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

100%

Morton N.

tom 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.

Stratigraphic distribution of brachiopods in the Lower Jurassic – Middle Jurassic transition of the Lusitanian Basin (Portugal) and a proposal of biozonation

86%

Andrade B. , Joral F. , Henriques M.

tom Vol. 4, no. 1

143-143

A detailed revision of the brachiopods of the Lower-Middle Jurassic transition in the Lusitanian Basin (Andrade 2006) has enabled the establishment of the stratigraphical distribution of this fauna. More than 2,000 specimens were collected at 11 sections throughout the basin, including the Bajocian GSSP in Murtinheira (Cabo Mondego). In all, 24 species, belonging to 14 genera, have been recognized along a stratigraphical interval that includes the Upper Toarcian, the Aalenian, and the Lower Bajocian. The Toarcian associations are characterized by species also recorded in neighbouring basins, such as Stroudithyris stephanoides, Sphaeroidothyris vari, Pseudogibbirhynchia bothenhamptonensis and Soaresirhynchia renzi; as well as species endemic to the Lusitanian Basin, such as Choffatirhynchia alcariensis, Nannirhynchia delgadoi, N. cotteri, Praemonticlarella conimbriguensis, Neozeilleria duartei and Pamirorhynchia(?) jorali. This mixed palaeobiogeographical character persists in the Aalenian, in which the associations include, together with widely distributed species such as Neozeilleria anglica, Pseudogibbirhynchia mutans or Lophrothyris withingtonensis, other species known in neighbouring basins, such as Sphaeroidothyris uretae and Neozeilleria sharpei, and other species recorded only in the basin, such as Soaresirhynchia minor, S. murtinheirensis and Sphaeroidothyris henriquesae. In the Lower Bajocian, excluding Loboidothyris perovalis, only endemic species are present (belonging mainly to endemic genera), such as Lusitanina bituminis, Stroudithyris choffati, Lusothyris atlantica and Mondegia limica. The interpretation of these distributions also enables to propose a brachiopod based biozonation for the studied interval. Three zones have been erected: 1. the Renzi Zone, for the Upper Toarcian, with two subzones: Renzi and Duartei; 2. the Anglica Zone, that ranges from the Aalensis Biochronozone of the Toarcian to the base of the Bajocian. It has been subdivided in 3 subzones: Nuskae, Anglica and Uretae; 3. the Choffati Zone, which comprises the main part of the Discites, Laeviuscula and Sauzei biochronozones, with two subzones: Bituminis and Limica; 4. this proposal of biozonation can be correlated with other established in neighbouring basins, such as the Iberian Range in Spain or the French Basins.