Comprehensive research on Late Eocene Hulgana cf. H. ertnia (Rodentia: Ischyromyidae)

doi:10.19615/j.cnki.2096-9899.250620

Abstract

Abstract:

A relatively well-preserved rodent fossil, including its incisors, cheek teeth, and postcranial skeleton, was collected from the Baiyin Obo in Siziwang Banner, Nei Mongol. A multifaceted research approach was undertaken in this study to conduct a comprehensive analysis on the newly discovered specimen. Based on a morphological comparison, the new specimen was identified as Hulgana cf. H. ertnia within the Ischyromyidae family. Incisive enamel microstructure analysis revealed the typical pauciserial enamel structure of Hulgana. Bone histological analysis indicates that the specimen represents a juvenile individual, which is consistent with the ontogenetic stage indicated by dental developmental stage and wear pattern. The application of geometric morphometrics to the calcaneus and bone histology of the femur and phalanx further substantiates the taxonomic classification of Hulgana as a terrestrial and cursorial rodent, exhibiting a degree of fossorial ability. This classification is analogous to that of certain extant cricetids and ground squirrels.

Key words: Baiyin Obo, Nei Mongol, Late Eocene, Hulgana, geometric morphometrics, locomotion, enamel microstructure, bone histology

摘要：

内蒙古四子王旗白音敖包地区新发现一保存较为完整的啮齿类化石，材料包含门齿、颊齿及头后骨骼。运用多种研究手段对其开展综合研究。根据形态对比，这一新化石被归入壮鼠科蒙语鼠属并鉴定为古蒙语鼠相似种(Hulgana cf. H. ertnia)。对其门齿釉质微观结构进行分析，结果显示为典型的散系釉质结构。骨组织学分析指示该标本为一年轻个体，这一结果也与其颊齿发育阶段以及磨蚀程度相符。新标本跟骨的几何形态测量学分析结果和股骨与指骨的骨组织学证据均支持蒙语鼠为一种擅奔跑的地栖型啮齿类，同时具备一定掘穴能力，与部分现生仓鼠及地松鼠等相似。

关键词: 内蒙古白音敖包, 晚始新世, 蒙语鼠属, 几何形态测量学, 运动方式, 釉质微观结构, 骨组织学

CLC Number:

Q915.873

LI Lan-Xin, LI Qian. Comprehensive research on Late Eocene Hulgana cf. H. ertnia (Rodentia: Ischyromyidae). Vertebrata Palasiatica, 2025, 63(3): 189-209.

李岚馨, 李茜. 2025, 63(3): 189-209, 对晚始新世古蒙语鼠相似种(啮齿目：壮鼠科)的综合研究. 古脊椎动物学报.

Figures/Tables 9

Fig. 1 Materials of Hulgana cf. H. ertnia from the Baiyin Obo, Nei Mongol, China A. IVPP V33668.1, left M1-2 with fragmentary maxilla and palate; B. V33668.2, right M2; C. V33668.3, right M1 with root of P4, fragmentary right maxilla, jugal and palate; D. V33668.4, left m3; E. V33668.5, left m2; F. V33668.6, right m2

Fig. 2 Morphology of left calcaneus (IVPP V33668.7) (A) and right navicular in Hulgana cf. H. ertnia (V33668.8) (B) from the Baiyin Obo, Nei Mongol, China Bones shown in dorsal (A1, B3), ventral (A2), lateral (A3), medial (A4), proximal (B1) and distal (B2) views Abbreviations: ast. astragalar facet; cacu. calcaneocuboid articulation facet; cb. calcaneal body; cff. calcaneal groove for tendon or flexor fibularis muscle; cnl. calcaneonavicular ligament; ct. calcaneal tuber; dpt. distal plantar tubercle; ect. ectocuneiform facet; ef. ectal facet; gtpb. groove for tendon of peroneus brevis muscle; gtpl. groove for tendon of peroneus longus muscle; lpct. lateral process of calcaneal tuber; mes. mesocuneiform facet; mpct. medial process of calcaneal tuber; php. plantar heel process; pp. peroneal process; su. sustentaculum tali with sustentaculum facet

Table 1 Measurements of cheek teeth of Hulgana cf. H. ertnia and H. ertnia (mm)

