~oologiralJournal of the Linnean SocieQ (1984), 82: 21 7-227. With 2 figures
The lower jaw of an aegialodontid mammal
from the Early Cretaceous of Mongolia
D. DASHZEVEG
Geological Institute, Mongolian Academy of Sciences, Ulan Bator, hfongolinn People’s
Republic
AND
Z. KIELAN-JAWOROWSKA”
Institut de Palhontologie, Mushurn National &Histoire Naturelle, 8 rue a‘e B u f o n , 75005
Paris, France
Received October 1983, accepted f o r publication Nouember 1983
A partial lower jaw is described of an argialodontid mammal, Kielantheriurn gobien.iis Dashzeveg,
1975, from the Guchin Us beds of Mongolia (TAptian or ?Albian). The jaw has foir molars and
four or five premolar loci. A count of P5 M4 is argued to be primitive for the lribosphenida.
McKenna’s interpretation of the postcanine dentition of Perumus as P5 M 3 is accepted. I t follows
that the Peramura could not lead to the Trihosphenida, which apparently arose from unknown
‘pantotheres’ with not less than nine postcanine terth.
KEY WORDS:-Tribosphenida
-
Kielantheriun - Aegialodontia
-
dentition
-
phylo5:eny
CONTENTS
Introduction . . .
Description .
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Kielantherium gobiensis
Comparisons . . .
Discussion . . . .
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References.
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Dashzeveg, 1975 .
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INTRODUCTION
Since 1960 Prof. Kenneth A. Kermack and his co-workers in the Department
of Zoology, University College, London, have searched for fossils in the
Wealden bone-bed a t Cliff End, near Winchelsea in Sussex, England. The year
1965 brought the discovery of a tiny worn lower molar, a little over 1 mm in
* Permanent address: Polska Akademia Nauk, Zaklad Paleobiologii, 02-089 Warszawa, al. Zwirki i Wigury
93, Poland.
0024-4082/84/090217
+ 1 1 $03.00/0
217
0 1984 The Linnean
Society of London
218
D. DASHZEVEG AND 2 . KIELAN-JAWOROWSKA
length, which was dubbed Aegialodon dawsoni Kermack et al., 1965. This
meticulous work demonstrated that Aegialodon possessed the oldest and most
primitive tribosphenic molar ever found. I t differs from the lower molars of
‘pantotheres’ in having a basined talonid, whose structure shows that the
protocone must have been present in the upper molars. Kermack et al. (1965:
535) in their abstract stated, “From Aegialodon can be derived the teeth of
Trinity Sands ‘Theria’ and Endotherium by a further increase of the size of
talonid and protocone. A continuation of the same process would give rise to the
.teeth of marsupials and placentals”. Thus Aegialodon is a long-sought missing
link between the Jurassic ‘Pantotheria’ and Cretaceous Theria with tribosphenic
molars.
Kermack et al. (1965) did not assign Aegialodon to any family, but
subsequently Kermack (1967) erected the Aegialodontidae. Crompton (197 1)
reconstructed the upper molar of Aegialodon.
Ten years after Prof. Kermack’s discovery, a single lower molar, similar to
Aegialodon, was found by the first author in Guchin Us, near Khovboor, in the
Gobi Desert, in beds of presumable ?Aptian or ?Albian age (Clemens et al.,
1979). This fossil was assigned (Dashzeveg, 1975) to a new genus and species
Kielantherium gobiensis (see also Crompton & Kielan-Jaworowska, 1978).
Fox (1976) claimed that Kielantherium is a junior synonym of Aegialodon, but
tentatively accepted the specific distinctness of the Asiatic and English forms.
However, Butler (1978), Clemens et al. (1979) and Kielan-Jaworowska et al.
(1979) used the name Kielantherium for the Asian form. Clemens et al. (1979: 30)
stated, “The Khovboor assemblage is probably significantly younger than the
Cliff End fauna. Not only are the two localities geographically disparate, but
they were probably isolated by seaways. . . The possibility that the very real
similarities between the known teeth is due to the retention of primitive features
from an older common ancestor cannot yet be eliminated. We therefore consider
Fox’s synonymy to be premature. . .” We follow this opinion in the present
paper.
