SMF R 4970 — A Legendary Specimen

The genus Psittacosaurus was a dinosaur that lived in East Asia during the Early Cretaceous. As many as twelve valid species have been assigned to this genus, with several additional forms that may represent distinct species. More than four hundred specimens have been discovered, making Psittacosaurus one of the most diverse genera among non-avian dinosaurs.

Among these more than four hundred specimens, some are fragmentary while others are nearly complete. One specimen in particular—SMF R 4970—is extraordinarily well preserved and has acquired an almost legendary reputation. Because of this exceptional preservation, paleontologists have been able to extract an impressive amount of biological information from it. Here we explore several of the remarkable insights that this single fossil has revealed about dinosaurs.

Image of specimen SMF R 4970（圖片來源：Bell, P. R et al.(2022)，採用 CC BY 4.0 授權）

Countershading Camouflage

Countershading is a common camouflage pattern found across many animal groups. Anyone who regularly observes wildlife will notice that numerous animals possess darker coloration on their backs and lighter coloration on their undersides. Many fish, for example, show the classic combination of a dark blue dorsal surface and a pale or white belly. This pattern is a typical form of countershading.

The principle behind countershading is relatively simple. When an animal is viewed from above, observers often see it against the background of its surrounding environment. In the ocean, for instance, the water appears blue, so a fish with a darker dorsal surface blends more effectively with that background. In contrast, when the animal is viewed from below, the observer's gaze is directed toward the bright sky or sunlight. Under such conditions, a pale underside makes the animal less conspicuous.

Because SMF R 4970 preserves exceptionally complete skin impressions, researchers were able to analyze fossilized pigment structures known as melanosomes. These microscopic bodies preserve traces of original pigmentation and allowed scientists to reconstruct the coloration of Psittacosaurus. The results indicate that this dinosaur possessed a clear countershading pattern, with a darker back and lighter underside.

Comparisons with modern ungulates show that countershading generally occurs in two major forms. Animals inhabiting open environments such as grasslands receive direct overhead sunlight. In such cases the transition between dark dorsal coloration and light ventral coloration is usually sharp and clearly defined. By contrast, animals living in closed environments such as forests experience diffuse lighting filtered through vegetation. In these animals the transition between dark and light pigmentation tends to be gradual.

The countershading pattern preserved in SMF R 4970 belongs to the second type. The pigmentation changes gradually rather than abruptly, suggesting that Psittacosaurus inhabited dimly lit forest environments. This interpretation matches paleoenvironmental evidence from the Jehol deposits, which indicates forests dominated by evergreen conifers surrounding the lakes where these fossils were buried.

Considering that Psittacosaurus includes many species, it is likely that coloration varied among them to match different habitats.

Tail Bristles

Bristles on the tail of specimen SMF R 4970（圖片來源：Bell, P. R et al.(2022)，採用 CC BY 4.0 授權）

The idea that birds are dinosaurs is now widely accepted. If birds belong within Dinosauria, then it follows that at least some dinosaurs possessed feather-like structures.

For many years feathers were thought to occur only in theropods, the dinosaur group most closely related to birds. Later discoveries revealed that ornithischian dinosaurs—the other major dinosaur lineage—also possessed filamentous integumentary structures. One of the most striking examples comes from SMF R 4970.

This specimen preserves a distinct tuft of filamentous structures on the tail. These filaments are extremely primitive, consisting of simple hollow bristles rather than complex feathers. Because they are restricted to the tail, they probably did not serve insulation or aerodynamic functions. Instead, they may have functioned as ornamental structures used during courtship displays or social signaling.

The discovery of these tail bristles has influenced many modern reconstructions of ceratopsian dinosaurs, in which similar tail filaments are often depicted.

The Cloaca

Reconstruction of Psittacosaurus, with particular emphasis on the cloacal region. The skin surrounding the cloaca is reconstructed as noticeably darker. The illustration also depicts the countershading pattern, with a darker dorsal surface, as well as the tubular bristles present on the tail.。 圖片由Bob Nicholls提供

Comparison of the cloaca of SMF R 4970 and that of a crocodile（圖片來源：Bell, P. R et al.(2022)，採用 CC BY 4.0 授權）

The reproductive anatomy of reptiles differs from that of mammals. In reptiles, the digestive, excretory, and reproductive systems all open into a single chamber known as the cloaca.

