CHACO MOUSE OPOSSUM Cryptonanus chacoensis FAUNA PARAGUAY

Cryptonanus chacoensis (Tate 1931) Image Gallery
TAX: Class Mammalia; Subclass Theria; Infraclass Metatheria; Order Didelphimorphia; Family Didelphidae; Subfamily Didelphinae, Tribe Monodelphini (Myers et al 2006, Gardner 2007). The genus Cryptonanus was defined by Voss, Jansa & Lunde, 2005. There are five known species according to the latest revision (Gardner 2007) one of which is present in Paraguay. This is a recently recognised species in a recently described genus that has for many decades been consistently confused with Gracilinanus agilis in the published literature. See the similar species section for distinguishing features. The generic name Cryptonanus is taken from the Greek meaning "hidden dwarf", in reference to the fact that this cryptic genus had been overlooked for so long. The species name chacoensis refers to the Chaco biome which makes up part of this species range. The species is monotypic. Tate (1933) reported both Marmosa agilis agilis and Marmosa agilis chacoensis (=Cryptonanus chacoensis) from Sapucaí, Departamento Paraguarí, Paraguay but made no effort to explain how two distinct subspecies could occur in the same locality. Recognising the anomaly Cabrera (1958) synonymised the two under M.a.agilis but apparently did not study the type material in doing so. In fact the characters stated for Tate´s "subspecific" differentiation closely correspond to the characters that distinguish Gracilinanus agilis from Cryptonanus chacoensis and his split was good, though it belonged at the generic and specific level rather than the subspecific level (Voss, Lunde & Jansa 2005). Synonyms adapted from Gardner (2007):
Marmosa agilis chacoensis Tate 1931:10. Type locality "Sapucay" =Sapucaí, Deparatmento Paraguarí, Paraguay.
Marmosa [(Thylamys)] agilis agilis Cabrera 1958:27. In part. Not Grymaeomys agilis Burmeister (1854).
Gracilinanus agilis Gardner & Creighton 1989:5. In part. Not Grymaeomys agilis Burmeister (1854).
Cryptonanus chacoensis Voss, Lunde & Jansa 2005:1. First use of current name combination.
ENG: Chaco Mouse Opossum (Gardner 2007).
ESP: No known names.
GUA: No known names.
DES: A tiny slender mouse opossum with short, smooth pelage and very short, inconspicuous guard hairs. Dorsally the pelage is uniformly brownish-grey, lacking patternation. Ventral pelage buffy-white, self-coloured to the base (ie not grey-based) and with at least the median line whitish. Head scarcely paler than dorsum, rarely with slight suggestion of a median line and with narrow black periocular patches. Males possess a gular gland. Ears moderately large and rounded, light fuscous-brown in colour. Vibrissae short. Plantar epithelium of tarsus naked. Central plantar surface of manus only sparsely tuberculated. Manus digits 3 and 4 of subequal length and longer than digits 2 and 5. Claws shorter than fleshy digital pads. Lateral carpal tubercles are present in adult males. Pedal digit 4 longer than adjacent digits 3 and 5. Tail greyish-brown, lightly bicoloured (darker above and paler below) and 1-1.2x head and body length. It is prehensile, lacking hair on the ventral surface at the tip. Caudal scales are arranged in annular series and bearing sparse, almost invisible hairs, three per scale. Females lack a pouch. Abdominal-inguinal mammae 4-1-4 = 9. (Tate 1933, Voss, Lunde & Jansa 2005). CR - Maxillary palatal vacuities, rostral process of the premaxillae and a secondary foramen ovale all absent. Palatal process of premaxilla reaches C1 on each side. Nasal wider anteriorly than posteriorly with tips extending to I1. Maxillary turbinals large and branched. Supraornital margins large and unbeaded. Interorbital and postorbital constrictions present in juveniles and young adults. No sagittal crest. Parietal and alisphenoid contact. Petrosal exposed laterally in fenestra between parietal and squamosal. Maxillopalatine fenestrae large and palatine fenestrae present. Posterolateral palatal foramina does not extend lingual to M4 protocones. Posterior palate with conspicuous lateral corners. Maxillary and alisphenoid do not contact on orbital floor. Transverse canal foramen present. Paroccipital