Sabre-tooth phytosaur

Smilosuchus gregorii inhabited the tropical fluvial and lacustrine plains of the Chinle Formation in southwestern North America during the Norian. The environment was a mosaic of riparian forests dominated by araucarias and cycads, multi-channel rivers with muddy margins, and shallow lakes. The climate was hot and seasonally humid, with a marked dry season. It coexisted with large metoposaurids, aetosaurs, rauisuchians, and the first dinosaurs.

Generalist ambush predator, similar in strategy to modern crocodilians. The heterodontic dentition of S. gregorii, with large anterior tusks and cutting posterior teeth, indicates capacity to capture diverse prey: fish, metoposaurids, small reptiles, and potentially large herbivores like Placerias. The short and broad snout suggests capacity to generate high bite force, favorable for capturing resistant prey. The elevated nostrils between the eyes allowed breathing with the body submerged.

Based on the extensive bone pathologies described by Lessner and Stocker (2021), S. gregorii was an active predator subject to frequent injuries, possibly resulting from intraspecific combat or fights with large prey. Variation in nasal crest size among specimens suggests possible sexual dimorphism, with males having larger crests. There is no evidence of colonial behavior, but modern large semiaquatic predators tend to be territorial along water bodies.

As a basal archosauriform, Smilosuchus likely had ectothermic or mixed metabolism, dependent on ambient temperature for thermoregulation. The dermal osteoderms covering the back and belly served protective functions and possibly solar thermoregulation. The extremely robust skull, with short and broad proportions, indicates well-developed temporal and pterygoid muscles, generating considerable bite force for an animal of its size.

Camp's foundational monograph on North American phytosaurs, containing the original description of the specimen that would become Smilosuchus gregorii. The work details cranial morphology, heterodontic dentition with large anterior tusks and posterior cutting teeth, and available postcranial elements. Camp places the material in the genus Machaeroprosopus, naming the species after geologist Herbert E. Gregory. The holotype UCMP 27200 comprises a complete skull with mandible, vertebrae, and osteoderms from the Blue Mesa Member of the Chinle Formation in Arizona. This work establishes the morphological foundation that would guide all subsequent revisions of the taxon for decades.

Leptosuchus gregorii (= Smilosuchus gregorii), historical skeletal reconstruction published by Petrified Forest National Park. Camp (1930) described holotype UCMP 27200 from material collected in the Chinle Formation of Arizona.

The Tepees at Petrified Forest National Park, Arizona: formations of the Blue Mesa Member of the Chinle Formation (220-225 Ma), the same deposit where Camp collected the S. gregorii holotype in 1928.

Comprehensive systematic revision of Late Triassic tetrapods from the southwestern United States. Long and Murry erect the new genus Smilosuchus for Leptosuchus gregorii Camp 1930, diagnosing it by eight cranial characters including the large rostral crest that elevates the nostrils. This work is the starting point of modern nomenclature for the species. The authors demonstrate that S. gregorii is distinguished from other Leptosuchus species by the combination of a particularly developed nasal crest with the short and broad snout shape and skull proportions. The monograph also provides valuable stratigraphic data situating the species mainly in the Blue Mesa Member and lower portions of the Sonsela Member of the Chinle Formation.

Scientific illustration of Smilosuchus gregorii by Nobu Tamura. Long and Murry (1995) established the genus Smilosuchus based on unique cranial characters, especially the elevated nasal bony mound between the eyes.

Chinle Formation layers at Canyonlands National Park, Utah. Long and Murry (1995) documented the stratigraphic distribution of S. gregorii mainly in the Blue Mesa Member and Sonsela Member of this formation.

Stocker describes Pravusuchus hortus, a new phytosaur from the Sonsela Member of the Chinle Formation, and conducts the first rigorous phylogenetic analysis focused on the interrelationships of leptosuchomorphs. The work critically reassesses the validity of Leptosuchus and its relationship to Smilosuchus, with direct implications for the phylogeny of S. gregorii. The analysis demonstrates that Smilosuchus is a valid taxon with robust diagnosis, positioning it as sister group to Pravusuchus within Leptosuchomorpha. The study also provides important biostratigraphic data, showing that different leptosuchomorph species diversified before the Adamanian-Revueltian turnover.

Amniote cladogram showing the position of Phytosauria (including Smilosuchus) within Archosauriformes. Stocker (2010) conducted the first rigorous phylogenetic analysis of leptosuchomorphs.

