Eutherian mammals from the Early Cretaceous of Mongolia

Zoologica Scripta. Vol. 18. NO. 2. pp. 347-355.1989 Printed in Great Britain

03W325W89 %3.00+ .oo Maxwell Pergamon Macmillan plc

0 1989 The Norwegian Acadcmy of Science and httcrs

Eutherian mammals from the Early Cretaceous of Mongolia

ZOFIA KIELAN-JAWOROWSKA and DEMBERLYIN DASHZEVEG Accepted 1 Aiqpt 19M

Kiclan-Jawiirinvska. Z. Dashzeveg. D. 19%). Eutherian mammals from thc Early Cretaceous of Mongolia.-Zm/. Srr. 18: 347-355.

A collcction o f cuthcrian mammals consisting of 39 specimens (tccth and jaw fragments) from the ?Aptiun or Alhian Khoboor Beds in the Gobi Desert, Mongolia. is described. It contains 3 taxa: Pro&ennule.ste.s gen.n.. assigned to the Otlestidae Nessov. new rank. with 2 shrcw size species; P. trofimovi sp.n. and P. minor spa.: a heavily worn larger lower molar which will be described elscwhcrc. It cannot be cxcluded that P. trojimovi and P. minor arc only sexual morphs within the same spccies. Pmkennulestes and Bobolestes Ncswv are possibly the oldest known cuthcricin mammals. but Prokennulesfes is morphologically more primitive than Buboleses. It has a labial mandibukdr foramcn, 5 prcmolan and 3 molars, 3 cusps in parastylar region. paracone larger than mctawnc. unwingcd conulcs. no prc- and postcingula. and lower molars with a 3-cusped talonid, which is narrower than the trigonid. Wlesres and Kennalestes may be derived with littlc modification from Prvkennalestes. Prokennalcstes gen.n. is congeneric with Prokennalestes Trofimov and Prozakumbdulestes Trolimov, which ST(: both nomina nu&.

Zojia Kielun-Jaworow?rku. Puleontologisk Museum, Universiretet i Oslo, Surs gate I . N-0542 Oslo 5. Norwuy. Demherlyin Dushzeveg. Geologicul Institute, Mon&un Academy of Sciences, Ulan Buror. MonKoliu.

Introduction

Beliajeva et al. (1974) made known the Sovict-Mongolian Palacontological Expedition's discovery of a rich fauna of Early Cretaceous (?Aptian or Albian) mammals, at the Khoboor (referred to also sometimes as Khovboor) locality in Guchin Us somon (county), in the Gobi Dcscrt, Mongolian Pcople's Republic. According to this preliminary report, the collcction amounted to about 500 spccimens. which are housed in the Paleontological Institute, U.S.S.R. Academy of Sciences, Moscow (abbreviated PIN).

Morc recently, one of us (D. D.) has made a more modest collection of tceth and jaw fragments which are housed in the Institute of Geology, Section of Palaeontol- ogy and Strdtigrdphy. Mongolian Acadcmy of Sciences. Ulan Bator (abbreviated GI P S q .

From these two collections representatives of the following taxa havc been described: Triconodonta (Trofi- mov 1978); Symmetrodonta (Trofimov 1980); Multituber- culata (Trofimov 1980; Kielan-Jaworowska et al. 1987); Eupantothcria (Dashzeveg 1979); Acgialodontia (Dashzeveg I Y75; Dashzeveg & Kielan-Jaworowska 1984).

In addition, thcrc are representatives of Eutheria, the first description of which is the object of this paper, based on material housed in GI PST.

Trofimov (in Beliajeva et al. 1974) named but did not describe or figure the eutherians Prokennalestes kozlovi and Prvzulambdalestes simpsoni. Since this date numerous scientists have been able to examinc these twa through the courtesy of Dr B. A. Trofimov, and know- ledge of them has been spread among scientists in the field. The generic names Prokennalestes and Pro- zalnmbdalestes have been cited as nomina nuda in relevant

literature (e.g. McKennd 1975; Crompton & Kielan- Jaworowska 1978; Clcmens er al. 1WY; fielan- Jaworowska el ul. 1979; Kielan-Jaworowska 1981, 1982, 1984; Fox 1984; Clemens and Lillegraven 1986; Novacek 1986; Nessov 1987) and even in the textbook Vertebrate paleontology and evolution (Carroll 1987). Wc have identified 3 eutherian taxa in our collection

from Khoboor. One is known from a single worn lower molar, 2.8 mm long, which will be described elsewhere. Two others are small congeneric species, which, on the basis of drawings which one of us (ZKJ) made in PIN in 1974, can be identified as belonging to the above cited nomina nuda. We cannot, however, make more detailed comparisons. as recently we havc not had access to the PIN collection. To rctain the name Prokennalestes Trofi- mov nomen nudum, we erect the new genus Proken- nalestes Kiclan-Jaworowska & Dashzeveg with two new species: P. lrofimovi and P. minor.