process of exoccipital small. Dorsal margin of foramen magnum formed by supraoccipital and exoccipitals. Angular process acute and strongly inflected. The following measurements are taken from Voss, Lunde & Jansa (2005) for specimens from Paraguay (n=3 males, n=3 females): Condylobasal Length: male 24.7mm, female 23.97mm (23.5-24.7mm); Width of Nasals: male 3.17mm(3-3.3mm), female 2.7mm (2.3-2.9mm); Least Interorbital Width: male 4.6mm, female 4.2mm (4-4.3mm); Zygomatic Width: male 14.13mm(13.9-14.4mm), female 13.4mm (13.2-13.6mm); Palate Length: male 13.7mm, female 12.9mm (12.6-13.3mm); Palate Width: male 7.73mm(7.7-7.8mm), female 7.67mm (7.6-7.8mm). DF: I5/4 C1/1 P 3/3 M 4/4 = 50. Crowns of I2-I5 rhomboidal, increasing in width from front to back. C1 with accessory cusps, posterior being more distinct than anterior. P1 present but small. P2 < P3, the latter lacking a cutting edge. Upper molars dilambdodont and highly carnassialized, increasing in width from front to back. M1 with ectoflexus shallow, but becoming deeper towards M3. Lower incisors with lingual cusp. Lower c1 usually with small posterior accessory cusp, but rapidly lost even with minor wear. p2>p3. Lower m3 with hypoconulid labially salient and twinned with large entoconid (much taller than hypoconulid). Molars small. Incomplete anterior cingulum on M3. The following measurements are taken from Voss, Lunde & Jansa (2005) for specimens from Paraguay (n=3 males, n=3 females): Length of Maxillary Row of Molars: male 9.83mm(9.7-10mm), female 9.57mm (9.3-9.8mm); Length of Molars: male 5.3mm(5.2-5.4-10mm), female 5.23mm (5.2-5.3mm); M1-M3 Length: male 4.63mm(4.6-4.7mm), female 4.6mm.
TRA: No information.
MMT: The smallest of the Paraguayan Mouse Opossums. The following measurements are taken from Voss, Lunde & Jansa (2005) for specimens from Paraguay (n=3 males, n=2 females): HB: male 9.5cm (8.9-10cm), female 8.6cm (8.4-8.8cm); TA: male 11.93cm (11.5-12.6cm), female 11cm (10.9-11.1cm); FT: male 1.57cm (1.5-1.6cm), female 1.5cm; EA: male 1.77cm (1.7-1.8cm), female 1.65cm (1.6-1.7cm); WT: male 16g, female 15g (14-16g).
SSP: Long confused with Gracilinanus agilis, this species is separated with care when using external characters only. Measurements and examination of skull characteristics may be necessary in some cases. Typically the tail of Cryptonanus is shorter when compared to head and body length (usually <1.2x) than that of Gracilinanus (1.2-1.5x) though there may be some overlap at the extremes and this character should not be used alone for specific designation. Tail length is typically in the range 95-117mm for adult Crytponanus and 110-165mm for Gracilinanus. More reliable is the ratio of premolar heights, with P2<P3 in Cryptonanus and the two of approximately equal height in Gracilinanus - though be aware of the affects of teeth wear in older specimens. On the canine C1 accessory cusps are present basally in Cryptonanus that are absent in Gracilinanus. Dorsally this species is brownish-grey in colour, Gracilinanus is on average somewhat browner, though this character can be difficult to judge. Ventral pelage is usually somewhat greyish basally in Gracilinanus and buffy basally in this species. Upon direct comparison Gracilinanus has larger ears, longer vibrissae and broader ocular rings than Cryptonanus, but these characters are difficult to measure when presented with a single specimen. Cranially maxillary palatal vacuities, rostral process of the premaxillae and a secondary foramen ovale are all present in Graciliananus but absent in Cryptonanus. The species can be easily separated from the two species of Paraguayan Thylamys by the fact that members of that genus have distinctly tricoloured pelage, whereas Cryptonanus is uniformly-coloured dorsally. Thylamys also habitually exhibit some degree of incrassination (fat deposits) in the tail and have highly granular surfaces to the feet, neither character being exhibited by Cryptonanus. Furthermore the species occurring in eastern Paraguay, Thylamys macrurus, is considerably larger than Cryptonanus. Micoureus paraguayanus is much larger with thick woolly pelage and broadly pale-tipped, bicoloured tail. (Voss, Lunde & Jansa 2005).
