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Smilosuchus
Smilosuchus
Smilosuchus
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Smilosuchus
Temporal range: Late Triassic,
221.5–205.6 Ma
Skeleton of S. gregorii
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Phytosauria
Family: †Parasuchidae
Subfamily: †Mystriosuchinae
Clade: †Leptosuchomorpha
Genus: †Smilosuchus
Long & Murry, 1995
Type species
†Machaeroprosopus gregorii
Camp, 1930
Species
  • †S. adamanensis (Camp, 1930)
  • †S. gregorii (Camp, 1930)
  • †S. lithodendrorum (Camp, 1930)
Synonyms

Synonyms of S. adamanensis:

  • Machaeroprosopus adamanensis Camp, 1930
  • Rutiodon adamanensis (Gregory, 1962)
  • Leptosuchus adamanensis (Long & Murry, 1995)

Synonyms of S. gregorii:

  • Machaeroprosopus gregorii Camp, 1930
  • Phytosaurus gregorii (Gregory, 1962)
  • Nicrosaurus gregorii (Gregory, 1962)
  • Rutiodon gregorii (Ballew 1989)
  • Leptosuchus gregorii (Irmis, 2005)

Synonyms of S. lithodendrorum:

  • Machaeroprosopus lithodendrorum Camp, 1930
  • Rutiodon adamanensis (Gregory, 1962)

Smilosuchus (from Ancient Greek σμίλη (smílē), meaning "knife, chisel", and Σοῦχος (Soûkhos), meaning "Sobek") is an extinct genus of leptosuchomorph parasuchid phytosaurs from the Late Triassic of North America. Three species have been named: the type species S. gregorii, S. adamanensis, and S. lithodendrorum, all recovered from the Norian-aged Chinle Formation of Arizona.

History

[edit]
S. gregorii skull

The type species was first described in 1995 as a replacement generic name for Leptosuchus gregorii.[1] Because of the large rostral crest it possessed, it was considered to be distinct enough from other species of Leptosuchus (all of which had smaller and more restricted crests) to be within its own genus. Some studies seem to suggest that Smilosuchus is congeneric with Leptosuchus, as the enlarged crest could have been independently developed in Leptosuchus.[2] However, newer studies support the idea that Smilosuchus is distinct from the type species of Leptosuchus, Leptosuchus crosbiensis. Phylogenetic analyses suggest that Smilosuchus is more closely related to mystriosuchins than to Leptosuchus species.[3][4]

Description

[edit]
S. gregorii and S. adamanensis compared to a human

Like all phytosaurs, Smilosuchus had the nostrils close to the top of its head. The rostral crest and nasal bulge supporting these raised nostrils was larger in Smilosuchus than in many other phytosaurs. Its skull was extremely large, up to 155 cm long, although estimates for the overall length vary from 7 m (23 ft)[5] to 12 m (39 ft). The jaws are very short and broad and the teeth are heterodont, with large tusks at the anterior of the mouth for impaling prey and more blade-like teeth for slicing flesh closer to the back of the mouth. The tusks are mounted on a bulge at the tip of the snout present in nearly all phytosaurs. Its squamosal processes are short and deep, indicating a powerful bite. This coupled with its large size (it is one of the largest known phytosaurs) suggests that it hunted large prey such as Placerias.[6]

Phylogeny

[edit]

Below is a cladogram from Stocker (2012):[4]

Phytosauria

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Smilosuchus

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Smilosuchus is an extinct of large, crocodile-like phytosaurs belonging to the , characterized by their semi-aquatic lifestyle, long snouts, armored bodies, and predatory adaptations during the period in . These reptiles, which superficially resembled modern crocodilians but were more closely related to archosaurs like s and birds, inhabited fluvial and lacustrine environments of the in what is now and surrounding regions. Known species include S. gregorii, S. adamanensis, and S. lithodendrorum, with the former representing one of the largest phytosaurs, featuring a robust up to 1.5 meters long and an estimated total body length of 7–12 meters, weighing approximately 800–1,500 kg. Fossils date to the Adamanian substage of the , around 217–218 million years ago, revealing details such as a unique sacral anatomy with three vertebrae (two primordial and one incorporated dorsosacral) that supported a strong pelvic girdle for terrestrial and aquatic locomotion. As apex predators, individuals of Smilosuchus likely fed on fish, amphibians, and smaller reptiles in slow-moving river systems, with some specimens exhibiting severe paleopathologies like on limb bones, suggesting resilience to or . The type species, S. gregorii, was originally described in 1930 as Machaeroprosopus gregorii , later assigned to Leptosuchus , before being elevated to its own in 1995, highlighting ongoing refinements in phytosaur taxonomy based on cranial and postcranial features like retracted nares and prominent narial crests.