Terminology

In frokenndestes there occurs on thc labial sidc of the mandible. to thc rear of thc basc of the masseteric crest, a foramcn lading to thc mdndibUh canal, which we designate labial mandibular foramen. On the lingual side. in front of the mandibular foramcn. thcrc is in froken- nu1e.sre.s u shallow furrow. which is a remnant of thc rncckclian gmovc. Such a f u m w O C L ~ S in many McvaLoic mammals and has been referred to as the internal groove by Simpson (19%) and the inner groove by K r c h (1971). In Morganvrodnn Kiihnc the furrow is very decp and housed persistent Meckcl's cartilage. thus leading Kermack eral. (1973) ' to refer to it as the rneckelian groove. In our two new species the furrow is so shallow that it probably could not house Mcckcl'w cartilage; wc call it the remnant of the mcckclian groove.

The nomenclature of molar mwn morphology ildoptcd here follows thatofVanValen (lY66),modifidhy Rich(1971). Inthcparastyhraw in frokennulestes thcrc are three cusps, rather than the usual two (stylo~wnc and parastylc). Wc designate the additional third cusp, situated lingual to the purastyh, the PrCPZIrdstyk.

347 Zmlogica Scripta 18

348 Z. Kielan-Jaworowska and D. Dashzeveg

In primitive trihosphenic molars there often occurs a distinct ~vyp on the metacrista which Cnimpton (IY71) designated as cusp *c'. This term is misleading as cusp C (capital Icttcr) is a synonym of the mesostyle (Benslcy IyO.7) . We designate cusp 'c' of Crompton the mctacrista cusp.

The measurements of tccth have heen performed according to the method shown by Novacck (1976, fig. 2). All specimens referred to as maxillae and miindihles under Other material are fragmentary.

Wc use the following ahhrcviiition: long. longitudinil. parallel to the longitudinal axis of the skull: tr. transvernal. perpendicular to the long axis.

lnfraclass Eutheria Gill, 1872 Order Proteutheria Romer. 1966 Superfamily Kennalestoidea Kielan-Jaworowska, 1981; new rank

Family Otlestidae Ness~v, 1W, new rank

Emended diagnosis. Remnant of the coronoid bone and of the meckelian groove. Labial mandibular foramen. Upper molars with large stylar area, without pre- and postcingula. Preparastyle and metacrista cusp. Lingual part of the upper molars short (long.). Conules. Talonid narrower than trigonid. Remarks. Ncssov (1985~) crected Otlestinae within the Palaeoryctidae. Recently Dr L. A. Nessov informed us that he now thinks that the Otlestinae should have a family rank and we agree with this.

Included grtreru. Otlesteu Ncssov. 1985; hkennalestes gen.n.

Prokennalestes gen.n.

Prokennalesres Trofimov. IY74-nomen tiudum. PrazaIumbdu1este.v Trofimw. I974-namen nudum.

Diagnosis. Estimated skull length vanes between W and 27 mm. Mandibular foramen situated very low. pstrong, piercing, P-' semimolariform, without metacone, Ps with single main cusp. Upper molars with large parastylar area; preparastyle and metacrista cusp present, paracone larger than metacone; conulcs not winged; no pre- and postcingula. Paraconid lowest of trigonid cusps, connate at the base with metaconid. 3 talonid cusps, distance between hypoconulid and entoconid smaller than between hypoculid and hypoconid. Anterolingual cuspule and anterolabial cingulum.

Remark. Only two upper and lower premolars have been preserved in GI PST collection of Prokennalestes. As, however. there are 5 premolar loci in the Prokennalestes Trofimov (nomen nudum) mandible in the PIN collection (see McKenna 1975). and as Trofimov's nomen nudum is congeneric with Prokennalestes gen.n., we diagnose Pro- kennalestes as having 5 premolars.

Etymology. h t . pro before, earlier than Kennalesttzs Kichn- J a w o m k a .

Type species. Prokennalestes trvfimovi sp.n.

Additionul species. Prokennulestes minor sp.n.

Zoologicu Scripta I8

W

Fig. 1. I'rr~krrrrrok~.sti,.s tmfiriroi-i pen. CI sp.n.. rccoiisIruclion of the dentition.-A. P'-M". IILTIUS~II vicw.-l3. the same. lahiiil vicw.4'. P,-M,t occlusiil view.-D. 'I'hc siiiiic. liiiguiil view.-fi. 'I'hc siiiilc. Iihiiil view-I;. M: ;ulterior view--G. The siimc. posterior vicw.

