Type species

Thisbemys corrugatus Wood, Reference Wood1959.

Referred species

Thisbemys uintensis (Osborn, Reference Osborn1895); T. medius (Peterson, Reference Peterson1919); T. perditus Wood, Reference Wood1962; T. plicatus Wood, Reference Wood1962; T. elachistos Korth, Reference Korth1984; T. brevicrista Ostrander, Reference Ostrander1986; T. intermedius n. sp.

Diagnosis

See Wood (Reference Wood1962, p. 101).

Occurrence

Early to middle Eocene (Wasatchian to Uintan NALMA) of western North America.

Thisbemys intermedius new species
Figures 1–3; Table 1

Figure 1.

Maxilla and dentaries of Thisbemys intermedius n. sp. from the Washakie Formation. (1, 2) FMNH PM 41972, (1) right maxillary fragment with P4–M1, (2) left maxillary fragment with M1; (3) FMNH PM 55855, holotype, lateral view of left dentary; (4) FMNH PM 41999, right dentary; (5) FMNH PM 41999, cross-section of right i1 (anterior to bottom of page). All figures to same scale (below). Anterior to top of page for (1, 2), to left for (3), to right for (4).

Figure 2.

Lower dentitions Thisbemys intermedius n. sp. from the Washakie Formation. (1) FMNH PM 55085, right p4–m1 (reversed); (2) FMNH PM 55086, left m1–m3; (3) FMNH PM 55881, left p4–m2; (4) FMNH PM 55855, left p4–m3 (holotype); (5) FMNH PM 55812, left p4–m3 (partial). All figures to same scale. Anterior to the left on all figures.

Figure 3.

Upper dentitions of Thisbemys intermedius n. sp. from the Washakie Formation. (1) FMNH PM 55201, left P4; (2) FMNH PM 55809, left P4–M1(partial); (3) FMNH PM 56374, left dP4; (4) FMNH PM 55019, right M1 or M2; (5) FMNH PM 55020, right M1 or M2; (6) FMNH PM 56417, left M1–M3; (7) FMNH PM 55981, left P4–M1; (8) FMNH PM 55735, left M2–M3. All figures to same scale. Anterior to the right on (4, 5), to the left on all others.

Type specimen

FMNH PM 55855, right and left articulated dentaries with i1, p4–m3 (Figs. 1.2, 2.4).

Table 1.

Dental measurements of Thisbemys intermedius n. sp. from the Washakie Formation. Abbreviations L, anteroposterior length; W, transverse width; N, number of specimens; M, mean; Min, minimum measurement; Max, maximum measurement; SD, standard deviation; CV, coefficient of variation. All measurements given in mm.

Diagnosis

Intermediate sized (Table 1), larger than T. plicatus and T. perditus, similar in size to T. corrugatus, smaller than T. uintensis and T. medius (Wood, Reference Wood1962, tables 35–38, 41–44); enamel crenulations on cheek teeth higher and more persistent than T. plicatus and T. perditus (not fusing at the bases until later stages of wear), less persistent than in T. corrugatus.

Occurrence

Lower Adobe Town Member, Washakie Formation, (early Eocene; Br3), Greater Green River Basin, Sweetwater County, Wyoming (see Appendix for list of specific localities).

Description

The dentary is relatively deep (Fig. 1.2, 1.3), ranging from 12.5 mm to 13.5 mm in depth below m1. The diastema is short, ranging from 7.5 mm to 10 mm in length, averaging ~50–60% the alveolar length of the cheek teeth. The dorsal surface of the diastema is a sharp ridge that dips only slightly below the alveolar margin of the cheek teeth. Ventrally, there is a slight symphysial flange below the anterior edge of m1, marking the posterior extent of the articular surface. The masseteric scar is marked by the convergence of two ridges (dorsal and ventral) meeting near mid-depth of the dentary ventral to m2. There are two main mental foramina, a larger one is ventral to the center of the diastema, approximately one-third the depth of the dentary below the dorsal surface of the diastema. A second minute foramen is also present on all specimens, directly posterior to the larger foramen and ventral to the anterior margin of p4. On some specimens there are one or more minute foramina on the dorsal half of the dentary, ventral to the tooth row.

The lower incisor is nearly equal in length and width in cross-section (average anteroposterior length = 5.05 mm, transverse width = 4.60 mm). The anterior enamel surface is flat to slightly rounded, but always rounded at the anterobuccal corner of the cross-section (Fig. 1.5). Enamel extends only slightly onto the lateral surface, with little or no presence on the medial surface. The medial side of the cross-section is flattened. In cross-section, the incisor is wider anteriorly, and tapers posteriorly.

