Institute For Ice Age Studies

The first thing to remember about proboscidean tusks (Figure 1) is that they are merely specialized teeth (upper incisors), and they are composed predominately of dentine. In contrast to many other ivories, mammoth and elephant tusks have a complex structure formed by specialized cells known as odontoblasts. These odontoblasts produce new dentine along the lining of the pulp cavity. As new dentine is produced, the odontoblasts migrate to the new surface of the pulp cavity, leaving behind processes known as dentinal tubules (MacGregor 1985) or so-called Schreger lines. These "radiate outward from the pulp cavity and incline obliquely towards the tip" of the tusk. The result, in transverse section, is a complex three-dimensional structure that takes the form of arcs running counter to other across the width of the tusk (Figure 2:IV), and producing by their intersection what are frequently referred to as "engine-turnings" or the Schreger pattern (Espinoza and Mann 1992).

Fig. 1: The structure of proboscidian tusk (after Espinoza and Mann 1992)

In transverse section, this Schreger pattern cross-cuts broad concentric rings often referred to as laminae (Figure 2:I). These are actually growth interfaces between superimposed cones of ivory. In living tusk, the boundaries between these laminae are not areas of structural weakness, being bound together by the complex intersecting dentinal tubules composing the Schreger lines and by collagen fibres that fill the interstices between the dental tubules (Waters 1980). However, upon dessication and accompanying deterioration of dentinal collagen, tusks tend to split or spall along these concentric boundaries (Figure 2:I), which are far more developed in the outer zones of the tusk's diameter. The inner core of the tusk, in the area surrounding the central nerve canal (Figure 2:II), is highly compact, homogeneous and virtually immune to such spalling. As a result, such inner ivory is exceedingly difficult to work with stone tools. It is perhaps no surprise then that in ivory-rich Aurignacian sites in SW France, this "core ivory" is abundantly represented in the waste products of tusk reduction.

Fig. 2: Cross-section of mammoth-tusk from the Alaskan permafrost showing I:Slight concentric dessication fractures near the exterior of the tusk, II: The homogeneous inner core of the tusk with nerve canal and radial dessication fracture, III: concentric dessication fracturing cross-cut by radial hairline fractures,IV: Detail of Schreger lines showing how dessication fractures cross-cut the Schreger lines.