2001. J. Linn. Soc., 52: 85-95. With 6 figures

January 1973

The dentitions of the earliest mammals

F. R. PARRINGTON, F.R.S., F.L.S.

University Museum of Zoology, Cambridge

Accepted for publication September 1972

In 1971 accounts of the dentitions of two of the earliest known mammals were published independently by Mills and Parrington. These accounts differ in a number of important respects, notably over the existence of mutual canine wear, the nature of the roots of the postcanines, the position and extent of the wear of the molars, the degree of likeness of the two forms, and the dental formula of the form where this can be determined. The data have been examined afresh and attempts made to explain the differences of opinion. The significance of the data is discussed.

CONTENTS

Introduction 85 Canine wear . . . . . . . . . . . . . . . . . . . . . . 86 Postcanine roots . . . . . . . . . . . . . . . . . . . . . 86 Molar wear . . . . . . . . . . . . . . . . . . . . . . 89 Comparison betweenEozosrrodon andKuehneotherium . . . . . . . . . 91 Dental formulae . . . . . . . . . . . . . . . . . . . . . 93 Conclusions . . . . . . . . . . . . . . . . . . . . . . 94 References . . . . . . . . . . . . . . . . . . . . . . 95

. . . . . . . . . . . . . . . . . . . . . .

INTRODUCTION

Our understanding of the once very rare Mesozoic mammals has been greatly enhanced by the discoveries of Professor W. Kuhne which started in 1938. Having discovered two triconodont teeth in the Rhaetic fissure fillings of a Carboniferous limestone quarry in Somerset, Kuhne carried his explorations further west, first discovering tritylodont remains in abundance (thereby confirming the views of those who held that these animals were technically reptiles, not mammals) and later, in Wales, more triconodont teeth and also a trituberculate tooth.

In more recent years two large collections of fragmentary remains of Upper Triassic mammals have been made from fissures in Carboniferous limestone quarries in the Bridgend district of Glamorgan in Wales. The first was made by K. A. Kermack and his colleagues in 1955 and later, and housed in University College, London. The second collection, from a single quarry, was made by a team from Cambridge in 1966 and is now housed in the Cambridge University

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86 F. R. PARRINGTON

Museum of Zoology. In 1971 two separate accounts of the form Eozostrodon (Morgunucodon), together with remarks on the contemporaneous Kuehneo- therium, were published, one by Parrington (May) and one by Mills (September) the latter following a brief account he had given at a meeting of the Linnean Society the previous year. These two accounts are in mutual contradiction in certain matters of interpretation of data and in much of the resulting taxonomy. Since these earliest known mammals have a direct bearing on the origins of mammals as a whole, and even on the origin of the Theria (i.e. marsupials and placentals) proper, it is desirable that an attempt be made to reach agreement on at least the odontological evidence.

A fuller account of the history of the various discoveries, and of the discussions to which they have given rise, is available in the introduction of Parrington’s (1971) paper. Mills studied the material available in the London collection; Parrington that in Cambridge.

CANINE WEAR

Parrington (1971) claimed that the upper and lower canine teeth of Eozostrodon wore against each other, the posterior part of the outer face of the lower tooth abutting against the anterior region of the inner face of the upper tooth, causing well marked wear facets to develop in a characteristic mammalian fashion. Mills (1971), on the contrary, claimed that no such wear is apparent and the animal in this respect is reptilian rather than mammalian. The issue is readily resolved. Series of six upper and six lower canines of various sizes mounted to show the wear facets may be seen in the Cambridge collection (Fig. 1). Of 32 upper canines selected at random 24 show clear wear facets, two show dubious conditions (perhaps partly post mortem or the result of abnormal wear), three are young teeth with incompletely developed roots, and three only are fully grown teeth lacking any signs of wear. The condition of these last teeth is open to various explanations. They might belong to a form other than Eozostrodon, such as Kuehneotherium; they might have come from animals with full dentitions but lacking normal occlusal relationships; or they might have come from animals having lost the lower canines and having filled their sockets with bone (as happens in the premolars), or from animals which had failed to develop lower canines. But the general condition of wear is unquestionably mammalian, and since the Permian gorgonopsids wore their canines in this fashion (Kemp, 1969) this is hardly surprising.