The p4 is the smallest of the cheek teeth. It is narrower anteriorly than posteriorly. The total length average is nearly equal to the width of the talonid (Table 1). It is covered with crenulations in unworn specimens (Fig. 2.1), and the number of ridges decreases and they get thicker as the tooth wears (Fig. 2.3), until late stages when they are completely obliterated (Fig. 2.5; see Table 2 for definition of wear stages). This affect is due to the broadening of the crenulations near the bases, causing a merging of the ridges as they wear. The metaconid is the highest and most anteriorly placed cusp at the anterolingual corner of the tooth. The protoconid is greatly reduced in size relative to the metaconid and appears as a small curved ridge at the anterobuccal corner of the tooth. The trigonid basin is minute and disappears after moderate wear. A thick ridge runs posteriorly from the lingual side of the metaconid along the lingual margin of the tooth, ending near the center of the lingual side, where it is separated from the entoconid by a narrow, deep valley. In heavily worn specimens, this valley is obliterated and the ridge is continuous with the entoconid (Fig. 2.5). The hypoconid and entoconid are nearly equal in size, the former being only slightly larger. The hypoconid is round, but slightly obliquely compressed (anterolingual to posterolabial). The entoconid is slightly anteroposteriorly compressed. The ectolophid extends posteriorly from the posterior side of the protoconid, then angles slightly posterolingually, ultimately curving directly buccally, joining the anterolingual corner of hypoconid. There is only a slight swelling near the center of the ectolophid on some of the specimens indicating a mesoconid. The re-entrant valley between the hypoconid and ectolophid is short and directed posterolinguallly. The posterolophid is continuous along the posterior margin of the tooth from the posterolingual corner of the hypoconid to the posterobuccal corner of the entoconid. There is no distinct hypoconulid on the posterolophid, but the lophid appears to thicken near its center.

Table 2.

Definitions of wear stages of cheek teeth of Thisbemys.

The dp4 is smaller than p4 and typically lower-crowned. In overall morphology, it is similar to that of p4 (longer than wide, narrower anteriorly than posteriorly), but the cusps are smaller and less well defined. The crenulations of the enamel are just as distinct as those of p4.

The m1 and m2 are nearly identical, m1 being slightly smaller in specimens that have both (Fig. 2.2–2.5). However, isolated molars cannot be separated into m1 and m2 with confidence. As with the premolars, the number and size of the crenulations on the occlusal surface vary with attrition (Fig. 2). They are typically rhomboidal in occlusal outline. The trigonid is only slightly narrower than the talonid. Unlike p4, the protoconid is subequal in size to the metaconid and situated slightly more posteriorly than the metaconid. The anterior cingulid (=metalophulid I) runs the entire width of the anterior margin of the tooth from the metaconid to the protoconid. The metaconid is circular in occlusal outline, whereas the protoconid wears to a crescentic outline. The posterior arm of the protoconid (=metalophulid II) extends anterolingually from the protoconid, paralleling the anterior cingulid. A lophid running lingually from the apex of the metaconid also parallels the anterior cingulid. In some cases, these fuse to form a complete metalophulid II. Generally there is a minor break between their complimentary ends. In some specimens, the metalophid II is complete on m1 but not m2 (Fig. 2.3). A small transversely oriented trigonid basin is always formed between the metalophulid II and the anterior cingulid. The ectolophid is short and runs from the anterolingual corner of the hypoconid to the posterolingual corner of the protoconid. A minute mesoconid is marked by a central swelling of the ridge. In a few specimens, there is a short lophid extending buccally from the mesoconid (Fig. 2.4). The hypoconid is the largest of the cusps and obliquely compressed, as in p4. The remainder of the talonid is similar to that of p4. However, the hypoconulid is more distinct as a thickened swelling at the center of the posterior cingulid. The lophid extending posteriorly from the metaconid on the lingual side of the tooth is similar to that on p4, but the valley near its center is much deeper. The entoconid is obliquely compressed (anterobuccal-posterolingual). In more heavily worn specimens, the buccal cusps and ectolophid are replaced by wear facettids because they wear more quickly than the lingual cusps (Fig. 2.4, 2.5).

