Palaeictops bicuspis ( Cope, 1880 )

Velazco, Paúl M. & Novacek, Michael J., 2016, Systematics of the genus Palaeictops Matthew, 1899 (Mammalia: Leptictidae), with the description of two new species from the Middle Eocene of Utah and Wyoming, American Museum Novitates 2016 (3867), pp. 1-42 : 7-10

Palaeictops bicuspis ( Cope, 1880)

Figures 2–5A , table 1

Stypolophus bicuspis Cope, 1880: 746 .

Ictops bicuspis Cope, 1881: 192 ; Cope, 1885: pl. 29a, figs. 2, 3. Name combination.

Palaeictops bicuspis Matthew, 1899: 31 , 35. First use of current name combination.

Diacodon bicuspis Matthew, 1918: 574–576 . Name combination.

Diacodon pineyensis Gazin, 1952: 19 . Name combination.

Palaeictops pineyensis Van Valen, 1967: 232 . Name combination.

HOLOTYPE: AMNH 4802 (fig. 2A–B), consisting of a partial skull that retains C, P4–P5, M1–M3 on the left side; the right side retains two incisors possibly corresponding to I2 and I3 (both are broken), as well as P2 (broken), P4–P5, M1–M2, and M3 (broken). Left ramus with p5–m3 and roots of p1, p2, and p4 (fig. 3). Right ramus with p4–m2 and roots of i?, c, p1, and p2 (fig. 4).

TYPE LOCALITY: Wind River Basin, Wyoming. Wind River Formation, Lower Eocene (Wasatchian NALMA) .

DIAGNOSIS (revised from Novacek, 1977: 14): Differs from other species of Palaeictops in having a large, swollen and anteriorly extended paracone of P4 (paracone less anteriorly projecting in P. altimontis and P. matthewi ); tricuspid p4 with large anterior cusp on the heel and lack of a cusp anterior to main cusp (p 4 in P. multicuspis , P. matthewi , and P. altimontis with cusp anterior to main cusp, but lacking a large anterior cusp on the heel). Differs from P. bridgeri in its smaller size, in lacking a broad prefossid between paraconid and metaconid on p5, and in having a more lingually positioned hypoconulid on m3. P4 and p4 less tall and trenchant than in P. multicuspis (p4) and P. matthewi . P4 more anteriorly extended that in P. altimontis (fig. 5).

DISTRIBUTION: Wind River Formation (Lost Cabin and Lysite members), Wasatch Formation (Knight Member), Willwood Formation, Wyoming, Lower Eocene (Wasatchian NALMA) . REFERRED MATERIAL: AMNH 4255, left ramus with broken m1, m2–m3 and, provisionally, several other specimens from the Willwood Formation, Bighorn Basin, Wyoming, described by Bown and Schankler (1982: 19) (see remarks below). YPM VPPU 13436, jaws with upper and lower cheektooth dentition and several other specimens described by Guthrie (1971: 54–55) from the Lost Cabin Member, Wind River Formation, Wyoming. Provisionally, USNM 19204, left ramus with damaged p5, m1–m3, from the Knight Member, Wasatch Formation, Big Piney La Barge Fauna, Sublette County, Wyoming. Described as the type of Diacodon pineyensis by Gazin, 1952, and referred to Prodiacodon tauricinerei by Novacek (1977: 26). YPM VPPU 13419, upper and lower jaws with cheektooth dentitions and associated distal humerus fragment (described as Palaeictops pineyensis by Guthrie, 1967) from the Lysite Member, Wind River Formation, Wyoming.

REMARKS: The most distinctive feature of Palaeictops bicuspis is the enlarged, anteriorly positioned paracone on P4 (fig. 5). This tooth is unknown in P. borealis , P. bridgeri , P. multicuspis, and P. robustus , and the combination of features cited above must be used to separate P. bicuspis from all other members of the genus.

We support Guthrie’s (1971) identification of YPM VPPU 13436 from the Lost Cabin Member of the Wind River Formation as Palaeictops bicuspis . The characteristic P4 is clearly present in this specimen.

Bown and Schankler (1982: 16) gave statistical comparisons of tooth measurements for several species of Prodiacodon and Palaeictops . These suggest that the type of Palaeictops (Diacodon) pineyensis ( Gazin, 1952) is closer to P. bicuspis than to Prodiacodon tauricinerei , as suggested by Novacek (1977). The type is badly preserved, and assignment is uncertain. At present, we provisionally accept its transfer to P. bicuspis . Also referable to this species are the Palaeictops pineyensis specimens described by Guthrie (1967) from Lysite Member of the Wind River Formation.

Bown and Schankler (1982: 18) identified AMNH 48763, a ramus with p5–m1, as Palaeictops bicuspis . This assignment is contradicted by the morphology of the specimen. The p5 paraconid is lower, less anteriorly positioned, and has a slightly concave posterior face continuous with the lingually opened prefossid. The m1 has a very distinct entoconulid, a feature shared by Prodiacodon and Myrmecoboides . AMNH 48763 is retained within Prodiacodon tauricinerei , as proposed by Novacek (1977: 26).

The nomenclatural history of Palaeictops bicuspis is reviewed in Novacek (1977).

Figures

FIG. 2. The skull of Palaeictops bicuspis AMNH 4802 (holotype), containing C, P4–P5, M1–M3 on the left side; the right side retains two incisors possibly corresponding to I2 and I3 (both are broken), as well as P2 (broken), P4–P5, M1–M2, and M3 (broken). A, (opposite page) dorsal (top) and ventral (bottom) views and B, (above) right lateral (top) and left lateral (bottom) views. Scale bar = 2 cm.

FIG. 3. Left ramus of Palaeictops bicuspis AMNH 4802 (holotype), containing p5–m3 and roots of p1, p2, and p4, in (clockwise from upper left) medial, occlusal, and lateral views. Scale bar = 2 cm.

FIG. 4. Right ramus of Palaeictops bicuspis AMNH 4802 (holotype), containing p4–m2 and roots of i?, c, p1, and p2, in (clockwise from upper left) lateral, occlusal, and medial views. Scale bar = 2 cm.

FIG. 5. Right maxilla in occlusal view, scaled to approximately the same size for comparison. A, Palaeictops bicuspis AMNH 4802 (P1 [missing], P2 [broken], P4–P5, M1–M2, M3 [broken]); B, Palaeictops altimontis AMNH 96250 (P1–P2, DP4–DP5, M1–M3).