Neomicroxini, Pardiñas & Curay & Brito & Cañón, 2021

Pardiñas, Ulyses F J, Curay, Jenny, Brito, Jorge & Cañón, Carola, 2021, A unique cricetid experiment in the northern high-Andean Páramos deserves tribal recognition, Journal of Mammalogy (J. Mammal.) 102 (1), pp. 155-172 : 164-166

publication ID	https://doi.org/10.1093/jmammal/gyaa147
DOI	https://doi.org/10.5281/zenodo.10672239
persistent identifier	https://treatment.plazi.org/id/2D6B5219-D76D-FFC9-FEF2-1C18FB25FC7C
treatment provided by	Felipe
scientific name	Neomicroxini
status	trib. nov.

Treatment

Neomicroxini , new tribe

( Figs. 1–5 View Fig View Fig View Fig View Fig View Fig and 8 View Fig )

Type genus, by present designation.— Neomicroxus Alvarado-Serrano and D’Elía, 2013 .

Diagnosis.— A tribe of the subfamily Sigmodontinae , “clade Oryzomyalia” (sensu Steppan et al. 2004) grouping small-sized cricetids (head and body length ~ 85 mm; body mass ~ 17 g) characterized by the following combination of morphological traits: small head and minute eyes, ears small (~ 14 mm), rounded, semihidden by head fur; dorsal fur long (~ 10 mm), dense, soft but not woolly, from grizzled to dark chestnut or rufous, blackish brown, belly slightly paler, scarcely countershaded; tail long (~ 80 mm) slightly shorter than head and body length, covered by short hairs; manus and pes with moderately pointed claws at the ends of the toes; palmar pad composed of five pads with digits stocky and subequal in length; plantar pad composed of six pads with a large hypothenar pad, squamate surface, and with the ungual tufts surpassing the end of the claws; three pairs of mammary glands arranged in pectoral, abdominal, and inguinal, pairs; skull delicate, with domed profile, rostrum pointed with short “nasal-tube” and globular noteworthy braincase; conspicuous nasolacrimal capsules and foramina; shallow zygomatic notches; interorbital region broad and smooth; zygomatic plate low, poorly developed, almost without free upper border, but with a noticeable masseteric tubercle; infraorbital foramen ovoid, not-constricted basally; incisive foramina broad, reaching posteriorly the first upper molar protocones; palate short ( N. latebricola ) or long ( N. bogotensis ) and wide; toothrows parallel; broad mesopterygoid fossa and parapterygoid plates; temporal region enlarged with long and thin hamular process of the squamosal applied to a robust mastoid promontorium; tegmen tympani overlapping suspensory process of squamosal; alisphenoid strut absent; carotid circulatory pattern type 1 (stapedial foramen, sphenofrontal foramen, and squamosal–alisphenoid groove, all present); auditory capsules inflated with very short Eustachian tubes; large mastoid capsules; large and rounded foramen magnum directed ventrally; dentary slender and low, with the ascending ramus strongly displaced backwards leaving all the molars laterally well-exposed; capsular process of the incisor root absent; short and pointed angular process; orthodont (in N. latebricola ) to opisthodont (in N. bogotensis ) gracile and ungrooved upper incisors; molars hypsodont, with crested coronal surfaces, main cusps opposite, and noticeable basal cingulum closing the main flexus/ids; anterocone of M1 with two well-developed and subequal size conulids; anterolophs and mesolophs barely present (in N. bogotensis ) or fused (in N. latebricola ); M3 small but bilobed (in N. bogotensis ) or tending to peg-like form (in N. latebricola ); mesolophids typically reduced or absent; m3 anteriorly–posteriorly compressed; upper molars three-rooted; lower molars two-rooted; axial skeleton typically composed of 13 thoracic, 7 lumbar, 4 sacral, and 23–25 caudal vertebral elements; axis with squared spinous process; second thoracic with markedly enlarged dorsal process; stomach unilocular–hemiglandular, with both types of epithelium subequally distributed (in N. latebricola ); gall bladder present (in N. bogotensis ) or absent (in N. latebricola ); soft palate with three complete diastemal and five incomplete interdental rugae (in N. latebricola ); two pairs of basal prostate glands (in N. bogotensis ) (after Thomas 1895; Anthony 1924; Gyldenstolpe 1932; Ellerman 1941; Voss and Linzey 1981; Reig 1987; Voss 1991, 2003; Alvarado-Serrano and D’Elía 2013; Curay 2019; this paper).

Content.— A single genus, Neomicroxus Alvarado-Serrano and D’Elía, 2013:1008 .

Geographic distribution.— Neomicroxini rodents are distributed along high-Andean (typically above 3,000 m a.s.l.) Polylepis forests and shrubland–grassland Páramo environments from northern Ecuador, through Colombia to southwestern Venezuela ( Linares 1998; Alvarado-Serrano and D’Elía 2015; Fig. 9 View Fig ).

Biochron.— Recent in Ecuador, Colombia, and Venezuela.

Remarks.— Because Neomicroxini contains the single genus Neomicroxus , the diagnosis constructed for it (Alvarado-Serrano and D’Elía 2013:1008) should be sufficient to diagnosis the tribe, by monotypy. However, we prefer to provide a more comprehensive diagnosis to also incorporate soft anatomy and postcranial skeleton and to add a more accurate description of molars. Microxus is not a classic Latin or Greek word; etymologically, it probably highlights the small (micr-) and pointed (oxys) rostrum of mimus , their genotype. Because Neomicroxus is a neologism composed by the prefix “Neo” (new) and the generic epithet Microxus , the name of the tribe is derived from the addition of the tribal ending -ini, hence, Neomicroxini .

