Prosciurillus Ellerman, 1947

Musser, Guy G., Durden, Lance A., Holden, Mary Ellen & Light, Jessica E., 2010, Systematic Review of Endemic Sulawesi Squirrels (Rodentia, Sciuridae), with Descriptions of New Species of Associated Sucking Lice (Insecta, Anoplura), and Phylogenetic and Zoogeographic Assessments of Sciurid Lice, Bulletin of the American Museum of Natural History 2010 (339), pp. 1-260 : 1-260

Prosciurillus Ellerman, 1947

The generic name Prosciurillus was generated after Ellerman had measured the orbit of hundreds of European and Asiatic squirrel skulls and discovered that species of the Indomalayan pygmy squirrels in Nannosciurus , the Sulawesian ground squirrel Hyosciurus , and Sulawesian small-bodied tree squirrel murinus all had a short orbit compared to all other squirrel taxa examined, less than one-fourth of the occipitonasal length. He ( Ellerman, 1947: 259) proposed ‘‘ Prosciurillus as a new genus for Sciurus murinus ’’ and compared it to the South American pygmy squirrel Sciurillus , with which it has no close phylogenetic alliance as would be documented decades into the future ( Moore, 1959; Mercer and Roth, 2003). Eleven years later, Moore (1958) redefined Prosciurillus , describing several anatomical traits associated with the skull that he thought to be diagnostic in addition to the relatively short orbit, and expanded the contents of the genus to include the Sulawesian leucomus (and presumably all the taxa associated with it, although not explicitly stated by Moore; see table 11).

Moore’s (1959: 176) expanded report of relationships among living squirrels, which contained the results of his survey of cranial characters in the Sciurinae , brought together Prosciurillus with the Sulawesian Rubrisciurus and Hyosciurus , as well as the Sundaic-Philippine Exilisciurus in the subtribe Hyosciurina of the tribe Callosciurini . Except for Exilisciurus , Moore’s hypothesis of phylogenetic relationships for Prosciurillus has been unambiguously supported by analyses of the nuclear IRBP and mitochondrial 12S and 16S ribosomal DNA used by Mercer and Roth (2003) to recover a squirrel phylogeny that included a monophyletic group containing Rubrisciurus , Prosciurillus , and Hyosciurus separate from a cladistic cluster formed by Callosciurus , Sundasciurus , and their relatives. Within the Sulawesian clade, the species of tree squirrels in Prosciurillus are more closely related to Rubrisciurus than to the morphologically divergent ground squirrels in Hyosciurus . The phylogram of relationships generated by Mercer and Roth excludes the species of pygmy squirrels in Exilisciurus from the Sulawesian clade (see introductory discussion in the account of Rubrisciurus ).