POSTCANINE ROOTS

Mills (1971) claims that the roots of the postcanine teeth of Eozostrodon differ from those of Kuehneotherium but the writer believes that this view overlooks the manner of root growth and that the differences between the two forms are trivial and of small taxonomic importance.

Most of the isolated postcanine teeth in the Cambridge collection have their roots broken, but every stage of Eozostrodon tooth development is manifest, from mere crown caps to fully grown teeth with worn crown and complete roots. An example of a premolar tooth of the type identified as belonging to Kuehneotherium by Kermack, Kermack & Mussett (1968) is shown in Fig. 2

THE DENTITIONS OF THE EARLIEST MAMMALS

n 87

- I mm

Figure 1. Isolated right upper and left lower canines of Eozoshodon to show the wear resulting from the mammalian type of mutual abrasion. Upper canines in inner view; lower canines in outer view. Note that only the three upper canines with crimped roots are worn to their tips.

- I mrn

Figure 2. Two left lower premolars of the same overall height. A is of the form figured by Kermack et nl. (1959) as belonging to Kuehneotheriurn with a narrow crown and the larger accessory cusp on the side of the principal cusp. €3 is a typical premolar of Eozoshodon. The roots of the two teeth are virtually indistinguishable and are therian-like.

88 F. R. PARRINGTON

together with a typical Eozostrodon premolar of similar size. Whilst the former is a more slender tooth and has the principal accessory cusp on the side of the main cusp, the roots of the two teeth do not differ to any obvious extent. However, the dissection of a number of Eozostrodon dentaries has shown that, certainly regarding the posterior premolars, cementum may be added, an addition which may not have taken place in the two teeth figured. Cementum gives the roots an irregular outline which is difficult to define because it locks the roots into the bone of the jaw without any sharp intervening boundary. A slight expansion of the apices of the roots is possibly characteristic of the fully mature Eozostrodon roots, but the condition of the roots of mature Kuehneotherium teeth is uncertain since no premolars have yet been found in a dentary and no dissection appears to have been made of the fragments of the premolar region of specimens ascribed to Kuehneotherium which might retain root remains.

The roots of the molar teeth of Eozostrodon are illustrated in Fig. 3. A specimen with an unworn crown and short and incompletely grown roots, apparently lacking cementum, is figured (Fig. 3A) together with a mature, worn specimen with roots showing the “blunderbuss’ appearance emphasized by Mills (Fig. 3B). A section (Fig. 3C) (traced from coloured microphotographs kindly supplied by Professor A. D. Hitchen) shows a long root which, without the cementum, is almost identical with the root of the Somerset specimen called Eozostrodon problematicus (Parrington, 1941). A very slight thickening of the apical dentine and the addition of cementum to this specimen would make the Welsh and Somerset specimens about identical and certainly not taxonomically different. True, the crown of E. problematicus differs slightly from the Welsh material in the comparative height of the accessory cusps but, as Mills (1971) himself states, the teeth of “Morganucodon” vary very considerably.

Mills (1971) stated that the London collection contains no Kuehneotherium specimens with teeth in the jaw, being dependent for his views regarding their roots, therefore, on teeth which had been broken out of the jaws or had fallen out (being incompletely grown) escaping subsequent damage. But all such material is unsatisfactory. A beautiful Kuehneotherium molar with an unworn crown and with perfect, but incompletely grown roots, was depicted by Parrington (1971, fig. 16g). The roots are no different from those of a young Eozostrodon molar, and thus both forms might be said to be “therian”. A worn Kuehneotherium molar in a fragment of a dentary was dissected (Parrington, 1971, fig. 12b) and the root shown to be long and to be covered, apparently, by cementum, though no apical swelling was evident. A fragment of a dentary (Sy 66), believed to belong to Kuehneotherium because of the somewhat triangulated appearance of the alveoli, the thickness of the bone, and the depth of the cavity for the postdentary bones, was also dissected, and herein again the roots were found to be long and to be enclosed in cementum but to lack terminal swellings.

