insightLMU RESEARCH

insightLMU RESEARCH
Issue 01 · 2010
n at u r a l s c i e n c e s
SUSANNE WEDLICH
D i d T. r e x p i c k o n t h e l i t t l e g u y s ?
For most dinosaur fans, Tyrannosaurus rex is the undisputed showstopper. But recent
work by Dr. Oliver Rauhut and coworkers leaves its popular image as a perfect killing
machine looking rather tarnished. The LMU palaeontologist has been able to show that
tyrannosaurids were not always kingpins – and rather than tackling colossal herbivores, they preferred to ambush smaller fry.
The lumbering herbivores of Jurassic and Cretaceous times were certainly no pushovers.
The horns of a Triceratops or the club-like tail of an Ankylosaurus were capable of inflicting
considerable damage on even the largest predatory saurians. Indeed, rare fossil finds of
carnivorous dinosaurs with serious injuries confirm that such encounters were not always
without risk for the attacker. Pictures of epic battles between these ancient giants have long
fired the imaginations of their aficionados, and contributed to the perception of them as
the “terrible lizards” – a view enshrined in their very name. Whether it be Tyrannosaurus
rex confronting a huge horned opponent, or an Allosaurus pack circling in to administer
the coup de grâce, we usually think of carnivorous dinosaurs as perfectly adapted killers
who preyed on species that were worthy adversaries, and well able to defend themselves.
But how often did such titanic life-or-death battles actually occur? Probably quite rarely,
according to PD Dr. Oliver Rauhut. The palaeontologist, who is a member of the LMU’s
Department of Geological and Environmental Sciences and a curator at the Bavarian State
Collections for Geology and Palaeontology, summarizes the current scientific consensus
as follows. “We now believe that large-bodied carnivorous dinosaurs attacked fully grown
prey species only in exceptional cases. The very few fossils that can be taken as evidence
for attacks on large prey actually attest to failed attempts to kill them. The potential victim
either escaped or both parties were killed. Indeed, it is possible that attacks on mature prey
species were launched by young and inexperienced individuals.”
So what then did theropods, the group that includes mighty bipedal dinosaurs like Tyrannosaurus rex, actually feed on? Oliver Rauhut and his colleague Dr. David Hone, who now
01
works at the Chinese Institute of Vertebrate Paläontology and Palaeoanthropology in Peking, have re-examined the available fossil evidence with a view to answering this question.
They came to a surprising conclusion: the ferocious carnivores showed a marked preference for young prey. Careful investigation of fossilized gut contents and coprolites (fossil
faeces) revealed that most of the bones present had not yet reached the end of their growth
phase, and some of them had been swallowed whole.
“We now assume that Tyrannosaurus rex and the other large theropods were mainly active as baby killers, so to speak, picking off the younger members of their preferred prey
species“, says Oliver
Rauhut. There is nothing unusual about this
pattern of behaviour,
as even a cursory comparison with presentday carnivores shows.
Crocodiles
and
big
cats all tend to choose
the young, the old,
Skull of Proceratosaurus. The specimen had been incorrectly classified for a long time, but the first detailed
investigation of the skull, carried out under the leadership of Oliver Rauhut, then showed that this neglected
fossil is the oldest known member of the tyrannosauroids.
Source: LMU Munich
the weak or the sick
as potential victims.
Fully grown, healthy
individuals can defend themselves more effectively and simply pose too much of a risk.
“Young animals represent much less of a threat to a carnivore, so an attacker is less likely
to be injured in an encounter”, as Oliver Rauhut points out. The selective predation strategy
used by modern carnivores results in a high rate of mortality among the younger cohorts
in vertebrate species. If carnivorous dinosaurs also selectively killed young individuals, a
similar effect should be discernable. It is clear that many dinosaur species produced large
numbers of eggs, and this in itself is often taken to indicate that mortality early in the life
cycle must have been high. High levels of egg production would in turn lead one to expect
to find reletively large numbers of young individuals in the fossil record. In fact, with the exception of cases where large assemblages of fossils are found together, having been killed
instantaneously or within a short time of each other, remains of young dinosaurs are strikingly rare. As Rauhut emphasizes: “From a statistical point of view, immature dinosaurs are
significantly underrepresented in the record“. On the other hand, adult specimens of smaller species do turn up in the expected frequency, so the relative lack of immature examples
of large-bodied forms cannot be explained by assuming that smaller fossils generally have
less chance of being preserved than bigger ones.
It may do little for the perception of their fighting skills, but a predilection for tender young
prey might even have improved the quality of the large-bodied dinosaurs’ diet. This notion
02
is suggested by observations on extant carnivores. The gastric juices of crocodiles, for
example, are so active that they can completely digest practically all components of bone.
This in turn means that these reptiles can make optimal use of the mineral salts that are
such a prominent ingredient of bone for their own metabolism. Mammals also consume
bones, and dinosaurs too may well have exploited the small bones of their young prey
as valuable sources of minerals with which to build up their own skeletons. The mineral
remains found in petrified gut contents and coprolites support this idea. The notion that
theropods preferentially picked off youngsters and digested them completely also offers a
reasonable explanation for the paucity of juvenile dinosaurs in the fossil record.
