Asilisaurus

Asilisaurus is an extinct genus of silesaurid archosaur that represents one of the earliest known members of the ornithodiran lineage, living during the early Middle Triassic (Anisian stage) approximately 243 million years ago in what is now southern Tanzania.^[1]^[2] The type and only species, A. kongwe, was formally described in 2010 based on an extensive collection of more than 20 specimens, including isolated bones, partial skeletons, and nearly complete individuals from the Lifua Member of the Manda Beds, providing a detailed view of its anatomy.^[1] This small, quadrupedal reptile, roughly the size of a Labrador retriever and weighing up to about 65 pounds, featured a long tail, gracile limbs adapted for terrestrial locomotion, leaf-shaped teeth, and a beak-like tip on the lower jaw, suggesting an herbivorous or omnivorous diet distinct from its carnivorous archosaur ancestors.^[3]^[4] As a silesaurid, Asilisaurus belongs to a group of dinosauriforms that are the closest non-dinosaurian relatives to true dinosaurs, sharing key features such as an upright limb posture and certain pelvic structures but differing in details like the presence of osteoderms in some silesaurs and specialized dentition.^[1] Phylogenetic analyses place Silesauridae as the sister taxon to Dinosauria within Ornithodira, highlighting Asilisaurus as the oldest confirmed avian-line archosaur and evidence that the lineages leading to dinosaurs and their closest kin had already begun diversifying by the early Middle Triassic, predating the oldest undisputed dinosaurs by about 10 million years.^[1]^[2] The discovery of Asilisaurus has significantly advanced understanding of early archosaur evolution, demonstrating parallel ecological radiations in ornithodirans and crocodylomorphs during the Triassic, with silesaurs occupying niches as small to medium-sized herbivores or omnivores in continental environments.^[1] Subsequent studies on its bone histology and femoral ontogeny reveal rapid growth rates similar to those of early dinosaurs, supporting the idea that such patterns were primitive for the dinosaurian stem and underscoring the group's role in the ecological buildup to dinosaur dominance later in the Triassic.^[5]^[4]

Discovery and naming

Discovery

Fossils of Asilisaurus kongwe were first discovered in 2007 during field expeditions in the Ruhuhu Basin of southwestern Tanzania, led by a team of paleontologists including Christian A. Sidor of the University of Washington.^[1] The initial finds came from locality Z34 in the Lifua Member of the Manda Beds, a fluvio-lacustrine deposit near the village of Litumba Ndyosi, where remains were recovered from a concentrated bonebed in mudstone-sandstone sediments.^[1] This assemblage included bones from at least 14 individuals, suggesting either a mass death event or an attritional accumulation over time, with articulated segments and isolated elements collected across a roughly 3 km² area.^[1] The holotype specimen, NMT RB9, consists of the anterior portion of a left dentary bearing teeth, while paratypes encompass a range of elements from multiple individuals, such as NMT RB10 (a left scapulocoracoid), NMT RB11 (a partial sacrum), and various limb bones, allowing for reconstruction of much of the skeleton.^[1] These specimens, housed at the National Museum of Tanzania (NMT) in Arusha, provided the basis for the initial anatomical and phylogenetic analysis. The genus and species Asilisaurus kongwe were formally described and named in a 2010 paper published in Nature by Sterling J. Nesbitt, Christian A. Sidor, and colleagues, establishing it as an early member of the dinosauriform clade based on shared avian-line archosaur features.^[1] Subsequent fieldwork yielded additional material, including specimens from localities Z90 and Z137 in the same formation.^[6] Notably, locality Z137 produced NMT RB159, an articulated partial skeleton that preserves previously unknown details of the axial and appendicular skeleton.^[6] In total, 18 specimens of A. kongwe are now known, comprising eight partial to nearly complete skeletons and ten isolated elements, significantly expanding the understanding of intraspecific variation within this taxon.^[6] This expanded dataset was detailed in a comprehensive anatomical monograph by Nesbitt, Langer, and Ezcurra in 2020.^[6]

Etymology

The genus name Asilisaurus is derived from asili, the Swahili word for "ancestor" or "foundation," combined with saurus, the Greek term for "lizard," highlighting its status as an early relative of dinosaurs.^[1] The species epithet kongwe translates to "ancient" or "old" in Swahili, emphasizing the animal's Middle Triassic age and its position among the earliest known avemetatarsalians.^[1] This naming convention incorporates local Tanzanian linguistic elements, as the fossils were recovered from the Manda Beds in southern Tanzania, and was formally introduced in the original description published in 2010.^[1]

