The specimen was recovered from levels of the Las Curtiembres Formation (Campanian, Upper Cretaceous), which also yielded large number of individuals of the small pipid frog Saltenia ibanezi (Báez, 1981 and Narváez and Sabino, 2008). Recent field trips conducted by the senior author resulted in the discovery of basal teleostean fishes, probable podocnemidid pleurodiran turtles, and remains of mesoeucrocodylian crocodiles, and theropod and sauropod dinosaurs (Scanferla pers. obs.) (Fig. 1).

Aves Linnaeus, 1758

Ornithothoraces Chiappe and Calvo, 1994

Enantiornithes Walker, 1981

Euenantiornithes Chiappe, 2002

Avisauridae Brett-Surman & Paul, 1985

Intiornis inexpectatus gen. et sp. nov.

Holotype: MAS-P/2 1 (Paleontological Collection of the Museo de Antropología, Salta City, Salta province, Argentina), slab and counter-slab preserving a right foot, articulated with the distal end of the corresponding tibiotarsus

Diagnosis. Avisaurid enantiornithine with the following diagnostic features: 1) tall and cup-like intercotylar prominence, even more developed than in Lectavis; 2) phalanx IV-4 anteroposteriorly short, being subequal in length to phalanx IV-2; 3) phalanx I-1 proportionally short and stout

Etymology. Inti, sun in Quechua language; and ornis, bird in Greek. inexpectatus, in reference of the casual and weird technical situation of the finding of the holotype specimen

Intiornis is a sparrow-sized enantiornithine bird (metatarsal length, 21 mm.) with an elongate tarsometatarsus. As typically occurs in enantiornithine birds, the distal end of the tibiotarsus shows a large and anteriorly flat medial condyle, a narrow and deep intercondylar sulcus, and distal condyles proximally cut off. Correspondingly, the proximal end of the tarsometatarsus shows an enlarged medial cotyle and a well-developed and cup-like intercotylar tubercle, even more developed than in the peculiar enantiornithine Lectavis (Chiappe, 1993). However, in Intiornis this prominence is even more developed proximally, being reminiscent to the condition present of the Ornithothoraces (Morrison et al., 2005). The tarsometatarsal lateral cotyle is smaller than the medial one, but it is well excavated proximally. As occurs in most other Enantiornithes, the tarsometatarsus of Intiornis is fused only proximally (Martin, 1995). In cranial view, the proximal end of the tarsometatarsus is well excavated, as documented in other Enantiornithes (e.g. Lectavis, Soroavisaurus, Avisaurus (Chiappe, 1993 and Varricchio and Chiappe, 1995)). All metatarsals are nearly straight, with metatarsal II being tightly appressed to metatarsal III in cranial view, as also applies for Soroavisaurus and Avisaurus. The shaft of metatarsal III shows its cranial edge convex in cross-section, a feature interpreted as diagnostic of Avisauridae (Chiappe, 1993). The shaft of metatarsal II is wider than that of III, and both are broader than IV, as characteristically occurs in Enantiornithes (Chiappe and Walker, 2002). An elongate and narrow proximal fenestra exists between metatarsals IV and III, constituting a feature that Intiornis uniquely shares with Soroavisaurus (Chiappe, 1993). The distal trochlea of metatarsal II is incompletely preserved, but appears to be broader than that of metatarsals III and IV, another diagnostic trait of Enantiornithes (Serano, 2000). The distal trochlea of metatarsal III is poorly excavated and shows poorly defined trochlear rims, in contrast with the well developed trochlea exhibited by derived Ornithothoraces (Morrison et al., 2005). The distal trochlea of metatarsal IV is highly damaged, but the preserved portion indicates that it was thinner than metatarsals II and III (Fig. 2 and Fig. 3).

Pedal digits are gracile and elongate, and pedal unguals are acute and recurved. Intiornis shows a typical anisodactyl foot, its pedal digit III being the longest, followed in decreasing length by digits IV, II, and I, an arrangement also reported for Sinornis (Sereno et al., 2002). Similar to other birds, the phalangeal formula is 2-3-4-5-X.

