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Evolution of Graphoceratidae and the Aalenian/Bajocian boundary at Bearreraig, Isle of Skye, NW Scotland

Morton N.

Volumina Jurassica

2006

Vol. 4, no. 1

196-196

The GSSP for the base of the Bajocian Stage, and therefore the Aalenian/Bajocian boundary, has been defined at Cabo Mondego, western Portugal and ratified by IUGS. It was the first of the Jurassic Stages to be so defined. At the same time an Auxiliary Stratotype Point (ASP) at Bearreraig, Isle of Skye, NW Scotland was ratified by IUGS. The key marker event for the Aalenian/Bajocian boundary is evolution within the ammonite family Graphoceratidae. The original intention was to use as marker the first appearance of the genus Hyperlioceras (s.l. to include Toxolioceras and other mainly microconch “genera”) which evolved from the genus Graphoceras. However, in the light of detailed study of successions in various areas of western Europe and North Africa, this was modified. The earliest horizon of Hyperlioceras (H. incisum) proved to be too limited in distribution to be useful for correlation, so that the second Hyperlioceras horizon (H. mundum) was selected as the key marker for definition and correlation of the base of the Bajocian. The details of the evolution of Hyperlioceras from Graphoceras are best preserved and documented at Bearreraig, which is why this section was accepted as ASP. The succession is relatively thick (c. 24 m for the topmost Aalenian and lowermost Bajocian) and the ammonites are preserved mainly in a sequence of nodules. Each nodule contains an assemblage, dominated by juveniles, which approximates to a biological population. The dimorphic Graphoceras limitatum/carbatinum species remains virtually unchanged morphologically through some 15 m of strata (evolutionary equilibrium?) and overlap with the first two Hyperlioceras species. The main morphological change into the first Hyperlioceras species (dimorphic H. incisum/rotabilis) is in the shape of the venter (a punctuation event?). This is followed over 13 m of strata by gradational increases in size and involution through H. mundum/aspera and continuing into H. walkeri/contorta.