The short orbit (indicated by the lacrimal and posterior margin of the zygomatic plate situated even with the second upper molar) and lack of transbullar septa in more than 50% of large samples (see Moore, 1959: 163) are the strongest cranial traits distinguishing Prosciurillus from most genera of Nannosciurinae . But among the Sulawesi endemics, a short orbit is also common to Rubrisciurus and Hyosciurus , as is either the complete absence or low frequency of occurrence of transbullar septa. All three genera also possess third premolars and exhibit parallel upper tooth rows. Otherwise, Prosciurillus is easily distinguished from Rubrisciurus and Hyosciurus by the following combination of traits (see comparative measurements summarized in table 3, color plates of live poses in figs. 9 and 10, and compare skulls illustrated in figs. 6, 12–14, 36, and 37): (1) short face and much smaller in body size (table 2), the two smaller-bodied species often referred to as pygmy tree squirrels (but still much larger than the species in Nannosciurus and Exilisciurus , the true pygmy or dwarf tree squirrels); (2) upperpart dark brown or chestnut brown and without pattern, or lacking pattern except for black ear tufts, or possessing black ear tufts with nape patches behind ears, or exhibiting black ear tufts and prominent black middorsal stripe from neck to base of tail (no ear tufts, nape patches, or dorsal stripe in Hyosciurus ; reddish orange fore-legs and feet, shoulders, thighs, and hind legs and feet in Rubrisciurus combined with black ear tufts); (3) underparts reddish orange to ochraceous, brownish gray, or dark gray washed with pale buff or silver (white or cream in Hyosciurus , reddish orange in Rubrisciurus ; (4) hind foot short and broad, the typical configuration in tree squirrels, claws short and strongly recurved (hind foot long and slender in Hyosciurus , claws very long and only slightly curved); (5) tail slightly shorter than length of head and body (half the length of head and body in Hyosciurus ; table 3); (6) three pairs of teats, one postaxillary and two inguinal (two inguinal pairs in Rubrisciurus ; one abdominal pair and two inguinal pairs in Hyosciurus ); (7) skull small and gracile with marked cranial flexion (robust, with strikingly reduced cranial flexion in Rubrisciurus and Hyosciurus ); (8) rostrum short and wide, nasals and premaxillaries not forming a tube projecting beyond faces of upper incisors (rostrum moderately long in Rubrisciurus , very long in Hyosciurus where nasals and premaxillaries form a tube projecting anterior to faces of incisors); (9) nasals much shorter than length of frontals (53%–66% of length of frontals) and shorter than width of interorbital region (nasals as long as frontals or only slightly shorter, and nasals longer than width of the interorbital region in Rubrisciurus and Hyosciurus ); (10) temporal ridges barely evident and reach occiput without coalescing to form a sagittal crest (stronger temporal ridges that combine to form sagittal crests in Rubrisciurus and Hyosciurus ); (11) postorbital processes of the frontals about even with anterior wall of the braincase (well anterior to the braincase in Rubrisciurus ); (12) anterior opening of the infraorbital canal lies slightly posterior to the premaxillary-maxillary suture and is concealed behind bony flange slightly or markedly projecting from ventral root of the zygoma, the anteroventral corner of the flange slightly thickened and marked on outer surface by a barely perceptible roughened elliptical area for insertion of the superficial masseter; (13) posterior border of bony palate located anterior to backs of the tooth rows, at about middle of each third upper molar (even with end of tooth rows or slightly caudad of tooth rows in Hyosciurus ); (14) descending palatine vein transmitted through a notch (posterior maxillary notch; a foramen instead of a notch occurs in about 5% of all total skulls examined) at the posterolateral margin of bony palate just caudad and slightly medial to end of tooth row (vein is enclosed by bone forming a foramen in Hyosciurus ); (15) pterygoid fossa narrow and deep, its lateral margin forming a high ridge— Moore’s (1959) ‘‘ectopterygoid ridge’’(lateral margin outlined by a low, inconspicuous ridge in Hyosciurus ); (16) upper incisors proodont (procumbent), projecting forward (orthodont, oriented at right angle to rostrum in Hyosciurus ); (17) upper third molar roughly triangular in occlusal view, about as long as it is wide (oblong in Rubrisciurus , much shorter than wide). Traits shared with Rubrisciurus and Hyosciurus are summarized in the diagnoses for those genera.

We provide accounts for the seven species of Prosciurillus , which are divided into two groups based on contrasts in coat color and patterning combined with physical body size. Squirrels of medium build (length of head and body 5 155–195 mm) exhibiting a combination of dorsal and ventral pelage coloration along with patterns associated with ear tufts, nape patches, and middorsal back striping comprise the P. leucomus group ( P. leucomus , P. alstoni , P. weberi , P. topapuensis , and P. rosenbergii ). Small-size squirrels (length of head and body 5 102– 150 mm) with dark brown upperparts,

TABLE 11 Allocation of Scientific Names Associated with Species in the Prosciurillus leucomus Group as Presented in Primary Checklists of Mammals from 1940 to the Present The combination of generic and specific names as originally proposed is listed in their published chronological order. See the accounts of species for names of authors and dates of publication.

slightly contrasting underparts, and generally without ear, nape or back displays (except for white fur on backs of the ears in P. abstrusus ) form the P. murinus group ( P. murinus and P. abstrusus ).