In the intermediate part of the upper dentition of lamnid sharks, there are distinct small teeth, named the intermediate teeth. This intermediate tooth is the boundary between the anterior teeth in front of it and the lateral teeth behind it. The teeth located near the commissure are named the posterior teeth. Thus, the upper teeth of lamnid sharks are usually divided into four groups. We tried to examine whether these distinctions are anatomically valid.
The symphyseal tooth is excluded from this study.

Fig.1. Dentition of Carcharhinus obscurus

First, the upper dentition of Carcharhinus obscurus, a non-lamnid sharks, is examined as a control. In the upper dentition of C. obscurus, tooth growth is suppressed at two sites: symphysis and commissure. The structure of the upper jaw restricts and narrows the space for tooth growth at these two sites. The space where tooth growth is suppressed is named as SWTGI.

Fig.2. Dentition of Isurus oxyrinchus

Isurus oxyrinchus, which belongs to lamnid sharks, has a SWTGP in the intermediate part of the upper jaw in addition to the two anterior and posterior SWTGPs, for a total of three locations. This intermediate SWTGP is found only in the jaw of lamnid sharks. The origin of this characteristic maxillary deformation is unknown, but it probably evolved in relation to foraging behavior.

Fig.3. Separation of anterior and posterior teeth

SWTGP appeared in the intermediate location of the upper jaw of lamid sharks. This indicates that the upper teeth were separated into two types, the anterior teeth and the posterior teeth, for the first time.

Fig. 4. Appearance of reduced teeth

With the appearance of the intermediate SWTGP, the affected teeth shrink. These teeth can be identified as the anterior or the posterior teeth by comparing their sizes with those of their anterior and posterior counterparts.

Fig. 5. Dentition with many reduced teeth

Odontaspis ferox, with its many reduced teeth, can be identified as the anterior and the posterior teeth, as can Isuru oxyrinchus.

Intermediate tooth

It is common to describe the reduced tooth of laminid sharks as intermediate tooth. This is a very convenient term. However, this terminology is extremely ambiguous. Among other things, it is not logical to equate reduced anterior tooth with reduced posterior tooth without making a distinction.

Shape of the third upper tooth ( A3) in sharks of the genus Carcharodon

Ref. 1. Applegate, Shelton P. 1965. "Tooth terminology and variation in sharks with special reference to the sand shark, Carcharias taurus Rafinesque." Contributions in science 86, 1–18.

Ref. 2. Dentitions of lamnid sharks

Plate 1.

Plate 2.

ver. 1: Sep. 14, 2023

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