Discovery and taxonomy

Discovery

The initial discoveries of Smilosuchus fossils took place during expeditions led by Charles Lewis Camp of the Museum of Paleontology in the Norian-aged of , USA, spanning the 1920s and 1930s. Camp's teams targeted rich bone beds in areas like the Blue Hills near St. Johns and the Placerias Quarry in County, yielding the first substantial collections of phytosaur material that formed the basis for recognizing large, advanced forms within the group. These efforts uncovered disarticulated skulls, partial skeletons, and associated elements, often preserved in fluvial and lacustrine deposits indicative of riverine environments. Key specimens include the holotype of S. gregorii (UCMP 27200), consisting of a complete , , vertebrae, , and osteoderms, collected from the Quarry during Camp's fieldwork. Another significant find is USNM 18313, a large partial skeleton from Apache County, gathered in 1948 by USGS collector Guy Hazen from strata near St. Johns. Fossils of Smilosuchus are predominantly from the Adamanian substage of the , dated to approximately 223–213 Ma, with additional material reported from contemporaneous units like the Dockum Group in and correlative beds in . Preservation typically involves fragmented or partially articulated remains, reflecting depositional dynamics in a subtropical setting. Recent studies have highlighted pathologies in USNM 18313, including healed fractures on multiple and exostotic lesions on limb bones such as the and , suggesting the individual survived significant trauma before death. These features, analyzed through CT scans, provide evidence of resilience in archosauromorphs and represent one of the most documented cases of skeletal injury healing in phytosaurs.

Etymology and naming

The genus name Smilosuchus derives from the Greek smilos, meaning "" or "", and suchus, meaning "", alluding to the blade-like posterior teeth characteristic of the taxon. It was formally established in 1995 by paleontologists Robert A. Long and Philip A. Murry as a replacement generic name for Leptosuchus gregorii, which had been originally described by Charles L. Camp in 1930 as Machaeroprosopus gregorii based on a partial and skeleton from the Upper of . Following its initial assignment to Machaeroprosopus by Camp (1930), the species was transferred to Leptosuchus during the 1950s amid broader revisions of phytosaur , reflecting similarities in cranial morphology among Late Triassic parasuchids. Ongoing debates centered on whether Smilosuchus warranted separation from Leptosuchus, particularly due to pronounced variations in the rostral crest—larger and more ornate in S. gregorii compared to species like L. crosbiensis and L. studeri—with some authors proposing synonymy based on perceived ontogenetic or dimorphic variation. These issues were resolved in favor of generic distinction by Michelle R. Stocker in , whose phylogenetic analysis recovered Smilosuchus and a restricted Leptosuchus as separate monophyletic clades within Leptosuchomorpha, supported by shared derived cranial features excluding rostral crest shape. Subsequent key revisions include William G. Parker's 2002 proposal to formally elevate material previously assigned to Leptosuchus (such as L. adamanensis) to the genus Smilosuchus based on diagnostic skull proportions and crest morphology from specimens. Cladistic analyses in 2012 further bolstered this separation, distinguishing Smilosuchus from Leptosuchus crosbiensis through autapomorphic traits like expanded squamosal processes and shape, reinforcing the genus's validity within phytosaur systematics.

