Symmetrodonta, Simpson, 1925

O. Averianov, Alexander, 2002, Early Cretaceous “ symmetrodont ” mammal Gobiotheriodon from Mongolia and the classification of “ Symmetrodonta ”, Acta Palaeontologica Polonica 47 (4), pp. 705-716 : 710-714

“ Symmetrodonta ”

Woutersia Sigogneau−Russell, 1983 , from the Late Triassic (Rhaetian) of France was based on several isolated teeth and was originally referred to Kuehneotheriidae ( Sigogneau−Russell 1983) .When additional material become available, it was transferred to the monotypic Woutersiidae within “ Symmetrodonta ” and “a certain parallelism” between this family and Docodonta was noted (Sigogneau−Russell and Hahn 1995: 245).Subsequently, Butler (1997) referred Woutersia to Docodonta .In my opinion, molars of Woutersia could be structurally antecedent to Docodonta , but Woutersia itself should not be formally included in that group (see Martin and Averianov in press). Woutersia molar morphology is structurally intermediate between those in morganucodontids and docodonts, and this taxon is better maintain in the monotypic Woutersiidae .Dental synapomorphy for Morganucodontidae , Woutersiidae , and Docodonta would be presence of the cusp “g” (“kuhneocone”) on the lower molars. Woutersiidae and Docodonta form a more inclusive clade, sharing lingual shift of the cusp “c” (semitriangulation).The idea about similarity of Morganucodon Kühne, 1949 with docodonts was first proposed by Butler (in Kühne 1950) and later supported by Crompton and Jenkins (1968) and Crompton (1974), who, however, referred Morganucodontidae to the “ Triconodonta ”, a point of view shared by most of the recent authors (but see McKenna and Bell 1997: 512, who retain Megazostrodon Crompton and Jenkins, 1968 within the Docodonta ).

The possibility of docodont affinities for Woutersia suggests that the “reversed triangle” pattern, postulated as synapomorphy for Holotheria (e.g., Prothero 1981; Hopson 1994 and references therein), was derived at least twice: once in “holotherians” (“symmetrodonts”, “eupantotherians”, and boreosphenidans), and again in docodonts, possibly related to Australosphenida ( Asfaltomylos Rauhut et al., 2002 , Ambondro Flynn et al., 1999 + Ausktribosphenos Rich et al., 1997 + Monotremata ), which independently acquired a tribosphenic molar pattern ( Luo et al.2001, 2002; Rauhut et al.

2002).However, the recent cladistic analyses of Mammalia ( Luo et al.2001, 2002) supported the old believe that docodont molar pattern evolved from the linear, not triangulated cusp configuration. Pascual et al.(2000) and Pascual and Goin (2001) argued for the close affinities of Docodonta and Australosphenida, but their interpretation for the origin of “triangulated” molars and the cusps homologies in docodonts is radically different.

Docodonts retain a primitive, Morganucodon −like mandible with a trough for postdentary bones and anteriorly placed angular process (see Kermack and Mussett 1958; Lillegraven and Krusat 1991).These mandibular features were retained in primitive australosphenidans, such as Asfaltomylos , Ausktribosphenos , and Bishops Rich et al., 2001 , but reduced in living monotremes (an angle is still present in the fossil monotremes Teinolophos Rich et al., 1999 and Obdurodon Woodburne and Tedford, 1975 , see Musser and Archer 1998 and review by Luo et al.2002).In “symmetrodonts” the an − gular process is lacking; a posteriorly placed angular process, present in “eupantotheres” and more advanced therians, therefore appears to be secondarily derived.Summing up, among Mesozoic mammals the combination of an angular process with the trough for the postdentary bones is found only in Morganucodontidae , Docodonta , and Australosphenida and close relationship between these group should be seriously considered. Shuotherium lacks an angular process and in molar appearance more reminiscent that of primitive holotherian (“symmetrodont”) mammals and should be included to that group (see below).Although these ideas are not in line with the current parsimony analysis ( Luo et al. 2002), we should remember that this analysis is based on limited data and could be affected greatly by numerous dental parallelisms between two main non−allotherian mammal groups: Morganucodontidae – Docodonta –Australosphenida and Holotheria.These parallelisms include development of both “normal” (posterior) and “pseudo” (anterior) talonid in two groups: Australosphenida versus Boreosphenida and derived Docodonta ( Tegotheriidae ) versus Shuotherium .

