<i>Sussemionus</i>, a new subgenus of <i>Equus</i> (Perissodactyla, Mammalia)

Véra Eisenmann

doi:10.1016/j.crvi.2009.12.013

Résumés

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The new subgenus of Equus, Sussemionus, is defined by peculiar dental characters so far unknown, or exceptional in late Pleistocene and extant Equus; it was in consequence assumed to be restricted to the early and middle Pleistocene. During that period, it was highly successful, ranging from North America to Ethiopia, and included dry-adapted (E. granatensis-like) and more humid-adapted (E. coliemensis-like) species. Recent molecular and osteological analyses concurred to prove its survival until ca 45 KYBP in Khakassia, southwest Siberia, Russia.

Le nouveau sous-genre d’Equus, Sussemionus, très répandu au cours du Pléistocène moyen, est défini par l’originalité de ses caractères dentaires : plis caballins à large base, parfois multiples ou renflés, protocones parfois très courts ; développement extraordinaire des stylides, métaconides très allongés et parfois bilobés, sillons linguaux à peine marqués, sillons vestibulaires parfois très profonds, même sur les prémolaires. De telles morphologies, rarissimes chez les Equus actuels, étaient jusqu’à présent inconnues au Pléistocène supérieur ; on supposait donc le groupe éteint depuis le Pléistocène moyen. De récentes études moléculaires et paléontologiques ont montré que ce n’était pas le cas : une espèce appartenant à ce sous-genre est en effet documentée dans une grotte du sud-ouest de la Sibérie ; le matériel date d’environ 45 000 ans.

1 Description

The term “Sussemiones” was first used in an informal way to group early and middle Pleistocene equid species with characteristic teeth [1]. Discoveries of new data, both paleontological and molecular [2,3], recommend a formal description and extend the temporal range of this group.

Sussemionus, n. subgenus.

Derivatio nominis:

In reference to the mixture of osteological characters, some of which are observed (namely) in the fossil equids from Süssenborn, Germany, and some others in extant hemiones.

Diagnosis:

Upper cheek teeth (Fig. 1) with peculiar plis caballin: multiple and/or with a very large base (Fig. 1A, Fig. 4A, Fig. 6), sometimes club-shaped (Fig. 1B). Such morphologies are unknown in extant species of Equus and in Allohippus. The enamel is often very plicated and the postprotoconal valley may be very deep. Protocones may be extremely short.

Fig. 1
Upper premolars of Sussemionus. A. E. cf. verae, section of PIN 2998-243, Chukochya, loc. 26. B. E. hipparionoides, P4/ of Akha 100, Akhalkalaki. Illustrations of these teeth were also published in [1].

Fig. 4
E. coliemensis. A. Left P2/-M3/ of the type skull IA 1741. B. Right P/2-M/1 and M/3 of IA 1721. Illustrations of these teeth were also published in [1].

Fig. 6
Upper premolar from the Acheulean of Gomboré II (Melka Kunturé, Ethiopia), 73-1978. Illustration also published in [1].

On the lower cheek teeth (Fig. 2), the occurence of stylids, sometimes isolated, is remarkable. Isolated ectostylids are characteristic of late African hipparions but exceptional in extant Equus. Plis protostylids on P/2 (characteristic of extant Grevy's zebras [4]) occur frequently (Fig. 2A). Plis protostylids on P/3-M/3 may be extremely developed (Fig. 2B) as well as plis hypostylids (Fig. 2C); the latter may even be isolated on M/3. The shape of the double knot of many lower premolars resembles extant hemiones, sometimes in an extreme, caricatural way: the metaconid is elongated, sometimes bilobated, the lingual valley is shallow, at times nearly absent (Fig. 2D and E). Unlike Hemiones, another particularity is the frequency of very deep vestibular valleys, on molars and even on some premolars (Fig. 2F). But the depth of the vestibular valley is very variable: associated teeth may have very deep and very shallow valleys (Fig. 2G). Both features are uncommon in extant species.

Fig. 2
Lower cheek teeth of Sussemionus. A. P/2 of E. verae, Chukochya loc. 21, PIN 835-123. B. P/4 of E. hipparionoides, Akhalkalaki, no. 99. C. M/3 of E. cf. verae, Yukon, Old Crow loc. 9, NMC 32165. D. P/3 of E. granatensis, Venta Micena, VM 84/C3-B9-12. E. P/3 of E. verae, Krestovka, PIN 3020-47 (851-74/8). F. P/3 of E. suessenbornensis, Süssenborn, S 9281. G. Associated M/3 and M/1 of E. verae, Chukochya loc. 37, PIN 3100-333. Illustrations of B, C, D, E, and G were also published in [1].

