Fossil discoveries from the Morrison Formation

Zachary Sepulveda (USA)

The subject matter of this short article is a rare, agatised snail fossil from the Jurassic Period, which was found in the Morrison Formation, and a dinosaur bone of an unknown species from the same layers. Both specimens are from Colorado. The Morrison Formation is a sequence of sedimentary rocks, which hold many fossils from the late Jurassic. For example, iconic dinosaurs such as Allosaurus fragilis, Diplodocus longus, and Brachiosaurus altithorax have all been found in this layer of Jurassic sediment.

Valvata scadbria

The aquatic gastropod is about 165 million years old and is a member of the Valvatidae, a taxonomic family of small, freshwater gastropods known as the ‘valve snails’. These have an operculum that is like a trapdoor attached to the upper surface of the foot and which closes the opening of the shell when the soft parts of the snail are retracted inside. The snail is from the species Valvata scadbria (Humbolt State University, 2002) and lived in a Jurassic freshwater pond or lake.

The specimen shown in Fig. 1 is quite beautiful as it now consists of agate. Over millions of years, the shell of the snail was replaced by this mineral in a process known as agatisation, which also occurs in other types of fossils. It is a beautiful mineral that is often used in jewellery.

Dinosaur bone

The dinosaur bone (Fig. 2) is from an unknown species and yields clues as to where the dinosaur died. It has not yet been dated at the time this paper was written, however, it appears to be a fragment of a large bone and it is attached to an extremely fine-grained sandstone. This indicates that the dinosaur died on a Jurassic beach – part of the Western Interior Seaway – which was a large sea that covered much of the central region of what is now the USA. Agate crystals have formed within the Haversian canals and can be seen at either end of the fossil bone. These are the tiny, interconnecting, longitudinal channels in bone tissue through which blood vessels, nerve fibres and lymphatics pass.

The dimensions of the gastropod fossil and dinosaur bone are shown in Table 1.

Overall, these are two magnificent fossils that help us reconstruct ancient worlds that have long since passed into deep time.

References

The gastropoda. (n.d.). Retrieved from: http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php

Humbolt State University Natural History Museum: Jurassic period. (2002, October 15). Retrieved from: http://www.humboldt.edu/natmus/Case_indexes/Case_jpgs/Jurassic.web/