Classification

Phylogenetic position

Asilisaurus is classified as a silesaurid within the clade Dinosauriformes, which is part of Avemetatarsalia, the ornithodiran lineage of archosaurs.^[7]^[8] This placement positions it as a close relative to early dinosaurs, outside but adjacent to Dinosauria in most cladistic analyses.^[8] Key synapomorphies supporting its silesaurid affinities include an edentulous premaxilla and a downturned tip of the dentary.^[8] These derived traits align Asilisaurus with other silesaurids, though it retains primitive features such as a closed acetabulum and a crurotarsal ankle joint shared with more basal dinosauromorphs; some traits exhibit homoplasy within early dinosauriform evolution.^[8] The initial 2010 phylogenetic analysis, based on limited material, recovered Asilisaurus as the sister taxon to Dinosauria within Ornithodira.^[7] A more comprehensive 2019 analysis, incorporating an expanded dataset and additional Asilisaurus specimens, refined this to place it in a closer relationship with Silesaurus opolensis within Silesauridae, highlighting high levels of homoplasy in traits shared among early dinosauriforms.^[8] This positioning underscores the rapid diversification of avemetatarsalians during the Middle Triassic, with Asilisaurus predating the oldest known dinosaurs by approximately 10–25 million years and contributing to the early radiation of ornithodiran archosaurs.^[7]^[8] Debates persist on whether silesaurids represent stem-dinosaurs or a distinct clade separate from Dinosauria, with some analyses suggesting paraphyly or inclusion within Ornithischia; however, post-2019 studies incorporating new silesaurid specimens continue to support their position as a stem group to Dinosauria.^[9]

Stratigraphic context

The fossils of Asilisaurus kongwe are known exclusively from the Lifua Member of the Manda Beds, the lowermost unit of the lower Ruhuhu Formation (Songea Group) in the Ruhuhu Basin of southern Tanzania.^[8] This member consists of a fluvio-lacustrine sequence of red to purple mudstones, sandstones, and minor conglomerates, representing floodplain and pond deposits in a rift basin setting.^[8] The type locality (Z34) and most referred specimens, including the holotype, were collected from exposures near the village of Sangasanga in the Mbinga District.^[8] The Lifua Member is dated to the Anisian stage of the Middle Triassic, approximately 247–242 million years ago, primarily on the basis of biostratigraphic correlations with the Cynognathus Assemblage Zone (Subzone C) of the Karoo Basin in South Africa, facilitated by shared taxa such as the traversodont cynodont Angonisaurus cruickshanki. Supporting evidence comes from detrital zircon U-Pb dating of correlated units, such as the basal Omingonde Formation in Namibia, which provides maximum depositional ages consistent with an early Middle Triassic (Anisian) timeframe.^[8]^[10] However, the upper portions of the Manda Beds show some biostratigraphic overlap potentially extending into the early Carnian, though Asilisaurus specimens are restricted to the lower, definitively Anisian levels.^[8] The bonebed at locality Z34, which yielded the holotype and numerous referred elements, is interpreted as a low-energy fluvial or lacustrine deposit within a floodplain pond, characterized by rubified mudstones with reworked glaebule conglomerates and associated nonmarine bivalves (Unio-like forms) and coprolites. The assemblage includes disarticulated but anatomically associated skeletal remains of multiple individuals, alongside partial skeletons of Asilisaurus and the cynodont Scalenodonta, suggesting an attritional death assemblage accumulated in semipermanent water bodies rather than through predation or mass mortality events. Surface weathering and minimal transport indicate rapid burial in a stable, vegetated environment under a warm, seasonally arid climate. The Lifua Member correlates with other Middle Triassic Gondwanan tetrapod-bearing horizons, such as the Ntawere Formation in Zambia and the Cynognathus Zone equivalents in the Karoo, based on shared cynodont and dicynodont faunas that define the Subzone C biozone. No significant stratigraphic revisions specific to Asilisaurus-bearing horizons have been proposed since 2019, maintaining the established Anisian framework.^[8]

