Occurrence of “Hippotherium” in the Old World: a revision of two hipparion species in Eurasia

doi:10.19615/j.cnki.2096-9899.241120

Abstract

Abstract:

A controversial taxon, Hipparion plocodus, is reviewed in the present study. Hi. plocodus has been confirmed to be a valid species with definite diagnostic characteristics, represented by cranial specimens from Baode, Shanxi Province. The phylogenetic analysis performed in the present study, with a new matrix, shows that Hi. plocodus forms a monophyletic group with a European species, Hippotherium malpassii. Actually, no close relationship between so-called Hm. malpassii and the genus Hippotherium has been identified, and the record of stratigraphic range of this genus in late stage of Late Miocene is currently absent. Herein previously Hi. plocodus and Hm. malpassii have both attributed into “Hipparion” before the discovery of better material. Evolutionary stages and correlative absolute age showed that these two species should derive independently from some primitive clade. During the late stage of the Late Miocene, the development of the Asian summer monsoon enhanced the humidity of China, with forest and wood habitats expanding considerably under this setting. As the result, one Eurasian closed-habitat lineage thus extended its range into China, which had become very suited for it, give rise to “Hi.” plocodus.

Key words: Eurasian hipparion, systematic revision, phylogeny, paleozoogeography

摘要：

对一个具有争议的类群Hipparion plocodus进行了回顾。山西保德的头骨材料具有明确的鉴定特征，证实Hi. plocodus为一有效种。使用新的特征矩阵进行系统发育分析，表明Hi. plocodus和欧洲种Hippotherium malpassii构成一个单系群。事实上所谓的Hm. malpassii和Hippotherium属无较近的亲缘关系，而该属目前无晚中新世晚期分布记录。因此之前所定义的Hi. plocodus和Hm. malpassii在更好的材料发现之前暂时归入“Hipparion” 属当中。二者演化水平略有差异，时代相当，表明它们独立起源于某一更原始类群。在晚中新世晚期，亚洲夏季风的盛行加强了中国地区的湿润程度，森林和灌木林生境在这一背景下迅速扩张，最终令欧亚大陆某一喜好封闭环境的类群向中国扩散，在这片适应的土地上演化出了“Hi.” plocodus。

关键词: 欧亚三趾马, 系统分类修订, 系统发育, 古动物地理

CLC Number:

Q915.877

SUN Bo-Yang, LIU Yan, WANG Shi-Qi, DENG Tao. Occurrence of “Hippotherium” in the Old World: a revision of two hipparion species in Eurasia. Vertebrata Palasiatica, 2025, 63(1): 57-80.

孙博阳, 刘艳, 王世骐, 邓涛. 2025, 63(1): 57-80, 旧大陆“Hippotherium”的存续：两种欧亚大陆三趾马的修订. 古脊椎动物学报.

Figures/Tables 7

Table 1 Skull measurements of “Hipparion” plocodus (mm)

item	M 3824	M 3825	AM 146439	AM 146440	AM 146441	AM 146442	AM 146443	AM 146448
muzzle length	117.2	107.9	114.1	109.5		108		109
palatal length	101.3	104.4	99.3	99.2	103.5	103.8	104.1	96
vomerine length								92.2
premolar length	74.7	75.7	67.6	73.1	67.9	73.8	71.4	63.8
molar length	59.5	59	52.9	57.7	56.8	60.9	57.9	54.5
upper cheek teeth length	136	137	120.3	128.7	125.5	134.9	130.3	118.3
minimal breadth of choanae				33.7	34.1		30.3	31
maximal breadth of choanae				36.3	41	39	35.5	37.2
palatal breadth	66.6	64.9	58.3	59.3	59.4	63.2	57	53.1
minimal muzzle breadth	35.5	35.3	35.5	35.9		37.1		35.7
muzzle breadth	42	46.9		52.5		55.4	51.6	48.6
frontal breadth							140.5
anterior ocular line								311.8
facial height								83.7
antero-posterior orbital diameter								53.4
orbital diameter perpendicular to the former								50.2
length of the naso-incisival notch				113.3				109
cheek length				150.5			149.7	151.6
POB length			37.2	32.1	40.9		37	30.8
POF length			90.3	81.5			87.2	87.3
distance between the back of POF and IOF				60.6			57.9	60.6
height of the POF			42.4	43.5			43	52.3
distance between POF and the facial crest			24.3	41.3	36.2		40	27.6
height of back of IOF				47.9			43.1	45.6
height of the back of POF above the alveolar border			72.3	75.6	69.4		82.5	77.2

