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

The Maastrichtian Maevarano Formation of northwestern Madagascar has yielded an extraordinary skeletal record of vertebrate fossils that includes dinosaurs, crocodiles, mammals, birds, turtles, lizards, snakes, and fish. However, despite an increasingly abundant fish record (a wealth of fish fossils have been collected during recent expeditions), skeletal remains of lungfish have not been recovered from the formation, and this is somewhat paradoxical because lungfish tooth plates (and potential burrows) are known from the underlying Ankazomihaboka and Marovoay beds. Recent work in the Befendrana field area, which is located approximately 11 km to the northeast of the main Berivotra field area, has led to the discovery of abundant clustered burrows that exhibit morphological characteristics consistent with generation by lungfish. The majority of the burrows attributed to lungfish occur in cross-bedded fluvial sandstones of the Masorobe Member. Over 100 burrows were described and mapped in this unit. The vast majority of burrows are exposed on bedding planes, but several are represented in cross section. Burrow traces exposed on bedding planes are generally elliptical in shape and are filled by concentric rings of predominantly downward deflected foreset laminae. Long axes of the elliptical burrow traces range from 9-29 cm, and average 16 cm. Minor axes range from 6-16 cm, and average 11 cm. Interestingly, the long axes of the burrow traces tend to align with paleocurrents in the local outcrop belt. In cross section, the burrows are non-branching and generally straight, with smooth walls and distinct demarcation from surrounding sediment. The depth of penetration ranges from 15-65 cm, although this measure is an underestimate because erosion has exhumed the tops of the ancient burrows. Several burrow traces show indication of both entrance and exit, as evidenced by cross-cutting relationships and reversed deflection of laminae. Similar burrows interpreted as lungfish aestivation traces have also been discovered in the basal meter of the Anembalemba Member in the Befendrana field area. The absence of lungfish burrows in the Berivotra field area presumably reflects inhospitable depositional conditions, including the recurrent emplacement of fine-grained debris flow deposits in the channel belts.