Type species: E. coliemensis Lazarev 1980 [5].

Origin: Kolyma, NE Siberia, Russia.

Age: late Early Pleistocene.

1.1 Description of the type material

Although not perfectly preserved, the skull IA 1741, type of E. coliemensis (Fig. 3, Table 1) belongs to Equus. The skull is as large as a Grevy's Zebra but with proportions more like Hemiones [6].

Fig. 3
E. coliemensis skull, IA 1741. From left to right: ventral view, profile, dorsal view.

		Chukochya
		E. coliemensis
		IA 1741
1	Basilar length	538
2	Overall palatal length	282
2–5	Palatal length s.s.	140
3	Vomerine length	[133]
4	Post-vomerine length	[123]
5	Muzzle length	[140]
6	Diastema	112
7	P2/-P4 length	[101]
7bis	M1/-M2/ length	82
8	P2/-M3/ length	[180]
10	Greatest choanal breadth	53
10bis	Least choanal breadth	42
11	Facial breadth	164
12	Length from Basion to anterior borders of P2/	[398]
13	Frontal breadth	230
14	Bizygomatic breadth	210
15	Cranial breadth	105
16	Breadth of supra-occipital crest	53
17	Muzzle breadth at posterior borders of I3/	70
17bis	Least muzzle breadth (between the crests)	55
18	Greatest length	590
19	Height of the infra-ocular bar	11
20	Height of the external auditive meatus	16.5
21	Antero-posterior orbital diameter	62
22	Dorso-ventral orbital diameter	58.5
23	Anterior ocular line	440
24	Posterior ocular line	225
29	Breadth at the occipital condyles	90
30	Breadth of foramen magnum	34

The upper cheek teeth of the type are plicated and have plis caballins wide at their base (Fig. 4A, Table 2). There are no associated lower cheek teeth but some were referred to E. coliemensis by Lazarev (Fig. 4B, Table 3). On the lower cheek teeth, the enamel is plicated and the hypostylid very developed on M/3.

			P2/	P3/	P4/
			L	Prot L	W	L	Prot L	W	Ht	L	Prot L	W	Ht
E. süssenbornensis	Süssenborn	Type Wüst	44.2	11.4	30	33.6	12.6	30		35.4	15	33
E. süssenbornensis	Süssenborn	S 514		8.5	29	31	13.5	33		31.5	15	33
E. cf. süssenbornensis	Akhalkalaki	Akha 1285				36	14	32	27	30.4	12	32	26
E. coliemensis	Kolyma	IA 1741	39.7	8.3	27.2	32	12.7	30.5		29.6	12.2	29.6
E. granatensis	Venta Micena	C3-87	38.5	8	27	29	9	27		28	11	28
E. hipparionoides	Akhalkalaki	Akha 100				29	[7.5]	28	39	25	6.0	26
			M1/	M2/	M3/
			L	Prot L	W	Ht	L	Prot L	W	Ht	L	Prot L	W	Ht
E. süssenbornensis	Süssenborn	Type Wüst	30.4	12.5	31.7		31	13.5	28.7		30	14.2	25.2
E. süssenbornensis	Süssenborn	S 514	28	14	30		29	13.1	30		33.9	16.6	27.2
E. cf. süssenbornensis	Akhalkalaki	Akha 1285	27.3	11.7	30	27	29.5	14.5	30.5	32	34	15	26	27
E. coliemensis	Kolyma	IA 1741	26.6	11.4	27.9		26.2	12.3	26.1		28.4	12.4	24.8
E. granatensis	Venta Micena	C3-87	26	8.5	27		27	11	27		26.5	8	22.5
E. hipparionoides	Akhalkalaki	Akha 100	26	7.0	24	42	26	10	23.5	43