Public sculpture of a phytosaur at Kings Park, Perth. The general body morphology, with long skull and osteoderm-covered body, is characteristic of the group that Stocker (2010) analyzed phylogenetically.

CT-based study of a sacrum from Smilosuchus adamanensis (PEFO 34852) revealing three sacral vertebrae, a dorsosacral plus two primordials. This is the first documentation of a phytosaur with more than two sacral vertebrae. The authors demonstrate that dorsosacral incorporation evolved at least eight times independently among Triassic archosauriforms, suggesting convergent evolution driven by developmental mechanisms involving Hox genes. Although the specimen is S. adamanensis, the implications are direct for S. gregorii, a sister species with comparable postcranial morphology. CT imaging confirmed the primordial sacral vertebrae are not co-ossified.

Size comparison reconstruction of Smilosuchus adamanensis. Griffin et al. (2017) studied the sacral anatomy of this species, discovering three sacral vertebrae, representing an anatomical novelty for phytosaurs.

Reconstructions of two Smilosuchus species by Jeff Martz/NPS. S. gregorii (right) and S. adamanensis (left) share the postcranial morphology studied by Griffin et al. (2017).

Comprehensive review of Phytosauria covering taxonomy, phylogeny, distribution, and paleoecology. The chapter synthesizes decades of research on phytosaur diversity, addressing the placement of Smilosuchus within a broader phylogenetic context. The authors review evidence on the aquatic ecology of phytosaurs, including skull adaptations for ambush and large prey capture. The review confirms that Smilosuchus gregorii was one of the largest semiaquatic predators of the Triassic of North America, with the specialized 155 cm skull being among the largest known for phytosaurs. The geographic and stratigraphic distribution of the taxon is mapped in detail.

Reconstruction of Smilosuchus gregorii by Jeff Martz/NPS. The nasal bony mound above the eyes, absent in true crocodilians, is the most visible feature of the group analyzed by Stocker and Butler (2013).

Diversity of pan-crocodilians (Crurotarsi). Phytosaurs like Smilosuchus occupied the aquatic ambush niche in the Triassic before being replaced by crocodyliforms in the Jurassic.

Revision of Mystriosuchus westphali with a broad assessment of phytosaur generic and specific limits. The work provides comparative anatomical data relevant to diagnosing Smilosuchus and other leptosuchomorphs, clarifying morphological characters used to distinguish genera within the family. Hungerbühler's analysis specifically addresses the skull characteristics that separate large long-snouted phytosaurs from short-snouted morphs like Smilosuchus, providing context for interpreting cranial shape evolution within Phytosauria. The study contributes to understanding the diversity of foraging strategies among Triassic phytosaurs.

Triassic paleogeography showing Pangaea. Phytosaurs like Smilosuchus inhabited the fluvial plains of southwestern North America, which during this period lay a few degrees north of the equator.

Late Triassic Araucaria fossil from the Chinle Formation, Arizona (Houston Museum of Natural Science). These trees coexisted with Smilosuchus in the riparian forests of southwestern Pangaea.

Phylogenetic analysis placing the Indian phytosaur Parasuchus hislopi within the broader phytosaur tree and testing its relationships with leptosuchomorphs including Smilosuchus. The analysis identifies key synapomorphies supporting the monophyly of Parasuchidae and clarifies biogeographic patterns of phytosaur dispersal across Pangaea. The results indicate that North American leptosuchomorphs, including Smilosuchus, form a distinct clade from European and Asian phytosaurs, suggesting regional diversification during the Late Triassic. The work contributes to understanding how phytosaurs colonized different regions of Pangaea from a common ancestor.

Holotype of the phytosaur Rutiodon manhattanensis (AMNH 4991) at the American Museum of Natural History. Rutiodon is a close relative of Smilosuchus within Leptosuchomorpha, sharing the same elevated nostril position.

Reconstruction of Smilosuchus adamanensis, sister species of S. gregorii. The biogeographic analysis by Kammerer et al. (2016) demonstrated that North American leptosuchomorphs like Smilosuchus form a distinct clade.