Kotatherium haldanei Datta, 1981 , based on single upper molar from the Early Jurassic Kota Formation, India, is usually placed in “ Symmetrodonta ” without attribution to a particularly family ( Datta 1981: 308; McKenna and Bell 1997: 44; Sigogneau−Russell and Ensom 1998: 457), or in Tinodontidae ( Prasad and Manhas 1997: 565) .It is generally simi − lar to Kuehneotherium , especially in the presence of a small, “extroverted” “metacone” [cusp C], but differs in having a much smaller stylocone. Sigogneau−Russell and Ensom (1998: 458) hint about “a possibly affinity [of K. haldanei ] with the Moroccan “peramurids”, but in the subsequent revision of the Moroccan peramurids (Sigogneau−Russell 1999) this question was not discussed.A second species, Kotatherium yadagirii Prasad and Manhas, 1997 , from the same formation and also based on an isolated upper molar ( Prasad and Manhas 1997), is probably not congeneric with the type species ( Sigogneau−Russell and Ensom 1998: 458). Kotatherium is restricted herein to the type species and is referred to Kuehneotheriidae .“ Kotatherium “ yadagirii may be an archaic amphilestid rather than a “symmetrodont”.

Delsatia Sigogneau−Russell and Godefroit, 1997 , from the Rhaetian of France was originally referred to Docodonta (Sigogneau−Russell and Godefroit 1997) .I agree with Butler (1997: 439) that Delsatia is not a docodont, but rather a “symmetrodont”.Except for the facts that paracristid and protocristid break at almost a right angle, and that the lingual cingulid is discontinuous, Delsatia is very similar to Kuehneotherium , and I refer it herein to Kuehneotheriidae .

Shuotherium Chow and Rich, 1982 , from the Middle Jurassic of China and England ( Chow and Rich 1982; Sigogneau−Russell 1998; Wang et al.1998), is similar to Kuehneotherium in retaining primitive dentary structure, with a trough for postdentary bones. Shuotherium may be derived from a kuehneotheriid such as Delsatia , from the Late Triassic of France (Sigogneau−Russell and Godefroit 1997), with which it shares a characteristic trigonid structure, wherein the paracristid and protocristid break at a right angle toward the paraconid and metaconid, respectively. Delsatia also has somewhat enlarged mesial cingulid cuspules (“e” and “f”), which appear to be incipient to the “pseudotalonid” of Shuotherium .

Amphidon Simpson, 1925 from the Upper Jurassic (Kimmeridgian–Tithonian) Morrison Formation, Wyoming, USA, is based on a single dentary with five cheek teeth (Simpson 1925).The postcanine formula is usually interpreted as p 1–4 m 1–4 (e.g., Simpson 1925, 1929; Cassiliano and Clemens 1979).However, the supposed p4 has essentially the same morphology as m1 and other molars.The molars are “func − tionally monocuspid” ( Cassiliano and Clemens 1979: 155), having weak paraconid and metaconid, although this weakness may be caused at least partly by the considerable wear on the only known specimen ( Cassiliano and Clemens 1979: 155).The occlusal view is “reconstructed” only for m1 (Sim − pson 1929: fig.17): it has an obtuse−angled crown with a trigonid angle of ~145°.This is the only feature that would place Amphidon within “ Symmetrodonta ”.However, it is equally plausible that Amphidon may be an aberrant amphilestid with a postcanine formula of p 1–3 m 1–5.Given these ambiguities, the genus is herein considered Mammalia incertae sedis.The concept of Amphidontidae is dubious and this taxon is regarded here as a nomen dubium.