Digit IV bears four short and stout pre-ungual phalanges. Pedal phalanx IV-4 is short and stout, being subequal in length to phalanx IV-2. In contrast, in remaining Enantiornithes, including Sinornis and Concornis phalanx IV-4 is slender and elongate, being much longer than phalanx IV-2 (Sereno et al., 2002). Pedal ungual IV is acute, elongate, and strongly curved, as in most Enantiornithes. An enlarged proximodorsal lip is present. The proximal articular surface is strongly concave, and the flexor tubercle is large and bulbous.

Digit III shows very elongate non-ungual phalanges, with phalanx III-1 being the longest, in contrast to Sinornis in which the longest phalanx is III-2, being the same proportions as present in Sinornis (Sereno et al., 2002). Phalanx III-1 shows a constricted phalangeal neck, and elongate distal flexor pits. Phalanx III-2 shows a laterodistally expanded flange.

Digit II is poorly preserved. The phalanx II-1 is short and stout, and shows on its proximal end subcuadrangular lateral and medial expansions, a feature also seen in Sinornis (Sereno et al., 2002). Phalanx II-2 is longer than II-1, and shows elliptical shaped flexor pits. Pedal ungual of digit II is elongate and shows a poorly developed flexor tubercle, but a well developed proximodorsal lip.

Digit I is reverted, as in most modern birds and other Enantiornithes (Chiappe and Calvo, 1994). Phalanx I-1 is short and stout, in contrast with the narrow and elongate morphology present in most Enantiornithes, including Sinornis, Soroavisaurus, Gobipteryx, and Vescornis (Kurochkin, 1995 and Zhou and Zhang, 2006). Phalanx I-1 shows a well-developed proximodorsal lip, and a wide and ovoid distal flexor pit. Pedal ungual of digit I is large and strongly curved, being larger than unguals of digits II and IV, at least, constituting a diagnostic trait of Enantiornithes (Kurochkin, 2000). Pedal ungual of digit I is poorly preserved. It shows a dorsoventrally extended proximal articular surface, features considered as diagnostic of Enantiornithes by some authors (Kurochkin, 2000). In addition, this phalanx exhibits a large and craniocaudally extended flexor tubercle.

We identify Intiornis as a member of Enantiornithes on the basis of the following traits: tibiotarsus with a bulbous and inflated medial condyle (much larger than the lateral one) with its cranial surface nearly flat, a condition that is mirrored by an enlarged medial cotyle of the tarsometatarsus; distal articular surface of metatarsal II expanded, being larger than that of the metatarsal III, and ungual of digit I very large, with enlarged proximal surface for articulation with the first phalanx (Kurochkin, 1995, Martin, 1995, Sereno, 2000 and Walker, 1981). In addition, Intiornis lacks several traits diagnostic of the more derived Ornithuromorpha, including metatarsal III not displaced plantarly with respect to metatarsals II and IV, absence of a proximal vascular foramen, and trochlea of metatarsal III poorly developed (Kurochkin, 2000). Within Enantiornithes Intiornis is referred to Euenantiornithes because it has a very narrow and deep intercondylar sulcus on distal tibiotarsus that clearly delimits distal condyles, and distal end of metatarsal IV narrower than metatarsals II and III (Chiappe and Walker, 2002).

