Fasolasuchus

Fasolasuchus tenax is an extinct genus of large-bodied carnivorous pseudosuchian archosaur belonging to the clade Loricata, specifically within the family Rauisuchidae, that inhabited South America during the Late Triassic epoch. Known primarily from fragmentary skeletal remains—including elements of the partial skull (such as the mandible) and postcrania—recovered from two specimens, it represents one of the largest terrestrial predators of its time, with estimated body lengths ranging from 8 to 10 meters and a robust build adapted for quadrupedal locomotion and active predation.^[1]^[2] The genus was formally described in 1981 by Argentine paleontologist José F. Bonaparte, based on fossils unearthed from the Upper Norian-aged Los Colorados Formation in La Rioja Province, northwestern Argentina, part of the Ischigualasto-Villa Unión Basin.^[1] These deposits, dating to approximately 227–208 million years ago, preserve a diverse array of Late Triassic vertebrates, in which Fasolasuchus likely occupied the apex predator niche, preying on early dinosaurs and other herbivores before the dominance of theropod dinosaurs in the Jurassic.^[2] The holotype specimen (PVL 3850) includes a femur measuring nearly 70 cm in length, supporting the large size estimates derived from comparisons with related rauisuchians.^[2] Phylogenetically, Fasolasuchus is positioned among paracrocodylomorph pseudosuchians, a group of extinct archosaurs more closely related to crocodylians than to dinosaurs or pterosaurs, though its exact interrelationships within Rauisuchidae remain somewhat unresolved in recent analyses due to the incomplete nature of the material.^[1] Osteohistological studies of its bones reveal rapid growth rates indicative of fibrolamellar tissue, with evidence of at least eight lines of arrested growth in osteoderms suggesting a minimum lifespan of nine years to reach skeletal maturity, highlighting its dynamic physiology as a fast-growing apex predator in a Pangean ecosystem.^[2] As one of the last and largest rauisuchians before their extinction at the end of the Triassic, Fasolasuchus provides critical insights into the faunal turnover that paved the way for dinosaurian dominance.^[1]

Discovery and naming

Etymology

The genus name Fasolasuchus was coined by Argentine paleontologist José F. Bonaparte in 1981 for a large Late Triassic pseudosuchian archosaur. It combines "Fasola," honoring Tomás H. Fasola, the fossil technician and collector who discovered the holotype during fieldwork in the Los Colorados Formation, with the Greek term suchus (Σοῦχος), referring to an Egyptian crocodile deity and commonly appended in taxonomy to denote crocodile-like reptiles.^[3] The specific epithet tenax derives from Latin, meaning "tenacious" or "holding fast," alluding to the species' robust skeletal structure that implies a strong grip and enduring predatory capability.^[4]

Type material and geological context

The fossils of Fasolasuchus were discovered in the early 1980s by a team led by Argentine paleontologist José F. Bonaparte in the Los Colorados Formation of the Ischigualasto-Villa Unión Basin, La Rioja Province, Argentina.^[1] The holotype specimen, PVL 3850, comprises a partial skeleton including premaxillae, nasals, fragmentary maxillae (one with 10 incomplete teeth), fragmentary pterygoid, posterior dentary with articular, 6 cervical vertebrae, 6 dorsal vertebrae, 8 caudal vertebrae, incomplete ischium, proximal pubis, complete right femur, radius, ulna, fibula, astragalus, calcaneum, rib fragments, and osteoderms.^[5] A referred specimen, PVL 3851, includes a left maxilla with teeth, left dentary with 5 teeth, axis, and incomplete vertebrae. No additional specimens beyond these have been referred to the genus as of November 2025.^[5] The Los Colorados Formation dates to the Norian stage of the Late Triassic, approximately 227–213 million years ago. This unit consists of red beds formed by fluvial and aeolian deposits, reflecting a semi-arid environment.^[6]