Manchurodon Yabe and Shikama, 1938 , from the Middle Jurassic Wafangdian Formation (see Zhang 1984; Zhou et al. 1991; Wang et al.2001) of eastern China, is based on a single specimen now lost, including a scapula and a dentary fragment with p 1–3 m 1–5 (Yabe and Shikama 1938). Manchurodon has generally been referred to Amphidontidae , because of its “functionally unicuspid” molars (Yabe and Shikama 1938: 355; Cassiliano and Clemens 1979: 155; McKenna and Bell 1997: 44).However, the dentition of Manchurodon was figured only in the labial view, and in this view molars of spalacotheriids may also look “functionally unicuspid” because of their acute−angled cusp pattern, in which paraconid and metaconid may be concealed by protoconid.Importantly, Manchurodon has a labial cingulid on lower molars (Yabe and Shikama 1938: 355, fig.1), characteristic for the majority of spalacotheriids (e.g., Cifelli and Madsen 1999), whereas in Amphidon the labial cingulum seems to be lacking (Simpson 1929: fig.17).At least one premolar (p1) in Manchurodon is single−rooted, which differentiates this taxon from Zhangheotherium and Spalacotheriidae , where all lower premolars are double−rooted. Patterson (1956: 29) considered the dental formula of Manchurodon as being p 1–4 m 1–4 because of “a noticeable break in size between the first four and the last four teeth”. This size break, however, does not necessarily reflect the premolar−molar boundary because of differing size relationships in the molar series.Whereas Spalacolestinae have an m1 that is nearly twice as long as m2 ( Cifelli and Madsen 1999), m1 is only 87% the length of m 2 in the “symmetrodont” Zhangheotherium (calculated from Hu et al.1998: fig.1).In Manchurodon , the fourth cheek tooth is 82% the length of the fifth.By analogy with Zhangheotherium , these teeth are thus reasonably interpreted as m1–2.These homologies are sup − ported by the fact that a distinct metaconid (lacking, so far as known, on premolars of “symmetrodonts”) is present on the fourth cheek tooth.Hence, the postcanine formula of Manchurodon appears to be p 1–3 m 1–5.Beyond this, little more can be said.The dental formula is not, in itself, particu − larly diagnostic.It is possible that the molar paraconids and metaconids were unreduced and there also exists significant doubt as to whether paraconid and metaconid of Amphidon are reduced or simply obliterated by wear.There is insuffi − cient basis to posit a special relationship between the two genera, and Manchurodon is considered herein as “ Symmetrodonta ” incertae sedis.

Nakunodon Yadagiri, 1985 from the Early Jurassic Kota Formation, India, is based on a single upper molar, possibly lacking a considerable part of the anterior crown (not only a “small portion”, as was stated in the original description, Yadagiri 1985: 415). Nakunodon was referred to Amphidontidae because of its “monodont [monocuspid] type ” (Yadagiri 1985: 415; McKenna and Bell 1997: 44; Prasad and Manhas 1997: 570).The tooth is indeed monocuspid: the metacone is absent and the stylocone is not present on the preserved crown, contra the description by Yadagiri (1985); the metastyle is very small.The most striking feature of Nakunodon is a complete cingulum, especially thick and robust lingually, with a protocone−like eminence at the junction of the pre− and postcingula.The affinities of Nakunodon remain problematic, but there exists no basis to classify it with Amphidon . Nakunodon is considered here as Holotheria incertae sedis.

Liaotherium Zhou et al., 1991 , from the Middle Jurassic of Liaoning Province, China, was originally referred to? Amphilestidae (Zhou et al.1991) ; McKenna and Bell (1997: 42) referred the genus to Amphilestidae without question. L. gracile Zhou et al., 1991 , the only known species, is represented by a nearly complete dentary preserving only one tooth, a crushed last molar, which is said to have had “three cusps arranging in a line” before suffering postmortem damage (Zhou et al.1991: 174).This was obviously the main (only?) reason for attribution of Liaotherium to Amphilestidae .However, a nearly linear arrangement of cusps can be observed on the last molar in some obtuse−angled “symmetrodonts”, such as Gobiotheriodon .There is no clear basis for dividing the postcanine tooth series of Liaotherium into premolars and molars by the alveoli preserved.Zhou et al.

