MORPHOLOGY AND RELATIONSHIPS OF SLAUGHTERIA ERUPTENS

A single replacement tooth sits in the jaw below a molariform deciduous precursor as is clear in the reconstructions from CT data of S. eruptens (Figure 1 and Figure 2). External morphology is presented in a QuickTime VR model from CT data (Figure 3) and in a movie constructed from SEM photos of the jaw (Figure 4).

One reinterpreted salient character of S. eruptens is the lack of molarization of the posterior premolars, in contrast to eutherian mammals (Kielan-Jaworowska et al. 2004). Slaughter (1968b) referred what he identified as submolariform premolars from the Trinity Group to Eutheria and later, by inference, to Pappotherium, which he considered a eutherian (Slaughter 1981). A submolariform lower premolar figured by Slaughter (1968b) differs in morphology from the dp4 S. eruptens. It either derives from a different taxon than S. eruptens, or may represent a deciduous premolar anterior to the dp4 position of S. eruptens or another taxon.

Molar length has been used as a character in phylogenetic reconstructions of Cretaceous eutherian mammals (Archibald and Averianov 2006). The length of the newly interpreted lower m1 of S. eruptens closely matches first molars of Eomaia scansoria (Ji et al. 2002) and Prokennalestes minor (Kielan-Jaworowska and Dashzeveg 1989).

Pairing of isolated upper and lower teeth of Trinity Group tribosphenidan mammals has been attempted since Patterson (1956) described, but did not name the first sample of these teeth. Despite new collections, little consensus has resulted (Slaughter 1965, 1971; Turnbull 1971; Butler 1978). Upper teeth of Trinity Group tribosphenidans are differentiated primarily on morphology of the stylar shelf and protocone. The first tribosphenidan taxa named from the Trinity Group (Pappotherium pattersoni Slaughter 1965 and Holoclemensia texana Slaughter 1968a) were based on upper molar teeth. These two mammals differ notably in size as well as stylar cusp development. Other Trinity Group tribosphenidan taxa that were named, based upon upper teeth, include Comanchea hilli (Jacobs et al. 1989), Atokatheridium boreni (Kielan-Jaworowska and Cifelli 2001), and Oklatheridium szalayi (Davis et al. 2008). The four remaining taxa are named from lower teeth or jaws.

Patterson (1956) recognized three distinct morphologies of therian lower molars (structural types 1-3) (Table 1). Based upon additional material, Slaughter (1965) added three more morphotypes (4-6) to Patterson's previous three. Several of these morphotypes were described originally from single teeth. In his first paper on the subject, Slaughter (1965) noted that his type 6 lower molar (one molar in a jaw fragment; SMU 61728) was of the size expected to match the uppers of P. pattersoni. This specimen was subsequently designated the type of a new genus and species Trinititherium slaughteri (Butler 1978). Slaughter (1971) described a left lower mammalian jaw with four teeth (SMU 61992), and tentatively allocated it to P. pattersoni based upon size and probable occlusal relationships. He did not comment on its relationship to the morphotypes of Patterson (1956) or Slaughter (1965). Turnbull (1971) assigned all the teeth of Patterson's types 1 and 2 and Slaughter's type 5 to Holoclemensia. One small tooth (PM 922) formed the basis for Patterson's (1956) morphotype 3. This tooth, along with Slaughter's types 4 and 6 (6 would become the type of T. slaughteri), and some more recent discoveries, were referred by Turnbull (1971) to Pappotherium. In contrast, Butler (1978) associated Patterson's type 2 lower molars with Pappotherium (and types 1, 4, 5 with Holoclemensia), but he argued that molariform teeth in the jaw SMU 61992 differed from other type 2 molars and named the lower jaw Slaughteria eruptens. Butler (1978) also believed that type 3 teeth (including PM 922) resembled Kermackia texana (Slaughter 1971), a taxon described from lower teeth nearly simultaneously with Turnbull's (1971) paper. Slaughteria eruptens was regarded by Butler (1978) as smaller than teeth that he referred to Pappotherium.

Thus, P. pattersoni, which is based upon upper molars, has been variously associated with lower molars of morphotypes 2, 3, 4, or 6 (two of which are now designated as separate taxa), and S. eruptens has been regarded as associated lowers of P. pattersoni or recognized as a distinct taxon (Table 1). The only area of real agreement in previous studies is that types 1 and 5 teeth are Holoclemensia (Turnbull 1971; Butler 1978).

Tooth size alone clearly differentiates at least two groups of lower molars from the Trinity Group (Figure 5, Table 1; Jacobs et al. 1989; Kobayashi et al. 2002).

Our objective is to constrain quantitatively the size of the lower molars that are compatible with the holotype and unequivocally referred upper molars of P. pattersoni. We also test the associations that have been made of isolated lower teeth with the holotype and unequivocal upper molars of H. texana. Within mammalian species, it is well established that cheek teeth exhibit patterns of size correlation (correlation fields of Kurten 1953). Adjacent and occluding pairs of teeth typically exhibit the strongest correlations, especially within fields such as premolars or molars (Kurten 1953; Van Valen 1962; Gould and Garwood 1969, Gingerich and Winkler 1979; Szuma 2000). A model for estimating opposing tooth sizes in Early Cretaceous mammals is limited by the lack of population size samples of skulls with associated dentitions. However, given the limited range of tooth morphologies found in Cretaceous mammals with tribosphenic molars and their similar dental formulae, it is possible to predict that corresponding pairs of occluding upper and lower molars (especially M/m1 M/m2) across a range of species will also be constrained to strong size correlation in order to maintain occlusal relationships. Assuming robust length correlations between occluding molar teeth, an estimate of lower molar length based upon a given upper molar length can be made using regression analysis. Archibald and Averianov (2006) use differences in upper M1/lower m1 length ratios as phylogenetic character states in analyzing Asian Cretaceous mammals. Such varying proportions could confound the assumptions in the following regression analysis. However, systematic differences in upper/lower molar length ratios would be apparent in the following analysis in scatter plots and measures of length relationships (see below).

