Lamottemys okuensis Petter, 1986

Denys, Christiane, Missoup, Alain Didier, Nicolas, Violaine, Fülling, Olaf, Delapré, Arnaud, Bilong, Charles Felix Bilong, Taylor, Peter John J. & Hutterer, Rainer, 2014, African highlands as mammal diversity hotspots: new records of Lamottemys okuensis Petter, 1986 (Rodentia: Muridae) and other endemic rodents from Mt Oku, Cameroon, Zoosystema 36 (3), pp. 647-690 : 653-660

Lamottemys okuensis Petter, 1986

Lamottemys okuensis Petter, 1986: 98 .

TYPE LOCALITY. — Cameroon, Mt Oku.

MATERIAL EXAMINED. — MNHN: 1984-493 (holotype); 1984-494, 2011-946 to 2011-966, 2013-49 to 2013-80. ZFMK: 69.171, 69. 183, 91.230 to 91.233 2003-891 to 899.

ZFMK (Lake Oku Cave owl pellet material): 91.116 to 91.129.

Morphological characters

This genus endemic of Mt Oku was known initially from two type specimens described by Petter and two supplementary broken specimens from Lake Oku initially attributed to Hybomys eisentrauti Van der Straeten & Hutterer, 1986 by Eisentraut and redescribed as Lamottemys by Dieterlen & Van der Straeten (1988). We collected 53 new specimens, Fülling collected 13 specimens and owl pellets yielded 13 skull fragments. This allowed us to describe for the first time, morphological and natural history parameters of this unique murine genus.We revise the genus diagnosis and compare it with the three closely related genera Hybomys , Desmomys Thomas, 1910 and Oenomys Thomas, 1904 . None of the previous studies compared the endemic genus with Hybomys . Preliminary molecular investigations confirm Lamottemys distinct generic status ( Missoup 2010).

Lamottemys okuensis is similar externally with a uniform brown fur and no dorsal stripe as Hybomys eisentrauti also recorded on Mt Oku. Moreover, the two species fit in the category of medium sized rats ( Table 2). Both have a tail length, which is slightly longer than the head and body length ( Lamottemys average:105%, Hybomys average:111%). According to Dieterlen & Van der Straeten (1988), Lamottemys and Desmomys have a nail on Digit 5 on the hand, while Oenomys and Hybomys have a claw.

The three newly collected pregnant females carried only one embryo each. The mammary formula is 0 + 2, as mentioned in Dieterlen & Van der Straeten (1988), but differing from that of Hybomys eisentrauti (1 + 2). According to Fülling (1992), the formula of the palatal ridges of L. okuensis is 2 + 6 = 8, which differs from that of Oenomys hypoxanthus albiventris Eisentraut, 1968 (2 + 5 = 7).

The skull of Lamottemys is massively built with a broad enlarged anterior part of the nasal and a wide, long interorbital constriction with marked frontal crests continuing onto the parietal bone in older individuals ( Fig. 2A ). The incisors are orange with traces of very shallow striation not always visible. The long nasal, ends before the incisors and is convex in lateral view ( Fig. 2C ). In the same view, the zygomatic process is narrow; its anterior border makes first an oblique and then a vertical crest. The zygomatic arch is inflated in the middle in the maxillary-jugal suture region. The optic foramen is large. The postglenoid fossa has a relatively large ovoid shape. Lamottemys okuensis has a long tympanic hook, which is oblique and directed toward the front of the skull. The mastoid process is large and slightly inflated. In the ventral view, the incisive foramen ends before the first root of the upper M1. The tooth rows are wide and parallel, and the narrow palate displays a small anterior palatal foramen ( Fig. 2B ). The tympanic bullae are not inflated but the periotic bones are well developed and the carotid canal is wide and well visible. The oval foramen is large. The middle lacerate foramen is small. On the mandible, the shape of the coronoid process is short with a curve aspect between coronoid and condyloid crests ( Fig. 2D ). The coronoid crest is slightly higher or at the same level with the condyloid crest. The mental foramen is situated in front of the massetric crest and the dental crest is poorly marked.