Butler ( 1 978) united the Aegialodontidae with the Kermackiidae and
Deltatheridiidae in an order Aegialodontia, and placed the order in a new
infraclass Tribotheria. Kielan-Jaworowska ( 1982) assigned the Aegialodontia to
an infraclass incertae sedis.
In 1981, in the same beds at Guchin Us that yielded Kielantherium gobiensis, the
first author found a lower jaw with four molars. T h e purpose of this paper is to
describe this lower jaw, justify its allocation to K. gobiensis, and to discuss the
affinities of the Aegialodontia.
This paper was prepared in the Geological Institute of the Academy of
Sciences of the Mongolian People’s Republic in Ulan Bator, and in the Institut
de Paltontologie, Muskum National d’Histoire Naturelle in Paris, during the
second author’s stay in France in 1983. The second author expresses her
gratitude to Prof. Philippe Taquet, Director of the Institut de Paltontologie in
Paris, for providing her with facilities in this Institute, to M . Denis Serette for
taking the photographs and Ms Christiane Weber-Chancogne for the scanning
electron microscope photographs. Both authors thank Dr Malcolm C. McKenna
(American Museum of Natural History, New York), Prof. William A. Clemens
(Department of Paleontology, University of California, Berkeley), Prof. Bernard
Krebs (Freie Universitat, Berlin) and Drs Denise Sigogneau-Russell and Donald
AEGIALODONTID MAMMAL
219
A. Russell (Institut de Paltontologie, Muskum National d’Histoire Naturelle,
Paris) for most helpful comments.
T h e specimen described here is housed in the Geological Institute, Academy
of Sciences of the Mongolian People’s Republic in Ulan Bator, for which the
abbreviation G I is used.
DESCRIPTION
Subclass Theria Parker & Haswell, 1897
Legion Tribosphenida McKenna, 1975, new rank
Infraclass incertae sedis
Order Aegialodontia Butler, 1978
Family Aegialodontidae Kermack, 1967
Genus Kielantherium Dashzeveg, 1975
Kielantherium gobzensis Dashzeveg, 1975
(Figs 1, 2)
MATERIAL:
Single partial lower jaw, GI PST 10-16, with base of the coronoid
process, four molars, eight alveoli or roots of four double-rooted premolars, and
one broken alveolus for one more premolar or canine.
Labial aspect (Fig. 1A). The jaw is slender with gently curved lower
margin, 1.7 mm deep below the posterior roots of M , and 1.9 mm deep below
the posterior root of M,. One mental foramen is visible below the posterior root
of P,. The labial upper margin of the coronoid process starts below the anterior
root of M, and extents posteriorly as a rounded ridge parallell to the lower
margin of the jaw at a distance of 2.5 mm, then it curves obliquely upwards and
backwards. T h e upper margin of the coronoid process behind 14, slopes very
gently upwards. There is a small foramen on the labial side behind the base of
the coronoid process. This foramen opens in the mandibular canal, whose main
entrance is on the lingual side of the jaw (see below). The presence of a n
additional entrance to the mandibular canal on the labial side of the jaw is rare
in mammals. It occurs in undescribed ‘Prokennalestes’ jaws from the Guchin Us
beds (GI PST 10-5 and GI PST 10-6) and in an undescribed edentulous
slender lower jaw of an unknown mammal from the same beds ( G I PST 10-4).
The presence of this foramen in different Guchin Us specimens that are of the
?Aptian or ?Albian age suggests that this is a primitive therian feature. T h e
foramen is not present in eutherian mammals (Kielan-Jaworowska, 1969,
1975a, 1981) from the Djadokhta and Barun Goyot formations, and probably
did not occur in the Deltatheroida (Kielan-Jaworowska, 198211. Gregory &
Simpson (1926) did not mention its presence in well preserved lower jaws of
Deltatheridium praetrituberculare, and the relevant parts of the lower jaws of
Deltatheridium and Deltatheroides housed in the Institute of Paleobiology in
Warsaw (Kielan-Jaworowska, 197513) have not been preserved. A similarly
placed foramen occurs also in a marsupial, Microbiotherium gallegoserise from the
Santa Cruz beds of Patagonia (Sinclair, 1906), but not in other species of
Microbiotherium. In some diprotodont marsupials, in the masseteric fossa, there
occurs a comparatively large foramen, regarded as a masticatory adaptation
(Abbie, 1939). However, the foramen in question in Kielantherium is not large
LOWER JAW:
220
D. DASHZEVEG AND Z. KIELAN-JAWOROWSKA
Figure I . Kielantheriurn gobiensis Dashzeveg, 1975. Guchin Us, Gobi Desert, Mongolian People’s
Republic, ?Aptian or ?Albian. GI PST 10-16. A, Incomplete right lower jaw with M,-M, in labial
view, x 9. B, The same in lingual view, x 9. C, Scanning electron microscope photograph of the
teeth and the posterior part of the jaw of the same specimen, in labial view, x 18. D, Scanning
electron microscope photograph of the same in lingual view, x 18.
