Neacomys marajoara, 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 : 19-26

Neacomys marajoara , new species

Marajoara Spiny Mouse Figures 4 , 7

HOLOTYPE: The holotype ( MPEG 40432 ) is an adult male (age class 3), collected on 19 January 2009, by R. V. Rossi (field number MAJ 23 ; fig. 4) in a pitfall trap. The specimen consists of a stuffed skin (missing the tip of the tail), skull, and skeleton; a tissue sample of this specimen is preserved in ethanol, and a partial cytochrome b sequence that we obtained from it has been deposited in Genbank with accession number MT 462070 .

TYPE LOCALITY: Tauari Farm , municipality of Chaves , Marajó Island, state of Pará, Brazil (0°39′S, 50°11′W, figs. 5, 6) .

DIAGNOSIS: Neacomys marajoara is a small species (table 3) that differs from congeneric taxa by the following combination of craniodental traits: skull delicate; interorbital region relatively broad; nasal bones straight anteriorly; supraorbital margins slightly convergent anteriorly; subsquamosal fenestra large (almost half the size of the postglenoid foramen on each side of the skull); paraoccipital processes separated from the auditory bullae; sphenopalatine foramen large; carotid circulation usually pattern 2 (sensu Voss, 1988); maxillary part of incisive septum (between the incisive foramina) narrow; M1 usually with flat and undivided anterocone; and M1 anteroloph usually fused with anterolabial conule.

MORPHOLOGICAL DESCRIPTION: Dorsal pelage dark brown finely sprinkled with orange (fig. 4); ventral pelage varying from pure white to yellowish white, separated from the dorsal pelage by a very 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 most specimens examined (except MPEG 40435 and MPEG 40446, in which ungual tufts are as long as the claws); 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, bicolored, and covered by short, spiny, and clearly visible hairs; white hairs present on ventral caudal surface from base to midlength; tail tip with very short (1 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. 7), with straight anterior nasal margins, notably broad rostrum, and shallow zygomatic notches; posterior nasal terminus usually slightly pointed, extending beyond maxillary-frontal suture; premaxillaries terminating anterior to nasals; lacrimal bone small and visible in dorsal view, usually in broad contact with frontal bones; supraorbital margins slightly convergent anteriorly; interorbital region relatively broad; supraorbital beads developed as projecting shelves; lateral expansion of the parietal restricted to the dorsal cranial surface. Incisive foramina small and usually teardrop shaped, not extending posteriorly to level of M1s; maxillary portion of incisive septum (dividing the left and right foramina) narrow. Zygomatic plate narrow. Palate with two posterolateral pits on each side. Auditory bullae small and globular, with short and narrow eustachian tubes; periotic bone extends anteriorly to internal carotid canal (except MPEG 40435, MPEG 40443, and MPEG 40439, in which the periotic does not reach the internal carotid canal), but does not enter the canal. Subsquamosal fenestra large (almost half the size of the postglenoid foramen); hamular process of the squamosal long. Paraoccipital process narrow and small and separated from the auditory bullae. Sphenopalatine foramen large; alisphenoid strut absent; carotid circulation pattern usually derived 7 (pattern 2, as identified by the presence of a large stapedial foramen and a large posterior opening of the alisphenoid canal, and by the absence of a squamosalalisphenoid groove and a sphenofrontal foramen; Voss, 1988).

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

KARYOTYPES: Karyotypes of specimens from the type locality were obtained by Silva et al. (2015), and karyotypes from specimens collected elsewhere on Marajó Island were obtained by Oliveira da Silva et al. (2019), who described a chromosomal complement of 2 n = 58 and a fundamental number (FN) = 70 (table 5).

TAXONOMIC COMPARISONS: Neacomys marajoara differs from N. dubosti in dorsal pelage color (dark brown finely sprinkled with orange versus light to dark brown finely sprinkled with orange in N. dubosti ), and by its bicolored tail (the tail is usually unicolored in N. dubosti ), narrower interorbital region, teardrop-shaped incisive foramina (the lateral margins of the incisive foramina are usually subparallel in N. dubosti ), globular auditory bullae (the bullae are usually flask-shaped in N. dubosti ), and carotid circulation usually pattern 2 (versus pattern 1 in N. dubosti ).

a We found variation in this character, please see Neacomys xingu description.

b We found variation in this character, please see Neacomys marajoara description.

Neacomys marajoara differs from N. xingu in dorsal pelage color (dark brown finely sprinkled with orange versus orange-brown sprinkled with black in N. xingu ), and by its bicolored tail (the tail is usually unicolored in N. xingu ), anteriorly straight (versus anteriorly expanded) nasal bones, broader interorbital region, narrower maxillary septum of the incisive foramina, larger subsquamosal fenestrae, and carotid circulation usually pattern 2 (versus usually pattern 1).

Neacomys marajoara differs from N. vossi in dorsal pelage color (dark brown finely sprinkled with orange versus brown sprinkled with orange and/or black in N. vossi ), and by its anteriorly straight (versus anteriorly expanded) nasal bones, broader interorbital region, paraoccipital processes separated from the auditory bullae (versus processes close to the auditory bullae), larger subsquamosal fenestrae, carotid circulation usually pattern 2 (versus pattern 1), and narrower maxillary septum of the incisive foramina.

Karyotypically, Neacomys marajoara differs from other species in the Dubosti Group by having a diploid chromosomal complement of 58 (versus 2 n = 64 in N. dubosti ; table 5), a uniquely large FN of 70 autosomal arms (versus FN = 64–68 in other group members), and submetacentric sex chromosomes (at least one of the sex chromosomes is acrocentric in N. dubosti and “species 2”).

DISTRIBUTION: Neacomys marajoara has been recorded only on Marajó Island, state of Pará, Brazil (fig. 5).

ETYMOLOGY: The specific epithet marajoara is to be treated as a noun in apposition. It is derived from Tupi Guarani (a language family that comprises many different indigenous Amazonian dialects) and denotes a native of Marajó Island (the type locality).

FIELD NOTES: Of the nine specimens for which trapping information is available, eight were captured in pitfall traps and one was taken in a Sherman trap placed on the ground. One paratype ( MPEG 40435 ) was pregnant with three fetuses.

REMARKS: Neacomys marajoara was previously reported in the literature as “ Neacomys sp.” (in part) by Silva at al. (2015) and as “ Neacomys sp. D” by Oliveira da Silva et al. (2019).

Figures

FIG. 4. The holotype of Neacomys marajoara, (MPEG 40432) in life. Photo by Rogério V. Rossi.

FIG. 7. Dorsal, ventral, and lateral cranial views and lateral view of mandible of Neacomys marajoara (MPEG 40432, holotype).