Description

Skull

The skull of Asilisaurus kongwe is short and triangular in shape, measuring approximately 15–20 cm in length, with a large circular orbit and a notably short antorbital region.^[8] The preorbital portion is shorter than the postorbital region, giving the cranium a compact profile similar in proportions to that of Silesaurus opolensis, though with distinct differences in dentition.^[8] The orbit represents the largest cranial opening, appearing circular in lateral view and emphasizing the relatively large eyes.^[8] The premaxilla is mostly edentulous, bearing only a single tooth at its posterior margin, which contributes to a beak-like structure at the snout tip.^[8] This element forms much of the external naris, featuring a shallow narial fossa on its lateral surface.^[8] In contrast, the maxilla possesses 10–12 recurved, leaf-shaped teeth housed in a shallow groove along its length; these teeth are peg-like with small, poorly developed serrations and become more strongly recurved and flattened posteriorly.^[8] The maxilla also bears a weakly defined antorbital fossa that extends posteroventrally, nearly reaching the ventral margin, with a vertically oriented palatal process.^[8] The antorbital fenestra itself is present but lacks pronounced details beyond this weak fossa, and the naris is smaller than the fenestra.^[8] The dentary exhibits an edentulous anterior margin that tapers to a downturned, sharp point, accommodating 8–10 teeth posteriorly that are ankylosed into alveoli along a convex dorsal margin; a possible keratinous beak may have covered the anterior jaw.^[8] The nasal bone contributes to the naris margin, while the frontal and parietal bones are lightly sculpted on their dorsal surfaces, with the frontal featuring a well-defined supratemporal fossa posteriorly.^[8] A 2019 reconstruction based on the well-preserved, disarticulated skull elements from specimen NMT RB159 confirms the premaxillary edentulism and provides unprecedented detail on cranial anatomy, highlighting contrasts with the dentulous premaxilla of Silesaurus opolensis.^[8] This specimen reveals additional nuances, such as the precise positioning of the single premaxillary tooth at the base of the postnarial process.^[8]

Postcranial skeleton

The postcranial skeleton of Asilisaurus kongwe is known from multiple partial specimens, providing insights into its axial and appendicular morphology as a basal dinosauriform. These elements indicate a lightly built animal with elongated vertebral centra and relatively gracile limbs, consistent with its position among early ornithodirans. Specimens range from small juveniles to larger adults, with limb bone lengths suggesting body sizes of 1–3 meters in total length.^[8] The axial skeleton features elongated vertebrae without prominent keels, a plesiomorphic condition relative to more derived dinosauriforms. Cervical vertebrae have parallelogram-shaped centra that are approximately 1.6 times longer than tall, lacking a median keel and with ventrally oriented diapophyses; the neural arch is low and lacks infradiapophyseal or prezygapophyseal laminae.^[8] Trunk vertebrae are spool-shaped, with anterior centra 1.5 times longer than high and middle to posterior centra exhibiting a 1.5–1.7 length-to-height ratio; they possess a hyposphene but no ventral keel.^[8] The sacrum consists of two vertebrae, with the first centrum 1.6 times longer than tall and the second 1.4 times, lacking ventral grooves or ridges.^[8] Caudal vertebrae increase in elongation posteriorly, from anterior centra 1.0–1.8 times longer than tall to middle ones 2.3–2.9 times and posterior ones up to 3.4 times, with a ventral ridge present in middle caudals.^[8] The pectoral girdle includes a laterally bowed scapulocoracoid, with the dorsal margin expanding to more than twice the shaft width and a subrectangular glenoid fossa; partial coossification is observed in some specimens.^[8] The forelimb is relatively short and straight, with a humerus measuring about 90 mm in length, featuring a deltopectoral crest positioned at approximately 20% of its length from the proximal end.^[8] The radius and ulna are subequal in length (83–92 mm), with the radius rod-like and the ulna bearing a prominent olecranon process.^[8] Metacarpals are short, averaging around 18 mm, representing approximately 30% of the length of metatarsal III.^[8] In the pelvic girdle, the ilium has a closed acetabulum, a prominent supraacetabular crest, and a brevis fossa, with medial scars confirming attachment to two sacral vertebrae.^[8] The pubis measures about 121 mm long, with an anteriorly bowed shaft and distal width of 16 mm.^[8] The ischium features a proximally broader body and a dorsoventrally expanded distal end.^[8] The hindlimb shows the femur longer than the tibia (144 mm versus 124 mm), with proximal tubera and a prominent fourth trochanter on the femur; the fibula is sigmoidal and slender at 120 mm long.^[8] Metatarsal III is the longest, comprising about 50% of tibia length, while metatarsals I and V are robust but shorter than II–IV; the proximal metatarsals are imbricated, and phalanges are retained on metatarsal V, a plesiomorphic trait differing from the reduced condition in many dinosaurs.^[8] Ungual phalanges are hoof-like, suggesting a digitigrade posture.^[8]

