Palaeoscincus, Leidy, 1856

Sahni, Ashok, 1972, The vertebrate Fauna of the Judith River formation, Montana, Bulletin of the American Museum of Natural History 147 (6), pp. 319-416 : 363-364

DINOSAUR EGG SHELL FRAGMENTS

The discovery of dinosaur egg shell fragments from Clambank Hollow in the Judith River Formation unfortunately was made after most of the material had been etched in 10 per cent formic acid. Surprisingly, the external and internal sculpture pattern is well preserved in the etched as well as in the nonetched material. The internal stratification of mineral and organic layers, however, is less clear in radial sections of the acid-treated material, indicating that these substances have been leached out.

At least five separate external sculptured patterns can be seen, including all three of the categories devised by Jensen (1966) for distinguishing fossil egg fragments from the North Horn Formation of Utah. According to Jensen's (1966) criteria, the egg shells probably belong to those of dinosaurs, although Jensen's Class A eggs bear certain resemblances to bird eggs. The egg shells described here are all believed to be dinosaurian.

The most common egg shell fragments from the Judith River Formation are those that belong to Jensen's Class B. Fragments from Jensen's Class A and Class C are rare in the Judith River Formation.

CLASS A: Shells in this class are characterized by a smooth external surface with little or no sculpturing. No color mosaic can be noticed. Pores are distinct and well spaced and their density in AMNH 8544 is about 10/sq. cm. (fig. 9X). The pore openings are either subcircular or pentagonal with straight sides, and do not appear to be elongated parallel to the length of the shell as mentioned by Jensen (1966, p. 61). The internal surface consists of mammillae which reach a maximum diameter of 0.1 mm. The thickness of the shell is 1.0 mm. Stratification, in radial section, is indistinct or, at places, absent.

CLASS B: These shells are ornamented with nodose or linear ridges raised above the general surface of the shell. This, by far, is the most common class in the Judith River Formation. AMNH 8545 is a fairly large fragment (1.5 x 2.0 cm.) with external sculpturing of small, wellspaced nodes (fig. 9P, Q). Pores occur in the valleys between the nodes, are indistinct and few in number. The thickness of the shell is 1.0 mm.

The nodes in specimen AMNH 8546 coalesce to form meandering ridges (fig. 9R, S). There is a possibility that this type is identical to the one described above and that the differences are positional in nature. The thickness of the shell is 1.2 mm.

Different from the other nodular types is AMNH 8547 (fig. 9V, W). The nodes are not circular in outline but have squared off bases and are relatively more closely spaced. Pores do not occur between the nodes but are deep gutter-like depressions that incompletely encircle the nodes. The thickness of the shell is 1.0 mm. As in some North Horn Formation forms, the internal surface consists of small mammillae with an average diameter of about 0.01 mm.

CLASS C: Specimen AMNH 8548 probably belongs to the Class C category ofJensen (1966). The sculpture pattern is impressed into the external layer of the shell and consists of deeply incised grooves and irregular pits (fig. 9T, U). Two distinct shell layers are visible in side view, as well as in radial section. The upper layer is 0.7 mm. thick and appears to be porous and spongy; the lower layer is 1.2 mm. thick, quite dense and extremely hard. No distinct pores are present, and their function must have been carried out by the irregular grooves and pits. On the internal surface, the mammillae are small and relatively well separated.

The presence of fragile dinosaur egg shell pieces in the Clambank Hollow Quarry certainly indicates that the eggs were laid quite close to the site of deposition. The variety of the shells also indicates that their discovery at the locality is not a chance occurrence, but that the stream bank was a suitable laying area for a number of different dinosaurs. No generic names have been assigned to the egg shell fragments and as yet little can be said even about the familial affinities of the eggs. The presence of grossly similar types in the Campanian and in the Maestrichtian suggests that similar reptiles existed during the two epochs. Although members of the major groups of dinosaurs were more diverse during the Campanian, all the families persisted into the Maestrichtian.

The material from Campanian Clambank Hollow adds yet another locality where eggs of dinosaurs have been found. Eggs and egg shell fragments have been reported from Mongolia ( Van Straelen, 1928), Southern China ( Young, 1965), India ( Sahni, 1957), France and Brazil ( Thaler, 1965), and North America ( Jepsen, 1931; Jensen, 1966).