Mindomys, WEKSLER & PERCEQUILLO & VOSS, 2006

WEKSLER, MARCELO, PERCEQUILLO, ALEXANDRE REIS & VOSS, ROBERT S., 2006, Ten New Genera of Oryzomyine Rodents (Cricetidae: Sigmodontinae), American Museum Novitates 3537 (1), pp. 1-29 : 16-18

Mindomys , new genus

TYPE SPECIES: Nectomys hammondi Thomas, 1913 .

CONTENTS: hammondi Thomas, 1913 .

DISTRIBUTION: Currently known from just nine specimens, eight of which were collected in the vicinity of Mindo in the western Andean foothills of Pichincha province, Ecuador. Another specimen, labeled as having been collected in the eastern (Amazonian) lowlands of Ecuador, represents a remarkable and somewhat problematic range disjunction. In consequence, the geographic distribution of this taxon is difficult to interpret. 5

MORPHOLOGICAL DIAGNOSIS: Dorsal pelage coarsely grizzled yellowish-brown; ventral pelage abruptly paler (whitish or yellowish superficially) in most specimens, but ventral hairs always gray-based. Pinnae small, not reaching eye when laid forward. Mystacial and superciliary vibrissae very long, extending posteriorly well beyond caudal margins of pinnae when laid back. Pes with sparse tufts of rather short ungual hairs at bases of claws on dII–dV; plantar surface sparsely covered with indistinct squamae distal to thenar pad; hypothenar pad present and distinct; claw of dI extending to or just beyond first interphalangeal joint of dII; claw of dV extending to middle of phalange 2. Tail unicolored (all dark), and much longer than combined length of head and body.

Skull with long, stout rostrum flanked by very shallow zygomatic notches; interorbital region anteriorly convergent, with strongly beaded supraorbital margins; braincase elongate, with well-developed temporal, lambdoidal, and nuchal crests developed in older adults. Posterior margin of zygomatic plate dorsal to M1 alveolus; jugal present and large (the maxillary and squamosal zygomatic processes widely separated, usually non-overlapping in lateral view). Nasals not extending posteriorly beyond lacrimal bones; lacrimals equally sutured to maxillary and frontal bones. Frontosquamosal suture anterior to

5 The eight specimens of ‘‘ Oryzomys ’’ hammondi from Mindo (0 ° 02 9 S, 78 ° 48 9 W, 1264 m above sea level; Paynter, 1993) were taken by three different collectors—G. Hammond, L. Söderström, and R.S. Voss—from 1913 to 1980; seven of these specimens are at the BMNH and the eighth is at the UMMZ. The single Amazonian record is based on MCZ 52543, an adult female whose skin label states that it was collected by the Olallas (a family of professional collectors) on 27 July 1929 at ‘‘Concepción, Oriente, Ecuador’’. According to Paynter (1993), Concepción is at 0 ° 48 9 S, 77 ° 25 9 W, about 50 km NE of Tena between 300 and 500 m above sea level in Napo province. Several aspects of these distributional data are problematic. First, we are not aware of any other species of small nonvolant native mammal that occurs below about 1500 m on both sides of the Ecuadorean Andes. Second, the lowlands and foothills around Tena have been intensively worked over for many years by numerous collectors, none of whom have taken additional material of this species. Thus, there is some reason to question whether Mindomys really occurs in Amazonia. Additionally, it is not known if Mindo represents the upper or the lower limit of the elevational distribution of this taxon in western Ecuador, or if Mindo is somewhere in the middle of its elevational range. If Mindomys is predominantly a lowland taxon, then it might more appropriately be classified as a trans-Andean rather than as an Andean clade (sensu Weksler, 2006). Only future fieldwork can resolve such uncertainties, which are obviously relevant to reconstructing oryzomyine historical biogeography.

frontoparietal suture (dorsal facet of frontal in broad contact with squamosal). Parietals with broad lateral expansions. Incisive foramina short, not extending posteriorly to level of M1 alveoli, usually widest posteriorly and converging anteriorly (teardrop-shaped). Posterolateral palatal pits small and unrecessed in some specimens but larger and recessed in moderately deep fossae in others; mesopterygoid fossa extending anteriorly between maxillae but not between molar rows; bony roof of mesopterygoid fossa usually completely ossified (some specimens have very narrow sphenopalatine vacuities flanking the presphenoid or the presphenoid/basisphenoid suture). Alisphenoid strut absent (buccinator– masticatory foramen and accessory oval foramen confluent). Stapedial foramen, squamosal–alisphenoid groove, and sphenofrontal foramen present (5 carotid circulatory pattern 1 of Voss, 1988). Postglenoid foramen small and dorsoventrally compressed; subsquamosal fenestra vestigial (not patent) or absent. Periotic broadly exposed posteromedially between ectotympanic and basioccipital, extending anteriorly to carotid canal; mastoid completely ossified, not fenestrated. Distinct capsular process of lower incisor alveolus absent; superior and inferior masseteric ridges converge anteriorly as open chevron below m1.

Labial and lingual flexi of M1 and M2 interpenetrating. First upper molar (M1) anterocone not divided into labial and lingual conules (anteromedian flexus absent); anteroloph well developed, fused with anterostyle on labial cingulum, and separated from anterocone by persistent anteroflexus; protostyle absent; paracone connected by enamel bridge to posterior moiety of protocone. Second upper molar (M2) protoflexus absent; mesoflexus present as two internal fossettes. Third upper molar (M3) with posteroloph and persistent hypoflexus. Labial accessory root of M1 absent.