Valid species

The genus Smilosuchus currently encompasses three valid species, all of which were originally described under other genera before being reassigned based on shared cranial and postcranial synapomorphies within Leptosuchomorpha. Although traditionally including three species, recent analyses (e.g., 2018) suggest S. lithodendrorum may not be closely related to S. gregorii and S. adamanensis, highlighting ongoing taxonomic debate. The , S. gregorii, was originally named Machaeroprosopus gregorii in 1930 by Charles Lewis Camp, with the specimen UCMP 27200 consisting of a complete , , partial vertebrae, , and osteoderms recovered from the Upper in . Later transferred to Leptosuchus and then to Smilosuchus in 1995. This species is diagnosed by a prominent, blade-like rostral crest extending along the dorsal surface of the premaxillae and maxillae, as well as an elongated that occupies over half the length of the . Early taxonomic placements linked S. gregorii to genera such as Phytosaurus and Pseudopalatus, but these synonymies have been rejected in favor of a distinct leptosuchomorph identity supported by phylogenetic analyses. S. adamanensis was originally described in 1930 as Machaeroprosopus adamanensis by Charles L. Camp, based on UCMP 26699, a partial from the in , and later formally transferred to Smilosuchus in 2010 by Stocker. It differs from the in possessing a shorter and broader with a reduced rostral crest height, reflecting adaptations potentially linked to varying predatory behaviors in fluvial environments. Initial debates questioned its separation from S. gregorii due to overlapping morphological variation, but this distinction was affirmed by quantitative phylogenetic data in 2018, which recovered S. adamanensis as a taxon within a monophyletic Smilosuchus. S. lithodendrorum was originally described in 1930 as Machaeroprosopus lithodendrorum by Camp from multiple specimens, including partial skulls and postcrania, collected from the Bluewater Formation (equivalent to the Chinle) in , and reassigned to Smilosuchus in 2010 by Stocker. Diagnostic features include an intermediate rostral crest morphology between S. gregorii and S. adamanensis, along with unique sacral characteristics such as the fusion of the first three sacral vertebrae into a rigid complex, as detailed in a 2017 osteological study of referred material PEFO 34000. This species further contributes to the exclusion of broader synonymies with outdated taxa like Phytosaurus, emphasizing Smilosuchus as a coherent genus.

Description

Skull

The skull of Smilosuchus gregorii is among the largest known for phytosaurs, reaching up to 155 cm in length in the specimen (UCMP 27200). The rostrum is elongate and narrow, approximately twice the length of the post-rostral portion of the , contributing to the overall slender yet robust cranial profile. Distinctive features include the external nares positioned dorsally and medially to the antorbital fenestrae, near on a raised —a derived trait in phytosaurs that contrasts with the terminal position in crocodilians. A prominent rostral crest, formed by the nasals and prefrontals, extends anteriorly from the nares, providing structural reinforcement along the dorsal surface. The antorbital fenestrae are large and positioned anterior to the retracted nares, while the infratemporal fenestrae are similarly expansive, facilitating adductor muscle attachment and reducing cranial weight. Dentition in Smilosuchus is , featuring enlarged caniniform tusks along the premaxillary and anterior maxillary margins suited for impaling prey, transitioning to ziphodont posterior teeth that are labiolingually compressed with carinae for slicing . The posterior exhibits robust squamosals with prominent descending processes that anchor large jaw adductor muscles, indicating a capacity for substantial bite force consistent with its role as an .

Postcranial skeleton

The vertebral column of Smilosuchus consists of approximately 20 presacral vertebrae, including a series of nine preserved with elongated centra and low neural spines that permitted flexibility in the neck region. The dorsal vertebrae, with at least six preserved, are robust and support the body's mass, while the in S. adamanensis comprises three fused vertebrae—a pair of primordial sacrals plus an incorporated dorsosacral from the trunk—enhancing stability for weight transfer to the hindlimbs during . The tail includes more than 40 caudal vertebrae, which are mediolaterally compressed and taper posteriorly, features consistent with aquatic adaptations in phytosaurs. The features robust forelimbs with humeri measuring around 425–435 mm in length, equipped with five digits and structured for activities such as walking or . In contrast, the hindlimbs are longer and more powerful, with femora up to 510 mm long and correspondingly elongated tibiae and fibulae, proportions that parallel those in modern crocodilians for in semi-aquatic environments. Dermal armor in Smilosuchus is extensive, comprising numerous rectangular to diamond-shaped osteoderms arranged in double paramedian rows along the dorsal surface of the back and , providing protective reinforcement against predation or injury. These osteoderms overlap and exhibit pitted ornamentation, overlapping with the underlying vertebrae for integrated structural support. Specimen USNM 18313, a nearly complete of S. gregorii, preserves evidence of including proliferative exostoses and cavitated lesions on eight limb bones (such as humeri, femora, and fibulae) and , with features like remodeling and draining tracts indicating healed trauma possibly from intraspecific , demonstrating the animal's resilience to .

