Dryptosaurus: didactyl or tridactyl?
Skye McDavid, July 23, 2022
Dryptosaurus aquilunguis, the "eagle-clawed tearing reptile" is one of the most famous dinosaurs from Eastern North America. This tyrannosaur was immortalized in Charles R Knight's famous painting Leaping Laelaps (Dryptosaurus was originally named Laelaps, but that name was preoccupied by a mite.) Of course the crocodilian-like integument is unlikely on Dryptosaurus. While integument of derived Tyrannosauroids is controversial, the most widely accepted "compromise hypothesis" (which I personally find most likely) is that they had scaly skin with sparse feathering. But there is another inaccuracy in this painting that I want to discuss: the hands. Knight reconstructed Dryptosaurus with five digits on the hand. This is simply wrong. Most theropods, including ancestral tyrannosaurs and the basal Proceratosaurids had three manual digits. For a long time, it was assumed that Dryptosaurus had three manual digits as well.
The famous Knight painting even inspired a museum mount. Prominently displayed in the New Jersey State Museum in Trenton are two skeletons of Dryptosaurus aquilunguis in a fighting pose inspired by the painting. The mount even sacrifices anatomical accuracy to a certain extent in order to resemble the painting. Unlike the painting however, this mount has three fingers.
Three fingers on Dryptosaurus isn't an unreasonable assumption. However, unlike most theropods, the derived tyrannosaurs known as Tyrannosaurids only had two functional digits, with a third vestigial digit represented only by a metacarpal. Dryptosaurus is not a Tyrannosaurid though; it is a Dryptosaurid. The only Dryptosaurids are Dryptosaurus itself and an unnamed species from the Merchantville formation, both of which are fragmentary. However, the Dryptosaurid material we do have is sufficient to place them as very close relatives of the Tyrannosaurids. Based on this, most recent representations of Dryptosaurus are didactyl.
Delaware recently designated Dryptosaurus as its State Dinosaur, and the Delaware Museum of Nature and Science recently revamped their paleontology exhibit, with a beautiful mounted skeleton of Dryptosaurus as its centerpiece. This mount follows the more recent interpretation of Dryptosaurus as a Tyrannosaurid relative and most likely didactyl, which I appreciated. Don't get me wrong: I do love the New Jersey Mount, but the Delaware Mount is more accurate.
At this point, we don't definitively know whether Dryptosaurus was didactyl or tridactyl, as it is more basal than didactyl Tyrannosaurids, but more derived than tridactyl Tyrannosauroids. Personally I find the didactyl hypothesis more convincing, but we are far from certain. We may find out in the future, either through more discoveries of Dryptosaurus itself or close relatives that will allow us to figure it out through phylogenetic bracketing. I wish I had a more solid conclusion to give you, but when you're dealing with fragmentary fossils from millions of years ago, sometimes "probably" or "I think" is the best you can do.