Lower first molar (m1) anteroconid without an anteromedian flexid; anterolabial cingulum present on m1 and m2, occasionally absent on m3; anterolophid present on m1 but absent on m2 and m3; ectolophid present on m1 but not on m2 and m3; mesolophid well developed on all lower molars; m2 hypoflexid short. Accessory roots absent on m1; m2 and m3 each with one large anterior root and one large posterior root.

Postcranial skeletal characters unknown.

Stomach without extension of glandular epithelium into corpus. Male reproductive tracts not examined.

COMPARISONS: Weksler (2006) recovered ‘‘ Oryzomys ’’ hammondi either as the most basal oryzomyine lineage (as in fig. 1 ) or as a member of clade B. Within clade B, ‘‘ O. ’’ hammondi was sometimes recovered as the sister taxon to Oecomys , but its relationships were unresolved in other analytic permutations. Because the phylogenetic position of ‘‘ O. ’’ hammondi was not strongly supported in any analysis, no compelling evidence exists for the membership of this taxon in any monophyletic group less inclusive than the tribe Oryzomyini . The following comparisons are therefore motivated in part by historical concepts of taxonomic affinity.

Among other characters, Mindomys differs from both Nectomys (the genus to which hammondi was originally referred by Thomas [1913]) and Oryzomys (the genus to which hammondi was transferred by Hershkovitz [1948]) by its much longer vibrissae (the vibrissae do not extend posteriorly behind the pinnae in Nectomys or Oryzomys ); possession of a distinct hypothenar pad on the hind foot (the hypothenar is absent or vestigial in Nectomys and Oryzomys ); unwebbed pedal digits ( Nectomys and Oryzomys have partially webbed hindfeet); very long fifth digit (the claw of dV does not extend beyond the first interphalangeal joint in Nectomys or Oryzomys ); much shallower zygomatic notches; much larger jugals; small, simple, unrecessed posterolateral palatal pits ( Nectomys and Oryzomys have large, complex posterolateral palatal pits that are deeply recessed in conspicuous fossae); complete stapedial circulation (the stapedial circulation is absent in Nectomys and Oryzomys ); absence of accessory roots on M1 and m1 (upper and lower first molars have accessory roots in Nectomys and Oryzomys ); and possession of an ectolophid on m1 ( Nectomys and Oryzomys lack ectolophids). Given the large number of additional characters by which Mindomys differs individually from Nectomys (represent- ed by N. squamipes in Weksler [2006: table 5]) and Oryzomys (represented by O. couesi and O. palustris ), the morphological distinctness of these taxa is not arguable.

Several authors ( Ray, 1962; Hershkovitz, 1970; Steadman and Ray, 1982) have suggested a close relationship between ‘‘ Oryzomys ’’ hammondi and extinct Antillean giant rats of the genus Megalomys . We have not examined material of Megalomys , but Musser and Carleton’s (2005) statements that its metatarsal pads are vestigial, that accessory roots are present on M1 and m1, and that it has a derived carotid circulation suggests that the genus is a member of clade D and not, in fact, a close relative of Mindomys .

Mindomys appears to differ consistently from Oecomys (a speciose genus that exhibits taxonomic variation in many characters) by its much smaller interdigital pads on the hind foot (the interdigital pads are very large in Oecomys ); indistinct plantar squamae (the sole of the hind foot is entirely smooth in Oecomys ); sparse tufts of short ungual hairs on pedal digits II–V (ungual tufts are denser and longer in Oecomys ); much longer rostrum (all species of Oecomys have short rostrums); frontosquamosal suture anterior to the frontoparietal suture (the two sutures are more nearly colinear in Oecomys ); shorter palate (the mesopterygoid fossa does not extend anteriorly between the maxillae in Oecomys ); small, simple, unrecessed posterolateral palatal pits ( Oecomys has large, often complex posterolateral palatal pits that are usually deeply recessed in conspicuous fossae); more lophodont upper molars (the labial and lingual flexi do not interpenetrate deeply on the upper molars of Oecomys ); suppression of the protoflexus on M2 (the protoflexus is distinct on unworn M2s in Oecomys ); absence of a paralophule on M2 (a distinct paralophule is present on the M2 of all examined species of Oecomys ); and absence of an anterolabial cingulum from m3 (the anterolabial cingulum is distinct on the unworn m3 of Oecomys ).

REMARKS: Hershkovitz (1948) designated hammondi as the type species of Macruroryzomys , but the latter was not diagnosed and the name is therefore unavailable ( Pine and Wetzel, 1975). Although Hershkovitz (1970) acknowledged this situation, he effectively did nothing to correct it, so Macruroryzomys remains a nomen nudum.

The possibly basal position of this extraordinary rat within the oryzomyine radiation, together with its enigmatic distribution and the absence of preserved tissues suitable for DNA extraction and sequencing, will hopefully impel future collectors to make special efforts to obtain more material.

ETYMOLOGY: For Mindo, a tiny agricultural community in the western Andean foothills of Pichincha province, Ecuador.

Figures

Fig. 1. Phylogenetic relationships of oryzomyines based on a heuristic parsimony analysis of sequence data from the Interphotoreceptor Retinoid Binding Protein (IRBP, 1266 bp from exon 1) and 99 morphological characters (after Weksler, 2006: fig. 37). Numbers above and below branches represent jackknife support (.50%) and decay indices (.1), respectively. Vertical bars on the right-hand side of the figure indicate taxon membership in clades A–D. See Weksler (2006: 14–17) for methodological details.