They’re stunning and a little off-putting, those strange, hook-snouted, basal archosauriforms like Proterosuchus and Archosaurus. Yes, this otherwise obscure specimen lent its name to that famous clade).
The origin of the hooked premaxilla may go back as far as Orovenator in which the premaxilla tilted down and extended only slightly beyond the dentary. This same degree of down tilt can be seen in Prolacerta and Protorosaurus, sisters to the ancestors of the archosauriforms.
Next we come to two taxa without a premaxilla preserved, Youngina BPI 3859 and Youngoides UC1528. And yet, considering the break and the tip of the dentary, there had to be a small or large premaxilla overhanging as shown in Figure 1. The BPI specimen nested at the base of all archosauriformes and otherwise resembled Proterosuchus.
All of the basal euarchosauriformes had an enhanced hooked premaxilla. The premaxilla was smaller in Proterosuchus, much larger in Archosaurus. In the former the tip above the teeth was so curved over it was visible ventrally, but not in Archosaurus. Likewise in Fugusuchus the tip of the premaxilla was not visible ventrally.
The premaxilla was much deeper beneath the naris in Gargainia, giving it a shark-like appearance. The other erythrosuchids did not have such a hooked premaxilla. In Euparkeria and Vjushkovia the premaxilla did not descend.
That’s the pattern. The hooked premaxilla developed, had its heyday, then disappeared.
Why Hook the Premaxilla?
Good question. Modern analogs are not readily apparent other than, perhaps the hooked beak of certain raptorial birds and turtles known to rip flesh from their prey. With no dentary teeth to oppose the premaxillary teeth one wonders if the hooked premaxilla might have acted like a scoop or scraper. If a fish was caught at the apex of the hook and at the tip of the dentary, there was no escape. These Komodo Dragon-sized reptiles may have lain in wait underwater in shallows for prey to come close for a drink, much like alligators do. In such a case a hooked premaxilla may have indeed worked like a hook to collar wading prey and drag them underwater.
Baby and juvenile proterosuchids are not known, so a growth series showing any sort of enlargement of the hook during maturity has not been demonstrated.
Early Reptile Convergence Among Planteaters
Captorhinids, like Labidosaurus, developed a very similar premaxilla by convergence. This is a character derived from the earliest of all reptiles, Cephalerpeton, in which the anteriomedial premaxillary teeth were longer than the lateral ones, emphasizing the appearance of the hook, reaching an acme in Limnoscelis. Many of these were plant eaters. Such a hook may have been useful in gathering food or may have served as their only defense, inflicting a sharp bite in an oncoming predator. On the other branch of basal Reptilia, the early insectivorous archosauromorphs did not have a descending premaxilla.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Avanzini M and Mietto P 2008. The occurrence of the vertebrate ichnogenus Synaptichnium in the Anisian (Middle Triassic) of Southern Alps. Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 259-265.
Broom R. 1903. On a new reptile (Proterosuchus fergusi) from the Karroo beds of Tarkastad, South Africa. Annals of the South African Museum 4: 159–164.
Gower DJ and Sennikov AG 1997. Sarmatosuchus and the Early History of the Archosauria. Journal of Vertebrate Paleontology 17(1):60-73.
Sennikov AG 1994. Pervyj srednetriansovyj proteroscchid iz Vostochnoy Evropy. Doklady Akademii Nauk 336:359-661.
Tatarinov LP 1960. Otkrytie pseudozhukhii v verkhnei permi SSSR: Paleontologischeskii Zhurnal, 1960, n. 4, p. 74-80.