Termastherium flacoensis, Wyss & Flynn & Croft, 2018
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publication ID |
https://doi.org/ 10.1206/3903.1 |
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persistent identifier |
https://treatment.plazi.org/id/03F087CC-FF90-3823-FE82-FF4EEA9FFA7B |
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treatment provided by |
Carolina |
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scientific name |
Termastherium flacoensis |
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genus et species novum |
Termastherium flacoensis , genus et species novum
Figures 5–7 View FIGURE 5 View FIGURE 7
HOLOTYPE: SGOPV 2987 ; right maxillary fragment bearing P2–M3.
AGE: As for Eomorphippus bondi .
TYPE LOCALITY: As for Eomorphippus bondi .
PARATYPES: SGOPV 2886 : left maxillary fragment bearing lingual portion of P3, all but the labial portion of P4, the alveolus and fragments of M1, and labially damaged M2–3 . SGOPV 3015 : right maxillary fragment bearing two little-worn cheekteeth, probably M1–2 . SGOPV 2996 : left mandibular fragment bearing m1–2 (or, less likely, m2–3) .
REFERRED SPECIMENS: SGOPV 2919: partial right upper molar, probably M2. SGOPV 3064: isolated, unworn Rm3 (tentatively referred). SGOPV 2883: right maxillary fragment, bearing portions of three teeth, identified as a posterior sliver of one tooth (P3) and the lingual three-quarters of the succeeding two teeth (P4–M1). SGOPV 3091; left mandibular fragment including the labial half of p4, m1, m3 plus remnants of two additional teeth (p3, m2); and SGOPV 3008: right maxillary fragment with three unworn teeth, probably dP2–4, provisionally referred (see below).
ETYMOLOGY: In reference to the summer resort town of Termas del Flaco, Chile, near which the holotype and referred material were collected .
DIAGNOSIS: Small-bodied leontiniid differing from Anayatherium, Ancylocoelus, Gualta , Leontinia , Scarrittia , and Taubatherium (the currently recognized Deseadan leontiniids, fide Shockey, 2005; see also Cerdeño and Vera, 2015), two new species from the La Cantera locality at the Gran Barranca ( Scarrittia barranquensis and Henricofilholia vucetichia , from the Deseadan or pre-Deseadan, Ribeiro et al., 2010), and Martinmiguelia and Coquenia (from the Eocene; see Bond and López, 1995 and Deraco et al., 2008, respectively) particularly in the strong unilateral hypsodonty of its upper cheekteeth, and the tendency for their ectolophs to wear into a blade standing high over the remainder of the occlusal surface.
DESCRIPTION: Termastherium flacoensis is approximately the same size as Eomorphippus bondi (particularly in upper and lower molar length), but is nevertheless easily distinguishable from the latter as well other notohippid species, particularly in the upper dentition. Although known from less complete material than is E. bondi , a sizable number of anatomically overlapping specimens provides a reasonably complete record of the upper dentition of this new leontiniid. The indirect proposed association of the upper and lower cheekteeth is based on: (1) both displaying features diagnostic of the Leontiniidae ; (2) size and morphological compatibility; and (3) congruent degree of hypsodonty. Overall, the upper cheekteeth of Termastherium flacoensis are much less hypsodont, especially lingually, than their counterparts in Eomorphippus obscurus and E. bondi .
UPPER DENTITION: No single specimen preserves representatives of all upper tooth positions, making the description below more composite than others in this study. The type specimen , SGOPV 2987 , bears the most complete dentition of Termastherium currently known; its M3 was inadvertently sliced transversely with a rock saw during trimming prior to preparation, resulting in a 3–4 mm gap within this tooth .
Resolving which tooth positions are represented by the six cheekteeth preserved in the holotype (P2–M3 versus dP1–M2) merits comment. A crescent of enamel ~ 2 mm wide abuts the posterolabial margin of the posterior tooth of SGOPV 2987 (projecting laterally at a right angle). At first glance this slice of enamel would seem to be a detached portion of the overlapping parastyle of a succeeding tooth, implying that the last fully preserved tooth of SGOPV 2987 is M2. Closer inspection, however, reveals that this protruding sliver is a preservational artifact, the result of crushing and displacement of the posterior portion of the last tooth preserved in the specimen, which is largely complete. Thus, there is no reason to conclude that a still more posterior tooth once existed but was not preserved. If we assume, therefore, that the posterior tooth of SGOPV 2987 is M3, counting forward in the toothrow indicates that the anterior element is P2. The little-worn condition of this tooth and its fairly complex crown morphology are consistent with its being P2, since the first premolar in notoungulates is generally simple in form and unreplaced, and thus is typically heavily worn .