Size

Smilosuchus was a large phytosaur, with adult individuals reaching total body lengths of approximately 5–8 m. The largest species, S. gregorii, attained lengths up to about 7.5 m, while S. adamanensis was smaller, around 4–6 m; S. lithodendrorum is intermediate in size based on cranial material. Skull lengths ranged from 120 cm to 155 cm, with the of S. gregorii (UCMP 27200) exceeding 1.5 m. Body mass estimates for adults range from 500 kg to approximately 2 tons, derived from allometric scaling of and limb measurements calibrated against extant crocodilians. For example, a specimen of S. gregorii with a length of 114.9 cm (USNM 18313) yielded an estimated of 500–800 kg using orbito-dorsal cranial length , indicating that larger individuals would scale proportionally higher. Volumetric models, informed by postcranial proportions from associated skeletons, support these ranges for mature specimens; estimates derived from scaling associated skeletons and cranial calibrated to modern crocodilians, with maximum sizes based on (UCMP 27200). Ontogenetic growth in Smilosuchus was rapid, as evidenced by juvenile specimens preserving incomplete cranial crests that developed more prominently in maturity. Fossil evidence, including partial juvenile skulls from the , shows accelerated crest formation during later growth stages, but no confirmed in size or morphology. Among phytosaurs, Smilosuchus ranked as one of the largest genera, but generally smaller than contemporaneous large pseudosuchians like .

Classification

Historical classification

When first described in 1930, the material now referred to Smilosuchus was assigned to the genus Machaeroprosopus within the family Phytosauridae by Charles L. Camp, based on cranial specimens from the of . This placement reflected the broad, catch-all nature of Phytosauridae at the time, which encompassed most known phytosaurs without finer subfamily distinctions. In the , Joseph T. Gregory's comprehensive review of phytosaur genera reassigned much of the material to or related taxa within Phytosauridae, emphasizing cranial morphology but noting palatal features shared with Pseudopalatus; this contributed to later proposals for a Pseudopalatidae (or Pseudopalatinae) grouping based on those palatal similarities. By the 1970s and 1980s, Karen L. Ballew's phylogenetic analysis incorporated the material into the subfamily Leptosuchinae (within Phytosauridae or emerging Parasuchidae), treating it as a of Leptosuchus (L. gregorii) due to shared traits like elongate nasal crests and rostral proportions. In the 1990s, Robert A. Long and Philip A. Murry elevated the taxon to the distinct genus Smilosuchus within Parasuchidae (sometimes termed Angistorhinidae or Rutiodontidae), distinguishing it from Rutiodon and Leptosuchus via unique antorbital fenestra shapes and jaw features; they also proposed Pseudopalatinae as a Norian-specific subfamily including Smilosuchus and Pseudopalatus. During the 2000s, it was further positioned within the clade Leptosuchomorpha (defined as the last common ancestor of Leptosuchus and Pseudopalatus plus descendants), amid debates over the monophyly of subfamilies like Rutiodontinae and Pseudopalatinae. These debates were largely resolved by 2010, when M. Ryan Stocker's analysis confirmed Smilosuchus as a monophyletic separate from Rutiodon, assigning additional like S. adamanensis based on squamosal and posttemporal characters, while solidifying its placement in Parasuchidae as a leptosuchomorph. Earlier 20th-century views had sometimes included phytosaurs like Smilosuchus within Crocodylotarsi (a stem-crocodylomorph group), but this has been refuted by consensus recognizing them as non-archosaurian archosauriforms basal to Archosauria.