Two regression models (see Materials and Methods), one for upper versus lower first molars (M1/m1; Figure 6) and one for second molars (M2/m2; Figure 7), are used to estimate the length of lower molars (dependent variable) expected from the lengths of upper molars in the types and referred specimens of P. pattersoni and H. texana (natural logs are used to equalize variance among the species sizes). Using these regression equations, the lengths of penultimate lower molar predicted from the type (SMU 61725) upper penultimate molar of P. pattersoni are 1.49 mm (M1/m1 model; ln m1L = 0.016 + 1.100(lnM1L)) and 1.33 mm (M2/m2 model; ln m2L = -0.186 + 1.336 (ln M2L). The two predicted lengths for P. pattersoni lower molars bracket the length of the last tooth preserved in S. eruptens (m1; L = 1.44 mm). A well-preserved left upper molar (molar position uncertain) that can be referred unquestionably to P. pattersoni (SMU 73069) is smaller than the holotype specimen (L = 1.28 mm x W = 1.4 mm). Using this upper molar in the regression model for M1/m1 predicts a corresponding lower molar length of 1.33 mm for P. pattersoni. If the M2/m2 model is employed, the predicted lower molar length for SMU 73069 is 1.15 mm.

Using the length of the holotype (SMU 61997) upper molar of H. texana, a lower molar length of 1.85 mm (M1/m1 model) or 1.71 mm (M2/m2 model) is predicted. Most of the lower molars that have been and are here referred to H. texana (Slaughter 1971; Butler 1978; Jacobs et al. 1989; Kobayashi et al. 2002; but see Turnbull 1971) are longer than 1.85 mm (Figure 5). The largest lower molars of Trinity Group tribosphenidans (including Patterson's 1956 type 1 teeth) can be distinguished clearly from the smaller teeth on morphological grounds. These are most parsimoniously referred to H. texana. All other known lower molars are distinctly smaller with a single exception - PM 965 is only slightly shorter and narrower than any of the specimens here referred to H. texana, and it matches the latter's predicted lower molar size (Figure 8). This tooth was separated by Patterson (1956) from larger "type 1" teeth and allocated to his "type 2," along with PM 948 among others (Figure 9). Despite the arguments of Turnbull (1971), PM 965 does not share characters here associated with lower molars of H. texana, e.g., a relatively large metaconid, a relatively small paraconid, and an enlarged hypoconid. We agree with Patterson (1956) that this tooth is distinct from type 1 teeth that are referred here to H. texana, although it is larger than any other lower tooth of type 2. Lower molars referred to H. texana are morphologically distinct from the molar (m1) of S. eruptens. The smaller lower molar length predicted for H. texana by the model may indicate a different upper/lower molar size relationship in this taxon, or that the holotype is a small individual.

The last tooth (= m1) preserved in S. eruptens is compatible with the size of lower tooth predicted from the type upper molar of P. pattersoni. Furthermore, the m1 of S. eruptens matches one type 2 tooth (PM 948; Figure 5, Figure 9) nearly exactly in size. However, the m1 of S. eruptens shows weaker development of an anterolingual accessory cusp on the base of the paraconid, a character originally associated with type 2 teeth (PM 948 and 965; Patterson 1956). Development of this "antero-internal" cuspule (Patterson, 1956) or "mesial" cuspule (Kermack et al. 1965) may be lacking on first lower molars (Turnbull, 1971), or may be of taxonomic significance. Its presence in Aegialodon (Kermack et al. 1965) suggests that the cuspule is a primitive character state. The protoconid of m1 in S. eruptens is broken, and it is difficult to assess its size relative to the metaconid (type 2 teeth were, in part, differentiated from those of type 1 by a relatively smaller protoconid). Talonid cusp size and arrangement differ between the teeth assigned here to H. texana and all the smaller Trinity Group tribosphenidans. Wear on the hypoconid of S. eruptens m1 makes its size relative to other talonid cusps unclear, but it is evident that the hypoconid was not so enlarged nor the entoconid relatively so small as in molars referred to H. texana.

There is clearly a group of lower molars from the Trinity Group (Figure 5) that are smaller than the m1 of S. eruptens and other teeth of similar morphotype (e.g., PM 948; Figure 9). Small lower molars that are referred to Trinititherium, Kermackia, and Comanchea differ morphologically from the dp4 of S. eruptens, which fits in their size class. Those lowers referred to Comanchea hilli trend along a somewhat narrower width to length relationship than the others (Figure 5).

Deciduous Teeth. Recognition of a deciduous tooth in S. eruptens affords an insight not previously available; the diagnosable morphology of a molariform deciduous premolar in Trinity Group tribosphenidan mammals. Important characters, in addition to small size and narrowness, include the relatively low, anteroposteriorly elongate trigonid with a small paraconid rotated to the midline of the tooth. These characters were noted in other deciduous teeth of Cretaceous mammals (Butler 1977). Several teeth from the Trinity Group fit this description including PM 922 (Figure 10; Patterson's 1956, "type 3") and Kermackia texana, and invite further examination of the status of these specimens to identify them as permanent or replacement teeth. More than one taxon of smaller therians exist within the Trinity Group, but how many remains open to question. It is also not clear whether any "molarized" premolars from the Trinity Group (Slaughter, 1968b) represent permanent teeth.