AECIALODONTID MAMMAL
221
enough to transmit slips of the masseteric muscle, and it is doubthl if i t may be
regarded as homologous to that of the diprotodont marsupials. I n some extant
mammals, e.g. elephants, hyracoids, lagomorphs and some others, there is a
foramen on the labial side of the lower jaw, in front of the base of the coronoid
process; this foramen, however, cannot be regarded as homologous to the above
described foramen in Kielantherium.
Lingual aspect (Fig. 1B). O n the broken posterior margin of the jaw there are
two concavities, the lower one possibly corresponding to the foramen leading to
the mandibular canal, the upper one interpreted as a break. Extending
anteriorly from the lower margin of the mandibular canal foramen, parallel to
the lower margin of the jaw, is a narrow, very distinct groove, 4 mm long, which
corresponds to the internal groove of Simpson (1928) and the inner groove of
Krebs (1971). The groove possibly served as a housing of Meckel’s cartilage
persisting in the adult. A similar groove has been found in IKimmeridgian
dryolestids from Guimarota in Portugal (Krebs, 197 l ) , and in the paurodontids,
Arnphitherium and Peramus (Simpson, 1928; Clemens & Mills, 1971 ). Its structure
in Kielantherium most closely resembles the condition in Peramus. Krebs (197 1)
suggested that the groove in question in Kimmeridgian dryolestids housed, in
addition to Meckel’s cartilage, an artery and mylohyoid nerve. I n the Early
Cretaceous dryolestid Crusafontia the groove is not present. In the edentulous
jaw, mentioned above, from the Guchin Us beds (GI PST 10-14) and in
‘Prokennalestes’ (GI PST 10-6) a remnant of this groove is distinguishable in the
posterior part of the jaw, much shallower and shorter than in Kielantherium.
Another characteristic feature of the lingual side of the lower jaw of
Kielantherium is a distinct articular facet for the coronoid bone at the base of the
coronoid process. T h e facet is wider than long (length preserved =: 0.7 mm) and
it tapers downwards. The presence of an articular facet for the coronoid bone in
Kielantherium was to be expected, because, of all ‘reptilian’ bones. the coronoid
was the longest-persisting in the mammalian jaw. T h e coronoid bone was
present not only in numerous Rhaeto-Liassic and Late Jurassic mammals
(Kermack at al., 1973; Crompton &Jenkins, 1979; Cassiliano & Clemens, 1979;
Krebs, 1969, 1971; Clemens & Mills, 1971; Hahn, 1977), but a n articular facet
for the coronoid has also been found in Early Cretaceous ‘Prokenrtalestes’, and a
faint trace of the articular facet persisted in the Late Cretacecus eutherians
Kennalestes and Asioryctes (Kielan-Jaworowska, 198 1 ) .
Posterior aspect. The posterior view of the jaw (Fig. 2A) shows that the
mandibular canal was very large and that the foramen on the labial surface of
the jaw led to it. When viewed posteriorly, the section of the canal has an
irregular shape, possibly due to breakage, and it is not thought desirable to
describe it in detail. The upper part of the jaw above the mandibular canal is
perforated by a minute foramen, possibly for a blood vessel.