The upper molar teeth of both forms have shorter roots than the lower, those of Eozostrodon being compressed, the anterior root along the line of the jaw, the posterior one across it. Both roots tend to be expanded at their apices, as are sometimes the roots of the premolars and apparently of even the posterior incisor. N o useful information has been obtained about the roots

THE DENTITIONS OF THE EARLIEST MAMMALS a9

- I mm

---

I mm Figure 3. Three lower molars of Eozostrodon to show the form of the roots. A is a freshly erupted right molar in inner view, with an unworn crown and incompletely grown roots. B is a fully grown right molar in inner view, with a worn crown and fully grown roots with swollen apices. C is a sketch of the section of a lower molar with a long root showing the cementum contributing to the apical swelling. The length of the root resembles that of Eozostrodon problematicus.

(C sketched from photographs of a section prepared by Professor A.D. Hitchen. Scale uncertain.) c, Cementum; d, dentine; pc. pulp cavity.

of the upper molars of Kuehneotheriurn, well preserved specimens being extremely few.

Taxonomic relationships adduced from the root forms in this connection rest solely on the small terminal swellings present upon the apices of the roots of Eozostrodon, but absent in Kuehneotheriurn-scarcely strong evidence of lack of affinity, and itself resting on very limited evidence with respect to Kuehneotheriurn.

MOLAR WEAR

The fact is strongly emphasized (Mills, 1971) that in Eozostrodon the principal cusp of the upper molars wears down between the major cusp and the

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90 F. R. PARRINGTON

posterior accessory cusp of the lower molar, whereas in Kuehneotherium it wears down between the posterior accessory cusp and the fourth cusp. This difference is held to be so fundamental as to accord the two forms independent origins from the reptiles, a view which ignores the Kuehneotheriurn molar illustrated by the writer (Parrington, 1967, fig. 5e) wherein the wear has taken place against the posterior face of the principal cusp.

Examination of the Cambridge material shows that, whilst the type of wear claimed by Mills can occur in Kuehneotherium (e.g. Fig. SB), the typical condition shows the principal wear to be against the outer face of the posterior accessory cusp (e.g. Sy 39, 47) though it may terminate in the valley between this cusp and the fourth (talonid) cusp. Specimen Sy 37 supports specimen Sy 6 in showing wear against the face of the principal cusp. These differences may well be due to some variation in the site of the start of wear which itself may well be correlated with the degree of angulation of the cusps (which varies along the jaw). In general, therefore, the site of initial wear is similar in the two genera and this is consistent with the full analysis of the matter worked out by Crompton (1971). However it must be recalled that Kermack (1967) has claimed that at least two species of Kuehneotherium are present in the London material, and Kermack et al. (1968) claimed that two genera of trituberculates were to be found in South Wales. (In neither case, however, were reasons for these claims adduced.)

Upper molar wear starts against the face of the anterior accessory cusp (Fig. 4A) and wear against the posterior accessory cusp develops later (Fig. 4B). Another difference between Eozostrodon and Kuehneotherium believed to exist by Mills (1971) is that the inner cingulum of the upper molars functions differently in the two forms. Mills’ contention is that in Kuehneotherium the cingulum acts as a “stop” against which the lower molars bite. But as in Eozostrodon, the wear in Kuehneotherium can be severe, the cingulum becoming extremely worn, and in regions of severe wear, obliterated. Thus in seven teeth in the Cambridge material the enlarged cingulum cuspule (called the parastyle by Kermack et al. (1968), has been so obliterated. Two upper molars showing severe wear are illustrated (Fig. 4); in these little remains of the inner cingulum which was patently ineffective as a “stop”.

I 1

I rnm

Figure 4. Two worn right upper molars of Kuehneotherium. A shows the principal wear against the leading accessory cusp. B is a more severely worn tooth. Note the fact that wear obliterates the cingulum.

THE DENTITIONS OF THE EARLIEST MAMMALS 91

CO W A R ISON BETWEEN EOZ OSTR ODON AND K UEHNEO THER I UM

Attention has already been called, briefly, to likenesses between the teeth of the two Triassic mammals (Parrington, 1967). Moreover Crompton & Jenkins (1968) have compared the molar crowns, cusp by cusp, finding five matching cusps in both upper and lower molars. Such likenesses cannot be ignored.