Description

Size and proportions

Fasolasuchus tenax is estimated to have attained a total body length of 8–10 meters (26–33 feet), positioning it as the largest known member of the Loricata clade. This estimation derives from scaling the approximately 70 cm femur length of the type specimen (PVL 3850) to the body proportions observed in comparably preserved rauisuchians such as Postosuchus kirkpatricki.^[2]^[1] Body mass estimates for Fasolasuchus range from 1.5 to 2 metric tons, calculated via volumetric modeling of the preserved skeletal elements and extrapolation from related pseudosuchians.^[2] The overall build of Fasolasuchus was long and low-slung, characteristic of quadrupedal loricatans, with notably elongated hindlimbs relative to the forelimbs and a robust axial skeleton supporting a terrestrial predatory lifestyle. The fragmentary cranial remains suggest a large, robust skull similar to other apex carnivorous pseudosuchians.^[2]^[1] In scale, Fasolasuchus approached the dimensions of the largest extant saltwater crocodiles (Crocodylus porosus), which can exceed 6 meters and 1 ton, but its morphology indicates stronger adaptations for terrestrial rather than semiaquatic locomotion.^[1]

Anatomical features

The preserved skeletal elements of Fasolasuchus tenax include fragmentary cranial material (premaxillae, nasals, fragmentary maxillae bearing up to 10 incomplete conical teeth, and a fragmentary pterygoid) from the holotype specimen (PVL 3850), along with three dorsal vertebrae, a partial right ilium, robust pubis and ischium, a complete right femur, dorsal ribs, and osteoderms. A referred specimen (PVL 3851) preserves additional cranial elements including a maxilla, anterior dentary, and an axis vertebra.^[7]^[8] The fragmentary cranial elements indicate a large skull, approximately 1 meter long based on comparisons, with conical teeth suited for puncturing and holding prey. The three dorsal vertebrae of the holotype feature tall neural spines that are anteroposteriorly expanded, similar to those observed in related loricatans such as Saurosuchus and Prestosuchus.^[7] The pelvic girdle includes a partial right ilium with an elongated preacetabular process extending anteriorly beyond the acetabulum, alongside a robust pubis and ischium that indicate strong hip musculature capable of supporting powerful thrusts. The complete right femur measures approximately 70 cm in length and possesses a straight shaft with a prominent fourth trochanter positioned on the posteromedial surface for attachment of the caudofemoralis muscle, facilitating strong retractive forces during locomotion.^[8] The fragmentary nature of the material, including the absence of complete skull, cervical (beyond the axis), or caudal vertebrae, restricts comprehensive anatomical reconstructions and comparisons.

Classification

Phylogenetic position

Fasolasuchus tenax is classified as a member of Loricata within the paraphyletic assemblage Rauisuchia, a group of large, carnivorous pseudosuchian archosaurs that dominated terrestrial ecosystems during the Late Triassic.^[9] Phylogenetic analyses place Fasolasuchus within Rauisuchidae in Loricata, closely related to more derived taxa such as Prestosuchus and Saurosuchus; this situates Loricata overall as the sister group to Crocodylomorpha within Pseudosuchia.^[9]^[2] The original description by Bonaparte (1981) tentatively allied it with rauisuchians, while the comprehensive cladistic analysis by Nesbitt (2011) using 412 morphological characters across 80 archosaur taxa confirmed its loricatan status and refined its placement among pseudosuchians.^[9] Subsequent studies have largely upheld this topology, though its exact interrelationships within Rauisuchidae remain somewhat unresolved due to the limited and fragmentary nature of available material, with no major revisions as of 2025.^[2] Within broader archosaur evolution, Fasolasuchus represents one of the terminal radiations of rauisuchians, occurring near the close of the Triassic before the group's extinction during the end-Triassic mass extinction event, which cleared ecological space for the rise of crocodylomorphs and dinosaurs.^[9]

Comparisons to other archosaurs

Fasolasuchus tenax exceeded the size of other rauisuchians, including Postosuchus kirkpatricki at 4–6 m in length and Saurosuchus galilei at 6–7 m, attaining up to 10 m itself.^[2]^[10]^[11] Its femur, measuring 69.9 cm in length, was more robust than that of Postosuchus (approximately 38–50 cm) while sharing a similar overall structure indicative of powerful hindlimb support.^[2]^[12] In comparison to early dinosaurs from the same Late Triassic ecosystems, Fasolasuchus outscaled Herrerasaurus ischigualastensis, which reached 3–6 m, and exhibited a more crocodile-like, quadrupedal build in contrast to the bipedal posture of these early theropods.^[2]^[13] As a loricatan pseudosuchian, Fasolasuchus shared ancestral traits with crocodylomorphs, such as the presence of an antorbital fenestra, but lacked the derived proal (forward-sliding) jaw motion characteristic of later crocodylomorphs.^[14]^[2] Fasolasuchus represents the most massive non-dinosaurian terrestrial predator of the Triassic, filling an apex predatory niche that bridged pseudosuchians and the rising dominance of large theropod dinosaurs.^[2]