A second maxillary fragment, SGOPV 2886 , bears cheekteeth in an earlier wear stage than the holotype. As with the holotype, there is some ambiguity as to the tooth loci preserved. The similar relative proportions of the teeth in both specimens, as well as the shape and morphology of the penultimate tooth (which tapers posteriorly and bears a strong lingually directed posteroloph) indicate that SGOPV 2886 most likely preserves P3–M3. The posteriormost tooth in SGOPV 2886 is even more tapered than the inferred M2 and has a somewhat triangular occlusal outline overall, consistent with its identification as M3 .
A third maxillary fragment, SGOPV 3015 (fig. 6), bears two beautifully preserved, littleworn cheekteeth, more likely M1–2 than P4–M1. Heavier wear on the holotype and SGOPV 2886 makes direct comparison to SGOPV 3015 difficult, as does incomplete preservation of M1 on the two other known maxillary dentitions. The anteriormost tooth of SGOPV 3015 is somewhat trapezoidal (an outline that also can be inferred for the incomplete M1 of the holotype), rather than transverse like P 4 in the holotype. The M 1 of the holotype and the anterior tooth of SGOPV 3015 are also similar in size, measuring ~ 17 mm anteroposteriorly at the lingual margin; the labial comparison is somewhat more subjective, given breakage of the holotype and uncertainty about the degree of imbrication of the parastyle, but is it clear the anterior tooth of SGOPV 3015 would fit well into the space created by removal of M 1 in the holotype. Moreover, the posterior tooth of SGOPV 3015 closely resembles the penultimate tooth of SGOPV 2886 (M2, which is less worn than its counterpart in the holotype), leading us to identify the two teeth in SGOPV 3015 as M1–2. In addition to other minor distinctions, the teeth in SGOPV 3015 seem to differ slightly in relative size compared to those of the other two maxillary specimens (with M1 appearing slightly larger and M2 slightly smaller in SGOPV 3015 ), but substantial differences in wear cloud the significance of these slight proportional disparities. We note these and other small morphological differences, ascribed to intraspecific variation and differences in wear, in the following descriptions .
The holotype preserves a moderately worn P2. This tooth, like all the succeeding cheekteeth on the holotype, is characterized by an ectoloph that towers over the remainder of the occlusal surface. The high-standing ectoloph seems to reflect three factors: the strongly bowed nature of the cheekteeth (less pronounced on P2 than on the more posterior teeth), their remarkable degree of unilateral hypsodonty (the crowns being as much as to 3–4× deeper labially than lingually), and an unusual pattern of wear (resulting in tremendous disparity in crown height; see below). The premolars are roughly rectangular in outline, with their long axes transverse, while the molars more resemble posteriorly canted trapezoids with longitudinal long axes. The P2 protocone consists of a steep face anteriorly and an inclined ramp that merges into the transverse loph posteriorly. The anterior base of the protocone arises from the deeply excavated anterior cingulum, the latter of which (at the early wear stage seen in holotype) is continuous with a valley projecting posteriorly along the lateral face of the protocone and that nearly meets the posterior cingulum. It is unclear whether the large depression anterior of the protocone is formed exclusively of the cingulum, or whether it also incorporated the “leontiniid depression,” regarded as diagnostic of the clade (Colwell, 1965; Villarroel and Colwell Danis 1997; Shockey, 2005). In either case, no pit or nearly circular depression distinct from the cingulum is present in the anterolingual corner of the upper premolars as it is in Leontinia sp. (illustrated in Patterson, 1934: fig. 10). Rather, the anterolingual portions of the upper premolars of Termastherium bear only an open, labiolingually directed trough that is confluent with the central fossa and lacks any manifestation of a pit-defining ridge as occurs in Leontinia . The lingual faces of the premolar protocones show no indication of the anteroposterior division seen in Leontinia ( Patterson, 1934) . Labially, a gutterlike cingulum traverses the base of the crown, descending ventrally along the posterolabial corner of the tooth in the position of the metacone column on the molars. A deep posterior cingulum has not yet united with the transverse loph.
