Smilosuchus gregorii

Smilosuchus gregorii (Camp, 1930) Long & Murry, 1995 is a phytosaur from the Late Triassic (Norian age, approximately 227–210 Ma), identified from levels of the Blue Mesa and Sonsela Member of the Chinle Formation in Arizona, USA. It is distinguished as one of the most robust and largest species of the Leptosuchomorpha clade, known from skulls up to 1.55 m long and total body length estimates ranging from 7 to 12 m. The holotype UCMP 27200 includes a complete skull, mandible, eight vertebrae, a femur, and osteoderms; other referenced specimens include nearly complete skeletons (USNM 18313 and AMNH FR 3060) with articulated skull and postcranial elements. .

The skull of S. gregorii features a highly developed rostral crest and an elongated nasal sac , supporting raised nostrils close to the eyes, a characteristic emblematic of the genus Smilosuchus and differentiating it from Leptosuchus sensu lato. Its mouth is relatively wide and short, with heterodont dentition: large anterior fangs for impaling prey and blade-like posterior teeth for sectioning flesh. The short, deep scaly processes indicate a powerful bite , associated with a predation style that included large prey such as Placerias.

Phylogenetically, Smilosuchus gregorii was initially assigned to Leptosuchus gregorii , but Stocker's (2010) revision reassigned it to the genus Smilosuchus based on a large rostral crest and several diagnostic cranial characters . Recent phylogenetic analyses show that Smilosuchus may not be monophyletic and that the exact position of S. gregorii varies depending on the set of morphometric or discrete characters employed. . In general, it is placed as a leptosuchomorph closer to Mystriosuchini than to other Leptosuchus.

The complete skeletal analysis of USNM 18313 has been key to understanding anatomical details beyond the skull. For example, its "crocodile-normal" ankle , investigated by Parrish (1986) and Sereno (1991), supports the interpretation of semi-aquatic locomotion with the capacity for efficient terrestrial movement. .

A recent investigation in Palaeontologia Electronica (Viner, Carrano & Heckert, 2021) described a specimen with extensive evidence of bone paleopathologies : outgrowths on eight postcranial long bones compatible with osteomyelitis or metabolic disease, making this Smilosuchus the most pathological case known among Triassic archosauromorphs. The injuries suggest that the animal survived for several months or years with reduced mobility, which reinforces the idea that, despite its large size, it could support itself in water or soft ground where the weight was not fully supported by the limbs .

From a paleoecological point of view, S. gregorii fit as one of the dominant apex predators in the Chinle Formation ecosystems, hunting large prey in both fluvial and riparian environments, possibly including giant amphibians, dicynodonts and other large archosaurs.

Approximate measurements of Smilosuchus:

• Scale 1:35 Complete
• Length 237 mm
• Height 42 mm
• Width 67 mm
• Snout-tail length 257 mm