EXCEPTIONAL LUNGFISH BURROWS IN THE UPPER CRETACEOUS MAEVARANO FORMATION, MAHAJANGA BASIN, NORTHWESTERN MADAGASCAR

The Upper Cretaceous (Maastrichtian) Maevarano Formation (Mahajanga Basin, northwestern Madagascar) is renowned for its exceptional vertebrate fossil record, but to date skeletal remains of lungfish have not been recovered from the unit. Lungfish fossils do occur in immediately underlying strata. Here we report the first unequivocal lungfish trace fossils known from the Maevarano Formation. More than 100 localized burrows occur in a pervasively cross-stratified sandstone body in the upper 5 m of the Masorobe Member, and several additional burrows occur at the base of the overlying Anembalemba Member. In cross section the burrows exhibit a non-branching, smooth-walled, vertical channel. In map view, the burrows have three predominant morphologies: circular, elliptical, and figure eight shaped. Circular cross-sections have diameters of 10-15 cm. Elliptical cross-sections have minor diameters of 6-14 cm, and an average minor to major axis ratio of 0.7. The figure eight cross-sections have minor diameters of 7-12 cm, and an average minor to major axis ratio of 0.5. These dimensions and architectures are consistent with those of lungfish burrows reported in previous studies, and parallel an actualistic study in Gambia that documented identical morphologies in excavated burrows. The figure eight shaped traces are particularly diagnostic, and represent cross-sections taken through the midsection of flask-shaped chambers that hosted lungfish folded in a U-shape (a position commonly assumed during aestivation). The edges of the burrow cross-sections are defined by differential oxidation or weathering of the infilled sediment, and the major axes of the burrows are generally aligned with the trend of the paleocurrents in the host facies. Two symmetrical projections from one burrow cross-section likely represent fin marks. The occurrence of burrows within pervasively cross-stratified facies reveals the impact of burrowing lungfish on surrounding sediment, and strata are deformed several centimeters beyond the outer walls of the burrows. This record of burrows confirms the presence of lungfish in the Upper Cretaceous Maevarano Formation, and also serves to expand our understanding of lungfish burrow morphology in the ichnological record.