Spinosaurus mirabilis

Name and Etymology

The genus name Spinosaurus derives from the Latin spina ("spine, thorn") and the Greek sauros ("lizard"), meaning "spine lizard" — a reference to the dramatically elongated neural spines forming the dorsal sail. The specific epithet mirabilis is Latin for "astonishing" or "wondrous," chosen in allusion to the remarkably enlarged nasal-prefrontal crest that distinguishes this species (Sereno et al., 2026). The research team also gave it the informal nickname "hell heron," reflecting the ecological interpretation that it hunted large fish from shallow river margins in the manner of a modern heron, albeit at a vastly greater scale.

Taxonomic Status

S. mirabilis is currently a valid species placed within Spinosauridae > Spinosaurinae > Spinosaurini > Spinosaurus. In the Bayesian phylogenetic analysis published by Sereno et al. (2026), S. mirabilis was consistently recovered as the sister taxon to S. aegyptiacus, supporting their assignment to the same genus. The genus Spinosaurus now comprises two species: S. aegyptiacus (Egypt, Morocco) and S. mirabilis (Niger).

One-Sentence Summary

A large semi-aquatic piscivorous theropod from inland Late Cretaceous river systems of central North Africa, distinguished by the tallest cranial crest known in any non-avian dinosaur.

Temporal Range

S. mirabilis dates to the Cenomanian stage of the Late Cretaceous, within an interval of approximately 100–95 Ma. The Farak Formation is assigned an Early Cenomanian age of roughly 100.2–97.2 Ma (LeBlanc, 2022). The describing paper summarizes the age as approximately 95 Ma.

Formation and Lithology

The Farak Formation lies within the Iullemmeden Basin of Niger and sits stratigraphically above the Echkar Formation within the Tégama Group. It represents a transitional sequence between the continental deposits of the underlying Tégama Group and the marine deposits of the Cenomanian–Turonian. Its lithology comprises a shaly sequence with kaolin-cemented sandstone intercalations: fine-grained sandstone and massive clays interbedded with coarse sandstone lenses, grey-greenish shales, fine-grained white micaceous sandstones, and argillaceous mottled sandstones, reaching a total thickness of approximately 190 m (Faure, 1962; Greigert, 1966; LeBlanc, 2022). The uppermost portion contains lacustrine limestones that have yielded fish, crocodylomorph, and dinosaur fossils.

Paleoenvironment

The depositional setting of the Farak Formation is interpreted as continental to transitional continental-marine. The S. mirabilis fossil localities at Jenguebi are specifically fluvial (river) deposits, situated approximately 500–1,000 km from the nearest Cenomanian marine shoreline. Co-occurring fossils include partial skeletons of rebbachisaurid and titanosaurian sauropods, all buried in river sediments, indicating a forested inland habitat dissected by rivers (Sereno et al., 2026). Freshwater bivalves (Unio, Cyrena) from the upper Farak Formation further support a continental fluvial-lacustrine paleoenvironment.

Holotype and Referred Specimens

The holotype is MNBH JEN1 (Musée National Boubou Hama, Niamey, Niger), comprising a fragmentary skull: right premaxilla, both maxillae, fused nasal crest, part of the right dentary, and five maxillary teeth. It was collected from the Jenguebi locality (Sirig Taghat, Agadez Region, Niger) from fluvial sandstones of the Farak Formation during the 2019 and 2022 expeditions.

Additional specimens include:

Specimen MNBH IGU11, an anterior dorsal centrum originally collected in 2000, was initially identified as a mid-cervical centrum of Carcharodontosaurus iguidensis (Brusatte & Sereno, 2007), then reinterpreted as Sigilmassasaurus (Evers et al., 2015) or Spinosaurus sp. (Sereno et al., 2022), before its definitive referral to S. mirabilis in 2026. The Iguidi material is temporarily on loan to Sereno's Fossil Lab at the University of Chicago, while all other specimens including the holotype are held at Abdou Moumouni University in Niger.

Diagnostic Features (Autapomorphies)

The following autapomorphies distinguish S. mirabilis from all other spinosaurids (Sereno et al., 2026):

• A massive scimitar-shaped midline cranial crest formed by the fused nasal and prefrontal bones — the tallest cranial crest in any non-avian dinosaur
• A proportionately low snout relative to other spinosaurids, with dorsal and ventral margins parallel in lateral view
• Greater spacing of the posterior maxillary teeth compared to S. aegyptiacus
• A more arched premaxillary tip than in S. aegyptiacus
• A subquadrate (rather than oval) expansion of the anterior dentary

Limitations of the Material

All known specimens are subadult or immature individuals, so the full adult morphology and size remain uncertain. Postcranial material is fragmentary; while elongated neural spines of the dorsal vertebrae imply the presence of a trunk sail, its precise shape is unknown.