The most obvious likenesses between the molars of Eozostrodon and the more anterior, linear, molars of Kuehneotherium are the following:

(i) The crown is dominated by a row of three principal cusps, more or less in a straight line.

(ii) The central cusp is the largest, the posterior accessory cusp the second largest.

(iii) A fourth cusp, or enclarged cuspule, has been added from the cingulum to the back of the row.

(iv) The upper molars have both external and internal cingula; with rare exceptions in Kuehneotherium, the lower molars have an internal cingulum only.

(v) The lower molars have proportionately much higher crowns than the upper molars.

These features, together with a similar size pattern, collectively indicate teeth of fundamentally identical constitution. The principal differences are the following:

(i) The cingulum cuspules of Eozostrodon, distinctly variable in size and number (Parrington, 1967, 1971), are matched in the upper teeth of Kuehneotherium by sporadic cuspules only and by one called the parastyle by Kermack, Kermack & Mussett (1968), and in the lower teeth by one or two anteriorly placed cuspules.

(ii) While in Eozostrodon the posterior accessory cusp is always the larger of the two, in Kuehneotherium the accessory cusps are more equal sized and the anterior one may be slightly the larger in the more angulated teeth.

(iii) The crowns of the Kuehneotherium molars are proportionately taller than those of Eozostrodon ; a phenomenon probably concerned with the angulation of the cusps.

As molarization develops posteriorly the angulation of the cusps presumably becomes more pronounced in Kuehneotherium and the likeness to the triconodont condition becomes correspondingly less.

An Eozostrodon upper molar and two moderately angled Kuehneotherium molars are illustrated in Fig. 5 , each in five aspects, allowing detailed comparisons.

By Mills (1971) the lower molars of Eozostrodon are interpreted as biconodont teeth with an enlarged cingulum cuspule added anteriorly (Fig. 62). This possible interpretation occurred also to the writer but it was dismissed because, while many theriodonts known from the Lower Trias and Upper Permian have triconodont teeth, none has triconodont upper teeth and biconodont lower teeth. The cingulum is one of the features which marks Eozostrodon as different from any known theriodont (hence the name). Some

92 F. R. PARRINGTON

5

5 - I rnrn

C ( Rev 1

Figure 5. Right upper molars of A Eozoszrodon and B and C two specimens of moderately angled Kuehneotheriurn molars to show the overall resemblances. 1, Outer view; 2, anterior view; 3, internal view; 4, posterior view; 5, crown view.

THE DENTITIONS OF THE EARLIEST MAMMALS

quite early cynodonts develop additional cuspules on the crown but these are outgrowths from the cusps since no ledge, or true cinguum, is present. The outstanding difference between the lower molars of Eozostrodon and the linear lower molars of Kuehneotherium is the asymmetry of the Eozostrodon tooth (Fig. 6 ) . To evolve a molar of the Kuehneotherium type from a typical Eozostrodon tooth would require an elimination of most of the cingulum cuspules and a development of more equal-sized cusps. In the posterior part of the jaw, an angulation of the cusps and the addition of an anterior cuspule would also have to occur. Such changes are modest requirements when viewed in the light of the profound changes undergone by the molars of the Tertiary mammals.

93

2

A

e

3 I

I mm

Figure 6 . Left lower molars of A Eozostrodon and B Kuehneotherium to show the general resemblances. 1 , Outer view; 2, crown view; 3 , inner view.

Mills (197 1) claims that the teeth of his three groups of Triassic mammals (the Morganucodontidae, Sinoconodontidae, Kuehneotheridae) have nothing in common except the triconodont condition, which, he rightly states, is widespread amongst theriodonts and early mammals. That the triconodont condition is basic was recognized by Osborn nearly a century ago and later work has amply demonstrated the truth of this.

DENTAL FORMULAE

There are considerable differences in the dental formulae given by Kermack & Mussett (1959), Parrington (1971) and Mills (1971). Some of the differences

94 F. R. PARRINGTON

may stem from the very fragmentary nature of the material, which has resulted in pieces of the symmetrodont Kuehneotherium, or of some as yet un- recognized form, being accepted as parts of the common Eozostrodon. Formulae have been given as follows:

5 1 2 4 I r C 1 PM 2-3 M 42 Kermack & Mussett (1959)

Parrington ( 197 1)

Mills (197 1)

I 4 1 ? C i P M 7 M 5 4 5

I - 3 1 C - P M - M - > 4 4-6 1 4-5 4-5

Kermack & Mussett (1959) claimed that the total number of lower postcanines was always seven. Parrington (1971) suggested that in some cases the number of lower molars might be four only. Mills apparently unaware of the formula published by Kermack & Mussett (1959) wrongly ascribed their incorrect number of two upper premolars to Crompton & Jenkins (1968) who, however, were never in a position to work out the formula and merely adopted the only one then published.