TEETH AND ALVEOLI (Figs lC, D, 2B): Four preserved teeth are recognized as
M,-M,. M , and M, are of the same size and shape and will be described
together. M , as preserved is shorter than M,, due to the wear of the trigonid
cusps, especially of the protoconid. However, it cannot be excluded that it was
originally slightly lower than M,. The trigonid is relatively large with regard to
the low and small talonid. The latter is arranged somewhat obliquely to the
longitudinal axis of the jaw. Of the three trigonid cusps the protoconid is the
Figure 2. Kielantherium gobienszs Dashzeveg, 1975. Guchin Us, Gobi Desert, Mongolian People’s
Republic, ?Aptian or TAlbian. GI PST 10-16. A, Stereo-photograph of the right lower jaw in
posterior view, x 20. B, Stereo-scanning electron miscroscope photograph of the same jaw in
occlusal view, x 14.4.
AEGIALODON 1 ID MAMMAL
223
highest, and the paraconid is higher than the metaconid and clearly separated
from the latter at the base. O n the anterior surface of the trigonitl there are two
distinct cuspules, the antero-buccal one (cingulum) situated lower and less
prominent than the antero-lingual. The talonid is very small and basined, well
defined by the cristid obliqua and by the postcristid-entocristid, which meet at
the posterior surface of the metaconid below its tip. There were probably only
two talonid cusps, hypoconid and hypoconulid, which are hardly recognizable
due to wear.
M, is smaller than M , and M,. It is only slightly worn and both anterior
cuspules are more prominent than on M , and M,. T h e hypoconulid is badly
damaged, but the hypoconid is well preserved. The difference between the
height of the paraconid and metaconid is more conspicuous than in the anterior
molars, and the metaconid is relatively shorter and less clearly separated at the
base from the protoconid than in M and M,.
M, is very little worn and is much lower than the remaining molars, its height
being less than two-thirds that of M,. The difference between the height of the
paraconid and the metaconid is relatively greater than in the preceding molars.
The anterior cuspules are less prominent than in the preceding molars. The
hypoconid and hypoconulid are well preserved, closely pressed against each
other. As in all remaining molars there is no trace of the entoconid, and the
talonid basin is deep, surrounded by the prominent cristid obliqua and the
pos tcristid-en tocris tid.
In all the molars the posterior root is clearly bigger than the anterior one.
Premolar alveoli and roots. I n front of M , there are alveoli for four doublerooted teeth, tentatively recognized as P,-P,. T h e last ones (for ?P,) are the
largest, with the posterior root bigger than the anterior, the tooth being
probably of the size of M , . The roots of ?P, are preserved, both of the same size,
and ?P, is smaller than ?P,. The roots of ?P, are slightly smaller than those of
?P,, the alveoli for ?P, are the smallest. I n front of the anterior alveolus for ?P,
there is a broken off alveolus, which might be for P , or for the catnine.
We should make i t clear why we regard the alveoli in front of the molars as
belonging to P,-P,, rather than to P,-P,. The structure of the lower jaw and of
the molars described above clearly shows that Kielantherium is a very primitive
therian mammal. I n ‘Prokennalestes’, an eutherian occurring in the same beds as
Kielantherium, there are five premolars. Five premolars are also present in
juvenile Late Cretaceous Kennalestes (Kielan-Jaworowska, 1981), in Gypsonictops
(Lillegraven, 1969), and in Eocene sirenians (Domning el al., 1982). It seems
that a premolar count of five was a primitive feature of eutherian mammals and
the number of four premolars was only secondarily achieved by the loss of one
tooth. It seems reasonable to assume that the ancestors of eutherian mammals,
to which Kielantherium certainly was close, also had five rathler than four
premolars.
COMPARISONS
The M,-M, described above are the same size and structure ,as in the type
specimen of Kielantherium gobiensis-GI PST 10-14, which is a single lower molar
(Dashzeveg, 1975). T h e type specimen might be M , , M, or M,; it differs from
the M,-M, in GI PST 10-16 in being less worn and in having the hypoconid
more prominent.
224
D. DASHZEVEG AND Z . KIELAN-JAWOROWSKA
The MI-M, of K. gobiensis are very similar to Aegialodon dawsoni (Kermack et
al., 1965) and are of approximately the same length, but differ from it in being
higher; this, however, may be due to the greater wear of Aegialodon. The bases of
the protoconid and metaconid are more clearly set apart in Kielantherium and in
Aegialodon. I n Kielantherium the talonid is placed lower with regard to the base of
the roots than in Aegialodon and has a different shape. I n Aegialodon the talonid
basin is roughly triangular, relatively wider than in Kielantherium and provided
with three or four cusps. I n Kielantherium the talonid basin is very narrow,
roughly rectangular, with only two cusps situated close to each other, at the
distal margin.