Stratigraphic and geographical range. Early Cretaceous (?Aptian or Albian) of Mongolia.

Prokennalestes hlfinrovi spa. (Figs. 1, 2, 11-14, 19-21, 25-27)

Holotype. GI PST 104 . posterior part of the right mandible with MrMl (Fig. 20-21).

Other material. All numbers have the acronym GI PST. 10-5. right mandible with Ml-M3; I&%, right mandible with Ml-M3; 10-5c, left mandible with MrMI; 1 M a . right mandible with M,-M,; IMb. right mandible with MTM3; 10-8~. left mandible with M M 3 ; 10--14a, right MI ur M,; 10-14c. left mandible with PmrMI and root of P4; 10-14d. right mandible with PrMl and alveolus for P,; 10-10, left maxilla with P5-M'; 1&14e, right maxilla with partial M2 and M'; 10-17, right M I ; 10-24, r$ht M'; IW24ia right maxilla with P'-M'; 10-24b. left maxilla with P-M'; 10-24c. right maxilla with MI-M'; 10-28c, left M'; 10-228e. left maxillu with worn P!IM'.

Type horizon und locality. ?Aptian or Albian Khoboor Beds. Khoboor, Guchin Us somon. Gobi Desert. Mongolia.

Diagnosis. Estimated length of skull 24-27 mm. Mandible relatively robust, 2-2.2 mm deep below M2 in labial view. Two-lobed, relatively large coronid bone united to the dentary. Remnant of meckelian groove convex upwards, medial flange very prominent. P4 with small lingual extension and sharp posterior cutting edge. M' with strongly elongated parastylar area and distinct ectoflexus.

Early Cretaceous cutherian mammals 349

Etymology. Named in honour or our wlleaguc and friend, Dr Boris A. 'I'nfimov (Paleontological Institutc. Moscow).

Description (see Table I for measurements) Tooth Taxon n OR n OK n OR

Table 1. Mea..runments (in mm) vltcrh re..erable to Prokcnnabstw

Length Ant. width Post. width

Maxilla. Only small fragments of maxilla have been preserved. showing the posterior margin of the infraorbital foramen. situated above the posterior part of p.

Mandible. Anterior part of the mandible in unknown. The mandible consists of a relativcly robust body (about 2-2.2 mm dccp below M2 in labial view), and large ramus, strongly elongated postcriorly. Thc only prcserved mental foramcn is placed bclow the posterior root of P4. The lower margin is almost straight opposite the alveolar border and concave below the masseteric fossa. The coronoid process slopes upwards at an angle of about 45-50'. The masseteric crest is wide when seen in anterior view and widens towards the base. There is a short, nearly transverse upper margin, which continues posteriorly sloping down to the robust condyle. The margin between the condyle and angular process is concave. Therc is a labial mandibular foramen situated to the rear of the base of thc masseteric crest, partly obscured in lateral view. In inner view the two-lobed coronoid bone, united to the dentary . is sccn. Extending postero-ventrally from the mronoid bone toward the angular process there is a medial flange, which widens (in dorsal view) posteriorly. A relatively large. oval mandibular foramen (depression) is situated very low at the posterior part of the medial flange and is partly obscured in medial view. It contains two small foramina, the anterior one pointing forwards and the posterior one pointing posteriorly and down- wards. The angular process has been broken in all the spccimens, but the prescrvcd parts show that it was bent medially. The rcmnant of the meckelian groove starts a short distance below the end of the mandibular foramen and continues anteriorly in an irregular convex upwards curvc. It is relatively wide in its posterior part and tapers and disappears below the talonid of M3. Its entire length in the holotype specimen is about 5 mm. There is an additional narrower groove above it, starting just in front of the mandibular foramen; its length in the holotype specimen is about 1.7 mm. Size and shape of both grooves varies.

Upper dentition. Incisors, upper canine and P'-p are unknown. is the highest of the preserved series, with a piercing cusp. the posterior part of which is compressed laterally, to form a sharp, cuttingpostcrior edge. The cusp is surrounded by a small cingulum, which widens lingually. P' is submolariform. generally similar to the molars, from which it differs in the lack of the metacone, in having 2. rather than 3, cusps in the parastylar area (stylocone and parastyle) in the molars and in having a less deep ectoflexus. Along the ectoflexus there are 3 minutc styles. Then: are two cusps in the metastylar region. The metasty- Par region is separated from the rest of the stylar area by an oblique groove. In lateral view the paracone in lower than P' and than the paracone of MI. The preparacrista and postmetacrista (postparacrista) form strong ridges, the metacrista cusp being present. The protocone is lower

