Neacomys xingu, Semedo & Silva & Gutiérrez & Ferreira & Nunes & Mendes-Oliveira & Farias & Rossi, 2020

Semedo, Thiago Borges Fernandes, Silva, Maria Nazareth Ferreira Da, Gutiérrez, Eliécer E., Ferreira, Daniela Cristina, Nunes, Mario Da Silva, Mendes-Oliveira, Ana Cristina, Farias, Izeni Pires & Rossi, Rogério Vieira, 2020, Systematics of Neotropical Spiny Mice, Genus Neacomys Thomas, 1900 (Rodentia: Cricetidae), from Southeastern Amazonia, with Descriptions of Three New Species, American Museum Novitates 2020 (3958), pp. 1-43 : 30-33

Neacomys xingu , new species

Xingu Spiny Mouse Figures 11 , 12

HOLOTYPE: The holotype ( UFMT 1268 ) is an adult female (age class 5), collected on 28 August 2009 by Cleuton Lima Miranda (original field number PSA 242 ) in a pitfall trap. The specimen is preserved as a skin, skull, and skeleton in good condition; additionally, a tissue is preserved in alcohol, and a partial cytochrome b sequence that we obtained from it has been deposited in Genbank with accession number MT 462060 .

TYPE LOCALITY: Flona Tapirapé-Aquiri , Marabá, state of Pará, Brazil (5°46′S, 50°32′W, fig. 5) .

DIAGNOSIS: Neacomys xingu is a small species (table 3) that differs from congeneric taxa by the following combination of craniodental traits: skull delicate; interorbital region narrow; nasal bones expanded anteriorly; supraorbital margins convergent anteriorly; subsquamosal fenestra small (about ¼ the size of the postglenoid foramen on each side of the skull); paraoccipital process separated from the auditory bullae; sphenopalatine foramen small; carotid circulation usually pattern 1 (sensu Voss, 1988); maxillary part of incisive septum (between the incisive foramina) wide; M1 usually with flat and undivided anterocone; and M1 anteroloph fused with the anterolabial conule.

MORPHOLOGICAL DESCRIPTION: Dorsal pelage orange-brown, sprinkled with black (fig. 11); ventral pelage varying from pure white to buffy white and separated from the dorsal pelage by a thin orange lateral line. Superciliary, genal, and mystacial vibrissae blackish and long (extending behind ears when laid back alongside the head); submental vibrissae absent; interramal vibrissae short and white; Ears small and rounded; postauricular hairs gray based with orange tips, forming an orange tuft behind each pinna. Ungual tufts white, longer than claws in length; fore- and hind feet covered dorsally with buffy-cream hairs; hind feet narrow and elongate with small interdigital membranes present. Tail about the same length as head and body, usually unicolored (except in PSA 069, MPEG 39901, and MPEG 42019, in which the tail is dark above and paler below), and covered by short, spiny, and clearly visible hairs; tail tip with very short (1–2 mm) terminal tuft; caudal scales small, arranged in annular series; each caudal scale with three subequal hairs inserted along its posterior margin.

Skull small and delicate in dorsal view (fig. 12); with anteriorly expanded nasal margins; notably broad rostrum and shallow zygomatic notches; posterior nasal terminus slightly pointed, extending beyond the maxillary-frontal suture; premaxillaries terminating slightly anterior to nasals; lacrimal bone small and visible in dorsal view, equally contacting the maxillary and frontal bones; supraorbital margins convergent anteriorly; interorbital region narrow; supraorbital beads developed as projecting shelves; lateral expansion of the parietal restricted to the dorsal cranial surface (except in MCN-M 1404 and MPEG 39901, in which the parietal is slightly expanded ventrally near the squamosal root of the zygomatic arch). Incisive foramina small with subparallel lateral margins, not extending posteriorly to level of M1s; maxillary portion of incisive septum (dividing the left and right foramina) usually wide. Zygomatic plate varying from broad to narrow. Palate with two posterolateral pits on each side. Auditory bullae small and usually globular, with short and narrow eustachian tubes; periotic bone extends anteriorly to the internal carotid canal but does not enter it (except in MPEG 42715 and MCN-M 1404, in which the periotic does not reach the internal carotid canal). Subsquamosal fenestra small (about ¼ the size of the postglenoid foramen); hamular process of the squamosal long. Paraoccipital process narrow and small and usually separated from the auditory bullae ( Sánchez-Vendizú et al., 2018: fig. 3C). Sphenopalatine foramen small (except in MPEG 41991, whose sphenopalatine foramen is large); alisphenoid strut absent; carotid circulation pattern usually primitive 8 (pattern 1, as identified by retaining a well-developed squamosal-alisphenoid groove and sphenofrontal foramen, both indicative of the presence of the supraorbital branch of the stapedial artery; Voss, 1988: fig. 18A, B).

First upper molar (M1) usually with slightly flat and undivided anterocone; anteroloph usually fused with the anterolabial conule (such that the anteroflexus is not distinguishable); M3 small; labial cusps (paracone, metacone) usually taller than lingual cusps (protocone, hypocone); m1 anteroconid undivided.

Mandible with mental foramen opening laterally; capsular process of lower incisor alveolus present, but indistinct (reduced to a slight rounded elevation), approximately at same height of coronoid process.

TAXONOMIC COMPARISONS: Neacomys xingu differs from N. dubosti in dorsal pelage color (orange-brown sprinkled with black versus light to dark brown finely sprinkled with orange), and by its narrower interorbital region, globular auditory bullae (the bullae are usually flask shaped in N. dubosti ), and carotid circulation usually pattern 1 (versus always pattern 1 in N. dubosti ).

Karyotypically, Neacomys xingu differs from all other members of the Dubosti Group by having a diploid chromosomal complement of 58 (versus 2 n = 64 in N. dubosti and 2 n = 54 in “species 2”), a uniquely small FN of 64 autosomal arms (versus FN = 66– 60 in other species), and submetacentric sex chromosomes (at least one sex chromosome is acrocentric in N. dubosti and “species 2”).

DISTRIBUTION: Neacomys xingu has been collected on the right bank of the lower Xingu River and in the region of Serra de Carajás, in southeastern Pará state (fig. 5). According to cytogenetic data ( Di-Nizo et al., 2017; Oliveira da Silva et al., 2019), the species also ranges southward into Vila Rica in northeastern Mato Grosso state. These records suggest that the species is restricted to the Xingu center of endemism ( Silva et al., 2005).

ETYMOLOGY: The specific epithet xingu is to be treated as a noun in apposition. The species name refers to the Xingu center of endemism, delimited by the Xingu and Tapajós rivers, where the species occurs.

REMARKS: Neacomys xingu was previously reported in the literature as “ Neacomys clade 7” by Patton et al. (2000), “ Neacomys sp.” (in part) by Silva at al. (2015), “ Neacomys sp.” by Di-Nizo et al. (2017) and Brandão et al. (2019), and “ Neacomys sp. C” by Oliveira da Silva et al. (2019).

Figures

FIG. 11. Dorsal and ventral views of the skin of Neacomys xingu (UFMT 1268, holotype).

FIG. 12. Dorsal, ventral, lateral cranial views and lateral view of mandible of Neacomys xingu (UFMT 1268, holotype).