Smilosuchus gregorii is an extinct species of phytosaur. Phytosaurs look like crocodiles, but evolved independently. S. gregorii is an early relative of the archosaur clade. Archosauria is the group that contains birds, crocodiles, and the evolutionary line leading back to their common ancestor (1). S. gregorii is among the largest phytosaurs, with a skull length of approximately 1.5 m (2) and an estimated total length of 12 meters. It lived 221.5-205.6 million years ago in the Late Triassic and is found in the Chinle Formation of Arizona (3). S. gregorii was a semi-aquatic predator with a long snout, armored form, and sprawling posture similar to modern crocodiles.
Though S. gregorii has many physical and lifestyle similarities with modern crocodiles, their relationship is currently thought to be an example of convergent or parallel evolution rather than ancestry. That is, S. gregorii has many features similar to a modern crocodile because it filled a similar niche and evolved those characteristics independently, rather than because it is closely related to crocodiles. S. gregorii has been traditionally classified as an archosaur and a member of the Pseudosuchia clade ("false crocodiles"), but according to Nesbitt (4), S. gregorii evolved before the common ancestor of crocodiles and birds, and is better classified in Archosauriformes.
Smilosuchus gregorii is considered to be brachyrostral, a “short snouted” form of phytosaur. The strong and broad jaws of S. gregorii are specialized for capturing and holding struggling prey. Smilosuchus would have likely employed a strategy of ambush predation, waiting motionless in the water and taking unknowing prey by surprise. It could have preyed upon medium to large prey, and this feeding strategy is similar to that seen in modern crocodilians today (1).
S. gregorii has three different tooth types, each with different functions. The front set of teeth are highly enlarged, and are hypothesized to be used for killing smaller-sized prey instantly with one blow. The next set of teeth is strongly arched and very firmly anchored. The strength of these teeth would have allowed S. gregorii to seize, hold, and drown prey of larger sizes. The teeth in the back of the jaw had sharp cutting edges, used to slash flesh off prey animals. The combination of these three tooth subtypes would have allowed S. gregorii to prey upon a diversity of food sources, giving it a more flexible feeding strategy (1).
Differences between phytosaurs and crocodiles
Though phytosaurs such as Smilosuchus gregorii bear a superficial resemblance to modern crocodilians, many differences separate the two groups. One of the major differences is the position of the nares, or nostrils. In crocodilians, the nostrils are at the end of the snout. This is not the case in phytosaurs, where they are often in the middle of the snout, positioned at the level of the eyes or higher. Additionally, phytosaurs lack the bony palate that facilitates breathing in crocodiles when the mouth is full of water. In humans, the bony palate is the roof of the mouth. While it is possible that phytosaurs could have approximated this feature with a fleshy palate, like seen in the back of the mouth in humans, it would have been structurally different from crocodiles. The structure of the ankle joint is the final difference between phytosaurs and crocodilians, with the phytosaur’s ankle far more primitive than their distant modern relatives (1).
Smilosuchus gregorii was a top predator in a rich environment, as preserved by the Chinle Formation. The Chinle formation is the result of Late Triassic (221.5 to 205.6 million years ago) deposition of aquatic sediments. This could have included fluvial sediments, deposited by by river or stream, lacustrine sediments, deposited by lake, and flood sediments (1). This diverse assemblage contains a range of taxa, including early archosaurs, dinosaurs, phytosaurs, and crocodylomorphs (2). During this time period, the first representatives of many modern groups first appeared in the fossil record, such as dinosaurs and mammals (3). The climate throughout most of the Triassic was very hot and dry, though by the time S. gregorii evolved, there was a shift toward higher levels of humidity (4).
Names and Taxonomy
Until recently, Smilosuchus gregorii and all other phytosaurs were classified as the earliest group in Crurotarsi, the evolutionary branch containing modern crocodiles and their extinct relatives (1, 2). There is still support for this grouping, but new evidence suggests that this long-held relationship may not be accurate. According to Nesbitt (3), phytosaurs may not be archosaurs at all, but an evolutionary offshoot prior to the common ancestor between crocodiles and birds. This leads to the collapse of Crurotarsi as a useful grouping, as it was previously defined with the inclusion of phytosaurs and would therefore include dinosaurs as well after the phylogenetic rearrangement (1).
S. gregorii has also had its share of taxonomic reshuffling. Leptosuchus is another genus of phytosaur, and there has been much discussion on whether Leptosuchus is congeneric with Smilosuchus. If they are, all species withing the genus Leptosuchus and all species within the genus Smilosuchus would need to be combined under one genus name (4). The crested snout of S. gregorii is not possessed by any Leptosuchus members, leading to them being them being definined as differente genera (5). However, the redefining of S. gregorii is still a matter of some debate, and not accepted by all researchers. Some still classify the taxon as Leptosuchus gregorii and argue that the nose crest was independently developed (4).
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