I’m sure this is going to come as a huge shock to you all, but I was absolutely enthralled by dinosaurs as a child. The problem was, however, that the only place that they still “lived” was in my imagination, some dark corners, and maybe in a museum basement somewhere. Living crocodilians, however, were the next best thing, and one of the first movies I ever remember seeing was the John Sayles-penned 1980 horror flick Alligator, which utterly terrified and mystified me. Part of my reaction to the film probably was due to being a kid (“Why can’t alligators be living in the sewers?”), but a far greater part of my fascination was reinforced by paleontology. Even before seeing the wonderful b-film, I had come across black-and-white photos of Deinosuchus (formerly Phobosuchus) from the American Museum of Natural History, and the prospect that there were once enormous crocodilians that could swallow you hole (as in one memorable scene in an alley in Alligator) seemed very real.
Unfortunately, I don’t think I ever got to see the huge skull of Deinosuchus on display, and the skull is no longer on exhibit in the fossil halls of the AMNH. Looking up the paper describing the reconstruction the AMNH scientists had made, there seemed to be surprisingly little material from the skull of the giant crocodilian, most of the skull actually being a conjecture as to what the animal looked like in life (something like a Cuban Crocodile, Crocodylus rhombifer). This doesn’t mean that there are no giant toothy creatures in the display cases of the AMNH, however, and one of the most impressive “near-crocodile”* specimens is the skull of Machaeroprosopus gregorii.
*To the best of my knowledge no one is calling phytosaurs “near crocodiles”, but I think it’s an appropriate pop-monkier for them to help remember their overall close relationship despite their differences.
The gigantic (4′ 8″ long) skull of Machaeroprosopus gregorii, from Colbert et al., 1947. Studies of the phytosaurs Machaeroprosopus and Rutiodon. Bulletin of the AMNH ; v. 88, article 2
Part of the confusion surrounding phytosaurs is that they look so much like modern crocodiles even though they are not. Much like nimravids and true saber-toothed cats, phytosaurs and crocodiles share a close common ancestor, but there are some minor (but important) differences that separate the groups. Outside some features like a more primitive ankle structure, one of the sure-shot ways to tell you’ve got a phytosaur on your hands is if I has its nostrils back on the skull just above or just anterior-to the eyes, as is the case with Machaeroprosopus (and the obligate popular paleo-book inclusion, Rutidon). Although these animals seemed to have some measured success living in the semi-aquatic niches they occupied, they left no living descendants, dying out at the end of the Triassic. Crocodiles as we know them today, by contrast, would not appear until the Cretaceous, their ancestors occupying almost entirely terrestrial or marine niches.
What is interesting about the AMNH Colbert et al. paper is that it puts forth some hypotheses about the many “species” of Machaeroprosopus found in one particular area. Rather than representing various species evolving over time, it seems more likely to the authors that various stages of life of the giant phytosaur are represented, although the other specimens are not illustrated for comparison. Further, the authors contemplate some variations in the “robustness” of the skulls and prominence of perceived nasal ridges, which would seem to suggest some sort of sexual dimorphism. I’m a bit dubious about which would end up being the male or female (naturally the females are attributed the more refined or less-prominent characters), but they could be on to something there. In fact, such sexual differentiation based on visual cues would have been important, as recent research has suggested that phytosaurs (like many extinct archosaurs) could not have smelled, and therefore used, pheromones to find mates, define boundaries, etc. (Senter, 2002). After a quick search for papers, however, it seems that Machaeroprosopus has been all but forgotten, little to no resolution coming of the growth series of the Chinle Formation or possible sexually-selected characters. This lack of information, however, does leave open the door for anyone interested enough to pick up the trail, and I certainly hope that someone does.
Update: And I nearly forgot to say that this post was initially inspired by a story that Walt of the ever-wonderful Prehistoric Pulp mentioned today called “Gator” by Robert J. Sawyer. I would have much rather seen a Deinosuchus making its way through Manhattan sewers, but it’s a quick read if you’re into paleo-oriented SF. Be sure to check out some of Walt’s other recommendations, too.