Paleobiology

Locomotion

Asilisaurus kongwe exhibits skeletal features indicative of a primarily quadrupedal gait with potential for facultative bipedality during bursts of speed. The forelimbs are approximately 70% the length of the hindlimbs, based on measurements of the humerus (90.6 mm) and radius (83.3 mm) relative to the femur (144.4 mm) and tibia (124.2 mm), suggesting they were capable of supporting a portion of the body weight. The humerus features a nearly straight shaft with only slight posterior bending at the proximomedial corner, a morphology that implies weight-bearing capability during quadrupedal locomotion, unlike the more curved humeri of obligate bipeds. Additionally, the closed acetabulum provides enhanced pelvic stability, further supporting a quadrupedal stance by distributing weight across all four limbs.^[11] The hindlimbs of Asilisaurus display adaptations for terrestrial locomotion but lack specializations for sustained high-speed running. The ankle joint is crurotarsal, with the astragalus bearing a short anterior ascending process that articulates with the tibia, enabling a spring-like action for efficient stride propulsion similar to that in early crocodilians. However, the metatarsals are relatively short, with metatarsal III measuring about 48-50% of tibia length, indicating limited cursoriality and reduced capacity for rapid, bipedal pursuits compared to early dinosaurs.^[11] A phylogenetic analysis incorporating vertebral proportions and limb ratios supports a predominantly quadrupedal gait for Asilisaurus, with the potential for brief bipedal excursions, contrasting with the more obligately bipedal locomotion of early dinosaurs.^[12] This assessment derives from ancestral state reconstructions within Silesauridae, where the Asilisaurus node is inferred as quadrupedal, differing from the facultative bipedality in descendant silesaurids like Sacisaurus.^[12] Relative to other dinosauromorphs, Asilisaurus appears more quadrupedal than Silesaurus opolensis, which shows pillar-erect hindlimbs but forelimbs primarily adapted for support rather than propulsion, and less so than the obligately bipedal lagerpetids, which have even more reduced forelimbs. No direct trackway evidence exists for Asilisaurus, leaving interpretations reliant on osteological data.^[12]

Diet

The cranial morphology of Asilisaurus kongwe, particularly its beak-like premaxilla and reduced dentition, indicates a likely omnivorous or herbivorous diet, with adaptations for cropping vegetation or consuming small invertebrates. The premaxilla is largely edentulous along its anterior margin, forming a keratinous beak-like structure similar to that in ornithischian dinosaurs, while bearing only a single posterior tooth; this configuration, combined with a ventrally deflected anterior mandible, suggests capabilities for ground-level foraging and processing soft plant matter or small prey.^[11] The teeth, numbering 8–12 per maxillary or dentary bone, are peg-like or leaf-shaped with minimal serrations and lack strong curvature, differing from the recurved, blade-like dentition of carnivorous archosaurs and pointing toward a feeding strategy focused on less abrasive foods rather than active predation.^[1]^[11] Comparative analyses position Asilisaurus as more herbivore-leaning than contemporaneous carnivorous early dinosaurs, with its edentulous beak and leaf-shaped teeth evolving independently from similar traits in ornithischians and sauropodomorphs, implying convergent adaptations for folivory or omnivory from a plesiomorphic carnivorous ancestor.^[1] The downturned jaw and poorly occluding anterior dentition further support inferences of a diet involving soft vegetation, akin to the cropping mechanisms in early herbivorous ornithodirans, though direct evidence such as gut contents remains absent.^[11] There is no indication of specialized predation, as the modest tooth reduction and lack of robust carnassials exclude tearing of vertebrate flesh, and advanced analyses like dental microwear or stable isotopes have not yet been applied to confirm precise dietary preferences.^[1]

Growth and development

Fossil evidence from the Manda Beds bonebed reveals significant ontogenetic variation in Asilisaurus kongwe, with femoral lengths ranging from approximately 87 to 200 mm across specimens, corresponding to body sizes from juveniles around 0.5-1 m in total length to adults up to approximately 1.5-2 m.^[13]^[14] This size disparity suggests rapid postnatal growth, akin to that observed in modern crocodilians, where individuals achieve substantial increases in length within the first few years of life.^[15] Morphological variations in femoral features, such as the presence and development of scars on the proximal end, show no evidence of sexual dimorphism; instead, these differences are primarily attributed to individual age and developmental plasticity rather than fixed polymorphisms.^[14] A detailed analysis by Griffin and Nesbitt (2016) reconstructed growth sequences using bone scar evidence across 27 femora, indicating that Asilisaurus followed an ontogenetic trajectory with high intraspecific variation, a pattern shared with early dinosaurs and supporting the Middle Triassic radiation of avemetatarsalians.^[14] Growth rates in Asilisaurus were comparable to those of basal dinosaurs, implying efficient developmental strategies that facilitated diversification during this period.^[15] Histological examination of long bones from Asilisaurus specimens reveals a microstructure consistent with ectothermic to mesothermic metabolism, characterized by woven bone tissue indicative of rapid deposition but lacking annual growth marks, which suggests continuous rather than seasonal growth.^[14] The presence of multiple growth stages represented in the bonebed assemblage further points to gregarious behavior, where individuals of varying ages likely coexisted in social groups.^[11] As of 2025, no subsequent histological studies have altered these interpretations, reinforcing the view of Asilisaurus as exhibiting flexible life history traits ancestral to dinosaurian development.^[11]