ps min 1

min 1 MI tmf 6

min 1 Ma tmf 7

min 7 M1 tmf 5

min P tn>f 1

min 2 ps trof 3

min 2 M' tmf 4

min 3 M2 trof 5

min 4 M I trof 2

min 2

Ps truf 1 1 .u 1.5 1.26 1.7-1.9 1.4 1.65-1 .Y I .>I .5 1.7-1.8

1.6 1.2-1.3 1.5-1.55 1.1-1.2 1 .H5-2.0 1.3-1.4 1.61.8 1.34.5 1 .>I .4 1.1-1.2

I 1 1 5 I 7 7 4 1 I 2 3 2 4 2 5 4 2 2

0.5 0.7

0.8

O.SI .n

0.6 1.1-1.3

I .&I .3

1.1-1.3 0.Y 1 .o 0.7 1.65-1.9 1.2-1.4 2.2-2.5 1.65-1.7 2.4-2.5 1.7-119 2.6 1.7

0.80.9

0.75-1 .n 0.6-0.65 0.65-1 .n

0.6

0.6

I .tt-2.0 1.2-1.5 2.1-2.4 1.6-1.75 2.2-2.4 1.7-1.9 1.7-1.8 1 . 1

Abbreviations: tmf P. trvfimvvi; rnin P. minor; N number of specimens; OR observed range of variation.

than the paracone. There is a large, conc-shaped paraconule and one or two small metaconules. The groove for the protoconid is deep. MI and M2 differ slightly in proportions. The stylar area

is very prominent with 3 cusps: stylocone, parastyle and preparastyle; it is longer in MI than in Mz. The preparastyle is missing in some specimens in MZ, which may possibly bc due to the wear or state of preservation. The ectoflexus is deeper in MZ than in MI, with 3 minute styles. The paracone is larger than the metacone. They are connate at the base and reach far lingually, the trigon basin being relatively small. The protocone is lower than thc paracone and metacone. There is a very prominent cone-shaped paraconule and one or two minute metaconules. The groovc for the protoconid is deep. M2 is shorter (long.) and wider (tr.) than M'. M' has an elongated parastylar region, with the usual 3 styles. There is a strong postparacrista. no metastylar region and the paracone is much larger than the metacone. The protocone is placed asymmetrically in the anterior part of the trigon. There is a distinct paraconule. though less con- spicuous than that in the preceding molars, and a metaconule is lacking.

Lower dentition. Incisors, canine and Pl-P4 are not known. Ps is not molarifom. There is a single trenchant cusp, an incipient not-bashed talonid with a single prominent cusp, and a minute anterior basal cusp. There is no obvious cingulum. The lower molars are much higher labially than lingually. they are set in the mandible so that the cusps are more vertical than the roots, which slope lingually (Fig. 1G). MI and M2 are similar. The protoconid is the highest of the trigonid cusps, the paraconid the lowest. Paraconid and metaconid are connate at the base. In anterior view there is an oblique anterolabial cingulum and a small anterolingual cusp; the hypoconulid of the preccding tooth fits between them. In outer view a dcpression separates the trigonid from the talonid. The

Zaologica Scripla 18

350 Z. Kielan-Jawarawska and D. Dashzeveg

k p 2-14-2. i'r~~karncrk.sres /roJiiwvi gcn. ct s p n . Kight maxilla with PI-M'. kihiiil view. IWk.--3-/fL 1'. minor sp.n.-3. Right maxilkr with I . kihial view. W S a . 4 . lafl maxilla with p-M-. lrhial view. I(L18h.-S. MI-M'olthc siimc. CXTlUSiil v i c w . 4 . lhc wamc in lingual and slightly cwxlus;il view.-7. LcR I.'-M1 cxxhiil view. 10-17h.4. The wrmc in lingual and sli *htly c ~ ~ l u w l v i c w . 4 . Lclt Ma. cxx'lus;il view. IOUxi.-lO. Ilc

wmc spccimcn as in Fig. I.-1.3. The wmc in cwx.luwl and slightly lahial view.-/J. Lcfi PF-M'. iKY'IuwI view. M L h . wmc in anterior and slightly (x~1usill vicw.-//-/4. /'. /,fiwovi.-ll. Right MI-M 5 . C K T ~ U ~ ~ view. IOLlc.-/2. Kight P'-M'. o r ~ l u w l view o f the

k7g.v. 2 4 xX. Figs. S-I4 SEM micn~grtiphs. X IS. All spccimcns arc houscd in Cil ISr.

cristid obliqua extends up the posterior face of the metaconid. The talonid is narrower than the trigonid and strongly elongated longitudinally. It is bounded by hypoconid, hypoconulid and entoconid, the hypoconid placed more anteriorly than the entoconid. The hypoconulid when unworn is as high as the hypoconid. The distance between the hypoconulid and entoconid is smaller than between the hypoconulid and hypoconid. In posterior view the talonid is twisted lingually. M3 differs from MI and M2 in having longer talonid and higher hypoconulid.