(1991) interpreted the postcanine tooth formula of Liaotherium as p 1–3 m 1–5 or p 1–4 m 1–4, but the interpretation p 1–2 m 1–6 is equally possible.This would give a postcanine formula similar to that of the spalacotherioid “symmetrodont” Zhangheotherium , from the Early Cretaceous of the same region, Liaoning Province, China ( Hu et al.1997).

More important, Liaotherium is similar to Zhangheotherium in having a long, thin, plate−like coronoid process, not exceeding the condylar process in width.In the amphilestids Phascolotherium and Amphilestes (Simpson 1928: fig.19), the coronoid process is much wider, considerably exceeding the condylar process in width. Liaotherium should be considered as Mammalia incertae sedis, because attribution to Amphilestidae or “ Symmetrodonta ” is equally possible.

Thereuodon Sigogneau−Russell, 1989 , based on isolated upper teeth from the Early Cretaceous (Berriasian) of Morocco and England, was originally described as a “symmetrodont” ( Sigogneau−Russell 1989; Sigogneau−Russell and Ensom 1998).However, this taxon may be based on the milk dentition of a dryolestid or a stem−group zatherian, as was recently suggested by Martin (2002) based on new materials from the Jurassic of Portugal. Thereuodon and the similar Atlasodon Sigogneau−Russell, 1991 from Morocco are herein excluded from “ Symmetrodonta ”.Similarly, Mictodon simpsoni Fox, 1984 , based on a single tooth from the early Campanian of Canada ( Fox 1984a), may represent milk dentition of the spalacolestine symmetrodont Symmetrodontoides canadensis Fox, 1976 , known from the same formation ( Fox, 1972, 1976, 1985) (R.Cifelli personal communi − cation 2002).This tooth is similar by almost aligned (“ob − tuse”−angled) cusp arrangement, low lateral cusps, well separated from the main cusp, and slender roots with the deciduous teeth of the mid−Cretaceous and Turonian spalacolestines (Cifelli 1999; Cifelli and Gordon 1999).

Bonaparte (1990) described five “symmetrodont” taxa from the Late Cretaceous (Campanian) Los Alamitos Formation of Argentina: Bondesius Bonaparte, 1990 ( Bondesiidae ), Casamiquelia Bonaparte, 1990 (family indet.), Brandonia Bonaparte, 1990 (? Spalacotheriidae ), Barberenia Bonaparte, 1990 , and Quirogatherium Bonaparte, 1990 (both Barbereniidae ).Later ( Bonaparte 1994) the four latter taxa were re − ferred to the Dryolestida and only Bondesius was left within the “ Symmetrodonta ”.In spited of this, McKenna and Bell (1997) kept Barbereniidae within the “ Symmetrodonta ”, considered Casamiquelia as “ Symmetrodonta ” incertae sedis, and placed Bondesius to the Tinodontidae ; only Brandonia was referred to the Dryolestida. Sigogneau−Russell and Ensom (1998: 465) noted the great similarity between upper molars of Barberenia and Thereuodon and decided that the former taxon “could still be considered a symmetrodont”.Indeed, the similarity between these two taxa is great and Barberenia may represent the milk dentition of one of the dryolestid known from the Los Alamitos fauna. Casamiquelia and Brandonia with the typical median ridge between the paracone and stylocone on the upper molars are best placed within the Dryolestida.The systematic position of Quirogatherium is less certain, it also could be based on upper milk tooth of a dryolestid.The lower molar, the holotype of Bondesius ferox Bonaparte, 1990 ( Bonaparte 1990: fig.3A–F), by its almost transverse protocristid and transverse talonid crest is more approximating condition of dryolestids ( Ensom and Sigogneau−Russell 1998; Martin 1999) than any of “symmetrodonts”.Its slender roots, en − larged talonid, and anteriorly projecting paraconid suggest that it is rather deciduous than permanent tooth (compare with Martin 1999: taf.12A–C).Consequently, Bondesius is removed here from the “ Symmetrodonta ” and placed within the Dryolestida.Currently there is no unambiguous record of “symmetrodonts” in the Late Cretaceous of South America.

