DISEASE NEVER COMES SINGLY: A PATHOLOGICAL CASE STUDY ON A CRETACEOUS AMMONOID MENUITES JAPONICUS (AMMONITIDA, PACHYDISCIDAE) FROM HOKKAIDO, JAPAN

In general, the hard part of animal bodies has more possibility of fossilization than the soft part and tends to record the pathological deformation easier. Particularly molluscan shells often record the progression of the disease because of their additional growths. A representative of Menuites japonicus (MCM-A2010) from the Upper Cretaceous in Hokkaido, northern Japan, has two types of pathological deformations that correlated to each other.

First, one of the pathological deformations is “forma aegra alternospinata”. This shows asymmetrization of the whorl ornamentation. Menuites japonicus usually has one row of tubercles on the umbilical margin per one side of the juvenile whorl and then two rows of those on the ventrolateral shoulder and the umbilical shoulder the late stage. However, MCM-A2010 becomes breaking of left-right symmetry of tubercles near the changing point of ornamentation. Deviating of the symmetry gets worse through its growth.

Second, “forma aegra juxtalobata” is also appeared in MCM-A2010. This pathological deformation shows a shifted the siphuncle to one side. The maximum veering of the siphuncle within the outside is located on the whorl covered by the aperture and shifting right side in 1.7 mm from the median plane.

We scanned MCM-A2010 by using X-ray Micro-CT and reconstructed inner structures by using Avizo, a 3D data visualization software. The 3D data reconstructed from CT data shows the siphuncle veering from the median plane frequently in the inner whorl. And the maximum shifting occurred in the phragmocone covered by the aperture, and then the siphuncle returns to the median plane according to the growth.

Considering the length of the body chamber, exacerbation of forma aegra alternospinata will coincide with remission of forma aegra juxtalobata. This means two types of different diseases in ammonoid have a great relationship. Two types of deformations we observed might be caused by some endogenetic factors because there is no trauma such as bite marks or parasite scars on the surface of this specimen. Besides, it is notable that the deformation of ribs has scarcely occurred, whereas the symmetry of tubercles is broken. This suggests the formation mechanism of tubercles might be independent of that of ribs.