Because the cloaca is composed of soft tissue, it rarely fossilizes. Remarkably, SMF R 4970 preserves the external opening of the cloaca, making it the only known non-avian dinosaur specimen with this structure preserved.

Among living reptiles, three general cloacal morphologies exist. Lizards and snakes typically possess a transverse opening, crocodilians have a longitudinal slit, and birds usually have a rounded opening. Comparisons show that the cloacal anatomy of Psittacosaurus most closely resembles that of crocodilians, including the presence of paired lateral lips surrounding the opening.

Unfortunately, the cloaca alone cannot reveal the sex of the individual. In reptiles, male reproductive organs remain hidden inside the cloaca except during copulation, so the external opening does not provide reliable evidence of sex.

Interestingly, the sides of the cloacal region appear swollen in SMF R 4970, resembling the position of paracloacal glands in crocodilians. These glands produce odoriferous secretions used in social communication. The cloacal region also shows distinct pigmentation, which may have functioned as a visual signal during courtship displays. Among living reptiles, similar visual signaling around the cloaca is known only in birds, which may suggest that this trait can be traced back to the most recent common ancestor of all dinosaurs.

The Umbilicus

Umbilicus of specimen SMF R 4970（圖片來源：Bell, P. R et al.(2022)，採用 CC BY 4.0 授權）

In mammals, embryos receive nutrients through an umbilical cord connected to the placenta. After birth, the cord detaches and leaves a scar known as the navel.

Although reptiles do not possess a placenta, their embryos still connect to the yolk sac through a structure that leaves a similar scar after hatching. Thus reptiles can also possess a navel-like mark.

In SMF R 4970, researchers identified two symmetrical rows of enlarged scales along the midline of the abdomen. After eliminating other possibilities, these structures were interpreted as the fossilized trace of an umbilical scar. If this interpretation is correct, it represents the first discovery of an umbilicus preserved in the fossil record of an amniote.

In most living reptiles the umbilical scar heals soon after hatching and becomes invisible. However, SMF R 4970 represents an adult individual, suggesting that in Psittacosaurus the umbilical mark remained visible throughout life.

A Brief Conclusion

In reality, SMF R 4970 has yielded many more discoveries than the four described here. Space limitations simply prevent discussing them all. These four examples were selected because they provide particularly intriguing insights into dinosaur biology.

At the same time, the extraordinary preservation of SMF R 4970 creates a challenge: many of its anatomical features are unique among known non-avian dinosaur fossils, making comparisons with other specimens difficult.

Nevertheless, researchers continue to study this remarkable fossil with great interest. Many of the discoveries mentioned above have been made only in recent years—the identification of the umbilicus, for example, was reported as recently as 2022—suggesting that SMF R 4970 may still hold further surprises.

SMF R 4970 — Specimen Information

The fossil specimen was discovered in the Jehol Biota deposits of Liaoning Province, China. It was later smuggled out of the country and eventually became one of the major exhibits of the Senckenberg Natural History Museum in Frankfurt. At present, the specimen cannot be confidently assigned to a specific species, and it is therefore classified simply as Psittacosaurus sp.

Author: Bai Leng

Reference:

1. Vinther, J., Nicholls, R., Lautenschlager, S., Pittman, M., Kaye, T. G., Rayfield, E., Mayr, G., Cuthill, I. C. (2016). 3D Camouflage in an Ornithischian Dinosaur. Current Biology.

2. Vinther, J., Nicholls, R., Kelly, D. A. (2021). A cloacal opening in a non-avian dinosaur. Current Biology.

3. Bell, P. R., Hendrickx, C., Pittman, M., Kaye, T. G., Mayr, G. (2022). The exquisitely preserved integument of Psittacosaurus and the scaly skin of ceratopsian dinosaurs. Communications Biology.

4. Bell, P. R., Hendrickx, C., Pittman, M., Kaye, T. G. (2022). Oldest preserved umbilical scar reveals dinosaurs had ‘belly buttons’. BMC Biology.