Compared to Hybomys , the skull of Lamottemys is larger and displays a longer interorbital constriction and a more developed rostrum. Hybomys retains an amphora-shaped skull with a more globular braincase and less developed fronto-parietal crests. In ventral view, the very large size of Lamottemys molars constitutes an immediate diagnostic character for the genus. A comparison with Desmomys har- ringtoni (Thomas, 1902) from Ethiopia, Hybomys eisentrauti and Oenomys hypoxanthus albiventris CVL specimens using canonical analyses shows that the representatives of the four genera are well discriminated along axes 1 and 2 ( Fig. 3 ). Hybomys is smaller and proportionally has a wider interorbital constriction compared to others genera, all having a larger skull than Hybomys . The plot of the two first components of the PCA shows that Oenomys can be distinguished from Lamottemys and Desmomys by its broader nasal and longer incisor foramen. Althought we have a relative small sample of Desmomys , our morphometric data seems to suggest a very close relationship with Lamottemys . All individuals of Oenomys were well classified, except one, which was a posteriori classified among Lamottemys .

A striking feature of Lamottemys are the molar teeth, which are very large and broad, and can be described as macrodont (ratio of upper teeth row length/greatest skull length:0.185 to 0.216; Table 3). In this aspect, Lamottemys resembles D. harringtoni and can be easily distinguished from H. eisentrauti . There is some overlap with Oenomys hypoxanthus albiventris .

The molars were well illustrated and described in Petter’s (1986) original description. We provide here SEM pictures for the paratype of Lamottemys ( Fig. 4A, E ).

According to Petter (1986), the upper incisors are opisthodont and display no or only a tiny groove. The lower incisors have a shallow groove that was figured by Petter (1986) but it is not found in all the new individuals collected. Cusps are well separated from each other and the third upper molar (M3/), although not very reduced, has a smaller size than the M2/. As evidenced by Petter (1986), stephanodont crests are well developed on all cusps; there is no t7, and the number of cingular cusplets is relatively low. The posterior cingulum is small both on the upper and lower M/1. There is a median antero cusp (tma) on the first lower molar related by an oblique crest at the junction of tE and tF on the lower M/1.

Stephanodonty, is characterized by the development of longitudinal crests at the back of the cusps and relating the internal and external rows of cusps. It is well developed and visible in all the wear age classes identified. Indeed, following an earlier work on Aethomys Thomas, 1915 ( Denys &Tranier 1992) we classified the new Lamottemys specimens into six molar wear stages. The description of the different molar wear stages can be made as follow:

– stage 0 (unerupted M3/3) and stage 1 (very fresh molars) are not represented in our sample (not figured);

– stage 2 ( Fig. 5A, E ): specimens display high cusps with stephanodont crest visible, but the in- ternal and external row of cusps are not connected longitudinally by crests. On the lower molars the cusps are well separated and the posterior cingulum is small;

– stage 3 ( Fig. 5B, F ): the longitudinal link occurs between t3-t5 and t9. There is a very small posterior cingulum on the upper M1 and M2. The wear facets are broad and the cusps are lower on the molar rows;

– stage 4 ( Fig. 5C, G ): the longitudinal links are wider between external cusps. Cusps show lower and broader wear facets, but they are still wellindividually defined;

– stage 5 ( Fig. 5D, H ): besides the external link, a longitudinal crest occurs between t1-t4 and t8. Cusps are low, very wide, linked longitudinally on the lower M1 and poorly individualized;

– stage 6 (not figured): Cusps are very low and unrecognizable.

