As sharks grow in size, one can reasonably assume that their teeth grow in size too. So if sharks were spending their entire lifespan in this area, or visited this area regularly throughout their lifespan, we should be finding smaller and larger teeth from the same species of shark. In some animals, and in some sharks too, the detailed shape of the teeth changes as they get older. In some shark species, juveniles have more cusps or serrations, while in other species it is the oposite. But in all cases you can still recognize that the teeth came from the same species.
When I looked at the species in my collection I noticed two striking examples, the "Tiger" shark Physogaleus contortus (formerly known as Galeocerdo contortus) and the Snaggletooth shark Hemipristis serra. From both species I have collected a few hundred teeth at Brownie's Beach.
As can be seen in the picture below, the teeth from Contortus are almost all the same size. There are a few smaller teeth, but these are clearly (para)symphysials, which are smaller. If we assume that these teeth belong to adult sharks, we could conclude that they were born elsewhere, and only moved to, or visited this area once they were larger.
 About one hundred Physogaleus contortus teeth, a representive sample from my collection. The smaller teeth on the left are all typical stubby parasymphyseals with very swollen roots, NOT juveniles. The other teeth are mostly lower jaw teeth separated in left/right and sorted by position in the jaw from the midline, or anterior teeth (top) to the rear, or posterior teeth (bottom + upper right). |
The Hemipristis teeth vary greatly in size. Of course, within the dentition of a single Hemipristis there is a great variation in tooth shape and size. However, just because tooth shape varies so much, it is very easy to tell whether a tooth is from the upper or lower jaw, and it is even possible to tell exactly from which position a particular tooth came from! So, if we compare teeth that have a very similar shape, we can assume they are from a similar position in the jaw. Any variation in size must then mean we are looking at sharks of different ages. This is exactly what can be seen in the figure below.
 Figure illustrating the variation in size for five Hemispristis teeth from a very similar location in the lower left jaw. Sizes as measured along the longest edge from the tip to the root: 38mm, 29mm, 25mm, 19mm, 13mm. |
All five teeth are from the 4th or 5th position in the lower left jaw (or at least very close to each other), but they vary in size from 38mm for the largest to 13mm for the smallest! From this we can conclude that Hemipristis spent most of its life in this area, or was a regular visitor throughout its lifespan.
Of course, I may be completely wrong! If you know of any literature that confirms or contradicts my conclusions I would be really interested in a reference!