The third premolar closely resembles P2, apart from its slightly greater size. The rim of the anterior cingulum is somewhat stronger on P3, and the area it encircles at the base of the protocone is somewhat larger. No specimen known to date preserves the labial face or complete ectoloph of P3. FIGURE 6. Two little to unworn right upper
The last upper premolar is subequal in size to cheekteeth of Termashippus flacoensis SGOPV P 3 lingually and very strongly bowed labially but 3015 (cast), probably M 1–2 in A, labial, B, occluotherwise still clearly premolariform. The labial sal, and C, lingual views. face is gently convex anteroposteriorly, with its generally smooth contour disrupted by a small parastyle (demarcated posteriorly by a faint cleft) and subtle undulations that mark the paracone and metacone columns (the former of which is more pronounced). The labial face shows no trace of a cingulum. The posterior cingulum remains prominent on the holotype, not yet united with the metaloph by wear. Similarly, the protoloph has yet to bridge the anterior margin of the tooth, with the result that the median valley remains open anteriorly. On SGOPV 2886, a small protoloph has just made contact with the protocone, resulting in the incipient isolation of the central fossette.
Two general aspects of the upper molars of Termastherium are relevant to this taxon’s assignment to the Leontiniidae . First, their degree of unilateral hypsodonty is remarkable. For example, the crown of the moderately worn M1 of SGOPV 2987 measures ~ 10 mm in height lingually (from the center of the lingual extremity of the protoloph to its base) and ~ 24 mm labially (from the base of the crown below the anterior root to the edge of the ectoloph in the paracone region). Increased wear would only have accentuated this lingual and labial disparity in crown height. Upper molar hypsodonty is strongly unilateral in many leontiniids but tends to be bilateral in notohippids (such as Eomorphippus bondi ).
A second notable feature of the upper molars of Termastherium arguing for this taxon’s referral to leontiniids rather than notohippids (in the traditional sense of that term) is the unusual form of the labial faces of these teeth. Rather than forming sheer, nearly vertical walls, as in Eomorphippus for example, these portions of the teeth (i.e., the interior faces of the terminations of the protoloph and metaloph) are somewhat swollen basally; higher on the crown these faces are convex labially. In lingual view (with occlusal surfaces directed up), the lingual faces of little worn upper molars somewhat resemble ocean waves that are about the break, i.e., the crest of the wave (the lingual edge of the occlusal surface) is curved beyond vertical. Wear of the protoloph and metaloph progressively removes the overhanging portion of these “waves,” such that a more typical vertical lingual face is achieved once the protoloph and metaloph merge at their bases. A more subdued version of this arrangement has been noted and well illustrated in the leontiniid Huilatherium pluriplicatum from the Laventan SALMA of Colombia ( Villarroel and Colwell Danis, 1997). Specimens of Anayatherium fortis from Salla, Bolivia (Deseadan SALMA), resemble Huilatherium pluriplicatum in this respect. A similar basal, shelflike bulge also occurs in various specimens of an unidentified species of Leontinia from the Deseadan of Patagonia represented by MLP 55-VIII-10-27, a maxillary fragment bearing M1–2 from La Flecha.
The molars differ substantially from the premolars in being much larger, much less transverse, more elongate anteroposteriorly, and tapering posteriorly (wider anteriorly than posteriorly, particularly in M2–3) rather than having anterior and posterior faces of roughly equal width. The molars are marked anteriorly by a short parastyle that is set off posteriorly from a low paracone ridge by a shallow groove on M1 (as judged from SGOPV 3015 ) and a shallower trough on M2–3. Clear evidence of a corresponding metacone ridge is lacking. On SGOPV 3015 , the labial faces of these teeth are doubly convex, bowed in the dorsoventral plane as well as anteroposteriorly (these faces are somewhat distorted or not preserved in the other two specimens). The height of the labial faces of these teeth (24 mm, M1;> 26 mm, M2) slightly exceeds their anteroposterior length (23 mm, M1; 24 mm, M2); this is true even in the heavily worn holotype due to the highstanding, bladelike ectolophs noted previously. This degree of labial hypsodonty contrasts strikingly with the much more brachydont lingual portion of the crown .