Analysis of cranial variation in a phytosaur assemblage from the Canjilon quarry in New Mexico, testing the hypothesis of sexual dimorphism. The study examines differences in crest size, snout proportions, and overall skull shape, with implications for understanding intraspecific variation in leptosuchomorphs including Smilosuchus. The authors identify two morphotypes: one with a more developed rostral crest and more robust skull (interpreted as male) and another with a smaller crest and more gracile skull (interpreted as female). This work has direct relevance for S. gregorii, where the large rostral crest was used as a diagnostic character by Long and Murry (1995).

Skull of Parasuchus hislopi (ISI R42) in lateral view with numbered anatomical characters. Zeigler et al. (2003) analyzed cranial variation in phytosaurs from the Canjilon quarry, including differences in nasal crest size possibly linked to sexual dimorphism.

Pseudosuchia phylogeny with branch lengths indicating evolutionary rates. Phytosauria, the group of Smilosuchus, represents one of the early radiations of pseudosuchians in the Triassic.

Comprehensive phylogenetic analysis of basal Archosauriformes, including a revised matrix with extensive new data. Phytosauria including Smilosuchus is placed as a basal archosauriform outside crown Archosauria, providing a robust phylogenetic framework for interpreting the evolutionary history of the group. Ezcurra's work is the largest analytical effort to resolve the relationships of primitive archosauriforms, with important implications for understanding why phytosaurs convergently developed so many characteristics of modern crocodilians despite being only distant relatives. The analysis recovers phytosaurs as either outgroup to all other archosauriforms or as basal pseudosuchians, depending on character weighting.

Amniote cladogram showing relationships among Archosauriformes. Ezcurra (2016) conducted the most comprehensive phylogenetic analysis of basal archosauriforms, including the position of phytosaurs.

Rutiodon, Late Triassic North American phytosaur, close relative of Smilosuchus. Ezcurra's (2016) analysis clarified the placement of all Phytosauria within Archosauriformes.

Description of a large, pathological skeleton of Smilosuchus gregorii from the Chinle Formation, including extensive periosteal reactions and healed bone fractures. The study analyzes paleobiological implications of paleopathology in fossil archosauromorphs, providing data on injury patterns, survival, and life history. The specimens exhibit extensive periosteal reactions in multiple skeletal elements, indicating bone infections (osteomyelitis) that the animal survived. The analysis suggests high-risk behaviors, possibly resulting from intraspecific combat or fights with large prey. This is one of the most detailed paleopathological studies ever conducted for any phytosaur, demonstrating that S. gregorii was an active predator capable of surviving serious injuries.

Skull of phytosaur Redondasaurus bermani (CM 69727) at the Carnegie Museum of Natural History, Pittsburgh. Phytosaurs like Smilosuchus exhibited bone pathologies similar to those described by Lessner and Stocker (2021) in S. gregorii, indicating high-risk behaviors common to the group.

Skull of phytosaur Machaeroprosopus andersoni, a close relative of Smilosuchus in the Chinle Formation. The robust cranial morphology of this group is related to the aggressive behaviors documented by the bone pathologies described by Lessner and Stocker (2021).

Figure 2: Average mortalities for controls and treatments in three bioassays. Average mortalities for controls and treatments assay 1 (A), assay 2 (B), assay 3 (C). Average mortalities and Standard Error. Download full-size image DOI: 10.7717/peerj.11101/fig-2

Figure 1: Millet grains fully mycotized by B. bassiana conidia. Photo credit: Maryam Nouri-Aiin. Download full-size image DOI: 10.7717/peerj.11101/fig-1

Detailed osteological description of the postcranial skeleton of the phytosaur Machaeroprosopus mccauleyi from the Chinle Formation of Petrified Forest National Park. Provides comparative data relevant to understanding postcranial anatomy across leptosuchomorphs, including Smilosuchus. The description includes the shoulder girdle, forelimbs, pelvic girdle, hindlimbs, and osteoderms. Parker demonstrates that the postcranial skeleton of large leptosuchomorph phytosaurs is surprisingly similar to modern crocodilians, indicating functional convergence for the semiaquatic lifestyle. The work complements knowledge of S. gregorii, whose postcranium is partially known.

Pangaea paleogeography at 250 Ma showing continental positions in the early Triassic. The Chinle Formation, where Parker (2012) studied phytosaur postcranial anatomy, was deposited in southwestern Laurasia during the Late Triassic.

Geographic distribution of aetosaurs in the Triassic (present-day positions). These archosaurs coexisted with Smilosuchus in the Chinle Formation, in the fluvial environment of southwestern Pangaea studied by Parker (2012).

