Labidolemur kayi

Silcox, Mary T., Bloch, Jonathan I., Boyer, Doug M. & Houde, Peter, 2010, Cranial anatomy of Paleocene and Eocene Labidolemur kayi (Mammalia: Apatotheria), and the relationships of the Apatemyidae to other mammals, Zoological Journal of the Linnean Society 160 (4), pp. 773-825 : 776-780

LABIDOLEMUR KAYI SIMPSON, 1929

Holotype: CM 11703 , left dentary with p4–m3 from the Eagle Mine locality ( Simpson, 1929), near Bear Creek, Montana (early Clarkforkian North American Land Mammal Age; Gingerich , 1982).

Material examined: The cranial anatomy is described from three new specimens of Labidolemur kayi from the Clarks Fork Basin, Wyoming. These include: (1) USMN 530221, a nearly complete, articulated skeleton with a skull and dentaries from UM locality SC-26 (Houde site 14), Clarks Fork Basin, north-eastern Wyoming, Willwood Formation, early Wasatchian, lowermost Cardiolophus radinskyi interval zone, early Eocene (between 54.92 and 54.70 Mya; Fig. 1 ); (2) UM 41869, a partially prepared, semiarticulated skeleton with skull and dentaries from UM locality SC-327, Clarks Fork Basin, northwestern Wyoming, lower Willwood Formation, late Clarkforkian, Phenacodus –Ectocion acme zone, late Paleocene (between 55.36 and 55.0 Mya; Fig. 2 ); and (3) USNM 530208, an associated rostrum and basicranium from UM locality SC-62 (Block Z), Clarks Fork Basin, north-western Wyoming; lower Willwood Formation, middle Clarkforkian, Uppermost Plesiadapis cookei range zone, late Paleocene (between 55.68 and 55.36 Mya; Fig. 3 ). See Gingerich (2003) for age model.

USNM 530221 and UM 41869 can be confidently attributed to Labidolemur on the basis of the presence of a p3 and a short talonid on m3 ( Gingerich & Rose, 1982). As for L. kayi , the molar teeth in these specimens are larger than in A. chardini and smaller than in Labidolemur serus (Appendix 4), and the lower molars are higher crowned than those of A. chardini (Gingerich, 1982) . The only apatemyid species currently documented from the Clarkforkian is L. kayi ( Gingerich & Rose, 1982) , which makes this a likely identification for USNM 530208. The morphology and size of the teeth in this specimen are similar to those of UM 41869 and USNM 530221, and also to previously published specimens of L. kayi (e.g. UM 73616; Gingerich & Rose, 1982: text and fig. 1).

Occurrence: Late Paleocene (Clarkforkian NALMA) through early Eocene (Wasatchian NALMA).

Discussion: All three new specimens of L. kayi preserve at least portions of the skull. USNM 530208 is the best-preserved cranium, with an almost intact basicranium and a well-preserved rostrum ( Fig. 3 ). The specimen is broken into two pieces at the back of the orbital region, and has experienced some crushing both near the site of the break, and rostrally and ventrally on the left side. USNM 530208 preserves an almost completely intact tympanic roof on the right side of the specimen, with well-demarcated sutures ( Fig. 4 , Table 1). The left basicranium of USNM 530208 has some crushing to the promontorium, causing damage to the rostral pole and adjacent tympanic roof, and a break into the promontorium laterally, revealing part of the cochlea ( Fig. 5 ).

USNM 530221 preserves the entire skull including the dentary ( Fig. 1 ). The right side of the skull is partially caved in, however, and the right mandible has become displaced so that it overlies much of the midline of the specimen. The rostral-most portions of this skull were embedded in epoxy during preparation, and are not fully visible. In the basicranium, portions of both ectotympanics are still in place, as are fragments of the left malleus and incus ( Fig. 6 ). However, the ultra high resolution X-ray computed tomography (uhrCT) data reveal that there is considerable damage to the auditory region in this specimen, including a break running mediolaterally through the promontorium on the left side, and some damage to the tympanic roof on both sides. The sutures between the bones of the tympanic roof are also less clearly demarcated than in USNM 530208.