The cleft between the protoloph and metaloph—and therefore the central valley—remains open lingually on M 1 in SGOPV 3015 but is closed by wear in the holotype (and not preserved in SGOPV 2886). Only a few millimeters of additional wear would have closed this gap in SGOPV 3015, producing an isolated central fossa. This contrasts with the condition in Eomorphippus , wherein the lingual cleft extends nearly to the base of the crown and closes only after considerable wear. The protoloph of M1 is strongly oblique posteriorly and consists of two segments: a labial half, oriented posterolingually, and a lingual half that bends into a parasagittal plane and points directly posteriorly. M1 bears a pronounced anterior cingulum, as do the other upper molars, in sharp contrast to Eomorphippus in which no trace of such structures exists. The anterior cingulum, well exposed on M1 of SGOPV 3015 (due to the lack of a tooth preceding it), is situated low on the crown, remaining distinct from the occlusal surface even on the heavily worn M1 of SGOPV 2887. In SGOPV 3015, the anterior cingulum on M1 begins low (ventrally) from a point just dorsal to the tip of parastyle, slopes posterodorsally, and becomes horizontal about 5 mm below the base of the crown at a point roughly in line with the anterior projection of the central valley. This cingulum continues lingually to a point above the obtuse-angled corner formed by the protoloph bend. From this point, the cingulum bends toward the occlusal surface a short distance while diminishing in relief, and disappears before reaching the occlusal surface (of the protoloph) itself. The shelf behind this cingulum is considerably broader on the holotype than on SGOPV 3015. The metaloph is short and transverse and is only beginning to show wear on M1 of SGOPV 3015. A short but pronounced cingulum rings the M1 metaloph posteriorly, extending from the posterolabial margin of the tooth to the lingual tip of the metaloph or to a point just posterodorsal to the hypocone (in little-worn to unworn teeth). The posterior cingulum sits much lower (ventrally) on the crown than the anterior cingulum; as a consequence, the former merges with the occlusal surface much earlier in wear than the latter. With wear, the posterior cingulum of M1 forms a distinctive circular postfossette, which occurs widely in Tinguirirican and younger notohippids and leontiniids.
The second molar generally resembles M1 but is slightly larger, more anteroposteriorly elongate, and more strongly tapered posteriorly (i.e., proportionally narrower posteriorly compared to M1). An anterior cingulum on M2 persists even on the fairly worn holotype; its course is very similar to that on M1 (so far as is visible) but slightly wider. The M 2 of SGOPV 3015 retains a graceful, anterolingually curling, unworn hypocone and an unworn protoloph. The posterior arm of the protoloph slopes posterodorsally into the notch that forms the lingual opening of the central valley; like the hypocone, it curls lingually, making the lingual face of this portion of the tooth (and the hypocone) resemble two lingually concave “soon-to-break ocean waves” as mentioned (fig. 5). The distinctive shape of the little-worn to unworn protoloph of the M2s of SGOPV 3015 and SGOPV 2886 (and the presence of the concave lingual face even on the heavily worn holotype M2) are among the most compelling resemblances among the specimens assigned to this taxon. The cleft between the protoloph and metaloph is deeper on M2 than on M1, even after accounting for wear .
The M 3 is preserved in the fairly worn holotype and in SGOPV 2886 (where it is damaged labially). Correcting for postmortem distortion in the holotype, the external face of this tooth shares the same highly bowed and doubly convex labial face as the anterior molars. Owing to the tooth’s lesser wear, the M3 ectoloph on the holotype projects only about half as far beyond the main occlusal surface as on M2 of the same specimen (6.5 mm vs. 3 mm). The anterior cingulum is similarly disposed as on M1–2, but the posterior cingulum is positioned much higher on M3, very near the base of the crown. A metaloph is only very weakly developed, but the cleft between it and the protoloph is much deeper than on the preceding molars, reaching nearly the base of the crown .
