Anteavis crurilongus

Anteavis crurilongus is an early-diverging theropod dinosaur from the late Carnian stage of the Late Triassic, discovered in the lower Cancha de Bochas Member of the Ischigualasto Formation, northwestern Argentina. Represented by holotype PVSJ 1085—a partial skeleton of a young adult including cranial and postcranial elements—this taxon highlights unexpected morphological diversity among basal theropods, possessing derived features previously considered exclusive to Neotheropoda. Estimated at 1.8–2.0 m (5.9–6.6 ft) in length and 6–10 kg (13–22 lb) in mass, it supports evidence for a rapid early diversification of small- to medium-sized dinosaurs during a climatic shift to semi-arid conditions around 231 Ma, preceding a faunal turnover and record gap until the Norian.

Etymology

The generic name Anteavis combines Latin “ante” (before, preceding) and “avis” (bird), referencing its ancestral traits relative to modern birds, the only extant theropods. The specific epithet crurilongus derives from Latin “crus” (leg) and “longus” (long), alluding to the proportionally elongated tibia and metatarsus characteristic of the hindlimb.

Physical Characteristics

Anteavis crurilongus was a lightly built, agile theropod with a body plan optimized for speed and predation in a diverse Late Triassic ecosystem. The holotype specimen (PVSJ 1085) represents a young adult individual – approaching full skeletal maturity at a minimum age of 12 years, as shown by slowed bone growth (an external fundamental system in the rib cortex) and fused elements like the shoulder girdle and sacral vertebrae. Though not a maximum-sized adult, it provides a reliable snapshot of the species near the end of its growth phase.

The skull was lightly constructed, with a long, narrow snout and sharp, recurved teeth suited for grasping small prey. The maxilla featured an elongated front process and a distinct promaxillary opening, while the lower jaw (dentary) was slender and reinforced. The neck consisted of low, elongated cervical vertebrae—over twice as long as tall—lacking the air sacs seen in later theropods, giving it a flexible but sturdy posture. The back included ten dorsal vertebrae with triangular transverse processes, similar to those of early coelophysids, supporting a lightweight torso.

The forelimbs were short and delicate, with a fused scapulocoracoid forming a compact shoulder girdle and humeri showing a deep groove near the head. The hands, though incompletely preserved, suggest reduced functionality – likely used for grasping rather than weight-bearing. In contrast, the hindlimbs were strikingly elongated and powerful, a hallmark of the species. The tibia measures exactly 175 mm, with a distinctive ridge along its lower shaft, and the metatarsus (especially metatarsal III, 105.8 mm long) reached 63% of tibial length – among the highest ratios in early theropods. This cursorial design, combined with a subrectangular astragalus and unfused tarsometatarsus, indicates Anteavis was a fast runner, capable of quick bursts to chase down prey.

The pelvis was robust for its size, with a thickened iliac blade, a short preacetabular process, and a notched postacetabular shelf. The pubis nearly matched the femur in length and featured paired openings (obturator foramen and pubic fenestra), while the ischium had a prominent obturator process—features previously thought unique to more advanced theropods. These pelvic traits, along with co-ossified sacrals, confirm the specimen’s advanced ontogenetic stage.

Body size estimates center on the holotype’s preserved elements. The tibia measures exactly 175 mm (Martínez et al. 2025, Extended Data Table 2), and scaling from comparable early theropods (Eodromaeus murphi, Buriolestes schultzi) yields a total length of 1.8–2.0 m (5.9–6.6 ft) and a mass of 6–10 kg (13–22 lb), placing it at the very lower end of the “medium-sized predator” niche (30–200 kg) described in the original study. A fully mature adult may have reached ~2.2 m (7.2 ft) and 12–15 kg (26–33 lb). The paper’s ecological classification as “medium” likely reflects its role as a top predator of small vertebrates rather than absolute body mass.

Overall, Anteavis combined primitive traits (e.g., non-pneumatic vertebrae, short arms) with surprisingly advanced features (e.g., pubic fenestra, extreme hindlimb elongation), making it a morphological “bridge” in early theropod evolution—nimble, bird-like in proportions, and unexpectedly derived for its time.

Diet and Feeding Habits

Carnivore; as a small basal theropod, likely preyed on small vertebrates including early sauropodomorphs, cynodonts, and sphenodontians; inferred from dentition (recurved teeth in maxilla/dentary) and lightweight build suited to quick-snapping attacks.

Habitat and Distribution

South America, Argentina, San Juan Province, Ischigualasto Provincial Park, Ischigualasto Formation (Cancha de Bochas Member).

Paleoenvironment

Fluvial alluvial plain with semi-arid to arid conditions post-Carnian Pluvial Episode; characterized by rivers, paleosols, and seasonal aridity; co-occurring taxa include theropods (Eodromaeus murphi, Herrerasaurus ischigualastensis), sauropodomorphs (Eoraptor lunensis, Panphagia protos, Chromogisaurus novasi), pseudosuchians (Proterochampsa barruei, Lagerpeton chanarensis), and therapsids (Exaeretodon riograndensis, Hyperodapedon sp.).

Behavior and Social Structure

Inferred from clade and ontogeny: solitary or small-group predator ambushing smaller prey; elongated hindlimb suggests agile cursorial locomotion for pursuit; young adult maturity implies rapid growth in resource-variable semi-arid setting.

Discovery and Research

Holotype PVSJ 1085 collected from lower Cancha de Bochas Member; described in 2025. Phylogenetic analyses (parsimony and Bayesian) recover it as sister to Lepidus + Neotheropoda, diverging after Eodromaeus murphi; supports basal Theropoda with derived neotheropod-like traits (e.g., sacral co-ossification, pubic fenestrae).

Discovery Context

Excavated from ~231.4 Ma fluvial sediments via U-Pb dating of tuff; part of HEH biozone yielding >2,400 vertebrate specimens, reflecting high-resolution sampling over four decades.

Significance and Interesting Facts

• First Carnian theropod with neotheropod-like features outside Neotheropoda, challenging gradual diversification models.
• One of the smallest known Carnian theropods despite young adult, demonstrating extreme cursoriality early in dinosaur evolution.
• Elongated hindlimb (tibia/metatarsus proportions among highest in early dinosaurs) indicates advanced running ability.
• Histology confirms young adult at death (12+ years), providing growth insights for early theropods.
• Highlights climate-driven turnover: post-228.9 Ma humid return caused dinosaur record gap until Norian larger-bodied recovery.
• Enhances Ischigualasto as key site for early dinosaur evolution.

Conclusion

Anteavis crurilongus exemplifies the rapid, morphologically experimental early theropod radiation in late Carnian southwestern Pangaea, blending primitive and derived traits to occupy medium-predator roles amid climatic flux. Its discovery in the richly sampled Ischigualasto Formation underscores a punctuated diversification event, where small- to medium-sized dinosaurs achieved ecological prominence before a turnover erased their record, paving the way for Norian giants. This taxon not only refines theropod phylogenetics but also illuminates the environmental drivers shaping Mesozoic dinosaur dominance.