The skull of UM 41869 is much more fragmentary ( Fig. 2 ). Both right and left promontoria are preserved, although isolated, as are portions of the right and left alisphenoids from the midline of the rostral basicranium (containing the foramen ovale), a fragment of the right squamosal, and both jugals. Elements of the right maxilla and premaxilla are preserved in articulation with the right mandible, which is almost complete. However, the midline region of the rostrum is crushed and displaced ventrally, and the left side of the rostrum is very fragmentary. The left mandible preserves fragments of p4 and the root of i1, all three molars, and is fairly complete distally. The description of the cranium will acf anterior carotid foramen as alisphenoid

bp basisphenoid tympanic process

bs basisphoid cat canal for the auditory tube

cp caudal tympanic process of the petrosal den dentary ec ectotympanic er epitympanic recess ex exoccipital

fcf fenestra cochleae

ff facial foramen fi fossa incudis fo foramen ovale

fr frontal

fv fenestra vestibuli

gf glenoid fossa

hf hypoglossal foramen hu humerus hy hyoid icc groove for internal carotid artery (stem)

if incisive foramen inc incus ju jugal ma mastoid

ml malleus map mandibular angular process

mc mandibular condyle

mx maxilla na nasal

nf nutrient foramen oc occipital occ occipital condyle pa parietal

pl palatine

pgf postglenoid foramen

pgp postglenoid process

plf posterior lacerate foramen

pmx premaxilla

pr promontorium

prc groove for promontorial artery

ptc opening to the pterygoid canal (= vidian foramen) ra radius ri canal for the ramus inferior of the stapedial artery

rp rostral tympanic process of the petrosal

sf stapedius fossa

sq squamosal

sqf subsquamosal foramen

stc groove for stapedial artery

sw epitympanic wing of the sphenoid

th tympanohyal ul ulna be based primarily on USNM 530208, augmented by observations on USNM 530221 and UM 41869.

Two specimens ( USNM 530221 and 530208) were scanned with the OMNI-X Industrial Scanner at the Center for Quantitative Imaging ( CQI), Pennsylvania State University, which produced uhrCT data. For USNM 530221, only the skull (and bones in its vicinity) was scanned. Only the caudal portion of USNM 530208 was scanned (i.e. the portion caudal to the break in the specimen). These specimens were immobilized in floral foam (i.e. ‘oasis’) and scanned in volume mode, in which 21 individual two-dimensional slices were created for each rotation. The axial fan angle was small enough to assume parallel beam reconstructions. Each rotation consisted of 2400 views of the object spanning 360°. The post-acquisition reconstruction process included all 2400 views, and each individual slice was stored as a 1024 ¥ 1024 matrix of 16-bit integers in tiff format. For USNM 530221 the reconstructed pixel size was 0.053 mm and the interslice distance was 0.058 mm; the data set included 697 images. For USNM 530208, the reconstructed pixel size was 0.033 mm, the interslice distance was 0.036 mm, and the data set consisted of 361 images.

Images were studied using Scion Image Beta 4.02 ( Scion Corporation, 2002) and ImageJ 1.27w ( Rasband, 2002). Parts of the data set were cropped and stacked using cropvoi and strip2raw, which are DOS programs developed by Nathan Jeffrey (University of Liverpool). Reslicing of the data in arbitrary planes, 2D image linking, and 3D reconstructions were performed using Voxblast for Unix (Vaytek, Inc.) on an SGI Octane 2 workstation. Anatomical terminology for the basicranium follows MacPhee (1981) and MacPhee, Novacek & Storch (1988). Terminology used to describe the rest of the skull follows Novacek (1986), unless otherwise noted.

DESCRIPTION

Figures

Figure 1. Photographs of Labidolemur kayi, USNM 530221. Skull in (A) dorsal, (B) left, (C) right, and (D) right view tilted ventrally. As this skull is part of a semiarticulated skeleton, it is not possible to take a photograph from a strictly ventral perspective. Scale bar: 5 mm.

Figure 2. Photographs of Labidolemur kayi, UM 41869. (A) right maxilla and dentary in lateral view. (B) left zygomatic in medial (internal) view. (C) fragment of right and left alisphenoids in ventral view. (D) fragment of the right petrosal in ventral view. Scale bar: 5 mm.

Figure 3. Photographs of Labidolemur kayi, USNM 530208. Cranium in (A) rostral, (B) caudal, (C) ventral, (D) dorsal, (E) left, and (F) right. Scale bar: 5 mm.

Figure 4. Right auditory region of USNM 530208. The specimen was tilted laterally from a strictly ventral view to facilitate viewing the medial portion of the tympanic cavity: (A) labelled; (B) unlabelled. In (A) the dotted portions of the paths for the alisphenoid canal and route for the ramus inferior of the stapedial artery (ri) indicate passage through a bony canal. There are grooves rather than bony canals for the branches of the internal carotid artery as they cross the promontorium (i.e., prc, icc, and stc). See Table 1 for abbreviations. Scale bar: 1 mm.

Figure 5. Ventral view of the left auditory region of USNM 530221: (A) labelled; (B) unlabelled. See Table 1 for abbreviations. Scale bar: 1 mm.

Figure 6. The auditory region of USNM 530221: (A) ventrolateral view of left ear; (B) ventral view of right ear and adjacent structures. In (B) the fine splint of bone of the anterior crus of the ectotympanic is outlined. See Table 1 for abbreviations. Scale bar: 5 mm.