MISSING MUZZLE FOUND: NEW SKULL MATERIAL REFERABLE TO APATOSAURUS AJAX (MARSH 1877) FROM THE MORRISON FORMATION OF MORRISON, COLORADO

In 1877, the first giant Jurassic dinosaurs were discovered in Morrison, Colorado by Arthur Lakes. Although North America did not produce the first sauropodomorph dinosaur, Apatosaurus ajax was the first sauropod that provided insight into the form of the most iconic family of long-necked dinosaurs. Crowning the neck with the correct skull proved challenging, particularly for the type sample of Apatosaurus, as only the rear of the skull had been identified -- until now.

From Lakes' Quarry 5, the most productive Morrison Formation sandstone quarry, we have recovered a virtually uncrushed, disarticulated but associated diplodocid maxilla and premaxillae. We assign these specimens to Apatosaurus ajax because the general proportions are most like Apatosaurus as described by Berman and McIntosh and differ from those of other diplodocids. A. ajax appears to be the only Apatosaurus species documented from the sample area.

The new Quarry 5 maxilla - premaxilla is broader across the muzzle than in Diplodocus, and agrees with that of Apatosaurus louisae. The undistorted new maxilla suggests a deeper eye/cheek region than previously reconstructed for A. louisae. Another feature linking the new maxilla to that of A. louisae is the proportionately large antorbital fenestra with an anterior end far deeper and blunter than that of Diplodocus.

The proportions of the Quarry 5 specimen are unique in the extreme depth of the anterior end of the antorbital fenestra, indicating that A. ajax was more derived in this feature than Apatosaurus louisae and to a lesser extent, Apatosaurus excelsus. A partial dentary with erupted teeth from Quarry 5 was assigned to Diplodocus lacustris in 1884; however the specimen was excavated in the same sandstone and within 10 m of the new apatosaur muzzle. Hence we refer the jaw to A. ajax. With this new partial snout, combined with the paired quadrates and braincase from the 1877 excavation at Morrison’s Quarry 10, we can more accurately reconstruct the skull of the first known species of Apatosaurus. With further preparation, these specimens from Quarry 5 help to illuminate the distinction between Apatosaurus ajax and other diplodocids.