		Hulgana cf. H. ertnia (V33668.1-6)			H. cf. H. ertnia Li and Li, 2023			H. ertnia Dawson, 1968
		n	range	mean	n	range	mean	n	range	mean
P4	L				1	3.15	3.15	2	3.50-3.90	3.70
	W				1	3.85	3.85	2	4.60-4.80	4.70
M1	L	2	3.42-3.63	3.53	7	3.40-3.70	3.53	2	4.10-4.40	4.25
	W	2	3.71-3.76	3.74	7	3.75-4.75	4.14	2	4.80	4.80
M2	L	2	3.62-3.71	3.67	2	3.35-3.55	3.45	2	3.70-3.90	3.80
	W	2	4.04-4.13	4.09	1	4.45	4.45	2	5.00-5.20	5.10
M3	L				1	3.40	3.40
	W				1	4.00	4.00
dp4	L				1	3.20	3.20
	W				1	2.60	2.60
p4	L				3	3.50-3.95	3.65	1	3.90	3.90
	W				4	3.00-3.40	3.18	1	3.90	3.90
m1	L				1	-	-	2	3.70-4.20	3.95
	W				1	-	-	2	3.90-4.30	4.10
m2	L	2	3.94-4.02	3.98	2	4.15-4.20	4.18	2	3.90-4.30	4.10
	W	2	3.47-3.61	3.54	2	3.75	3.75	2	4.10-4.40	4.25
m3	L	1	4.28	4.28	1	4.20	4.20	1	4.80	4.80
	W	1	3.65	3.65	1	3.50	3.50	1	4.60	4.60

Fig. 3 Results of principal component analysis of 15 calcaneal measurements for 17 taxa Circles mark 3 different known ecological types (terrestrial, arboreal, semiaquatic) of the 16 taxa Results support Hulgana cf. H. ertnia as a generally robust terrestrial ecomorph Colors for boxplots: highlighted red for Hulgana cf. H. ertnia; firebrick for terrestrial taxa; gold for generalists; deepskyblue for arboreal taxa

Fig. 4 PCA loading plot of calcaneal morphometrics showing variable contributions to principal components Note that percentage of explained variances is approximate

Fig. 5 SEM micrographs of transverse and longitudinal sections of upper incisive Schmelzmuster of Hulgana cf. H. ertnia (uncoded incisive specimen) from Baiyin Obo In transverse sections, EDJ is to the bottom while OES is to the top; in longitudinal sections, EDJ is to the bottom while the tip of the incisor is to the left A. transverse section; B. close-up view of transverse section; C. longitudinal section; D. close-up view of longitudinal section. Abbreviations: EDJ. enamel-dentine junction; HSB. Hunter-Schreger bands; IPM. interprismatic matrix; OES. outer enamel surface; P. prism; PE. portio externa; PI. portio interna; PLEX. prismless external layer

Fig. 6 SEM micrographs of transverse and longitudinal sections of lower incisive Schmelzmuster of Hulgana cf. H. ertnia (uncoded incisive specimen) from Baiyin Obo In transverse sections, EDJ is to the bottom while OES is to the top; in longitudinal sections, EDJ is to the bottom while the tip of the incisor is to the right A. transverse section; B. close-up view of transverse section; C. longitudinal section; D. close-up view of longitudinal section. Note that PLEX (prismless external layer) is hardly seen in the lower incisor Abbreviations see Fig. 5

Fig. 7 Histological features evident in femur of Hulgana cf. H. ertnia (V33668.9) from Baiyin Obo A. whole cross-section of the femur under normal light with posterior side at the top, showing suspected linea aspera; B. partial cross-section under normal light, showing Haversian bone, medullary cavity (MC), primary osteon (PO), resorption cavity (RC) and secondary osteon (SO); C. whole cross-section of the femur under polarized light with posterior side at the top, displaying resorption lines (RL); D. partial cross-section under polarized light, showing compact coarse cancellous bone (CCCB), drifted secondary osteon, Haversian bone, lamellar bone (LB), Sharpey's fiber, trabecular bone (TrB), woven bone (WB) and vascular canal (VC) Same plotting scale is utilized separately for A and C as well as for B and D

Fig. 8 Histological features evident in phalanx of Hulgana cf. H. ertnia (V33668.10) from Baiyin Obo A. whole cross-section of the phalanx under normal light with dorsal side at the top; B. partial cross-section under normal light, showing medullary cavity (MC), primary osteon (PO), secondary osteon (SO), Sharpey's fiber, vascular canal (VC) and suspective vascular notch (VN?); C. whole cross-section of the phalanx under polarized light with dorsal side at the top, displaying resorption cavity (RC), resorption line (RL) and Haversian bone; D. partial cross-section under polarized light, showing compact coarse cancellous bone (CCCB), endosteal bone (ELB), lamellar bone (LB), periosteal bone (PLB) and woven bone (WB) The style of usage for plotting scales is the same as in Fig. 7

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