Microderson Sigogneau−Russell, 1991 , based on a single upper molar from the Early Cretaceous (Berriasian) of Morocco, was originally attributed to the Spalacotheriidae ( Sigogneau−Russell 1991; followed by McKenna and Bell 1997: 45), but later ( Sigogneau−Russell and Ensom 1998) considered family incertae sedis within “ Symmetrodonta ”.How − ever, presence of three roots and a vestigial metacone at the lingual base of the cusp “C” ( Sigogneau−Russell and Ensom 1998: 461, fig.9) clearly set this taxon apart of “Sym − metrodonta” and allow us to compare it with the Peramura, which are well diversified in the Anoual fauna (Sigogneau−Russell 1999). Microderson is a stem−group zatherian, more primitive by the metacone development than Nanolestes Martin, 2002 from the Late Jurassic of Portugal ( Martin 2002).

Donodon Sigogneau−Russell, 1991 , from the Early Cretaceous (Berriasian) of Morocco, was originally referred to the monotypic Donodontidae within Dryolestoidea ( Sigogneau−Russell 1991), and later transferred to a new sublegion within Cladotheria ( Ensom and Sigogneau−Russell 1998).The holo − type of Donodon perscriptoris Sigogneau−Russell, 1991 , an isolated upper molar, seems to belong to a dryolestoid, but the referred specimen, a dentary fragment with the two?last molars ( Sigogneau−Russell 1991: pl.1, fig.4), certainly does not belongs to this group.These lower molars have acute−angled trigonids (trigonid angle ~65° and ~72°) with small, roughly equal paraconid and metaconid, quite small talonid, and apparently unreduced posterior root.From their origin at the protoconid, the paracristid and protocristid extend almost anteriorly and posteriorly, respectively, then abruptly turn inwards towards the paraconid and metaconid respectively.It appears that a transverse shearing surface, characteristic for Cladotheria, was lacking. Ensom and Sigogneau−Russell (1998: 43) were fully aware that “the characteristics of the protoconid and of the roots of the attributed lower molars [of Donodon ] are not those of dryolestoids”, but preferred to retain the upper and lower molars in the same taxon.I think that the dentary specimen referred to D. perscriptoris should be attributed to “ Symmetrodonta ”.The reduction of the lingual cingulid and mesial cingulid cuspule “f” are similarities shared with Gobiotheriodon .

Excluding Zhangheotherium, Cretaceous Spalacotheriidae form a well−supported monophyletic group, within which are placed the more inclusive Spalacolestinae, containing endemic taxa of a North American radiation ( Cifelli and Madsen 1999).The main trend of spalacotheriid evolution is toward development of an acute−angled molar pattern, bringing mesial (prevallum) and distal (postvallum) shearing surfaces into a more transverse position, which considerably increases the total length of the shearing surface for the same molar series length.Correspondingly, the molar formula in − creases to M1–6 (or 7) m1–6 (or 7), which also increases the total shearing surface length.The dentary was quite derived, eventually lacking the Meckel's groove and having an hypertrophied pterygoid crest or process and efflected posteroinferior border of dentary for m.pterygoideus medialis and m.masseter attachment respectively, which provide lateral translation and rotation of the dentary during the masticatory cycle ( Cifelli and Madsen 1999).This lateral translation and rotation was responsible for development of continuous prevallum and postvallum shearing surfaces on molars ( Crompton 1971).An additional wear surface was formed along the enlarged and somewhat transversely (obliquely) oriented, crest−like distal stylar cusp on upper molars ( Fox 1985: 22, fig.1, 1 and 2), which apparently sheared against food parti − cles.In the development of transversely oriented continuous shearing surfaces, spalacotheriids are analogous to tribosphenic and dryolestoid mammals.Evidently, spalaco − theriids were highly specialized; an untestable speculation explaining their long survival in the Cretaceous of North America is that they were able to consume food resources not readily available to other mammals in the ecosystem.To speculate further, it is possible that progressive evolution and specialization of Spalacolestinae was permitted by the rarity of eutherian mammals: eutherians are not known from Cenomanian–Santonian faunas of North America, and do not become abundant on the continent until the early Campanian ( Fox 1984b; Cifelli 1990, 2000).Radiation of eutherian mammals in North America at this time may have led to the rapid extinction of endemic Spalacolestinae, which are “not known in North America later than about the early late Campanian” ( Cifelli and Gordon 1999: 11).

