Albertosaurus libratus

Paul, G. S., 1988, Predatory Dinosaurs of the World, Predatory Dinosaurs of the World, New York: Touchstone Books, pp. 323-349 : 329-332

ALBERTOSAURUS LIBRATUS (Lambe, 1914) synonyms— Gorgosaurus libratus , Albertosaurus sternbergi TYPE— NMC 2120

BEST SPECIMENS—type, TMP 85.62.1, AMNH 5458 , FMNH PR308 ?, AMNH 5336 ?, USNM 12814 ? (juvenile?), AMNH 5664 ? (juvenile?), ROM 1247 ? (juvenile?)

SPECIMENS on display at—AMNH, FMNH, TMP, ROM, NMC, USNM

time—late Campanian of the late Late Cretaceous

horizon and distribution—Judith River Formation, western North America

main anatomical study—Lambe, 1917

	AMNH 5664	AMNH 5458
SKULL LENGTH—	678 mm	1040
TOTAL LENGTH—	5.8 m	8.6
FEMUR LENGTH—	700 mm	1025
HIP HEIGHT—	1.9 m	2.8
MASS—	700 kg	2.5 tonnes

This is the best known 0f the tyrannosaurs in terms 0f known remains, which include a number of fine skulls and skeletons of varying ages. Many more are being found. First discovered in the Canadian section of the Judith River (formerly Oldman) Formation in the late 1800s, Lawrence Lambe named and described the type skeleton in the WW I years. Unfortuantely, he characterized it as a sluggish scavenger. Just to look at the form of the Lambe’s skeleton as it was found in the ground belies this image.

It is interesting and important that two types of A. “libratus ” heads have been found. In both the preorbital horn, which is much larger than the postorbital horn, is rather cylindrical. In the type and some others the cylinder points up and forward and forms a shorter triangle. In others the horn is more horizontal, rectangular, and longer; this second kind is seen in the FMNH skull. The suture patterns of the skull roof bones also differ, and the first form has bigger, and perhaps fewer, teeth. It may be that these types represent “gracile” and “robust” forms of the sort seen in the primitive coelophysians. But the skeletons of tyrannosaurs do not show as much dual divergence as the coelophysians do, and it has not yet been shown that theropod species in general are split into two such variants. Whether the differences indicate sexes or very similar species—a la lions and tigers—is not obvious at this time. Certainly the variation in A. libratus is more than within T. rex and some other theropod species. So those specimens that may, or may not, belong to the second type have been indicated with question marks. There are more of the second type than the first, yet most of the second group seem to be juveniles.

This brings us to another point. During 1970, Dale Russell noted that the much smaller A. sternbergi , based on skeleton 5664, is probably a juvenile. Also observe (see pages 334-35) that its orbital horns are of the FMNH skull type. One thing Russell did not notice is that the teeth of this two-thirds-sized albertosaur are literally larger than those of the big specimen. This may mean that they are different species after all. Yet, a few skulls of differing sizes do seem to show that the teeth get smaller as they approach full size, and this supports the possibilty that these skulls do represent a growth series. If so, such a dramatic tooth reduction is rare. That it can happen at all is because the teeth are continually replaced by new sets. If juveniles were abandoned by their parents at half size, their big teeth may have helped them get along in what was a very hard world. Or perhaps, like the needle-sharp teeth and claws of lion cubs, the youngsters’ big teeth allowed them to protect themselves against nonrelations that wished them harm. Possibly it was just a genetic quirk of no particular meaning.

Other possible growth changes include a moderate decrease in relative limb length, especially the extremities, with increasing size. The transverse crest atop the braincase did not become large until adulthood, and the adult’s skull was relatively bigger. There does not appear to be a consistent change in the length and depth of the snout relative to the rest of the head between different specimens.

Most A. libratus skulls have been flattened from side to side by the pressure of overlaying sediments, and this obscures the breadth of the back of the skull and the forward-facing of the eyes. This is shown by AMNH S336 and the new skull TMP 85.62.1. Somewhat crushed from top to bottom, they show the truly heavy build of the back of the head and the good binocular vision. Even worse is the oblique down and forward crushing a few skulls have experienced. Some artists have innocently failed to account for this, and drawn A. libratus with a weird, sort of pig-like snout. Like all big albertosaurs, it has smaller forelimbs than Tyrannosaurus . A few adult skulls show the beginnings of the kind of orbital process that becomes so well developed in later tyrannosaurs.

The adult skeleton is restored after the nearly complete New York specimen, whose skull horns are like the type’s. FMNH PR308 is used to show the alternate skull form. The juvenile skeleton is one of the most complete dinosaur skeletons known, it lacks only a few tail tip vertebrae. I find this a particularly attractive dinosaur. With its big size, long limbs, long upturned skull, compact body, and long bulldog neck, it combines grace, speed, and power in an elegant hunting machine.

White-rhino-sized Albertosaurus was the dominant predator of the Judith River, making up about 75 percent of the big-predator fauna. The equally big Tyrannosaurus torosus was its main competitor, the smaller gracile albertosaur less so. The Judith River’s most numerous inhabitants were rhino-sized duckbills, and they were probably the main prey of A. libratus . The duckbill’s main defense was to run, perhaps into dense brush to try and lose the albertosaurs. However, the more powerful horned dinosaurs were by no means immune to the depredations of this tyrannosaur either.