Megaleptictis altidens, Meehan & Martin, 2012

Meehan, Tj & Martin, Larry D., 2012, New large leptictid insectivore from the Late Paleogene of South Dakota, USA, Acta Palaeontologica Polonica 57 (3), pp. 509-518 : 510-515

Megaleptictis altidens sp. nov.

Figs. 1–5 ; Tables 1, 2.

Etymology: Though the teeth are not hypsodont, this species was named altidens , or tall tooth, for its much higher−crowned premolars and molars compared to other leptictids.

Holotype: KUVP 2568 ; a nearly complete skull and mandibles with right P3–M3, left P3–M3, right p3–m2, and left i2–m3. There are no other recognized specimens.

Type locality: South Dakota Locality 2 of Kansas University (KU− SD−002), Custer County .

Type horizon: This specimen was collected from the tan siltstone floodplain deposits of the White River Group in an 1894 Kansas University expedition. Based on other material collected with it, KUVP 2568 is most likely latest Eocene (Chadronian NALMA), but may be earliest Oligocene (Orellan NALMA).

Diagnosis.—A leptictid that differs from Leptictis in being larger−bodied, having higher−crowned teeth, and parasagittal crests with a long postorbital constriction. The skull is over 10% longer than in Leptictis haydeni Leidy, 1868 and Leptictis dakotensis Leidy, 1868 . The nasofrontal sutures form a W−shape, as opposed to a broad U−shape as in contemporaneous Leptictis and Blacktops species. The anterior squamosal sinus foramen is large, and the suprameatal foramen is small, while L. dakotensis has the opposite morphology. Unlike in other Late Paleogene leptictids, the P3 is very tall, lacks cingula and cuspules, and has a vestigial metacone. In the upper molariform teeth (P4–M3), relative crown height on the lingual side is approximately 1/3 taller than in L. dakotensis . These upper teeth are incipiently unilaterally hypsodont. Compared to L. dakotensis , the lower premolars and molars are much higher−crowned as well, p1–p3 lack cuspules, and p4–m2 are more anteroposteriorly compressed, as in the upper molariform series.

Measurements.—See Tables 1 and 2.

Description and comparisons.—The skull of KUVP 2568 is undistorted, but is missing its zygomatic arches, left postglenoid process, auditory bullae, right occipital condyle, portion of the supraoccipital, small portions of the nasals, maxillae, and lateral braincase, anteriormost portion of the rostrum, and the teeth anterior to P3 ( Fig. 1A ). Since I1 is lost evolutionarily in leptictids, the upper teeth missing in this specimen are I2, I3, C, P1, and P2. The dentaries are mostly complete, with heavy damage to the rami ( Fig. 2B ). The right dentary of KUVP 2568 has p3–m2 well preserved, but almost all of the posterior ramus is broken away. The left dentary is more complete with i2–m2 well preserved, and the base of i1 present, as well as the posterior portion of the m3

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talonid. The left dentary has most of the ramus preserved, but the condyloid portion is a separate fragment, and the angle of the jaw is broken away.

The mandible of Megaleptictis mainly conforms to the unspecialized morphology of leptictids listed by Novacek (1986): the dentary is long, its ventral margin is convex, the deepest portion of the jaw is at the level of m1, the mandibular condyle is wide and cylindrical, and the masseteric fossa is moderately deep. The dentary of KUVP 2568 differs in being relatively more robust, particularly below the molars, as one would expect with higher−crowned teeth. Most distinctly, the anterior edge of the coronoid process is straight, and its apex forms a slender triangle, while in Leptictis dakotensis the anterior edge is convex, and the coronoid apex is hook−like ( Novacek 1986; Fig. 2A, B 1 ). Leptictis dakotensis possesses anterior and posterior mental foramina, which are located ventral to p1 and the anterior root of p3, respectively ( Novacek 1986; Fig. 2A ). In KUVP 2568, both dentaries have three mental foramina. On the left, the anterior and middle foramina are ventral to the anterior and posterior edges of p2, respectively, and the posterior mental foramen is ventral to the anterior edge of p4 ( Fig. 2B 1 ). The right dentary differs in having the anterior mental foramen lying ventral to p1 and the posterior foramen ventral to the center of p4 ( Fig. 2B 3).

The lower incisors (i1–i3) and canine are damaged in KUVP 2568, but they appear to be typical for leptictids. The sectorial premolars (p1–p3) are very tall; the left p2 crown height as measured on the lingual side is 5.5 mm, and the right p3 is 5.8 mm. The second premolar is twice as large as p1 and somewhat smaller than p3 ( Fig. 2B , Table 1). These premolars lack cuspules, unlike in Leptictis dakotensis . There are small diastemata along i3–p4 similar to L. dakotensis ; the largest (1.9 mm) in the mandible of KUVP 2568 lies between p2 and p3.

