Emended definition—Medium to large-sized arboreal cricetid rodents with tail slightly longer than head and body; four mammae arranged as two inguinal pairs; hindfoot short and broad, digit V nearly equal to II-IV, plantar pads large and closely approximate, ungual tufts present; interorbit cuneate, supraorbital shelves pronounced, dorsally reflected, continuing (in adults) as well-defined temporal ridges; interparietal conspicuous, long and wide, laterally contacting squamosal; zygomatic plate relatively narrow, dorsal notch absent or weakly suggested; tegmen tympani adnate to squamosal (Voss, 1993); alisphenoid strut present, postglenoid foramen tiny, subsquamosal fenestra absent, hamular process undefined (Carleton, 1980); palatal conformation short-wide (Hershkovitz, 1962), parapterygoid fossa shallow and relatively narrow; mesopterygoid fossa typically fully ossified, sphenopalatine vacuities absent or inconspicuous slits (Carleton, 1980); molars brachyodont, strongly cuspidate with principal cones opposite, mesoloph(id) and other accessory enamel ridges well developed; M2 four-rooted, m3 large and in size and coronal topography resembling m2; 1st rib articulating only 1st thoracic vertebra, humerus with entepicondylar foramen, trochlear process of calcaneum broad and positioned proximally (Carleton, 1980); stomach unilocular-hemiglandular, gall bladder absent, caecum long and elaborately infolded (Carleton, 1973, 1980); glans penis short and wide with rudimentary to deep crater, spines large and widely spaced, baculum shorter than glans, with moderate cartilaginous spine but without lateral bacular digits (Hooper, 1960; Hooper and Musser, 1964a). Contents—Nyctomys Saussure, 1860; Otonyctomys Anthony, 1932; Ototylomys Merriam, 1901; Tylomys Peters, 1866.
Vorontsov (1959) arranged these genera as members of Oryzomyini (including "thomasomyines"), within a broadly defined Cricetinae. Hooper (1960) first advanced a family-group concept for these Middle American endemics, then comprising only Tylomys and Ototylomys which he associated as one group of four within North American genera having a "simple" phallus, the neotomine-peromyscines (= Neotominae sensu Reig, 1980, and others). Subsequently viewed either as early offshoots of a lineage leading to Neotoma (Hooper and Musser, 1964a) or as an older clade, possibly including Nyctomys, basal to the divergence of neotomine-peromyscines and sigmodontines (Carleton, 1980). Reig (1984) nomenclaturally formalized the rank as subfamily and listed its generic contents; McKenna and Bell (1997) used it as tribe within Sigmodontinae sensu lato.
Although tylomyines can be morphologically circumscribed, as here defined, abundant traits indicate that Tylomys Ototylomys (Tylomyini; see emended definition under Tylomys account) and Nyctomys Otonyctomys (Nyctomyini new tribe; see Nyctomys account) are themselves distantly related (see below). While various kinds of evidence do point to the antiquity of certain of these genera (Arata, 1964; Carleton, 1980; D’Elía et al., 2003; Engel et al., 1998; Haiduk et al., 1988; Hooper and Musser, 1964a; Sarich, 1985; Steppan, 1995; Voss and Linzey, 1981), no recent study has mustered the taxonomic sampling needed to critically test the several questions of monophyly at issue, whether early diverging members of Neotomini (Hooper and Musser, 1964a), a primitive clade apart from Neotominae and Sigmodontinae (Carleton, 1980; Reig, 1984), or another phyletic topography as yet unrecognized in our classification.