DIS: Occurs from northwest Argentina (Provincias Chaco and Jujuy) and Paraguay, south and east to Provincia Buenos Aires, Argentina, Rio Grande do Sul in Brazil and probably northern and western Uruguay, though the species has yet to be recorded in that country (Gardner 2007). Flores (2006) considers that all records of Gracilinanus agilis for Argentina refer to this species, though Chebez (2009) notes that skulls of at least one Gracilinanus species (agilis or microtarsus) are known from Provincia Misiones. Anderson (1997) reported the species from extreme southern Bolivia at Tarija but Voss, Lunde & Jansa (2005) stated that at least one of the four specimens mentioned was actually an unidentified species of Marmosops, whilst the other three specimens were unavailable for examination. The species is apparently very widespread in Paraguay and likely occurs throughout the country in suitable habitat. Voss, Lunde & Jansa (2005) list the following specimens from Paraguay: Estancia Doña Julia, Departamento Alto Paraguay (TK 61053, 61072, 61074, 61103); Estancia Dos Marias, Departamento Caazapá (GD 521); 13.3 km north of Curuguaty by road, Departamento Canendiyú (UMMZ 137143); Palmar de las Islas, Departamento Alto Paraguay (TK 65331); Concepción, Departamento Concepción (BMNH 11.11.19.23), Río Aquidaban at Paso Horqueta, Departamento Concepción (UMMZ 134552); 1.6 km south of Tobatí by road, Departamento Cordillera (UMMZ 126105); Sapucaí, Departamento Paraguarí (BMNH 4.1.5.48, 5.8.1.8); Estancia La Victoria, Departamento Presidente Hayes (TK 60201).
HAB: There is some suggestion that this species prefers open, grassy and often wet habitats. Paraguayan distributional records reflect areas of palm savanna (humid Chaco and Pantanal), marsh and seasonally-inundated grassland. Voss, Lunde & Jansa (2005) describe capture sites in Paraguay as in a hollow log (Sapucaí), on a wood pile (Sapucaí), on wet ground in a marsh (Tobatí), on the ground in high grass at the edge of a marsh (Paso Horqueta) and on the ground at the base of fruiting bromeliad stalks (Curuguaty). A specimen at PN San Rafael was taken close to human habitation at the interface of secondary humid forest and agriculture (P.Smith pers. obs.). This data suggests that they are able to tolerate the close proximity of humans more so than other small marsupials.
ALI: Presumably largely insectivorous, no published data exists that can be unequivocally attributed to this species.
REP: No data unequivocally referring to this species is available. Massoia & Fornes (1972) stated that Marmosa agilis chacoensis in Rio Grande do Sul, Brazil gave birth to litters of up to 12 young, but given that the number of teats is nine the possibility of misidentification means that the data must be treated with caution.
