique orbital (C), and transparent left oblique orbital (D) views Abbreviations: ld. lacrimal duct; ioc. infraorbital canal; iof. infraorbital foramen; laf. lacrimal foramina; ozf. orbital zygomaticofacial foramen; rac. rostral alveolar canal; zfc. zygomaticofacial canal; zff. zygomaticofacial foramen

Fig. 7 The stereo-images of the three anterior postcanines of Yuanotherium minor (IVPP V15335) A, B. the three anterior postcanines in occlusal (A) and lingual (B) views; C-F. PC1 in occlusal (C), dorsal (D), anterior (E), and posterior (F) views; G-J. PC2 in occlusal (G), dorsal (H), anterior (I), and posterior (J) views; K-N. PC3 in occlusal (K), dorsal (L), anterior (M), and posterior (N) views Abbreviations: ac. accessory cuspules; B1. anterior cusp in the buccal row; B2. posterior cusp in the buccal row; L1, L2 and L3. anterior, median and posterior cusps in the lingual row, respectively; M1, M2, M3 and M4. anterior, median, posterior and the last cusps in the median row, respectively The numbers and arrows in D, H, L indicate the position of each root, whereas the arrows in E, I, M indicate the curvature in the anterior buccal root. Scale bars=0.5 mm

Fig. 8 The scanning electron microscope (SEM) photo of PC teeth of Yuanotherium minor (IVPP V15335), showing the wear facets and striations A. tooth row with the position of each zoom in B-H; B-H. close-up looking of the lingual cingulum of PC1 (B), the L2 cusp of PC1 (C), the lingual cingulum of PC2 (D), the M2 cusp of PC2 (E), the L3 + L4 cusps of PC2 (F), the lingual cingulum of PC3 (G), and the B2 cusp of PC3 (H) Blue arrows show the moving direction of the lower cusps when chewing, pink arrows show the major wear facets, red arrows show the position and direction of the wear scars on the functional cusps Note some abnormal dark areas on the photo are caused by glue

Fig. 9 Suggested cladogram of the Tritylodontidae based on the mode of reduction of the cusps of the upper postcanines Modified from Setoguchi et al. (1999). Figure not scaled

Fig. 10 Three patterns showing the maxillary evolution in tritylodontids A-C. diagrams showing ventral views of palatal elements of Kayentatherium, Yuanotherium, and Bienotheroides, respectively; D-F. cross sections corresponding to positions of D-F in A-C Abbreviations: av. alveolus; fap. facial process of maxilla; icf. incisive foramen; palp. palatal process of maxilla; zyp. zygomatic process of maxilla

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