DECIDuOuS UPPER DENTITION: The collection of Tinguiririca Fauna specimens from Termas del Flaco includes an enigmatic maxillary fragment, SGOPV 3008, bearing three distinctive, nearly unworn cheekteeth, all of which are at least one-third smaller than their most plausible counterparts in the holotype and other specimens referred to Termastherium flacoensis . Still, this specimen more closely resembles Termastherium than any other taxon from the Tinguiririca Fauna. Although it conceivably represents three successional cheekteeth of a second species of Termastherium , we interpret it as the partial deciduous dentition (dP2–4) of T. flacoensis rather than the sole specimen of a distinct and morphologically unusual leontiniid. If, on the other hand, the teeth of this specimen are successional, the three tooth loci most likely are P3–M1, and so this specimen would likely represent a second species of Termastherium given the substantial size difference from the same tooth loci in T. flacoensis . This alternative interpretation reflects the marked size disparity between the central and posterior tooth on SGOPV 3008, a disparity also seen between unquestioned P4 and M 1 in T. flacoensis . In addition, the strong parastylar groove on the central tooth of SGOPV 3008 compared to the posterior tooth suggests that the former is P4.
On balance, several lines of reasoning favor identification of SGOPV 3008 as deciduous rather than successional teeth (i.e., dP2–4 rather than P3–M1), and assignment to T. flacoensis . First, SGOPV 3008 shares some similarities with the deciduous dentition of Leontinia gaudryi described by Patterson (1934). These include details of the lingual faces of the teeth, including the apparent posterior position of the protocone. The resemblance between the posterior tooth of SGOPV 3008 and dP4 of FMNH P14659 ( L. gaudryi ; Patterson, 1934: fig. 22) is especially strong.
Secondly, the generally bizarre form of this dentition argues for its deciduous nature, particularly in the shapes of the anterior two teeth, which are not transverse, as would be expected for successional leontiniid premolars, but rather triangular to anteroposteriorly elongate with asymmetrically curved lingual margins. The deep parastylar notches of all three teeth, especially the anterior two, are also curious, being considerably deeper than those of the permanent premolars of most other toxodontians. The anterior two teeth of SGOPV 3008 lack the single, massive, centrally positioned lingual cusp typical of the successional premolars in leontiniids and other toxodontians); instead, the anterior tooth bears two small cusps near its anteroposterior midline, with the posterior one the larger of the two, as is typical of leontiniids. On the central tooth, a single indistinct posterocentral cusp is incorporated into a sweeping loph that connects to the parastyle anteriorly. The overall semimolariform morphology of the two anterior teeth is not markedly dissimilar from the molars of many other toxodontians, and the structures near the posterior margins of all three teeth look as though they would have worn into features resembling the posterior fossette of typical leontiniid molars (features not found in permanent premolars of leontiniids or other toxodontians). The “cresting wavelike” lingual surface of the posterior tooth also is reminiscent of this region in the two molars represented by SGOPV 3015.
Thirdly, the presence of three equally little-worn to unworn cheekteeth is more consistent with a young animal that died early in life than an adult notoungulate that would show substantial and differential wear across these three teeth. The central tooth of SGOPV 2008 does show some slight evidence of wear, particularly on the protoloph, where the original enamel crest has been breached and the underlying dentine exposed. It is more difficult to assess the degree of wear of the anterior tooth, but some must have occurred. The posterior tooth is pristine, with the lophs showing no evidence of abrasion. Based on comparisons with other notoungulates, if the posterior tooth were a freshly erupted M1, the two teeth anterior to it should be much more heavily worn.
Taken together, the lines of evidence presented above suggest that SGOPV 3008 preserves only deciduous teeth (dP2–4), thereby hindering its comparison to related forms despite its obvious distinctiveness. Although SGOPV 3008 could conceivably represent a previously unrecognized taxon (in the Tinguiririca Fauna or in the South American mammal record in general), it more likely pertains to a Tinguirirican notoungulate already recognized on the basis of nonoverlapping dental material. Various peculiarities noted in the description below strongly suggest that this specimen pertains to Termastherium flacoensis .
At the same time, dP3 of FMNH P14659 ( L. gaudryi ) exhibits a strong separation of the protoconule and protocone (the terminology of Patterson), whereas no such division exists on dP3 of SGOPV 3008. The dP3–4 (ICN-P-328) of Huilatherium pluriplicatum ( Villarroel and Guerrero Diaz, 1985) exhibit a deep lingual fold between these structures, which also contrasts with the condition seen in SGOPV 3008. Additionally, the deep parastylar notches noted above, which are not typical of permanent premolars of most toxodontians, are also not seen in the deciduous leontiniid dentitions mentioned previously (FMNH P14659 and DGP ICN-P-328).