As suggested by Dieterlen & Van der Straeten (1988), the molar pattern of Lamottemys seems to be very similar to that of Oenomys , especially due to the development of stephanodont crests ( Fig. 4B, F ). However, Oenomys molars are larger ( Table 3) and their proportions relative to the total length of the skull are smaller. Three main differences allow us to distinguish between the two genera. First, the stephanodont crest of the t3 on upper M1 never reaches the t 5 in Oenomys , while it is always fused with the t 5 in Lamottemys . Second, the cusps of the second lobe of the upper M3 are never fused or only at very advanced wear stages with the ones of the first lobe in Lamottemys , while in Oenomys there is always a link between the t5 and the t9. Third, on the lower M1, the main cusps are more alternated in Oenomys than in Lamottemys . Compared to Hybomys , Lamottemys displays a more median cusp and a larger labial cingular margin, as well as a more developed stephanodonty. Desmomys does not display such pronounced stephanodonty and cusps are more transversally aligned than in Lamot- temys ( Fig. 4C, G ). Both have macrodont molars. H. (Typomys) trivirgatus (Temminck, 1853) possesses some small stephanodont crests on the t3 and t1 of the first upper molar, and has alternate cusps ( Fig. 4D, H ). It has shorter sized molars rows and more transversally fused cusps. Hybomys univittatus (Peters, 1876) molars display a very transverse first lobe of the M1/ and trace of a stephanodont crest on the t4 and t6, as well as a wide M3/ with well-fused cusps. The first lower molar displays a stephanodont crest and an antero-median cusp (tma) on the prelobe. The molars of H. eisentrauti are not macrodont (ratio UTR /LGT: 0.166) compared to those of Lamottemys (0.199) and Desmomys (0,225)( Table 3).

In conclusion we can summarize here some diagnostic characters for Lamottemys okuensis : medium sized rat with uniform brown-yellow dorsal and ventral coloration (no stripe, no red nose), with a tail equivalent to HB size. Females have two pairs of pectoral mammae. There are 2 + 6 palatal ridges. The skull displays a wide and long rostrum, a long interorbital constriction with fronto-parietal crests well marked on old adults, large stephanodont molars, macrodonty, upper incisors slightly grooved and opisthodont ( Table 4). The stephanodont character, as well as molar cusps disposition has allowed Ducroz et al. (2001) to postulate an affiliation between Lamottemys and the tribe Arvicanthini . Further molecular and morphological phylogenetic analyses are ongoing to answer the question of the affinity of this endemic genus.

Our new collections confirm that L. okuensis lives in mountain forest between 2100 and 2900 m ( Petter 1986; Dieterlen & Van der Straeten 1988). It is a terrestrial species and its stomach contents analyzed by one of us ( OF) revealed exclusively green plant matter, suggesting an entirely herbivorous diet.

Figures

FIG. 2. — Lamottemys okuensis Petter, 1986 Holotype, paratype specimen (MNHN 1984-493, 1984-494): A, dorsal view of the skull; B, ventral view; C, lateral view; D, detail of the mandible. Scale bar: 1 mm.

FIG. 3. — Canonical analysis based upon 12 skull distances including Hybomys eisentrauti Van der Straeten & Hutterer,1986, H. badius Osgood, 1936, H. univittatus (Peters, 1876), Desmomys harringtoni (Thomas, 1902) (Ethiopia), Lamottemys okuensis Petter, 1986, Oenomys hypoxanthus albiventris Eisentraut, 1968 from CVL and immediate surroundings. Symbols: ®, Hybomys Thomas, 1910; •, Oenomys Thomas, 1904; p, Lamottemys Petter, 1986; ¢, Desmomys Thomas, 1910.

FIG. 4. — SEM pictures of upper and lower molar rows: A-D, top row; E-H, bottom row; A, E, Lamottemys okuensis Petter, 1986 CG MNHN1984-494 paratype; B, F, Oenomys hypoxanthus albiventris Eisentraut, 1968, Ethiopia CG1981-538; C, G, Desmomys harringtoni (Thomas, 1902), Ethiopia BMNH 8.6.88; D, H, Hybomys univittatus CAR CG 1966-97. Scale bar: 1 mm.

FIG. 5. — Wear stages for upper and lower molars of different Lamottemys okuensis Petter,1986 specimens:A-D, top row;E-H, bottom row;A, E, CG 2011-961 stage 2; B, F, CG 2011-956 stage 3; C, G, CG 2011-947 stage 4; D, H, ZFMK91.233 stage 5. Scale bar:1 mm.