After accounting for heavy wear, the trigonids of p4–m 2 in KUVP 2568 were likely erect and tall. The unworn height of p4 was likely around 4.5 mm, while in Leptictis dakotensis it is on the order of 3.5 mm (e.g., KUVP C−2225 and F:AM 108194). In Fig. 2 , note how much taller the crown bases are in KUVP 2568 compared to L. dakotensis , as well as the flat, heavy wear of the trigonid and talonid cusps. The longest molariform tooth in KUVP 2568 is p4, but m1 is wider, so these two teeth are subequal in overall size. The fourth lower premolar has a well−defined paraconid, which is shorter and smaller than the protoconid and metaconid. The metaconid is the tallest cusp and is subequal in size to the protoconid. This trigonid morphology is similar to Leptictis dakotensis . The talonid has a large hypoconid, medium−sized entoconid, and a small hypoconulid with a twin cuspule between it and the entoconid. Except for the twinned cuspule, Leptictis has the same talonid morphology.

The lower molariform teeth of KUVP 2568 are slightly more compressed anteroposteriorly than in Leptictis dakotensis . In m1 and m2, the trigonid and talonid lengths are subequal, while in L. dakotensis the talonids are longer than the trigonids. In m1 and m2 of KUVP 2568, the paraconid is likely highly vestigial; it may have been lost, but the presence of the paraconid is difficult to discern owing to wear. As in Leptictis , the hypoconulids of p4 and m1 impinge on m1 and m2, respectively, creating indentations. In m1 and m2, the protoconid and metaconid are directly across from each other, and the entire trigonid is worn down to one slightly concave surface. The second molar is slightly smaller than m1; their talonids have a large hypoconid, medium−sized entoconid, and small hypoconulid, as seen in Leptictis .

The right m3 of KUVP 2568 is broken at the base, and the left m3 is broken away with just the posteriormost talonid preserved. It can only be observed that the preserved portions of m3 conform to Leptictis morphology ( Novacek 1976, 1986: table 2): an elongate m3 outline with a bulbous hypoconulid.

On the left side of the KUVP 2568 skull, the bases of I2–P2 are present and exhibit no deviations from typical leptictid morphology ( Fig. 1A 2 , A 3). The P3 is typical in being sectorial with a somewhat isolated protocone abutting posterior to the paracone, yielding a triangular occlusal outline of the tooth ( Fig. 3). P3 is unique in having a tall sectorial ridge, tall protocone, no cingula, no cuspules, and a small metacone; there is wear along the posterior tooth edge—it appears that an enlarged paracone dominates the sectorial portion of P3 and that the metacone is vestigial.

Except for their greater height, P4–M2 of KUVP 2568 are close in morphology to Leptictis dakotensis . The lingual crown height of P4 is 3.5 mm in KUVP 2568, while it is 1.7 mm in KUVP C−2225, L. dakotensis (both specimens have moderate wear). In KUVP 2568, the molariform teeth lack lingual cingula, have moderate ectocingula and precingula, have well−developed postcingula with hypocones, and have paracones that are slightly larger than the metacones (as is typical in leptictids). The protocones are all well worn and appear to be typical in shape and position. The precingula are situated centrally along the anterior tooth margins, while the postcingula meet the lingual edges of the teeth, being even with the protocones. The precingula and postcingula have buccal edges that taper into the tooth walls and prominent lingual edges that are blocky to rounded in occlusal view ( Fig. 3). The postcingula are tall next to the protocones (owing to the large hypocones), and they taper and slant buccally towards the tooth bases, while the precingula are nearly level and a little closer to the tooth bases in lingual view. Leptictis dakotensis has similar postcingula, but the precingula tend to be lens−shaped (tapered at both ends). Where there is less tooth wear on KUVP 2568, the precingula have a few cuspules, and the large hypocones of the postcingula have neighboring hypoconules that are nearly as large. Closer to the linhttp://dx.doi.org/10.4202/app.2011.0035

gual tooth bases of P4–M2 and dorsal to the hypocone (or dorsal to the point between the protocone and hypocone), there is also a cuspule. The stylar shelves of P4–M 2 in L. dakotensis are larger, and the ectocingula are larger and continuous from the paracones to metacones. In P4–M2 of KUVP 2568, the ectocingula are discontinuous, tapering to an end at the posterior paracones and are then moderately developed around the metacones. M1 and M2 are relatively wider and shorter in KUVP 2568 than in L. dakotensis .