The shaft of metatarsal III, with its cranial margin convex in cross-section, supports the inclusion of Intiornis within the euenantiornithine clade Avisauridae (Chiappe, 1993). Three genera are currently recognized as composing this clade: Avisaurus, from the Campanian and Maastrichtian of North America (Varricchio and Chiappe, 1995), Soroavisaurus from the Maastrichtian of NW Argentina (Chiappe, 1993), and Neuquenornis, from Santonian deposits of NW Patagonia (Chiappe and Calvo, 1994). More recently, Dyke and Ösi (2010) described the peculiar avisaurid genus Bauxitornis from the Late Cretaceous of Hungary. Recent works by Cau and Arduini (2008) indicate that the genera Concornis (Osi, 2008), Halimornis (Chiappe et al., 2002), and Enantiophoenyx (Cau and Arduini, 2008) may also belong to this clade. Regrettably, in Halimornis the tarsometatarsus is unknown, thus direct comparisons with Intiornis are not possible; however, the size difference between both taxa may serve as a distinguishing feature (Chiappe et al., 2002). With regards to Concornis, Intiornis clearly differs from this genus, among other anatomical details, in having shorter phalanx IV-1, more elongated phalanx IV-3, and in the less distally located metatarsal II which is distant to the distal end of metatarsal III, and in some other minor details (Sanz and Buscalioni, 1992 and Sanz et al., 1995). On the other hand, Intiornis may be distinguished from the poorly known genus Enantiophoenyx in having metatarsal III thinner at mid-shaft, the metatarsal IV relatively broader, and the phalanx I-1 stouter (Cau and Arduini, 2008). Intiornis is clearly distinguishable from Bauxitornis in lacking the proximodistally abbreviate metatarsal II and the elongate metatarsal IV present in the latter genus (Dyke and Ösi, 2010). Moreover, Intiornis lacks the laterally bowed proximal end of metatarsals II and III exhibited by Bauxitornis (Dyke and Ösi, 2010).

Among conventional avisaurids, the sparrow-sized Intiornis is distinguished from the pigeon-sized Neuquenornis in that the first one has a stouter tarsometatarsus, stouter phalanx I-1, and long and narrow fenestra between proximal halves of metatarsals III and IV (Fig. 4B, D). Intiornis differs from Neuquenornis and resembles Soroavisaurus and Avisaurus (Chiappe, 1993) in having a medially directed metatarsal II (Varricchio and Chiappe, 1995) and broad distal trochleae of metatarsals II and III, suggesting that these three taxa may conform a group within Avisauridae, exclusive of Neuquenornis.

Intiornis differs from Avisaurus in lacking a crescent-shaped trochlea of metatarsal IV (Varricchio and Chiappe, 1995), and in having a more elongated tarsometatarsus, and a well defined intercotylar knob on its proximal end (Fig. 4).

On the other hand, among avisaurids, Intiornis more nearly resembles Soroavisaurus in having an elongate tarsometatarsus and a long and narrow fenestra between proximal metatarsals IV and III (Chiappe, 1993). However, Intiornis is clearly distinguishable from Soroavisaurus in having shortened pedal phalanx I-1, and ungual of digit 2 more elongate and acute, among other minor details (see generic diagnosis of Intiornis) (Fig. 4).

Zhou and Zhang (2006) hypothesized that there was a tendency towards increasing body size among the derived bird clade Ornithurae, being the size of early basal taxa smaller than more derived and latter forms. These authors indicated that, in contrast to Ornithurae, the Enantiornithes retained minute size until the Late Cretaceous, because they must retain small body size in order to obtain a flapping flight with a less perfect flight apparatus. However, although several Late Cretaceous Enantiornithes showed medium to large body sizes, some taxa still exhibited minute body proportions, as is the case of Intiornis, Alexornis and an unnominated taxon from Hungary (Brodkorb, 1976 and Osi, 2008). This indicates that although several latest Enantiornithes were large-sized, there also existed an array of minute forms coexisting with them (Gong et al., 2004). Hone et al. (2005) indicate that the small size of the Latest Cretaceous modern birds (i.e. Neornithes) probably allowed this clade to survive the Cretaceous–Palaeocene boundary, since smaller organisms are the more likely survivors of extinction events at the expense of its larger counterparts. Nevertheless, Intiornis and Alexornis, both coming from Latest Cretaceous beds are very small sparrow-sized taxa, rivalizing the size of the smallest Late Cretaceous Neornithes (see Chiappe and Dyke, 2002). They suggest that size was probably not a relevant feature for the extinction of Enantiornithes nor survivorship of Neornithes at the Cretaceous–Palaeocene boundary.