			P/2	P/3
			L	L DN	L postf	W	Ht	L	L DN	L postf	W	Ht
E. süssenbornensis	Süssenborn	S 9281	39	17.7	17	18	38	35	23	13	19.2	46
E. cf. süssenbornensis	Akhalkalaki	Akha 1	42	16.7	19	16	44	35	20	17.5	18	55
E. verae	Chukochya, loc. 21	PIN 835-123	41	20.5	18	18.7	[65]	34	20.5	17.5	22	70
E. granatensis	Venta Micena	VM 84 C3.B9.12	36.6	14.2	17.4	16		32.7	19.2	16.8	17.2
E. hipparionoides	Akhalkalaki	Akha 99	37			17	23.5	33	18.3	14	17	21
			P/4	M/1
			L	L DN	L postf	W	Ht	L	L DN	L postf	W	Ht
E. süssenbornensis	Süssenborn	S 9281	33	21	11.1	20	51	30	17.5	11	16	44
E. cf. süssenbornensis	Akhalkalaki	Akha 1	33	18.5	15.3	18	[67]	30	17	11	17
E. verae	Chukochya, loc. 21	PIN 835-123	32.5	20	16	22	77	30	18.3	16	19	67
E. granatensis	Venta Micena	VM 84 C3.B9.12	29.5	18	13.2	16.8		28.4	16.8	10.2	14.5
E. hipparionoides	Akhalkalaki	Akha 99	30.5	16.1	12.8	16.2	28	28.3	16.1	8.7	16.8	20
			M/2	M/3
			L	L DN	L postf	W	Ht	L	L DN	L postf	W	Ht
E. süssenbornensis	Süssenborn	S 9281	30	17.2	11	16	48	35	14.5	16.2	50
E. cf. süssenbornensis	Akhalkalaki	Akha 1	31.7	16.5	11	16	50	37	16		15	45
E. verae	Chukochya, loc. 21	PIN 835-123	29.5	17	14	20	67	35	15.5	14	17	69
E. granatensis	Venta Micena	VM 84 C3.B9.12	28	15.6	8.9	14.3		33	15.2	9.3	13.7
E. hipparionoides	Akhalkalaki	Akha 99	29	14.2	9	14.7	25

Other species already referred to Sussemiones – now the new subgenus (see description and illustrations in [1]):

– E. suessenbornensis (Süssenborn, Germany), E. cf. suessenbornensis (Akhalkalaki, Georgia; Cueva Victoria, Spain), E. verae (NE Siberia), E. cf. verae (Alaska, Yukon). The metapodials are large and robust;
– E. granatensis (Venta Micena, Spain), E. hipparionoides of Akhalkalaki, the very poorly defined E. altidens and the slightly better founded E. marxi of Süssenborn. Teeth and/or metapodials of this group are found in the British Forest Beds (Trimingham), and in Spain (Cueva Victoria, Cullar de Baza, Huescar). The metapodials are slender.

1.2 New data

Orlando et al. [2] analysed some late Pleistocene samples from Proskuriakova cave (Khakassia, southwest Siberia, Russia) believed to belong to a kind of E. hydruntinus. Unexpectedly, instead of an E. hydruntinus or some hemione, they found a new clade of Equus with no extant relative, and distinct from E. hydruntinus.

A more detailed osteological study of the fossils from Proskuriakova cave confirmed the biomolecular findings: the Equus from Proskuriakova is neither an hydruntine nor an hemione; it shows dental features of Sussemiones [3] (description in press). In the course of the same study, it was discovered that another late Pleistocene Equus, believed to belong to a hydruntine, should also be referred to Sussemiones. It was excavated at the Mousterian site of Tsopi, Georgia.

2 Discussion and conclusions

2.1 Origin of Sussemionus

In the late Blancan of Arizona, there is a very small species [7] reminding of Sussemionus (Fig. 5A). Another, much larger species, with marked Sussemionus characters (Fig. 5B) was found in Alaska inside probably Pliocene deposits (more than 2 Ma, A. Sher pers. comm.). It seems thus that, like most equids, Sussemiones originated in North America.

Fig. 5
Lower cheek teeth series tentatively referred to Sussemionus. A. AMNH 116502, Dry Mountains, Arizona, adapted from Azzaroli and Voorhies 1993. B. Lost Chicken, Alaska. Illustrations of these teeth were also published in [1].

2.2 Geographical and chronological ranges

The subgenus Sussemionus was very successful, judging by its geographical distribution from Arizona to possibly Ethiopia (Fig. 6), and its chronological range from ca 2 Ma to about 45–50 ka (Proskuriakova cave).

2.3 Environments

Dental and skeletal morphologies of extant Equus allow one to make assumptions as to the environment of fossil equids. Usually, a simple enamel is observed in species feeding on hard vegetation, while a more complicated enamel is found in species feeding on a softer one [8, p. 98]. Slenderness and cursorial proportions are linked to open landscapes, while robustness and ‘graviportal’ proportions are observed in opposite cases [9].

The extremely rich collection of equids from Venta Micena (Spain) include no other species than E. granatensis. This equid had a simple enamel, slender limb bones, and cursorial proportions [10]. On the other hand, in Chukochya, from where E. coliemensis and E. verae (possibly a junior synonym) were described, there is no evidence of a slender species. E. coliemensis had a very plicated enamel and robust limb bones.

Things are more complicated for the other findings where both slender and robust equids were excavated: the time encompassed by Süssenborn deposits is very long; Cueva Victoria may have been equally heterogeneous. It seems that Akhalkalaki is the only place where a robust form (E. cf. suessenbornensis) and a slender one (E. hipparionoides) have coexisted, at least in a broad paleontological sense.