M3 is also close in morphology to Leptictis dakotensis . It is similarly reduced, being about two−thirds the size of M2 ( Fig. 3). The M3 precingulum and postcingulum are the same as in the other molariform teeth, yielding a bulbous outline to the lingual side of the tooth, as in L. dakotensis . On the M3s of KUVP 2568, the paracones and left metacone are broken away; the parastylar shelf is prominent, the metacone is reduced, and the ectocingulum is highly reduced, as in L. dakotensis .

In ventral view, the palate of KUVP 2568 conforms to Leptictis dakotensis morphology ( Novacek 1986: fig. 14; Figs. 1A 3, 3), with the palatine bone extending anteriorly to P4 and having a large postpalatine foramen, small middle palatine foramen, and an apparently elongate anterior palatine foramen (there is some crushing in this region). The premaxillae are broken away at the incisive foramina.

In the basicranium of Leptictis dakotensis , the somewhat inflated entotympanic bulla covered most of the tympanic chamber ( Novacek 1986), but preservation of this fragile bone is rare. The entotympanics are broken away in KUVP 2568, and there are no associated ectotympanic rings. The roof of the middle ear cavity is comparable to that of L. dakotensis , with the epitympanic recess and fossa for the tensor tympani being slightly deeper ( Novacek 1986: figs. 20–23; Fig. 1A 3, B 3). KUVP 2568 is missing the right occipital condyle and a portion of the supraoccipital bone just dorsal to the foramen magnum; its occipital region is blockier in outline than in L. dakotensis ( Fig. 1A 4, B4 ), with the petromastoid bone having a deeper fossa for neck muscle attachment. This region has at least three foramina—cracking and mineralization may be obscuring other foramina. Though the nuchal crest is broken along much of its edge, its prominence is slight and comparable in development to L. dakotensis .

The left postglenoid process of KUVP 2568 is chipped laterally, but appears to have been as broad and thick as in Leptictis dakotensis . Novacek (1986) stated that in leptictids the mandibular condyle does not fill the glenoid fossa, so that some horizontal motion is possible. The condyloid and glenoid in KUVP 2568 appear to form a tighter articulation than in L. dakotensis , but some horizontal motion is certainly possible since the glenoid fossa is open anteriorly. Just dorsal to the postglenoid process, running posteriorly to the nuchal crest, is the suprameatal fossa with an anteriorly positioned suprameatal foramen, as in L. dakotensis ( Fig. 4 ). The suprameatal fossa is taller in KUVP 2568 than in L. dakotensis , and its ridged boundaries are more pronounced anteriorly and dorsally. The dorsal ridge runs the complete length to the nuchal crest ( Fig. 4 ), while in L. dakotensis it tapers out a few millimeters anterior to the nuchal crest. The ventral ridge of the suprameatal fossa is the roof of the external auditory meatus. In ventral view, the concavity of the external auditory meatus is as in L. dakotensis , as is the short auditory canal. Dorsal to the suprameatal fossa on the right side of cranium of KUVP 2568, there are three foramina for the squamosal venous sinus, and the anterior one is quite large. On the left side, only this large squamosal sinus foramen is present ( Fig. 4 ), but mineralization and preparation work may be obscuring any smaller openings. As described by Novacek (1986: figs. 2 and 17) in L. dakotensis , there are four foramina: three grouped posteriorly and one anteriorly, which lies dorsal to the postglenoid process. These three posterior ones are subequal in size and not as large as the anterior one in KUVP 2568. One might expect high individual variation in these foramina, but other specimens of L. dakotensis have three subequal foramina of medium size (about 0.7–1.0 mm in diameter), while KUVP 2568 has a major squamosal sinus foramen that is much larger (maximal diameter of 1.8 mm) than neighboring foramina, including the suprameatal foramen; in L. dakotensis , the suprameatal foramen is much larger than the squamosal sinus foramina. A more pronounced feature of the posterior squamosal region dorsal to the suprameatal fossa is that it is deeply recessed in KUVP 2568, unlike in L. dakotensis .

The more anterior foramina along the lateral braincase cannot be described owing to cracking and some loss of bone, but the infraorbital region is well preserved. The infraorbital foramen is relatively larger in KUVP 2568 than in Leptictis dakotensis , and the posterior rostrum is taller, reflecting its higher−crowned teeth. The root of the zygomatic arch ventral to the infraorbital foramen has a shallow fossa, as in L. dakotensis , and anteriorly along the ventral rostrum, the alveolar processes are more pronounced in KUVP 2568 than in L. dakotensis .