From the same beds at Guchin Us that yielded Kielantherium, Dashzeveg
(1979) described a new genus Arguimus, assigned to the Peramuridae.
Kielantherium differs from Arguimus in the presence of the talonid basin and a n
entocristid. The paraconid and metaconid in Kielantherium are situated closer to
each other than in Arguirnus. T h e tooth identified by Dashzeveg as M,, and
regarded now as M , (see below) is of the same length (1.3 mm, measured
exteriorly) as M , of Kielantherium, but the width of the trigonid is 0.8 mm in
Kielantherium and 1 mm in Arguimus.
DISCUSSION
I t follows from the foregoing description that Kielantherium had four molars
and at least four, but possibly five, premolar loci. McKenna (1975: 26) stated,
“ . . , a premolar locus count of 5, rather than 4,is judged here to be primitive
for the Theria”. We concur with this opinion regarding mammals with
tribosphenic molars (Tribosphenida of McKenna), but we shall argue that four
(and not three as postulated by McKenna) molars were primitive for the
Tri bosphenida.
Clemens & Mills (1971) recognized in Peramus four molars and four
premolars, while McKenna identified these teeth as five premolars and three
molars. We concur with this interpretation of McKenna and we add the
following argument to support it. M: in Peramus (as denoted by Clemens &
Mills) are very different from M i and M:, both in number and arrangement of
the cusps, as well as in shape. M i is short transversely and more similar to the
preceding premolar than to the succeeding molars, which are wide transversely.
Also M , has not acquired a three-cusped triconid characteristic of the molars
(see Clemens & Mills, 1971: fig. 2 ) . Although it is impossible to demonstrate
that M: are premolars because they are not visibly replaced, they are better
classified as premolars than as molars on the basis of accepted morphological
criteria.
I t is also interesting to compare the dentition of Peramus with those of the
oldest known Asian eutherians: ‘Prokennalestes’ from the ?Aptian or ?Albian
(which was named by Beliajeva et al., 1974, but not described and is cited here
as a nomen nudum) and the Late Cretaceous Kennalestes, Asioyctes and
Zalambdalestes (see Kielan-Jaworowska, 1969, 1975a, 1981 ) . All these Asian
genera are similar to each other in the organization of the dentition when
examined in lateral view. Originally they had five premolars, of which three
anterior ones, both upper and lower, were small and double-rooted (one of them
lost in most Late Cretaceous genera, but present in ‘Prokennalestes’ and in
AEGIALOUONTID MAMMAL
225
juvenile Kennalestes). The strongest upper premolar of the whole series was the
penultimate one, which was a piercing tooth, higher than the comparatively
large ultimate premolar. If one accepts McKenna’s interpretaiion of Peramus
dentition, one finds a striking similarity in the lateral arrangement of postcanine
teeth in Peramus and in Asian Cretaceous Eutheria.
The interpretation of the dentition of Arguimus, assigned by Dashzeveg (1979)
to the Peramuridae, poses difficulties, as Dashzeveg recognized four premolars in
it. I t seems, however, possible, albeit not certain, that the tooth identified by
Dashzeveg as M, is in fact P,, being slightly more molariform than P, in
Peramus.
McKenna ( 1975) regarded his infraclass Peramura, erected to include
Peramus, as the sister-group of the Tribosphenida, and encompassed there two
infraclasses in a sublegion Zatheria. According to him the synapomorphous
feature of this group is the presence of eight postcanine loci: five premolars and
three molars. Zatheria have been subsequently diagnosed by P’rothero (1981:
321) as follows, ‘reduce to three molars, basined talonid, add hypoconulid and
entoconid, reduce stylocone, lose anterior cingulum on lower molars.”
McKenna (1975) consequently postulated that in the Marsupialia and in the
Deltatheridiidae (which he placed in Eutheria, using Van Valen’s term
Deltatheridia in the new meaning) the teeth commonly designated M I , M2, M 3
and M4, are dP5, M I , M2 and M3.