The m3 is longer than m1 and m2. The posterior width is equal to or slightly narrower than the anterior width. The differences between m3 and the other molars are due to the elongation of the talonid (Fig. 2.2, 2.4). The hypoconid is smaller than in m1 and m2, making the tooth narrower posteriorly than anteriorly. The posterior cingulid bows much farther posteriorly than in m1 and m2. The entoconid is still prominent.

The only cranial material are a few fragments or maxillae that retain the anterior base of the zygomatic arch (Fig. 1.1,1.2). The base of the zygomatic arch preserves the ridge for the attachment of the masseter (=protrogomorphy). The posterior margin of the zygomatic arch ranges slightly in position from being even with the posterior margin of P4 (Fig. 1.1), to being even with the metaloph of P4 (Fig. 1.2). No specimens retain any of the palatal surface medial to the lingual edge of the cheek teeth.

There are no specimens that contain P3, but based on its alveolus, it is clearly single-rooted and positioned anterior to the lingual root of P4 (Fig. 1.2). P4 is slightly smaller than M1 or M2 (Table 1). There is a distinct parastyle at the anterobuccal corner of the tooth that is continuous with an anterior cingulum that runs lingually from the parastyle along the anterior margin of the tooth until it reaches the anterior arm of the protocone, slightly more than half the width of the tooth (Fig. 4.1, 4.2). The paracone is separated from the anterior cingulum and parastyle by a narrow, deep valley. The protoloph runs lingually from the paracone, joining the anterior arm of the protocone just posterior to the lingual end of the anterior cingulum. There is no indication of a paraconule on any specimens. The mesostyle is prominent and anteroposteriorly compressed. Frequently, a low loph extends lingually from the mesostyle into the central valley between the protoloph and metaloph (Fig. 2.2). The metacone is continuous with the metaloph that curves slightly anteriorly, meeting the crescentic protocone just posterior to its center. A minute hypocone is a minor swelling on the posterior cingulum posterior and slightly lingual to the apex of the hypocone. The posterior cingulum runs along the posterior margin of the tooth from the hypocone, fusing at its buccal end, to the center of the posterior side of the metacone. The valleys between the cingula and the major lophs are relatively shallow and filled with crenulated enamel in unworn and little-worn specimens (Fig. 3.1, 3.2). As the teeth wear, due to the broadening of the crenulations near the bases, they become broader and merge with one another, resulting fewer recognizable crenulations. As dentine is exposed (more prominently on the lingual cusps first), the evidence of the enamel wrinkling is eliminated.

Figure 4.

Cheek teeth and dentary of Thisbemys sp., cf. T. uintensis from the Washakie Formation. (1) FMNH PM 54964, right M3; (2, 3) FMNH PM 55083, (2) occlusal view of left m2–m3; (3) lateral view of dentary; (1, 2) are to same scale (above), (3) to scale below.

DP4 is similar to P4 in occlusal morphology, differing only in being smaller, more nearly triangular in outline, and slightly more molariform with a better development of the anterior cingulum and hypocone (Fig. 3.3).

As with the m1 and m2, M1 and M2 are nearly identical in morphology, varying only slightly in size, making it nearly impossible to separate these molars as isolated teeth. Again, the enamel surfaces of the unworn specimens bears minute crenulations that fill the basins (Fig. 3.4, 3.5), only to be reduced in number but broadened with wear (Fig. 3.6, 3.8) until they are completely eliminated at the latest stages of wear (Fig. 3.7). In overall morphology, the first two molars are similar to that of P4, but the cups of protoloph are larger and more distinct. The parastyle and anterior cingulum are as in P4, but the cingulum extends farther lingually, making the tooth wider anteriorly and overall more quadrate in occlusal outline. The anterior cingulum joins the anterior arm of the protocone. Unlike P4, there is a distinct, but small protoconule on the protoloph just buccal to the protocone. The protoloph joins the crescentic protocone at the same point where the anterior cingulum ends. A short loph extends buccally into the central valley of the tooth from the lingual border, but ends before reaching the center. The paracone has a short loph extending posteriorly from its apex along the buccal edge of the tooth. This loph is continuous with the mesostyle on some, but not all specimens. The mesostyle is transversely compressed, and frequently attached to a lophule extending lingually into the central basin of the tooth for a short distance. This lophule is always present, but not always continuous with the mesostyle. The metaloph and metacone are as in P4, but the metaloph is not complete to the hypocone until the later stages of wear. The hypocone is small, but larger than in P4, and situated posterior and slightly buccal to the apex of the protocone. The posterior cingulum is continuous from the hypocone to the posterobuccal corner of the tooth as in P4.

