"Long before it's in the papers"
August 17, 2009

RETURN TO THE WORLD SCIENCE HOME PAGE

Chicken-hearted tyrants? Dinos may have sought easy prey

Aug. 10, 2009
Courtesy Ludwig-Maximilians-Universität München
and World Science staff

Huge, meat-eating di­no­saurs like the fa­mous Ty­ran­no­saur­us rex ap­par­ently weren’t quite the fear­less hunters they are of­ten thought of as, re­search­ers claim.

Rath­er than pick­ing on prey their own size, palaeon­tol­o­gists say, the mega-pred­a­tors may have aimed down­ward, pre­fer­ring to at­tack young di­no­saurs.

“These young an­i­mals hardly posed any risk to the preda­tors,” said Ol­i­ver Rauhut of Ludwig-Maximilians Un­ivers­ity in Mu­nich, Germany. “And their ten­der bones would have added im­por­tant min­er­als to a [preda­tor’s] di­et,” where­as adult bones might have been in­di­gest­ible. 

More to the point, he said, large adult prey were often tough, well-armed and able to in­flict con­si­der­able suf­fer­ing on an at­tacker. Pre­datory dino­saurs were not in busi­ness to be action he­roes: they just wanted to eat.

“An­i­mals such as Ty­ran­no­saur­us are of­ten seen as the per­fect ‘killing machi­nes’ with ex­tremely pow­er­ful bites, which were able to bring down even the larg­est pos­si­ble prey,” added Rauhut. Ty­ran­no­saur­us has a spe­cial place in lore about dino­saurs. Not even re­cent finds of slightly big­ger – and may­be even more ter­ri­fy­ing – spe­cies like Gigan­oto­saurus could dent the au­ra of “T-Rex.”

But the fossil record reveals a not-so-invincible ani­mal, he added, a crea­ture that like any­one else had to pick its bat­tles. “The very few fos­sils that re­flect the hunt of pred­a­to­ry di­no­saurs on large her­bi­vores tell a tale of fail­ure – the prey ei­ther got away, or both prey and pred­a­tor were killed.”

The find­ings are pub­lished in the Aug. 3 on­line is­sue of the re­search jour­nal Lethaia.

Di­rect ev­i­dence re­veal­ing the di­et of pred­a­to­ry di­no­saurs is rare. But what lit­tle there is—fos­silized stom­ach con­tents or fe­ces—points to ju­ve­niles or an­i­mals much smaller than the at­tacker, said Rauhaut, a co-author of the stu­dy.

Such find­ings should­n’t be a sur­prise, giv­en that “e­ven mod­ern pred­a­tors pre­fer old and sick an­i­mals or un­ex­pe­ri­enced young in­di­vid­u­als,” said Da­vid Hone of the In­sti­tute of Ver­te­brate Pa­le­on­tol­ogy and Pa­le­oan­thro­po­l­ogy in Bei­jing, an­oth­er co-author.

An­oth­er look at re­cent pred­a­tors re­veals an ad­di­tion­al ben­e­fit of young prey: Crocodiles, the clos­est liv­ing rel­a­tives of di­no­saurs, have ex­tremely strong acids in their stom­achs. These can com­pletely dis­solve the soft bones of young an­i­mals, adding im­por­tant nu­tri­ents to the rep­tiles’ di­et. 

The fos­sil finds of ju­ve­nile di­no­saurs that have been swal­lowed whole by theropods sup­port the idea that di­no­saurs might have prof­ited from this as well, Hone added. Theropods were mem­bers of a sub­or­der, or ev­o­lu­tion­ary fam­i­ly, of di­no­saurs that in­clud­ed T. rex and some of the oth­er most fierce car­ni­vores.

Mis­sing fos­sils lend additional plau­si­bil­ity to this account, the re­search­ers added. “Finds of di­no­saur nest­ing sites in­di­cate that they con­tained large num­bers of eggs which should have re­sulted in a high num­ber of off­spring,” said Rauhut. “But lit­tle of this is re­flected in the fos­sil rec­ord. Ju­ve­nile di­no­saurs are sur­pris­ingly rare – may­be be­cause many of them have been eat­en by pred­a­tors. Hope­fully there will soon be more ev­i­dence to help us really un­der­stand the theropods’ hunt­ing be­hav­ior.”

* * *

Huge, meat-eating dinosaurs like the famous Tyrannosaurus rex apparently weren’t quite the fearless hunters they are often thought as, researchers claim. Rather than picking on prey their own size, palaeontologists say, the mega-predators may have aimed downward, preferring to attack young dinosaurs. “These young animals hardly posed any risk to the predators,” said Oliver Rauhut of Ludwig-Maximilians University in Munich, Germany. “And their tender bones would have added important minerals to a [predator’s] diet.” In contrast, he said, larger adult prey were well-armed and could inflict a good deal of suffering on an attacker. “Animals such as Tyrannosaurus are often seen as the perfect ‘killing machines’ with extremely powerful bites, which were able to bring down even the largest possible prey,” added Rauhut. Not even recent finds of slightly bigger – and maybe even more terrifying – species like Giganotosaurus could dent the aura of “T-Rex.” “But the very few fossils that reflect the hunt of predatory dinosaurs on large herbivores tell a tale of failure – the prey either got away, or both prey and predator were killed.” The findings are published in the Aug. 3 online issue of the research journal Lethaia. Direct evidence revealing the diet of predatory dinosaurs is rare. But what little there is—fossilized stomach contents or feces—points to juveniles or animals much smaller than the attacker, said Rauhaut, a co-author of the study. Such findings shouldn’t be a surprise, given that “even modern predators prefer old and sick animals or unexperienced young individuals,” said David Hone of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, another co-author. Another look at recent predators reveals an additional benefit of young prey: Crocodiles, the closest living relatives of dinosaurs, have extremely strong acids in their stomachs. These can completely dissolve the soft bones of young animals, adding important nutrients to the reptiles’ diet. The fossil finds of juvenile dinosaurs that have been swallowed whole by theropods support the idea that dinosaurs might have profited from this as well, Hone added. Theropods were members of a suborder, or evolutionary family, of dinosaurs that included T. rex and other vicious carnivores. Missing fossils lend even more plausibility, the researchers added. “Finds of dinosaur nesting sites indicate that they contained large numbers of eggs which should have resulted in a high number of offspring,” said Rauhut. “But little of this is reflected in the fossil record: Juvenile dinosaurs are surprisingly rare – maybe because many of them have been eaten by predators. Hopefully there will soon be more evidence to help us really understand the theropods’ hunting behavior.”