The three teeth preserved in SGOPV 3008 are strongly unilaterally hypsodont (labially), their lophs persisting as sharp, uninterrupted enamel ridges, reflecting little to no wear and providing an unusually clear view of occlusal morphology (fig. 7). The anterior tooth, interpreted as dP2, is substantially smaller than dP3, but otherwise the two teeth show a strong resemblance. They are subquadrangular with fairly straight and complete ectolophs but somewhat more rounded lingual margins. Both have short (2–3 mm long) but distinct parastyles; on the labial faces of these teeth, the parastyles are demarcated posteriorly from a strong paracone ridge by a deep vertical cleft. An equally pronounced cleft separates the paracone ridge from the remainder of the labial face posteriorly. Lingually, a prominent cingulum sweeps from near the parastyle/paracone column junction to the base of the protocone on both teeth. On dP2, the protocone appears to form a distinct, isolated cusp rather than being incorporated into a protoloph, but minor breakage obscures whether a protoloph was in fact developed. A vertical ridge forms the posterolingual margin of the protocone; this ridge flattens into a fairly broad posterior cingulum. The protocone base is slightly swollen, resulting in a smooth face separating the anterior and posterior cingula. The ectoloph of dP2 is elevated well above the remainder of the occlusal surface; this unusual pattern (a hallmark of the permanent dentition of T. flacoensis ) is even more accentuated on dP3–4.
The protoloph is unmistakably present on dP3, extending from near the parastylar notch almost to the posterior margin of the tooth. Wear has breached the apex of the original protoloph ridge, resulting in a 1–2 mm wide enamel-bordered loph. As on dP2, a sharp vertical ridge connecting with the posterior cingulum climbs from the protocone tip. A short transverse loph extends labially from the protocone to the ectoloph, mimicking a molar metaloph. The base of the protocone is more inflated than on dP2, and its enamel face is more crenulated. Nearly the entire length of the dP3 parastyle overlaps the posterolabial face of dP2.
The dP4 is nearly as long as the other two deciduous premolars combined, approximating an isosceles trapezoid in outline more than a triangle (as in dP2–3). Relative to tooth size, the parastyle of dP4 is considerably smaller than on dP2–3, and the vertical groove separating the parastyle from the paracone column is much shallower. Moreover, no groove sets off the paracone column from the remainder of the labial face posteriorly, in contrast to the condition on dP2–3; rather, the paracone column is smoothly convex posteriorly. Only a very subdued undulation marks the position of the metacone column on the labial face. The labial face of the tooth is 13 mm high (measured from the base of the crown between the two labial roots to the ectoloph edge), roughly twice the equivalent span on the lingual face of the tooth. As on dP2–3, a distinct anterior cingulum is present, but it terminates nowhere near as far posteriorly as on dP2–3. Rather, the anterior cingulum ends in line with a distinct shoulder formed by a bend in the lingual face of the protoloph. A very short cingulum rings the metaloph posteriorly, extending from the posterior margin of the ectoloph to the posterior end of the unworn, comma-shaped metaloph, the intersection of which corresponds to the position of the hypocone. This posterior cingulum sits very near the occlusal surface of the tooth and with wear that soon would have formed a circular posterior fossette, as discussed for the molars of T. flacoensis . In this and other respects, dP4 resembles the little-worn M1 of SGOPV 3015 ( T. flacoensis ) described above, save for its smaller size. As in the molars of T. flacoensis , the lingual face of dP 4 in SGOPV 3008 is strongly convex, curling in a “wavelike” fashion. This pattern is less pronounced on dP3, in part because this tooth is more worn than dP4 and thus has lost some of its original height lingually, but also because the lingual face of dP3 is much less broad anteroposteriorly than dP4. In contrast to M1 of E. bondi , where a deep cleft separates the protoloph and metaloph lingually, only a shallow indentation separates these structures on the lingual face on the molariform dP4 of SGOPV 3008 (as in the molars of T. flacoensis ).