Pmkennalestes minor sp.n. (Figs. 3-10,15-18,22-24,28)

HokfJtypc. GI PST 10-7a. fragment of the posterior part of the left mdndihk with MI (Figs. 2.V24).

Zookogit'u Scriptu I#

Other material. All numhers have the acronym GI PST. 1&7b. right mandible with broken MI. M1. alveolusforM3; 10-9. left mandible with P,-M,; 10-14. edentulow mandible with alveoli for PrM3; llL14b. left mandible with broken M2; I&&, left MI or M,; 1&28g. right MI or M2; 1&32b; I d t mandible with Ml-M2; IO-Sa, right maxilla with P'. alveoli for P' and MI; I 0 - k left MI; W17b. left maxilla with P-M'; l&llla. left maxilla with M*-M'; I&18bb, left maxilla with P'-M'; 10-19, left MI; 1&28. left MI; lo4UI. hrokcn left M'; 10-28i. right M'; l&32a. hrokcn left M'; 1&32c, right mandible with M,.

Type hurimn and lacalily. ?Aptim or Alhian Khobiwr Bcds. Khoboor. Gushin Us somon, Gobi Desert. Mongolia.

Diagnosis. Estimated lcngth of the skull about 21 mm. Mandible very slcnder, 1.4 mm deep below M2 (labial view), remnant of the coronoid bone developed as a small, hardly discernible inflation. Remnant of thc rneckelian groove only slightly bent, may be divided posteriorly into two parallel furrows. Medial flangc weak. P" with

Early Cretaceous eutherian mammals 35 1

Figs. /%27.-/S-/N. /’rnki~rrrrekr.~ir.s rniriovgcn. ct sp.n.-/S. Left miindihlc with PFM2 (moliirs strongly worn). liiliiiil view. lO4.-/h. The samc in lingiial v i c w . 4 7. I’csterior part of thc right msndihle. lingiial view. IO-l.lh.-/X. Entire specimen o f thc samc. lnhirl vicw.-fC2/. -I*. tm&wwi 9.”. 1Y. I’istcrior part of thc right mandihlc with M,-M3. part of MI cut off on the photograph. lilhial view. IWS.-20. I’ostcrior prt of the right ni;mdiblc. lingurl view. holotypc. I(M.-2/. Labial view o f thc cntirc spccimcn of the srmc. showing M2 and MI. 22-24. P. irIirwr.-22. Right M1. lahiiil vicw.--23. k f t rnandihlc with MI. lingurl view. halotypc I0-21.-3. LAii;i l view o f the siimc.-2.%27. 1’. lroflriwri.-2S. Right MI and M: lrbil view. l(h,a.-2h. Lingual and slightly cnu.luwl view o f the sarnc.-27. O~u.lusrl vicw o f thc mmc. Figs. 15-3. 23-24 X8. Figs. 22.3-27 x IS. F i p . 17.20.22.2S-27SEM micrographs. All spccimcns arc houscd in Cil PSI.

rounded main cusp and without lingual extension. M3 with relatively smell parastylar area and shallow cctoflexus. Othcr teeth as in P. rrofiimovi, but smaller.

Efw1010gy. La. minor smaller (smaller than P. rrofmovi).

Description (SCC Tablc I for measurements)

Prokennalestes minor is similar to P. frofimovi. except for thc structurc of the mandible. and differs primarily in being smaller.

Zoologica Scripla 18

352 Z. Kieinn-Jaworowska and D. Dushztwg

B

Pmm

Maxilla. Only fragments of maxilla have been preserved, showing the infraorbital foramen placed above the €'-P5 embrasure. In the lower part of the depression leading to the foramen there is a convexity, with a groove above it. Minute nutrient foramina are present both on the convexity and in the groove.

Mandible. The body of the mandible is very slender, about 1.4 mm deep below Mz (labial view). The mental foramen is placed below the embrasure between P4 and P5 in the middle of mandible height in GI PST 10-9 (Fig. 15), and opposite the posterior root of P4 and higher in GI PST 10-14b (Fig. 18). The lower margin is slightly concave opposite the masseteric fossa. The masseteric crest is less prominent than in P. trof imvi and the masseteric fossa is less deep. The remnant of the coronoid bone forms a minute inflation of the dentary. The medial flange is less prominent than in P. trofiovi and the remnant of the meckelian groove starts below the middle part of the mandibular foramen, is only slightly bent and may be divided by a longitudinal ridge into two grooves of sub- equal length (about 3.5 mm in GI PST 10-14), the upper one narrower than the lower. The mandibular foramen is shaped as in P. trofimovi and the angular process, which is broken, was bent inwards.