Body Plan and Size

The holotype (MNBH JEN1) is a subadult estimated at approximately 8 m total body length. This is roughly 15% smaller than FSAC-KK-11888, the proposed neotype of S. aegyptiacus (also a subadult), and about 61% the size of MSMN V4047 (a partial snout), one of the largest known S. aegyptiacus specimens (Sereno et al., 2026). Adult size estimates for S. mirabilis vary: New Scientist reported approximately 10–14 m, Reuters cited roughly 12 m (40 ft) and 5–7 tonnes, and Smithsonian Magazine reported 4,500–6,350 kg (10,000–14,000 lbs). However, the original paper explicitly did not fix an adult size estimate because all specimens are immature.

Skull and Crest

The defining feature of S. mirabilis is the scimitar-shaped midline cranial crest, formed by fusion of the nasal and prefrontal bones. This crest curves posterodorsally (upward and rearward) and reaches approximately 50 cm (20 inches) in bony height alone. Its surface is covered with longitudinal and crisscrossing striations and grooves, while internally it contains a dense network of vascular canals. These features closely parallel the condition seen in extant birds with bony casques — such as the cassowary (Casuarius), helmeted guinea fowl (Numida meleagris), and maleo (Macrocephalon maleo) — in which the bony core is encased in a keratinous sheath that significantly extends the structure's total height and may have been vividly colored (Sereno et al., 2026). The crest was almost certainly a display structure, functioning in sexual selection, territorial competition, and species recognition. It appears too fragile to have served as a weapon, despite being composed of solid bone without the pneumatic air sacs found in some other dinosaur crests.

Dentition and Snout

The snout is elongated and narrow, resembling that of a crocodilian, but differs from S. aegyptiacus in the more arched premaxillary tip and lower overall snout profile with parallel dorsal and ventral margins. The upper and lower tooth rows exhibit interdigitation: the lower jaw teeth protrude outward and between the upper teeth during occlusion, forming a "fish trap" optimally designed for piercing and retaining slippery prey. The teeth are large, conical, and lack serrations — classic adaptations for piscivory shared convergently with ichthyosaurs, crocodilians, and pterosaurs (Vidal in Sereno et al., 2026). A diastema (gap) in the upper jaw accommodates three large lower jaw teeth.

Retracted Nostrils

The external nostrils are positioned further back on the snout than usual, enabling the animal to submerge most of its snout underwater while breathing normally — a key adaptation for a stalking ambush predator that needed to wait motionless for fish.

Hindlimb Proportions and Dorsal Sail

Specimen MNBH JEN4, which preserves a dentary and tibia, allows direct comparison of hindlimb proportions. The tibia-to-dentary length ratio suggests that the hindlimbs of S. mirabilis may have been proportionally slightly longer than those of S. aegyptiacus (Sereno et al., 2026). The morphology of preserved dorsal vertebrae implies the presence of a trunk sail formed by greatly elongated neural spines, though the sail's precise height and shape remain unknown due to the fragmentary nature of the postcranial material.

Piscivory

S. mirabilis exhibits some of the most extreme piscivorous adaptations of any dinosaur (Vidal in Sereno et al., 2026). Unserated large conical teeth, interdigitating upper and lower tooth rows, a long crocodilian-like snout, and retracted nostrils all point to a specialized fish-eating ecology. Co-occurring fish fauna include the large coelacanth Mawsonia, which was depicted as prey in the paleoart reconstruction accompanying the Science cover.

Semi-Aquatic Ecology and the "Hell Heron" Model

Sereno et al. (2026) described S. mirabilis as a "hell heron" — a massive wading predator that "had no problem wading on its sturdy legs into two meters of water but probably spent most of its time stalking shallower traps for the many large fish of the day." This interpretation directly challenges the "fully aquatic pursuit predator" hypothesis advanced by Ibrahim et al. (2020), which was based on the elongated tail neural spines and chevrons of S. aegyptiacus. The key counter-evidence includes: (1) the inland fluvial setting of the S. mirabilis fossils, 500–1,000 km from any marine shoreline; (2) bone density and tail morphology arguments against effective aquatic propulsion (Sereno et al., 2022); and (3) the general body plan being ill-suited for open-water swimming. Baumgart (UF Health, 2026) noted that both whales and plesiosaurs underwent complete anatomical transformation for aquatic life (developing flippers and streamlined bodies), whereas Spinosaurus retained a fundamentally terrestrial body plan.