M3 is nearly triangular in occlusal outline and slightly smaller than M1 and M2 (Fig. 3.6, 3.8). The anterior half of the tooth is as in the anterior molars, but the metaloph and hypocone are greatly reduced. The metaloph consists of an anteroposteriorly compressed metacone at the posterobuccal corner of the tooth and is separated from a large, circular metaconule. The metaconule is not continuous with the hypocone. On heavily worn specimens, the metaloph is complete from the metacone to the protocone (Fig. 3.8). A hypocone is either completely lacking, or a minor swelling on the lingual end of the posterior cingulum (Fig. 3.4, 3.8).

Etymology

Latin, intermedius, in between.

Remarks

Morphologically, T. intermedius n. sp. differs little from the other known species of the genus, other than the size of the dentition and the persistence of the crenulations of the enamel on the cheek teeth. In size, it is larger than the earlier occurring T. plicatus (Br2: Wood, Reference Wood1962, tables 41, 42) and nearly equal in size to the contemporaneous T. corrugatus (Br3: Wood, Reference Wood1962, tables 35, 36). Statistical comparisons of the sample of T. intermedius n. sp. with both T. plicatus and T. corrugatus (Students t-test) demonstrated that T. intermedius n. sp. differed consistently from both the other species in nearly all dimensions of length and width of the cheek teeth (Table 3). The accessory lophule on M1 and M2, diagnostic of T. brevicrista (Ostrander, Reference Ostrander1986, fig. 1), is not present on any of the available specimens of T. intermedius n. sp.

Table 3.

Results of t-test comparison of measurements of lower cheek teeth of Thisbemys intermedius n. sp. with T. corrugatus and T. plicatus. Measurements for T. corrugatus and T. plicatus from Wood (Reference Wood1962, tables 35, 41). Abbreviations: DF, degrees of freedom; L, anteroposterior length; W, transverse width.

In morphology, the crenulated enamel of the cheek teeth of T. intermedius n. sp. is less persistent than that of T. corrugatus, and more so than T. plicatus and all other earlier occurring species of the genus. Although not necessarily diagnostic, the m1 and m2 of T. intermedius n. sp. average longer than wide in occlusal measurements; >70% of the specimens (N = 66) have m1 and or m2 that are longer than wide. Interestingly, among other species, the m1 and m2 of T. corrugatus (N = 172) and T. plicatus (N = 56) average wider than long (Wood, Reference Wood1962, tables 35, 41), and in T. peditus (including specimens referred to “T. nini;” N = 11) m1 averages longer than wide, and in m2 the average width and length are equal (Wood, Reference Wood1962, tables 37, 39).

The large sample of T. intermedius n. sp. allows for a better understanding of the variation in the structure of the crenulations of the enamel of the cheek teeth based on wear and has allowed for the definition of wear stages (Table 2). Due to the sequence of eruption of the cheek teeth, it quite often occurs that p4 and m3 are much less worn (at an earlier wear stage) than m1 and m2 (Fig. 2.4). Each tooth in every specimen was valued at its wear stage (see Appendix).

Occurrence

Late Bridgerian (early Eocene; Br3), Lower Adobe Town Member, Washakie Formation, Sweetwater County, Wyoming.

Description

Only a fragment of the dentary is preserved on FMNH PM 55803 (Fig. 4.3). The dorsal ridge of the masseteric scar is preserved and extends anteriorly to a point ventral to the anterior root of m2, slightly more anterior than in other species (Wood, Reference Wood1962, fig. 36C).

Both of the preserved molars (m2, m3) are little worn and preserve a complex of enamel lophulids (Fig. 4.2) within the basins, as well as multiple furrows on the external sides of the teeth, particularly the buccal and posterior sides.

