Pachysuchus Actually a Hidden Dinosaur

Paleontologists are naming new dinosaurs at an extremely rapid pace. This past week alone, we’ve seen the announcement of Philovenator and Ichthyovenator, and the next new dinosaur is undoubtedly only a few days from publication. But we have also lost a few dinosaurs. Some of these, such as Dracorex, Anatotitan and Torosaurus, might get folded into other genera thanks to our changing understanding of how dinosaurs grew up. And as paleontologist Bill Parker pointed out at Chinleana, creatures once thought to be dinosaurs have been recategorized as very different, distantly related sorts of archosauriforms (the major group to which dinosaurs, crocodiles and many related lineages belong). Shuvosaurus, for example, was originally described as a Triassic iteration of the “ostrich mimic” dinosaurs such as Ornithomimus but turned out to be a strange, bipedal creature that was more closely related to crocodiles. And Revueltosaurus, an animal originally cast as a dinosaur because of its teeth, is now known to be more closely related to the well-armored “armadillodile” aetosaurs.

Yet reinterpretations can go the other way. Parker points out that a paper just published in Vertebrata PalAsiatica reports that a fossil thought to represent a superficially crocodile-like animal is actually part of a dinosaur jaw.

In 1947, paleontologist Yang Zhongjian—better known to many by the name C.C. Young—mentioned a fragment of a sauropodomorph dinosaur’s snout discovered in the roughly 195-million-year-old, early Jurassic deposits near Lufeng, China. He referred the specimen to Lufengosaurus, one of the many long-necked, small-skulled dinosaur cousins of the more famous sauropods. A few years later, Young changed his mind. He redescribed the battered fragment as a piece of a phytosaur skull. These archosaurs, found in older Triassic strata, generally resembled crocodiles but were actually a different group. (The easiest way to tell the difference is that the nasal openings of phytosaurs sat far back on their snouts, near their eyes.) Young named the animal Pachysuchus imperfectus, and although heavily damaged, the fragment became an important milestone for phytosaurs. The fossil was discovered in early Jurassic rock, so it lived millions of years after phytosaurs disappeared elsewhere. Young’s phytosaur seemed to represent the last of these trap-jawed aquatic predators.

Not everyone agreed with Young’s conclusion. While some paleontologists followed Young’s phytosaur ID, others said that the fragment was too uninformative to tell exactly what kind of archosaur it belonged to. The specimen was somehow lost in the collections of China’s Institute of Vertebrate Paleontology and Paleoanthropology, hindering efforts to figure out exactly what sort of animal Pachysuchus was.

Paul Barrett and Xu Xing relocated and re-examined Pachysuchus, but they didn’t see a phytosaur. Young was much closer to the mark with his original determination. The damaged skull piece exhibits many traits never seen in phytosaurs but that closely match what paleontologists have documented among sauropodomorph dinosaurs. Exactly what species of dinosaur the jaw belonged to is impossible to say—the appropriate traits for a species identification may be missing—but the best fit is certain some variety of sauropodomorph.

There were no Jurassic phytosaurs in Asia. And the proposed occurrences of Jurassic phytosaurs elsewhere are highly questionable, at best. These creatures, which lived alongside and probably preyed on early dinosaurs, were wiped out at the end of the Triassic, just before dinosaurs rose to global dominance.

Reference:

Barrett, P. M., and X. Xu. 2012. The enigmatic reptile Pachysuchus imperfectus Young, 1951 from the lower Lufeng Formation (Lower Jurassic) of Yunnan, China. Vertebrata PalAsiatica 50:151-159

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.