Upper dentirim. Incisors. canine and P1-p are not known. is high, piercing and differs from that in f. trofimovi in having the main cusp rounded in cross section and in having a very small internal extension. P5 and M'-M are similar to those in P. trof imvi . They have only one metaconule, while there are sometimes two in P. trofimovi. M3 differs in having a less elongated parastylar region and a shallower ectoflexus.

Lower dentition. Incisors, canine and P,-P3 are not known. P4 and P5 are similar in shape, but the anterior

Zdagicu Scripta I8

cusp of P4 is less prominent than that of Ps, and an incipient talonid is not developed in P4. Ps and the lowcr molars are almost identical to those in P. trofimovi. In a worn specimen, GI PST 10-9, the talonidin Ps is basined.

Cornparisom

It appears from the foregoing description that P. trofimovi and P. minor are very close to one another, differing principally in size and structure of the mandible. As they are represented by similar numbers of specimens (19 and 19) it cannot be excluded that they are sexual morphs of the same species, P. rroy'irnovi being presumably a male and P. minor a female. The differences in mandible structure primarily concern the ooronoid bone and the shape of the remnant of the meckelian groove. Both of these structures are in the process of being lost phylogenetically. and it can be presumed, for example, that the remnant of the ooronoid bone unites earlier with the dentary in females than in males (analogy to the well known case in human anatomy, where the coracoid process unites with the scapula in the fourteenth year in the female and seventeenth year in the male). However, taking into account the differences in the structure of the mandible and the size difference of about 1.3-1.5 times, which is greater than usual within species of small mammals, we describe the two forms of Prokennalesres as separate species.

Recently, Nessov (1982,1984,1985a, b, 1987). Ncssov & Gureyev (1981) and Nessov and Trofimov (1979) described numerous eutherian taxa from the Albian, early Late Cretaceous and Paleogene of the Soviet Middle Asia. Orlestes Nessov. 1985 from the Late Cenomanian of Uzbekistan is closely related to Prokennalesles. Proken- nalestes differs from Otlestes in being smaller, in having a relatively larger labial mandibular foramen, a more prominent parastylar region with 3 styles (the preparastyle is very small or absent in Otlestes), 2 distinct styles in the metastylar region (in Orlesres the metastylar region is smooth) and both mnules cusp-like (the metaconule is winged in Oflestes). The lower canine is double-rooted in Prokennulestes (alveolus preserved in a mandible in PIN collection) and single-rooted in Oilestes, P4 has an anterior basal cusp in Prokennalestes hut not in Otlestes, and Ps has a single main cusp with anterior and posterior cuspules and a scarcely developed talonid in Prokennalesres while the trigonid has a small protoconid and a relatively large talonid in Otlestes. Otherwise, the twogenera share family characters of the Otlestidae and Prokennulestes may be regarded as ancestral to Orlestes.

A genus of tribotherian grade (Butler 1978; Clemens & Lillegraven 1986) which invites a comparison with Pro- kennalestes is Pappotherium Slaughter from the Albian of Texas (Slaughter 1465, 1971; Fox 1975; Butler 1978). Prokendestes resembles Puppotherium in having a large parastylar area, which, however, is differently shaped in the two genera, as there is no preparastyle in Pap- pothcrium. The stylar area is wider in Puppotherium than in Prokennrrlesres and in this respect resembles members of the Deltatheridiidae (see Kielan-Jaworowska 1975b, fig. 8). The conules are not winged but are differently shaped in the two genera, and the metastylar cusp is

Early Cretaceous eutherian mammals 353

present in both. See further comments on Pappofherium under Discussion.

The oldest undoubted cutherian so far described is Bobolestes Nessov from the late Albian of Uzbekistan (Nessov 1985~). Bobolesres lacks lingual cingula, as in members of the Otlestidae, but otherwise it is a relatively advanced cutherian for the Latc Albian. It differs from Prokennalestes in having relatively short (tr.) uppcr molars, and in having well developed winged conules, no preparastyle and no metacrista cusp. Bobolesres resembles Pofumofelses Fox (Fox 1972) from the early Campa- nian of Alberta in the general shape of the crown of M2, but differs from it in having winged mnulcs, while the conules are absent fmm Potamotelses.