BEH: Activity Levels All Paraguayan specimens have been taken on the ground despite simultaneous trapping efforts in trees, suggesting an at least partially terrestrial existence, whilst the prehensile tail and digit arrangement indicate that they are adapted for climbing. Enemies González et al (1999) report remains of this species in Barn Owl Tyto alba pellets in Rio Grande do Sul, Brazil, though it was reported as Gracilinanus agilis.
VOC: No information.
HUM: None.
CON: Globally considered to be of Low Risk Least Concern by the IUCN, click here to see their latest assessment of the species on account of its wide distribution, large population, occurrence in protected areas, and tolerance of habitat modification. Unlike other small marsupials this species has been frequently trapped in areas close to human habitation, suggesting that they are less susceptible to habitat modification than other species. They are apparently adaptable and occur in a variety of open habitats. At this stage it is difficult to draw any firm any conclusions about population trends in Paraguay owing to the confusion in the literature between this species and Gracilinanus agilis, but it would seem to be under less threat than most other Didelphids. The species is one of the more commonly captured small marsupials in Paraguay and may even be benefitting from deforestation of the Atlantic Forest and expanding its range.
Citable Reference: Smith P (2009) FAUNA Paraguay Online Handbook of Paraguayan Fauna Mammal Species Account 34 Cryptonanus chacoensis.
Last Updated: 30 June 2009.
References:
Anderson A 1997 - Mammals of Bolivia: Taxonomy and Distribution - Bulletin AMNH 231.
Burmeister H 1854 Systematische Uebersicht der Thiere Brasiliens: Welche während einer Reise durch die Provinzen von Rio de Janeiro und Minas Geraës Gesammlt oder beobachtet wurden Vol 1 - G.Reimer, Berlin.
Cabrera A 1958 - Catálogo de los Mamíferos de América del Sur - Revista Museo Aregntino de Ciencias Naturales Bernadino Rivadavia Zoology 4: p1-307.
Chebez JC 2009 - Otros que Se Van - Editorial Albatros, Buenos Aires.
Flores DA 2006 - Orden Didelphimorphia in Bárquez R, Díaz, MM, Ojeda RA eds Mamíferos de Argentina, Sistemática y Distribución - SAREM, Buenos Aires.
Gardner AL 2007 - Mammals of South America Volume 1: Marsupials, Xenarthrans, Shrews and Bats - University of Chicago Press.
Gardner AL, Creighton GK 1989 - A New Generic Name for Tate´s microtarsus Group of South American Opossums (Marsupialia: Didelphidae) - Proceedings of Biological Society of Washington 102: p3-7.
Gonzalez EM, Claramunt SJ, Sralegui AM 1999 - Mammífers Hallados en Egragrópilas de Tyto alba (Aves: Strigiformes: Tytonidae) en Bagé, Rio Grande do Sul, Brasil - Iheringia Serie Zool. 86: p117-120.
Massoia EJ, Fornes A 1972 - Presencia y Rasgos Etoecológicos de Marmosa agilis chacoensis Tate en las Provincias de Buenos Aires, Entre Rios y Misiones (Mammalia: Marsupialia: Didelphidae) - Revista de Investigación Agropecuaria INTA 9: p71-82.
Myers P, Espinosa R, Parr CS, Jones T, Hammond GS, Dewey A 2006 - The Animal Diversity Web (online). Accessed December 2007.
Tate GHH 1931 - Brief Diagnoses of Twenty-six Apparently New Forms of Marmosa from South America - AMNH Novitates 493.
Tate GHH 1933 - A Systematic Revision of the Marsupial Genus Marmosa - Bulletin AMNH 66.
Voss RS, Lunde DP, Jansa SA 2005 - On the Contents of Gracilinanus Gardner & Creighton 1989 with the Description of a Previously Unrecognised Clade of Small Didelphid Marsupials - AMNH Novitates 3482.

ACKNOWLEDGEMENTS
Special thanks to Juan Carlos Chebez for providing important literature and Nilton Cáceres for very kindly reviewing texts and providing a copy of his book Os Marsupiais do Brasil.