Mills (1971) established the number of upper incisors on a perfect premaxilla, describing a large gap near the midline. Parrington (1971) wrongly restored a tooth in this region in an imperfect specimen. All the material in the Cambridge collection either shows or suggests four lower incisors. Mills (1971) states there may be four or five and quotes M 24563 as showing six alveoli anterior to the canine. But he does not state if this latter specimen, or any of those showing evidence of five incisors, have postcanine teeth present to identify them. Mills claims four or five lower premolars; Parrington four. There now seems agreement that the number of lower molars may be either four or five. The revised formula would appear to be:

I - C - P M q M 4 - 5 3 1 5 4 4 1

CONCLUSIONS

The supposed absence of canine abrasion, the nature of the postcanine roots, the positions and nature of the molar abrasions, and the form of the molar crowns-these, together with the incorrectly supposed marsupial-like tooth replacement in Eozostrodon, are Mills’ reasons for claiming that the early mammals can be divided into three groups of independent origin from the reptiles. Not one of these reasons withstands close examination. If it is deemed necessary to split mammals into two major groups, as some would, the “therian” and “non-therian”, this is probably best done, on present knowledge, by emphasizing the triangulation of the molar cusps in the former group alone as suggested by Patterson (1956). But the likenesses between Eozostrodon and Kuehneotherium remain to demonstrate that the trituberculates evolved from the triconodonts ( s .1 . ) and are therefore closely related. A major division of the Mammalia appears to the writer to be unsatisfactory unless it is based on the

THE DENTITIONS OF THE EARLIEST MAMMALS 95

detrahens muscle of the monotremes as opposed to the digastric muscle of the Theria, a difference which may well reflect independent origins.

Full accounts of the forms Erythrotherium and Megazostrodon, now in preparation, and a redescription and reconsideration of Sinoconodon, are necessary before any serious discussion of the interrelationships of the various eozostrodonts and their allies is possible.

ACKNOWLEDGEMENTS

I am indebted to Professor T. Weis-Fogh and Dr K. A. Joysey for kindly hospitality in the Museum of Zoology at Cambridge, and to Mr R. D. Norman, as ever, for various photographs and for the drawings.

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CROMPTON, A. W., 1971. The origin of the tribosphenic molar. In D. M. Kermack & K. A. Kermack

CROMPTON, A. W. & JENKINS, F. A,, 1968. Molar occlusion in late Triassic mammals. Biol. Rev., 43:

HOPSON, J. A. & CROMPTON, 1969. Origin of mammals. In Th. Dobzhansky e t al. (Eds), Evolutionary

KEMP, T. S., 1969. On the functional morphology of the gorgonopsid skull. Phil. Trans. R . SOC., 8256.

KERMACK, K. A., 1967. The interrelations of early mammals. J. Linn. SOC. (Zool.). 47: 241-249. KERMACK, D. M., KERMACK, K. A. & MUSSETT, F., 1959. The Welsh pantothere Kuehneotherium

KERMACK, K. A. & MUSSETT, F., 1959. The first mammals. Discovery, 20: 144-151. MILLS, J. R. E., 1971. The dentition of Morganucodon. In D. M. Kermack & K. A. Kermack (Eds), Early

PARRINGTON, F. R., 1941. On two mammalian teeth from the Lower Rhaetic of Somerset. Ann Mag.

PARRINGTON, F. R., 1967. The origins of mammals. Advmt Sci. Lond., 24: 165-173. PARRINGTON, F. R., 1971. On the Upper Triassic mammals. Phil. Trans. R . SOC., 8261: 231-272. PATTERSON, B., 1956. Early Cretaceous mammals and the evolution of mammalian molar teeth.

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