From among thc eutherians with developed lingual cingula, Kennalestes invites a comparison with Proken- nulesres. The two genera resemble onc another in having a double-rooted lower canine (as does Asiorycfes Kielan- Jaworowska), in the arrangement of the cheek teeth, in having the penultimate upper premolar developed as a strong piercing tooth and in having semimolariform ultimate premolar with a low paracone. There are 5 prcmolars in Prokennalesres and 4 in adult Kennalesfes, although a juvcnile Kennalestes possesses 5 premolar loci (Kielan- Jaworowska 1981). The upper molars are similar in both genera in having a large, petaloid parastylar region with 3 cusps. The differences concern the prescnce of pre- and postcingula in Kennalesfes, different shape of the conules, which are winged in Kennulestes rather than cone-shapcd (as in Prokennalestes), and the lack of a metastylar cusp in Kennalestes. M3 is shorter (tr.) is Kennalestes than in Prokennalesfes. The lower cheek teeth are similar in both genera, but thc ultimate premolar is providcd with a talonid in Kennalesfes which is only incipient in Proken- nulesres. Prokennulestes has a relatively large labial mandibular foramen. which is hardly discernible in Ken- nnlesfes, a more prominent medial flange and a distinct rcmnant of the meckelian groove, which has disappeared in Kennalestes. The similarities between these genera are so great that it would be easy to derive Kennulestes from Prokennulesfes.

One can also argue that Prokennalestes should be assigned to the Kennalestidae rather than to the Otles- tidac. as the features common to Otlestes and Proken- nalestes are primitive characters. If it should be shown that the penultimate and ultimate upper premolars are differently shaped in Oflestes than in Prokennalestes, then the assignment of Prokennulestes to the Kennalestidae rather than to the Otlestidae would appear justified. For the time being. however, we classify Prokennulestes as a member of the Otlestidae. regarding an appearancc of prc- and postcingula as an apomorphy of the Kcnnales- tidae.

Other genera assigned to the Kcnnalestidae (Nessov 1982. 1985~) from the early Late Cretaceous of Soviet Middle Asia are more advanced than Kennulesfes and do not invite a close comparison with Prokennulesfes.

Prokennalestes should also be compared with primitivc members of the Palaeoryctidac. of which the most similar is the Late Cretaceous Asian Asioryctes (Kiclan- Jaworowska 1975~. 1981, 1984). Prokennalesfes differs from Asiarycfes in having 5 , rathcr than 4, premolars, but

rcscmbles it in thc general arrangement of the upper and lower chcck teeth as sccn in lateral view (as docs Ken- nulesres). It differs from Asioryctes in having the upper molars less elongated transversely, a larger 3-cusped parastylar region and differently shaped conules. The similarities are: a deep ectoflexus (but without styles in Asiorycfes) and the absence of prc- and postcingula. In the lower molars, the protoconid is more reduced in Asioryctes. The North American Late Cretaceous palacoryctids. such as, for examplc, Cirnolestes Marsh and Bafodon Marsh (Lillcgraven 1969; Clemens 1973; Butler 1977). are more advanced than Prokennalesres and do not invite close comparison.

Discussion

The origin of eutherian and mctatherian mammals has been widely discussed from the viewpoints of biology and palaeogeography (see Lillcgraven 1974, 1975; Clemcns and Lillegraven 19%; Lillegravcn el ul. 19877; Fox 1984; Kielan-Jaworowska 1982.1984 for summaries).

The oldest mammal with a tribosphenic dentition is Aegialodon Kermack ef af . from the Wcalden of Great Britain, known from a single lower molar (Kermack et ul. 1965). Kielantherium Dashzcveg (Dashzcvcg 1975), which is close to Aegialodon and is assigned to the Aegialodontidae, occurs in the Khoboor Beds which yield Prokennulesfes and is apparently a latc survivor of the aegialodontid line.

Kielunfherium has the labial mandibular foramen, which as we have argued (Dashzeveg & Kielan- Jaworowska 1984) is probably a primitive therian feature. It is rctained in the Otlcstidae and in the Zalambdales- tidae, although it has not becn recognized previously in members of this family, having been regarded as due to damagc (Kielan-Jaworowska 1969,1975a. 1984). It is best seen in mandibles of Zalambdalestes lechei Gregory & Simpson (Kielan-Jaworowska & Trofimov 1981: pl. 2) and Z. nymbulakensis Nessov (Nessov 198556: pl. II.2a). The foramen is relatively larger in Zalambdalestidae than in Otlestidae and is situated higher. In the Late Cretacc- ous Asian eutherian Kennalestes and Asioryctes the foramen is in the process of disappearing; it is very small and hardly discernible, being not much larger than the nutrient foramina. The labial mandibular foramcn is absent in the Deltatheridiidae.

As demonstrated above, Prokennulestes has an additional stylar cusp (preparastyle), which otherwise occurs only in Kennulestes. As the upper molars of Aegialodon and Kielantherium are not known. and as the parastylar region in all the known molars of Peramus Owcn and other Eupantotheria is incompletely prcscrved (Clemens iyr Mills 1971), it is difficult to ascertain whethcr 2 or 3 parastylar cusps are charactcristic for earliest Eutheria.