Review of the Triassic record of early ornithischian dinosaurs and other archosaurs, including phytosaurs, in the context of Late Triassic faunal diversity. Provides data on the co-occurrence of Smilosuchus with other archosaurs in the Chinle Formation biotas. The study documents that phytosaurs like Smilosuchus were the dominant aquatic predators in communities where the first dinosaurs were just beginning to diversify. The faunal review shows that Smilosuchus coexisted with aetosaurs, rauisuchians, metoposaurid amphibians, cynodonts, and the first dinosaur representatives in the Chinle Formation of Arizona during the Norian.

Dinosauria cladogram with integument types by group (scales, primitive feathers, modern feathers). Irmis et al. (2007) documented the co-occurrence of Smilosuchus with the first dinosaurs in the Chinle Formation, where the aquatic predator and newly diversifying dinosaurs inhabited the same ecosystem.

Araucaria fossil from the Chinle Formation, Arizona. The associated flora and fauna documented by Irmis et al. (2007) show a subtropical riparian forest ecosystem where Smilosuchus was a top predator.

Review of progress and future directions in archosaur phylogenetics, addressing the unresolved placement of Phytosauria relative to other archosauriforms. The paper discusses convergent evolution between phytosaurs and crocodilians and implications for functional morphology. Brochu highlights that convergence between Smilosuchus and modern crocodilians is one of the most extraordinary cases of convergent evolution in vertebrates, with the nostrils being the only immediately visible morphological distinction for non-specialist observers. The review also addresses how the uncertain phylogenetic position of phytosaurs complicates common ancestor reconstructions and interpretations of shared characters.

Phylogeny of Crocodyliformes, the group that replaced phytosaurs in the aquatic predator niche. Brochu (2001) analyzed convergence between phytosaurs like Smilosuchus and crocodilians, groups that evolved similar morphologies completely independently.

Evolution of hyolaryngeal elements in Archosauria. The morphological convergence between phytosaurs like Smilosuchus and crocodilians, analyzed by Brochu (2001), is even more remarkable when one observes the deep anatomical differences in the vocal and respiratory apparatus of both groups.

Comprehensive analysis of archosaur phylogenetics using 443 characters and 80 taxa, including thorough treatment of Phytosauria. Places Smilosuchus and other phytosaurs as archosauriforms outside the crown archosaur node, providing the most complete phylogenetic framework for the group. Nesbitt's monograph is the broadest analysis ever conducted on archosaur relationships, representing a fundamental reference point for all Triassic reptile paleontology. For Smilosuchus specifically, the work confirms that phytosaurs are basal archosauriforms, definitively clarifying the long-standing controversy about their phylogenetic placement.

Amniote cladogram showing relationships among Archosauriformes. Nesbitt (2011) produced the most comprehensive phylogenetic analysis of archosaurs, definitively positioning phytosaurs as basal archosauriforms outside the crown Archosauria.

Life restoration of Protome batalaria, a phytosaur from the Sonsela Member of the Chinle Formation. Nesbitt (2011) clarified the relationships of all major phytosaurs like Smilosuchus and Protome within Archosauriformes.

Description of a new aetosaur from the Late Triassic of Brazil, providing biogeographic context for Pseudosuchia diversification in Pangaea. The paper documents faunal exchange between North and South American Triassic ecosystems, relevant for understanding the biogeography of phytosaurs like Smilosuchus. The biogeographic analysis indicates that while aetosaurs reached South America during the Carnian, leptosuchomorphs like Smilosuchus remained predominantly in North America. The work contributes to understanding why Smilosuchus and its relatives did not disperse to other regions of Pangaea, unlike other pseudosuchians.

Crocodilia cladogram showing the diversification of modern crocodilians, descendants of the crocodyliforms that replaced phytosaurs in the Jurassic. Desojo et al. (2012) biogeographic analysis contextualizes how phytosaurs remained restricted to North America while other pseudosuchians dispersed across Pangaea.

Skull of phytosaur Machaeroprosopus (NMMNH P-50040) from New Mexico, representing the North American lineage that coexisted with the South American aetosaurs studied by Desojo et al. (2012). The restricted distribution of leptosuchomorphs to North America contrasts with the wide dispersal of aetosaurs across Pangaea.

Full-size T-rex animatronic from the Jurassic Park franchise, with the iconic red Jeep — Amaury Laporte · CC BY 2.0