Phylogenetic analyses

Phylogenetic analyses place Smilosuchus within the clade Phytosauria, specifically as a basal member of Leptosuchomorpha, a subgroup of Parasuchidae characterized by advanced cranial features such as an elongated narial opening and a prominent squamosal spine. In the 2012 cladistic analysis by Stocker, incorporating 43 taxa and 43 cranial characters analyzed via maximum parsimony, Smilosuchus is positioned crownward of basal phytosaurs like Paleorhinus but basal to more derived leptosuchomorphs such as Mystriosuchus, forming a polytomy with Leptosuchus and Rutiodon within a monophyletic Parasuchidae. These shared synapomorphies, including the posterior position of the external nares and antorbital fenestra morphology, support its placement as a transitional form during the Norian stage of the Late Triassic. Subsequent studies have refined this position, confirming Smilosuchus as more derived than early phytosaurs such as Diandongosuchus from the of , which represents a basal parasuchid outside Leptosuchomorpha, but less advanced than pseudopalatine-bearing forms like those in Mystriosuchinae. The 2018 comprehensive phylogeny by Jones and , utilizing 43 operational taxonomic units (OTUs) and 109 characters (94 discrete, 10 continuous, and 5 geometric morphometric) in TNT software with implied weighting, recovered Smilosuchus species as a paraphyletic or weakly supported monophyletic group basal to Mystriosuchini, often sister to Pravusuchus or Leptosuchus, with Bremer support values of 1–2 indicating moderate stability. This analysis highlights in semiaquatic adaptations, as Smilosuchus shares no direct synapomorphies with crocodylians despite superficial similarities in skull elongation and body plan, emphasizing the position of Phytosauria as the to all other pseudosuchians. Recent examinations of pathological specimens have further corroborated these relationships. For instance, the 2021 study of a large, osteomyelitis-afflicted of S. gregorii (USNM 18313) referred the specimen to the based on cranial morphology consistent with prior diagnoses (e.g., Jones and 2018), confirming its placement within Smilosuchus despite pathological distortions in postcranial elements. This reinforces the 's basal leptosuchomorph status without altering broader Phytosauria topology, underscoring the robustness of diagnostic characters even in compromised material.

Paleobiology

Locomotion and habitat

Smilosuchus was a semi-aquatic predator adapted to the riverine floodplains of the in western during the Adamanian substage of the , approximately 217–218 million years ago. This environment featured a warm, humid climate with seasonal monsoons, supporting perennial rivers, lakes, marshes, and lush vegetation in a tropical setting near Pangaea's western margin. The formation's fluvial systems, characterized by low- to high-sinuosity streams and episodic flooding, provided ideal habitats for aquatic and semi-aquatic taxa, with evidence of fluctuating water tables from burrows and bivalve growth bands. Fossils of Smilosuchus are known primarily from sites in and , indicating a distribution restricted to the Chinle paleoenvironment without evidence of long-distance migration. These localities, including the Petrified Forest National Park in , suggest endemism to this regional fluvial-lacustrine system influenced by seasonal precipitation patterns. On land, Smilosuchus employed a crocodile-like sprawling , limiting its terrestrial speed to an estimated 5–10 km/h for sustained movement, though it could achieve brief high walks for short bursts using its robust hindlimbs and . Its sacral , featuring three vertebrae with strong rib- articulations, enhanced pelvic stability and supported weight-bearing during occasional terrestrial forays, balancing its primarily aquatic lifestyle. In water, a powerful, laterally compressed provided for ambush predation, while forelimbs with robust and shorter radii relative to the humerus facilitated maneuvering in shallow rivers and lakes. Within these fluvial systems, Smilosuchus coexisted with aetosaurs such as Paratypothorax and early dinosaurs, occupying a top predatory niche amid a diverse assemblage adapted to wet, seasonal conditions. The 2017 analysis of its sacral structure underscores this ecological balance, revealing adaptations for efficient load distribution that permitted both aquatic dominance and viable land excursions in a dynamic, monsoon-driven .