From the orbit to the nuchal crest along the dorsal skull, KUVP 2568 has double parasagittal crests, as do all other known leptictids from the White River Group, including the unpublished genus “Frictops” from the latest Eocene (Chadronian NALMA; Novacek 1978). The parasagittal crests form the dorsal border of the temporal fossa. In KUVP 2568, the parasagittal crests differ from all other leptictids in having a long postorbital constriction ( Fig. 1A 1 ), rather than being fairly straight and parallel as in Leptictis ( Fig. 1B 1 ) and Blacktops ( Meehan and Martin 2010) , or wavy and posteriorly converging as in “Frictops” ( Novacek 1978). Another differentiating trait of the dorsal skull is the shape of the nasofrontal sutures. In KUVP 2568, they are W−shaped, while Leptictis has broad U−shaped sutures ( Fig. 5 ), as does Blacktops ( Meehan and Martin 2010) . “Frictops” also has W−shaped nasofrontal sutures, but they are more elongated, and “Frictops” is easily separated from Megaleptictis in being small−bodied for a White River leptictid and in possessing a much narrower snout and broader frontoparietal region ( Novacek 1978).

Statigraphic and geographic range.— The type specimen, KUVP 2568 , is from the lower or middle White River Group, southwestern South Dakota, which is latest Eocene or earliest Oligocene (Chadronian or Orellan NALMA). There are no known referred specimens .

Figures

Fig. 1. Leptictid insectivore skulls. A. Megaleptictis altidens gen. et sp. nov., holotype (KUVP 2568), from the latest Eocene or earliest Oligocene of South Dakota’s White River Group, compared to the common species of this time; A1, skull in dorsal view, note the postorbital constriction of the parasagittal crests; A2, lateral view, the teeth and rostrum are taller than in L. dakotensis; A3, stereo pair, ventral view, palate shape, middle ear region, and general tooth features are similar to L. dakotensis; A4, occipital view, the occipital region is blockier than in L. dakotensis. A portion of the supraoccipital region is broken away. B. Leptictis dakotensis Leidy, 1868 (F:AM 108194) from the earliest Oligocene of North Dakota’s White River Group; B1, dorsal view, the parasagittal crests are nearly straight and parallel; B2, lateral view; B3, stereo pair, ventral view; B4, occipital view. Photographs (B1, B2, and B4) from Novacek (1986: fig. 2). Scale bars 10 mm.

Fig. 2. Leptictid insectivore mandibles. A. Leptictis dakotensis Leidy, 1868 (F:AM 108194) from the earliest Oligocene of North Dakota’s White River Group, left lateral mandibular drawing (modified from Novacek 1986: fig. 1). B. Megaleptictis altidens gen. et sp. nov., holotype (KUVP 2568), from the latest Eocene or earliest Oligocene of South Dakota’s White River Group. B1. Left dentary in lateral view; i2–m2 are well preserved, except for the fractured crown of p3; the base of i1 and posterior talonid of m3 are present; the posterior ramus is broken away, except for a portion of the condyloid region shown in its approximate position. Note the highercrowned teeth of M. altidens and the flat wear of the p4–m2 trigonids. In contrast to the hook−like process of L. dakotensis, the coronoid process of M. altidens is triangular with a straight coronary ridge. B2. Occlusal view of mandibles placed in articulation.B3. Right dentary in lateral view; complete p3–m2 and the fractured bases of i2–p2 and m3 are present.

Fig. 4. Left squamosal regions of leptictid insectivores. A. Megaleptictis altidens gen. et sp. nov., holotype (KUVP 2568), from the latest Eocene or Early Oligocene of South Dakota’s White River Group in oblique lateral view. The suprameatal fossa of M. altidens is larger than in L. dakotensis, as is the anterior venous foramen from the squamosal sinus. The dorsal ridge of the fossa extends posteriorly all the way to the nuchal crest, unlike in L. dakotensis. B. Leptictis dakotensis Leidy, 1868 (F:AM 108194) from the earliest Oligocene of North Dakota’s White River Group in lateral view (modified from Novacek 1986: fig. 2).

Fig. 5. Nasofrontal sutures of leptictid insectivores. A. Megaleptictis altidens gen. et sp. nov., holotype (KUVP 2568), from the latest Eocene or earliest Oligocene of South Dakota’s White River Group compared to the common species of this time; rostrum in oblique dorsal view, showing the W−shaped nasofrontal sutures. B. Leptictis dakotensis Leidy, 1868 (F:AM 108194) from the earliest Oligocene of North Dakota’s White River Group; rostrum in dorsal view, showing the U−shaped nasofrontal sutures (modified from Novacek 1986: fig. 2). White and black lines just posterior to the sutures outline their shape.