Science Article

July 2000

Exceptional Soft-Tissue Preservation in A Fossilized Dinosaur
By A.A. Snelling
Reproduced CEN Tech., vol. 12, no 2, 1998

Only rarely are the soft tissues of animals preserved by burial and fossilization, because such exquisite preservation requires exceptional circumstances. Yet it is becoming increasingly obvious from fossils that are being unearthed that such exceptional circumstances were probably not all that rare.

Amongst the fossil deposits best known for preservation of the soft tissues of dinosaurs are the Santana Formation of Brazil (1) and the Yixian Formation of China (2). However, now the Lower Cretaceous Pietraroia Plattenkalk in the Benevento Province of southern Italy can be added to this list of lagerstatten (known as conservative deposits). This rock unit has been known since the 18th century for its beautifully-preserved fossil fishes, but now the full preparation of a fossil theropod specimen found preserved in this marly limestone, with details of soft anatomy never seen previously in any dinosaur, has been reported (3).

The specimen (see Figure 1) has been named as Scipionyx samniticus gen. et. Sp. nov., a new genus and species in the Maniraptoriformes family of coelurosaurs in the suborder Theropoda, and the first dinosaur ever to be found in Italy.3 Its skeleton, 237 mm long from the tip of its jaws to the last preserved tail vertebra, lies on its left side in near perfect anatomical articulation. The head is upturned with respect to its position in life, and most of the skelton is intact (only the distal portions of the hindlimbs, most of the tail and the second claw on the right forelimb are missing). The body proportions indicate that this animal was little more than a hatching (4,5).

A unique, striking feature of this specimen is the exceptional preservation of soft parts (see figure 1).3 Muscles are present in the chest area, with scattered acicular (needle like) fibers visible under 50x magnification. Large muscular fibers are also preserved at the tail base. Most of the intestine (5.2mm average diameter) is positioned further forward than was generally thought, whereas the colon passes through the pelvic canal close to the vertebral column. The gut is described as being surprisingly short and deep in section. Immedidately above the furcula (wishbone), there appear to be some tracheal rings. A large, reddish, welldelimted hematitic (iron-oxidestained) halo is tentatively interpreted as liver traces, mainly because of its location.

The gastralia (abdominal ribs), still in life position, allow estimation of the abdominal depth and reveal their contribution to an effective support for the posterior intestinal tract. The presence of a furcula in this articulated specimen eliminates every doubt about the interpretation of similar structures in other theropods (6,7). Despite this being the best preserved theropod specimen found (thus far), there is also no evidence of feathers or remnants of any other outer covering (for example, skin) (3). Thus this specimen provides no support whatsoever for the evolutionary 'just so' story about dinosaurs being the immediate ancestors of birds.

The discovery of this specimen can only be described as remarkable, considering the scarcity of juvenile theropod dinosaurs in the fossil record. Furthermore, it's the exceptional preservation of soft tissue that makes this juvenile theropod specimen unique. It has been suggested that to accomplish such exceptional preservation, the fine-grained marly limestone containing this fossilized dinosaur must have been deposited in a shallow lagoonal environment affected by cyclic periods of low oxygen levels (3).

However, this suggestion ignores the evidence in comparable modern environments with low oxygen levels and water-saturated lime muds that anaerobic bacteria will rapidly destroy soft tissues. The solutions extremely rapid burial of the animal in the lime muds followed by rapid diagenesis (hardening) to marly limestone. Indeed, rapid diagenesis of lime muds can be observed today on the north Norfolk (England) coast, (8,9) and has been given as the explanation for the Santana Formation lagerstatten (Brazil (10).

Thus one can only marvel at the inability of so many scientists to see and comprehend what such exceptional soft-tissue preservation implies - not slow and gradual sedimentation and burial, but extremely rapid geological processes.

REFERENCES

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2. Ji, Q. and Ji. S., 1996. On discovery of the earliest bird fossils in China and the origin of birds. Chinese Geology, 233:33.

3. Dal Sasso, C. and Signore, M., 1998. Exceptional soft-tissue preservation in a theropod dinosaur from Italy. Nature, 392:383-387.

4. Norell, M. A., Clark, J. M., Dashzeveg, D., Rhinehen, B., Chiappe, L. M., Davidson, A. R., Mckenna, M. C., Altangerel, P. and Novacek, M. 1994. A theropod dinosaur embryo and the affinities of the Flaming Cliffs dinosaur eggs. Science,266:779-782.

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6. Bryant, H. N. and Russell, A. P., 1993. The occurrence of clavicles within Dinosauria: implications for the homology of the avian furcula and the utility of negative evidence. Journal of the Vertebrate Paleontology, 13:171-184.

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10. Martill, D. M., 1989. The Medusa effect: instantaneous fossilization. Geology Today, 5:201-205.