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Gli squali fossili

Archaeolamna

SIVERSON 1992
Extinct mackerel shark
Lwr - Upr Cretaceous
Ordine Famiglia
Lamniformes BERG 1958 Cretoxyrhinidae GLIKMAN 1958

Referred to by most North American collectors as Plicatolamna arcuata, or more recently, Cretodus arcuatus, these common Cretaceous teeth should now be ascribed to Archaeolamna kopingensis. Davis (1890) originally named these teeth Odontaspis kopingensis however, Woodward's 1894 description of Lamna arcuata came into common usage. Cappetta and Case 1975 moved the species to Herman's newly erected genus - Plicatolamna. Subsequently, Herman (1977) moved it to Cretodus. Cappetta (1987) questioned this move, and Kent (1994) appears to have reluctantly followed Herman and utilized Cretodus. When studying tooth sets, Siverson found that these lamnoid teeth were part of a primitive, odontaspid-like dentition. He erected a new genus (Archaeolamna) and reverted to the senior synonym (kopingensis).

This genus is known from Albian to Maastrichtian sediments of Europe, North America and Australia, generally from isolated teeth only. A single species have been attributed to the western North Atlantic --Archaeolamna kopingensis (DAVIS, 1890). Teeth from this species dating to the Maastrichtian of the Western Interior Seaway (Wyoming and Montana) vary from their Atlantic counterparts and have been placed in the subspecies A. k. judithensis.

As alluded to earlier, the genus does not have the relatively simple tooth-set seen in living lamnoids. Rather, an odontaspid-like, complex grouping of teeth is present. It incorporates small symphyseal?/parasymphyseals, intermediates and posterior teeth and enlarged anterior and laterals.

In general, the teeth of A. kopingensis have bilobate roots with a U-shaped basal margin and a lingual protuberance. Some teeth show evidence of a very weak nutrient groove. The cusp is triangular and elongated with a pair of triangular cusplets and complete cutting edge. Except for some posterior teeth, the crown are smooth (without wrinkles).

The upper anteriors have a weak distal bend with symmetrical roots and the lowers are more erect with less symmetrical roots. Upper laterals bend more strongly towards the commissure and the roots are shorter with an obtuse basal margin. Lower laterals are more erect than the uppers and the mesial lobe is often elongated. Posterior teeth are much smaller than the anteriors and laterals with a distally directed cusp and often lacking a cusplet on the mesial shoulder.
 

Fig. 1 - Archaeolamna kopingensis
from left to right, 14 x 12.5, 17.5 x 12.5 & 20 x 14.5 mm
Big Brook, Monmouth Co., NJ

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