LOWER DENTITION: Several lower jaws and teeth are assigned to Termastherium flacoensis ; this description relies most heavily on SGOPV 2996 (fig. 8), with supplementary information provided by the other paratype and referred specimens. A left mandibular fragment (SGOPV 2996) bears two well-preserved, fairly hypsodont molars. The hypolophid of the posterior tooth curves fairly far lingually and is not unusually elongate, suggesting that it is probably m2 rather than m3. If these teeth are in fact m1–2, they are very nearly the same size as the corresponding teeth in SGOPV 2891, referred above to the notohippid Eomorphippus bondi . The primary reason for our referral of SGOPV 2996 and related lower dental elements to the Leontiniidae rather than to E. bondi concerns the unusual manner in which the fossettid in the anterior portion of the talonid is formed. This process is well illustrated in the posterior tooth of SGOPV 2996 (m2), where an anterior spur of the entoconid pinches off against the lingual wall of the anterior portion of the hypolophid to enclose a fossettid (fig. 8). The same configuration may be seen on the somewhat damaged occlusal surface of the m 3 in SGOPV 3091. This condition, which characterizes all leontiniids, except the early diverging Coquenia bondi from the late Casamayoran (Barrancan) or Mustersan of northwestern Argentina (Deraco et al., 2008) and possibly the roughly coeval Martinmiguelia (Bond and Lopéz, 1995) , contrasts with the condition in archaeohyracids and notohippids wherein the trigonid-talonid fossettid forms through the union of a posteriorly directed spur of the metalophid with the anterolingual portion of an unbranched entoconid. A second, less persuasive line of evidence for referral of SGOPV 2996 to the Leontiniidae rather than to E. bondi (or some other notohippid) concerns the development of lingual cingula beneath the trigonid and trigonid-talonid junction in both teeth. Lingual cingula are common in leontiniids but seldom seen in Mustersan and later notohippids. (It should be noted, however, that in leontiniids—including Leontinia —the lingual cingula generally span the full length of most cheekteeth rather than being restricted anteriorly, as occurs in SGOPV 2996.) We are aware of two exceptions to this pattern: SGOPV 2891, referred above to E. bondi , exhibits a subdued, papillate lingual cingulum on the anterior half of m2 (the only molar well enough preserved to show the condition) that is highly reminiscent of the one seen in SGOPV 2996 and the m3 of SGOPV 3091; SGOPV 2891also has a very pronounced lingual cingulum on p3, the only premolar that preserves this region clearly. The second exception, noted above, is Rosendo pascuali , which displays lingual cingula on the lower premolars but not the molars. Thus, the presence of lingual cingula in SGOPV 2996 suggests, but does not unambiguously argue for, its referral to the Leontiniidae rather than Eomorphippus bondi . The argument for referral of SGOPV 2891 to Notohippidae has been presented previously.
In addition to the features noted above, the two molars in SGOPV 2996 are characterized by short trigonids that are separated from the talonids labially by vertical (m2) or nearly vertical (m1) clefts. The labial faces of these teeth show no hint of cingula, even on m1, where the base of the crown is fully exposed. This contrasts with Leontinia , where strong labial cingula span essentially the entire length of all lower cheekteeth. The greatest height of the m1 crown labially is 16.6 mm (below the protoconid), slightly less than the anteroposterior length of the tooth (17.6 mm). The occlusal surface of the m1 trigonid is damaged in SGOPV 2996, but the paralophid and metalophid were evidently much more closely spaced than on m2. The entolophid has already worn into an anteroposteriorly broad surface on m1. It shows no evidence of the entolophid-formed fossettid seen on m2, evidently having already been removed by wear. The cleft between the anterior margin of the entolophid platform and lingual half of the posterior face of the trigonid remains open on both teeth. If a second trigonid-talonid fossettid were to have developed on m1, it would not have done so until after considerable wear. A shallow notch demarcates the posterior end of the entolophid from the posterior terminus of the hypolophid on m1; on m2, this region is marked by a deep triangular excavation, reflecting the tooth’s lesser wear and longer talonid.