The Chinese Zhangheotherium , having rounded, conical molar cusps that lack connecting crests ( Hu et al.1997), ap − parently did not achieve the dental specialization characteristic for Spalacotheriidae , described above, and should be excluded from this taxon, although it may be a sister taxon to Spalacotheriidae ( Cifelli and Madsen 1999: fig. 19).

Shalbaatar Nessov, 1997 was based on single specimen (an edentulous dentary fragment) from the lower Bissekty Formation at Dzharakuduk, Uzbekistan. Shalbaatar was originally referred to Multituberculata (?Plagiaulacoidea), because of the “relatively anterior position of the coronoid process” ( Nessov 1997: 162). Nessov (1997: 162–163) also noted prominent “hystricognathy” of this specimen; i.e., lateral deflection of the posteroventral border of the dentary.He thought that Shalbaatar belongs to a peculiar group of multituberculates, retaining a primitive posterior position of the masseteric fossa and unenlarged p4, but derived in “hystricognathy” of the mandible (in parallel with some rodents).According to Z.Kielan−Jaworowska (personal communication 2000), the posteriorly placed masseteric fossa excludes Shalbaatar from Multituberculata , because this indicates absence of the “backward masticatory power stroke” that is characteristic of the group ( Gambaryan and Kielan−Jaworowska 1995).

My investigation of ZIN 82622, the holotype of Shalbaatar bakht Nessov, 1997 , reveals several features characteristic of spalacotheriid (or, even, spalacolestine: Cifelli and Madsen 1999) “symmetrodonts”: molars small, less than 1 mm in length; the last molar is smaller than the penultimate molar; the molar alveoli are obliquely set on the dentary, canting labially; molar roots anteroposteriorly short and transversely wide; prominent pterygoid crest, placed relatively high and extending anteriorly to the alveolar border; a pocket posterior to the mandibular foramen and above the pterygoid crest; and strong lateral deflection of the angular region.The extraordinary structure of the dentary, found only in some spalacotheriids and in Shalbaatar , unambiguously places the latter taxon within the “symmetrodont” family Spalacotheriidae , at least.If possible attribution to Spala − colestinae is corroborated by further specimens, Shalbaatar would be the only non−American member of this subfamily. It is quite possible, because the Bissekty vertebrate fauna,

*taxa excluded from “ Symmetrodonta ”: Amphidon Simpson, 1925 ( Mammalia incertae sedis), Nakunodon Yadagiri, 1985 (Holotheria incertae sedis), Woutersia Sigogneau−Russell, 1983 ( Woutersiidae , sister group for Docodonta ), Thereuodon Sigogneau−Russell, 1989 (Dryolestida, or Zatheria indet., milk dentition), Bondesius Bonaparte, 1990 (Dryolestida indet., milk dentition), Casamiquelia Bonaparte, 1990 (Dryolestida), Brandonia Bonaparte, 1990 (Dryolestida), Barberenia Bonaparte, 1990 (Dryolestida indet., milk dentition), Quirogatherium Bonaparte, 1990 (Dryolestida indet.,?milk dentition), Atlasodon Sigogneau−Russell, 1991 (Dryolestida, or Zatheria indet., milk dentition), Microderson Sigogneau−Russell, 1991 (stem−group zatherian), “ Kotatherium “ yadagirii Prasad and Manhas, 1997 (? Amphilestidae ).

Uzbekistan, is rather similar to that from the coastal plains of the North American Late Cretaceous.Several taxa are shared; including, notably, the eutherian mammal Paranyctoides (see Archibald and Averianov 2001).