Predatory Dinosaurs of the World Author Paul, G. S. text 1988 1988-12-31 Touchstone Books New York Predatory Dinosaurs of the World 323 349 book chapter 10.5281/zenodo.1095032 b97fd151-e825-47df-8472-cf628646d260 1095032 ALBERTOSAURUS MEGAGRACILIS new species type and best specimen— LACM 23845 (subadult?) time—latest Maastrichtian of the latest Late Cretaceous horizon and distribution—Hell Creek Formation of Montana MAIN ANATOMICAL STUDY— Molnar, 1978

SKULL LENGTH— —	Type ~ ~900 900 mm
TOTAL LENGTH— —	—7.5 m m
TONNAGE— —	~ ~ 1.7 1.7

In describing the one partial skeleton, Ralph Molnar tentatively assigned it to the contemporary A. lancensis. While looking over the remains I became convinced that they are much too big and too immature—the poorly ossified elements and moderate sized transverse crest atop the braincase suggest it was not fully grown—to belong in the much smaller species. This animal is clearly not Tyrannosaurus either. The next question is whether it is A. libratus or A. arctunguis. The LACM animal’s extremely atrophied forelimbs, down-bent nasals, very long snout, and long hind limbs strongly indicate that it is not. A new species is therefore named, one that describes its combination of large size and gracile build. In fact, this species probably got as big as A. libratus . Not enough is known to allow a skeletal restoration. A . megagracilis is similar to and may be a direct descendant of the earlier A . arctunguis, which in turn may be a direct descendant of the yet earlier A . libratus . So these three species may represent a lineage in which size and basic design remained remarkably consistent, but the legs became increasingly long, the arms ever smaller, the snout longer, and the form overall more gracile. Not only are the hand claws small, but their very small tubers for muscle insertion show that the arm was very weak. A . megagracilis is more advanced than even Tyrannosaurus rex in forelimb reduction, and this indicates that given a little more time albertosaurs would have abandoned them altogether. Albertosaurus libratus ? AMNH 5664juvenile Time it did not have, for the rarity ofA. megagracilis relative to T. rex suggests that, like many other latest Cretaceous dinosaurs, it was in trouble. If so, then the big albertosaur lineage may have been doomed even if the great extinction had not taken place. This lineage’s decline seems to have been due to the lessening numbers of their preferred prey, duckbills, in Maastrichtian time, not because the genus was intrinsically inferior to Tyrannosaurus . Aside from T. rex , the competitor of A. megagracilis was the small and equally rare A. lancensis .