Investigating hadrosaurian crests

One obvious use of CT scanning can be demonstrated by referring
to the extravagant range of crests seen on some hadrosaurian
ornithopods. These dinosaurs were very abundant in Late
Cretaceous times and have remarkably similarly shaped bodies;
they only really differ in the shape of their headgear, but the reason
for this difference has been a long-standing puzzle. When the first
‘hooded’ dinosaur was described in 1914, it was considered likely
that these were simply interesting decorative features. However, in
1920 it was discovered that these ‘hoods’, or crests, were composed
of thin sheaths of bone that enclosed tubular cavities or chambers of
considerable complexity.

Theories to explain the purpose of these crests abounded from the
1920s onwards. The very earliest claimed that the crest provided an
attachment area for ligaments running from the shoulders to the
neck that supported the large and heavy head. From then on,
ideas ranged from their use as weapons; that they carried highly
developed organs of smell; that they were sexually specific (males
had crests and females did not); and, the most far-sighted, that the
chambers might have served as resonators, as seen in modern birds.
During the 1940s, there was a preference for aquatic theories: that
they formed an air-lock to prevent water flooding the lungs when
these animals fed on underwater weeds.

Most of the more outlandish suggestions have been abandoned,
either because physically impossible or they do not accord with the
known anatomy. What has emerged is that the crests probably
performed a number of interrelated functions of a mainly social/
sexual type. They probably provided a visual social recognition
system for individual species; and, in addition, some elaboration of
the crests undoubtedly served a sexual display purpose. A small
number of hadrosaur crests were sufficiently robust to have been
used either in flank or head-butting activities as part of pre-mating
rituals or male–male rivalry competitions. Finally, the chambers
and tubular areas associated with the crests or facial structure are
thought to have functioned as resonators. Again, this presumed
vocal ability (found today in birds and crocodiles) can be linked to
aspects of social behaviour in these dinosaurs.

One of the greatest problems associated with the resonator theory
was gaining direct access to skull material that would allow detailed
reconstruction of the air passages within the crest, without breaking
open prized and carefully excavated specimens. CT techniques
made such internal investigations feasible. For example, some
new material of the very distinctively crested hadrosaur
Parasaurolophus tubicen was collected from Late Cretaceous
sediments in New Mexico. The skull was reasonably complete,
well preserved, and included a long, curved crest. It was CT
scanned along the length of the crest, then the scans were digitally
processed so that the space inside the crest, rather than the crest
itself, could be imaged. The rendered version of the interior cavity
revealed an extraordinary degree of complexity. Several parallel,
narrow tubes looped tightly within the crest, creating the equivalent
of a cluster of trombones! There is now little doubt that the crest
cavities in animals like Parasaurolophus were capable of acting as
resonators as part of their vocal system.

Soft tissues: hearts of stone?

In the late 1990s, a new partial skeleton of a medium-sized
ornithopod was discovered in Late Cretaceous sandstones in South
Dakota. Part of the skeleton was eroded away, but what remained
was extraordinarily well preserved, with evidence of some of the
soft tissues, such as cartilage, which are normally lost during
fossilization, still visible. During initial preparation of the specimen,
a large ferruginous (iron-rich) nodule was discovered in the centre
of the chest. Intrigued by this structure, the researchers obtained
permission to CT scan a major part of the skeleton using a large
veterinary hospital scanner. The results from these scans were
intriguing.

The ferruginous nodule appeared to have distinctive anatomical
features, and there appeared to be associated nearby structures.
The researchers interpreted these as indicating that the heart
and some associated blood vessels had been preserved within the
nodule. The nodule appeared to show two chambers (interpreted
by the researchers as representing the original ventricles of the
heart); a little above these was a curved, tube-like structure that
they interpret as an aorta (one of the main arteries leaving the
heart). On this basis, they went on to suggest that this showed that
dinosaurs of this type had a very bird-like, fully divided heart, which
supported the increasing conviction that dinosaurs were generally
highly active, aerobic animals.

As early as 1842, and the extraordinarily prophetic speculations of
Richard Owen, it had been supposed that dinosaurs, crocodiles, and
birds had a relatively efficient four-chambered (i.e. fully divided)
heart. On that basis, this discovery is not so startling. What is
astonishing is the thought that the general shape of the soft tissues
of the heart of this particular dinosaur might have been preserved
through some freak circumstance of fossilization.
Soft tissue preservation is known to occur under some exceptional
conditions in the fossil record; these generally comprise a mixture
of very fine sediments (muds and clays) that are capable of
preserving the impressions of soft tissues. Also, soft tissues, or
rather their chemically replaced remnants, can be preserved by
chemical precipitation, usually in the absence of oxygen. Neither of
these conditions apply to the ornithopod skeleton described above.
The specimen was found in coarse sandstone, and under conditions
that would have been oxygen-rich, so from a simple geochemical
perspective, conditions would appear to be very unlikely to preserve
soft tissues of any type.

Not surprisingly, the observations made by the researchers have
been challenged. Ironstone nodules are commonly reported in these
deposits and are frequently found associated with dinosaur bones.
The sedimentary conditions, the chemical environment in which
the structures might have been preserved, and the interpretation of
all the supposedly heart-like features have been contested. At
present, the status of this specimen is therefore uncertain, but
whatever else is claimed, if these features are simply those of an
ironstone nodule, then it is extraordinary that they are so heart-like.