Crompton dk Kickan-Jaworowska (1978) reconstructed hypothctieal primitivc eutherian upper and lowcr second molars. Prokennalesfes is close to this hypothetical form, differing only in having unwinged conules. Butler (1977) discussed the evolution of the last premolars and of the molars in Cretaceous Eutheria. He listed the features he regarded as primitive when comparing the molars of

Zoologica Scripta I8

354 Z. Kielun-Jawuruwska and D. Dashzeveg

Kennulestes and Citnolestes. Prokennalestes shares most of thcse fcatures. cxcept that it has distinct cuspules betwen the stylocone and metastyle (point 4), and that thc cristid obliqua extends up the posterior face of the rnetaconid. rather than mccting thc trigonid midway between thc protoconid and the metaconid (point 9). Subsequently Fox (1984) gavc an extcnsivc list of hypothetical character states of carly cuthcrians. Proken- nnlesres agrecs with most of the character statcs visualized by Fox. although it is more primitive in certain respects. Fox (point 3 of his list) cites at least one cusp in the parestylar region. Prokennalestes has 3 cusps, which might be il primitivc feature. but at least 2 cusps are a primitive charactcr state. Point 18: "talonid basined, at widest (m2) little widcr than trigonid. and tri-cuspatc", whereas in Prakennalestes thc talonid is narrower than the trigonid, as it is in Kennulesres. Point 22: "ultimate lower premolar having incipient mctaconid and incomplctely basined talonid, bearing two cusps ( hypoconulid and cntoconid)" while in Prokennulestes this tooth has only one main cusp and a barcly developed talonid with a single cusp.

The question arises whethcr Prokennalestes is the oldest known and most primitive eutherian mammal? There are no doubts that it is a cuthcrian as it has 5 premolars and 3 moliirs and the last uppcr premolar is submolariform. Then: is a gradual morphological changc between the prcmolars and molars. as is characteristic of Eutheria. Thc exact age of the beds yielding Prokennalestes cannot at prcsent be cstablished with any certainty. It has been refcrred to as of ?Aptian or Albian age, but one of us (D. D.) has new cvidcncc that it may be of Neocomidn age. As wc cannot publish this cvidence yet, we continue to refer to it as ?Aptian or Albian.

As dcmonstrated above. the Latc Albian Bobolestex is morc advanced than Prokennalestes. The other candidate for most primitivc eutherian is the Albian Puppotherium from Texas. Thcre is strong disagreement as to the qualifi- cation of Pupputherium for this position. Slaughter (1965, 1971) and Fox (1975. 1984) have argued that Pap- potheriitm is a cuthcrian. It is truc that isolated molars of Pappotherium arc similar to thosc of Prokennalestes, which certainly is a eutherian. However, it cannot be excludcd that the fcaturcs of resemblance may be due to the retention of primitive therian characters, rathcr than to relationship. especially as thc upper molars of Pap- potherium also show a rcsemblance to Deltatheridium Grcgory & Simpson. which is not a eutherian (Kielan- Jaworowska 19756). As long as the dental formula of Pappotherium is not known. it is impossiblc to demon- strate unequivocally whether or not it is a cuthcrian or a pappothcridan (Butler 1978; see also Clemens and Lillcgraven 19%). We chmse to follow the latter view, thus regarding Prokennalesres as the most primitive and possibly the oldest know eutherian mammal.

Acknowledgements

Dr €3. A. l'rofimov (Palcontological Institute, Mmcow) and Dr L. A. Ncs!wv (Leningrad State Univcrsity) kindly allowed ZKJ to study the collertions in their charge. Dincussion with Dr Ncssuv was of special value for these investigations. Dr P. M. Butlcr (Royal tiolloway and Bcdford New Collcg) and Prof. J. A. Hopson (University of Chicago)

Zdogiccr Srriptu I8

kindly read the manuscript of this paper and offered most useful comments and criticism. The drawings were made by Mr Magnus Ranheim after the pencil sketches of ZKJ. the photographs were taken by Mr Per A. Aas (both Palcontologink Muscum, Univcrsitctet i Oslo) the SEM photographs by MsTurid Winje (Institutt for Gcologi. Univer- sitctet i Oslo). Rescarch was supported 'by funds from Norges Allmennvitenskapcligc Forskningsmd. grant ABULM D.40.28.032 to ZKJ. To all these persons and institutions we express our sincere thanks and gratitude. This p p e r is paleontological rmtribution number 346 from the University tS0sIo.

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Eutherian mammals from the Early Cretaceous of Mongolia