STALK-AND-AMBUSH HUNTERS
“Many modern carnivores give chase until their prey is exhausted and then close in for
the kill”, says Oliver Rauhut. “In the course of the pursuit, the prospective victim may
also sustain so many
small
wounds
that
escape becomes impossible.
most
Theropods
probably
did
not hunt in this way.
Instead they relied on
ambush tactics − lying in wait, attacking
suddenly and, after a
short sprint, inflicting
Skull of Tyrannosaurus rex compared with a skull of Proceratosaurus (top right).
a serious wound, then
Souce: LMU Munich
biding their time until
the prey died.“ But when he had successfully waylaid and killed his prey, even the fiercest attacker could not relax. He had to feed quickly, otherwise he ran the risk that a larger
competitor or a hunting pack might appear and drive him from his hard-won meal. It would
certainly have been impossible to polish off a huge herbivore in a short time. Going after
smaller, younger individuals, was a better proposition, promising less risk and greater gain.
“There are now some other indications that support our hypothesis“, reports Oliver Rauhut.
“Nevertheless, we must hope that further discoveries of coprolites and fossilized gut contents will shed more light on the hunting behaviour of theropods. The fossil evidence that
has a bearing on this issue is simply too sparse at the moment.“
However, it is not beyond the bounds of possibility that crucial pieces of the puzzle are
waiting to be found in the world’s palaeontological museums. After all, an almost complete
dinosaur skull that had lain overlooked in the collection of the Natural History Museum in
London for the past 100 years recently turned out, on closer inspection, to be a sensational
find. The specimen had been incorrectly classified, but subsequent study revealed it to be
03
the first known representative of a new genus, which has been named Proceratosaurus. The
first detailed investigation of the skull, carried out under the leadership of Oliver Rauhut,
then showed that this neglected fossil is the oldest known member of the tyrannosauroids,
the group of predatory dinosaurs that would later give rise to the tyrannosaurids, which
include the iconic Tyrannosaurus rex. These bipedal carnivores shared a basic body plan,
characterized by short arms, a massive trunk ending in a powerful tail, and razor-sharp
teeth set in an extremely robust skull. The best-known members of the family, like T. rex,
appear in the Late Cretaceous, although some less imposing specimens are known from
an earlier geological era, the Jurassic. However, the origins and early diversification of
this important group of saurians remain obscure. Proceratosaurus could cast new light
on this phase of their evolution. “We first have to prepare those parts of the skull that are
still encased in the rock matrix“, says Rauhut. “It is really astonishing that this fossil has
received so little attention, because it is one of the best-preserved dinosaur skulls ever found in
Europe.“ He and his team used advanced imaging techniques to peer through the surrounding
matrix and visualize the bony elements of the skull. “Computer tomography is a marvellous
method, because it allows us to examine the interior of a fossil without disturbing or damaging the specimen in any way”, says Angela Milner who, as the curator responsible for the
specimen, personally accompanied the fossil on its journey from London to Texas, where
the tomographic scans were done.
The detailed anatomical examination of the skull and the analysis of the tomographic data
were subsequently carried out back at the Natural History Museum. This work revealed
many clear resemblances in the structure of the teeth and jaws and the cranial cavity between Proceratosaurus and the mighty Tyrannosaurus rex – although Proceratosaurus is
about 100 million years older and far less massive. Its skull is only about one-fifth the size
of that of its more famous relative, and the whole animal weighed about 40 kg. An adult Tyrannosaurus, in contrast, weighed up to 8 tons. Because the Proceratosaurus skull already
displays features characteristic of later tyrannosaurids, one can assume that its powerful
jaws were its most important weapon. “Within the tyrannosaurid group, the typical hunting
strategy probably developed first“, says Oliver Rauhut. “The later forms then brought the
requisite toolkit to perfection, gradually strengthening both skull and jaw muscles, while at
the same time growing enormously in size”. Proceratosaurus also tells us that the Tyrannosauridae evolved over a very long stretch of time, giving rise to a wide diversity of species. It
is therefore quite probable that other early members of the family remain to be discovered.
Indeed, one has already made its appearance.
Almost simultaneously with the rediscovery and reassessment of Proceratosaurus, a further member of the T. rex clade was described for the first time. This specimen, Raptorex
kriegsteini, clearly shows that the successful T. rex design was invented very early, albeit in
miniature form. In fact, Raptorex looks very like its distant relative, which it closely resembles in body form and skeletetal proportions, and it has the same number of sharp teeth in
04
its jaws. Luckily for R. kriegsteini, it went on the prowl about 60 million years before T. rex
made its entrance. For even the adult form of the older species – 2 metres long and weighing a paltry 65 kg – would have been nothing more than a tasty appetizer for the largest
tyrannosaurid – just enough to tide him over until it was time for dinner.
Priv.-Doz. Dr. Oliver Rauhut has been a Curator at the Bavarian State Collections for Palaeontology and Geology
since 2004, and is a member of the Department of Geological and Environmental Sciences at LMU. In 2004, he
received the Albert Maucher Prize in Geoscience, awarded by the German Reserach Foundation (DFG).
http://www.palaeontologie.geowissenschaften.uni-muenchen.de/personen/dozenten/rauhut/index.html
[email protected]
05