The question arises, if we accept McKenna’s (1975: fig. 2) interpretation of
the Peramus dentition, why we do not concur with his consequent hypothesis of
the primitive dental formula in Tribosphenida?
The lower molars of Kielantherium ( M , and M,) are nearly identical with the
single known lower molar of Aegialodon, and it cannot be excluded, as postulated
by Fox (1976), that the two genera are synonyms. If Fox is right, the dental
formula of Aegialodon would be the same as that of Kielantherium. If the two
grnera differ in dental formulae, that of Aegialodon (which possihly was slightly
smaller and occurs in beds significantly older than Kielantherzum) would be
probably more primitive than that of Kielantherium. This would mean that
Aegialodon might have a greater number of postcanine teeth than Kielantherium. If
one accepts McKenna’s hypothesis of 5 3 postcanine teeth in early
Tribosphenida, the morphological (or evolutionary) line: common ancestor of
Peramura and Aegialodontia with P5 M3 + Aegialodontia -+ Eutheria is
unacceptable, as it would involve a reversal of evolution: a change of P5 of a
common ancestor into a completely molariform tooth in Aegialodontia, and a
subsequent return to an incompletely molariform stage of the same tooth in
earliest Eutheria. If McKenna’s hypothesis is right, one should derive the
Eutheria directly from the Peramura, and all the remaining Tribclsphenida from
the Aegialodontia. This requires an assumption that the tribosphenic molar
developed twice in the evolution of the Theria: in a line leading kom unknown
‘pantotheres’ to the Aegialodontia, and in a line leading from the Peramura to
the Eutheria. It follows that the Tribosphenida would be a group of
polyphyletic origin.
It is, however, possible to propose a more parsimonious hypothesis of the
evolution of the Tribosphenida, accepting that the postcanine count in
Aegialodontia was five premolars and four molars. I n this case Peramura would
form a side line, not leading to the Tribosphenida, which would arise from
+
226
D. DASHZEVEG AND Z. KIELAN-JAWOROWSKA
unknown ‘pantotheres’ with not less than nine postcanine teeth. With such an
assumption it would be easy to derive all later Tribosphenida from the
Aegialodontia: the Marsupialia by loss of two premolars, the Eutheria by loss of
one molar, the Pappotherida by unknown changes in premolar number and (if
Butler, 1978, is right) by retaining four molars, and the Deltatheroida (KielanJaworowska, 1982) by loss of two premolars and a tendency to lose one molar.
In the present state of knowledge of early Tribosphenida it would be
misleading to erect to many higher category taxons, such taxons being based not
on a real knowledge of the anatomy of these groups, but on speculations derived
from scanty information, and to propose a cladogram. I t has been discussed by
one of us elsewhere (Kielan-Jaworowska, 1982) why Butler’s ( 1978) infraclass
Tribotheria (encompassing the orders Aegialodontia, Pappotherida and the
Deltatheridiidae) is unacceptable. Future knowledge of these groups (for the
Deltatheridiidae a separate order Deltatheroida Kielan-Jaworowska, 1982 has
been erected), may show that each of them merits an infraclass of its own, and
that perhaps some of them should be classified together, but we think that for
the time being it is more reasonable to leave them in an infraclass incertae sedis, as
proposed earlier (Kielan-Jaworowska, 1982).
Neither do we accept McKenna’s (1975) infraclass Zatheria, as it has been
shown that one cannot be sure that the Peramura and the Tribosphenida are
sister-groups; it seems therefore premature to us to include them in the Zatheria.
We accept herein McKenna’s taxon Tribosphenida, which he erected as an
infraclass, and to which we attribute a higher rank, possibly that of a legion.
When more data on the anatomy of the early Theria are known, the rank of the
Tribosphenida might be changed again. We find the name Tribosphenida well
chosen and useful, as it includes in its name the diagnosis of the group, the
characteristic feature of which is the acquisition of a tribosphenic molar, which
we regard as a synapomorphous feature for the Tribosphenida. If, however, it
would be shown in future that the Eutheria are derived directly from the
Peramura and not from the Aegialodontia, then it would be necessary to regard
the Tribosphenida as a group of polyphyletic origin.
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