The m2 is rectangular to rhomboidal in occlusal outline. The metaconid is the tallest cusp of the trigonid and positioned slightly more anteriorly than the protoconid. The anterior cingulid is continuous across the anterior border of the tooth from the anterobuccal point of the metaconid to the anterolingual corner of the protoconid. The metaconid is obliquely compressed (anterobuccal to posterolingual), and a low metastylid crest extends posteriorly from its apex along the lingual edge of the tooth for half its length, where it ends in a deep, narrow valley. The metalophulid II is in two parts: a lophid extending buccally from the apex of the metaconid (posterior to, and paralleling the anterior cingulid), and a complimentary lophid extending lingually from the protoconid. The protoconid is crescentic in occlusal outline. The posterior arm of the protoconid continues posteromedially along the buccal edge of the tooth, then curves buccally, joining the anterior arm of the hypoconid. There is no indication of a mesoconid. There is a minor, low lophid extending anteriorly from the anterobuccal side of the hypoconid, partially blocking the buccal valley between the hypoconid and protoconid. The posterior cingulid (=posterolophid) is continuous from the posterolingual side of the hypoconid to the posterobuccal corner of the entoconid. Among the crenulations in the center of the tooth, is a small, lingually directed lophulid from the hypoconid that nearly reaches the center of the tooth. The entoconid is transversely compressed. A distinct lophulid extends buccally from the center of its buccal side, but ends approximately halfway across the tooth.

The m3 is typically much longer than m2, and narrower posteriorly than anteriorly. The anterior half of the tooth is similar to that of m2, but the complimentary lophulids from the metaconid and protoconid are not directly aligned as in m2, and the gap between them is larger. As is typical, m3 is elongated posteriorly compared to m2, with slightly smaller entoconid and hypoconid. The ectolophid is slightly longer and thinner than in m2.

The M3 referred here is also similar in morphology to M3s of other species of Thisbemys, but is assigned here due to its larger size (Fig. 4.1). The posterobuccal corner of the tooth is expanded (relative to M1 and M2). The anterior half of the tooth is similar to that of M1 and M2 in other species, but the metacone and hypocone are greatly reduced in size relative to the anterior molars. The metaconule is large and circular in outline. The metacone is reduced to a slight transversely compressed swelling at the posterobuccal corner of the tooth. There are multiple minute mesostyles along the buccal edge of the tooth between the paracone and metacone.

Measurements

FMNH PM 54964, M3: anteroposterior length = 5.26 mm; transverse width = 5.20 mm. FMNH PM 55083, m2: anteroposterior length = 4.20 mm, transverse width = 4.29 mm; m3: anteroposterior length = 5.14 mm, transverse width = 4.13 mm.

Remarks

Based on the highly crenulated enamel of the molars, these specimens are referable to Thisbemys. However, they are distinctly larger than the Washakie specimens referred here to T. intermedius n. sp. (Table 1). Wood (Reference Wood1962) diagnosed T. uintensis from the Uintan of Utah as having a hypolophid on m1 or m2 that was continuous with the ectolophid, but not the hypoconid (Wood, Reference Wood1962, fig. 39A). On the Washakie specimen, FMNH 55083, there is a short lophid extending buccally from the entoconid, and another short lophid extending lingually from the center of the ectolophid. While not complete across the entire tooth, it appears to be the same structure as in the holotype of T. uintensis. The referred M3 has a hypocone that is more distinct than in T. intermedius n. sp., and similar to that of the holotype of T. medius (Wood, Reference Wood1962, fig. 39E).

The holotype and only other known specimen of T. uintensis are from the Wagonhound Member of the Uinta Formation, Utah (Osborn, Reference Osborn1895; Wood, Reference Wood1962). This horizon is referred to the middle Uintan (Ui2: Robinson et al., Reference Robinson, Gunnell, Walsh, Clyde, Storer, Stucky, Froehlich, Ferrusquia-Villafranca, McKenna and Woodburne2004), however, Anderson (Reference Anderson, Janis, Gunnell and Uhen2008) mistakenly listed it as being from the later Myton Member (Ui3). The occurrence of these specimens from the late Bridgerian possibly extends the range of T. uintensis earlier into the late Bridgerian (Br3).

Thisbemys nini

Anderson (Reference Anderson2015, fig. 5.11) figured a specimen of m2–m3 mistakenly identified as T. nini. Although this specimen was part of the original hypodigm of this species (Wood, Reference Wood1962), it was previously reassigned to T. perditus, and the former species (limited to the holotype) was transferred to the genus Paramys (Korth, Reference Korth1984).

Thisbemys perditus

Wood (Reference Wood1962) differentiated this species from other species of the genus by its smaller size and less-persistent crenulations on the cheek teeth (Wood, Reference Wood1962). In occurrence, it was limited to the late Wasatchian NALMA, earlier than other species of the genus (Wood, Reference Wood1962; Korth, Reference Korth1984; Anderson, Reference Anderson, Janis, Gunnell and Uhen2008). Anderson (Reference Anderson2015, fig. 2) figured this species as occurring throughout the Bridgerian as well. There is no record of this species later than the Wasatchian NALMA.