Paleoecology

Habitat and environment

Fasolasuchus inhabited the Ischigualasto-Villa Unión Basin in northwestern Argentina during the Norian stage of the Late Triassic, approximately 227 to 213 million years ago. The Los Colorados Formation, from which its fossils derive, records a continental depositional environment dominated by fluvial systems within a broad alluvial plain. This setting featured moderately sinuous rivers that deposited thinning-upward cycles of coarse- to fine-grained sandstones, siltstones, and mudstones, transitioning upward into overbank deposits including ponds and crevasse splays. Sedimentary structures such as large-scale trough and tabular cross-bedding, horizontal lamination, and ripple lamination indicate active fluvial processes with episodic high-energy flows. The absence of evaporites, desiccation cracks, or eolian dunes, combined with the extensive development of floodplain deposits, points to a seasonally humid or subhumid climate rather than persistently arid conditions. The characteristic red coloration of the sediments stems from early diagenetic oxidation under oxidizing conditions, not direct evidence of aridity. This environment likely experienced periodic heavy rainfall supporting vegetation and faunal diversity, though no direct paleobotanical remains are preserved due to oxidative degradation. The formation's tetrapod assemblage reflects a Late Triassic (Norian) ecosystem in which early dinosaurs were abundant and ecologically significant, coexisting with basal archosaurs including aetosaurs, sphenosuchian crocodylomorphs, and other rauisuchids; early dinosaurs such as the sauropodomorph Riojasaurus and tetanuran theropods; derived synapsids like cynodonts; and primitive chelonians such as Palaeochersis. Taphonomic evidence shows that fossils, including those of Fasolasuchus, were typically preserved as partially or fully articulated skeletons in light gray to white bone material contrasting the red matrix, indicating limited post-mortem transport and rapid burial in low-energy fluvial or overbank contexts.

Predatory role and behavior

Fasolasuchus tenax is inferred to have occupied the role of an apex predator in the Late Triassic ecosystems of the Los Colorados Formation in northwestern Argentina, where it was the largest known terrestrial carnivore, reaching lengths of up to 10 meters.^[2] As such, it likely preyed upon large herbivores abundant in its habitat, including dicynodonts, aetosaurs, and early sauropodomorph dinosaurs like Lessemsaurus, or scavenged their remains to fulfill its carnivorous diet.^[2]^[15] Its sharp, serrated teeth were adapted for slicing flesh, supporting a predatory lifestyle focused on medium- to large-bodied vertebrates rather than small prey.^[16] The hunting strategy of Fasolasuchus is reconstructed as that of an ambush or short-burst pursuit predator, enabled by its quadrupedal locomotion and powerful hindlimbs, which allowed for rapid acceleration to subdue prey.^[2] A robust pelvis and overall skeletal build suggest it was capable of tackling sizable herbivores, potentially using its strong jaws to deliver crushing bites once prey was immobilized, akin to the predatory mechanics observed in modern crocodilians.^[2] Bone histology reveals rapid growth rates and skeletal maturity achieved by around 9 years of age, indicating a high metabolic rate consistent with an active, energetically demanding predatory existence.^[2] Behavioral inferences point to a solitary lifestyle, with no fossil evidence supporting pack hunting among rauisuchians like Fasolasuchus; instead, its size and territorial demands likely favored individual hunting ranges.^[2] Rapid somatic growth and the presence of an external fundamental system in its ribs further imply a potentially territorial behavior to secure resources in a competitive environment.^[2] A single row of dorsal osteoderms may have served for protection during confrontations or displays of dominance, though direct evidence is limited.^[16] Fasolasuchus declined alongside other rauisuchians toward the end of the Norian stage of the Late Triassic, around 210 million years ago, representing one of the last members of the group before their extinction.^[17] This extinction is attributed to ecological competition with emerging theropod dinosaurs, which began to dominate carnivorous niches, compounded by environmental changes preceding the end-Triassic mass extinction event.^[18]^[19]