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Development and diversity of the Lithiotis-type bivalves in the Lower Jurassic carbonates of the Central and Southern Velebit Mt., Croatia

Lithiotis-type bivalves are characteristic faunal element of many Lower Jurassic successions in southern Europe, western Arabia, western and central Asia, as well as western margin of North and South America. These large bivalves were most significant buildup-makers in Early Jurassic shallow-marine environments of many Tethyan carbonate platforms. The study of carbonates with Lithiotis-type bivalves was carried out in Central Velebit Mt. (Kubus section) and Southern Velebit Mt. area (Libinje and Mali Alan sections), which are 50 km apart. Benthic foraminifera assemblage (Lituosepta recoarensis, Paleomayncina termieri, Lituosepta compressa, Orbitopsella primaeva, O. praecursor, Pseudocyclammina liasica, Socotraina serpentina) indicates Late Sinemurian to earliest Toarcian age. The thickness of sections with Lithiotis-type bivalves varies from 210 m on Kubus, to 171 m on Libinje and 145 m on Mali Alan. The oldest Lithiotis-type bivalves were found in the lowermost Upper Sinemurian beds at Kubus and youngest in the Lower Toarcian beds at all three sections. Lithiotis-type bivalve shells mostly occur in micritic limestones indicating growth on muddy substrates, as typical mud-stickers fauna, in low- to moderate-energy shallow-marine lagoonal environments of the inner carbonate platform. Shells are mostly reworked, and therefore rarely preserved in their primary life position. All studied sections were characterized by similar shallow-marine environments during the Late Sinemurian and beginning of the Early Pliensbachian. Relative deepening started in late Early Pliensbachian at Kubus (as indicated by occurrence of coarser-grained limestones with slumps), while in Southern Velebit sections shallow-marine environments persisted until the latest Pliensbachian. The Pliensbachian–Toarcian transition and earliest Toarcian were characterized by deposition of dark grey micritic limestones with Lithiotis-type bivalves and clayey limestones with thin marly beds. Intensely bioturbated micritic limestones, known as “spotted limestones” sharply overlay the last beds with Lithiotis-type bivalves and S. serpentina indicating middle Early Toarcian age. Lithiotis-type bivalves are most abundant in the Upper Pliensbachian and lowermost Toarcian deposits in all sections and are generally more frequent in Southern Velebit Mt. Preliminary results show that Lithioperna scutata occurred as the first among Lithiotis-type bivalves, and is the most common species in the Upper Sinemurian and Lower Pliensbachian beds. First occurrence of Cochlearites loppianus can be noticed in the middle Lower Pliensbachian beds. L. scutata and C. loppianus were equally frequent during the Late Pliensbachian. Lithiotis problematica was certainly determined only in the Lower Toarcian beds of the Kubus section. However, most of the Early Toarcian Lithiotis-type bivalves cannot be determined on generic level. The trend of increasing shell size is clearly seen in all Lithiotis-type bivalves: Late Sinemurian L. scutata shells were smaller (average shell length 4–5 cm) comparing to the Pliensbachian ones (9–12 cm) and generally, they are smaller than shells of C. loppianus, which measured 15–30 cm in length from the Late Pliensbachian and 25–30 cm from the Toarcian. Rapid development and wide diversity of these unique bivalves during the Early Jurassic indicate recovery of optimal conditions within shallow-marine environments before their final demise caused by the worldwide early Toarcian Oceanic Anoxic Event (T-OAE).

Lithiotis-type bivalves ; Lower Jurassic ; Velebit ; Croatia

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