Table 2 Upper cheek tooth measurements of “Hipparion” plocodus (mm)

Teeth	N	Measures	Avg	Min	Max
P2	4	L	27.4	24.9	28.6
	2	W	20.8	19	22.5
	3	PL	6.7	5.6	8.4
	2	PW	3.7	3.4	4
P3	6	L	21.3	18.6	22.6
	5	W	21.6	20.6	23
	6	PL	6.3	4.9	8
	5	PW	4.1	3.1	5.2
P4	6	L	20.0	18.7	21.3
	5	W	21.0	20.2	22.4
	6	PL	6.0	4.8	7
	6	PW	4.4	3.3	5.7
M1	6	L	17.7	16.2	19.2
	5	W	19.4	18.9	20.1
	6	PL	6.1	4.6	6.7
	6	PW	4.0	2.7	5.1
M2	6	L	18.0	16.4	19.1
	4	W	19.0	18.3	19.8
	6	PL	5.7	4.7	6.4
	5	PW	4.1	3	4.8
M3	6	L	20.4	19.1	22.9
	5	W	18.4	15.7	24
	6	PL	5.6	5.1	6
	5	PW	3.6	2.5	4.9

Table 3 Lower cheek tooth measurements of AMNH F:AM 146708 (mm)

Tooth	item	Measurement	Tooth	item	Measurement
p2	L	26.6	m1	L	19.8
	W	12.7		W	11.7
	FL	11.2		FL	8.1
	FI	42.1		FI	40.9
p3	L	22.9	m2	L	20.3
	W	14.7		W	10.8
	FL	10.1		FL	7.2
	FI	44.1		FI	35.5
p4	L	21.2
	W	14.4
	FL	8.9
	FI	42.0

Fig. 1 “Hipparion” plocodus in Lok. 49, Baode, Shanxi A. ventral view of lectotype PMU M 3824; B. left view of AMNH F:AM 146448; C. AMNH F:AM 146708: C1. left view, C2. occlusal view of cheek tooth row; D. AMNH F:AM 146443: D1. left view, D2. occlusal view of left cheek tooth row. Scale bars = 2 cm

Fig. 2 Comparison on occlusal morphology of upper cheek teeth of “Hipparion” plocodus and “Hi.” malpassii A. left cheek tooth row of lectotype of “Hi.” plocodus, PMU M 3824; B. left cheek tooth row of “Hi.” plocodus, AMNH F:AM 146443; C. left cheek tooth row of holotype of “Hi.” malpassii, IGF 9400V (Bernor et al., 2011:fig. 13b); D. left P2 of “Hi.” malpassii, NHMB JH 126A (Bernor et al., 2011:fig. 15b); E. left P3 of “Hi.” malpassii, NHMB JH 126B (Bernor et al., 2011:fig. 15c); F. left M1 of “Hi.” malpassii, NHMB JH 126D (Bernor et al., 2011:fig. 15e);G. right P4 of “Hi.” malpassii, NHMB JH 126F (Bernor et al., 2011:fig. 15f). Scale bar = 4 cm

Fig. 3 Correlation of geographical distributions and phylogenetic relationship of Cormohipparion and Eurasian hipparion lineage based on the strict consensus tree from most-parsimonious trees Abbreviations: CI. consistency indece; G. Galerian; Gao. Gaozhuangian; Maz. Mazegouan; MN. European Neogene Mammal units; RI. consistency indece; Rus. Ruscinian; S. Salawusuan; Z. Zhoukoudianian

Fig. 4 Paleozoogeographical comparison of hipparion taxa in Bahean (A) and Baodean (B) Dispersal routes are referred to Garcés et al. (1997) and present research Greens and yellows distributed in Mid-latitude Asia, Europe and North Africa after Fortelius et al. (2014), respectively show the relatively close and open habitat

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