Paleoecology

Habitat and environment

The fossils of Asilisaurus kongwe were recovered from the Lifua Member of the Manda Beds in the Ruhuhu Basin of southern Tanzania, a Gondwanan rift basin during the Middle Triassic (?Anisian stage) approximately 245 million years ago.^[16] This stratigraphic unit consists primarily of fluvial-lacustrine deposits, including floodplain mudrocks and sandy siltstones, indicative of riverine and pond settings within subsiding basins fed by rivers originating from rift scarps.^[16] The depositional environment reflects low-energy conditions, with the lower Lifua representing distal crevasse splay complexes and the middle to upper Lifua featuring pond and sheetwash deposits characterized by rubified, calcified mudstones and glaebule conglomerate lenses.^[16] The paleoclimate of the Lifua Member was warm and seasonally wet, with evidence of increasing mean annual temperature and rainfall from the Early to Middle Triassic, fostering vegetated floodplain depressions and semipermanent water bodies along riparian zones.^[16] Semiarid influences are suggested by pedogenic calcretes and strongly oxidized sediments, pointing to periodic dry seasons within an overall humid tropical framework.^[16] These conditions align with broader Pangean climatic patterns of the Middle Triassic, though site-specific data emphasize a localized rift basin setting without volcanic or marine signatures.^[16] Taphonomic evidence from Asilisaurus and associated tetrapod remains indicates burial in quiet-water environments, such as overbank ponds, where partially articulated skeletons were preserved alongside nonmarine bivalves and coprolites, highlighting stable, low-oxygen depositional sites conducive to fossil accumulation.^[16]

Contemporaneous fauna

The vertebrate assemblage of the Lifua Member of the Manda Beds, where Asilisaurus kongwe occurs, is characterized by a diverse array of early archosauromorphs, synapsids, and amphibians, reflecting the post-Permian recovery and early diversification of terrestrial tetrapods in Gondwana during the Middle Triassic (Anisian stage). Cynodonts, particularly traversodontids such as Scalenodon angustifrons, represent prominent herbivores in this ecosystem, with some individuals scaling to large body sizes (estimated at over 1 meter in length) and likely occupying grazing niches in floodplain environments.^[16] These synapsids coexisted with dicynodonts like Sangusaurus and Angonisaurus, which were also herbivorous and contributed to the dominance of non-archosaurian tetrapods in the food web.^[17]^[18] Archosauromorphs formed a significant component of the fauna, including rhynchosaurs such as Stenaulorhynchus stockleyi, a common quadrupedal herbivore with specialized dentition for processing tough vegetation, and early pseudosuchians like Nundasuchus songeaensis and Teleocrater rhadinus, which may have filled carnivorous or omnivorous roles as precursors to crocodylomorphs. No other dinosauromorphs beyond silesaurids like Asilisaurus are known from the formation, underscoring the rarity of avemetatarsalian-line taxa at this time and highlighting Asilisaurus as a key early representative of the lineage leading to dinosaurs. Temnospondyls, such as Stanocephalosaurus pronus, occupied semi-aquatic predatory niches, preying on fish and smaller vertebrates in riverine and lacustrine settings. Actinopterygians, though less commonly preserved, are inferred to have filled aquatic invertebrate- and detritus-feeding roles based on associated sedimentary contexts with nonmarine bivalves like Unio karooensis.^[16] The overall assemblage indicates an emerging dominance of archosaurs in southern Gondwanan ecosystems, with at least six archosaur lineages co-occurring and adapting to varied terrestrial habitats.^[1] Bonebeds containing Asilisaurus remains, comprising multiple individuals alongside other taxa, occur in the middle to upper Lifua, with coprolites in these deposits pointing to active biological processing. This biotic community highlights ecological partitioning, with Asilisaurus as a rare but phylogenetically pivotal element amid a broader synapsid-amphibian-dominated landscape.