Hypsodonty and volcanic ash fall in the northern Andes.— Sigmodontines display varied morphological contrivances to increase the functional longevity of their molars. According to the traditional view of the subfamily’s evolution, low-crowned (= brachydont) founding population(s) radiated into a remarkable diversity of medium- (“mesodont”) to high-crowned (hypsodont) taxa to face the abrasiveness of South American environments, particularly during the transition from forests to grasslands ( Hershkovitz 1962). However, the topic remains problematic, despite hypsodonty perhaps being the key innovation droving diversification. Madden’s (2015) integrative approach to understanding hypsodonty and its causes over the entire sigmodontine radiation demonstrated a significant increase in the proportion of hypsodont taxa among folivores and herbivores in faunas occurring on andisols (volcanic ash soils), particularly in montane environments. This fact presumably reflects the increased mobility of abrasive minerals (in particular, tephras) as contaminants in mammalian foodstuffs. Madden (2015) explored the possible contribution of ash-rich vulcanism and mountain uplift in the Cenozoic by analyzing the association among hypsodont sigmodontines, active volcanoes, duststorms, and glaciers, as potential sources. He concluded ( Madden 2015:79–80) that “hypsodonty is a complex phenomenon involving correlations with herbivorous diets, large-scale climate variation in temperature and rainfall, and both the sources of sediment and the surface processes that mobilize environmental abrasives through the animal’s environment.” He found a clear association between a core of sigmodontine hypsodonty and the “Northern Volcanic Zone– NVZ in southern Colombia and Ecuador (between 7ºN and 0.5ºS latitude)” ( Madden 2015:figure 3.8).

The Northern Volcanic Zone concentrates several of the most important examples of active and extinct volcanoes in South America ( Hall 1977). According to Hall and Calle (1982:235)

the most evident indication of magmatic activity during the Pleistocene and Holocene are the numerous large stratovolcanoes, both extinct and active, that cap the northern Andes. These volcanoes form two principal parallelrowsthatextendfromtheColombianbordersouthward to about latitude 2°30’S, south of which they are absent... The western row, that forms the Western Cordillera, includes the volcanoes Chimborazo, Carihuairazu, Quilotoa, Iliniza, Corazón, Atacazo, Pichincha, Pululahua, Cotacachi, Cuicocha, Yana–urcu, and Chiles. The eastern row, that crests the Cordillera Real, includes the volcanoes Sangay, Altar, Tungurahua, Cotopaxi, Sincholagua, Antisana, Las Puntas, and Cayambe. Between the two volcanic rows, the Interandean Valley contains somewhat more eroded volcanoes...

Plio-Pleistocene ash fall deposits cover much of the Andes, including the Interandean Valley, and have been largely recognized in classic geological formations as the Cangahua Formation, a thick sequence of ash material (Hall and Calle 1982; Mothes et al. 1998). An explosive period of widespread vulcanism, beginning in Late Pliocene and increased during the Plio-Pleistocene interval, accompanied the rise of both cordilleras in the Ecuadorian Andes ( Hungerbühler et al. 2002). It is tempting to connect hypsodonty in Neomicroxus with the continuous ash fall in northern Andes during the Neogene. The emergence of a small-bodied, markedly high-crowned lineage of sigmodontines as an evolutionary response to an increasing tephra input in the context of Andean orogeny is rather speculative. However, additional facts provide some support for this hypothesis. For example, hypsodonty is less developed in N. bogotensis than N. latebricola , and this species occurs in areas where Neogene ash fall has been more limited ( Soriano et al. 1999). The diet of N. latebricola is unknown, but unsystematic observations suggest that the species is a surface-dweller, feeding on plant material, roots, and few invertebrates, and, therefore, ingesting abrasive soil particles ( Brito 2013). Finally, it is striking to note that some tendency to high-crowned molars is present in other taxa that share the northern Ecuador high-Andean ranges with Neomicroxus , in particular, several species of Thomasomys . Although these sigmodontines are mainly treated as brachydont ( Pacheco 2003), a moderate degree of hypsodonty is found in several other Ecuadorian species. When diagnosing T. ucucha , a high-Andean (crest of the Cordillera Oriental between ca. 3,400 and 3,700 m) species, Voss (2003:10) noted “small, hypsodont molars lacking well-developed cingula and stylar cusps” and then “hypsodont [molars] when unworn (by comparison with more brachydont congeners)” ( Voss 2003:11). In resurrecting Thomasomys cinnameus Anthony, 1924 , another high-Andean Ecuadorian form and one of the smallest known species of the genus (known masses range from 14 to 19 g— Voss 2003:table 5), Voss (2003:29) indicated “larger and more hypsodont molars with weakly developed cingula and stylar cusps [than in Thomasomys gracilis ].” The group of species associated with Thomasomys aureus ( Tomes, 1860) , widespread along the Andes from Bolivia to Venezuela ( Pacheco 2015; Brito et al. 2019) also is characterized by having high-crowned molars (see Supplementary Data SD2). Evidently, hypsodonty deserves further attention as a plausible evolutionary response in northern Andean sigmodontines.