New light on Iguanodon

The resurgence in palaeobiology in the 1960s, and the new insights
into dinosaurs prompted by John Ostrom’s important work,
provided a spur to reinvestigate some of the earliest discoveries.
Louis Dollo’s description of the incredible discoveries of Iguanodon
at Bernissart created the image of a giant (5 metres tall, 11 metres
long) kangaroo-like creature. It had:
powerful back legs and a massive tail that helped it to balance . . . [and]
was a plant eater . . . it grasped bunches of leaves with its long tongue,
then pulled them into its mouth to be clipped off with the beak.
The picture of Iguanodon was of an animal that was the dinosaur
equivalent of a ‘tree browser’, represented in the recent past by the
giant South American ground sloths and today by giraffes. Dollo
himself referred to Iguanodon as a ‘girafe reptilienne’. Rather
surprisingly, nearly every aspect of this vision of Iguanodon is
incorrect or seriously misleading.

Bernissart: a ravine where Iguanodon perished?
Some of the earliest work at Bernissart focused on the extraordinary
circumstances of the original discovery. The dinosaurs had been
unearthed in a coal mine at depths of between 356 and 322 metres
below the surface (Figure 18). This was unexpected, as the coal
seams being excavated were known to be Palaeozoic in age and
dinosaurs are of course unknown in rocks of such antiquity.
However, the Iguanodon skeletons were not found in the coal seams
themselves, but in a pocket of shale of Cretaceous age that cut
across the more ancient coal-bearing rocks. Mining geologists had a
commercial interest in discovering the extent of these clays, and the
degree to which they might affect coal extraction, so they began
mapping the area.

Cross sections of the mine, created during these geological
investigations, suggested that the horizontal layers of Palaeozoic
rocks (with their valuable coal seams) were occasionally cut through
very steeply by beds of Mesozoic shale (finely laminated clays). The
cross sections gave the first impression of steep-sided ravines cut
into the ancient rocks, and formed the basis for a graphic and rather
appealing notion that the Bernissart dinosaurs represented a herd
that had tumbled to their deaths (Figure 18). Dollo, himself no
geologist, was more inclined to the idea that these dinosaurs had
lived, and died, in a narrow gorge. However, the more dramatic
story had the greater impact, and was further embellished by
suggestions that they had been stampeded into the ravine by huge
predatory dinosaurs (megalosaurs), or by some freak event such as a
forest fire. This was not entirely wishful thinking: extremely rare
fragments of a large predatory dinosaur were discovered within
the Iguanodon-bearing beds; and charcoal-like lumps of coal
were recovered from some of the rubble-like deposits found in the
region between the coal-bearing rocks and the dinosaur-bearing
shaly beds.

The discoveries at Bernissart presented a huge logistic challenge
in the 1870s and early 1880s. Complete skeletons of dinosaurs
measuring up to 11 metres in length had been discovered at
the bottom of a deep mine; they were the focus of worldwide
interest at the time, but how were they to be excavated and
studied? A cooperative venture was arranged between the Belgian
government, funding the scientists and technicians of the Royal
Natural History Museum in Brussels, and the miners and engineers
at the colliery in Bernissart. Each skeleton was carefully exposed
and its position in the mine recorded systematically on plan
diagrams. Every skeleton was divided into manageable blocks
approximately 1 metre square. Each block, protected by a jacket of
plaster of Paris, was carefully numbered and recorded on plan
drawings (Figure 19) before being lifted and transported to Brussels.
Back in Brussels, the blocks were reassembled from the records,
rather like a gigantic jigsaw puzzle. The plaster was painstakingly
removed to reveal the bones of each skeleton. At this point an artist,
Gustave Lavalette, specially commissioned for the project, drew the
skeleton in its death pose before any further preparation or
extraction was undertaken (Figure 20). Some skeletons were
completely extracted from the shale and mounted to create a magnificent display that can be seen to this day at the (renamed)
Royal Institute of Natural Sciences, in Parc Léopold, Brussels. Other
skeletons were cleared of the shale matrix on one side only and
arranged in their burial pose on wooden scaffolding supporting vast
banks of plaster. This display mimics their entombed positions
when they were first discovered in the mine at Bernissart.
The original plans of each excavation, and some crude geological
sections and sketches of the discoveries, are preserved in the
archives of the Royal Institute in Brussels. This information has
been ‘mined’, this time for clues concerning the geological nature of
the dinosaur burial site.