An isolated, unworn, right lower molar, SGOPV 3064, perfectly matches the size and form of what would be expected in SGOPV 2996 if it included an m3, and is consistent with the size expected based on upper tooth dimensions for T. flacoensis . The hypolophid of SGOPV 3064 is considerably longer (both proportionally and absolutely) than its counterpart on the posterior tooth of SGOPV 2996, consistent with the former’s identification as an m3. Since this tooth is completely unworn, is not lodged in a mandibular fragment, and has been removed entirely from the matrix, it provides an excellent perspective on how occlusal morphology of this tooth would have changed during wear. SGOPV 3064 is remarkably hypsodont; the maximum height of the crown, 3.0 cm (measured labially), matches the tooth’s greatest anteroposterior dimension (near the crown base). The roots of SGOPV 3064 had yet to form, but convergence of the crown’s base indicates that they would have closed later in ontogeny. In any case, it is clear that the lack of roots cannot be ascribed to breakage. The tall, largely enamelless anterior face of the tooth consists of a shallow vertical trough into which the rear of the preceding talonid would have projected. The labial edge of this basally widening groove is made up of a bladelike, anterolabial margin of the protolophid, while its lingual margin consists of a more rounded column below the paraconid. The labial face of SGOPV 3064 consists of four tall, basally expanding spires separated to varying degrees by clefts. The anterior two spires (the paraconids and metaconids) merge basally about midway along the height of the tooth. The cusps are continuous with the paralophids and metalophids, respectively. A short, generally subdued but partly rugose cingulum slopes steeply down the anterolingual base of the paraconid spire and crosses the base of the trigonid. Comparably expressed lingual cingula occur on the two lower molars of SGOPV 2996 (referred to Termastherium flacoensis ), but also on SGOPV 2891 (referred to E. bondi ).
The tip of the third lingual spire on SGOPV 3064 (entoconid) forms a free-standing pinnacle with respect to the hypolophid. This spire broadens rapidly toward its base, merging with the remainder of the talonid at various levels vertically along the crown. A curtain of enamel extends anterolabially from 4 mm below the tip of the entoconid to join the lingual wall of the hypolophid immediately posterior of the junction of the hypolophid with the trigonid. Immediately posterior to this junction, a sliver of enamel connects the entoconid spire to the hypolophid; wear would have resulted in isolation of an “intra-entolophid” fossettid, as is characteristic of leontiniids. The anterior face of the entoconid pillar is separated from the rear of the trigonid by a narrow (anteroposteriorly) but deep (transversely and vertically) cleft, indicating that no fossettid of the kind seen in archaeohyracids and notohippids (formed through union of the metalophid and entolophid) would have formed until perhaps the crown had been worn>75% of the way to the roots. Posteriorly, the entoconid pillar merges with the remainder of the talonid slightly below the midpoint of the crown’s height. The fourth spire, less freestanding than the others, consists of a narrow ridge that climbs to the posterior end of the unworn hypolophid blade. This ridge flattens toward its base, terminating as a small anteriorly projecting cingulum. This cingulum extends anteriorly to a point nearly directly below the entoconid, projecting somewhat below the level of the posterior end of the trigonid cingulum (with which it is discontinuous). A concave, bowl-shaped area occupies the basal portion of the rear of the talonid posterior to this fourth spire. Posteriorly the tooth forms an inclined ridge that thickens transversely toward its base. One effect of this inclination is that the maximum anteroposterior length of the wear surface of the talonid would have increased substantially (by ~25%) with wear. The cristid obliqua meets the trigonid just below the midpoint of the posterolingually slanting metalophid. The cleft dividing the trigonid and talonid labially persists to the base of the crown.
A left mandibular fragment bearing p3–m3 from Cañadón Blanco, MLP 52-XI-4-167, is also relevant to this discussion. Although labeled in the MLP collections as “ Notohippidae indet.,” the molars of this specimen bear an anterior spur off the entoconid, which entirely forms the fossettid posterior to the trigonid; it also bears lingual cingula. In these respects, MLP-52-XI-4-167 resembles SGOPV 2996 and other leontiniids, but MLP-52-XI-4-167 is roughly one-third larger than SGOPV 2996 in all dental dimensions. MLP 52-XI-4-167 may ultimately be found to correspond to a species of Termastherium distinct from T. flacoensis , but this potential action awaits a comprehensive revision of related material from Cañadón Blanco.
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