Diet and feeding

Smilosuchus was a carnivorous that occupied the top in the fluvial ecosystems of the , targeting large terrestrial and semi-aquatic vertebrates including aetosaurs and dicynodonts. Direct evidence of predation includes bite marks on osteoderms of the aetosaur Typothorax coccinarum, consisting of subparallel pits and striated scores that match the compressed, posteriorly positioned teeth of phytosaurs; these traces indicate attacks or scavenging by large semi-aquatic predators such as Smilosuchus. Aetosaurs like Typothorax were substantial herbivores capable of defensive armored carapaces. Dental microwear textural analysis of Smilosuchus lithodendrorum reveals a generalist carnivorous-piscivorous diet, incorporating , tetrapods, and harder , with no evidence of strong dietary specialization or partitioning along the tooth row. The rougher microwear textures observed in this robust species suggest frequent processing of larger, tougher food items compared to more gracile phytosaurs. As a semi-aquatic , Smilosuchus likely utilized riverine habitats to launch surprise attacks on prey near edges, employing enlarged anterior tusk-like teeth to seize victims and posterior ziphodont teeth—laterally compressed with fine serrations—for slashing and dismembering flesh. In paleocommunities, Smilosuchus dominated as the primary large vertebrate predator, with co-occurrence data indicating interactions with dicynodonts such as Placerias and early theropods like Coelophysis. Pathological evidence from specimens, including healed fractures and bite marks on postcranial elements attributed to phytosaurs or conspecifics, points to potential cannibalistic or intraspecific during territorial disputes or seasons. The powerful jaw adduction, facilitated by robust quadrate and adductor musculature, supported effective prey capture and subdual without reliance on herbivory or extensive scavenging.

References

  1. https://ucmp.berkeley.edu/taxa/verts/archosaurs/phytosauria.php
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC5696133/
  3. https://www.facts.app/dinosaur/smilosuchus
  4. https://palaeo-electronica.org/content/pdfs/1123.pdf
  5. https://ucmp.berkeley.edu/about-ucmp/archival-collections/camp-papers/
  6. https://palaeo-electronica.org/content/2021/3385-pathological-phytosaur
  7. https://pubs.usgs.gov/of/1981/0039/report.pdf
  8. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GC008474
  9. https://www.researchgate.net/publication/229451280_A_new_taxon_of_phytosaur_Archosauria_Pseudosuchia_from_the_Late_Triassic_Norian_Sonsela_Member_Chinle_Formation_in_Arizona_and_a_critical_reevaluation_of_Leptosuchus_Case_1922
  10. https://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2010.00983.x
  11. https://pubs.geoscienceworld.org/books/book/1745/chapter/107605099/phytosauria
  12. https://pmc.ncbi.nlm.nih.gov/articles/PMC6292387/
  13. https://onlinelibrary.wiley.com/doi/10.1111/joa.12681
  14. https://biodiversitypmc.sibils.org/collections/plazi/357D771BFFBAFFB2EDC4FB72FE74FD81
  15. https://www.researchgate.net/publication/313203445_Heterodonty_in_the_European_phytosaur_Nicrosaurus_kapffi_and_its_implications_for_the_taxonomic_utility_and_functional_morphology_of_phytosaur_dentitions
  16. https://bioone.org/journals/bulletin-of-the-american-museum-of-natural-history/volume-2011/issue-352/352.1/The-Early-Evolution-of-Archosaurs--Relationships-and-the-Origin/10.1206/352.1.full
  17. https://doi.org/10.2475/ajs.260.9.652
  18. https://doi.org/10.7717/peerj.5901
  19. https://doi.org/10.26879/1123
  20. https://digitalcommons.unl.edu/usgsstaffpub/218
  21. https://palaeo-electronica.org/content/pdfs/1162.pdf
  22. https://doi.org/10.1111/joa.12681
  23. https://digitalrepository.unm.edu/eps_etds/349
  24. https://digitalrepository.unm.edu/cgi/viewcontent.cgi?article=1353&context=eps_etds
  25. https://pubs.geoscienceworld.org/palaios/article/36/1/28/586300/Bite-Marks-on-an-Aetosaur-Archosauria-Suchia
  26. https://onlinelibrary.wiley.com/doi/10.1111/pala.12515
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