The geology of the coal-mining area of the Mons Basin, in which
lies the village of Bernissart, had been the subject of study before
dinosaurs were ever discovered. A major review in 1870 pointed out that the coal-bearing strata of the Mons Basin were pock-marked by
‘cran’ (naturally formed subterranean pits). Each ‘cran’ was of
limited extent and filled with shales. It was concluded that
these had formed by the dissolution of Palaeozoic rocks deep
underground. The roofs of such caverns collapse periodically under
the sheer weight of the overlying rocks, so the spaces become filled
with whatever lies above: in this case soft clays or shales. The
collapse of such sediments had been recorded locally in the Mons
area as rather alarming, earthquake-like shocks. By amazing
coincidence, a minor ‘earthquake’ of this type took place while the
dinosaurs were being excavated in August 1878 at Bernissart. Minor
collapses in the galleries were noted, as well as flooding, but the
miners and scientists were soon able to resume their work once
the flood water had been pumped out.
Despite all the local geological knowledge, it is very curious that the
scientists from the Museum in Brussels incorrectly interpreted the
geological nature of the ‘cran’ at Bernissart. The mining engineers
produced crude geological sections from the tunnels that yielded
the dinosaurs. These showed that immediately beyond the
coal-bearing seams there was a section of 10–11 metres of breccia
(broken beds containing irregular blocks of limestone and coal
mixed with silt and clay, the ‘collapsed coal-bearing rocks’ of Figure
18) before entering steeply dipping, but more regularly stratified,
shales that yielded the fossils. Toward the middle of the ‘cran’ the
clay beds were horizontally bedded, and as the tunnel approached
the opposite side of the ‘cran’ the beds once again became steeply
tilted in the opposite direction before passing again into a
brecciated region and finally re-entering the coal-bearing deposits.
The symmetry of the geology across the ‘cran’ is exactly what would
be expected if overlying sediments had slumped into a large cavity.
The sediments in which the dinosaurs are embedded also directly
contradict the ravine or river-valley interpretations. Finely stratified
shales containing the fossils are normally deposited in low-energy,
relatively shallow-water environments, probably equivalent to a
large lake or lagoon. There is simply no evidence for catastrophic
deaths caused by herds of animals plunging into a ravine. In fact,
the dinosaur skeletons were found in separate layers of sediment
(along with fish, crocodiles, turtles, thousands of leaf impressions,
and even rare insect fragments), proving that they definitely did not
all die at the same time and therefore could never have been part of
a single herd of animals.
Study of the orientation of the fossil skeletons within the mine
suggests that dinosaur carcasses were washed into the burial
area on separate occasions and from different directions. It was
as if the direction of flow of the river that carried their carcasses
had changed from time to time, exactly as happens in large,
slow-moving river systems today.
So, as early as the 1870s, it was clearly understood that there
were neither ‘ravines’ nor ‘river valleys’ in which the dinosaurs
at Bernissart might have perished. It is fascinating how the
dramatic discovery of dinosaurs at Bernissart seems to have
demanded an equally dramatic explanation for their deaths,
and that such fantasies were uncritically adopted even though
they flew in the face of the scientific evidence available at
the time.

The image of Iguanodon as a gigantic kangaroo-style creature has
become iconic because of the generous distribution of full-sized
skeletal casts to many museums around the world. But does the
evidence for this restoration survive further scrutiny?