However, the debate is not entirely settled. Fabbri et al. (2022) published bone density data suggesting subaqueous foraging capability, and Matt Lamanna (Carnegie Museum of Natural History) proposed that the truth may lie in a middle ground: "What if it's doing both? What if it's wading sometimes? What if it's getting into the water and swimming around some?" (Smithsonian Magazine, 2026).

Co-occurring Fauna

The Farak Formation has yielded a diverse vertebrate assemblage alongside S. mirabilis:

Fossil Localities

S. mirabilis is known from two localities in central Niger, Agadez Region:

Both localities expose outcrops of the Farak Formation and lie in the heart of the modern Sahara Desert.

Paleogeographic Context

During the Cenomanian (approximately 95 Ma), the Niger fossil sites lay near the equator at approximately 5°N paleolatitude. Africa was still in the process of separating from South America as the Atlantic Ocean widened. The region lay hundreds of kilometers inland from the southern Tethys Sea margin, in a warm, humid environment with extensive river systems and forests. The inland distribution of S. mirabilis demonstrates that spinosaurids were not restricted to coastal settings but also inhabited interior fluvial ecosystems.

Phylogenetic Analysis

Sereno et al. (2026) conducted a Bayesian phylogenetic analysis using an updated version of the character matrix from Sereno et al. (2022). The results are summarized as follows:

• Spinosauridae divides into two major subclades: Baryonychinae and Spinosaurinae.
• Within Spinosaurinae, the tribe Spinosaurini includes Oxalaia quilombensis, an undescribed Brazilian spinosaurine (UFMA ITA1), and the genus Spinosaurus.
• Within Spinosaurus, S. mirabilis was consistently recovered as the sister taxon to S. aegyptiacus, firmly supporting their congeneric status.

Stepwise Spinosaurid Radiation Model

The paper identifies three evolutionary phases in spinosaurid evolution:

• Jurassic Origins: Development of elongated fish-snaring skulls and early divergence into the Baryonychinae and Spinosaurinae lineages.
• Early Cretaceous Radiation: Circum-Tethyan expansion, with spinosaurids becoming dominant predators in coastal ecosystems across Africa, Europe, South America, and Asia.
• Late Cretaceous Specialization: Maximum body size achieved; shallow-water ambush specialists confined largely to northern Africa and South America as the Atlantic widened.

S. mirabilis represents the culmination of this third phase — one of the last-surviving and most derived spinosaurid species.

Alternative Hypotheses and Debate

The ecology of Spinosaurus remains one of the most actively debated topics in dinosaur paleontology. Ibrahim et al. (2020) proposed an "aquatic pursuit predator" model based on the high neural spines and chevrons of the S. aegyptiacus tail. Sereno et al. (2022) countered with bone density and hydrodynamic arguments favoring a semi-aquatic wading model, a position strongly reinforced by the inland discovery of S. mirabilis. Fabbri et al. (2022) published bone compactness data suggesting subaqueous foraging capability. David Hone (Queen Mary University of London), who was not involved in the 2026 study, acknowledged the importance of the crest discovery and inland habitat confirmation but cautioned that "we need to discover better preserved specimens to find out more about these poorly known dinosaurs" (NHM, 2026).

Confirmed

• S. mirabilis is a valid species within the genus Spinosaurus (consistently recovered as sister to S. aegyptiacus in phylogenetic analysis)
• Presence of a massive scimitar-shaped cranial crest (confirmed by three separate crest specimens)
• Interdigitating conical dentition adapted for piscivory
• Inland fluvial depositional setting (Farak Formation, 500–1,000 km from the Cenomanian shoreline)

Strongly Supported

• Keratinous sheath covering the crest, likely brightly colored (based on vascular canals, surface texture, and extant avian analogs)
• Semi-aquatic shallow-water ambush predation ecology (the "hell heron" model)
• Presence of a dorsal sail structure (inferred from dorsal vertebra morphology, though exact shape unknown)

Hypothetical / Uncertain

• Precise adult body size (all specimens are immature)
• Exact height and shape of the dorsal sail
• Degree of swimming ability (position on the fully aquatic to purely wading spectrum)
• Coloration and patterning of the crest (no direct evidence preserved)

Popular Media vs. Scientific Record

Some media reports described S. mirabilis as having a "horn" on its head; more accurately, it possesses a midline cranial crest. Additionally, the widely reported adult size figures (12 m, 5–7 tonnes) derive from media estimates and general comparisons, not from the original paper, which explicitly did not fix adult dimensions given the immaturity of all known specimens.

Specimen Summary

Stratigraphic Summary